Post on 06-Jul-2020
N° d’ordre 2010-ISAL-0101 Année 2010
Thèse
présentée devant
l’Institut National des Sciences Appliquées de Lyon
pour obtenir
le grade de Docteur
École Doctorale : Matériaux de Lyon
Spécialité : Matériaux Polymères et Composites
par
Sébastien LIVI
----------------------
IONIC LIQUIDS MULTIFUNCTIONAL AGENTS OF THE POLYMER MATRICES
---------
Soutenue le 02 décembre 2010 devant la Commission d’Examen :
JURY
DUCHET-RUMEAU Jannick Professeur (INSA Lyon) – Directrice de thèse GALY Jocelyne DR CNRS (INSA Lyon) – Examinateur GANTILLON Barbara Dr (Société TEFAL) – Examinateur GERARD Jean-François Professeur (INSA Lyon) – Co-directeur de thèse PHAM Thi Nhàn Maître de Conférences (Université de Caen) – Examinateur PLUMMER John Christopher Professeur (Ecole Polytechnique Fédérale de Lausanne) – Rapporteur SEGUELA Roland DR CNRS (Université de Lille 1) – Rapporteur
INSA de Lyon – Liste des Ecoles Doctorales
III
INSA Direction de la Recherche – Ecoles Doctorales – Quadriennal 2007-2010
SIGLE ECOLE DOCTORALE NOM ET COORDONNEES DU RESPONSABLE
CHIMIE
CHIMIE DE LYON http://sakura.cpe.fr/ED206
M. Jean Marc LANCELIN
Insa : R. GOURDON
M. Jean Marc LANCELIN Université Claude Bernard Lyon 1 Bât CPE 43 bd du 11 novembre 1918 69622 VILLEURBANNE Cedex Tél : 04.72.43 13 95 Fax : lancelin@hikari.cpe.fr
E.E.A.
ELECTRONIQUE, ELECTROTECHNIQUE, AUTOMATIQUE http://www.insa-lyon.fr/eea M. Alain NICOLAS
Insa : C. PLOSSU ede2a@insa-lyon.fr Secrétariat : M. LABOUNE AM. 64.43 – Fax : 64.54
M. Alain NICOLAS Ecole Centrale de Lyon Bâtiment H9 36 avenue Guy de Collongue 69134 ECULLY Tél : 04.72.18 60 97 Fax : 04 78 43 37 17 eea@ec-lyon.fr Secrétariat : M.C. HAVGOUDOUKIAN
E2M2
EVOLUTION, ECOSYSTEME, MICROBIOLOGIE, MODELISATION http://biomserv.univ-lyon1.fr/E2M2 M. Jean-Pierre FLANDROIS
Insa : H. CHARLES
M. Jean-Pierre FLANDROIS CNRS UMR 5558 Université Claude Bernard Lyon 1 Bât G. Mendel 43 bd du 11 novembre 1918 69622 VILLEURBANNE Cédex Tél : 04.26 23 59 50 Fax 04 26 23 59 49 06 07 53 89 13 e2m2@biomserv.univ-lyon1.fr
EDISS
INTERDISCIPLINAIRE SCIENCES-SANTE Sec : Safia Boudjema M. Didier REVEL Insa : M. LAGARDE
M. Didier REVEL Hôpital Cardiologique de Lyon Bâtiment Central 28 Avenue Doyen Lépine 69500 BRON Tél : 04.72.68 49 09 Fax :04 72 35 49 16 Didier.revel@creatis.uni-lyon1.fr
INFOMATHS
INFORMATIQUE ET MATHEMATIQUES http://infomaths.univ-lyon1.fr M. Alain MILLE
Secrétariat : C. DAYEYAN
M. Alain MILLE Université Claude Bernard Lyon 1 LIRIS - INFOMATHS Bâtiment Nautibus 43 bd du 11 novembre 1918 69622 VILLEURBANNE Cedex Tél : 04.72. 44 82 94 Fax 04 72 43 13 10 infomaths@bat710.univ-lyon1.fr - alain.mille@liris.cnrs.fr
Matériaux
MATERIAUX DE LYON M. Jean Marc PELLETIER
Secrétariat : C. BERNAVON 83.85
M. Jean Marc PELLETIER INSA de Lyon MATEIS Bâtiment Blaise Pascal 7 avenue Jean Capelle 69621 VILLEURBANNE Cédex Tél : 04.72.43 83 18 Fax 04 72 43 85 28 Jean-marc.Pelletier@insa-lyon.fr
MEGA
MECANIQUE, ENERGETIQUE, GENIE CIVIL, ACOUSTIQUE M. Jean Louis GUYADER
Secrétariat : M. LABOUNE PM : 71.70 –Fax : 87.12
M. Jean Louis GUYADER INSA de Lyon Laboratoire de Vibrations et Acoustique Bâtiment Antoine de Saint Exupéry 25 bis avenue Jean Capelle 69621 VILLEURBANNE Cedex Tél :04.72.18.71.70 Fax : 04 72 43 72 37
mega@lva.insa-lyon.fr
ScSo
ScSo*
M. OBADIA Lionel
Insa : J.Y. TOUSSAINT
M. OBADIA Lionel Université Lyon 2 86 rue Pasteur 69365 LYON Cedex 07 Tél : 04.78.69.72.76 Fax : 04.37.28.04.48 Lionel.Obadia@univ-lyon2.fr
*ScSo : Histoire, Geographie, Aménagement, Urbanisme, Archéologie, Science politique, Sociologie, Anthropologie
VII
«En science, la phrase la plus excitante que l’on peut entendre,
celle qui annonce des nouvelles découvertes,
ce n’est pas Eureka, mais c’est drôle.»
Isaac Asimov
Résumés
IX
Résumé : Une excellente stabilité thermique, une faible pression de vapeur saturante, une
ininflammabilité, une bonne conductivité ionique ainsi que les différentes combinaisons cations/anions possibles font des liquides ioniques l'objet d'un engouement grandissant de la Recherche. De part ces avantages, les LI se présentent comme une nouvelle voie dans le domaine des polymères, et en particulier dans le milieu des nanocomposites où leur utilisation est essentiellement limitée à la fonction de surfactant des silicates lamellaires. Néanmoins, avant de pouvoir prétendre à un statut d'alternative, il est nécessaire de mettre en évidence les effets bénéfiques de leur utilisation sur les propriétés finales des matériaux polymères.
Dans un premier temps, l’objectif de ce travail a été de synthétiser des liquides ioniques différents par la nature de leur cation et anion mais tous porteurs de longues chaînes alkyles afin de permettre une meilleure compatibilité avec la matrice. Ensuite, les excellentes propriétés intrinsèques des liquides ioniques ont motivé leur utilisation comme agents structurants dans une dispersion aqueuse fluorée. Ainsi, leur rôle d’agents ioniques sur la morphologie, les propriétés physiques, thermiques et mécaniques a été étudié. Dans une seconde partie, les liquides ioniques ont été utilisés comme agents intercalants des silicates lamellaires puis confrontés aux surfactants conventionnels dans le but de préparer des argiles thermiquement stables pour la préparation de nanocomposites thermoplastiques/argiles.
Dans une dernière partie, une faible quantité de ces argiles organiquement modifiées ont été introduites par intercalation à l'état fondu dans deux matrices différentes afin de mettre en évidence les effets de ces nouveaux agents interfaciaux sur les propriétés finales du matériau.
Mots-clefs : Liquides ioniques ; Nanocomposites ; Silicates lamellaires ; Agents structurants ; CO2
supercritique.
Ionic Liquids : Multifunctional agents of the polymer matrices
X
Ionic Liquids: Multifunctional agents of the polymer matrices Abstract : An excellent thermal stability, a low saturated vapor pressure, a no flammability, a
good ionic conductivity and the different cations / anions combinations possible of ionic liquids are currently the focus of the research. Because of its various benefits, they are as a new alternative in the polymer science, and particularly in the field of the nanocomposites where their use is currently limited to the function of surfactant for the layered silicates. However, before claiming the status of an alternative, it is necessary to highlight the benefits of their use on the final properties of polymer materials.
Initially, the objective of this work was to synthesize different ionic liquids by the nature of cation and anion, but all bearing with long alkyl chains to allow greater compatibility with the matrix. Then, the excellent intrinsic properties of ionic liquids have motivated their use as structuring agents in a fluorinated aqueous dispersion. Thus, their role in ionic agents on the morphology, physical, thermal and mechanical properties was studied. In a second part, ionic liquids have been used as agents intercalating layered silicates and then confronted with conventional surfactants in order to prepare thermally stable clays for the preparation of nanocomposite thermoplastic / clay.
In the last section, a small amount of organically modified clays were introduced by melt intercalation in two different matrices in order to highlight the effects of these new interfacial agents on the final properties of the material.
Keywords : Ionic Liquids ; Nanocomposites ; Layered silicates ; Building blocks ; Supercritical
CO2.
Sommaire
XI
Sommaire :
Pages
INTRODUCTION GENERALE ____________________________________________________ 1
RESUME ETENDU ____________________________________________________________ 3
Chapter I Ionic liquids: State of the art _______________________________________ 27
I.1 Ionic liquids ______________________________________________________________ 29 I.1.1 Origin of ionic liquids _________________________________________________________ 29 I.1.2 Properties of ionic liquids ______________________________________________________ 30 I.1.3 Structure and synthesis of ionic liquids ____________________________________________ 30
I.1.3.1 Effect of cation _____________________________________________________________ 31 I.1.3.2 Effect of anion _____________________________________________________________ 32 I.1.3.3 Synthesis of ionic liquids _____________________________________________________ 33
I.1.4 Applications of ionic liquids ____________________________________________________ 36 I.1.4.1 New alternative to conventional solvents _________________________________________ 36 I.1.4.2 Electrochemistry ____________________________________________________________ 36 I.1.4.3 Homogeneous and heterogeneous catalysis _______________________________________ 36 I.1.4.4 Metal ion capture ___________________________________________________________ 37 I.1.4.5 Chemistry in supercritical medium ______________________________________________ 37 I.1.4.6 Surfactants, plasticizers, and lubricants in polymer science ___________________________ 38
I.1.5 Main limitation of ionic liquids __________________________________________________ 39 I.1.6 Conclusion__________________________________________________________________ 39
I.2 Ionic liquids: A new way in the preparation of polymer/layered silicates nanocomposites ________________________________________________________________________ 40
I.2.1 Introduction _________________________________________________________________ 40 I.2.2 Ionic liquid-polymer interactions ________________________________________________ 41
I.2.2.1 Lubricants _________________________________________________________________ 41 I.2.2.2 Plasticizers ________________________________________________________________ 42 I.2.2.3 Polymer electrolytes _________________________________________________________ 43 I.2.2.4 Preparation of porous polymer _________________________________________________ 44 I.2.2.5 ILs supported on organic polymers ______________________________________________ 45 I.2.2.6 Preparation of supramolecular polymers based on ILs _______________________________ 45
I.2.3 Intercalating agents for layered silicates ___________________________________________ 47 I.2.3.1 Structure and properties of layered silicates _______________________________________ 47 I.2.3.2 Organic modification of layered silicates _________________________________________ 48 I.2.3.3 Conclusions ________________________________________________________________ 59
I.2.4 Polymer/layered silicates _______________________________________________________ 60 I.2.4.1 Preparation methods of PLS nanocomposites ______________________________________ 60 I.2.4.2 Characterization of PLS nanocomposites _________________________________________ 62 I.2.4.3 ILs treated-Layered silicates for polymer nanocomposites ____________________________ 63
I.2.5 Conclusions _________________________________________________________________ 68
Conclusions of chapter I ________________________________________________________ 69
References of chapter I _________________________________________________________ 70
Ionic Liquids : Multifunctional agents of the polymer matrices
XII
CHAPTER II POLYMER/IONIC LIQUID INTERACTIONS _____________________________ 75
II.1 New building blocks _____________________________________________________ 77 II.1.1 Introduction _________________________________________________________________ 77 II.1.2 Experimental ________________________________________________________________ 78
II.1.2.1 Materials __________________________________________________________________ 78 II.1.2.2 Processing and characterization of the IL/PTFE films _______________________________ 78 II.1.2.3 Synthesis of ionic liquids _____________________________________________________ 80
II.1.3 Morphology and mechanical performances of polymer/IL blends _______________________ 83 II.1.4 Conclusions _________________________________________________________________ 85
II.2 Nanostructuration of ionic liquids in fluorinated matrix: Influence on the mechanical properties ____________________________________________________________________ 86
II.2.1 Introduction _________________________________________________________________ 86 II.2.2 Results and discussion _________________________________________________________ 87
II.2.2.1 Effect of ionic liquids on the structuration of fluorinated polymer films _________________ 87 II.2.2.2 Effect of ionic liquids on the thermal properties of fluorinated polymer-based blends ______ 90 II.2.2.3 Effect of ionic liquids on the PTFE crystallinity ____________________________________ 91 II.2.2.4 Effect of ionic liquids on the mechanical properties of fluorinated polymer ______________ 93
II.2.3 Conclusions _________________________________________________________________ 99
Conclusions of chapter II ______________________________________________________ 100
References of chapter II _______________________________________________________ 101
Chapter III IONIC LIQUIDS AS NEWS INTERCALATING AGENTS FOR LAYERED SILICATES _ 103
III.1 A comparative study on ionic liquids used as surfactants: Effect on thermal and mechanical properties of high-density polyethylene nanocomposites __________________ 105
III.1.1 Introduction _____________________________________________________________ 105 III.1.2 Experimental ____________________________________________________________ 107
III.1.2.1 Materials _________________________________________________________________ 107 III.1.2.2 Synthesis of phosphonium and imidazolium salts _________________________________ 107 III.1.2.3 Organic modification of montmorillonite ________________________________________ 108 III.1.2.4 Processing and characterization of the HDPE/clay nanocomposites ___________________ 110
III.1.3 Results and discussion _____________________________________________________ 111 III.1.3.1 Characterization of modified montmorillonites ___________________________________ 111 III.1.3.2 Thermal stability of modified montmorillonites ___________________________________ 114 III.1.3.3 Structural analysis by WAXD _________________________________________________ 116 III.1.3.4 Surface energies of modified montmorillonites ___________________________________ 118 III.1.3.5 Influence of ionic liquid content _______________________________________________ 118
III.1.4 HDPE/clay nanocomposites _________________________________________________ 120 III.1.4.1 Thermal properties of nanocomposites __________________________________________ 120 III.1.4.2 Mechanical properties of nanocomposites _______________________________________ 121 III.1.4.3 Morphology of nanocomposites _______________________________________________ 122
III.1.5 Conclusions _____________________________________________________________ 123
Sommaire
XIII
III.2 Supercritical CO2-Ionic Liquid Mixtures For Modification of Organoclays ______ 124 III.2.1 Introduction _____________________________________________________________ 124 III.2.2 Experimental ____________________________________________________________ 125
III.2.2.1 Organic modification _______________________________________________________ 125 III.2.3 Results and discussion _____________________________________________________ 127
III.2.3.1 Effect of supercritical carbon dioxide as a exchange solvent on the thermal degradation of the modified MMT ___________________________________________________________________ 127 III.2.3.2 Structural analysis __________________________________________________________ 133 III.2.3.3 Surface energies ___________________________________________________________ 136
III.2.4 Conclusions _____________________________________________________________ 136
Conclusions of chapter III _____________________________________________________ 137
References of chapter III ______________________________________________________ 138
Chapter IV POLYMER/LAYERED SILICATES NANOCOMPOSITES _____________________ 141
IV.1 Synthesis of new surfactants: Effect of the ionic liquids on the thermal stability and the mechanical properties of high density polyethylene nanocomposites _______________ 143
IV.1.1 Introduction _____________________________________________________________ 143 IV.1.2 Experimental ____________________________________________________________ 144
IV.1.2.1 Materials _________________________________________________________________ 144 IV.1.2.2 Processing and characterization of HDPE/clay nanocomposites ______________________ 145 IV.1.2.3 Synthesis of imidazolium and phosphonium salts _________________________________ 146 IV.1.2.4 Organic modification of montmorillonite ________________________________________ 149
IV.1.3 Results and discussion _____________________________________________________ 150 IV.1.3.1 Thermal stability of ionic liquids ______________________________________________ 151 IV.1.3.2 Thermal stability of ionic liquid modified montmorillonites _________________________ 153 IV.1.3.3 Surface Energy of ionic liquid modified montmorillonites ___________________________ 154 IV.1.3.4 PE/modified-montmorillonites nanocomposites ___________________________________ 155
IV.1.4 Conclusions _____________________________________________________________ 160
IV.2 Ionic Liquids as Interfacial Agents in PVDF-based nanocomposites ____________ 161 IV.2.1 Introduction _____________________________________________________________ 161 IV.2.2 Experimental ____________________________________________________________ 162
IV.2.2.1 Materials _________________________________________________________________ 162 IV.2.2.2 Synthesis of ionic liquids ____________________________________________________ 162 IV.2.2.3 Organic modification _______________________________________________________ 163
IV.2.3 Results and discussion _____________________________________________________ 164 IV.2.3.1 Characterization of ILs exchanged montmorillonites _______________________________ 164 IV.2.3.2 Effect of interfacial interactions on the material physical properties ___________________ 168
IV.2.4 Conclusions _____________________________________________________________ 175
Conclusions of chapter IV _____________________________________________________ 176
References of chapter IV ______________________________________________________ 177
CONCLUSION GENERALE ____________________________________________________ 179
Abréviations et symboles
XV
Abréviations et symboles 1) Nomenclature
ILs : Ionic Liquids RTILs : Room Temperature Ionic Liquids EtNH3NO3 : Ethylammonium nitrate BF4
- : Tetrafluoroborate PF6
- : Hexafluorophosphate CF3SO3
- : Trifluoromethanesulfonate N(SO2CF3)2 : Bistrifluoromethanesulfonylimide HF : Hydrofluoric acid PLLA : Poly-L-lactide PMMA : Poly-(methylmethacrylate) PVC : Poly-(vinylchloride) PA6 : Polyamide PLA : Polylactide PEO : Poly(ethylene oxyde) PAN : Poly(acrylonitrile) PVDF : Poly(vinylidene fluoride) PVDF (HFP) : Poly(vinylidene fluoride-hexafluoropropylene) MMA : Methylmethacrylate PVA : Poly(vinyl) alcohol PE : Polyethylene PP : Polypropylene PET : Polyethylene terephtalate PEEK : Polyether ether ketone ABS : Acrylonitrile-butadiene-styrene PS : Polystyrene DOP : Dioctyl phtalate γ-APS : Aminopropyltriethoxysilane MMT : Montmorillonite PLS : Polymer Layered Silicates CEC : Cation Exchange Capacity ScCO2 : Supercritical Carbon Dioxide 2) Structural Characterization
TGA : Thermogravimetric Analysis DSC : Differential Scanning Calorimetry WAXD : Wide Angle X-ray Diffraction SAXS : Small Angle X-ray Scattering XPS: X-ray Photoelectron Spectroscopy TEM : Transmission Electron Microscopy DMTA : Dynamic Mechanical Thermal Analysis Tm : Melting Temperature (°C) Tc : Crystallization Temperature (°C) ∆Hm : Melting Enthalpy (J/g) Xc : Crystallinity percentage E’ : Storage Modulus (MPa) E : Young’s Modulus (MPa) tanδ : E’’/E’ (sans unité) 2θ : Diffraction angle (°)
Introduction générale
Page 1
INTRODUCTION GENERALE
Dans la science des matériaux, un des objectifs de la recherche est de développer des
matériaux polymères à haute performance. Afin d'y parvenir, l'incorporation de charges de
dimensions nanométriques dans une matrice polymère a été une voie privilégiée. Ces agents
nanométriques de différente nature : copolymères à blocs, composés du carbone (graphène,
nanotubes de carbone), oxydes métalliques (alumine, titane, zircone, silice) ou silicates
lamellaires (montmorillonite, mica) sont couramment utilisés. En raison de leur faible coût,
d'une surface spécifique (400 à 700m2/g) et d'un facteur de forme (100 à 1000) considérables,
les silicates lamellaires sont les nanocharges les plus communément utilisées dans
l'élaboration des nanocomposites. Néanmoins, la différence de polarité entre les argiles
(hydrophiles) et les polymères (hydrophobes) mène à de mauvaises interactions entre les
charges et la matrice avec comme conséquences une mauvaise dispersion des nanoargiles
dans la matrice accompagnée de propriétés finales diminuées. Il est alors nécessaire de
modifier la surface des silicates lamellaires afin d'améliorer la compatibilité vis-à-vis de la
matrice polymère. Le greffage d'organosilanes est un traitement de surface rendu possible par
la présence de groupements hydroxyle sur les bords des feuillets mais le traitement de surface
le plus répandu reste l'échange cationique qui consiste à remplacer les cations compensateurs
situés entre les feuillets d'argiles par des cations organiques nommés agents intercalants, le
plus souvent des alkylammonium quaternaires. Les différentes méthodes de préparation des
nanocomposites à base de silicates lamellaires modifiées par des sels d'ammonium (voie
solvant, polymérisation in situ, intercalation à l'état fondu) ont été largement étudiées.
Récemment, de nouveaux composés organiques sont apparus comme une nouvelle
alternative aux sels d'ammonium conventionnels. Ces sels fondus appelés liquides ioniques
ont l'avantage d'être ininflammable, de posséder une meilleure stabilité thermique, une faible
pression de vapeur saturante et une bonne conductivité ionique. Toutefois, malgré ces
nombreux avantages, leur utilisation dans le domaine des nanocomposites reste encore
limitée.
L'objectif de ce travail est de synthétiser des liquides ioniques différents par la nature
de leur cation et anion mais tous porteurs de longues chaînes alkyles permettant la
compatibilisation avec la matrice. Le liquide ionique sera ajouté dans la matrice polymère soit
en tant qu’agent structurant soit en tant qu’agent compatibilisant de la charge lamellaire. Le
rôle du liquide ionique sur la morphologie de la matrice et les propriétés physiques,
thermiques et mécaniques, sera étudié et discuté.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 2
Le manuscrit est divisé en 4 chapitres. Le premier chapitre comporte l’étude
bibliographique menée en 2 parties. La première partie présente un état de l’art sur les
liquides ioniques et leurs diverses applications dans le domaine de la synthèse organique, la
catalyse, l'électrochimie, les fluides supercritiques, ou l'énergie. La deuxième partie porte sur
le volet nanocomposite et sur l'apport des liquides ioniques comme nouveaux agents
intercalants des charges lamellaires sur les propriétés finales du matériau.
Dans un deuxième chapitre, les excellentes propriétés intrinsèques des liquides
ioniques ont motivé leur utilisation comme agents structurants dans une dispersion aqueuse
fluorée. Ce deuxième chapitre a été consacré dans un premier temps à la synthèse des liquides
ioniques pyridinium, imidazolium et phosphonium et à la structuration de ces agents ioniques,
assimilée à celle observée pour les ionomères et dépendante de la nature chimique des cations
et des anions utilisés, dans une matrice polymère. Dans un deuxième temps, l'influence des
liquides ioniques sur les propriétés mécaniques a été caractérisée en régime statique et
dynamique. Les effets de la vitesse de déformation sur la morphologie des domaines ioniques
ont été étudiés par microscopie électronique à transmission et par la diffusion des rayons X
aux petits angles. La stabilité thermique et la cristallinité des films préparés ont également fait
l'objet de cette étude.
Le traitement de surface des silicates lamellaires par les liquides ioniques utilisés
comme agents intercalants est décrit dans le troisième chapitre. Dans une première partie, les
propriétés des liquides ioniques sont confrontées à celles des ammoniums quaternaires
conventionnels. L'importance du rôle des espèces physiquement adsorbées à la surface des
argiles sur les propriétés physico-chimiques des silicates lamellaires a aussi été étudiée. Dans
une seconde partie, une nouvelle méthode de préparation des argiles organiquement
modifiées, respectueuse de l'environnement et basée sur la combinaison du CO2 supercritique
et des liquides ioniques a été envisagée ainsi que les conséquences de cette association sur la
stabilité thermique et le taux d'intercalation des argiles.
La préparation de nanocomposites polymères/argiles thermiquement stables par
intercalation à l'état fondu a fait l’objet de ce quatrième et dernier chapitre. En effet,
l'influence de la nature chimique des agents intercalants sur la morphologie ainsi que sur les
propriétés physiques et mécaniques des nanocomposites a été étudié sur deux matrices
différentes: le polyéthylène haute densité (PEhd) et le polyfluorure de vinylidène (PVDF).
Résumé étendu
Page 3
RESUME ETENDU
Chapitre 1 : Les liquides ioniques
Au cours de ces dernières années, les liquides ioniques (LI) ont suscité un intérêt
grandissant dans la Recherche académique et industrielle. Les principales raisons de cet
engouement s’expliquent par leurs propriétés attractives telles que leur stabilité thermique,
leur faible pression de vapeur saturante, leur ininflammabilité, leur bonne conductivité et une
température de fusion inférieure à 100°C. Tous ces éléments font des LI d’excellents
candidats pour un grand nombre d'applications dans des domaines aussi variés que
l’électrochimie, la catalyse, les matériaux polymères, ou encore dans la fabrication de
nanomatériaux comme agents tensioactifs pour les silicates lamellaires et silice, les composés
du carbone ou les oxydes métalliques. Dans la partie bibliographique, nous rappellerons ce
qu’est réellement un liquide ionique et nous décrirons ses diverses utilisations.
• Structure des liquides ioniques
- Cations
Les LI sont des sels fondus composés d'un cation organique associé à un anion
organique ou inorganique et qui possèdent des températures de fusion inférieure à 100°C.
Les cations organiques les plus rencontrés sont les ammonium, imidazolium,
phosphonium, pyridinium, pyrazolium, thiazolium, oxazolium ou pyrolidium. Des
fonctionnalisations différentes (amine, acide, thiol, ester, acrylate, nitrile) permettent
d'augmenter la gamme de cations disponibles et ont une influence significative sur les
propriétés physico-chimiques des LI. Ainsi, nous savons qu'une augmentation de la longueur
de chaîne peut augmenter la température de fusion du sel qui définit la gamme d'utilisation
des LI [1]. Les différentes structures possibles sont résumées sur la Figure 1. Malgré ce large
choix, les cations les plus couramment utilisés restent les cations imidazolium associés à un
nombre infini d'anions.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 4
X-NN
R1 R3
R5 R4
R2
+
X-
N NR1R2
+X-N
N NR1
R4 R3
R2
+X-
Y N
R1 R2
R4
R3+
N
R
+X-
NR2R1
+ [PRxH(4-x)]+ [NRxH(4-x)]
+
pyrazolium triazolium oxazolinium (Y = O)thiazolium (Y = S)
phosphonium ammoniumpyrolidiumpyridinium
imidazolium
Figure 1 – Les différents cations qui composent le liquide ionique
- Anions
Il existe deux types d'anions que l'on retrouve régulièrement dans la littérature: les
anions hexafluorophosphate (PF6-), tétrafluoroborate (BF4
-) et trifluorométhanesulfonate
(CF3S03-) dit anions fluorés et les anions conventionnels comme le brome (Br-), le chlore
(Cl-), l'iode (I-) et le chloroaluminate (AlCl4-) pour ne citer que les plus connus. La nature de
l'anion utilisé joue un rôle déterminant sur les propriétés finales des liquides ioniques,
notamment, en ce qui concerne la stabilité thermique des sels. Par exemple, lorsqu'un sel
imidazolium est associé à un anion fluoré (BF4-), sa stabilité thermique est considérablement
améliorée comparée au même LI combiné à un anion bromure (Br-) [2]. Il en est de même sur
la solubilité des liquides ioniques, l'utilisation du 1-méthyl-2-butyl-imidazolium
tétrafluoroborate est soluble dans l'eau alors que le même cation avec l'anion PF6-est non
miscible à l'eau. Dans le domaine des électrolytes et des batteries de piles à combustible, les
anions les plus communément utilisés sont les anions fluorés (BF4-) et (PF6
-). Les différents
anions sont résumés dans le Tableau 1.
Tableau 1 – Les différents anions inorganiques ou organiques les plus souvent rencontrés Inorganic anions Organic anions
F-, Br
-, Cl
-, I
-
BF4-, PF6
-, SbF6
-, AsF6
-
NO3-, ClO4
-
CuCl2-, AuCl4
-, SnCl3
-
CH3CO2-, CH3SO4
-, C6H5SO3
-
CF3CO2-, C(CF3SO2)3
-
N(SO2CF3)2
CF3SO3-
Résumé étendu
Page 5
• Applications des liquides ioniques
- Catalyse
Leur capacité à dissoudre de nombreuses substances comme les catalyseurs ainsi que
leur immiscibilité avec les réactifs et les produits confèrent aux LI un net avantage, très utile
en catalyse homogène et hétérogène. On les retrouve ainsi dans plusieurs réactions: les
réactions de couplage Suzuki-Heck [3], d'oxydation [4], sulfonation [5], isomérisation où les
LI imidazolium et ammonium sont couramment utilisés.
- Elimination des métaux
Les liquides ioniques sont de plus en plus utilisés en remplacement des solvants
organiques traditionnels dans les procédés d'extraction des métaux, en particulier dans le
domaine des déchets nucléaires [6] et de la contamination de l'eau [7]. Par exemple, Les
liquides imidazolium, notamment associés aux anions fluorés PF6-, BF4
- sont utilisés dans
l'extraction des ions sodium, cesium, lithium ou potassium [8].
- Science des polymères
L'utilisation de solvants organiques dans la préparation d'électrolytes de polymères est
fréquente. Néanmoins, des problèmes de volatilité et d’inflammabilité sont générés lorsqu'il
est nécessaire de travailler sur des gammes de température élevées. Ces inconvénients ont
conduit les chercheurs à se tourner vers les liquides ioniques qui contrairement aux solvants
conventionnels possèdent une excellente stabilité thermique, une faible volatilité, une bonne
conductivité et sont ininflammables. Les liquides ioniques imidazolium et pyridinium associés
aux anions PF6-, BF4
-, CF3SO3- et N(CF3SO3)
2- [9-11] ont été largement étudiés. Ainsi, les LI
ont été associés aux polymères électrolytes soit directement par polymérisation à partir du LI
[12] soit par solubilisation du polymère électrolyte dans le LI [13].
Dans l'industrie, les LI sont également utilisés comme plastifiants. Dans ce domaine,
les LI sont de bons substituts des plastifiants traditionnels dans des polymères comme le
PLLA, le PMMA et le PVC [14]. Des études ont également été menées sur l'utilisation des
liquides ioniques à température ambiante (RTIL) et leur grande capacité à réduire le
frottement et l'usure des polymères contre les métaux [15].
Dans le domaine des nanocomposites polymères à base de silicates lamellaires
(montmorillonite), l'utilisation des liquides ioniques ammonium comme agent compatibilisant
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 6
des charges a fait l’objet de nombreux travaux [16]. Toutefois, la faible stabilité thermique
des sels d'ammonium due à l'élimination d'Hoffmann limite grandement leur utilisation lors
de la mise en œuvre à haute température de nanocomposites polymères/argiles. Afin de
contourner cette limitation, l'utilisation de liquides ioniques thermostables basés sur les
cations pyridinium, imidazolium ou phosphonium a été envisagée [17]. Pourtant, l'usage de
ces agents intercalants reste limité en raison du coût très élevé des LI mais également du
manque de choix disponible.
Cette partie bibliographique vise à donner une simple description des LI et de leurs propriétés
attractives. L’utilisation des LI dans le domaine des nanomatériaux reste encore limitée
compte tenu de leur coût. Le grand nombre de combinaisons possibles entre cation, anion et
nature du ligand permet d’envisager par une synthèse à façon une large palette de LI
répondant à un grand nombre d’applications et offrant de nombreuses perspectives.
• Références
[1] C. Chiappe, D. Pieraccini, J. Phys. Org. Chem. (2005); 18:275–297. [2] W.H. Awad, J.W. Gilman, M. Nyden, R.H. Harris, T.E. Sutto, J. Callahan, P.C. Trulove, H.C. DeLong, D.M. Fox, Thermochimica Acta (2004); 409:3. [3] M.J. Earle, S.P. Katdare, World Patent WO 2002030862 (2002). [4] M.J. Earle, S.P. Katdare, World Patent WO 2002030865 (2002). [5] J.F. Brennecke, E.J. Maginn, AIChE J. (2001); 47:2384–2388. [6] R.A. Bartsch, S. Chun, S.V. Dzyuba, Ionic Liquids Industrial Applications for Green Chemistry, American Chemical Society, Washington, DC, (2002); 58–68. [7] S. Chun, S.V. Dzyuba, R.A. Bartsch, Anal. Chem. (2001); 73:3737–3741. [8] H. Luo, S. Dai, P.V. Bonnesen, A.C.I. Buchanan, J.D. Holbrey, N.J. Bridges, R.D. Rogers, Anal.Chem. (2004); 76:3078–3083. [9] Noda, A.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B (2001); 105:4603. [10] Sakaebe, H.; Matsumoto, H. Electrochem. Commun. (2003); 5:594. [11] Lewandowski, A.; Swiderska, A. Solid State Ionics (2004); 169:21. [12] Ohno, H. Electrochim. Acta (2001); 46:1407. [13] Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electroanal. Chem. (1998); 459:29. [14] M. Rahman and C. S. Brazel, Polym. Degrad. and Stab. (2006); 91:3371–3382. [15] J. Sanes, F. J. Carrión, A. E. Jiménez and M. D. Bermúdez, Wear (2007); 263:658–662. [16] H.L. Tyan, Y.C. Liu, K.H. Wei, Chem Mater. (1999); 11:1942. [17] V. Mittal, European Polymer Journal. (2007); 43:3727–3736.
Résumé étendu
Page 7
Chapitre 2 : Interactions LI/polymère
Dans le domaine des matériaux polymère, les liquides ioniques ont souvent été utilisés
en tant que solvant vert et conducteur dans les gels electrolytes ou en tant que surfactant pour
les charges lamellaires. Mais jusqu’à présent, aucun travail à notre connaissance ne mentionne
l’utilisation des LI en tant qu’agent structurant d’une matrice polymère.
Dans ce deuxième chapitre, nous avons cherché à étudier l’impact du LI, introduit en
tant qu’additif dans la matrice polymère sur la morphologie et les propriétés physiques et
thermomécaniques du polymère. Les effets induits par le LI peuvent être modulés par le large
choix de combinaisons possibles cations/anions. Dans ce travail, nous avons choisi
d’introduire le LI dans une suspension aqueuse fluorée composée de polytetrafluoroéthylène
(PTFE) stabilisée. Le cahier des charges est difficile car le PTFE présente déjà une excellente
stabilité thermique, une résistance élevée aux acides et aux bases et un faible coefficient de
frottement. Quelle sera la plus value apportée par le liquide ionique introduit à un faible taux
(1%) dans la matrice PTFE après filmification ?
• Morphologie des LI dans le PTFE
Dans ce chapitre, différents liquides ioniques ont été synthétisés à partir de cations
pyridinium, imidazolium ou phosphonium et associés soit à des anions halogénés de type
iodure (I-), bromure (Br-) ou hexafluorophosphate (PF6-). Le Tableau 2 présente un récapitulatif
des différents sels synthétisés au cours de cette étude.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 8
Tableau 2 – Structure chimique des LI synthétisés
Cation Anion Designation
N NH37C18 C18H37
I-
C18C18Im I-
PC18H37
I- Br- PF6
-
C18P I- C18P Br- C18P PF6
-
I-
C18Py I- N
C18H37
- Influence du cation
La microscopie électronique à transmission a révélé des morphologies différentes en
fonction de la nature du cation introduit dans la matrice fluorée. Avec seulement 1% de
liquide ionique, une structuration volumique apparaît dans la matrice. Le liquide ionique
imidazolium (C18C18Im I-) génère deux types de morphologies coexistantes: la première
correspondant à la formation d’agrégats de domaines ioniques tandis que la seconde est
semblable à une morphologie co-continue, similaire à celle du liquide ionique pyridinium
(C18Py I-). A l’opposé, dans le cas du LI phosphonium, une excellente dispersion est observée
avec une structuration à l’échelle du nanomètre. Les différentes structurations des LI dans la
matrice fluorée en fonction de la nature du cation sont représentées sur la Figure 2.
200 nm 200 nm 200 nm
Figure 2 – Influence du cation sur la structuration de la matrice PTFE (imidazolium, pyridinium, phosphonium)
Résumé étendu
Page 9
Malgré la présence des longues chaînes alkyle (18 carbones) comme ligands sur les
différents liquides ioniques qui doivent interagir favorablement avec la matrice hydrophobe
du polytétrafluoroéthylène, la miscibilité des LI dans la matrice polymère est médiocre ce qui
conduit à la création d’une morphologie de phases séparées. De telles morphologies ont déjà
été observées et sont comparables à celles des ionomères dans les mélanges de polymères. En
effet, il est bien connu que le regroupement de paires d'ions dans un milieu de faible constante
diélectrique est responsable de la formation de micro ou de nanostructures qui peuvent être
prédites théoriquement [4, 5]. Le principal paramètre contrôlant la micro séparation de phase
dans un milieu non-polaire sont les interactions dipôle-dipôle entre les paires ce qui induit la
formation d’agrégats ioniques [6, 7]. Dans ce travail, une analogie peut être faite avec la
formation des agrégats de LI, dépendante des interactions entre le polymère et les différentes
combinaisons possibles cations/anions.
- Influence de l’anion
La nature de l’anion a également une influence significative sur la morphologie finale.
La Figure 3 illustre le rôle de l’anion sur la structuration générée dans la matrice PTFE à partir
d’un liquide ionique phosphonium associé soit à un contre anion iodé ou bromé soit à un
fluoré. Les anions bromé et fluoré conduisent à une morphologie grossière composée
d’agrégats. Cette structuration à l’échelle du micron contraste avec la morphologie à l’échelle
nanométrique obtenue avec l’anion iodure.
200 nm200 nm200 nm
Figure 3 – Structuration de l’échelle du micron au nanomètre de mélanges PTFE/LI (1% LI en poids) (C18P I-, C18P Br-, C18P PF6
-)
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 10
• Influence des liquides ioniques sur les propriétés mécaniques des films
PTFE/LI
- Influence du cation
Dans cette partie, les propriétés mécaniques des films structurés en relation avec
leur morphologie ont été également étudiées. Le comportement mécanique est très
dépendant de la nature chimique des liquides ioniques. En effet, l’addition d’1% en poids
de liquides ioniques à base de pyridinium et d’imidazolium associés à l’anion iodure
présentent des performances mécaniques similaires en traction uniaxiale, conformes à
leurs morphologies co-continues identiques. Si des augmentations de module de l’ordre de
38 et 41% sont obtenues respectivement pour les LI pyridinium et imidazolium, une légère
diminution de l’allongement à la rupture de 11% est observée. En revanche, pour le LI
phosphonium qui conduisait à une spectaculaire structuration à l’échelle nanométrique, le
compromis propriétés à rupture/rigidité était fortement amélioré puisque des
augmentations de la rigidité et de la déformation à la rupture sont obtenues avec des
hausses respectives de +160% et +190% comme le montre la Figure 4.
0
2
4
6
8
10
12
14
16
0 100 200 300 400 500 600
Strain at break (%)
Str
es
s (
MP
a)
PTFE
PTFE C18P I-
Figure 4 – Effet du liquide ionique phosphonium (1% en poids) sur les propriétés mécaniques à une vitesse de
déformation de 0.004 s-1 à température ambiante
De meilleures interactions c'est-à-dire une forte cohésion des interfaces due aux
interactions ioniques semblent avoir lieu entre la phase LI phosphonium et la matrice
fluorée ce qui conduit à une augmentation du module PTFE/LI. La phase LI agit comme
un agent de renforcement en formant un réseau de forte cohésion s’apparentant à celui des
réseaux percolants de nanocharges (noir de carbone, silice) mais qui est également capable
d’accomoder des déformations extrêmement importantes (délai de la rupture à haute
déformation).
Résumé étendu
Page 11
- Influence de l’anion
Les propriétés mécaniques peuvent également être modulées par la nature
chimique de l’anion. Ainsi, nous avons observé que le LI phosphonium associé aux anions
bromure et hexafluorophosphate conduit à des augmentations du module de 84% à 115%,
respectivement, en référence au PTFE non chargé. A l’opposé du LI C18P I-, la mauvaise
distribution des agrégats de LI C18P Br- et C18P PF6- dans la matrice fluorée a comme
conséquence une diminution de l’allongement à la rupture comprise entre 22% et 84%
respectivement.
• Influence des vitesses de déformation sur le comportement mécanique
des films polymère
En raison de l’excellente dispersion du LI phosphonium associé au contreanion
iodé dans la matrice fluorée ainsi que du très bon compromis rigidité/plasticité obtenu,
nous avons décidé d’étudier l’effet de la vitesse de déformation sur la morphologie de la
phase LI après déformation ainsi que sur les propriétés mécaniques qui en découlent.
Une des premières observations que nous avons faites est que l’addition du LI
phosphonium dans le PTFE conduit à une cristallisation à très haute vitesse de
déformation (0.2 s-1) avec une augmentation de la cristallinité de 10% en comparaison à
celle de la matrice seule. Pour une même vitesse de déformation, au niveau des propriétés
mécaniques, seule une légère augmentation de la rigidité est obtenue (200 MPa pour le
PTFE/C18P I- vs 170MPa pour le PTFE) qui peut être attribuée à une compétition entre la
réorganisation de la phase LI dans la matrice et la cristallisation sous déformation.
Pour corroborer cette hypothèse, nous avons utilisé de nouveau la microscopie
électronique à transmission (MET) pour révéler les domaines LI dans la matrice sous
différentes vitesses de déformation (0, 0.004 s-1, 0.2 s-1) (Figure 5).
PTFE/C18P I-
Initial state 0.004 s-1 0.2 s-1PTFE/C18P I-
0.004 s-1 0.2 s-1
200 nm 200 nm200 nm
Figure 5 – Effet de la vitesse de déformation sur la morphologie du LI phosphonium
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 12
Lorsque la vitesse de déformation augmente, la nanostructure co-continue du LI est
maintenue et orientée dans l’axe de la déformation ce qui signifie que le réarrangement
des domaines ioniques nécessite un temps de relaxation plus important que le temps
caractéristique du processus de déformation. Ce phénomène est très proche de celui
observé par Visser et al [8], qui a proposé un modèle pour les ionomères mettant en
évidence un réarrangement spatial des domaines ioniques au sein de la matrice polymère.
Les auteurs ont également souligné le rôle de la nature des paires ioniques sur la
mécanique et le comportement de déformation.
Pour la première fois, une structuration à l’échelle nanométrique des liquides ioniques
dans un film polymère a été mise en évidence. Nous avons également démontré que les effets
de la nature chimique du LI déterminés par le choix du cation organique : pyridinium,
imidazolium ou phosphonium aussi bien que le choix de l’anion (halogènes ou fluorés)
peuvent affecter la structuration et les propriétés physiques et mécaniques du polymère. En
effet, une combinaison cation/anion adéquate génère une flexibilité sans précédent ainsi
qu’une amélioration significative de la rigidité. Les LI offrent ainsi une nouvelle alternative
pour structurer à l’échelle nanométrique les matériaux polymères.
• Références
[1] M. P. Scott, M. Rahman and C. S. Brazel, Eur Polym J. (2003); 39:1947–1953. [2] F. Avalos, J. C. Ortiz, R. Zitzumbo, M. A. López-Manchado, R. Verdejo and M. Arroyo, App.Clay Sci.
(2009); 43:27–32 [3] Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electrochem. Soc. (1997); 144:L67. [4] A.R. Khokhlov, E.F. Dormidontova, Phys. Uspekhi (2005), 118, 73. [5] I.A. Nyrkova, A.R. Khokhlov, Y.Y. Kramarenko, Polym. Sci. USSR (1990), 32, 852. [6] A. Eisenberg, B. Hird, R.B. Moore, Macromolecules (1990), 23, 4098. [7] I.A. Nyrkova, A.R. Khokhlov, M. Doi, Macromolecules (1993), 26, 3601. [8] S.A. Visser, S.L. Cooper, Polymer (1992), 33, 4705-4710.
Résumé étendu
Page 13
Chapitre 3 : Utilisation des liquides ioniques comme agents intercalants de silicates lamellaires
Depuis les années 80 et depuis les premiers travaux réalisés par l’équipe de Toyota sur
les nanocomposites polyamide-montmorillonite (PA-MMT), le domaine des nanocomposites
à charges lamellaires est en plein essor. En effet, le challenge lié à ces nouveaux matériaux,
vise à améliorer les propriétés finales des matériaux, notamment les propriétés thermiques,
mécaniques et barrière [1] avec un très faible taux de charge inorganique. La clé du succès
réside dans le contrôle de la dispersion des feuillets individuels, décrit comme l’état
d’exfoliation. Mais le manque de compatibilité entre les argiles (hydrophiles) et les
polymères, le plus souvent hydrophobes rend difficile l’obtention de cet état d’exfoliation.
Pour contourner cette difficulté et améliorer la compatibilité entre les argiles et le polymère,
l'utilisation d'espèces organiques nommées agents intercalants ou surfactants, est nécessaire
afin de réduire l'énergie de surface des silicates lamellaires et augmenter les distances
interfoliaires de façon à promouvoir la dissociation des feuillets en vue d’obtenir un état de
dispersion exfolié, plus propice à l'amélioration des propriétés finales des nanocomposites [2,
3]. Jusqu’alors, les sels d’ammonium sont classiquement utilisés.
Toutefois, la faible stabilité thermique des ammoniums quaternaires, qui se dégradent
dès 180°C [4], limite considérablement leur utilisation pour l’élaboration de nanocomposites
polymères/argiles nécessitant des températures de mise en œuvre élevées. Les liquides
ioniques apparaissent alors comme une nouvelle alternative aux ammoniums conventionnels.
L’objectif de ce chapitre a donc été de préparer des argiles organiquement modifiées par des
cations organiques thermostables, en particulier imidazolium et phosphonium connus pour
leur excellente stabilité thermique, et fonctionnalisés par de longues chaînes alkyle afin de
diminuer l’énergie de surface des argiles. En raison d'une offre limitée de liquides ioniques
commerciaux à longues chaînes alkyle (> 14 carbones), nous avons été amenés à synthétiser
des LI à façon.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 14
• Nomenclature des montmorillonites modifiées
Dans une première partie, afin de démontrer la supériorité thermique des liquides
ioniques sur les sels d’ammonium, une étude comparative sur quatre montmorillonites
modifiées a été décrite. Tout d’abord, nous avons sélectionné deux montmorillonites
commerciales (Nanofil 15 et Nanofil 919) traitées par les agents intercalants diméthyl ditallow
ammonium (MMT-DMDT) et diméthyl benzyltallow ammonium (MMT-DMBT) et nous les
avons comparés aux montmorillonites modifiées par les LI N-octadécyl-N’-
octadécylimidazolium (MMT-I) et octadécyltriphenylphosphonium (MMT-P). La Figure 6
décrit les différents cations organiques qui ont été comparés.
N+
Tallow
CH3
TallowH3C
N+
H3C
H3C Tallow
N
N
C18H37
C18H37
I
P C18H37
I
Versus
Figure 6 – Ammonium vs imidazolium et phosphonium
- Stabilité thermique des MMT modifiées
D’après la littérature [5-7], lorsque l’on modifie la surface des argiles par des cations
organiques, deux types d’interactions interviennent:
- (I) Les interactions de Van der Waals correspondant aux espèces organiques
physiquement adsorbées sur la surface de l'argile.
- (II) Les interactions ioniques correspondant aux espèces réellement intercalées entre
les feuillets de la montmorillonite.
Afin d’identifier et de quantifier le taux d’intercalation des espèces adsorbées et
intercalées, des lavages successifs au méthanol ont été effectués. Ainsi, nous avons démontré
par analyse thermogravimétrique (ATG) que les MMT-I et MMT-P ont des températures de
dégradation correspondant aux espèces physisorbées comprises entre 320°C et 340°C
(évaporation des LI), respectivement comparées aux MMT-DMDT et MMT-DMBT qui se
dégradent à plus basse température 220°C et 270°C.
Concernant les espèces intercalées, les montmorillonites modifiées par les liquides
ioniques imidazolium et phosphonium présentent une meilleure stabilité thermique que les
DMDT P DMBT I
Résumé étendu
Page 15
montmorillonites traitées ammonium puisque les températures de dégradation des nouveaux
surfactants s’étendent sur une plage comprise entre 420-490°C (MMT-I) et 510°C (MMT-P)
contrairement aux ammonium qui commencent à se dégrader à partir de 340-440°C (MMT-
DMDT) et de 300-400°C (MMT-DMBT). La Figure 7 représente le comportement de
dégradation thermique des montmorillonites organiquement modifiées.
N
N
C18H37
C18H37
I
P C18H37
I
N+
H3C
H3C Tallow
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- Analyse structurale et propriétés de surface des argiles modifiées
Ensuite, nous avons caractérisé par la diffraction des rayons (DRX) et par la méthode
de la goutte posée les distances inter feuillets ainsi que les énergies de surface qui résultent de
la modification organique des silicates lamellaires. Le Tableau 3 récapitule les différents
résultats obtenus.
Tableau 3 – Energie de surface et distances interfoliaires
Montmorillonite Distance interfoliaire
(nm)
γ total (mN.m-1)
MMT-Na+ 1,2 73 MMT-DMDT 3,0 40 MMT-DMBT 1,9 49
MMT-P 4,2 37 MMT-I 3,7 32
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- 34
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 16
Avant le traitement de surface, l'espace interfoliaire de la montmorillonite sodique est
égale à 1,2 nm. Après la procédure d’échange cationique, le LI imidazolium conduit à une
distance interfoliaire de 3,7 nm, caractéristique d’une conformation paraffinique en position
trans-trans des chaînes alkyle alors que le LI phosphonium implique une distance de 4,2 nm
due à l’encombrement stérique des trois fonctions benzyle et de la chaîne alkyle. Ces
distances interfoliaires sont à comparer à celles de 1,9 nm et 3,0 nm obtenues respectivement
pour les MMT-DMBT et MMT-DMDT. Concernant les énergies de surface, l’utilisation des
LI mène à des valeurs très proches de celles des polyoléfines ce qui nous laisse suggérer une
excellente compatibilité de ces charges vis à vis des matrices hydrophobes comme le
polyéthylène (PE) ou encore le polypropylène (PP) [9].
• Association CO2 supercritique/LI pour la modification organophile des montmorillonites
Dans une seconde partie, notre but a été de reproduire les mêmes modifications
chimiques de la montmorillonite par les liquides ioniques mais cette fois-ci en utilisant le CO2
supercritique (ScCO2) afin de permettre un traitement de surface des silicates lamellaires
respectueux de l’environnement, c'est-à-dire sans l’utilisation de solvants organiques. En
effet, les avantages du dioxyde de carbone en condition supercritique sont nombreux: grande
diffusivité similaire à un gaz, faible tension de surface, viscosité et densité similaire à celles
d'un liquide ce qui lui confère un pouvoir de solvabilité élevé ajustable par contrôle de la
pression [10]. L’échange cationique s’effectue dans des conditions de pression et de
température de (75 bar, 80°C) pour la MMT-I et (80 bar, 90°C) pour la MMT-P.
- Stabilité thermique des MMT traitées LI
Nous avons montré que la combinaison du CO2 supercritique, des liquides ioniques
imidazolium et phosphonium permet la modification des silicates lamellaires avec des
résultats similaires à un échange cationique standard. Néanmoins, la faible solubilité des LI
dans le CO2 supercritique nécessite l’utilisation d’un co-solvant [11]. Dans notre cas, le co-
solvant utilisé est l’eau. L’utilisation de la combinaison CO2 supercritique-LI-eau lors de la
modification des silicates lamellaires conduit à une meilleure stabilité thermique ainsi qu’à un
taux d’intercalation des cations organiques entre les feuillets d’argiles plus important.
Résumé étendu
Page 17
Ainsi, la dégradation thermique des espèces intercalées des montmorillonites
modifiées par les sels d’imidazolium et phosphonium en milieu supercritique est retardée. En
effet, des températures de dégradation de 540°C et 570°C sont obtenues contre des
températures de 420-490°C et 510°C pour les MMT-I et MMT-P étudiées dans la première
partie de ce chapitre. Les courbes ATG de la montmorillonite traitée par le LI imidazolium
par échange cationique classique et sous ScCO2-eau sont représentées sur la Figure 8.
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Universal V4.2E TA Instruments Figure 8 – Stabilité thermique des MMT-I modifiées par échange standard (a, a’ dérivée de ∆m/m) et sous
ScCO2-eau (b, b’ dérivée de ∆m/m)
Ces résultats ont pu être expliqués par une diminution de la température de fusion des
LI lors de l’exposition au CO2 supercritique en présence d’eau due à la mise en place de
faibles interactions acide-base de Lewis entre la partie basique des LI et la partie acide du
CO2 [12].
En conclusion, nous avons clairement démontré la meilleure stabilité thermique des
silicates lamellaires modifiées par les LI imidazolium et phosphonium comparés aux argiles
traitées par les ammoniums quaternaires conventionnels et l’utilisation du CO2 supercritique
associée à l’eau et aux liquides ioniques permet un échange cationique propre avec une
augmentation significative de la stabilité thermique des argiles modifiées pour des
caractérisations structurales comparables.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 18
• Références [1] E. Jacquelot, E. Espuche, J.F. Gerard, J. Duchet, P. Mazabraud, J. Polym. Sci. Part B: Polym. Phys. 44 (2) (2006) 431. [2] S.Y. Lee, W.J. Cho, K.J. Kim, J.H. Ahn, M. Lee, J. Colloid Interface Sci. 284 (2) (2005) 667. [3] H. He, J. Duchet, J. Galy, J.F. Gerard, J. Colloid Interface Sci. 295 (2006) 202. [4] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Chem. Mater. 13 (9) (2001) 2979. [5] L. Le Pluart, J. Duchet, H. Sautereau, J.F. Gérard, J. Adhes. 78 (7) (2002) 645. [6] W. Xie, R. Xie, W.P. Pan, D. Hunter, B. Koene, L.S. Tan, R. Vaia, Chem. Mater. 14 (11) (2002) 4837. [7] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Thermochim. Acta 367– 368 (2001) 339. [8] S. Boucard, J. Duchet, J.F. Gérard, P. Prele, S. Gonzalez, Macromol. Symp. 194 (1) (2002) 241. [9] C.M. Hansen, A. Beerbower, Kirk-Othmer Encyclopedia of Chemical Technology, second ed., Interscience, New York, 1971. p. 889. [10] S.P. Nalawade, F. Picchioni, L.P.B.M. Jansen, Prog. Polym. Sci. 31 (2006) 19-43. [11] S. Keskin, D. Kayrak-Talay, U. Akman, Ö. Hortaçsu, J. of Sup. Fluids 2007, 43, 150-180. [12] A.M. Scurto, E. Newton, R.R. Weikel, L. Drauker, J.Hallett, C.L. Liotta, W. Leitner, C.A. Eckert, Ind. Eng. Chem. Res. 47 (2008) 493.
Résumé étendu
Page 19
Chapitre 4 : Nanocomposites polymères/silicates lamellaires
Dans ce dernier chapitre, nous avons testé les montmorillonites rendues
thermiquement stables en les incorporant à l’état fondu dans deux matrices différentes, le
polyéthylène haute densité (PEhd) et le polyfluorure de vinylidène (PVDF) lors de
l’élaboration de nanocomposites. L’influence du ligand, sa nature chimique, le rôle du cation,
imidazolium vs phosphonium et de l’anion (Br-, I-, PF6-) ont été étudiées sur les propriétés
thermiques, physiques et mécaniques ainsi que sur la morphologie des nanocomposites
résultants.
• Utilisation des liquides ioniques comme agents interfaciaux dans les nanocomposites PEhd
Dans une première partie, l’influence de la nature chimique des cations (imidazolium,
phosphonium) et des anions (Br-, I-, PF6-) sur la stabilité thermique des liquides ioniques eux-
mêmes aussi bien que sur les montmorillonites modifiées par les LI a fait l’objet de ce travail.
La Figure 9 résume les différents liquides ioniques synthétisés et utilisés dans la préparation de
nanocomposites PEhd/MMT.
N NH37C18 C18H37
N N
H37C18C22H45
BrIN N
C22H45 C22H45
Br
3a 3b 3c
X = I
Br
PF6
1a
1b
1c
PC18H37
X
Imidazolium 3a-3c
Phosphonium 1a-1c Figure 9 – Liquides ioniques imidazolium et phosphonium synthétisés
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 20
- Stabilité thermique des liquides ioniques
D’après la littérature, les cations imidazolium et phosphonium sont connus pour
posséder une excellente stabilité thermique [1, 2]. Dans le cas des sels d’imidazolium,
l’influence de la longueur des chaînes alkyle (C18 ou C22) est négligeable et leurs températures
de dégradation, proches de 320°C, sont similaires. A l’opposé, la combinaison de l’anion
associé au cation organique joue un rôle important sur la stabilité thermique des liquides
ioniques [3]. En effet, l’utilisation de l’anion hexafluorophosphate (PF6-) combinée avec le
cation phosphonium provoque une augmentation de la température de dégradation de 140°C
par rapport aux LI phosphonium contenant les anions bromure (Br-) et iodure (I-) qui se
dégradent à des températures proches de 330°C. La Figure 10 met en évidence cette stabilité
thermique améliorée.
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Universal V4.2E TA Instruments
Figure 10 – Effet de la nature chimique de l’anion associé à des LI
phosphonium : Evolution de la perte de poids en fonction de la température (TGA, DTG)
(●) C18P Br-, (■) C18P I-, (○) C18P PF6-
(Vitesse de montée:20°C/min, N2)
Résumé étendu
Page 21
- Caractérisation physico-chimiques des MMT traitées LI
Les montmorillonites modifiées par les LI imidazolium et phosphonium ont été
également caractérisées par analyse thermogravimétrique (ATG), diffraction des rayons X et
par la méthode de la goutte posée. Ainsi, au niveau du comportement thermique, on observe
que ni la longueur de la chaîne alkyle ni la nature de l’anion utilisé dans le cas des
imidazolium n’ont d’influence sur les températures de dégradation des charges lamellaires qui
sont identiques aux températures constatées dans le chapitre 3, c'est-à-dire des températures
de 320, 410 et 480°C. Il en est de même pour les MMT traitées par les LI phosphonium
combinés aux anions halogénés tandis que dans le cas du LI phosphonium associé à l’anion
hexafluorophosphate, seule une augmentation de la température de dégradation correspondant
aux espèces physiquement adsorbées est améliorée passant de 330°C à 410°C. Ce résultat
s’explique par la meilleure stabilité thermique intrinsèque du liquide ionique. Par contre, la
gamme de température correspondant aux espèces intercalées (400-500°C) entre les feuillets
d’argiles est conservée. Par diffraction des rayons X, le changement d’anion ou une
fonctionnalisation différente n’influe aucunement sur les distances inter feuillets obtenues. Au
contraire, la nature de l’anion joue un rôle important sur les énergies de surface des MMT
modifiées. Les distances interfoliaires et les énergies de surface sont résumées dans le Tableau
4.
Tableau 4 – Distances interfoliaires et énergies de surface des montmorillonites modifiées par les liquides ioniques
Montmorillonite Distances interfoliaires
(nm)
γ total (mN.m-1)
MMT-Na+ 1.2 73 MMT-P I- 4.2 37
MMT-P Br- 4.1 43 MMT-P PF6- 4.2 36
MMT-C18C18Im 3.7 32 MMT-C18C22Im 3.7 39 MMT-C22C22Im 3.8 33
Le choix du liquide ionique, en particulier de son cation et de son anion, induit des
MMT modifiées avec des propriétés spécifiques adaptées à la matrice polymère utilisée. Dans
ce cas de figure, la modification des silicates lamellaires par les LI imidazolium et
phosphonium génère une bonne affinité avec les polyoléfines [5].
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 22
- Comportement mécanique et thermique des nanocomposites PE/MMT-
LI
En termes de propriétés thermiques et mécaniques des matériaux mis en œuvre par
intercalation à l’état fondu, des améliorations ont été observées. Tout d’abord au niveau de la
stabilité thermique, on constate que l’addition de seulement 1% en poids des montmorillonites
traitées par les liquides ioniques imidazolium et phosphonium dans la matrice polyéthylène,
conduit à une augmentation de 10 à 15°C de la température de dégradation des
nanocomposites PEhd/MMT modifiées. Sur la Figure 11, le comportement thermique des
nanocomposites PEhd à base des MMT traitées par les sels d’imidazolium est représenté.
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Universal V4.2E TA Instruments Figure 11 – Stabilité thermique des nanocomposites PE/MMT modifiées par les LI imidazolium :
(●) PE/MMT-C18C18Im, (■) PE/MMT-C22C22Im (♦) Neat PE, (○) PE/MMT-C18C22Im (Vitesse de montée:20°C/min, N2)
En comparaison du comportement mécanique de la matrice non chargée, l’ajout de
seulement 1% de charges organiquement modifiées dans le polyéthylène conduit à des
augmentations de la rigidité de 25 à plus de 40% dans le cas des LI imidazolium C18C18Im et
C22C22Im, respectivement et de +30% en ce qui concerne le LI phosphonium C18P I-.
En conclusion, nous avons démontré que l’utilisation des montmorillonites traitées par
les LI imidazolium et phosphonium permet une amélioration de la stabilité thermique et la
rigidité des nanocomposites PE/argiles.
Résumé étendu
Page 23
• Utilisation des liquides ioniques comme agents interfaciaux de nanocomposites PVDF
Dans une seconde partie, l’influence de la nature chimique des cations organiques
imidazolium et phosphonium sur la structure polymorphique du PVDF, les propriétés
mécaniques ainsi que sur la morphologie des nanocomposites a été étudiée. Le Tableau 5
résume les LI synthétisés et les montmorillonites modifiées (1% en poids) pour ce type de
nanocomposites à base de MMT.
Tableau 5 – Nomenclature des LI et MMT modifiées Nom
commercial
Références
Intercalant
Nanofil 757
MMT-Na+
MMT-I
MMT-P
PC18H37
I
MMT-IC12F
N NC18H37
I
C18H37
N N(CH2)2(CF2)9CF3
I
H3C
- Caractérisations thermiques des MMT-LI
La fonctionnalisation du cation imidazolium par une chaîne perfluorée n’améliore pas
la stabilité thermique des montmorillonites par rapport à celles modifiées par les LI
fonctionnalisés par les chaînes alkyles. Une diminution de la température de dégradation des
espèces physisorbées, attribuée à la volatilisation des courtes chaînes fluorées, de 330°C pour
les LI à longues chaînes alkyles à 280°C est obtenue. Pour les espèces intercalées, une
température de 460°C est alors observée. Les différentes températures déterminées par ATG
correspondant aux espèces physisorbées et intercalées sont rapportées dans le Tableau 6. On
notera cependant que ces 3 MMT conservent toutes un caractère thermostable amélioré par
rapport aux MMT modifiées par les ions ammonium.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 24
Tableau 6 – Nomenclature des LI et MMT modifiées Sample Température de
degradation des espèces
physisorbées (°C)
Température de degradation
des espèces intercalées
(°C) MMT-P 330 510 MMT-I 320 420/480
MMT-IC12F 280 460
- Morphologie des nanocomposites PVDF/MMT-LI
Lorsque les silicates lamellaires organiquement modifiées sont introduits dans la
matrice PVDF, des morphologies différentes ont été observées en fonction de la nature
chimique du LI. En effet, dans le cas des LI imidazolium et phosphonium à longues chaînes
alkyle, une structure intercalée des nanocomposites est constatée contrairement à la
montmorillonite traitée par le cation imidazolium fluoré qui semble plus apte, en partie par
son caractère hydrophobe à induire une meilleure dispersion des feuillets d’argiles dans la
matrice fluorée. Les différentes morphologies obtenues sont représentées dans la Figure 12.
1 µm 1 µm 1 µm Figure 12 – Morphologie des nanocomposites PVDF/MMT-I, PVDF/MMT-P, PVDF/MMT-IC12F
(Gauche à droite)
- Comportement mécanique des nanocomposites PVDF/MMT-LI
Les propriétés mécaniques sont dépendantes de la morphologie des nanocomposites.
Un effet plastifiant est obtenu avec une augmentation de la déformation à la rupture en
traction uniaxiale de 100% pour la MMT-I et de 700% pour la MMT-IC12F tandis qu’une
légère diminution du module de Young de l’ordre de 15% est constatée dans les deux cas. En
conséquence, l’état de dispersion le plus abouti conduit au meilleur comportement mécanique,
c'est-à-dire à une augmentation importante de la déformation suivie d’une légère diminution
de la rigidité.
Résumé étendu
Page 25
- Influence des MMT-LI sur la structure cristalline polymorphe du PVDF
En raison des nombreuses phases cristallines du PVDF, nous avons étudié par
diffraction des rayons X l’effet de ces agents interfaciaux utilisés comme surfactant des
silicates lamellaires sur la cristallinité de la matrice fluorée. D’après la littérature [7, 8], nous
savons que les phases les plus couramment rencontrées sont les phases α et β. Il est toutefois
possible de les différencier puisque les pics de diffraction caractéristiques de la forme α sont
localisés à 17.8, 18.4, 19.2°2θ et ceux correspondant à la formation de la phase β peuvent être
détectés à 20.7°2θ. La Figure 13 montre les diffractogrammes des nanocomposites
PVDF/MMT.
10 15 20 25 30
0100020003000400050006000
2θ
PVDF
0500
10001500200025003000
PVDF/MMT
0
500
1000
1500
2000
Inte
nsit
y (
u.a
)
PVDF/MMT-P
0500
10001500200025003000
PVDF/MMT-I
0
500
1000
1500
2000 PVDF/MMT-IC12F
Figure 13 – Diffractogrammes des nanocomposites (a) PVDF/MMT-IC12F, (b)
PVDF/MMT-I, (c) PVDF/MMT-P, (d) PVDF/MMT et (e) PVDF non chargé Nous avons mis en évidence que la nature chimique du cation organique joue un rôle
important sur la structure cristalline polymorphe du PVDF. L’utilisation du LI imidazolium
fonctionnalisé par la chaîne perfluorée ainsi que le LI phosphonium génère la formation de la
phase β, généralement obtenue sous une déformation mécanique ou l’application d’un champ
électrique [9].
a)
e)
d)
c)
b)
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 26
Dans ce travail, nous avons démontré que la compatibilité entre la matrice polymère et
les silicates lamellaires liée à la nature du cation organique est le paramètre principal
contrôlant les interactions physico-chimiques au sein de la matrice et contribue aussi à une
meilleure distribution et dispersion des argiles dans la matrice polymère. Ainsi, nous avons
observé que l’addition de seulement 1% en poids de MMT modifiée par l’imidazolium fluoré
conduit à une exfoliation des feuillets dans le polymère résultant en une amélioration du
compromis rigidité/déformation. En outre, nous avons également mis en évidence que la
nature chimique des agents interfaciaux, en particulier les LI à base de phosphonium et
imidazolium fluoré, favorise la formation de la forme β. Cette observation peut ouvrir de
nouvelles perspectives dans le domaine des membranes pour piles à combustible.
• Références
[1] H.L. Ngo, K. Le Compte, L. Hargen, A.B. McEven Thermochim. Acta 97 (2000) 357-358 . [2] W.H. Awad, J.W. Gilman, M. Nyden, R.H. Harris, T.E. Sutto, J. Callahan, P.C. Trulove, H.C. DeLong, D.M. Fox, Thermochimica Acta 409 (2004) 3. [3] W.H. Awad, J.W. Gilman, M. Nyden, R.H. Harris, T.E. Sutto, J. Callahan, P.C. Trulove, H.C. DeLong, D.M. Fox, Thermochimica Acta 409 (2004) 3. [4] C. Byrne and T. McNally, Macromolecular Rapid Communications 2007, 28, 780-784. [5] S. Boucard, J. Duchet, J.F. Gérard, P. Prele, S. Gonzalez, Macromol. Symp. (2002) 194 (1):241. [6] H.A. Patel, R.S. Somani, H.C. Bajaj, R.V. Jasra, Applied Clay Sci. (2007) ; 35:194-200. [7] T. U. Patro, M. V. Mhalgi, D. V. Khakhar and A. Misra, Polymer (2008), 49, 3486-3499. [8] D.J. Lin, C.L. Chang, F.M. Huang, L.P. Cheng, Polymer 44 (2003) 413-422. [9] J. Scheinbeim, C. Nakafuku, B.A. Newman, K.D. Pae, J. Appl. Phys. 50 (1979) 4399-4405.
Chapter I: Ionic Liquids: State of the art
Page 27
Chapter I Ionic liquids: State of the art Since the first discovery of the nanocomposites described by the Toyota team in the
80s, the industrial and academic research have focused particularly on the processing of
nanocomposites based on lamellar silicates of nanometer size. The promise of these
nanocomposites lies in their multifunctionality, the possibility of realizing unique
combinations of properties unachievable with conventional materials. What are the reasons
that allow in reaching this promise? The first one is linked to the confinement of polymer
chains into nanofillers due to the small size of fillers compared to polymer chains
dimensions. Among these nanofillers, lamellar or platelet–like fillers are made of platelets
having different composition and spatial arrangement. The most widespread lamellar fillers
belong to the family of smectite that are montmorillonite. The dimensions of each platelet
typically range in the following scales: few nanometers (thickness), tens of nanometers
(width) and from tens of nanometers to few micrometers (length). Their reduced dimensions
are responsible for a high specific surface (from 400 up to 700 m2.g-1) and their particular
morphology confer them a high aspect ratio (from 100 up to 1000). The second one is the
large amount of interfacial zones, i.e the multiplication of surfaces and interphases that
implies a strong synergy between the polymer matrix and inorganic nanofillers and the third
one is the spatial structuration of nanofillers. The well known challenges to get
nanocomposites include control over the distribution in size and dispersion of the nanofillers.
The montmorillonites are expandable clays: their interlayer inorganic cations can be replaced
by other cations having a higher affinity for the fixed ionic sites on the platelets. One
important consequence of the charged nature of the clay surfaces is that they are generally
highly hydrophilic and therefore naturally incompatible with a wide range of polymer. A
prerequisite for successful formation of polymer-clay nanocomposites is therefore screening
of the clay polarity to make the clay “organophilic”. This can be readily achieved through ion-
exchange reactions which is the most used modification method to modify clay nature (from
hydrophilic to hydrophobic) to an extent which depends on the nature of the organic
molecule. This surface treatment generates organically-modified clays (or organoclays)
characterized by an improved compatibility with most of the polymers.
The innovative part of this work is the use of a new generation of surfactants that are
the ionic liquids based on the pyridinium, imidazolium, or phosphonium cations. In the first
part, an overview of ionic liquids will be made. Then, in a second step, the use and the
influence of different ionic liquids as surfactants for nanofillers, plasticizers in polymers and
reinforcing agents will be described.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 28
Pages
I.1 Ionic liquids ............................................................................................................................. 29 I.1.1 Origin of ionic liquids ................................................................................................................... 29 I.1.2 Properties of ionic liquids.............................................................................................................. 30 I.1.3 Structure and synthesis of ionic liquids ......................................................................................... 30
I.1.3.1 Effect of cation .......................................................................................................................... 31 I.1.3.2 Effect of anion .......................................................................................................................... 32 I.1.3.3 Synthesis of ionic liquids .......................................................................................................... 33
I.1.3.3.1 Imidazolium ionic liquids ............................................................................................... 33 I.1.3.3.2 Pyridinium ionic liquids .................................................................................................. 33 I.1.3.3.3 Phosphonium ionic liquids .............................................................................................. 34 I.1.3.3.4 Anionic exchange ............................................................................................................ 34
I.1.4 Applications of ionic liquids ......................................................................................................... 36 I.1.4.1 New alternative to conventional solvents ................................................................................. 36 I.1.4.2 Electrochemistry ....................................................................................................................... 36 I.1.4.3 Homogeneous and heterogeneous catalysis .............................................................................. 36 I.1.4.4 Metal ion capture ...................................................................................................................... 37 I.1.4.5 Chemistry in supercritical medium ........................................................................................... 37 I.1.4.6 Surfactants, plasticizers, and lubricants in polymer science ..................................................... 38
I.1.5 Main limitation of ionic liquids ..................................................................................................... 39 I.1.6 Conclusion..................................................................................................................................... 39
I.2 Ionic liquids: A new way in the preparation of polymer/layered silicates nanocomposites .................................................................................................................................................. 40
I.2.1 Introduction ................................................................................................................................... 40 I.2.2 Ionic liquid-polymer interactions .................................................................................................. 41
I.2.2.1 Lubricants ................................................................................................................................. 41 I.2.2.2 Plasticizers ................................................................................................................................ 42 I.2.2.3 Polymer electrolytes .................................................................................................................. 43 I.2.2.4 Preparation of porous polymer .................................................................................................. 44 I.2.2.5 ILs supported on organic polymers ........................................................................................... 45 I.2.2.6 Preparation of supramolecular polymers based on ILs ............................................................. 45
I.2.3 Intercalating agents for layered silicates ....................................................................................... 47 I.2.3.1 Structure and properties of layered silicates ............................................................................. 47 I.2.3.2 Organic modification of layered silicates ................................................................................. 48
I.2.3.2.1 Grafting of organosilanes ................................................................................................ 49 I.2.3.2.2 Cationic exchange ........................................................................................................... 50
I.2.3.3 Conclusions ............................................................................................................................... 59 I.2.4 Polymer/layered silicates ............................................................................................................... 60
I.2.4.1 Preparation methods of PLS nanocomposites ........................................................................... 60 I.2.4.1.1 Solution intercalation ...................................................................................................... 61 I.2.4.1.2 In situ intercalative polymerization ................................................................................. 61 I.2.4.1.3 Melt intercalation ............................................................................................................ 61
I.2.4.2 Characterization of PLS nanocomposites ................................................................................. 62 I.2.4.3 ILs treated-Layered silicates for polymer nanocomposites ....................................................... 63
I.2.4.3.1 Polystyrene/IL-modified clays nanocomposites ............................................................. 63 I.2.4.3.2 PVDF/IL-modified clays nanocomposites ...................................................................... 65 I.2.4.3.3 Polyolefins/IL modified clays nanocomposites .............................................................. 66 I.2.4.3.4 Polyester/IL modified clays nanocomposites .................................................................. 67 I.2.4.3.5 Polyamide/IL-modified clays nanocomposites ............................................................... 67 I.2.4.3.6 PVC/IL-modified clays nanocomposites ........................................................................ 68
I.2.5 Conclusions ................................................................................................................................... 68
Conclusions of chapter I ................................................................................................................. 69
References of chapter I ................................................................................................................... 70
Chapter I: Ionic Liquids: State of the art
Page 29
I.1 Ionic liquids Over the last few years, the ionic liquids (ILs) have been of a large interest both for
the academic and industrial fields, because they have been widely promoted as a green
solvent. Indeed, their unique properties, such as their chemical stability, thermal stability, low
saturated vapor pressure, non-flammability, and good ionic conductivity make them as ideal
candidates in a wide variety of applications in the chemical industry. Their application
domains concerned more recently polymer science. Indeed, they have been used mainly as
polymerization media in several types of polymerization processes to prepare functional
polymers [1]. ILs are also investigated as components of the polymeric matrices, lubricants,
plasticizers, or components in the class of polymer gels [2-4], as templates for porous
polymers [5]. ILs are found both in the energy field, specifically in the electrolyte batteries
and fuel cells since ILs are considered as novel electrolytes for electrochemical
polymerizations [6, 7]. In the manufacture of nanomaterials, ILs are used as surfactants for
different fillers such as lamellar silicates or metal oxides.
I.1.1 Origin of ionic liquids
In recent years, research on ionic liquids was intensified, even if these ones are known
since the early twentieth century. In fact, according to the literature [8], the first ionic liquid
displaying a melting temperature below 100°C has been described for the first time was an
ammonium salt, i.e ethylammonium nitrate (formula EtNH3NO3). But it's really in the 70s that
the first ionic liquids were synthesized and used in batteries for nuclear warhead [9]. Wilkes
and others scientists have continued to use ionic liquids as electrolytes in batteries and study
different properties [8-10]. Then, during the 80s, another class of ionic liquids has emerged:
the imidazolium salts. Chloroaluminate anions associated with these salts have prevented their
development, due to the high reactivity between these anions and water and in
air. Nevertheless, it is only in the early 90s, after the synthesis of imidazolium salts,
particularly the 1-butyl-3-methylimidazolium tetrafluoroborate (BF4-) and 1-butyl-3-
methylimidazolium hexafluorophosphate (PF6-) that were stable in water and in the air that the
interest of researchers has focused on ionic liquids. The most commonly ionic liquids used are
imidazolium and pyridinium salts containing BF4- and PF6
- anions [11-13].
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 30
Indeed, research into ionic liquids is booming and they could be found in various
fields: homogeneous and heteregeneous catalysis, solvent replacement for the synthesis
applications and polymer science.
I.1.2 Properties of ionic liquids
Unlike conventional solvents based on organic molecules, ionic liquids are a
combination of cations and anions, respectively, positive and negative. Furthermore, ionic
liquids have melting temperatures below 100°C, and could be denoted as RTILS (Room
Temperature Ionic Liquids) since they are liquid at room temperature. This advantage offers
them many application areas. According to the literature, the relatively low melting
temperature ionic liquids are the resulting combination of large and asymmetric organic
cations and inorganic anions which leads to a decrease of the lattice energy. Today, they are
considered as ‘green’ solvents and environmental friendly components due to their
recyclability and their ability to be re-used several times. They have many advantages such as
excellent thermal stability, i.e about 300°C and even higher when using fluorinated anion, and
low saturated vapor pressure to prevent evaporation. They are also non inflammable and
polar. In addition, they have good thermal and electrical conductivities that makes them
excellent candidates elsewhere in the electrochemical environment. Finally, it is also possible
to synthesize ionic liquids for applications covered by varying combinations of anions and
cations. ILs could be also used as solvents for many substances such as proteins,
carbohydrates, polysaccharides, DNA, crude oil, and plastics materials.
I.1.3 Structure and synthesis of ionic liquids
The ionic liquids (ILs) can reduce the use of many organic solvents because of their
many qualities and take part in different types of synthesis. They are the most often made of
an organic cation associated with an organic or inorganic anion. These many combinations
cation / anion make ionic liquids tunable for a specific application.
Chapter I: Ionic Liquids: State of the art
Page 31
I.1.3.1 Effect of cation
There are several types of ILs as a function of the chemical nature of cation. The
cation is usually a bulk organic structure with low symmetry. The most of ionic liquids are
based on different organic cations such as: ammonium, imidazolium, phosphonium,
pyridinium, pyrazolium, thiazolium, oxazolium, or pyrolidium. In the litterature, many other
cations have been studied and functionalized by various groups: amine, acid, thiol, ester,
acrylate and nitrile. The structure of the frequently used organic cations is given in Figure I-1.
X-NN
R1 R3
R5 R4
R2
+
X-
N NR1R2
+X-N
N NR1
R4 R3
R2
+X-
Y N
R1 R2
R4
R3+
N
R
+X-
NR2R1
+ [PRxH(4-x)]+ [NRxH(4-x)]
+
pyrazolium triazolium oxazolinium (Y = O)thiazolium (Y = S)
phosphonium ammoniumpyrolidiumpyridinium
imidazolium
Figure I-1 – Different types of cations considered for ionic liquids
So far, the most commonly used cations in the literature are mainly based on
imidazolium cations associated with an infinite number of anions. The nature of the cation
and the chemical nature of ligand play a key role in the physico-chemical properties of salts.
For example, the length of alkyl chains and the symmetry of the molecule have a significant
influence on the melting temperature. Chiappe and Pieraccini [14] indicated that as the size
and asymmetry of the cation increase, the melting point increases. Further, an increase in the
branching on the alkyl chain increases the melting point. The melting point of ILs is a key
issue as it represents the lowest limit of the liquid state. Melting temperature and thermal
stability define the range of temperature for which ILs could be used as solvents.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 32
I.1.3.2 Effect of anion
The change of anion results in an increasing number of alternative ILs with various
properties. There are two types of anions that are found commonly in the literature: the
fluorinated anions, such as hexafluorophosphate (PF6-) et tetrafluoroborate (BF4
-)
trifluoromethanesulfonate (CF3S03-), and the conventional anions i.e bromide (Br-), chloride
(Cl-), iodide (I-) and chloroaluminate (AlCl4). The final properties of ionic liquids depend on
the nature of the anion used. Indeed, whatever the type of cation used, significant differences
are observed according to the associated anion. In some cases, anion is responsible for the
decrease or increase of melting temperature and thermal stability [15]. For example, the ionic
liquid, 1-methyl-2-butyl-imidazolium tetrafluoroborate, has a better thermal stability than the
same salt in the presence of a bromide anion. The chemical nature of anion also plays on the
solubility of ionic liquids. For example, 1-methyl-2-butyl-imidazolium tetrafluoroborate is
soluble in water whereas the same cation with the anion PF6- is totally immiscible in water.
The choice of the anion also has an influence on the viscosity and the density of molten salts.
BF4- and PF6
- anions are the most commonly used in numerous applications, more especially
in electrolytes and batteries. Despite some advantages, they have important limitations due to
the formation of HF when heated and placed in the presence of water. It is for these reasons
that researchers have focused on the use of other anions such as CF3SO3- and (CF3SO3)2N
- by
bonding fluorine to carbon to prepare a C-F bond inert to hydrolysis. But fluorinated anions
tend to be expensive and in response to cost and safety concerns, new ILs with non-
fluorinated anions have been prepared such as alkylsulfate that are considered as nontoxic and
biodegradable [16-18]. The structures of anions commonly used are gathered in Table I-1.
Table I-1 – Inorganic and organic anions.
Inorganic anions Organic anions
F-, Br
-, Cl
-, I
-
BF4-, PF6
-, SbF6
-, AsF6
-
NO3-, ClO4
-
CuCl2-, AuCl4
-, SnCl3
-
CH3CO2-, CH3SO4
-, C6H5SO3
-
CF3CO2-, C(CF3SO2)3
-
N(SO2CF3)2
CF3SO3-
Chapter I: Ionic Liquids: State of the art
Page 33
I.1.3.3 Synthesis of ionic liquids
As the number of cation/anion combinations is almost infinite, we have considered in
this bibliographical part a small part in restricting itself primarly to a general description of
the synthesis of imidazolium, pyridinium, and phosphonium salts. In general, the anion
exchange and the quaternization reaction by alkyl chains were studied. Many papers describe
the synthesis of ionic liquids [19, 20].
I.1.3.3.1 Imidazolium ionic liquids
Many methods for the synthesis of imidazolium ionic liquids with short or long alkyl
chains containing halogenated anions, tosylates, or triflates have been reported [21].
Imidazole undergoes a deprotonation step with sodium methoxide then reacts with a halide
anion functionalized with alkyl chains. The both reactions are performed in solvent medium
(acetonitrile, tetrahydrofurane, etc...) at reflux temperature and for reaction time from several
hours to several days, under inert atmosphere. The chemical nature of R1,2 group allows tuning
of ILs towards organic medium. For example, perfluorinated chains can be introduced. Figure
I-2 describes the general synthesis of imidazolium salts.
N
NH
N
N
N
N
R1
R1X N
N
R1
R2
X
R2X
- NaX
MeO- Na+
R1=R2 = alkyl chains X = Br-, Cl-, I- Figure I-2 – Synthesis of imidazolium salts
I.1.3.3.2 Pyridinium ionic liquids
Over the last thirty years, pyridinium ionic liquids have been widely used by
researchers, especially in catalytic processes, such as Diels Alder reactions [22] and Friedal-
Crafts alkylations [23]. The pyridinium salts containing the anions Cl-, Br-, I-, BF4-, PF6
-, and
N(SO2CF3)2 are the best known. Recently, these ionic liquids were re-considered with new
chiral pyridinium salts and nitrile functionalized ones. The reaction scheme shown in Figure
I-3 for pyridinium ionic liquids is similar to that of imidazolium IL.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 34
N N
R
X
X = Cl, Br, I
RX+Heat MY
-MX N
R
Y
R = alkyl chain
MY = NaBF4, HBF4, NaPF6, HPF6 Figure I-3 – Synthesis of pyridinium salts
I.1.3.3.3 Phosphonium ionic liquids
Compared to imidazolium, ammonium, and pyridinium ionic liquids, research on the
phosphonium ionic liquids is more limited and the fields of applications are less numerous.
Phosphonium cations commonly used are based on triphenyl-, trihexyl-, or tridecylphosphine
and the associated anions are most often halide anions Cl-, I-, Br-, C6H5SO3-, CF3SO3
- and
conventional fluoroanions BF4-, PF6
- and N(SO2CF3)2. However, the number of patents on
phosphonium ionic liquids is increasing which gives an evidence of an ever-growing interest
by the industry [24]. For example, the chemical reaction of ionic liquids based on
triphenylphosphonium is reported in Figure I-4.
X
P
Heat
RXP
R MY
-MX
Y
PR
X = Cl, Br, I MY = NaBF4, NaPF6, HPF6, HBF4 Figure I-4 – Synthesis of phosphonium salts
I.1.3.3.4 Anionic exchange
The anion exchange is also possible, by using Lewis acids or organic salts (sodium
salts) or by metathesis reactions [25]. Whatever the methods used, the reaction yields are
excellent. The main disadvantage of these methods is that it requires a purification step which
is important to remove all traces of impurities due to incomplete exchange. The anions
commonly introduced after exchange are the followings: NO3-, AlCl4
-, BF4-, PF6
-, CF3SO3-,
(CF3SO2)2-, or CF3CO2
-. Figure I-5 and Figure I-6 show the two routes to perform exchange
reaction either with Lewis acids or organic salts from an imidazolium salt as an example.
Chapter I: Ionic Liquids: State of the art
Page 35
N
N
R1
R2
X
X = Cl, Br, I
N
N
R1
R2
MXn+1MXn
M = Al, Cu, Fe, Zn, Sn
Figure I-5 – Example of anionic exchange with Lewis acids
N
N
R1
R2
X
X = Cl, Br, I
N
N
R1
R2
YMY
MY = NaBF4, NaPF6, LiNTf2
-MX
Figure I-6 – Example of anionic exchange with organic salts
More recently, new fluorinated anions with low melting temperatures, i.e.
fluorohydrogenates [26], perfluoroalkyltrifluoroborates [27], trifluorophosphates [28], and
perfluoroalkyl-β-diketonates [29] were introduced by anionic exchange. The use of these new
fluorinated anions and perfluorinated alkyl chains containing cations [30] has consequences
on the physico-chemical properties of ionic liquids such as viscosity, density, solubility,
melting temperatures, and conductivity. According to the literature, when using cations
functionalized with perfluorinated chains or fluorinated anion or combination of both, ionic
liquids with specific properties are obtained. Excellent thermal stability, chemical resistance
to acids and bases, or a total inertia with conventional organic solvents were achieved [31].
Until now, these ionic liquids have numerous applications in catalysis, as surfactants to
stabilize the perfluorocarbons dispersion in ionic liquid media [32].
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 36
I.1.4 Applications of ionic liquids
The physico-chemical properties of ionic liquids are opening a broad field of
applications.
I.1.4.1 New alternative to conventional solvents
The different properties of ionic liquids as their non flammability, low saturation
vapor pressure and the fact that ILs are not explosive is a valable asset for substitution of
conventional organic solvents. Currently, their ability to dissolve a wide variety of organic
substances or solutes in liquid-liquid extractions is retained. According to the literature, ionic
liquids are used for the extraction of aromatic compounds or alcohols of different binary
mixtures [33, 34].
I.1.4.2 Electrochemistry
Imidazolium ionic liquids are commonly used as electrolyte for the electrodeposition
of metals on the surface of a conductive material. The advantage of such salts is that they are
not easily reduced allowing the reduction of many metals at room temperature. In addition,
the use of hydrophobic ionic liquid prevents infiltration of hydrogen gas and formation of
holes linked to the formation of hydrogen bubbles in the metal during the evaporation step
[35]. On the other hand, it is known that ammonium and imidazolium salts used in
electrochemical reactions allow a better diffusion of organic compounds than in conventional
organic solvents.
I.1.4.3 Homogeneous and heterogeneous catalysis
In literature, many articles deal with various applications of ionic liquids in catalytic
reactions [36, 37]. Indeed, this area represents a large part of the activity of these salts. The
first ionic liquids used in the alkylation reactions of Friedal-Craft reactions or Diels-Alder
reactions are pyridinium salts. The imidazolium and ammonium salts are used in the reactions
of oxidation [38], nitration [39], sulphonation [37], isomerization, hydroformilation, and the
coupling reactions of Suzuki and Heck [40]. For example, Hermann et al. [41] showed that
using an ionic liquid instead of a conventional solvent improves the yield of the Heck
Chapter I: Ionic Liquids: State of the art
Page 37
reaction. Xiao [42] showed an increase in selectivity when using ionic liquids. In addition, ILs
combine several advantages. In fact, their ability to dissolve most of catalysts and their
immiscibility with the reactants and products give them both the benefits of a liquid and a
solid necessary in homogeneous and heterogeneous catalysis.
I.1.4.4 Metal ion capture
Ionic liquids are increasingly used instead of conventional organic solvents in
extraction processes and metal removal [43, 44] and particularly in the field of nuclear waste.
Many studies have focused on the extraction of radioactive metals such as lanthanides and
actinides [45]. Others authors have studied the behavior of uranium in several ionic liquids.
Water contamination by metals led scientists to use ionic liquids to allow the removal of
contaminants. In most cases, the imidazolium ionic liquids are the most often used : Visser et
al. [46], Chun et al. [47], and Luo et al. [48] have studied the use of imidazolium IL
containing anion PF6- to extract Na+, Cs+, Li+ and K+. Other researchers have considered
imidazolium ionic liquid associated to a TF2N- anion to extract strontium [49].
I.1.4.5 Chemistry in supercritical medium
One of the major challenges of our time is to eliminate the conventional organic
solvents commonly used in chemical industry to develop a green chemistry. To achieve this
objective, more and more studies suggest a novel alternative to conventional solvents.
Supercritical carbon dioxide (ScCO2), water, and ionic liquids are expected media of this new
chemistry. In particular the combination of ionic liquids and supercritical CO2 was studied in
the literature: In fact, this combination allows to couple the unexpected properties of
supercritical CO2 such as a high diffusivity and viscosity (as a gas), a low surface tension,
density and solvency (as a liquid) at tunable by adjusting pressure. Supercritical carbon
dioxide offers an acceptable combination of pressure and temperature to achieve supercritical
conditions, with a temperature above 31°C and a pressure above 73 bar. Moreover, it is not
toxic and its cost is low. When combined with the low volatility and the high polarity of ionic
liquids compared to apolar and volatile supercritical CO2, an excellent and complementary
combination is obtained. For these reasons, many studies focus on the behavior and the
understanding of interactions between supercritical CO2 and ionic liquids in biphasic systems
[50, 51]. The most studied of ionic liquids in these studies is the imidazolium salt containing
the anion PF6- due to the solubility of fluorinated anion in ScCO2 [52, 53].
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 38
I.1.4.6 Surfactants, plasticizers, and lubricants in polymer science
In the field of polymer nanocomposites based on inorganic fillers, especially layered
silicates (montmorillonite, mica, laponite), ionic liquids could play a key role. Indeed, a
number of current issues of nanocomposites is the polarity difference between the hydrophilic
clays and the hydrophobic polymer matrix [54]. To avoid this difficulty, it is necessary to
modify the clay surface to fit the interfacial interactions and expand their layered structure.
Currently, ammonium ionic liquids are the most commonly used as intercalating agents into
layered silicates. The use of these organic salts can increase the interlayer space and reduces
the surface energy of clay which makes them more compatible with the polymer matrix.
Many studies examine the use of alkylammonium ionic liquids as surfactants and their
beneficial contributions on the thermal, mechanical and barrier properties of final
nanocomposites [55, 56]. However, alkylammonium salts have a serious disadvantage: their
low thermal stability. Indeed, according to the literature, they begin to degrade from 180°C
[57] due to the Hofmann elimination (Figure I-7) which limits their use in the nanocomposites
processing at higher temperatures (especially when intercalation is done in molten state) [58].
C C
H
+NR3
∆C C + H+ + NR3
Figure I-7 – Hofmann elimination on the quaternary ammonium
Recently, new surfactants have emerged with the use of pyridinium, imidazolium, and
phosphonium salts that have a better thermal stability (at about 300°C) than ammonium ionic
liquids. Moreover, the use of ionic liquids have a positive impact on mechanical, thermal and
electrical conductivity properties of nanocomposites [59]. Despite these benefits, the use of
these salts is limited because of the cost of ionic liquids and the fact that the organic salts with
long alkyl chains necessary to increase interlayer distances are not commercially available.
In industry, ionic liquids are also used as plasticizers to ameliorate the processability,
the flexibility and the ductility of rigid polymers. In this field, ionic liquids are excellent
candidates to replace conventional plasticizers used in PLLA, PMMA, and PVC [60]. The
advantages of ionic liquids include an excellent thermal stability and a low volatility
compared to conventional plasticizers such as polyethylene glycol or phtalate. Studies have
also been conducted on the use of ionic liquids at room temperature (RTILS) and their ability
to lower friction and wear of polymers against steels [61].
Chapter I: Ionic Liquids: State of the art
Page 39
I.1.5 Main limitation of ionic liquids
Despite the many advantages and unique properties of ionic liquids as well as the
multiple combinations of cations and anions that can be proposed, the price of these new
materials is their main disadvantage. Today, ionic liquids per kilogram costs tens of thousands
more than common organic solvents such as acetone. However, this cost can be significantly
reduced [62]. In the case of imidazolium ionic liquids, Wagner et al. [63] have anticipated
price of around 50-100 euros per kilogram if the industries produce large quantities.
Another limit is that the synthesis of ionic liquids involves the use of conventional
organic solvents such as acetonitrile, toluene, THF whereas such molten salts are intended to
replace the conventional organic solvents. However, in next year’s, the emergence of new
synthesis methods for avoiding the use of conventional solvents will appear, such as
supercritical medium and microwave synthesis [64].
Another limit is the viscosity significantly higher than that of organic solvents. This
problem can be offset by changing the nature of the anion, increasing temperatures or using
the supercritical medium at very high pressure.
Despite these drawbacks, these challenges can be overcome and ionic liquids are
really promising in many areas, especially in the case of polymer processing.
I.1.6 Conclusion
This work aims to summarize the information present in the literature about ILs. A
brief description of many properties of ionic liquids as well as a overview of beneficial
contributions of the use of these salts in different fields of applications, including that of
polymers have been described in this first part. ILs are receiving more and more attention
every day both in academic research and commercial applications.
The use of ILs in polymer science has quickly advanced from use as solvents and has
become focused an using ionic liquid as functional additive to polymer chains or to hybrid
materials.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 40
I.2 Ionic liquids: A new way in the preparation of polymer/layered silicates nanocomposites
I.2.1 Introduction
Nanocomposites are the focus for materials engineers to overcome the limitations of
traditional micrometer-scale polymer composites. The field of nanocomposites involves the
study of multiphase materials where at least one of the constituent phases has one dimension
less than 100 nm. Although some composites nanofilled with carbon black or silica have been
used for a very long time, research and development of polymer layered silicate
nanocomposites (PLS) has greatly increased since the eighties. The promise of PLS lies in
their multifunctionality, the possibility of realizing unique combinations of properties
unachievable with traditional materials. What are the reasons that allow in reaching this
promise? The first one is linked to the confinement of polymer chains within nanofillers due
to the small size of fillers compared to polymer chains dimensions. The second one is the
large increase of interfacial zones which could lead to increase the polymer-inorganic surface
interactions and the third one is the spatial structuration of nanofillers. The well known
challenges to get high performance nanocomposites include control over the distribution in
size and dispersion of the nanofillers which are strongly dependent on tailoring the interfaces
between organic and inorganic phases. Lamellar silicates are generally highly hydrophilic
species and therefore incompatible with a wide range of polymer. A necessary prerequisite for
successful formation of polymer-clay nanocomposites is therefore alteration of the clay
polarity to make the clay “organophilic”. This can be readily achieved through ion-exchange
reactions which is the most used modification method to modify clay nature (from hydrophilic
to hydrophobic) to an extent which depends on the nature of the organic molecule. The
cationic exchange is usually carried out with alkylammonium ions [65] resulting of the
protonation of aliphatic or aromatic amines in an acidic medium. Their basic formula is CH3-
(CH2)n-NH3+ with n varying between 1 and 18. The length of the ammonium ions has a strong
effect on the resulting structure of nanocomposites. The amine functionality has also an
impact on the final properties of materials [66]. However, the great disadvantage of these
ammonium salts is their poor thermal stability as degradation starts from 180°C which is a
Chapter I: Ionic Liquids: State of the art
Page 41
serious limitation to the preparation of nanocomposites processed at high temperatures.
Recently, a new alternative can be proposed with Ionic Liquids (ILs) which are subject of
many research in various areas, due to their excellent thermal stability, their non-
flammability, a low saturated vapor pressure, and good thermal and electrical conductivities.
However, in the field of nanocomposites, their use is limited as intercalating agents of layered
silicates and very few studies have actually investigated the effects of ionic liquids on the
final properties of the material. In this second part of the state of the art, the use of ionic
liquids as surfactants, plasticizers and lubricants in the nanocomposite field is described.
I.2.2 Ionic liquid-polymer interactions
The tunability of Ionic Liquids (ILs) via the different cation / anion combinations is a
huge advantage to design new surfactants with specific physico chemical interactions towards
polymer medium. Indeed, it is possible to synthesize ILs with suitable functionalities (epoxy,
fluorinated groups, and alkyl groups). Despite this impressive capability, the use of ionic
liquids is not well developed in the field of nanocomposites compared to their applications as
lubricants, plasticizers, or electrolyte gels.
I.2.2.1 Lubricants
In the field of lubricants, imidazolium ionic liquids are commonly used in different
assemblies steel-steel [67-69], aluminum-steel [70-72], polymer-steel [73] or in coatings
based on nickel and chromium [74-75]. The advantage of ionic liquids, in addition to being
very good lubricants is that they can be used on a wide range of temperature due to their
excellent thermal stability. Yao et al. [76] have synthesized series of 1,3 dialkylimidazolium
with long alkyl chains to be used as lubricants for steel-steel joints in a temperature range
between 25 and 150°C. They concluded that the addition of the ionic liquids functionalized
with long alkyl chains generated a reduction of friction and anti-wear properties, mainly at
high temperatures. Jimenez et al. [77] studied the influence of the chemical nature of the
cation or anion. Three ionic liquids were compared to improve the wear behavior of steel-steel
contacts, i.e. imidazolium ionic liquids denoted 1-methyl-3-octylimidazolium
tetrafluoroborate (BF4-) and 1-methyl-3-hexylimidazolium hexafluorophosphate (PF6
-), versus
a quaternary ammonium containing halide anion (Cl-). Then, the ionic liquids have been
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 42
compared to a conventional oil at room temperature. They showed that the use of mineral oil
or ammonium ionic liquids as lubrifiant on steel gives high friction coefficients and wear
rates. On the other side, ionic liquids based on imidazolium cation functionalized with
hexafluorophosphate and tetrafluoroborate show better lubricating performance than the
ammonium salt from 25 to 200°C which is due to higher thermal stability of ionic liquids.
Recently, ionic liquids have been introduced as an internal or external lubricant in
polymer-metal assemblies. Sanes et al. [73] studied the influence of the ionic liquid, 1-hexyl-
3-methylimidazolium hexafluorophosphate, on the tribological properties of polyamide PA6.
The addition of ionic liquid (3wt.%) in the polyamide matrix does not affect the storage and
loss moduli whereas the presence of imidazolium ionic liquid plays a decisive role on the
tribological properties of the polymer. Indeed, at values of -35 ° C and +67 ° C, the neat PA-6
matrix already displays a reduced friction but the PA-6 combined with 3wt.% of IL leads still
lower friction constant values over the temperature range. They explain these performances
by the formation of stable adsorbed layers of the highly polar ionic liquid molecules on the
steel surface.
I.2.2.2 Plasticizers
In recent years, ionic liquids have represented a new alternative as substitute for
traditional plasticizers for many polymers; such as polyvinylchloride (PVC) and
polymethylmethacrylate (PMMA). However, very few studies have investigated these new
plasticizers for economic reasons. In fact, the cost of ionic liquids is currently too high for a
large diffusion of these products in the industry. Sankri et al. [78] have prepared
thermoplastic starch by melt processing using 1-butyl-3-methylimidazolium chloride as
plasticizer. The authors compared the influence of imidazolium ionic liquid to glycerol
plasticizer on the mechanical properties of thermoplastic starch. The results are significant in
the case of imidazolium since a increase of the strain at break from 100 to 400% is observed.
They attributed this modulus decrease to a reduction of hydrogen bonds between starch
molecules. Scott et al. [79] used room temperature ionic liquids (RTILs) based on
imidazolium cations as plasticizers for poly(methylmethacrylate). Generally, dioctyl phthalate
(DOP) is used as plasticizer. Butylmethylimidazolium and hexylmethylimidazolium
hexafluorophosphate were considered as excellent plasticizers for PMMA because they
reduce the glass transition temperature while improving the thermal stability.
Chapter I: Ionic Liquids: State of the art
Page 43
Rahmann et al. [80] have used different ionic liquids varying the cation chemical
nature, imidazolium, phosphonium, or ammonium, as novel plasticizers for
poly(vinylchloride) matrix. A higher decrease in the glass transition temperature of PVC
compared to conventional plasticizers was obtained with an improved flexibility of material.
A better leaching and migration resistance than the conventional plasticizers has been shown
as well. Nevertheless, the behavior of ionic liquids and conventional plasticizers remain
identical when subjected under far-range UV exposure. Park et al. [81] have investigated the
influence of phosphonium ionic liquids on the degradation of polylactic acid (PLA). In this
work, two phosphonium ionic liquids containing decanoate and tetrafluoroborate as anion
have been studied. Initially, they found that phosphonium ionic liquids were well dispersed
and were partially miscible in the PLA matrix. Then, they observed that the lubrication and
hydrolytic degradation were more pronounced when the ionic liquid based on decanoate anion
was used whereas the phosphonium ionic liquid with fluorinated anion led to an increase of
the thermal stability of PLA.
I.2.2.3 Polymer electrolytes
There are several types of polymer gel electrolytes that are used in various fields of
applications such as in secondary batteries, sensors, and various ionic devices [82, 83].
Typically, the preparation of polymer electrolytes from solutions requires polar organic
solvents and electrolyte salts in a polymer matrix. The term “gel electrolyte” is used to
describe these materials. In this case, the properties of solvents used such as viscosity and
dielectric constant as well as concentration of salts play a key role on the conductivity of the
electrolyte [84]. However, the use of organic solvents which are volatile generates
flammability problems when used at high temperatures.
These drawbacks have prompted the research to find a new alternative to conventional
organic solvents. For replacing these solvents, ionic liquids have recently been selected and
are of great interest in research on the electrochemical applications. These ionic liquids have
been chosen for their unique characteristics, including excellent thermal stability,
nonflammability, non volatibility, low melting temperature, and very high ionic conductivity
[85, 86].
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 44
The most important and widely studied cations are the imidazolium and pyridinium
ionic liquids combined with anions such as PF6-, BF4
-, CF3SO3-, and N(CF3SO3)2
- [87-89]. In
the literature, different polymer electrolytes containing ionic liquids obtained by various
methods were described, i.e. polymerization of ionic liquids [90, 91], inclusion of different
polymers in ambient temperature ionic liquids [89, 92], and preparation of polymer gel with
hydrophilic and hydrophobic ionic liquids [93]. All research reported previously, have
demonstrated high conductivity suitable for applications.
Usually, the majority of polymers used in polymer electrolyte systems have been
based on high-molecular weight poly(ethylene oxide), PEO [83]. Recently, various polymer
as poly-(methylmethacrylate) PMMA, poly(acrylonitrile) PAN, poly(vinylidene fluoride)
PVDF, and poly(vinylidene fluoride-hexafluoropropylene) PVDF(HFP) have been studied
[94, 95]. For example, Susan et al. [96] have synthesized polymer gel by polymerization of
methyl methacrylate (MMA) in 1-ethyl-3-methylidazolium bis(trifluoromethylsulfonyl)imide
ionic liquid with a small amount of cross linker. They obtained ionic gels that exhibit ionic
conductivities at room temperature, high mechanical strength, transparency and flexibility
required for polymer electrolytes. For the preparation of electrolyte membrane with improved
mechanical properties and higher ionic conductivity in the temperature range from 20 to
140°C, Sing et al. [97] have used imidazolium ionic liquid, denoted 2,3-dimethyl-1-
octylimidazolium, with triflate anions into a PVDF(HFP) matrix.
In the field of polymer electrolytes, ionic liquids have shown their importance. All
these promising results as plasticizers, lubricants, or polymer electrolyte allows to expect new
insights in the field of nanocomposites.
I.2.2.4 Preparation of porous polymer
The use of ionic liquids in the preparation of porous polymer is a path that has also been
studied. Indeed, the elimination of ILs from a mixture of polymer is simple. Zhu et al. [98]
have prepared polyurea with pore sizes of 100-500 nm by the interfacial polymerization
between hexane and a series of imidazolium combined with hexafluorophosphate and
tetrafluoroborate anions. Other authors have prepared by in situ polymerization a series of
porous composites containing polymers and ionic liquids [99].
Chapter I: Ionic Liquids: State of the art
Page 45
I.2.2.5 ILs supported on organic polymers
The use of ionic liquids supported on hybrid materials is a new area of application of
ILs. Indeed, the idea to use recoverable and reusable catalysts is very important that it is
economically and environmentally interesting. According to the literature, the hybrid organic-
inorganic materials containing imidazolium based on silica are most often encountered [100].
However, some recent works mention the use of polystyrene to support ILS as shown in
Figure I-8 [101].
Figure I-8 – PSIL: Polymer supported imidazolium salts [101]
Chi et al. [101] studied the catalytic properties of this material for nucleophilic
fluorination and different substitutions. They have demonstrated that the longest linker
(dodecyl) associated with tetrafluoroborate counteranion (BF4-) leads to the best catalytic
activity unlike to PSIL associated with other anions [102]. Despite a slight development in
catalytic reactions [103], the synthesis of poly (ethylene) glycol (PEG) functionalized ionic
liquids showed interesting physical and chemical properties [104].
I.2.2.6 Preparation of supramolecular polymers based on ILs
The preparation of supramolecular structures is governed by hydrogen bonds [105],
host-guest interactions [106], metal-ligand coordination [107] as well as ionic interactions
[108]. These, last ones, the most frequently encountered in the field of electrolytes [109] have
been widely used to create chemical/physical cross-link in polymer matrix such as alginates
[110], halatopolymers [111] and a wide variety of ionomers [112].
Recently, Wathier and Grinstaff [113] suggested that ionic liquids may play an
important role in the formation of ionic networks based on coordinating ion pairs. Indeed, the
authors have synthesized an ionic liquid composed of a dication, tetraalkylphosphonium
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 46
covalently linked and tetraanion ethylenediaminetetraacetate. They have demonstrated that
Coulomb interactions are governed by pairwise interactions between cation and anion and the
extended structure of the ionic liquid may lead to a supramolecular ionic network (Figure I-9).
+
Anion Cation Figure I-9 – Schematic diagram of a supramolecular ionic network
The authors have also suggested that the ramification of multivalent ions could lead to
networks while the presence of large ions would allow mobility which lead to an increase of
the toughness of the resulting materials.
The combination of the mechanical properties linked to ionomer with the homogeneity
and high charge densities typical of ionic liquids could lead to a new range of optimized
materials. However, these ionic materials are often subject to phase separations, long range
interactions that may hinder the structure-properties relationships.
Recently, ionic liquids have been used in the preparation of the self-assembly of
dendrons into supramolecular columns and sphere [115]. These supramolecular structures
contain the ionic liquid part segregated as a core leading thus ILs nanoreactors with the
intention of making reaction in confined ionic liquid geometries.
In summary, the diversity of ionic liquids opens a new path in the field of
supramolecular polymers and it is possible to envisage ionic materials with enhanced
properties as is the case for the ionomers.
Chapter I: Ionic Liquids: State of the art
Page 47
I.2.3 Intercalating agents for layered silicates
I.2.3.1 Structure and properties of layered silicates
For the preparation of PLS nanocomposites, the most commonly used are layered
silicates (montmorillonite, hectorite, saponite) which belong to phyllosilicates 2:1. Their
crystal lattice consists of layers made up of an octahedral sheet of either aluminum or
magnesium hydroxide confined between two tetrahedrally coordinated silicon atoms layers.
Details concerning the structure and chemistry for these layered silicates are provided in
Figure I-10.
Figure I-10 – Structure of 2:1 phyllosilicates
The layer thickness is about few nanometers, and the width and length dimensions of
these platelets vary from tens of nanometers to few micrometers, depending of the nature of
the layered silicate. Their nanoscale dimensions are responsible for a high specific surface
(from 400 up to 700 m2.g-1) and their particular morphology confers them a high aspect ratio
(from 100 up to 1000). The consequence of the stack of the sheets leads to a Van der Waals
gap between the layers denoted interlayer space or clay gallery. In fact, isomorphic
substitutions (Al3+ by Mg2+) create locally negative charges on the layer surface that are
compensated by positive ions (alkali or alkaline cations) localized inside the clay galleries.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 48
These exchangeable cations are easily replaced by other organic cations which
separates the platelets and whose dimension (basal spacing d001) depends on the nature of the
cations and the degree of clay hydration. At a larger scale, clay is formed of particles and
particle arrangements with multiple organizations both from the nanoscale to micrometer
scale. Silicate platelets represent MMT elementary particles: MMT primary particles include
5 to 10 platelets (size 8 to 10 nm) and MMT aggregates consist in several MMT primary
particles stacked together without any preferential orientation (size 0.1 to 10 µm), as shown in
Figure I-11.
400 à 700 nm
400 à 700 nm
Aggregates
Ø de 1 à 30 µµµµm
Primary particles
e= 5 à 10 nm
Layers
e= 1 nm
400 à 700 nm
400 à 700 nm
Ø de 1 à 30 µµµµm
e= 5 à 10 nm
e= 1 nm
Figure I-11 – Structure of 2:1 phyllosilicates
In addition, the layered silicates, for example the montmorillonite with chemical
formula Mx (Al4-xMgx)Si8O20(OH)4 have hydroxyl groups on the edges of clay layers (or M is the
monovalent cation and x is the degree of isomorphous substitution between 0.5 and 1.3).
These two particular characteristics of layered silicates, exchangeable cations and hydoxyl
groups, are key parameters for the modification and the preparation of PLS nanocomposites.
I.2.3.2 Organic modification of layered silicates
The low cost of layered silicates, especially montmorillonite, is the driving force for
their use in the field of nanocomposites. However, it appeared that this nanofiller is only
compatible with the hydrophilic polymers such as polyethylene oxide (PEO) [116] or
poly(vinyl)alcohol (PVA) [117]. Regarding hydrophobic polymers, an immiscibility,
analogous to polymer blends is observed which results from the poor interfacial interactions
between organic and inorganic compounds. Such a poor affinity leads to poor thermal and
mechanical properties. It is therefore necessary to modify the surface of clays. The commonly
used surface treatments involve cationic exchange protocol and/or grafting of organosilanes.
Chapter I: Ionic Liquids: State of the art
Page 49
I.2.3.2.1 Grafting of organosilanes
This surface modification method is widespread in the case of metal oxides and silica
[118, 119] which has a surface chemistry allowing condensation reactions on the OH groups
of the surface. Such a chemistry could be transposed to layered silicates which possess
hydroxyl groups available on the edges of layers and between galleries [120]. This process
requires the use of organosilanes deposited in solution or gas phase. According to the
literature, the nature of the clay and the solvent are key parameters [121, 122]. In fact, He et
al. [122] have shown that grafting of aminopropyltriethoxysilane (γ-APS) on two clays, a
pristine montmorillonite and a synthetic fluorohectorite generates differences especially in
terms of reactivity and functionality of silane groups (Figure I-12).
Figure I-12 – The hypothetical diagram for the intercalation and silylation of g-APS
into clay interlayer and the possible structural models for T1, T2, T3 units [122]
Then, Shanmugharaj et al. [123] proposed a model similar to that established by
Negrete-Herrera [124] which shows that the localization of the silane groups (on the edges of
the layers or inside the galleries) is dependent on type of solvent used during the grafting
reaction.
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Page 50
I.2.3.2.2 Cationic exchange
• Definition
The intercalation of inorganic exchangeable cations of layered silicates by organic
cations in an aqueous medium has been shown for the first time by Giesekind [125] and
Hendricks [126]. The mineral host structure and the nature of the intercalating agents are two
important parameters determining the success of the cationic exchange.
• Influence of the mineral host structure and the compensating cation
This type of layered silicates is characterized by a specific property known as the
cation-exchange capacity (CEC), and generally expressed in milliequivalent per gram
(meq/g). The cationic exchange capacity corresponds at the substitution of the sodium or
calcium compensating cations initially present at the platelet surface by monovalent cations in
100 grams of clay. Generally, the typical value of the CEC is included between 60 and 200
meq/100g. Below 50 meq/100g, the CEC is too low to allow sufficient intercalation while at
200meq/100g, separation of layers is prevented by excessive interlayer attractive forces. The
hectorite and montmorillonite are the best compromise to get an efficient cationic exchange.
The nature of cation plays a crucial role on the silicate swelling. Indeed, smaller the
cation is, greater mobility is which facilitates the exchange process.
• Selection of the organic cation
For several years, many of organic salts have been used as modifiers agents of layered
silicates and have been studied for the preparation of polymer-clay nanocomposites. The
surfactants most commonly used are alkylammonium salts [127, 128]. Recently, nitrogen and
phosphorus compounds are emerging as the alkylpyridinium, alkylimidazolium or
alkylphosphonium. The role of these organic salts is a key issue. In fact, their uses reduce the
surface energy of clay and improve the characteristics of the polymer matrix [129]. Besides
the presence of long chains based on organic cations induces a larger interlayer spacing which
allows the diffusion of polymer chains between the clay layers and dissociate them [130,
131].
Chapter I: Ionic Liquids: State of the art
Page 51
Sometimes, the organic cations are functionalized in order to react with the polymer or
from the addition of monofunctional or multifunctional components of the same nature as
surfactant to achieve a better compatibilization between filler surface and the polymer
matrices. For example, Ladika et al. [132] have used a combination ammonium modified
montmorillonite/ammonium polymer with high molar weights (3000g.mol-1) to get to the
exfoliation of clay layers.
- Ammonium salts
Currently, an intensive research is done on the modification of layered silicates by
several varieties of modifiers associated with chloride and bromide anions [127, 128]. The
alkylammonium salts which are derived from the synthesis of complete alkylation of amine or
ammonia, are the most used ionic liquids to prepare organoclays. The main role of quaternary
alkylammonium is to lower the surface energy of the modified clays and to improve the basal
spacing. The chemical natures of cation, length of carbon chains, the size and the shape of the
polar head have major effects on the success of the cationic exchange.
In the case of polymer/clay nanocomposites, alkylammonium influences the affinity
between the polymer and the clay surface. For example, it is known that the use of clays
treated with dialkyl dimethylammonium halides, in particular with two tallow chains of about
18 carbon atoms, a surface energy similar to polyolefins such as polypropylene and
polyethylene is obtained [129] while for polar polymers as polyamide, alkylammonium
functionalized by a benzyl or hydroxy functions have been recommended for a better affinity.
The alkyl chain length is responsible for the increase in interlayer space required for the
intercalation of polymer chains. Zig et al. [66] showed that organoclay modification with
protonated primary amines gives a much better toughness/stiffness balance with respect to
those modified with protonated secondary and tertiary amines or quaternary ammonium
cations, respectively. Because of the non polar nature of their chain, they reduce the
electrostatic interactions between the silicate layers and lower the surface energy of the
layered silicates so that an optimal diffusion of the polymer during the exfoliation process can
be obtained to dissociate the assembled clay layers.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 52
The combination of a quaternary ammonium salt with another organic compound
(methacrylate, maleic anhydride) was also very investigated to modify clays [133]. For
example, Liu et al. [134] developed a new class of modified clays based on a combination of
conventional alkylammonium and epoxypropyl methacrylate resulting a larger basal spacing
(2.98 nm).
The amino acids having a primary amino group -(NH2) and an acidic carboxyl group
(COOH) have been also intercalated between the clay layers of montmorillonite and used in
the synthesis of polyamide 6-clay hybrids [135].
Despite the compatibility of MMT modified by long alkyl-chain quaternary
ammonium with hydrophobic polymers (PE, PP), the lower thermal stability of conventional
alkylammonium ions showing a onset decomposition temperature of approximately 180°C
severely limits their use in the preparation of PLS considering matrices processed at high
temperatures such as polyamide (PA), polyethylene terephthalate (PET), or polyether ether
ketone (PEEK) [136].
- Ionic liquids
Organosilicas, silicas, metal oxides, carbon nanotubes and recently layered silicates
for the preparation of polymer nanocomposites from thermoplastics processed at high
temperature or from thermosets with high cure temperatures have been studied [137]. Figure
I-13 summarizes the different ionic liquids that are possible to be used as intercalating agents
for layered silicates [138]. The most commonly used are organic cations containing nitrogen
with pyridinium and imidazolium ionic liquids [139]. Other chemical compounds, such as
pyridinium, quinolinium [140] or phosphonium [141] were also studied due to their excellent
thermal stability compared to ammonium compounds [142].
Chapter I: Ionic Liquids: State of the art
Page 53
Figure I-13 – Cations and anions commonly used for the formation of ionic liquids [143]
The use of pyridinium salts, as a surfactant for layered silicates was described from the
70s by Slade et al. [144] who described and used hexadecylpyridinium ionic liquid to modify
vermiculite for the adsorption of non polar molecules. Others studies have shown the
adsorption capacity of pollutants such as chlorobenzene or phenanthrene by
dodecylpyridinium-modified bentonite [145, 146]. In opposite, very few studies have
investigated the thermal stability of these clays modified with such modifiers agents and their
uses in the preparation of PLS nanocomposites. However, the importance of the chemical
nature of the anion on the thermal stability of the modified clays has been shown. In fact, Kim
et al. [147] have demonstrated that the combination of pyridinium cation with fluorinated
anion such as BF4- leads to an increase of the thermal stability of the treated clays.
The imidazolium ionic liquids, opposite to the pyridinium salts have been the subject
of several studies for the preparation of organically modified clays and compared to the
conventional quaternary ammonium treated ones [138, 148]. The research focused on the
thermal decomposition of imidazolium salts and the role of their chemical structure.
According to the literature [149, 150], at high temperature, the imidazole cation is resistant to
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 54
ring fission during thermal rearrangements of 1-alkyl and 1-aryl-imidazoles. These later
results explain a better thermal stability of the imidazolium cation compared to the
ammonium salts.
The influence of the anion type on the thermal stability of the imidazolium ionic
liquids have been studied by Awad et al. [139] that have demonstrated that for the
hexafluorophosphate PF6-, tetrafluoroborate BF4
- and bis(trifluoromethylsulfonyl)imide
N(SO2CF3)2, an increase of 100°C in the degradation temperature is obtained compared to use
of halide salts, i.e. bromide and chloride. Table I-2 summarizes the different imidazolium ionic
liquid used [139].
Table I-2 – Akyl imidazolium molten salts with different alkyl groups and counter ions
R1 R2 R3 X-
Methyl Methyl Propyl Br
Cl
BF4
PF6
Isobutyl
Hexadecyl
Eicosyl
Ethylbenzene
For clays modified with imidazolium salts as surfactant, the literature demonstrates a
large increase of degradation temperature under nitrogen and oxidative atmosphere compared
to ammonium-treated montmorillonites. In addition, despite the intrinsic stability of
imidazolium ionic liquids induced by the substitution of halide anions by fluorinated ones, no
significant improvement in the thermal stability of the MMT treated by ionic liquids having
tetrafluoroborate or hexafluorophosphate anions have been demonstrated.
Because of the well-known properties of phosphorous compounds, such as flame
retardancy and heat stabilization, the use of phosphonium ionic liquid as intercalating agents
for layered silicates has been widely studied [151, 152].
According to the literature the large increase (about 50°C) of the degradation
temperature for phosphonium-modified clays compared to ammonium-treated ones, implies a
large advantage for the preparation of PLS in the melt blending [153]. In 2007, Patel et al
[154] have prepared a series of montmorillonites modified with different bromide quaternary
phosphonium salts, denoted methyl, ethyl, propyltriphenylphosphonium,
Chapter I: Ionic Liquids: State of the art
Page 55
tetraphenylphosphonium, tributyltetradecylphosphonium, showing a higher thermal stability
than the ammonium treated montmorillonites. In particular, a large increase up to 300-400°C
of the thermal stability of the tetrabutylphosphonium- and tetraphenylphosphonium-modified
montmorillonites was shown.
• Consequences of the cationic exchange on lamellar silicates
Generally, the cationic exchange takes place in water or a mixture water / alcohol.
Indeed, the use of ultra-pure water is preferred in order to facilitate the clay swelling and to
get rid of no desired cation. Sometimes it is necessary to use an organic solvent as co-solvent
to dissolve fully intercalating agents. First, the layered silicates are mixed and stirred
vigorously in the aqueous media. Then, the modifiers agents based on the cation exchanged
capacity of the lamellar silicates are previously dissolved in the organic solvent and are added
to the clay suspension. However, several parameters such as temperature, organic modifiers
concentration and reaction time are all factors ensuring the success of the cationic exchange.
Indeed, the cationic exchange involves several types of interactions: first, electrostatic
interactions but also weak intermolecular interactions between the clay surface and the
surfactant. The nature of the compensating cations and the steric hindrance of the intercalating
agents play a crucial role in the distribution of the organic species shared between: i) the
physically adsorbed species on the clay surface ii) the intercalated species in the clay galleries
from 30 up to 40 wt% [155, 156].
- Effect of the long alkyl chain length on the organoclays structure
To characterize the orientation of the organic chains in the layered silicates, several
characterization methods, essentially X-Ray diffraction (XRD) and infrared (FTIR) have been
used in most of the papers [157, 158].
Initially, Lagaly and Weiss [157] deduced for the first time the orientations of organic
chains. They have demonstrated that the surfactant chain length and the charge density of the
layered silicate play a key role in the different arrangements of organic salts between the clay
layers. Figure I-14 highlights the different conformations adopted by the organic ions.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 56
Figure I-14 – Orientations of alkylammonium ions in the galleries of layered silicates: (a)
monolayer, (b) bilayers, (c) pseudotrimolecular layers, (d, e) paraffin-type arrangements of
alkylammonium ions with different tilting angles of the alkyl chains [160].
As illustrated in Figure I-14, as the chain length is less than 12 carbons, organic cations
retains a planar configuration, monolayer or bilayer type with a basal spacings of 1.35 nm and
1.75 nm, respectively [159, 65]. In contrary, when chain length increases, a
pseudotrimolecular configuration with the shift of the alkyl chains is obtained and leads to an
increase of intergalleries space (2.2 nm) [1]. When the organic cation has a long alkyl chain
(eighteen carbons), paraffin-type structures are observed with a larger interlayer distance (3.1
nm) [160].
A more practical method based on FTIR and XRD experiments has been proposed by
Vaia et al. [158] to observe the interlayer structure and the phase structure of organic salt.
Following the frequency shifts of the asymmetric and symmetric vibrations of methylene
groups (CH2)n of the aliphatic chain as a function of chain length, temperature, or the
interlayer density, different degrees of order of intercalated cations are evidenced. They have
showed that the intercalated chains can vary from liquid-like to solid-like state, with the
liquid-like structure resulting when the interlayer packing density or the chain length
decreases as well as the temperature increases. Overall, a more ordered structure is obtained
in the presence of long alkyl chain (eighteen carbons) with a liquid crystal behavior while the
chains with twelve or six carbons have a liquid-like and gas liquid-like, respectively. The
alkyl chain structuration models proposed by Vaia et al. [158] are presented in Figure I-15.
Chapter I: Ionic Liquids: State of the art
Page 57
Figure I-15 – Alkyl chain aggregation model proposed by Vaia et al [158]: a) short chain lengths, lateral
monolayer; b) medium chain lengths, in plane disorder and interdigitation to form quasi bilayers; c) long
chains lengths, interlayer order increases leading to a liquid-crystalline polymer environment.
Other methods have also been used to study the orientations of alkyl chains of treated
layered silicates such as molecular dynamics simulations [161] or solid state NMR. Hackett et
al. [161] have demonstrated that when the length of alkyl chains increases for a constant
interlayer distance, the stacking of the alkyl chains leads to increase pressure on the galleries
resulting in increased interlayer distances. Thus, a basal spacing of 1.32 nm is obtained for a
monolayer while bilayer and trilayer arrangement lead to basal spacing of 1.8 and 2.3 nm,
respectively. Figure I-16 shows the different arrangement of modifiers agents.
Figure I-16 – Molecular modeling of surfactant configurations with different chain lengths
intercalated in clays considering different cation exchange capacity by a) monolayer (CEC = 0.9
meq/g); b) bilayer (CEC = 1.2 meq/g); c) trilayer (CEC = 1.5 meq/g) [161]
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 58
- Effect of the temperature
The temperature at which the cationic exchange is carried out has a significant
influence on the arrangement of the surfactant between clay layers. Le Pluart et al. [155] have
clearly demonstrated the effects of temperature on the cationic exchange mechanisms. Indeed,
the montmorillonite treated with octadecylammonium as modifiers agents at 60°C displays
two different organizations on the X-Ray spectrum with two quite brad peaks at 3.3 nm and
2.0 nm, indicative of the coexistence of a paraffin-like and a pseudotrilayer structures. When
the cationic exchange is performed at 80°C, the spectra show an intense, thin and regular
diffraction peaks at 3.2 nm and 1.6 nm that suggest a long-range order of paraffinic structure.
- Effect of the cationic exchange capacity of the modifiers agents
The amount of organic ions to be introduced during the cationic exchange process is a
important parameter [162]. In fact, an excess of salt is necessary to complete the layers of ions
ionically bonded to the platelet surface. An excess of organic species corresponding to 2 times
of the CEC at 80°C in deionised water is the most common condition reported in the literature
[163, 164].
- Identification of the organic species
Thermogravimetric analysis (TGA) may complete modified clay characterizations by
investigating the degradation mechanisms and the identification of the interactions between
clay mineral and intercalating agents. According to the literature [165], two kinds of
interactions take place which can be evidenced from thermal degradation kinetics: i) first
degradations from 150°C to 300°C correspond to the organic species physically adsorbed on
the clay surface via Van der Waals interactions and ii) degradations from 330 to 550°C are
related to the intercalated species via ionic interactions in the clay galleries.
Chapter I: Ionic Liquids: State of the art
Page 59
I.2.3.3 Conclusions
It was demonstrated from the literature that the use of ionic liquids as intercalating
agents of layered silicates was developed since many years by using quaternary
alkylammonium. However, the use of these later ILs is limited by their low thermal stability
which implies to focus on more thermostable ionic liquids based on pyridinium, imidazolium
or phosphonium cations.
Nevertheless, does the addition of such highly-thermostable IL surfactants improve the
dispersion of nanofillers in polymer matrices as well as the final properties of resulting
nanocomposites?
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I.2.4 Polymer/layered silicates
In the last decade, the dispersion of nano-layered silicate in a polymer matrix was a
challenge and has opened up new interests in materials science. These organic-inorganic
hybrid materials have shown significant increases compared to the properties of conventional
composites or pure matrix. It has also been demonstrated that the degrees of dispersion of
nanofillers in a polymer matrix and the method of preparation play a key role on the final
properties of the materials. Another important objective of the research is to create
nanocomposites with enhanced properties for a very low filler amounts in the polymer matrix.
To achieve this, different modified-clays have been used [138, 166]. In fact, there are many
studies on the use of ammonium-treated layered silicates [166, 167] while in opposite, the use
of thermostable ionic liquids such as pyridinium, imidazolium or phosphonium is poorly
developed. However, their use into polystyrene [122], polyethylene [168], polypropylene
[169], poly(vinylidene fluoride) [170], and poly(ethylene terephtalate) matrices [171] have
been reported.
I.2.4.1 Preparation methods of PLS nanocomposites
The preparation of PLS nanocomposites is strongly dependent on matrix parameters
such as hydrophobicity, molar masses, presence of reactive groups as well as characteristics
of the silicates, such as chemical nature of the cation and anion of the intercalating agents.
Three main routes are well-known to incorporate the layered silicates into the polymer matrix
(Figure I-17).
In situ
polymerization
In situ
Melt
Intercalation
Solution Intercalation
Layered silicates
Monomer
Figure I-17 – Different routes for the preparation of the nanocomposites
Chapter I: Ionic Liquids: State of the art
Page 61
I.2.4.1.1 Solution intercalation
Solution intercalation requires working on a solvent system in which the polymer or
pre-polymer is firstly dissolved while modified-clays or pristine clays are dispersed in a
suitable solvent to allow the swelling of the clay and to facilitate the insertion of polymer
chains between clay layers. Then, the solvent is evaporated and to form the nanocomposites.
This procedure is used for water soluble polymers such as PEO, PAA or PVP [116, 117, 172].
However, this method is used to prepare epoxy-based nanocomposites [173].
I.2.4.1.2 In situ intercalative polymerization
In situ polymerization is the first method used by Toyota [174, 135]. In the presence of
an initiator, the monomer is introduced between clay layers which activate the
polymerization. A variety of polymer nanocomposites has been prepared using this method,
i.e. polyamide (PA) [175], polystyrene (PS) [176] or polyolefins (PP or PE)/layered silicates
[165]. However, poor compatibility between the monomer and modified clays may prevent
the use of this method.
I.2.4.1.3 Melt intercalation
The great advantage of the melt intercalation technique is that the use of solvent is not
required. In fact, layered silicates or modified layered silicates are mixed with the polymer
matrix in the molten state. The mixing methods are extrusion and/or injection molding and are
easily applicable in the industry. That is why, this method is the most commonly used. Then,
the polymer chains are then intercalated or exfoliated to form nanocomposites. A wide range
of PLS nanocomposites such as polyolefins [165], poly(vinylidene fluoride) [166] have been
prepared using this method.
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I.2.4.2 Characterization of PLS nanocomposites
To characterize the morphology of nanocomposites, two techniques are often
associated X-ray diffraction (XRD) and transmission electron microscopy (TEM). The X-ray
diffraction is the method commonly used to determine the structure of nanocomposites. This
technique allows the determination of the interlayer distance between clay layers with Bragg's
law (nλ = 2d.sin θ, where n is the diffraction order, λ the wavelength of the incident X-ray and
θ the diffraction angle). The type of structure is determined by the displacement of the
diffraction peak. In fact, if no displacement of diffraction peaks towards lower angles is
observed, an intercalated structure is preserved. In opposite, if the peak is shifted to smaller
angles or totally disappears, it can assume that the clay layers are well distributed in the
polymer matrix. However, if the interlayer distances are greater than 8.0 nm, which
corresponds to an angle < 1 degree, the X-ray diffraction is limited. Generally, an overview of
the layered silicates distribution in the polymer matrix is necessary. Transmission electronic
microscopy is a good alternative but the morphological observations are based on a small part
of the sample and it may not be representative of the sample. The three different types of
nanocomposites based on the WAXD patterns and TEM images combination are shown in
Figure I-18.
Figure I-18 – WAXD patterns and TEM images of three different types of nanocomposites [177]
Chapter I: Ionic Liquids: State of the art
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I.2.4.3 ILs treated-Layered silicates for polymer nanocomposites
I.2.4.3.1 Polystyrene/IL-modified clays nanocomposites
Blends for acrylonitrile-butadiene-styrene (ABS) and ammonium- or imidazolium-
modified layered silicates, with ratios from 5 to 15 wt.% have been prepared in boiling
acetone by ultrasonic processing to improve the efficiency of the mixture [178]. Although the
XRD spectra are similar for both blends ABS/ammonium- or imidazolium-treated
montmorillonites, a higher degree of spatial distribution with the presence of several single
delaminated layers for ABS/imidazolium modified montmorillonite were evidenced.
Regarding the mechanical properties analyzed by dynamical mechanical analysis (DMA), an
increase of the stiffness of 40% at 25°C was obtained for ABS/montmorillonite treated with
imidazolium salt. The authors have attributed this increase to the degree of dispersion of clay
layers in the polymer matrix and also to the excellent thermal stability of imidazolium cation
that is not affected by the hot pressing procedure at 200°C for 10 min compared to ammonium
modified clays which must have been degraded.
In another work, PS/imidazolium clays nanocomposites have been prepared using a
similar method, i.e. in solvent blending (chlorobenzene) [176]. For identical intercalating
agents i.e. imidazolium ionic liquid, the nature of the clays, montmorillonite and fluorinated
synthetic mica, as well as that the use or not of sonication have been studied on the degree of
the clay exfoliation. The results clearly showed that the nature of clay mineral and the mixing
method are two key parameters. In fact, without sonification, they have demonstrated that the
use of montmorillonite led to a better clay layers distribution in the polymer matrix compared
to fluorinated synthetic mica. These different results could be explained by the higher charge
density and higher aspect ratio of the clay mineral. However, the only way to achieve
nanocomposites with exfoliated morphology is to use sonification as dispersion process.
The same authors have developed a technique based on melt rheology to describe the
morphology and to differenciate the degree of exfoliation in the PS / modified clays
nanocomposites [179]. A better dispersion of nanoclays was evidenced when clays are treated
with imidazolium surfactant. A schematic representation of clay dispersion state is given in
Figure I-19 as a function of the surface treatment of the montmorillonite.
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Afterdispersion
Afterdispersion
Figure I-19 – Schematic representation of speculated clay dispersion mechanism into the PS matrix as a function of the surface treatment
(a) ammonium modified MMT (b) imidazolium treated MMT
However, the authors concluded that the correlation between the melt rheology data
and the degree of clay dispersion is very difficult and that further studies are needed. In
conclusion, these studies demonstrated that the nature of the surfactant, the clay mineral, and
the mixing method are three key parameters to control in the processing of PS based
nanocomposites with a very fine dispersion of nanofillers.
In the literature, other methods of preparation of the nanocomposites based on
polystyrene, by melt compounding [180] or in situ polymerization [181] have been used as
well as various modifiers such as pyridinium, quinolinium, or phosphonium ionic liquids
[140, 180] in order to study their effects on the thermal, physical and the mechanical
properties of the nanocomposites.
Recently, Bottino et al. [181] prepared PS/montmorillonites nanocomposites at 3wt.%
via in situ polymerization. The influence of the surfactant (ammonium or imidazolium) and
the effects of the alkyl chains length for the imidazolium salt (C12, C16, C18) have been studied
on the morphology and the photo-oxydation properties. TEM micrographs of
polystyrene/montmorillonite treated with an imidazolium salt with short alkyl chain (C12)
show an intercalated morphology with very few exfoliated layers in the polymer matrix
compared to imidazolium functionalized with long alkyl chains (C16 and C18) or a partially
or fully exfoliated morphology is obtained. On the other side, the effects of imidazolium
treated montmorillonite are identical to the ammonium treated montmorillonite ones. And the
photo-oxydation properties of the PS/modified layered silicates are degraded compared to the
neat PS.
a)
b)
Chapter I: Ionic Liquids: State of the art
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Poly(styrene-co-acrylonitrile) (SAN)/modified clays nanocomposites by melt
compounding have been prepared [180]. Chu et al. [180] demonstrated that the
functionalization of the surfactant such as ammonium, imidazolium, or phosphonium is very
important. In fact, they have concluded that ammonium carrier polar functions (OH) or
aromatic imidazolium and phosphonium carriers aromatic function are the most adapted ones
for SAN nanocomposite preparation with the better clay dispersion. They also showed that the
use of the ammonium surfactant functionalized with hydroxyl groups improves the thermal
and the flammability properties of the nanocomposites. These results highlighted the
importance of the organic cation, especially its functionalization (alkyl chains, hydroxyl
groups, epoxy, vinyl) as a function of the matrix considered.
I.2.4.3.2 PVDF/IL-modified clays nanocomposites
Poly(vinylidene fluoride) (PVDF) nanocomposites based on ammonium, pyridinium
or phosphonium modified montmorillonites have been prepared by melt intercalation [170].
The influence of the modified clays on the morphology as well as on the physical and
mechanical properties has been investigated. XRD and TEM analyses show different
morphologies as a function of the nature of the surfactant. Thus, an exfoliated structure is
obtained for ammonium-treated and pyridinium-treated montmorillonites while phosphonium
ionic liquid modified montmorillonite leads to a partially exfoliated nanocomposite
morphology. The mechanical properties of nanocomposites are superior to those of neat
PVDF. However, the influence of thermostable ionic liquids, pyridinium and phosphonium,
on mechanical properties is limited. In fact, the storage moduli of the neat PVDF and the
PVDF nanocomposites are the same. Only the elongation at break increases (+175%) with 5
wt% of pyridinium treated montmorillonite compared to +200% of strain at break obtained
for ammonium modified montmorillonite. The authors are also interested in the effects of
modified clays on the polymorphic structure of the PVDF mainly composed of the most
common α and β phases. They have noticed that the use of phosphonium clay increases the
melting and crystallization temperatures of the matrix. Moreover, phosphonium
montmorillonite is the most efficient nucleating agent and an excellent generator of β-phase,
that is the phase required for dielectrical applications.
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I.2.4.3.3 Polyolefins/IL modified clays nanocomposites
Polypropylene/imidazolium modified clays nanocomposites have been prepared using
melt compounding method [169]. Morphological analyses of the nanocomposites containing
3vol% of organically treated montmorillonite (OMMT) by transmission electronic
microscopy showed a mixed intercalated/exfoliated morphology with few tactoïds and a few
single clay layers. The tensile properties of both composites with different filler volume
fraction have been studied. The modulus increases of 35% for PP/OMMT containing 4 vol%
of fillers while a decrease of relative yield strain is observed whereas an improvement of the
relative oxygen permeation is noticed. TGA analyses shows that the addition of only 1 vol%
of imidazolium modified montmorillonite improves the thermal behavior but this
improvement is optimized for PP/OMMT with 4 vol% of fillers. In conclusion, Mittal has
shown that the use of imidazolium treated clay allows an increase of the thermal, barrier and
mechanical properties.
Other studies on polypropylene nanocomposites based on imidazolium-modified
montmorillonite have been reported [182, 173]. In fact, Ding et al. [173] have used
imidazolium salt as intercalating agents of montmorillonite for the preparation of
nanocomposites by in situ dissolution of isotactic polypropylene (xylene being required as
solvent). The influence of solvent (water vs xylene) during the montmorillonite modification
was also investigated. XRD and TEM analyses show a well exfoliated montmorillonite
platelets which are disordely dispersed in the PP matrix. The consequences of the addition of
OMMT is a significant increase of 166°C respect to virgin polypropylene when the treatment
surface of montmorillonite is carried out in xylene compared to + 62°C when the water is
used.
In addition, Mittal [169] showed that the use of trialkylimidazolium modified MMT
has a significant effect on the final properties of the PP/layered silicates nanocomposites. In
opposite, the remplacement of trialkylimidazolium by dialkylimidazolium cation in the
polyethylene matrix does not affect the thermal properties of the PE nanocomposites [164].
These results show that the use of ionic liquids as surfactant could be very promising.
Chapter I: Ionic Liquids: State of the art
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I.2.4.3.4 Polyester/IL modified clays nanocomposites
The preparation of the polyethylene terephthalate (PET)/IL-modified clays requires
working at high temperature. PLS nanocomposites with different ILs-modified
montmorillonites have been prepared by melt intercalation using a brabender internal mixer at
280°C [171, 183]. Thermally stable montmorillonites modified with pyridinium, quinolinium,
imidazolium and phosphonium surfactants were considered. In both cases, the thermal
decomposition of the intercalating agents, evidenced by the change of the color of the
nanocomposites has not modified the final properties.
Nanocomposite from Poly(ε-caprolactone) have been prepared by in situ
polymerization with dibutylin dimethoxide as an initiator [184]. The materials showed an
excellent dispersion of clay layers with a highly exfoliated nanocomposite morphology. The
authors have demonstrated that the use of imidazolium functionalized MMT with hydroxyl
function bearing a long flexible alkyl chain allowed the grafting of polymer chains on the
montmorillonite surface. This good affinity between the polymer and the modified clays leads
to a significant increase of storage moduli.
I.2.4.3.5 Polyamide/IL-modified clays nanocomposites
Polyamide (PA) nanocomposites based on imidazolium or phosphonium treated
montmorillonites have been produced by melt compounding [151, 175]. In the first study
[175], the use of imidazolium or ammonium with one long alkyl chain as surfactant of
montmorillonite leads to a poor dispersion of the organoclay in the matrix. This poor
distribution could be explained by the size and the geometrical structure of the organic cations
which screen the silicate surface, i.e. leading to a difference of polarity between polyamide
(polar) and treated clays (hydrophobic).
The use of phosphonium or ammonium with alkyl chains to treat clays in PA6 matrix,
leads to a mixed intercalated/exfoliated morphology in the both cases [151]. This
phenomenom can be explained by the preparation method of organically modified
montmorillonites which are modified under supercritical carbon dioxide. This method of
treatment of the organically modified layered silicates may have played a role on the polarity
of the clay which had the effect of improving the interactions between clay and polyamide
matrix. Moreover, the use of phosphorous compound-based MMT play a positive role on the
fire retardancy properties of the nanocomposites compared to ammonium compound. In
conclusion, the polarity of layered silicates is the key parameter.
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I.2.4.3.6 PVC/IL-modified clays nanocomposites
The influence of modified layered silicates on the thermal stability, especially the
thermal behavior of the nanocomposites has been also studied [185]. PVC nanocomposites
containing 2wt% of clays treated with ionic modifiers as imidazolium, ammonium or
phosphonium salts and non-ionic modifiers such as polypropylene glycol or glycerol
monostearate were compared. The non ionic compounds did not affect the thermal stability
whereas the use of cationic surfactants accelerates the dehydrochlorination of PVC in the
following order: phosphonium ion, imidazolium ion, alkyl ammonium ion, and ethoxy/alkyl
ammonium ion.
These results show the importance of the choice of the intercalant and the matrix. In
this case, no increase of the thermal stability is obtained but for the PVC plasticized with
phthalates, an improved thermal stability has been reported [186, 187]. Instead of using ionic
liquids as intercalating agents for lamellar silicates to improve the thermal and mechanical
properties of the PVC nanocomposites, it may be preferable to consider only the use of ionic
liquids as reinforcing and plasticizers agents [162].
I.2.5 Conclusions
Layered silicates are one-dimensional nanofillers less than 100 nm with a significant
aspect ratio and an high specific surface commonly used to overcome the limitations of
conventional polymer composites. However, pristine layered silicates are not compatible with
organic polymers, especially with hydrophobic polymers. This poor affinity between the clay
mineral and the polymer leads to composites resulting in poor properties. In order to improve
the properties, the distribution of layered silicates in polymer matrices and the interactions
between the surface of layered silicates and the polymer requires to be improved with organic
treatment of clays. The surface modification of layered silicates using modifying agents based
on organic cations, especially ammonium salts have widely been reported.
Nevertheless, a lower thermal stability of the quaternary alkylammonium salt due to
the Hofmann's elimination has a limiting effect on their use in the processing of the
thermoplastic and thermosetting nanocomposites requiring higher temperatures. Recently,
ionic liquids based on pyridinium, imidazolium, or phosphonium cations are emerging as a
new alternative for the preparation of thermally stable organically modified clays. The
Chapter I: Ionic Liquids: State of the art
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preparation, the characterization and the properties of different nanocomposites using these
new surfactants to modify the fillers have been discussed in this part. Thus, we have
demonstrated that the PLS nanocomposites with low ILs modified clays amounts (< 5 wt%)
have displayed properties far mostly superior to the neat polymer matrix ones.
Conclusions of chapter I Ionic liquids are organic salts with melting temperatures below 100°C and offer many
properties such as excellent thermal stability, negligible vapor pressure, non-flammable, high
ionic conductivity, tunable solubility for organic and inorganic molecules and multiple
combinations of cations/anions. These many distinct advantages of the ionic liquids are the
focus of academic and industrial research in various applications such as organic synthesis,
inorganic materials synthesis, electrochemistry, fuell cells, supercritical fluids, homogeneous
and heterogeneous catalysis and in the polymer science, particularly in polymer gel
electrolytes, lubricants and plasticizers. Despite their many advantages, due to their good
thermal stability, the use of ionic liquids in the field of the nanocomposites is mainly limited
to the role of modifiers agents for layered silicates. We have shown that the influence of ILs
modified clays on the PLS nanocomposites are mixed though. The true potential of ILs based
on the infinite cation/anion combinations is not really exploited. In fact, the reduced choice
and the high cost of the commercial ionic liquids functionalized with different groups (epoxy,
hydroxy, vinyl, and fluorinated chains) are the main causes of this limitation. To achieve
significantly optimized PLS nanocomposites, a lot of studies are needed to find the suitable
association between the specific IL and the matrix.
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Chapter II POLYMER/IONIC LIQUID INTERACTIONS In the field of polymer materials, ionic liquids have often been used as a green solvent
and conductors in the gel electrolyte or as a surfactant to the layered silicates. So far, to our
knowledge, no work mentions the use of IL as structuring agent of a polymer matrix.
In this second chapter, we sought to investigate the impact of the IL, introduced as an additive
in the polymer matrix on the morphology, physical and thermo-mechanical properties of the
polymer. The effects generated by the ionic liquids can be modulated by the wide range of
possible combinations of cations / anions. In this work, we have chosen to introduce the IL in
fluorinated aqueous suspension comprising polytetrafluoroethylene (PTFE) stabilized. The
specification is difficult because the PTFE have excellent thermal stability, high resistance to
acids and bases and a low friction coefficient. What are the contributions of using ionic
liquids introduced at a low rate (1%wt) in the PTFE matrix after film formation?
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Pages
II.1 New building blocks ........................................................................................................... 77 II.1.1 Introduction ................................................................................................................................... 77 II.1.2 Experimental ................................................................................................................................. 78
II.1.2.1 Materials __________________________________________________________________ 78 II.1.2.2 Processing and characterization of the IL/PTFE films _______________________________ 78 II.1.2.3 Synthesis of ionic liquids _____________________________________________________ 80
II.1.2.3.1 Synthesis of phosphonium salt ....................................................................................... 80 II.1.2.3.2 Synthesis of imidazolium salt ........................................................................................ 82 II.1.2.3.3 Synthesis of pyridinium salt ........................................................................................... 82
II.1.3 Morphology and mechanical performances of polymer/IL blends ................................................ 83 II.1.4 Conclusions ................................................................................................................................... 85
II.2 Nanostructuration of ionic liquids in fluorinated matrix: Influence on the mechanical properties ......................................................................................................................................... 86
II.2.1 Introduction ................................................................................................................................... 86 II.2.2 Results and discussion ................................................................................................................... 87
II.2.2.1 Effect of ionic liquids on the structuration of fluorinated polymer films _________________ 87 II.2.2.1.1 Nanostructures in bulk .................................................... Error! Bookmark not defined. II.2.2.1.2 Surface analysis of fluorinated polymer/IL blends ........................................................ 89
II.2.2.2 Effect of ionic liquids on the thermal properties of fluorinated polymer-based blends ______ 90 II.2.2.3 Effect of ionic liquids on the PTFE crystallinity____________________________________ 91
II.2.2.3.1 Influence of organic cation ............................................................................................. 91 II.2.2.3.2 Influence of halide and fluorinated anions associated on phosphonium cation.............. 92
II.2.2.4 Effect of ionic liquids on the mechanical properties of fluorinated polymer ______________ 93 II.2.2.4.1 Dynamical mechanical analysis of fluorinated polymer-IL blends ................................ 93 II.2.2.4.2 Mechanical properties of fluorinated polymer modified using ILs. ..... Error! Bookmark
not defined. II.2.2.4.3 Effect of strain rate on the uniaxial tension behaviour of polymer/IL films .................. 96 II.2.2.4.4 Effect of ionic liquids on the morphology after deformation ......................................... 97
II.2.3 Conclusions ................................................................................................................................... 99
Conclusions of chapter II ............................................................................................................. 100
References of chapter II ............................................................................................................... 101
Chapter II: Interactions Polymer/Ionic Liquids
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II.1 New building blocks The use of the multiple combinations of ionic liquids (ILs) as functional building
blocks based on pyridinium, imidazolium and phosphonium cations to achieve materials
combining a structuration at nanoscale with the dramatic mechanical properties of the
resulting ionomers has been successfully demonstrated for the first time in a fluorinated
matrix.
II.1.1 Introduction The ability to create regularly shaped nanoscale objects which serve as the building
block is an extremely important goal in materials science. One of the key issues is to design
and create new polymer materials with unprecedented improvements in their physical
properties. Different self-assembly pathways are described to lead to hierarchical structures
formed from heterogeneous chemical species, like organic molecules, polymers, organic-
inorganic nanobuilding blocks. Nanostructured thermosets may be obtained by the self-
assembly of amphiphilic block copolymers in a reactive solvent and freezing of the resulting
morphologies by crosslinking reactions [1]. The introduction of fillers with nanometer-scale
dimensions into a polymer matrix is another route to prepare nanostructured polymers [2].
Ionic liquids (Ils), which are organic salts with a melting point below 100°C and have
been widely promoted as “green solvents” are attracting much attention of academic and
industrial research in many fields of chemistry and industry due to their chemical stability,
excellent thermal stability, inflammability, low vapor pressure and high ionic conductivity
properties. The ionic liquids could present an advantageous and promising way in a wide
variety of applications: Over the last few years, ILs have been popularly used as solvents for
organic synthesis, as well as homogeneous and heteregeneous catalysis, electrochemical
applications, electrolyte batteries, fuel cells, and also been used as media for polymerization
processes [3]. ILs are also used as surfactants for lamellar silicates. The conventional ILs
frequently used in the layered silicates-based nanocomposite are mainly quaternary
ammonium [4].
Ionic Liquids : Multifunctional agents of the polymer matrices
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If few studies indicate the use of thermostable ionic liquids based on imidazolium or
phosphonium ions for layered silicates intercalation [5-7], no work has been published yet on
the use of ILs as new building blocks. This study reports for the first time the achievement of
a nanoscale structuration from ILs into a polymer matrix. This structuration can be tuned by a
wide choice of cation-anion combinations including pyridinium imidazolium, and
phosphonium as cation associated to iodide, bromide, or fluorinated anions. The preparation
of IL nanostructured films from a fluorinated polymer solution could open many applications
in energy and materials fields by creating new polymers with interesting properties.
II.1.2 Experimental
II.1.2.1 Materials
All chemicals necessary to the synthesis of ionic liquids, i.e. triphenylphosphine
(95%), imidazole (99.5%), pyridine (99%), iodooctadecyl (95%), and solvents (toluene,
sodium methanoate, pentane, and acetonitrile) were supplied from Aldrich and used as
received. The polytetrafluoroethylene used in this study, called PTFE, is considered as an
aqueous dispersion of PTFE from Solvay. The composition is as follows: PTFE (60%wt),
water (32-33%wt), octylphenol polyethoxylates Triton® (7% wt), and ammonium
perfluorooctanoate (0.1%wt). The pH of aqueous dispersion is 10 and the PTFE particle
average size is about 220 nm.
II.1.2.2 Processing and characterization of the IL/PTFE films
The aqueous suspensions of PTFE containing 1%wt of pyridinium or imidazolium or
phosphonium ionic liquids were stirred using a Rayneri® disperser with a speed of 1500 rpm
for 15 minutes. Then the suspension was spread on stainless steel plates using a bar coater
equipped to get 50 µm-thick wet layer. A thermal treatment at 400°C for 10 minutes was
applied to obtain the final polymer film (thickness of the dried and sintered film: 20 µm).
Thermogravimetric analyses (TGA) performed on the ionic liquids and on PTFE/IL
nanocomposites were characterized using a Q500 thermogravimetric analyser (TA
instruments). The samples were heated to 700°C at a rate of 20 K.min-1 under nitrogen flow of
90mL/min.
Chapter II: Interactions Polymer/Ionic Liquids
Page 79
DSC measurements were performed ousing a Q20 (TA instruments) from -20°C to
400°C. The samples were kept for 3 min at 400°C to erase the thermal history before being
heated or cooled at a rate of 10 K.min-1 under nitrogen flow of 50mL/min. For calculations of
enthalpy of melting and crystallization, the enthalpy of reference considered for PTFE is 82
J/g [19-20].
Wide angle X-ray diffraction spectra (WAXD) were carried out using a Bruker D8
Advance X-ray diffractometer at the H. Longchambon diffractometry center (Université de
Lyon) at room temperature. A bent quartz monochromator was used to select the Cu Kα1
radiation (λ = 0.15406 nm) and run under operating conditions of 45 mA and 33kV in Bragg-
Brentano geometry. The angle range analyzed is included between 1 and 30°2θ.
Small Angle X-ray Scattering was carried out on the D2AM beamline at the European
synchrotron Radiation Facility (ESRF, Grenoble, France). The incident photon energy was 16
keV. A bidimensional detector (CCD camera from Ropper Scientific) was used to collect the
scattered radiation. The contribution of the empty cell was subtracted from the scattering
images of the studied samples.
X-ray Photoelectron Spectroscopy experiments were carried out in a KRATOS AXIS
Ultra DLD spectrometer using a hemispherical analyzer and working at a vacuum better than
10-9 mbar. All the data were acquired using monochromated Al Kα X-rays (1486.6 eV, 150
W), a pass energy of 40 eV, and a hybrid lens mode. The area analysed is 700µm x 300µm.
The peaks were referenced to the C-(C,F) components of the C1s band at 284.6 eV.
Transmission Electron Microscopy (TEM) was performed at the Center of
Microstructures (Université de Lyon) using a Philips CM 120 field emission scanning
electron microscope with an accelerating voltage of 80 kV. The samples were cut using an
ultramicrotome equipped with a diamond knife, to obtain 60 nm thick ultrathin sections.
Then, the sections were set on copper grids for observation.
Uniaxial Tensile tests were carried out using a MTS 2/M electromechanical testing
machine at 22±1°C and 50±5% relative humidity at crosshead speed of 0.004 and 0.2 s-1. A
cookie cutter was used to obtain dumbbell-shaped specimens.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 80
Dynamic Mechanical Thermal Analysis were carried out using a Rheometric Solid
Analyzer RSA II operating at ± 0.01% tensile strain and a frequency of 1Hz. The heating rate
was 3 °K.min-1 from -130°C to 320°C.
II.1.2.3 Synthesis of ionic liquids
In this paper, a general and simple method for the synthesis of a serie of organic halide
and fluorinated salts is reported based on
i) Iodide (I-), bromide (Br-) and hexafluorophosphate (PF6-) combined
phosphonium cations with one long alkyl chain (1a-1c) denoted C18P I-, C18P Br-,
C18P PF6-; respectively.
ii) Iodide associated imidazolium cation with two long alkyl chains
denoted C18C18Im I- (2)
iii) Iodide combined pyridinium salt denoted C18Py I- (3). The synthesis of
pyridinium, imidazolium, and phosphonium ionic liquids were presented in Figure
II-1.
PC18H37
N N N NH37C18
HN N
MeO Na C18H37I
- NaI
N NH37C18
C18H37 I
X
X = I , Br
C18H37X
P
2
1a, 1b
25°C
HPF6
PC18H37 PF6
1c
N N
C18H37I
C18H37I
C18H37
I
3
Figure II-1 – Synthesis and structure of ionic liquids
II.1.2.3.1 Synthesis of phosphonium salt
Chapter II: Interactions Polymer/Ionic Liquids
Page 81
Two different procedures were used for the synthesis of phosphonium ionic liquids
combined to halide (I-, Br-) or fluorinated (PF6-) anions. The first one is the synthesis of
halide salts C18P I- and C18P Br-: In a 100 mL flask was placed under a positive nitrogen
pressure, triphenylphosphine (1 equiv., 20 mmol, 5 g) and a solid of alkyl iodide and bromide
[1 equiv., 20 mmol i.e; 13 g for octadecyl iodide (C18H37I) and 11 g for octadecyl bromide]
were diluted in toluene (20mL). The stirred suspension was allowed to react for 24 h at
120°C, and a yellow precipitate was formed. The reaction mixture was then filtered, and
washed repeatedly with pentane. The synthesis of salts was confirmed by 1H NMR, and 13C
NMR spectroscopies.
Octadecyltriphenylphosphonium iodide (C18P I-) 1a. White solid, Yield = 90%. 1H
NMR (CDCl3): δ 0.8-0.90 (m, 3H, CH3); 1.10-1.35 (m, 28, CH2Me); 1.50-1.70 (m, 4H, PCH2(CH2)2); 3.50-3.70 (m, 2H, PCH2); 7.70-7.90 (m, 15H, H arom). 13C NMR (CDCl3): δ 14.00 (CH3); 22.67 (CH2Me); 23.2; 29.37-29.66; 30.24; 31.85 (PCH2); 118.45; 130.43; 133.70; 135.15 (P-Carom.).
Octadecyltriphenylphosphonium bromide (C18P Br-) 1b. White solid, Yield = 90%. 1H NMR (CDCl3): δ 0.8-0.85 (m, 3H, CH3); 1.05-1.30 (m, 28, CH2Me); 1.55-1.75 (m, 4H, PCH2(CH2)2); 3.50-3.65 (m, 2H, PCH2); 7.70-7.85 (m, 15H, H arom). 13C NMR (CDCl3): δ 14.00 (CH3); 22.52 (CH2Me); 23.0; 28.90-29.66; 30.20; 32.00 (PCH2); 118.85; 130.80; 133.50; 135.45 (P-Carom.).
Octadecyltriphenylphosphonium hexafluorophosphate (C18P PF6-) has been prepared
by anionic exchange according to the following procedure : In a 100 mL flask,
octadecyltriphenylphosphonium iodide (C18P I-) (5.000 g, 13.14 mmol, 1 equiv.) was
dissolved into dichloromethane (25 mL). The mixture was stirred for 30 min at room
temperature. A solution of hydrogen hexafluorophosphate (HPF6) (3.830 g, 26.28 mmol, 2
equiv.) diluted in water (25 mL) was stirred for 30 min and added to the
octadecyltriphenylphosphonium iodide solution. The stirred suspension was allowed to react
for 24 h at room temperature. The reaction mixture was then introduced in a separatory funnel
and the organic layer was washed repeatedly with distilled water (4x 25 mL). The mixture
was dried over anhydrous magnesium sulfate, concentrated under reduced pressure. The
solvent was removed by evaporation under vacuum.
Octadecyltriphenylphosphonium hexafluorophosphate (C18P PF6
-) 1c. White solid, Yield = 90%. 1H NMR (CDCl3): δ 0.8-0.90 (m, 3H, CH3); 1.10-1.35 (m, 28, CH2Me); 1.45-1.70 (m, 4H, PCH2(CH2)2); 3.40-3.55 (m, 2H, PCH2); 7.55-7.85 (m, 15H, H arom). 13C NMR (CDCl3): δ 14.00 (CH3); 22.35 (CH2Me); 23.5; 29.12-29.74; 30.35; 31.75 (PCH2); 118.75; 130.22; 133.50; 135.05 (P-Carom.).
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 82
II.1.2.3.2 Synthesis of imidazolium salt
Three steps are necessary for the synthesis of N-Octadecyl-N'-octadecylimidazolium
iodide (C18C18Im I-). The first one is the deprotonation of the imidazole ring with a solution of
sodium methoxide prepared as follows : Sodium (1 equiv., 0.465 g, 20 mmol) is dissolved in
dry freshly distilled methanol (10 mL) in a septum sealed, 100 mL round-bottomed, three-
necked flask equipped with a condenser, a nitrogen inlet and a magnetic stirrer. Then,
Imidazole (1 equiv., 1.37 g, 20 mmol) and a small amount of acetonitrile (10 mL) were
introduced into the solution of sodium methoxide at room temperature. After 15 min, the
white suspension was formed and concentrated under reduced pressure. The dried white
powder was diluted in acetonitrile and a powder of octadecyl iodide (C18H37I) was added
under nitrogen atmosphere. The mixture was heated at 85°C during 24 hours. After the first
alkylation, the same procedure was used for the second alkylation. After cooling to room
temperature, the solvent was removed by evaporation under vacuum, and the beige solid
obtained was filtered, washed repeatedly with pentane and dried. The assignment of 13C, 1H
NMR resonance peaks are reported below.
Octadecyloctadecylimidazolium iodide (C18C18Im I-) 2. White powder, Yield =
97%. 1H NMR (CDCl3): δ 0.75-0.90 (m, 6H, 2CH3), 1.15-1.30 (m, 64H, 32 CH2], 1.80-1.90 (m, 2H, NCH2CH2), 4.30 (t, 2H, CH2N=), 7.45 (m, 1H, H arom), 7.65 (m, 1H, H arom), 9.15 [s (b), 1H, H arom]. 13C NMR (CDCl3): δ 14.10 (2CH3); 22.67 (2CH2Me); 26.23; 28.97; 29.35-29.69; 30.24; 31.91 (CH2); 50.10; (CH2N=); 50.32 (CH2N-); 121.69; 122.48 (=CN); 136.88 (N-C=N).
II.1.2.3.3 Synthesis of pyridinium salt
The general procedure for the synthesis of pyridinium iodide (C18Py I-) is the
following: In a 100 mL flask was placed under a nitrogen pressure, 10 mmol of alkyl iodide
[octadecyl iodide (C18H37I)] and distilled pyridine (1.5 equiv.). The stirred suspension was
allowed to react for 24 h at room temperature. A yellow precipitate was formed. The reaction
mixture was then filtered, washed repeatedly with pentane. Most of the solvent was removed
under vacuum. Pyridinium iodide was isolated after drying and fully characterized by 1H
NMR, 13C NMR spectroscopies.
Octadecylpyridinium iodide (C18Py I-) 3. White solid, Yield = 90%. 1H NMR
(CDCl3) δ: 0.84 (t, J = 7.3 Hz, 3H, CH3), 1.15-1.30 (m, 30H, 15 CH2), 1.90-2.02 (m, 2H, NCH2CH2), 4.90 (t, J = 7.5 Hz, 2H, NCH2), 8.05 (t, J = 7.2 Hz, 2H arom), 8.45 (t, J = 7.8 Hz, 1H, H arom), 9.35 (d, J = 5.4 Hz, 2H, H arom). 13C NMR (CDCl3) δ: 14.04 (CH3); 22.58; 25.91; 28.99; 29.25-29.60; 31.81-31.84 (CH2); 62.02 (CH2N=); 128.57 (C=C); 144.82; 145.50 (=CN).
Chapter II: Interactions Polymer/Ionic Liquids
Page 83
II.1.3 Morphology and mechanical performances of polymer/IL blends
To prepare new supramolecular ionic networks, various functional ionic components
have been introduced in very low quantities (1 wt%) in a fluorinated matrix. The influence of
the organic cation, i.e pyridinium, imidazolium, or phosphonium on the structuration of the
matrix, was studied. TEM micrographs reveal the presence of ILs into the fluorinated polymer
matrix due to the difference of electronic density between polymer and IL phases. Images
carried out with only 1 wt % of pyridinium, imidazolium, or phosphonium ionic liquids are
reported in Figure II-2 show different types of structuration which are tuned by the chemical
nature of cations.
(a) (b)
(c) (d)
200 nm
200 nm
200 nm
200 nm
Figure II-2 – TEM micrographs of the neat PTFE matrix and polymer/IL blends (a) PTFE; (b) PTFE/C18C18Im I-; (c)
PTFE/C18Py I-; (d) PTFE/C18P I-
By using C18C18Im I- as IL, two different structurations of ionic liquid are evidency
simultaneously: a co-continued morphology and many aggregates of ILs. A less achieved co-
continuous morphology is also obtained with C18Py I-. Only the phosphonium ionic liquid
leads to an excellent distribution and dispersion of IL since a structuration at nanoscale is
observed in the whole polymer.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 84
The chemical nature of the counteranion significantly influences the final morphology
as shown in Figure II-3 by using the same phosphonium cation combined with different anions,
i.e. : iodide (I-), fluoride (PF6-), and bromide (Br-).
200 nm 200 nm 200 nm
Figure II-3 – TEM micrographs of the nanocomposites (a) PTFE/C18P I-, (b) PTFE/C18P Br-, (c) PTFE/C18P PF6-
Even if both C18P I- and C18P Br- have an identical thermal stability (degradation at
330°C), the final morphology is quite different. A fine structuration at nanoscale is clearly
obtained with the iodide anion whereas with the bromide anion, a poor distribution of large
aggregates can be observed. A coarse morphology resulting from a good distribution of
aggregates is also obtained with the fluorinated anion. This final morphology can be
associated to the excellent thermal stability of hexafluorophosphate based on phosphonium [6,
7].
These nanoscale structurations have a dramatic impact on the mechanical behaviour of
polymer films as shown in Figure II-4. But the mechanical performance is very dependent on
the ionic liquids used, i.e. pyridinium, imidazolium, versus phosphonium. Better interactions
seem to take place between the phosphonium ionic liquid functionalized with long alkyl
chains and the fluorinated matrix.
0
2
4
6
8
10
12
14
16
0 100 200 300 400 500 600
Strain at break (%)
Str
es
s (
MP
a)
PTFE
PTFE C18P I-
Figure II-4 – Effect of the phosphonium ionic liquid (1wt%) on the mechanical properties determined by
uniaxial tensile tests at room temperature and 0.004 s-1: (♦) PTFE; (■) PTFE/C18P I-
Chapter II: Interactions Polymer/Ionic Liquids
Page 85
Indeed, in the case of phosphonium ionic liquid containing iodide anion, an excellent
dispersion in the polymer film provides a significant effect on the mechanical properties of
the polymer as both stiffness and strain at break are improved compared to the neat PTFE.
This behaviour could be explained by the existence of the web structure of IL which sustains
load during mechanical strain.
II.1.4 Conclusions
In conclusion, we have demonstrated that the chemical nature of cations and
counteranions plays a key role on the dispersion and the structuration of ionic liquids in the
polymer matrix. As a consequence, the morphologies and the physical properties of PTFE/IL
blends can be tuned by a suitable combination of cation-anion IL. This first work highlights
the huge potential of these new building blocks in the polymer materials.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 86
II.2 Nanostructuration of ionic liquids in a fluorinated matrix: Influence on the mechanical properties
In this work, Ionic liquids (IL) have been used for their intrinsic and unique properties.
Pyridinium, imidazolium, and phosphonium ionic liquids have been synthesized and used as
new synthetic building blocks in a polymer matrix. From a fine tuning of the IL cation-anion
combination, the influence of the chemical nature of buildings blocks was investigated on the
morphology by transmission electronic microscopy (TEM) and small-angle X-Ray scattering
(SAXS). The physical and mechanical properties of material were discussed in relationship
with the structuration. An incredible increase of both flexibility and stiffness was achieved
from a nanoscale structuration resulting good interactions between ionic liquid and the
fluorinated matrix.
II.2.1 Introduction
In the world of nanotechnology, the goal is to create new materials with significantly
improved physical properties by designing structuration of material at nanoscale. For many
years, several approaches have been described for preparing new structures from organic
molecules, polymers, or organic-inorganic hybrids. One of the well- known approaches
consists in the design of nanostructured thermosets obtained by the use of block copolymers
with amphiphilic block [1-3]. Another way for the development of nanostructured polymers is
the introduction of inorganic nano-objects having nanometer-scale dimensions such as silica
[4], layered silicates [5-10], or carbon nanotubes [11-13]. In addition, ionic liquids (IL)
appear as a new alternative and are subject to the attention of research in many applications
due to their unique properties such as excellent thermal stability, non-flammability, low
saturated vapor pressure, good electrical, and thermal conductivity. In most cases, ionic
liquids are used as green solvents in organic synthesis [14] and synthesis of nanoparticles
[15], homogeneous and heterogeneous catalysis [16], in polymer science as plasticizers [17],
or lubricants [18]. They can also be used as surfactants in the lamellar silicate nanocomposites
and the most frequently used are the ammonium salts [19-21]. In recent years, pyridinium,
imidazolium, and phosphonium ionic liquids known for their excellent thermal stability are
increasingly used [22-23]. Moreover, the wide range proposed of ionic liquid based on many
combinations cation/anion offer new perspectives in polymer science, particularly in the field
of energy. However, from our knowledge, no study describes the use of ionic liquids in order
Chapter II: Interactions Polymer/Ionic Liquids
Page 87
to design nanostructured materials from a polymer matrix. This paper will describe the
generation of nanostructured phase based on ionic liquids of different chemical natures in a
fluorinated polymer matrix as well as the consequences of this nanostructuration on the
physical and mechanical properties.
II.2.2 Results and discussion
II.2.2.1 Effect of ionic liquids on the structuration of the fluorinated matrix
II.2.2.1.1 Analysis of morphology by transmission electronic microscopy (TEM)
Transmission electron microscopy is the suitable tool to reveal the existence of IL as a
separated phase into the fluorinated matrix according to the difference in electronic densities.
With only 1 wt % of ILs, a structuration is reached (Figure II-5). In this study, the chemical
nature of the cation and halide anions plays a key role on the different morphologies obtained.
The dispersion of imidazolium ionic liquid (C18C18Im I-) generates two types of
nanostructurations: The first one corresponds to the formation of aggregates of ionic clusters
while the second one is similar to a co-continuous morphology. A less achieved co-
continuous morphology is also obtained with pyridinium ionic liquids (C18Py I-). In the
opposite of phosphonium ionic liquid, an excellent dispersion is achieved since a structuration
at nanoscale is observed in the polymer matrix.
(a) (b)
(c) (d)
200 nm
200 nm
200 nm
200 nm
Figure II-5 – TEM micrographs of neat (a) PTFE and blends
(b) PTFE/C18C18Im I-, (c) PTFE/C18Py I-, (d) PTFE/C18P I-
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 88
The chemical nature of the different counteranion halide (I-, Br-) or fluorinated (PF6
-)
associated to phosphonium cation significantly influences the final morphology as reported in
Figure II-6. A poor distribution of important aggregates can be observed with bromide anion
whereas a coarse morphology resulting from a good distribution of aggregates is also obtained
with the fluorinated anion. Both of these microscale structurations display a large contrast in
comparison to the nanoscale morphology reached with the iodide anion.
(a) (b) (c)
200 nm 200 nm 200 nm
Figure II-6 – TEM images of (a) PTFE/C18P I-, (b) PTFE/C18P Br-, (c) PTFE/C18P PF6-
Due to the hydrophobic nature of polytetrafluoroethylene and strong interactions
between ionic compounds in PTFE/IL blends, a phase-separated morphology is generated
spontaneously. Whatever the ionic liquid considered, their miscibility remains very poor even
with the presence of the octadecyl chain. Nevertheless, such morphologies could be compared
to the ones observed in ionomers for which ion-containing polymers provide a mean of
generation of various types of morphologies and subsequent properties especially in polymer
blends [29, 30]. In such materials, ionic species introduce specific interactions between
components and could lead to miscible or partially miscible blends of two immiscible
polymers from the control of the type of ionic acid group and/or conteranion. It is well known
that the clustering of ion pair in a low dielectric constant medium is responsible of different
nano- and microstructures which can be predicted theorically [31, 32]. The main parameter
controlling the microphase separation in a non-polar media are the dipole-dipole interactions
between pairs leading to formation of multiplet structures [33, 34], i.e. ionic aggregates. In
the present case, the same phenomena could be evoked with the formation of ionic liquid
aggregates which from different types of morphologies depending on the balance of the
interactions between polymer medium and anion-cation pairs.
Chapter II: Interactions Polymer/Ionic Liquids
Page 89
In conclusion, the ionic liquids have a similar behavior to ionomers which were well
described in the literature. As reported, ionic liquids form ionic clusters of varied
morphologies from nanoscale up to microscale, easily tuned by the wide variety of salts
achieved by a great number of combinations possible between cation and anion.
II.2.2.1.2 Surface analysis by X-ray Photoelectron Spectroscopy (XPS)
According to the chemical nature of the two components, i.e. PTFE and ILs, a surface
segregation could occur. In order to evaluate the interactions between ILs and fluorinated
matrix and a possible enrichment of one of the components at the surface of the films, X-ray
photoelectron spectrometry was used to analyze the surface of the neat PTFE and ILs/PTFE
films. The Table II-1 shows the percentage of C-F and C-C bonds detected on the sample
surface.
Table II-1 – Atomic percentage of species detected on the surface of fluorinated films
Atomic % C-F C-C PTFE 59 30
PTFE 1% C18P I-
97 3
PTFE 5% C18P I-
93 5
PTFE 1% C18C18Im I-
88 12
The neat PTFE composed only of (-CF2-CF2-) monomer units displays a ratio of two
between the C-F (60%) and C-C (30%) bonds. As the ionic liquid is added to the polymer
matrix, a significant decrease of C-C bonds in the presence of ionic liquids C18C18Im I-, C18P
I- is observed. With C18P I-, only 3% of C-C bonds were measured instead of 30% on neat
PTFE film which cannot be explained by a surface segregation. These results are an evidence
of the influence of ionic liquids on the chain scission inducing an increase of C-F bonds
during post-treatment at high temperature. Moreover, the migration of IL takes place in the
bulk of the material no trace of iodine, phosphorous, or nitrogen has been found at the surface
even at higher ILs contents, i.e. 5 wt%.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 90
II.2.2.2 Effect of ionic liquids on the thermal properties of the fluorinated matrix
The thermal behavior of the PTFE films containing 1 wt% of ionic liquids was
characterized by thermo-gravimetric analysis in order to study the effect of pyridinium,
imidazolium and phosphonium ions on the thermal stability of the blends. The
thermogravimetric results carried out on neat PTFE and on PTFE modified with pyridinium,
imidazolium or phosphonium salts are compared in Figure II-7.
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of PTFE/C18Py I- (a, a’), PTFE/C18P I- (b, b’), PTFE/C18C18Im I- (c, c’) and PTFE unfilled (d, d’) (heating rate : 20 K.min-1; nitrogen atmosphere).
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Chapter II: Interactions Polymer/Ionic Liquids
Page 91
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t (%
)
400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700
Temperature (°C)
� (a)� (b)� (c)� (a)� (b)� (c)
Universal V4.2E TA Instruments
Figure II-8 – Evolution of weight loss as a function of temperature (TGA) and derivative of TGA curves (DTG) of of PTFE/C18P with PF6
- (a, a’), Br- and I- (b, b’) and PTFE unfilled (c, c’) (heating rate : 20 K.min-1; nitrogen atmosphere).
In conclusion, the insertion of ionic liquid in PTFE at low IL content, i.e. 1%wt., does
not affect the thermal stability of the fluorinated matrix.
II.2.2.3 Effect of ionic liquids on the crystallinity of the fluorinated matrix
II.2.2.3.1 Influence of the organic cation
The effect of IL on the polymer physical characteristics such as crystallinity was
studied (the samples were analyzed at a heating and a cooling rate of 10K.min-1). Table II-2
gathers melting (Tm) and crystallisation (Tc) temperatures as well as the corresponding
enthalpies of melting, ∆Hm, and crystallization, ∆Hc, for the neat fluorinated matrix and with
the addition of 1%wt. of the different types of IL.
Table II-2 – Influence of organic cation on the physical properties of PTFE films
Samples Tm (°C)
Tc (°C)
∆Hm (J/g)
Xc (%)
PTFE 326 310 32 39 PTFE/C18P I- 328 307 28 34
PTFE/C18C18Im I- 328 307 29 35 PTFE/C18Py I- 329 308 36 44
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 92
The IL incorporation into polymer matrix has a minor effect on the thermal transitions.
The melting temperatures of neat PTFE or PTFE modified with pyridinium, imidazolium, or
phosphonium ionic liquids are included between 326°C (PTFE) to 329°C (PTFE/C18Py I-).
Regarding the crystallization temperatures, no significant difference is observed since the
values are nearly in the same temperature range, i.e. from 307°C to 310°C. Only, the chemical
nature of organic cation induces differences in the crystallinity of the PTFE matrix.
Indeed, the addition of phosphonium and imidazolium ionic liquids results in a slight
decrease in crystallinity. This decrease could be attributed to the steric hindrance of cations
caused by the presence of two long alkyl chains on imidazolium ion and the presence of three
benzyl groups and a long alkyl chain on phosphonium ion. On the other side,
octadecylpyridinium iodide that is less sterically hindered has a light nucleating effect on the
crystallization of fluoropolymer with an increase of 5%.
II.2.2.3.2 Influence of the conteranion
The effect of halide (Br-, I-) and fluorinated (PF6
-) anions combined to phosphonium
cation on the crystallinity of polymer films was studied by DSC (Table II-3).
Table II-3 – Influence of anion on the physical properties of fluorinated films
Samples Tm (°C) Tc (°C) ∆Hm (J/g) Xc (%) PTFE 326 310 32 39
PTFE/C18P I- 328 307 28 34 PTFE/ C18P Br- 327 307 28 34 PTFE/ C18P PF6
- 329 308 21 38
The melting and crystallization temperatures are not affected by the nature of the
anion since the DSC measurements are the same whatever the anion used. In terms of the
melting enthalpies, the cation has a predominant effect on the halide or fluorinated anions.
This phenomenom could be explained by the fact that the fluorinated nature of the anion
improves the compatibility with the matrix and contributes to the matrix crystallization.
Chapter II: Interactions Polymer/Ionic Liquids
Page 93
II.2.2.4 Effect of ionic liquids on the mechanical properties of the fluorinated polymer
II.2.2.4.1 Dynamical mechanical analysis of fluorinated films
As reported in Figure II-9, the dynamic mechanical spectra of PTFE and PTFE/C18P I-
display the storage moduli E’ and the main relaxation peaks. These peaks were previously
assigned by McCrum to the long range motions i.e. α relaxation at 120°C, to the β relaxation
attributed to the combination of a transition in crystal form at 20°C and at – 97°C, the random
chain motions is attributed to the γ relaxation [35].
-130.0 -34.0 62.0 158.0 254.0 350.0
106
107
108
109
1010
0.01
0.1067
0.2033
0.3
_]
Temperature (°°°°C)
Mo
du
liE
’(M
Pa
)T
an
δ (δ (δ (δ ( °° °°C
)) ))
γγγγ-97°C
ββββ20°C
αααα120°C
PTFE
PTFE/C18P I-
-130.0 -34.0 62.0 158.0 254.0 350.0
106
107
108
109
1010
0.01
0.1067
0.2033
0.3
_]
Temperature (°°°°C)
Mo
du
liE
’(M
Pa
)T
an
δ (δ (δ (δ ( °° °°C
)) ))
γγγγ-97°C
ββββ20°C
αααα120°C
-130.0 -34.0 62.0 158.0 254.0 350.0
106
107
108
109
1010
0.01
0.1067
0.2033
0.3
_]
Temperature (°°°°C)
Mo
du
liE
’(M
Pa
)T
an
δ (δ (δ (δ ( °° °°C
)) ))
-130.0 -34.0 62.0 158.0 254.0 350.0
106
107
108
109
1010
0.01
0.1067
0.2033
0.3
_]
Temperature (°°°°C)
Mo
du
liE
’(M
Pa
)T
an
δ (δ (δ (δ ( °° °°C
)) ))
γγγγ-97°C
ββββ20°C
αααα120°C
PTFE
PTFE/C18P I-
Figure II-9 – Evolution of storage moduli E’ and main an secondary relaxations determined from tan δ spectra
recorded at 1 Hz on the neat PTFE and on the blend PTFE C18P I-
The thermomechanical properties can be significantly improved thanks to the high
thermal stability of ionic liquids. The storage moduli, E', obtained by DMA at different
temperatures are summarized in Table II-4.
Table II-4 – Dynamical mechanical analysis of IL-modified PTFE: Evolution of storage modulus at various
temperatures at 1Hz
Sample E’ (*106 MPa)
25°C
E’ (*106 MPa)
150°C
E’ (*106 MPa)
250°C PTFE 505 84 42
PTFE/C18P I- 405 139 82 PTFE/C18P Br- 425 138 66 PTFE/C18P PF6
- 547 193 99 PTFE/C18C18Im I- 594 136 68
PTFE/C18Py I- 522 141 57
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 94
• Influence of the organic cation
The chemical nature of the organic cation plays a key role on the thermomechanical
properties of polytetrafluoroethylene. At 25°C, the film modified with the imidazolium ion
shows at room temperature the highest value in the elastic moduli E' with an increase of 17%
compared to unfilled film and 47% compared to the film filled with phosphonium ion. Then at
150°C, whatever the organic cation used, similar moduli but always higher than one measured
on PTFE are obtained. At higher temperatures like 250°C, the highest values in moduli are
measured on the PTFE films filled with the phosphonium ionic liquid because of its thermal
stability better than the imidazolium salt one.
• Influence of the conteranion
At room temperature, the addition of phosphonium ionic liquid associated to halide
anions, causes a decrease in modulus of about 20%. On the contrary, for PTFE/C18P PF6-, a
slight increase of modulus is observed. On the other side, as the temperature increases, the
moduli values for the PTFE/IL materials increase slightly compared to the ones of neat PTFE.
This improvement is more significant when the fluorinated anion is used. In fact, the
hexafluorophosphate anion contributes to better thermomechanical behaviour of IL-modified
PTFE. This phenomenom is even more pronounced at higher temperature because of the
better thermal stability of the fluorinated anion.
These changes of storage moduli can be explained by the fact that the ionic liquids
form in the PTFE medium a separated phase which displays a strong cohesion due to the ionic
dipole-dipole interactions. According to the temperature dependence of ionic interactions, the
multiplet aggregates which exist at low temperature could exist in a larger range of
temperature, i.e. to higher temperatures, before reaching the temperature at which ionic forces
become too weak to contribute to the stiffness of the material. This hypothesis could be
similar to the temperature dependence of storage modulus of ionomers with temperature [30].
II.2.2.4.2 High deformation mechanical analysis of fluorinated films
The mechanical properties determined on the fluorinated polymer films with 1 %wt of
ionic liquids are gathered in Table II-5.
Chapter II: Interactions Polymer/Ionic Liquids
Page 95
Table II-5 – Effect of ionic liquids on the tensile properties of fluorinated films
(1 wt %) (crosshead speed : 0.004 s-1).
Sample Young’s modulus (MPa)
Strain at break (%)
PTFE 65 180 PTFE/C18P I- 170 522
PTFE/C18P Br- 120 140 PTFE/C18P PF6
- 140 70 PTFE/C18C18Im I- 110 160
PTFE/C18Py I- 90 160
• Influence of the organic cation
The mechanical performance is very dependent on the chemical nature of ionic liquid
used. If 1 wt% of pyridinium and imidazolium ions within the polymer matrix leads to a
similar mechanical behaviour with an increase of modulus of 38% and 41% respectively and a
slight decrease of 11% for the strain at break, the phosphonium ionic liquid introduced in the
fluorinated matrix give an excellent stiffness/failure properties compromise. Indeed, an
increase of 160% of the stiffness and 190% for the strain at break are achieved. Better
interactions seem to take place between the phosphonium ionic liquid functionalized with
long alkyl chains and the fluorinated matrix. In fact, as mentioned before, the confined ionic
liquid phase has a strong cohesion due to the ionic interactions which can contribute to an
increase of the stiffness of the IL-modified PTFE, i.e. the ionic liquid phase acts a reinforcing
agent. As the interfacial interactions between ionic liquid phase and PTFE medium become
better, an efficient stress transfer at the interface could contribute to a higher Young’s
modulus.
• Influence of the conteranion
The mechanical properties can be also tailored from the chemical nature of the anion.
By using the phosphonium ionic liquid, differences are observed for Young’s modulus and
the strain at break as a function of the anion used. For the three anions, a strong increase of
modulus is obtained of 160% with C18P I-, 84% with C18P Br- and 115% with C18P PF6-. On
the other hand, for the strain at break, the phosphonium ionic liquid containing iodide anion
has a plasticizing effect with a large effect (190%) whereas for the C18P Br- and C18P PF6-, the
addition of these salts reduces the fracture behaviour of the fluoride matrix with a decrease of
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 96
22% and 84% respectively. These results are consistent with the morphologies shown
previously. In the case of C18P I-, a very fine structuration is achieved in the fluoride matrix.
Whereas for C18P Br-, the poor distribution of the phosphonium ionic liquid in the polymer
explains the increase in stiffness due to the presence of aggregates and the decrease of the
strain at break. For the PTFE/C18P PF6-films, many well-dispersed ionic aggregates in the
matrix explain the increase in modulus and the higher decrease at the strain at break as
observed in ionomers [36].
II.2.2.4.3 Effect of strain rate on the fluorinated films
• Effect of ionic liquids on the crystallinity and the mechanical properties
Due to the fine dispersion of the phosphonium ionic liquid in the fluorinated film and
the excellent mechanical compromise achieved, the effects of strain rate were investigated on
the Young’s modulus and ultimate properties as well as on the morphology changes after
uniaxial stretching at a given strain. In fact, DSC measurements after deformation at different
strain rates are listed in Table II-6 and the mechanical properties are summarized in Table II-7.
Table II-6 – DSC data on fluorinated strained at different strain rate (5 and 250 mm/min)
Samples Tm (°C) Tc (°C) ∆Hm
(J/g) Xc (%)
PTFE without strain
326 310 32 39
PTFE 0.004 s-1 327 309 32 39 PTFE 0.2 s-1
329 308 40 49
PTFE C18P I-
without strain 328 307 28 34
PTFE C18P I- 0.004 s-1
328 307 42 51
PTFE C18P I- 0.2 s-1
329 308 49 60
Chapter II: Interactions Polymer/Ionic Liquids
Page 97
Table II-7 – Effect of the strain rate on tensile properties of the polymer films
Sample Tensile modulus (MPa)
Strain at break (%)
PTFE 0.004 s-1 65 180 PTFE 0.2 s-1 170 450
PTFE/C 18 P I - 0.004 s-1 170 522 PTFE/C 18 P I - 0.2 s-1 200 715
First of all, one can remember that the PTFE films are not oriented due to the
processing method, i.e. drying from a water-based solution followed by heating at 400°C. At
low strain rates, the melting enthalpy of the neat PTFE is unmodified, whereas as the
fluorinated film is strained at high strain rates (0.2 s-1), a crystallization under strain takes
place. Indeed, the high strain rate applied promotes the chain extension which leads to an
increase of crystallinity in the polymer. This increase in crystallinity ratio can be associated to
the increase of Young’s modulus from 65 at 170 MPa.
The addition of phosphonium ionic liquid in the matrix enhances the crystallization
under strain with an increase of 10% compared to the neat polymer film. This increase could
be due to a rearrangement of the ionic liquid phase in the polymer matrix. In fact, the ionic
interactions have a reversible character and the ionic liquid phase based on multiplet
aggregates could be reorganized continuously during strain. Regarding mechanical properties,
this slight increase in the modulus at high strain rate could reflect the effect of competition
between relaxation for the re-organization of ionic liquid phase and deformation.
II.2.2.4.4 Effect of ionic liquids on the morphology after deformation
For a better understanding of the material change during uniaxial tensile test, SAXS
analysis were performed on the PTFE films with and without IL after deformation reached at
different deformation rates as reported by Visser et al. for ionomers [37].
The SAXS images performed on unfilled PTFE and PTFE/C18P I- at different strain
rates, i.e. 0, 0.004, 0.2 s-1 are shown in Figure II-10.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 98
PTFE Initial State
PTFE/C 18P I--
Initial state 0.004 s-1
0.004 s-1 0.2 s-1
0.2 s-1
PTFE Initial State
PTFE/C 18P I--
Initial state
PTFE Initial State
PTFE/C 18P I--
Initial state 0.004 s-1
0.004 s-1 0.2 s-1
0.2 s-10.004 s-1
0.004 s-1 0.2 s-1
0.2 s-1
Figure II-10 – SAXS images of neat PTFE and PTFE/C18P I- under different strain rates
Without deformation, the isotropic character of the PTFE film due to the processing
method used is observed. As a strain rate (even low) is applied, a configuration with four leaf
clover is observed and in presence of ionic liquid, a different organization is noticed. Indeed,
without strain, the PTFE/C18P I- sample shows an isotropic behavior that is kept at low strain
rates. At a strain rate of 0.2 s-1, this phenomenon disappears in favor of an anisotropic
behavior oriented in six directions. As a consequence, the presence of ionic liquid as a
dispersed nanophase which is based on multiplet clusters could be used to modify the
deformation phenomena of PTFE due to the dynamic character of this phase under strain.
The transmission electron microscopy is used to evidence the morphology of the film.
Figure II-11 shows the different morphologies observed on the strain sample after uniaxial test
carried out at different strain rates.
Chapter II: Interactions Polymer/Ionic Liquids
Page 99
PTFE/C18P I-
Initial state 0.004 s-1 0.2 s-1PTFE/C18P I-
Initial state 0.004 s-1 0.2 s-1
200 nm 200 nm200 nm
Figure II-11 – TEM images of PTFE/C18P I- under different strain rates
After a low strain rate applied, TEM micrographs reveal that the IL nanodomains
which are initially organized as a co-continuous nanostructure (‘spider web’-type) collapse to
form large domains whereas for an higher strain rate, the IL co-continuous nanostructure is
kept and is oriented towards the axis of tension. This means that the relaxation of the IL
nanostructures has a relaxation time larger than the characteristic time of the deformation
process. This phenomenon could be very close to the ones observed by Visser and al [37] who
purposed for ionomers a model invoking ionic aggregate spatial rearrangement within the
polymer matrix. The authors also pointed out the role of the nature of ionic pairs on the
mechanical and deformation behaviour.
II.2.3 Conclusions
For the first time, ionic liquids were used like new building block to achieve a
structuration at nanoscale into a polymer film. We have clearly demonstrated that the effects
of the chemical nature of ionic liquid determined by a choice of the cation: pyridinium,
imidazolium versus phosphonium and the choice of the anion halide versus fluorinated play a
significant role on the structuration and the physical properties of the polymer. A suitable
combination between cation and anion leads to a nanoscale structuration of polymer with an
unprecedented flexibility and a stiffness dramatic improvement. These new building blocks
can be a new alternative in material field for various applications.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 100
Conclusions of chapter II In this section, we have shown that the introduction of a low rate (1 wt%) of ionic
liquids in the polymer matrix has a behavior similar to that of ionomers. In fact, different
structuring of ILs, dependent on the chemical nature of the cation (pyridinium, imidazolium,
phosphonium) and anion (halide, fluorinated) were highlighted. Thus co-continuous
morphologies are obtained for pyridinium and imidazolium salts while an excellent dispersion
of phosphonium ionic liquid is observed.
Then, the influence of ionic liquids on the mechanical properties has been studied both
in static and dynamic mechanical or significant increases in modulus are observed. The
perfect distribution of the phosphonium IL in polymer film also provides increases in strain at
break. However, at high strain rates, a decrease of the effect of the ionic liquid is observed in
favor of the crystallization under strain.
Chapter II: Interactions Polymer/Ionic Liquids
Page 101
References of chapter II [1] S. Maiez-Tribut, J.P. Pascault, E.R. Soulé, J. Borrajo, R.J.J. Williams, Macromolecules (2007); 40:1268–1273. [2] A. Vermogen, S. Boucard, J. Duchet, K. Masenelli-Varlot, P. Prele, R. Seguela, Macromolecules (2005); 38:9661–9669. [3] J.Lu, F. Yan, J. Texter, Progress in Polymer Science (2009); 34:431–448. [4] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, and R. Vaia, Chem. Mater (2001); 13 (9):2979–2990. [5] S. Livi, J. Duchet-Rumeau, T.-N. Pham and J.-F. Gérard, J.colloid Interf Sci (2010); 349:424–433. [6] C.Byrne and T. McNally, Macromolecular Rapid Comm. (2007); 28:780–784. [7] W. H. Awad, J. W. Gilman, M. Nyden, R. H. Harris, T. E. Sutto, J. Callahan, P. C. Trulove, H. C. DeLong and D. M. Fox, Thermochimica Acta (2004); 409:3–11. [8] Q. Guo, J. Liu, L. Chen and K. Wang, Polymer (2008); 49:1737–-1742. [9] X. Yang, F. Yi, Z. Xin and S. Zheng, Polymer (2009); 50:4089–4100. [10] Z. Xu and S. Zheng, Polymer (2007); 48:6134–6144. [11] R. Yokoyama, S. Suzuki, K. Shirai, T. Yamauchi, N. Tsubokawa and M. Tsuchimochi, European Polymer
Journal (2006); 42:3221–3229. [12] L. Priya and J. P. Jog, Journal of Applied Polymer Science (2003); 89:2036–2040. [13] S. Pavlidou and C. D. Papaspyrides, Progress in Polymer Science (2008); 33:1119–1198. [14] S. Sharma and S. Komarneni, Applied Clay Science (2009); 42:553–558. [15] K. Stoeffler, P. G. Lafleur and J. Denault, Polymer Engineering & Science (2008); 48:1449–1466. [16] W. S. Wang, H. S. Chen, Y. W. Wu, T. Y. Tsai and Y. W. Chen-Yang, Polymer (2008); 49:4826–4836. [17] H.-K. Fu, C.-F. Huang, J.-M. Huang and F.-C. Chang, Polymer (2008); 49:1305–1311. [18] L. Li, B. Li, M. A. Hood and C. Y. Li, Polymer (2009); 50:953–965. [19] Z. Spitalsky, D. Tasis, K. Papagelis and C. Galiotis, Progress in Polymer Science (2010); 35:357–401. [20] S. Bose, R. A. Khare and P. Moldenaers, Polymer (2010); 51:975–993. [21] H. Vallette, L. Ferron, G. Coquerel, A.-C. Gaumont and J.-C. Plaquevent, Tetrahedron Letters (2004); 45:1617–1619. [22] A. Safavi and S. Zeinali, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2010); 362:121–126. [23] L. Xu, G. Ou and Y. Yuan, Journal of Organometallic Chemistry (2008); 693:3000–3006. [24] M. Rahman and C. S. Brazel, Polymer Degradation and Stability (2006); 91:3371–3382. [25] K. Park, J. U. Ha and M. Xanthos, Polymer Engineering & Science (2010); 50:1105–1110. [26] A. Vazquez, M. López, G. Kortaberria, L. Martín and I. Mondragon, Applied Clay Science (2008); 41:24–36. [27] H. He, J. Duchet, J. Galy, J.F. Gerard, J.colloid Interf Sci (2006); 295:202. [28] W. Xie, R. Xie, W.-P. Pan, D. Hunter, B. Koene, L.-S. Tan and R. Vaia, Chem Mater (2002); 14:4837–4845. [29] C.G Bazuin, A. Eisenberg, Ind. Eng. Chem. Prod. Res. Dev (1981); 16:41. [30] I. Capek, Adv. Coll. Interface Sci. (2005); 118:73. [31] A.R. Khokhlov, E.F. Dormidontova, Phys. Uspekhi (2005); 118:73. [32] I.A. Nyrkova, A.R. Khokhlov, Y.Y. Kramarenko, Polym. Sci. USSR (1990); 32:852. [33] A. Eisenberg, B. Hird, R.B. Moore, Macromolecules (1990); 23:4098. [34] I.A. Nyrkova, A.R. Khokhlov, M. Doi, Macromolecules (1993); 26:3601. [35] McCrum NG. J Polym Sci (1959);34:355–69. [36] R.F. Storey, D.W. Baugh, Polymer (2000); 41 (9):3205. [37] S.A. Visser, S.L. Cooper, Polymer (1992), 33:4705–4710.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 103
Chapter III IONIC LIQUIDS AS NEWS INTERCALATING AGENTS FOR
LAYERED SILICATES Since the 80s and the early work done by Toyota on polyamide-montmorillonite
nanocomposites, the field of layered silicates nanocomposites is booming. In fact, with these
new materials, we seek to improve the thermal, mechanical or barrier properties with a very
low ratio of inorganic filler. The key parameter is the control of the distribution of individual
sheets, described as the state of exfoliation. Nevertheless, the lack of compatibility between
the hydrophilic clay and mostly hydrophobic polymers makes it difficult to obtain this state of
exfoliation. To circumvent this difficulty and improve the compatibility between clay and
polymer, the use of organic species denoted intercalating agents or surfactants, particularly
ammonium salts is necessary to reduce the surface energy and increase interlayer distances of
the layered silicates in order to promote the separation of layers to obtain an exfoliated
dispersion state more conducive to improving the final properties of nanocomposites.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 104
Pages
III.1 A comparative study on ionic liquids used as surfactants: Effect on thermal and mechanical properties of high-density polyethylene nanocomposites ...................................... 105
III.1.1 Introduction ............................................................................................................................ 105 III.1.2 Experimental .......................................................................................................................... 107
III.1.2.1 Materials ................................................................................................................................. 107 III.1.2.2 Synthesis of phosphonium and imidazolium salts .................................................................. 107
III.1.2.2.1 Synthesis of octadecyltriphenylphosphonium salt ...................................................... 107 III.1.2.2.2 Synthesis of N-octadecyl-N'-octadecylimidazolium salt ............................................. 108
III.1.2.3 Organic modification of montmorillonite ............................................................................... 108 III.1.2.4 Processing and characterization of the HDPE/clay nanocomposites ...................................... 110
III.1.3 Results and discussion ............................................................................................................ 111 III.1.3.1 Characterization of modified montmorillonites ...................................................................... 111
III.1.3.1.1 Identification of interactions and effect of the washing .............................................. 111 III.1.3.2 Thermal stability of modified montmorillonites ..................................................................... 114 III.1.3.3 Structural analysis by WAXD ................................................................................................. 116 III.1.3.4 Surface energies of modified montmorillonites ...................................................................... 118 III.1.3.5 Influence of ionic liquid content ............................................................................................. 118
III.1.4 HDPE/clay nanocomposites ................................................................................................... 120 III.1.4.1 Thermal properties of nanocomposites ................................................................................... 120 III.1.4.2 Mechanical properties of nanocomposites .............................................................................. 121 III.1.4.3 Morphology of nanocomposites .............................................................................................. 122
III.1.5 Conclusions ............................................................................................................................ 123
III.2 Supercritical CO2-Ionic Liquid Mixtures For Modification of Organoclays ............. 124 III.2.1 Introduction ............................................................................................................................ 124 III.2.2 Experimental .......................................................................................................................... 125
III.2.2.1 Organic modification .............................................................................................................. 125 III.2.3 Results and discussion ............................................................................................................ 127
III.2.3.1 Effect of supercritical carbon dioxide as a exchange solvent on the thermal degradation of the modified MMT ..................................................................................................................................... 127
III.2.3.1.1 Thermal stability of imidazolium-modified montmorillonite ..................................... 127 III.2.3.1.2 Thermal stability of phosphonium-modified montmorillonite .................................... 131
III.2.3.2 Structural analysis ................................................................................................................... 133 III.2.3.2.1 Imidazolium modified montmorillonite ...................................................................... 133 III.2.3.2.2 Phosphonium modified montmorillonite ..................................................................... 134
III.2.3.3 Surface energies ...................................................................................................................... 136 III.2.4 Conclusions ............................................................................................................................ 136
Conclusions of chapter III ............................................................................................................ 137
References of chapter III .............................................................................................................. 138
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 105
III.1 A comparative study on ionic liquids used as surfactants: Effect on thermal and mechanical properties of high-density polyethylene nanocomposites
Dialkyl imidazolium and alkyl phosphonium salts were synthesized to be used as new
surfactants for cationic exchange of layered silicates, such as montmorillonite (MMT). The
synthesized phosphonium (PMMT) or imidazolium ion (I-MMT)-modified montmorillonites
display a dramatically improved thermal degradation with respect to commonly used
quaternary ammonium salts. This thermal degradation window can still be shifted toward
higher temperatures after washing of modified clays. Two kinds of organic species can be
identified onto clay: physically adsorbed species versus chemically adsorbed species. To
Evidence the impact of these thermally resistant ionic liquids, the modified montmorillonites
were introduced in a great commodity polymer, i.e., high-density polyethylene. Thermoplastic
nanocomposites with a very low amount of nanofillers were processed in melt by twin screw
extrusion. If the thermal stability of polyethylene is slightly increased with only 2 wt.% of
thermostable made clays, the stiffness–toughness compromise is well improved since a strong
increase in modulus is achieved with both thermostable clays without loss of fracture
properties. But these mechanical performances are mainly obtained with unwashed
thermostable clays because the physically adsorbed organic species onto clay surfaces behave
like a compatibilizer that helps both the dispersion into the PE matrix and improves the
clay/matrix interface quality.
III.1.1 Introduction Although the clays have been recognized for a long time, the attention of academic
and industrial researchers was recently focused on organically modified clays as nanoscale-
reinforcing agents for polymer materials [1–3]. Indeed, the insertion of these lamellar fillers in
polymers can have significant effects not only on the mechanical [4] and barrier performances
[5,6] but also on ablation and flammability resistances [7] due to nanometric dimensions and
high aspect ratios of layered silicates and also due to synergism between polymer and
inorganic nanofillers. Among layered silicates, montmorillonite (MMT) is commonly used
[8]. Nevertheless, to ensure a good compatibility between montmorillonite and polymer
during the processing of nanocomposites, a surface modification of pristine MMT is required.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 106
By exchanging sodium or calcium cations for organic cations, the surface energy of MMT
decreases and the basal spacing expands [9]. For such a purpose, cationic exchange using
alkylammonium salts is very often used [10,11].
Melt intercalation is by far the most promising method and is industrially preferred for
processing thermoplastic polymer (TP)-based nanocomposites as it can be performed without
use of solvents and by considering conventional tools for processing, i.e., the extrusion
process. However, the process involves working at high temperatures as melting of
conventional TPs requires temperatures above 190°C. Although the commonly used
ammonium salts have been gaining significant success in the processing of polymer/MMT
nanocomposites, a common shortcoming is their low thermal stability. The thermal
degradation of MMT modified by long-chain alkyl quaternary ammonium ions begins from
180°C as shown by TGA studies carried out by Xie et al. [12]. Indeed, the Hoffmann
degradation during processing described in the literature [13,14] could initiate/catalyze
polymer degradation and could affect the physical and mechanical properties of final
materials. To increase the thermal stability of organically modified clays, the use of more
thermally stable compounds, such as ionic liquids based on phosphonium and imidazolium
salts, can offer a new alternative to the ammonium salts [15,16]. These salts may be
considered as ionic liquids because their melting point is below 100°C, and their glass
transition temperature is around 10°C for phosphonium salt and 42°C for imidazolium salt.
Intensive thermal studies on imidazolium and phosphonium salts have shown a better
thermal stability than the alkyl ammonium cations [15,17,18]. Despite many benefits, few
studies using such cations for layered silicate intercalation have been reported in the literature
maybe because of the higher price of these surfactants compared to ammonium salts [19–21].
Moreover, commercially available thermally stable surfactants only incorporate short
aliphatic chains (up to C14).
In this work, efforts have been made to synthesize ionic liquids with long alkyl chains
based on imidazolium (two chains in C18) and on phosphonium salts with three benzyl
groups and only one long aliphatic chain (1 chain in C18). The long alkyl chains cause
expansion of the distance between the layers, and the aromatic groups help to generate a
better intercalation of the clay platelets because aromatic groups can be trapped in the
hexagonal cavities of layers. The thermal stability of the obtained organoclays was compared
with that of ammonium-modified clays. Each ammonium cation shows exactly the same
substituents as the imidazolium cation, either two linear octadecyl chains or an aromatic
group and an aliphatic chain for the phosphonium cation. Then, a high-density polyethylene
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 107
(HDPE) was selected to be mixed with lamellar silicates modified with highly thermally
stable ionic liquids. Finally, the morphology and the thermal and mechanical properties of
nanocomposites processed with the thermally stable ionic liquids were evaluated and
compared with ammonium-modified clay-based nanocomposites [22,23].
III.1.2 Experimental
III.1.2.1 Materials
A sodic montmorillonite, denoted Nanofil 757 (MMT), i.e., an aluminosilicate with
intercalated sodium was chosen as pristine clay and was provided by Süd Chemie (Germany).
The Nanofil 757 has a cation exchange capacity of 95 meq/100 g and is described by the
following formula Na0,65[Al,Fe]4Si8O20(OH)4. Two commercial organically modified
montmorillonites (Nanofil 15 and Nanofil 919) were purchased from Süd Chemie to obtain
reference organophilic clays. Both commercial organoclays are modified with quaternary
ammonium ion carriers of either tallow chains (Nanofil 15) or aromatic groups and tallow
chains (Nanofil 919). Tallow chains have the following composition: 65% C18, 30% C16,
and 5% C14. All chemicals necessary for the synthesis of ionic liquids, i.e.,
triphenylphosphine (95%), imidazole (99.5%), iodooctadecyl (95%), and all the solvents
(toluene, sodium methanoate, pentane and acetonitrile) were supplied from Aldrich and used
as received.
The polyethylene used in this study, called HDPE, is a high-density polyethylene from
Basell, with the trade name Hostalen GF 4750, showing a melt flow index of 0.4.
III.1.2.2 Synthesis of phosphonium and imidazolium salts
III.1.2.2.1 Synthesis of octadecyltriphenylphosphonium salt
In a 100-mL flask was, placed under a positive nitrogen pressure, 1 eq of
triphenylphosphine (5 g) and 1 eq octadecyl iodide (7.3 g). The stirred suspensions were
allowed to react for 24 h at 120°C in toluene (20 mL), and a yellow precipitate was formed.
The reaction mixture was then filtered and washed repeatedly with pentane. Most of the
solvent was removed under vacuum. The synthesis of salts was confirmed by 13C NMR
spectroscopy collected on a Bruker AC 250 (250 MHz) spectrometer. The assignment of 13C
NMR resonance peaks is reported below. 13C NMR (CDCl3): d 14.00 (CH3); 22.67 (CH2Me); 23.2; 29.37–29.66; 30.24; 31.85
(P–CH2); 118.45; 130.43; 133.70; 135.15 (P–C).
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 108
III.1.2.2.2 Synthesis of N-octadecyl-N'-octadecylimidazolium salt
A solution of sodium methoxide was prepared from 1 eq of sodium (0.465 g) using
dry, freshly distilled methyl alcohol (10 mL) in a sealed septum, 100 mL round-bottom, three-
necked flask equipped with a condenser, under nitrogen atmosphere and magnetic stirring.
Imidazole (1 eq, 1.37 g) diluted in acetonitrile (10 mL) was then added into the stirred
mixture of sodium methoxide previously cooled at room temperature. After 15 min, a white
precipitate was formed. The suspension was then concentrated under reduced pressure for 1 h.
The dried white powder was dissolved in acetonitrile, and a solution of octadecyl iodide (1 eq,
7.70 g) diluted in acetonitrile (10 mL) was then added under an inert atmosphere of nitrogen
at room temperature. The mixture was stirred for 1 h and then heated under reflux at 85 C for
about 24 h. A solution of octadecyl iodide (1 eq, 7.70 g) diluted in acetonitrile (10 mL) was
added to the mixture at room temperature. The stirred suspension was heated under reflux at
100 C for about 24 h leaving a brownish viscous oil in each case. After cooling to room
temperature, the solvent was removed by evaporation under vacuum, and the orange-colored
or beige solid was filtered, washed repeatedly with pentane, and dried. Purification of the
resulting imidazolium salts was accomplished by crystallization from ethyl
acetate/acetonitrile: 75/25 mixture. The assignment of 13C NMR resonance peaks is the
evidence of the success of the ionic liquid synthesis. 13C NMR (CDCl3): d 14.10 (2CH3); 22.67 (2CH2Me); 26.23; 28.97; 29.35–29.69;
30.24; 31.91 (CH2); 50.10; (CH2N); 50.32 (CH2N–); 121.69; 122.48 (CN); 136.88 (N–CN).
III.1.2.3 Organic modification of montmorillonite
The montmorillonite (2 g, 1.9 meq) was dispersed in 400 mL of deionized water. The
amount of surfactant added was about 2 CEC, based on the cation exchange capacity (CEC =
95 meq/100 g) of the MMT used [9]. This dispersion was mixed and stirred vigorously at
80°C for 6 h, followed by filtration and continuous washing at 80°C with deionized water
until no iodide ions were detected using an aqueous silver nitrate (AgNO3) solution. The
solvent was removed by evaporation under vacuum. The modified montmorillonite was then
dried for 12 h, at a suitable temperature (not greater than 80°C). The imidazolium,
phosphonium, and the quaternary ammonium ions used for the exchange reactions are
presented in Table III-1. The following abbreviations were used to design the different
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 109
montmorillonites: MMT-Na+ means the pristine montmorillonite. A phosphonium
montmorillonite denoted MMT-P was obtained when octadecyltriphenylphosphonium iodide
was used like surfactant. An imidazolium montmorillonite denoted MMT-I was obtained
when the N-octadecyl-N'-octadecylimidazolium iodide was used as an intercalation agent.
Both commercial montmorillonites modified with ammonium ions are MMT-DMDT for the
montmorillonite carrying a dimethyl ditallow quaternary ammonium as cation and MMT-
DMBT for the montmorillonite modified by a dimethyl (benzylmethyl) tallow quaternary
ammonium.
Table III-1 – Designation of pristine, commercial and synthesized ionic liquid modified montmorillonite
(MMT)
Designation Intercalant Trade name MMT-Na+ - Nanofil 757
MMT-DMDT
Nanofil 15
MMT-DMBT
Nanofil 919
MMT-I
MMT-P
N+
Tallow
CH3
TallowH3C
N+
H3C
H3C Tallow
P C18H37
I
N
N
C18H37
C18H37
I
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 110
III.1.2.4 Processing and characterization of the HDPE/clay nanocomposites
Nanocomposites were obtained by melt intercalation of modified montmorillonite into
a high-density polyethylene (2% by weight) using a twin screw DSM microcompounder. The
mixture was sheared for about 3 min with a 100 rpm speed at 190 C and injected in a 10-cm3
mold at 30 C to obtain dumbbell-shaped specimens. Different nanocomposite samples were
prepared by varying the surface treatment used to modify the montmorillonite and by
considering the nonwashed, exchanged MMT.
Thermogravimetric analyses (TGA) of organically modified clay and composites were
performed on a Q500 thermogravimetric analyzer (TA instruments). The samples were heated
from 30 to 800°C at a rate of 20 K min1 under nitrogen flow.
Surface energy of modified clays was determined with the sessile drop method on a
GBX goniometer. From contact angle measurements taken using water and diiodomethane as
test liquids on pressed modified clay disks, polar, and dispersive components of surface
energy were determined using the Owens–Wendt theory.
Bruker D8 Advance X-ray diffractometer at the H. Longchambon diffractometry
center. A bent quartz monochromator was used to select the Cu Ka1 radiation (k = 0.15406
nm) and run under operating conditions of 45 mA and 33 kV in Bragg–Brentano geometry.
The angle range scanned is 1–102h for the modified clays and 1–302h for the nanocomposite
materials.
Uniaxial tensile measurements were taken using a MTS 2/M electromechanical testing
system at 22 ± 1 C and 50 ± 5% relative humidity. Tensile tests were performed with a speed
of 10 mm min1.
The transmission electron microscopy (TEM) was carried out at the Center of
Microstructures (University of Lyon) on a Philips CM 120 field emission scanning electron
microscope with an accelerating voltage of 80 kV. The samples were cut using an
ultramicrotome equipped with a diamond knife, to obtain 60-nm-thick ultrathin sections.
Then, the sections were set on copper grids.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 111
III.1.3 Results and discussion
III.1.3.1 Characterization of modified montmorillonites
III.1.3.1.1 Identification of interactions and effect of the washing
According to the literature [9,12,17,24], as a layered clay is modified with an organic
cation, two kinds of interactions between organic cations and inorganic clay can take place:
(i) Van der Waals bonds as the organic species are physically adsorbed on the clay surface.
(ii) Ionic bonds as the species are intercalated in the montmorillonite galleries.
The effect of washing allows identification of the interaction intensity, chemical
versus physical, linking the organic species to the layered silicate surface. After checking the
solubility of ionic liquids in different solvents, methyl alcohol was chosen as solvent for
washing. The effect of the washing is clearly shown on TGA analysis reported in Figure III-1.
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0.2
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0.4
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riv.
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igh
t (%
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)
0 200 400 600 800
Temperature (°C)
� MMT-I aw1� MMT-I bw� MMT-I aw2
Universal V4.2E TA Instruments Figure III-1 – Derivative of TGA curves (DTG) of the MMT-I bw (before washing)
and MMT-I aw (after washing) (heating rate : 20 K.min-1). Three peaks of degradation are observed on the derivative curve of the weight loss
(DTG curve) of imidazolium-modified montmorillonite, before washing (called MMT-I bw).
After two successive washings (called MMT-I aw 2) with methanol, the first degradation peak
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 112
that corresponds to the species physically adsorbed on the montmorillonite surface almost
disappears for the benefit of the second peak. Unlike the first peak, the second increases
significantly. This can be explained by the gradual removal of salt excess on the clay surface
that creates a larger aperture of clay galleries allowing a higher quantity of the salt to be
intercalated between clay layers. Figure III-2 reports DTG curves obtained from the thermal
analysis realized on the phosphonium-treated montmorillonite before (bw) and after washing
(aw).
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0.8
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1.2
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riv.
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igh
t (%
/°C
)
0 200 400 600 800
Temperature (°C)
� MMT-P bw� MMT-P aw2� MMT-P aw1
Universal V4.2E TA Instruments Figure III-2 – Derivative of TGA curves (DTG) of the MMT-P bw (before
washing) and MMT-P aw (after washing) (heating rate: 20 K.min-1). The first weight loss still corresponds to a partial physisorption at the edges (bearing
polar SiOH groups) or on the external surface of the platelets since the peak decreases after
washing. The fact that it does not completely disappear means that a part of the ionic liquid is
well intercalated but in a peripheral position with respect to the clay gallery as reported by
Davis et al. [25] or is physisorbed from p–SiOH interactions at the edges of the lamellar
silicates. Such portion of surfactant cannot be washed away easily (since it underwent cationic
exchange), but it is not thermally stabilized by the presence of the inorganic silicate platelets
in a confined position. As a consequence, it degrades at the same temperature as the
physisorbed surfactant. On the other hand, the degradation which is evidenced at about 500°C
corresponds to the well-intercalated species between layers.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 113
The same observation is made on the MMT-DMBT and the MMT-DMDT. A single
washing with methyl alcohol is enough to eliminate the species physically adsorbed. Table
III-2 summarizes the relative mass losses of the physically adsorbed species and the
intercalated species for the modified montmorillonites before and after washing.
Table III-2 – Relative mass loss of physically adsorbed and intercalated species measured by TGA on the modified montmorillonites
Sample % physically adsorbed species
% intercalated species
MMT-P bw* 46 12 MMT-P aw* 9 18 MMT-I bw 31 18 MMT-I aw - 23
MMT-DMDT bw 2 25 MMT-DMDT aw - 25 MMT-DMBT bw 16 20 MMT-DMBT aw - 18
The weight percent of physisorbed species varies before washing and cannot be
measured after washing since it is negligible. On the other hand, the intercalated cation
amount is about 20 wt.% for all the organically modified clays.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 114
III.1.3.2 Thermal stability of modified montmorillonites
Thermogravimetric analysis (TGA) may complete modified clay characterizations by
investigating the degradation mechanisms and the effects of functionalization and washing on
the thermal stability. We have already reported that the first peak corresponding to the first
weight loss on the derivative curve of the weight loss as a function of temperature typically
indicates the presence of organics that must be simply physisorbed [9–24]. Such organic
fraction either did not undergo the cation exchange process or stayed unconfined. The second
weight loss corresponds to organics intercalated into clay galleries. Indeed, organics inside
clay galleries display higher temperatures of degradation. In this study, the montmorillonites
modified with distinct cations (ammonium versus phosphonium or ammonium versus
imidazolium) were compared.
First, the montmorillonite modified with the dimethyl (benzymethyl) tallow
quaternary ammonium was compared to one treated with the phosphonium salt since both
salts display alkyl chains and benzyl rings as ligands. Figure III-3 summarizes the data
extracted from these thermogravimetric (TGA) curves and their derivative (DTG).
0 100 200 300 400 500 600 700 800
60
65
70
75
80
85
90
95
100
105
% W
eig
ht
Temperature (°C)
MMT-DMBT
0 100 200 300 400 500 600 700 800
-0,025
-0,020
-0,015
-0,010
-0,005
0,000
0,005
0,010
Deri
v.W
eig
ht
(%°C
)
Temperature (°C)
220°C
300°C
390°C
430°C
MMT-DMBT
0 100 200 300 400 500 600 700 800
40
50
60
70
80
90
100
% W
eig
ht
Temperature (°C)
MMT-P
0 100 200 300 400 500 600 700 800
-0,08
-0,06
-0,04
-0,02
0,00
0,02
0,04
Deri
v.W
eig
ht
(%°C
)
Temperature (°C)
340°C
510°C
MMT-P
2b
Figure III-3 – TGA and DTG curves of the MMT-DMBT (a) and
MMT-P (b) (before washing) (heating rate: 20K.min-1).
On the DTG curves of MMT-DMBT (Figure III-3a), the first peak corresponding to the
degradation of the species physically adsorbed on the surface of clay starts at 220°C, whereas
the phosphonium-modifiedmontmorillonite is still thermally stable at this temperature.
Indeed, in Figure III-3b, the first peak of degradation of the physisorbed species is at 340°C, a
a)
b)
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 115
temperature corresponding to the evaporation one of neat ionic liquid. In this case, a dramatic
difference of 120°C is observed. For the intercalated species in the montmorillonite galleries,
the same trend is observed. Whereas the degradation takes place at about 500°C for MMT-P,
the degradation of intercalated species is extended between 300 and 400°C for MMT-DMBT.
These results show clearly the better intrinsic thermal stability of phosphonium salt compared
with the ammonium salt [12], and the confinement has a similar effect on both salts, i.e., the
temperature shift from adsorbed state to confined state.
Second, the imidazolium montmorillonite and the montmorillonite bearing a dimethyl
ditallow quaternary ammonium as cation was also compared, as both have only two similar
long alkyl chains. Figure III-4 reports the TGA curves and their derivatives.
0 100 200 300 400 500 600 700 800
60
70
80
90
100
% W
eig
ht
Temperature (°C)
MMT-DMDT
0 100 200 300 400 500 600 700 800
-0,10
-0,08
-0,06
-0,04
-0,02
0,00D
eriv.W
eig
ht
(%°C
)
Temperature (°C)
MMT-DMDT
270°C
340°C
440°C
0 100 200 300 400 500 600 700 800
40
50
60
70
80
90
100
% W
eig
ht
Temperature (°C)
MMT-I
0 100 200 300 400 500 600 700 800
-0,10
-0,08
-0,06
-0,04
-0,02
0,00
Deri
v.W
eig
ht
(%°C
)
Temperature (°C)
MMT-I
320°C
420°C
490°C
Figure III-4 – TGA and DTG curves of the MMT-DMBT (a) and MMT-P
(b) (before washing) (heating rate: 20K.min-1). By considering the thermal analysis carried out on MMT-DMDT and on MMT-I, the
difference is less significant. In Figure III-4a, a shoulder is observed at 270°C, followed by the
degradation of species intercalated at 340 and then at 440°C. In the case of MMT-I (Figure
III-4b), a first clear peak attributed to physically adsorbed species at 320°C is observed,
followed by the degradation of the species intercalated at 420 and 490°C. The increase in the
degradation temperature is much lower between the MMT-I and the MMTDMDT (50°C for
physisorbed species and 80°C for intercalated ones) because both alkyl chains as ligands
display a lower intrinsic thermal stability than benzyl groups. However, keep in mind that the
imidazolium or phosphonium-modified montmorillonites have a much better thermal stability
than the ammonium-treated ones.
a)
b)
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 116
III.1.3.3 Structural analysis by WAXD
The cationic exchange process is clearly detectable by X-ray diffraction as shown in
Figure III-5.
0 2 4 6 8 10
0
500
1000
1500
2000
2500
3000
3500
4000
Inte
nsity (
a.u
)
2θ
MMT-DMDT
d001
= 3.0 nm
0 2 4 6 8 10
0
200
400
600
800
1000
1200
Inte
nsity (
a.u
)
2θ
MMT-Id
001 = 3.7 nm
0 2 4 6 8 10
0
500
1000
1500
2000
2500
3000
3500
4000
Inte
nsity (
a.u
)
2θ
MMT-DMBT
d001
= 1.9 nm
0 2 4 6 8 10
400
450
500
550
600
650
700
750
800
850
Inte
nsity (
a.u
)
2θ
MMT-P
d001
= 4.2 nm
Figure III-5 – X-Ray diffraction spectra of ionic liquid modified MMT before washing:
(a) MMT-DMDT; (b) MMT-I; (c) MMT-DMBT; (d) MMT-P.
Before treatment, the basal spacing of the sodic montmorillonite is 1.2 nm, which
corresponds to the d-spacing of MMT-Na reported in the literature [9]. After organic
treatment in water by the imidazolium and phosphonium salts, the MMT-P displays a (0 0 1)
diffraction peak at 2.12h, corresponding to an interlayer distance of 4.2 nm. This value could
be explained by the swelling of layered silicates due to the steric volume occupied by the
three ring functions and the alkyl chain. For the MMT-I, the diffraction peak situated at 2.42h
is significant at a distance of 3.7 nm, a distance similar to one characteristic of a paraffinic
conformation with trans–trans positions of the alkyl chain. On the other hand, for the
montmorillonite modified with a ditallow quaternary ammonium, i.e., MMT-DMDT, the
lower intercalation distance of 3.0 nm is significant of a paraffinic chain tilted on the clay
surface. For the montmorillonite functionalized with a dimethyl benzyl tallow quaternary
ammonium, MMT-DMBT, the intercalation distance only of 1.9 nm is reduced by nearly half
because the organic chains adopt a pseudo trilayer conformation [26]. For the MMT-P and the
MMT-I, the spectra show intense, thin, and regular diffraction peaks that suggest a long-range
order.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 117
The effect of washing reported in Figure III-6 was studied by X-ray diffraction on
different montmorillonites.
0 2 4 6 8 10
0
500
1000
1500
2000
2500
3000
3500
4000
Inte
nsity (
a.u
)
2θ
MMT-DMDT awd
001 = 2.4 nm
0 2 4 6 8 10
0
2000
4000
6000
8000
Inte
nsity (
a.u
)
2θ
MMT-I aw
d001
= 2.7 nm
0 2 4 6 8 10
0
500
1000
1500
2000
2500
3000
3500
4000
Inte
nsity
(a.u
)
2θ
MMT-DMBT aw
d001
= 1.9 nm
0 2 4 6 8 10
0
500
1000
1500
2000
2500
3000
Inte
nsity
(a.u
)
2θ
MMT-P aw
d001
= 2.1 nm
Figure III-6 – X-Ray diffraction spectra of ionic liquid modified MMT after washing:
(a) MMT-DMDT; (b) MMT-I; (c) MMT-DMBT; (d) MMT-P.
The washing step leads to a shift of diffraction peaks toward higher angles. The
intercalation distance decreases from 3.0 to 2.4 nm for the MMT-DMDT, from 3.7 to 2.7 nm
for the MMT-I, and from 4.2 to 2.1 nm for the MMT-P after washing with methyl alcohol.
Only the MMT-DMBT does not have any change in gallery height after washing. Washing
with methyl alcohol causes a reorganization of chains that explains the decrease in distances.
But in all cases, washing performed on MMT-P, MMT-I and MMT-DMDT does not induce
any decrease in the intercalation distance below 2.0 nm. This observation is corroborated by
the no change in distance obtained for the MMT-DMBT. As a result, the cationic exchange
leads to a part of organic cations that is really intercalated between the sheets inducing about a
2.0-nm gallery height, while another part of organic cations causes the clay swelling only due
to steric volume of the organic ligands.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 118
III.1.3.4 Surface energies of modified montmorillonites
The contact angles and surface energy determined by the sessile drop method on
pressed powder are collected in Table III-3.
Table III-3 – Determination of polar and dispersive components of the surface energy on pristine and on exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders) Montmorillonite Θwater (°) ΘCH2I2 (°) γ polar
(mN.m-1) γ dispersive (mN.m-1)
γ total (mN.m-1)
MMT-Na+ 22.9 ±0.9 33.6 ±0.8 30 43 73 MMT-DMDT 72.8 ±0.3 55.4 ±0.6 9 31 40 MMT-DMBT 61.0 ±0.5 48.8 ±0.7 14 35 49
MMT-P 88.9 ±0.1 49.4 ±0.6 2 35 37 MMT-I 92.8 ±0.1 55.5 ±0.6 1 31 32
Polyethylene [28]
102.0 53.0 0.05 34.10 34.15
Both ionic liquids based on phosphonium and imidazolium salts make the
montmorillonite more hydrophobic with a surface energy similar to the surface energy of a
polyolefin [27]. The polar components are very low which evidenced that the hydroxyl groups
are well covered by the organic chains. The steric hindrance of imidazolium and
phosphonium ionic liquids causes an efficient screen of the hydrophilic surface of lamellar
silicates. A stronger hydrophobic character can be obtained with the synthesized ionic liquids
compared to the ammonium cations. Hence, an efficient compatibility must be generated
between ionic liquid-modified nanolayers and polyethylene matrix.
III.1.3.5 Influence of ionic liquid content
The washing effect shows that such a large amount of cationic liquid corresponding to
2 CEC used for cationic exchange is useless for clay treatment. As a result, the ionic liquid
amount can be reduced during cationic exchange. Instead of adding 2 CEC of surfactants,
only 0.5 CEC was added to perform the cationic exchange. The TGA analysis performed
before the washing step on phosphonium-modified montmorillonite with two different
surfactant amounts (2 CEC versus 0.5 CEC) is reported in Figure III-7.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 119
0 100 200 300 400 500 600 700 800
0
10
20
30
40
50
60
70
80
90
100
% w
eig
ht
Temperature (°C)
MMT-P bw2CEC
MMT-P bw 0,5CEC
Figure III-7 – Effect of the amount introduced of ionic liquid (2 CEC versus 0.5 CEC) used for cationic exchange on thermal stability measured by TGA
before the washing step (heating rate: 20K.min-1)
The similar thermal degradation for both exchange treatments highlights that a lower
amount of surfactant is sufficient to efficiently modify the clay and gives the clay a thermal
stability up to temperatures higher than 400 C. The X-ray diffraction is witnessed that the
amount of ionic liquid can be reduced since the same spectrum is obtained with a lower
surfactant content. The modified clay is characterized by the same intercalation distance as
shown in Figure III-8.
0 2 4 6 8 10
0
200
400
600
800
1000
1200
1400
1600
Inte
nsity (
a.u
)
2θ
MMT-P bw 2 CEC
MMT-P bw0.5 CEC
Figure III-8 – Effect of the amount introduced of ionic liquid (2 CEC versus 0.5 CEC) used for
cationic exchange on intercalation distance measured by WAXD before the washing step
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 120
III.1.4 HDPE/clay nanocomposites
III.1.4.1 Thermal properties of nanocomposites
The thermal behavior of the composites containing 2 wt.% of modified clay was
characterized by thermogravimetric analysis in order to study the effect of imidazolium and
phosphonium-treated montmorillonites on the thermal stability of the nanocomposites. The
thermogravimetric results carried out on polyethylene alone and the polyethylene filled with
modified montmorillonites are shown in Figure III-9.
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Universal V4.2E TA Instruments Figure III-9 – TGA curves of nanocomposites based on polyethylene matrix filled with 2%wt. of sodic montmorillonite (PE-MMT) and of phosphonium (PE-(MMT-P bw)) or imidazolium (PE-(MMT-I bw)) modified montmorillonite (heating rate : 20 K.min-1).
The improvement in thermal stability is not tremendous. The chosen polyethylene is
already thermally very stable up to about 450°C, which corresponds to the temperature from
which starts the modified montmorillonite degradation. Moreover, the samples are analyzed
under inert gas. The addition of only 2 wt.% of the unwashed imidazolium-modified
montmorillonite in the polyethylene matrix does not improve the matrix thermal degradation.
With 2 wt.% of phosphonium-modified montmorillonite, the thermal degradation of the
polyethylene matrix can be only improved by 10°C. By introducing two times more
phosphonium-modified clay, i.e., 5 wt.%, delay in the thermal degradation is also doubled as
shown in Figure III-10.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 121
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Universal V4.2E TA Instruments Figure III-10 – TGA curves of nanocomposites based on polyethylene matrix filled with 2 and 5
wt% of phosphonium modified montmorillonite (PE-(MMT-P)) (heating rate : 20 K.min-1).
III.1.4.2 Mechanical properties of nanocomposites
Mechanical properties of the PE/clays nanocomposites containing 2 wt.% are detailed
in Table III-4.
Table III-4 – Effect of clay washing on tensile properties of the 2 CEC ionic liquid modified montmorillonite-high density polyethylene nanocomposites (2wt%) crosshead speed: 10mm.min-1
*bw: before washing aw : after washing
A strong increase in modulus is obtained when the polyethylene was prepared with
phosphonium or imidazolium-modified montmorillonite: an increase of 40% for the MMT-I
and 50% for the MMT-P. However, this stiffness is reduced when the nanocomposites are
prepared with washed clays or with 0.5 CEC-modified clay. In both cases, only an increase of
20% is observed for both organically modified clays. This decrease in modulus could be
associated with the removal of physically adsorbed species on the edges of the silicate layers
Sample Tensile modulus (MPa)
Strain at break (%)
Stress at break (Mpa)
PE 740 18 84 PE-MMT 720 19 71
PE-(MMT-P bw) 1100 17 97 PE-(MMT-P aw) 887 18 88 PE-(MMT-I bw) 1041 17 96 PE-(MMT-I aw) 883 19 87
PE-(MMT-I 0.5 CEC) 835 18 74
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 122
by the successive washings or by the low amount of surfactant present. Indeed, the excess of
salts may help the compatibilization between the polyethylene and the modified
montmorillonites. About the strain at break, the modified clays do not reduce the fracture
behavior of the matrix since the values are found to be similar.
III.1.4.3 Morphology of nanocomposites
Transmission electron microscopy micrographs carried out on the nanocomposites
processed with 2 wt.% of phosphonium or imidazolium-modified clays and reported in Figure
III-11 show a varying dispersion state following whether the introduced clays are washed or
not.
a)
d)c)
b)a)
d)c)
b)
Figure III-11 – Influence of washing clay on TEM micrographs performed on 2
%wt ionic liquid treated montmorillonite-polyethylene nanocomposites: (a) PE-(I-MMT aw), (b) PE-(I-MMT bw), (c) PE-(P-MMT aw), (d) PE-(P-MMT bw)
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 123
When PE/clay materials are prepared with unwashed modified clays, the morphology
is more uniform, and the TEM pictures reveal much less contrast because the nanolayers are
well dispersed in the form of isolated layers. On the other hand, when the nanocomposites are
processed with washed clays, the morphology is composed of small tactoids and even of a
few isolated aggregates. The excess of salts present before washing in the nanocomposites
promotes a better layer exfoliation. The physisorbed and unremoved salts behave as an
efficient compatibilizer to create an intimate contact between filler and matrix. This very good
level of dispersion was obtained without the use of large amounts (about 20 wt.%) of maleic
anhydride-grafted polyethylene usually used as compatibilizer in the polyolefin
nanocomposite processing [28] but has a similar limiting effect in reducing the bulk matrix
mechanical performances, particularly stiffness. In this work, a very good stiffness–toughness
compromise was obtained in combination with a very high dispersion level.
III.1.5 Conclusions In this work, ionic liquids based on phosphonium and imidazolium salts with long
alkyl chains were synthesized and used as surfactants to modify a lamellar silicate surface and
to help its intercalation into a nonpolar polymer matrix. The thermogravimetric analysis
performed on the imidazolium and phosphoniumtreated montmorillonites shows an important
improvement in thermal stability of the salts compared to the conventional ammonium
cationsdue to their intrinsic thermal stability. These new surfactants behave as conventional
organic cations and are easily swollen inducing a high d-spacing. The washing of clays after
amodification step is not really required for processing nanocomposites based on HDPE
matrix. Indeed, the physically adsorbed surfactants on the layer edges have a key role in the
preparation of nanocomposites as they act as a compatibilizing agent. Polyethylene
nanocomposites were prepared by a melt process with only 2 wt.% imidazolium or
phosphonium-modified montmorillonites. The stiffness of PE matrix is clearly increased
without reducing its fracture behavior. The use of unwashed clays is the key parameter to
achieve a very fine dispersion state and the optimized mechanical properties because the
physically adsorbed surfactants act as a compatibilizing agent in situ generated at the
nanolayer/matrix interface.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 124
III.2 Supercritical CO2-Ionic Liquid Mixtures For Modification of Organoclays
The use of supercritical CO2 as solvent in the modification of montmorillonite by
imidazolium and phosphonium ionic liquids bearing long alkyl chains (C18) known for their
excellent thermal stability is described. The objective is to combine the environmentally
friendly character of ionic liquids and supercritical carbon dioxide for the organophilic
treatment of lamellar silicates. Dialkyl imidazolium and alkyl phosphonium salts were
synthesized to be used as new surfactants for cationic exchange of layered silicates. Then, the
synthesized phosphonium (MMT-P) or imidazolium (MMT-I) modified montmorillonites,
cationically exchanged under supercritical carbon dioxide with or without co-solvent, have
been analyzed by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) and
compared to montmorillonites treated by conventional cationic exchange.
III.2.1 Introduction Since the 80s, the industrial and academic research has a growing interest in the design
of polymer / clay nanocomposites. According to the literature, the insertion of lamellar
silicates in a polymer matrix can have beneficial effects on the final properties of the
nanocomposite. The most often reported improvements are for thermo-mechanical behaviour
[4] and gas barrier properties [5-6] due to nanometer-range dimensions and high aspect ratio
of layered silicates leading to a large amount of interfacial zones. Natural clays, such as
montmorillonite (MMT) are frequently used for that purpose [29].
However, the addition of non-modified clay due to the poor interfacial interactions is
very limited. Indeed, it is necessary to modify the surface of the pristine MMT, mostly by
cationic exchange using ammonium salts [10-11] to improve the compatibility between the
polymer and lamellar fillers during the processing step of nanocomposites, i.e. to improve the
final dispersion state and to get a full/large development of the interfaces.
Ionic liquids are organic salts with melting points below 100°C, which are considered as
green solvents. The most commonly used are imidazolium [21], pyridinium [8] and
phosphonium [17] salts. They are used in several applications: chemical reaction medium, i.e.
as solvents, electrolytes in batteries, lubricants, plasticizers, and catalysts. Recently, ionic
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 125
liquids were used as surfactants to replace the conventional alkyl ammonium salts that show a
lower thermal stability [15, 16, 18]. Despite many benefits, very few studies using such
cations for layered silicates intercalation have been reported in the literature may be due of the
actual higher cost of these surfactants compared to ammonium salts [19, 20, 21]. Moreover,
thermally stable and commercially available ionic liquids which could be considered as
surfactants only incorporate short aliphatic chains (up to C14).
In this work, ionic liquids bearing long alkyl chains based on imidazolium (two chains
in C18) and on phosphonium salts with three benzyl groups and only one long aliphatic chain
(1 chain in C18) were synthesized. As these ionic liquids used as clay intercalants, the long
alkyl chains and the aromatic groups could cause expansion of the distance between the layers
and could contribute to a better intercalation of the clay platelets. To perform an
environmentally friendly MMT surface treatment, properties of ionic liquids and supercritical
CO2 were combined. The use of CO2 in the supercritical state should make useless the
addition of any solvent for that the cationic exchange succeeds. The main interest of
supercritical CO2 is to design a clean surface treatment compared to the ones using
conventional solvents. In fact, carbon dioxide has a high diffusivity like a gas, low surface
tension (close to zero), viscosity and density like a liquid which gives it a high solvency
power tuneable by adjusting pressure [30]. In this work, the combination of supercritical CO2
and ionic liquids should lead to an environmentally benign cationic exchange process to
modify the MMT with imidazolium and phosphonium salts with long alkyl chains. The
objective of this work is to design a process without use of solvent and to identify the
supercritical CO2 exposure effects on the physico-chemical properties resulting of modified
clays.
III.2.2 Experimental
III.2.2.1 Organic modification
As supercritical carbon dioxide was used as solvent instead of water, the procedure
was the following one: 2g of untreated MMT and an excess of surfactant (2 CEC:
imidazolium and phosphonium ionic liquid) were placed into a 300 mL high pressure reactor.
Then, an initial loading of the autoclave at a pressure of 50 bars and at a temperature of 20°C
was made.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 126
The choice of temperature used was chosen based on the melting temperatures of the
imidazolium and phosphonium ionic liquid, i.e. 71 and 85°C, respectively. Initially, tests were
made at these melting temperatures without to give results because of their high viscosity.
Higher temperatures i.e. 80°C for the imidazolium ionic liquid and 90°C for the phosphonium
salt, were chosen to perform synthesis. Once the reactor temperature setpoint, the pressure
displayed was 70 bars for the imidazolium-modified montmorillonite and 80 bars for the
phosphonium-modified montmorillonite. After 6 hours of reaction and depressurization
different settings to minimize the losses of modified clays, the autoclave was depressurized at
a rate of 3.6 bar per second. A phosphonium-montmorillonite, denoted MMT-PCO2 and an
imidazolium-montmorillonite denoted MMT-ICO2, were obtained in the supercritical carbon
dioxide. When 10 mL of water was added as a co-solvent, the nomenclature is as follows for
MMT-P(CO2+Water) and MMT-I(CO2+Water).
The structure of synthesized phosphonium and imidazolium ionic liquids are described
in Table III-5.
Table III-5 – Designation of pristine and both synthesized ionic liquid modified montmorillonite (MMT)
Trade name
Intercalant Cationic Exchange process
Designation
Nanofil 757
MMT-Na+ MMT-Na+
N NH37C18 C18H37
Water
Supercritical CO2
Supercritical CO2
(+ 10% water)
MMT-I
MMT-ICO2
MMT-I(CO2+Water)
PC18H37
Water
Supercritical CO2
Supercritical CO2
(+ 10% water)
MMT-P
MMT-PCO2
MMT-P(CO2+Water)
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 127
III.2.3 Results and discussion
III.2.3.1 Effect of supercritical carbon dioxide as a exchange solvent on the thermal degradation of the modified MMT
III.2.3.1.1 Thermal stability of imidazolium-modified montmorillonite
ThermoGravimetric Analysis (TGA) may help to identify intercalated as well as
physisorbed species of modified clay by investigating the degradation mechanisms and the
effects of functionalization on the thermal stability. Figure III-12 shows the evolution of the
weight loss as a function of temperature and the corresponding derivative curves performed
on imidazolium modified montmorillonites either in water at room temperature and pressure
or in supercritical carbon dioxide. The thermal degradation of imidazolium modified MMT
reveal three weight losses whatever the cationic exchange process used. According to the
literature [7, 9, 20, 22], as a layered clay is modified with an organic cation, two kinds of
interactions between organic cations and inorganic clay can take place: i) Van der Waals
bonds as the organic species are physically adsorbed on the clay surface and; ii) Ionic bonds
as the species display ionic interactions inside galleries in the clays.
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Figure III-12 – Evolution of weight loss as a function of temperature (TGA) and derivative of TGA curves (DTG) of the MMT-I bw (a, a’) and MMT-I CO2 bw (b, b’) (heating rate : 20 K.min-1; nitrogen atmosphere).
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 128
The first weight loss corresponds to a partial physisorption on the edges (bearing polar
SiOH groups) or on the external surface of the platelets since the peak decreases after
washing. The fact that it does not completely disappear after intensive washing is associated
to a part of ionic liquid is well intercalated but in a peripheral position respect to the clay
gallery as reported by Davis et al. [25] or is physisorbed from π-SiOH interactions at the
edges of the lamellar silicates. Such part of surfactant can not be washed away easily (since it
underwent cationic exchange) but it is no thermally stabilized by the presence of the inorganic
silicate platelets in a confined position. As a consequence, it degrades at the same temperature
as the physisorbed surfactant. On the other side, the degradation which is evidenced in a
temperature range from 400 to 500°C corresponds to the well intercalated species. Indeed,
organics inside clay galleries display higher temperatures of degradation. As reported in
previous works [31].
Indeed, on the MMT-I DTG curves, the first peak at 340°C corresponds to physical
adsorption onto clay surface and the second and third peak at about 420°C and 480°C are
related to the imidazolium ionic liquid species really intercalated between clay layers. The
same signature on TGA curve is clear evidence that the cationic exchange of montmorillonite
with imidazolium ionic liquid is possible using only supercritical carbon dioxide as a solvent.
Up to now, only phosphonium ionic liquids with much shorter alkyl were considered in
ScCO2 to modify MMT but working at very high pressures while using a small amount of co-
solvent [32]. The influence of a co-solvent on the modification of lamellar silicates in
supercritical carbon dioxide was also considered. However, there is a significant drawback,
i.e. the formation of sticky powders due to the presence of ionic liquid excess adsorbed on the
surface of montmorillonite having a very high viscosity. As a consequence, the easiest
solution is to reduce the amount of ionic liquid introduced into the autoclave from the use of a
co-solvent as increasing pressure (75 bars) up to reduce the viscosity of ionic liquid required.
According to the literature [33], the solubility of ionic liquid in supercritical CO2
remains extremely low and it is necessary to use organic co-solvents. Wu et al. [34] studied
the effects of organic solvents as acetone or ethanol in ScCO2. Large increase of the solubility
of the ionic liquid in ScCO2 was reported. This phenomenon is explained by strong interaction
of solvent with the ionic liquid due mainly to their high polarity. For this knowledge, we
selected the most polar solvent, i.e. water, that has the huge advantage of being a green
solvent (such as supercritical CO2 and ionic liquids) to design an environmentally sustainable
cationic exchange process. However, compared to organic solvents, it has the disadvantage of
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 129
being CO2-phobic but water could be used as a real co-solvent due to the fact that the
synthesized ionic liquids are soluble in water.
With water as co solvent, the thermal degradation of imidazolium modified
montmorillonite is quite different from one of the modified montmorillonite by conventional
cationic exchanges, i.e. in aqueous solution or ScCO2 medium process (Figure III-13).
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Universal V4.2E TA Instruments Figure III-13 – Evolution of weight loss as a function of temperature (TGA) and
derivative of TGA curves (DTG) of the MMT-I bw (a, a’) and MMT-I (CO2 + Water) bw (b, b’) (heating rate : 20 K.min-1; nitrogen atmosphere).
The modified montmorillonite in supercritical carbon dioxide combined with co-
solvent shows both a much earlier degradation of physisorbed species (240°C versus 340°C)
but in the opposite a delayed degradation of intercalated species (540°C versus 420/490°C).
Washed with methyl alcohol that is the more suitable solvent of ionic liquids removes
the physically adsorbed species corresponding to the first degradation peak (Table III-6).
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 130
Table III-6 – Relative mass loss of physically adsorbed and intercalated species determined by TGA (imidazolium modified montmorillonites either in water or ScCO2.
Sample Cationic Exchange process
Physically adsorbed
species (%)
Intercalated species
(%) MMT-I bw Water
31 18
MMT-I aw - 23 MMT-ICO2 bw Supercritical
CO2
27 22 MMT-ICO2 aw - 25
MMT-I(CO2+Water)
bw Supercritical
CO2
(+ 10% water)
47 18
MMT-I(CO2+Water)
aw 9
33
Table III-6 summarizes the relative mass losses of the physically adsorbed species and
the intercalated species for the imidazolium modified montmorillonites either after water
solution or supercritical CO2 medium processes before and after washing with or without co-
solvent. One can observe that before and after washing, the results for imidazolium-modified
montmorillonite (MMT-I) and imidazolium-treated montmorillonite under supercritical CO2
(MMT-I (CO2 + Water)) are similar. In the case of MMT-I (CO2 + Water), when comparing with the
imidazolium modified MMT before washing with a standard cationic exchange process, the
weight percent of physisorbed species is significantly higher (a difference of 16%). We found
almost the same difference (10% instead of 16%) for the intercalated species. Thus, the use of
the supercritical CO2 leads to an increase of the intercalated species ratio up to 33%. This
means that the combined effect of supercritical CO2 and the solubility of imidazolium salts in
water play an important role on the intercalation process.
In order to have a better understanding of the role of the various components,
imidazolium ionic liquids were introduced in the autoclave, heated at 80°C under pressure of
70 bars for 6 hours. The melting temperature of the imidazolium ionic liquid after synthesis
denoted as C18I and after exposure to supercritical CO2, denoted as C18ICO2 are reported in
Table III-7.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 131
Table III-7 – Effect of exposure to supercritical CO2 on the melting temperature of imidazolium ionic liquid at 80°C
Ionic liquid Exposure under ScCO2
Time (h)
Melting temperature
(°C) C18I 0 71
6 38 C18ICO2 24 33
After 6 hours in supercritical carbon dioxide, a strong decrease of the melting point is
observed. Which remains about the same as the exposure time increases, this effect could be
explained by the presence of ScCO2 which remains soluble in the ionic liquid. In conclusion,
there is a solubility limit of the ionic liquid in the CO2 phase.
According to the literature reported on the influence of supercritical CO2 on ionic
liquids [35, 36], this phenomenon was also observed for example by Kazarian et al. [37] who
reported that the melting point of imidazolium ionic liquids based on a C16 chain and a
fluoride anion was reduced from 25°C after a ScCO2 treatment under a pressure of 70 bars.
Later, another study on the phosphonium and ammonium salts [38] showed in the both cases
an important decrease of 100°C but at higher pressure of exposure (150 bars). Recently,
Scurto et al. [39] concluded that CO2 interacted with the ionic liquid due to the establishment
of weak Lewis acid-Lewis base interactions between basic moieties of the organic salt and the
acidic carbon of CO2.
In conclusion, the decrease of the melting temperature of the ionic liquid after
exposure to the the supercritical CO2 in the presence of water as a co-solvent in which ionic
liquid is soluble allows an optimal cationic exchange and a better intercalation of imidazolium
salt in the clay layer galleries.
III.2.3.1.2 Thermal stability of phosphonium-modified montmorillonite
The same approach was considered for the modification of montmorillonite with
phosphonium ions. The cationic exchange was performed in water under atmospheric
pressure but also in supercritical carbon dioxide. The TGA analysis performed on
phosphonium-modified MMT before washing with methyl alcohol are reported in Figure
III-14.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 132
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Universal V4.2E TA Instruments Figure III-14 – Effect of ScCO2 on the thermal degradation of phosphonium
modified montmorillonite by TGA; derivative of the TGA curves MMT-P bw (a, a’) versus MMT-PCO2 bw (b, b’) (heating rate : 20 K.min-1, nitrogen atmosphere).
The same MMT modification realized under supercritical fluid shows the same TGA
analysis which evidences the success of the cationic exchange in the conditions.
As previously, we led the same experiment with the presence of co-solvent such as
water. Figure III-15 displays the TGA and DTG curves of phosphonium-modified
montmorillonites MMT-P and MMT-PCO2+ Water.
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Universal V4.2E TA Instruments Figure III-15 – Effect of ScCO2 combined with water as co-solvent on the thermal degradation of phosphonium-modified montmorillonite by TGA : the derivative of the TGA curves MMT-P (a, a’) bw versus MMT-PCO2+ Water (b, b’) bw (heating rate : 20 K.min-1, nitrogen atmosphere).
With the presence of a co-solvent, a shift of intercalated species is observed (570°C
versus 510°C) whereas the physically adsorbed species are the same degradation behavior
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 133
(320°C versus 330°C). The DSC data realized on phosphonium ionic liquid after synthesis
and after treatment with ScCO2 shows the same behavior in Table III-8.
Table III-8 – Effect of exposure to supercritical CO2 on the melting temperature of phosphonium ionic liquid at 90°C
Ionic liquid Exposure under ScCO2
Time (h)
Melting temperature
(°C) C18P 0 85
6 57 C18PCO2 24 55
The melting temperature of the phosphonium salt is significantly reduced after
treatment with supercritical carbon dioxide (28°C), in the same order of magnitude than for
imidazolium-based ionic liquid. We can expected that, according to the melting temperature
depression, the solubility of the two types of exchanged-montmorillonites ionic liquids in
ScCO2 are similar.
III.2.3.2 Structural analysis
III.2.3.2.1 Imidazolium modified montmorillonite
The effect of the cationic exchange process on the MMT intercalation was studied by X-
ray diffraction and reported in Figure III-16.
0 1 2 3 4 5 6 7 8 9 10
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nsity (
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(b)
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Figure III-16 – Effect of the cationic exchange process on the interlayers distance
measured by X-Ray diffraction spectra of phosphonium ionic liquid modified MMT: (a) MMT-I; (b) MMT-ICO2; (c) MMT-I(CO2 + Water)
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 134
After organophilic treatment by a conventional cationic exchange, the MMT-I displays
a (001) diffraction peak at 2.3°2θ, corresponding to an interlayer distance of 3.7 nm, distance
similar to one characteristic of a paraffinic conformation with trans-trans positions of the alkyl
chain. The CO2 step leads to a shift of diffraction peak towards lower angles with an
interlayer distance of 4.1 nm, corresponding to diffraction peak at 2.1°2θ. The use of water as
a co-solvent leads to reduce the effects of swelling by ionic liquid which is solubilized in
water. The interlayer distance was found to be similar to one of the imidazolium-modified
montmorillonite performed by conventional cationic exchange.
III.2.3.2.2 Phosphonium modified montmorillonite
Figure III-17 shows the X-ray diffraction spectra performed on MMT-P, MMT-PCO2 and
MMT-P(CO2+water).
0 1 2 3 4 5 6 7 8 9 10
0
500
1000
1500
2000
2500
3000
Inte
nsity (
u.a
)
2θθθθ
(a)
(b)
(c)
(a)
(b)
(c)
Figure III-17 – Effect of the cationic exchange process on the interlayers distance
measured by X-Ray diffraction spectra of phosphonium ionic liquid modified MMT: (a) MMT-P; (b) MMT-P CO2; (c) MMT-P(CO2 + Water)
Before treatment, the basal spacing of the sodic montmorillonite is 1.2 nm, which
correspond to the d-spacing of MMT-Na+ reported in literature [7]. After organic treatment in
water by the phosphonium salt, the MMT-P displays a (001) diffraction peak at 2.1°2θ,
corresponding to an interlayer distance of 4.2 nm. This value could be explained by the
swelling of layered silicates due to steric volume of the three ring structure and the alkyl
chain. For the MMT-PCO2, the diffraction peak located at 1.8°2θ, i.e. indicating a d001 of 4.9
nm, a slight increase in the interlayer distance can be explained by the ionic liquid swelling
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 135
under the effect of supercritical CO2. The diffraction spectrum displays also a small peak at
3.1°2θ, corresponding to physically phosphonium salt adsorbed on the surface of
montmorillonite. By using water as co-solvent, the diffraction peak is located at 2.0°2θ, i.e.
indicating a distance of 4.4 nm for MMT-P(CO2 + Water), close to the one of the phosphonium-
treated MMT by conventional cationic exchange. In all the cases, the spectra show intense,
thin, and regular diffraction peaks which suggest a modification on a long range order.
The resulting structure of the ionic liquid-modified montmorillonites, i.e. the d001
distances, as well as the amount of intercalated ionic liquids species can be explained from the
solubility parameters of the various components : ionic liquids, water, and supercritical CO2
the spreading coefficient. In fact, as a conventional route is used, i.e. water solution of ioni
liquid as exchange process medium, the intercalation of imidazolium or phosphonium alkyl-
modified species proceeds from the well-known exchange process described for quaternary
ammonium intercalants [40-41]. As supercritical CO2 is used as medium for intercalation, the
driving force is the spreading of ionic liquid species onto the MMT surface as the surface
polarity of montmorillonite matches the surface tension (or solubility parameter) of the ionic
liquid better than the ScCO2 medium. In the later case, i.e. involving water as co-solvent, as
this one is not soluble in ScCO2, the ionic liquids remain in the water phase leading to a
highly concentrated ionic liquids-water phase, which spreads onto the polar montmorillonite
surface. As a consequence, the process involving water as co-solvent is similar to the
conventional water-solution based protocol.
The solubility of ionic liquids in supercritical CO2 became of a charge interest as such
an understanding requires modelling approaches [42-44] which could be used for practical
purposes [45]. Melting temperature of the phosphonium salt is significantly reduced after
treatment with supercritical carbon dioxide (28°C), in the same order of magnitude than for
imidazolium-based ionic liquid. We can expected that, according to the melting temperature
depression, the solubility of the two types of exchanged-montmorillonites ionic liquids in
ScCO2 are similar.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 136
III.2.3.3 Surface energies
The contact angles and surface energy determined by the sessile drop method on
pressed powder are collected in Table III-9.
Table III-9 – Determination of polar and dispersive components of the surface energy on pristine and on exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders)
Montmorillonite Θwater (°) ΘCH2I2 (°) γ polar (mN.m-1)
γ dispersive (mN.m-1)
γ total (mN.m-1)
MMT-Na+ 22.9 ±0.9 33.6 ±0.8 30 43 73 MMT-P 88.9 ±0.1 49.4 ±0.6 2 35 37 MMT-I 92.8 ±0.1 55.5 ±0.6 1 31 32
MMT-P ScCO2 co 81.5 ±0.1 45.5 ±0.7 3.6 36.8 40.4 MMT-I ScCO2 co 125.2 ±0.5 41.9 ±0.7 3.6 38.5 42.1
Both ionic liquids based on phosphonium and imidazolium salts make the montmorillonite
more hydrophobic with a surface energy close to the surface energy of a polyolefin [28]. The
polar components are very low which is an evidence that the hydroxyl groups are well
covered by the organic species, especially C18 chains. The steric hindrance of imidazolium
and phosphonium ionic liquids causes an efficient screening of the hydrophilic surface of
lamellar silicates.
For imidazolium and phosphonium-modified montmorillonites under supercritical
carbon dioxide without co solvent, the values are identical. Whereas the montmorillonites
MMT-P ScCO2 co and MMT-P ScCO2 co, i.e. by using water as a co-solvent, the values of
surface energy are slightly higher.
III.2.4 Conclusions In this work, we demonstrated that it is possible to modify lamellar silicates by ionic
liquid phosphonium and imidazolium using solvents as water and supercritical CO2. The
resulting properties are improved thermal stability of intercalated species and better
intercalation between the layers of montmorillonite. This process can be improved and
requires many additional studies on the interactions between different components which are
considered. Nevertheless, this study highlights that several solvents, such as water, ionic
liquids, and supercritical CO2, which are among the most promising components of green
chemistry, could be used relevant surface treatment of layered minerals.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 137
Conclusions of chapter III
In Chapter III, the surface treatment of layered silicates by intercalating agents like
ammonium, imidazolium and phosphonium has been studied in two ways: the cation
exchange method involving the use of conventional organic solvents and a method
environmentally friendly using supercritical CO2 as solvent.
Firstly, better thermal stability of ILs on the conventional ammonium has been
demonstrated. Indeed, increases of 50°C to 120°C are observed for physisorbed and
intercalated species. In addition, the amount physically adsorbed on the surface of
montmorillonite is compatibilizing agent between the filler and the polymer which results in
thermal and mechanical properties increased.
In a second step, we showed that using supercritical CO2 associated with water as co-
solvent leads to a decrease of melting temperatures of ILs important which leads to a better
intercalation of imidazolium ILs and phosphonium between the clay layers with a sharp
increase of the degradation temperature.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 138
References of chapter III [1] E.P. Giannelis, Adv. Mater (1996); 8:29. [2] P.C. Le Baron, Z. Wang, T.J. Pinavaia, Appl. Clay Sci. (1999); 15:11. [3] Y.W. Mai, Z.Z. Yu, Polymer Nanocomposites, Woodhead, Cambridge, (2006). [4] L. Le Pluart, J. Duchet, H. Sautereau, Polymer 46 (2005); (26):12267. [5] E. Jacquelot, E. Espuche, J.F. Gerard, J. Duchet, P. Mazabraud, J. Polym. Sci. Part B: Polym. Phys. (2006); 44 (2):431. [6] M.A.Osman,V.Mittal,M.Morbidelli, U.W. Suter, Macromolecules (2003); 36:9851. [7] J.W. Gilman, C.L. Jackson, A.B. Morgan, R. Harris, E. Manias, E.P. Giannelis, Chem. Mater. (2000); 12:1866. [8] S.M. Auerbach, K.A. Carrado, P.K. Dutta, Handbook of Layered Materials, Taylor & Francis, New York, (2004). [9] L. Le Pluart, J. Duchet, H. Sautereau, J.F. Gérard, J. Adhes. (2002); 78 (7):645. [10] S.Y. Lee, W.J. Cho, K.J. Kim, J.H. Ahn, M. Lee, J.colloid Interf Sci. (2005); 284 (2):667. [11] H. He, J. Duchet, J. Galy, J.F. Gerard, J.colloid Interf Sci. (2006); 295:202. [12] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Chem. Mater. (2001); 13 (9):2979. [13] T.D. Fornes, P.J. Yoon, H. Keskkula, D.R. Paul, Polymer (2001); 42:9929. [14] J.W. Gilman, Appl. Clay Sci. (1999); 15:31. [15] W.H. Awad, J.W. Gilman, M. Nyden, R.H. Harris, T.E. Sutto, J. Callahan, P.C.Trulove, H.C. DeLong, D.M. Fox, Thermochim. Acta (2004); 409:3. [16] J. Zhu, A.B. Morgan, F.J. Lamelas, C.A. Wilkie, Chem. Mater. (2001); 13:3774. [17] W. Xie, R. Xie, W.P. Pan, D. Hunter, B. Koene, L.S. Tan, R. Vaia, Chem. Mater. (2002); 14 (11):4837. [18] H.L. Ngo, K. Le Compte, L. Hargen, A.B. McEven, Thermochim. Acta (2000); 97:357–358. [19] F.A. Bottino, E. Fabbri, I.L. Fragala, G. Malandrino, A. Orestano, F. Pilati, Macromol. Rapid Commun. (2003); 24:1079. [20] J.W. Gilman, W.H. Awad, R.D. Davis, J. Shields, R.H. Harris, C. Davis, Chem. Mater. (2002); 14:3776. [21] V. Mittal, Eur. Polym. J. (2007); 43:3727. [22] C. Lotti, C.S. Isaac, M.C. Branciforti, R. Alves, S. Liberman, R. Bretas, Eur. Polym. J. (2008); 44:1346. [23] S. Filippi, C. Marazzato, P. Magagnigni, A. Famulari, P.V. Arosio, S. Meille, Eur. Polym. J. (2008); 44:987. [24] W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, R. Vaia, Thermochim. Acta (2001); 339:367–368. [25] R.D. Davis, J.W. Gilman, T.E. Sutto, Clay Clay Miner. (2004); 52 (2):171. [26] F. Bergaya, B.K.G. Theng, G. Lagaly, Handbook of Clay Science, first ed., Elsevier, (2006). [27] C.M. Hansen, A. Beerbower, Kirk-Othmer Encyclopedia of Chemical Technology, second ed., Interscience, New York, (1971). p. 889. [28] S. Boucard, J. Duchet, J.F. Gérard, P. Prele, S. Gonzalez, Macromol. Symp. (2002); 194 (1):241. [29] Chigwada, G., D. Wang, et al, Polym Degrad. and Stab. (2006); 91(4):848–855. [30] S.P. Nalawade, F. Picchioni, L.P.B.M. Jansen, Prog. Polym. Sci. (2006); 31 19–43. [31] S. Livi, J. Duchet-Rumeau, T.-N. Pham and J.-F. Gérard, J.colloid Interf Sci (2010); 349:424–433. [32] E. Naveau, C. Calberg, C. Detrembleur, S. Bourbigot, C. Jérôme, M. Alexandre, Polymer (2009); 50:1438–1446. [33] S. Keskin, D. Kayrak-Talay, U. Akman, Ö. Hortaçsu, J. of Supercritical Fluids (2007); 43:150–180. [34] W.Wu, J.Zhang, B.Han, J.Chen, Z.Liu, T.Jiang, J.He, W.Li, Chem.Comm. (2003); 1412–1413. [35] V. Najdanovic-Visak, A. Serbanovic, J.M.S.S. Esperança, H.J.R. Guedes, L.P.N. Rebelo, M.Nunes da Ponte, Chem.Phys.Chem. (2003); 4:520. [36] M. Roth, J. of Chromatography, A (2009); 1216:1861–1880. [37] S.G. Kazarian, N. Sakellarios, C.M. Gordon, Chem. Comm. (2002); 1314. [38] A.M. Scurto, W. Leitner, Chem. Commun. (2006); 3681. [39] A.M. Scurto, E. Newton, R.R. Weikel, L. Drauker, J.Hallett, C.L. Liotta, W. Leitner, C.A. Eckert, Ind. Eng. Chem. Res. (2008); 47:493. [40] C.B. Hedley, G. Yuan, B.K.G. Theng, Applied Clay Science. (2007); 35:180–188. [41] A. Vasquez, M. Lopez, G. Kortaberria, L. Martin, I.Mondragon, Applied Clay Science. (2008); 41:24–36. [42] X. Ji, H. Adidharma, Fluid Phase Equilibria. In press, Accepted Manuscript. [43] J. Kumelan, A. Perez-Salado Kamps, I. Urukova, D. Turna, J.Chem. Thermodynamics. (2005); 37 (6):595–602.
Chapter III: Ionic Liquids as news intercalating agents for layered silicates
Page 139
[44] J.S. Torrecilla, J. Palomar, J. Garcia, E. Rojo, F.Rodriguez, Chemiometrics & Intelligent Laboratory
Systems. (2008); 93 (2):149–159. [45] M.G. Freire, C.M.S.S. Neves, S.P.M. Ventura, M.J Patras, I.M. Marrucho, J.Oliveira, J.A.P. Coutinho, Fluid Phase Equilibria, In press, accepted manuscrit.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 141
Chapter IV POLYMER/LAYERED SILICATES NANOCOMPOSITES In this last chapter, we have used these montmorillonites thermally stable during the
preparation of nanocomposites by melt intercalation in two different matrices, high density
polyethylene (HDPE) and polyvinylidene fluoride (PVDF). The influence of ligand, the
variation of chain length, the functionalization of perfluorinated chains, the role of the
imidazolium versus phosphonium cation, and the anion (Br-, I-, PF6-) have been studied on
thermal, physical and mechanical properties as well as on the morphology of the resulting
nanocomposites.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 142
Pages
IV.1 Synthesis of new surfactants: Effect of the ionic liquids on the thermal stability and the mechanical properties of high density polyethylene nanocomposites ................................ 143
IV.1.1 Introduction ............................................................................................................................ 143 IV.1.2 Experimental .......................................................................................................................... 144
IV.1.2.1 Materials ................................................................................................................................. 144 IV.1.2.2 Processing and characterization of HDPE/clay nanocomposites ............................................ 145 IV.1.2.3 Synthesis of imidazolium and phosphonium salts .................................................................. 146
IV.1.2.3.1 Synthesis of octadecylphosphonium bromide and iodide 1a-1b ................................. 146 IV.1.2.3.2 Synthesis of octadecylphosphonium hexafluorophosphate 1c .................................... 147 IV.1.2.3.3 General procedure for the synthesis of N-alkyl-N’-alkyl imidazolium salts 3a-3c. .... 148
IV.1.2.4 Organic modification of montmorillonite ............................................................................... 149 IV.1.2.4.1 Preparation of phosphonium-MMT ............................................................................ 149 IV.1.2.4.2 Preparation of imidazolium-MMT .............................................................................. 150
IV.1.3 Results and discussion ............................................................................................................ 150 IV.1.3.1 Thermal stability of ionic liquids ............................................................................................ 151 IV.1.3.2 Thermal stability of ionic liquid modified montmorillonites .................................................. 153 IV.1.3.3 Surface Energy of ionic liquid modified montmorillonites ..................................................... 154 IV.1.3.4 PE/modified-montmorillonites nanocomposites ..................................................................... 155
IV.1.3.4.1 Morphology of the nanocomposites ........................................................................... 155 IV.1.3.4.2 Thermal stability of the nanocomposites .................................................................... 157 IV.1.3.4.3 Mechanical properties of the nanocomposites ............................................................ 158
IV.1.4 Conclusions ............................................................................................................................ 160
IV.2 Ionic Liquids as Interfacial Agents in PVDF-based nanocomposites .......................... 161 IV.2.1 Introduction ............................................................................................................................ 161 IV.2.2 Experimental .......................................................................................................................... 162
IV.2.2.1 Materials ................................................................................................................................. 162 IV.2.2.2 Synthesis of ionic liquids ........................................................................................................ 162
IV.2.2.2.1 Synthesis of ILs with long alkyl chains ...................................................................... 162 IV.2.2.2.2 Synthesis of imidazolium functionalized with perfluorinated chain ........................... 163
IV.2.2.3 Organic modification .............................................................................................................. 163 IV.2.3 Results and discussion ............................................................................................................ 164
IV.2.3.1 Characterization of ILs exchanged montmorillonites ............................................................. 164 IV.2.3.1.1 Thermal stability of ionic liquid-modified montmorillonites ..................................... 164 IV.2.3.1.2 Structural analysis of ionic liquid-modified montmorillonites ................................... 166 IV.2.3.1.3 Surface energy of ionic liquid-treated montmorillonites ............................................ 167
IV.2.3.2 Effect of interfacial interactions on the material physical properties ...................................... 168 IV.2.3.2.1 On the morphology of the PVDF nanocomposites ..................................................... 168 IV.2.3.2.2 Crystallinity of PVDF based nanocomposites ............................................................ 170 IV.2.3.2.3 Mechanical properties of PVDF based nanocomposites ............................................. 174
IV.2.4 Conclusions ............................................................................................................................ 175
Conclusions of chapter IV ............................................................................................................ 176
References of chapter IV .............................................................................................................. 177
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 143
IV.1 Synthesis of new surfactants: Effect of the ionic liquids on the thermal stability and the mechanical properties of high density polyethylene nanocomposites
Ionic liquids based on alkyltriphenyl phosphonium and dialkyl imidazolium surfactant
salts with long alkyl chains have been synthesized and used as intercalating agents for the
preparation of highly thermally stable organophilic montmorillonites. These new surfactants
behave as conventional organic cations and are easily swollen inducing a high d-spacing.
Thermoplastic nanocomposites with a very low amount of nanofillers have been processed by
melt mixing using a twin screw extruder. The thermal stability of the phosphonium- (MMT-P)
or imidazolium- (MMT-I) modified montmorillonites has been enhanced by up to 100°C
compared with conventional quaternary ammonium cations, making melt mixing of such
modified nanoclays possible with high density polyethylene (HDPE) processed at high
temperature.
IV.1.1 Introduction Recent research dedicated to the introduction of layered silicate-(montmorillonite,
MMT) into polymer matrices demonstrates an increase of thermal stability [1], mechanical
properties [2-4], and reduced flammability [5-7] for numerous types of polymer-clay
nanocomposites. The lamellar and confined structure of inorganic layers in polymer matrix
and the nanoscale dimensions of particles could explain changes in polymer physics, such as
molecular mobility and thermal resistance. This later one is of course also associated to the
thermal stability of the surface modifiers, i.e. interfacial agents. In fact, increasing the thermal
stability of montmorillonite and resulting nanocomposites is a key issue in the developement
of polymer-clay nanocomposites at the industrial scale.
The limited thermal stability of the conventionally used alkylammonium cations [8]
intercalated into layered minerals and the degradation occurring during processing of some
thermoplastic polymers (for example polyolefins) in the presence of nanodispersed MMT
have motivated the development of improved organophilic treatments for layered silicates.
Such new organomodifiers should enable the preparation of polymer/layered silicate
nanocomposites based on polymers that require high melt-processing temperatures [9-10]
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 144
and/or long residence times under high shear as well as for thermoset reactive systems which
are cured at high temperatures.
Ionic liquids (ILs) are organic salts with melting temperature below 100°C. They are
known for their excellent thermal stability, non-flammability, low vapor pressure and high
ionic conductivity. Their properties can be tuned by different associations of cations and
anions. The cation is usually a bulky organic structure with a low symmetry, such as
pyridinium, imidazolium, phosphonium, and ammonium ions. The current research mainly
focused on Room Temperature Ionic Liquids (RTILs) composed of asymmetric N-N
dialkylimidazolium cations bearing short alkyl chains such as butyl-methyl or ethyl-methyl
functionalization. The original part of this work results in the synthesis of ILs functionalized
with long alkyl chains suitable for improving the compatibility and the dispersion of
nanolayered silicates within polyolefin matrix, i.e. hydrophobic medium.
The aim of this study is to synthesize new thermally stable intercalating agents and to
find the most relevant combination of cation and anion to design an ionic liquid efficient to
get organophilic montmorillonites and to improve the physical properties of resulting
nanocomposite. The effect of the chemical nature of cation, imidazolium versus
phosphonium, the influence of the alkyl chain length, i.e. octadecyl versus docosyl, for
imidazolium salts and the effect of the chemical nature of anion, i.e. halide versus fluorinated
for phosphonium salts on the morphology, the thermal, and mechanical properties of high
density polyethylene-based nanocomposites will be investigated.
IV.1.2 Experimental
IV.1.2.1 Materials
A sodic montmorillonite, denoted Nanofil 757 (MMT), i.e. an aluminosilicate with
intercalated sodium was chosen as pristine clay. This one was provided by Süd Chemie Co.
The Nanofil 757 has a cation exchange capacity of 95 meq/100 g and is described by the
following formula Na0,65[Al,Fe]4Si8O20(OH)4. All chemicals necessary to the synthesis of
ionic liquids, i.e. triphenylphosphine (95%), imidazole (99.5%), 1-iodooctadecane (octadecyl
bromide 95%), 1-bromodocosane, (docosyl bromide 96%) and solvents (toluene, methanol,
pentane and acetonitrile) were supplied from Aldrich and used as received.
The polyethylene used in this study, denoted HDPE, is a high-density polyethylene
from Basell, with the trade name Hostalen GF 7260 showing a melt flow index of 0.4.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 145
IV.1.2.2 Processing and characterization of HDPE/clay nanocomposites
Nanocomposites based on PE/organically modified montmorillonites (1% by weight)
were prepared using a 15g-capacity DSM micro-extruder (Midi 2000 Heerlen, The
Netherlands) with co-rotating screws. The mixture was sheared for about 3 min with a 100
rpm speed at 190°C and injected in a 10cm3 mould at 30°C to obtain dumbbell-shaped
specimens.
Thermogravimetric analysis (TGA) of organically modified clay and nanocomposites
were performed on a Q500 thermogravimetric analyser (TA instruments). The samples were
heated from 30 to 800 °C at a rate of 20 K.min-1 under nitrogen flow.
Differential scanning calorimetry (DSC) analysis were performed on a Q20 (TA
instruments). The samples were kept for 1 min at 200°C to erase the thermal history before
being heated or cooled at a rate of 10 K.min-1 under nitrogen flow. The crystallinity rate was
determined considering ∆H100% to be 102.7 J/g [30].
Surface energy of modified clays was determined with the sessile drop method using a
GBX goniometer. From contact angle measurements performed with water and
diiodomethane as probe liquids on discs obtained from clay powders by pressing, polar and
dispersive components of surface energy were determined by using Owens-Wendt theory.
Transmission Electron microscopy (TEM) was used at the Center of Microstructures
(Université de Lyon) on a Philips CM 120 field emission electron microscope with an
accelerating voltage of 80 kV. The samples were cut using an ultramicrotome equipped with a
diamond knife to obtain 60 nm thick ultrathin sections. Then, the sections were set on copper
grids.
Uniaxial Tensile Tests were carried out on a MTS 2/M electromechanical testing
system at 22±1°C and 50±5% relative humidity at crosshead speed of 10 mm.min-1.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 146
IV.1.2.3 Synthesis of imidazolium and phosphonium salts
IV.1.2.3.1 Synthesis of octadecylphosphonium bromide and iodide 1a-1b
The studied ionic liquids are not commercially available and their synthesis was
reported using the same protocol as the one described by Livi et al. [11].
Octadecyltriphenylphosphonium salts have been synthesized according to Figure IV-1 with the
corresponding iodide and bromide anions. Octadecyl hexafluorophosphate is not
commercially available and has been prepared by anionic exchange from octadecyl iodide and
hydrogen hexafluorophosphate (HPF6).
PC18H37
X
X = I , Br
C18H37XP
1a, 1b
+
Toluene
120 °C 25°C
HPF6
PC18H37 PF6
1c Figure IV-1 – Synthesis of the phosphonium ionic liquid 1a-1b-1c
General method
In a 100 mL flask were placed under a positive nitrogen pressure, triphenylphosphine
and octadecyl iodide or octadecyl bromide. The stirred suspensions were allowed to react for
24 h at 120 °C in toluene (20 mL), a yellow precipitate was formed. The reaction mixture was
then filtered, washed repeatedly with pentane. Most of the solvent was removed under
vacuum and the product was dried to a constant weight to give a white solid. The structure of
these salts was confirmed by 13C NMR spectroscopy.
a. Octadecyltriphenylphosphonium iodide 1a
1a was obtained by reaction of triphenylphosphine (5 g, 1 equiv.) and octadecyl iodide
(7.70 g, 1 equiv.). Yield = 90 %, white powder.
13C NMR (CDCl3): δ 14.00 (CH3); 22.67 (CH2Me); 23.2; 29.37-29.66; 30.24; 31.85
(PCH2); 118.45; 130.43; 133.70; 135.15 (P-Carom.).
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 147
b. Octadecyltriphenylphosphonium bromide 1b
1b was obtained by reaction of triphenylphosphine (5 g, 1 equiv.) and octadecyl
bromide ( 9.8 g, 1 equiv.). Yield = 88 %, brown powder.
13C NMR (CDCl3): δ 14.00 (CH3); 22.52 (CH2Me); 23.0; 28.90-29.66; 30.20; 32.00
(PCH2); 118.85; 130.80; 133.50; 135.45 (P-Carom.).
IV.1.2.3.2 Synthesis of octadecylphosphonium hexafluorophosphate 1c
In a 100 mL flask, octadecyl iodide (C18H37I) (5.0 g, 1 equiv.) was dissolved into
dichloromethane (25 mL). The mixture was stirred for 30 min at room temperature. A
solution of hydrogen hexafluorophosphate (HPF6) (3.8 g, 2 equiv.) diluted in water (25 mL)
was stirred for 30 min and added to the octadecyl iodide solution. The stirred suspension was
allowed to react for 24 h at room temperature. The reaction mixture was then introduced in a
separatory funnel and the organic layer was washed repeatedly with distilled water (4x 25
mL). The mixture was dried over anhydrous magnesium sulfate and concentrated under
reduced pressure. The solvent was removed by evaporation under vacuum and the product
was dried to a constant weight to give a white solid.
13C NMR (CDCl3): δ 14.00 (CH3); 22.35 (CH2Me); 23.5; 29.12-29.74; 30.35; 31.75
(PCH2); 118.75; 130.22; 133.50; 135.05 (P-Carom.).
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 148
IV.1.2.3.3 General procedure for the synthesis of N-alkyl-N’-alkyl imidazolium salts 3a-3c.
Dialkylimidazolium cation-based ionic liquids have been easily prepared by alkylation
of the commercially available imidazole with an alkyl iodide to give respectively the
corresponding N-alkyl-N’-alkylimidazolium halide (Figure IV-2).
N N N NR
HN N
MeO Na RI
- NaI
N NR R' I
R'I
C18
C22 C22
R = C18 R' = C18
C22
2a 3a
2b
2c
3b
3c Figure IV-2 – Synthesis of the imidazolium ionic liquids 3a-3c
A solution of sodium methoxide was prepared from sodium (1 equiv.) in dry freshly
distilled methyl alcohol (10 mL) in a sealed septum, 100 mL round-bottomed, threenecked
flask equipped with a condenser, under nitrogen atmosphere and magnetic stirring. Imidazole
(1 equiv.) diluted in acetonitrile (10 mL) was then added into the stirred mixture of sodium
methoxide previously cooled at room temperature. After 15 min, a white precipitate was
formed. The suspension was then concentrated under reduced pressure for 1 h. The dried
white powder was dissolved in acetonitrile and a solution of alkyl halide RX (1 equiv.) diluted
in acetonitrile (10 mL) was then added under an inert atmosphere of nitrogen at room
temperature. The mixture was stirred for 1 h, then heated under reflux at 85 °C for about 24 h.
A solution of alkyl halide R’X (1 equiv.) diluted in acetonitrile (10 mL) was added to the
mixture at room temperature. The stirred suspension was heated under reflux at 100 °C for
about 24 h leaving a brownish viscous oil in each case. After cooling to room temperature, the
solvent was removed by evaporation under vacuum, the beige coloured powder was filtered,
washed repeatedly with pentane and dried. Purification of the resulting imidazolium salts was
accomplished by crystallization from ethyl acetate/acetonitrile: 75/25 mixture. After drying,
alkyl imidazolium salts 3a-3c were fully characterized by spectroscopy. The assignment of 13C NMR spectroscopy resonance peaks is an evidence of the success of the ionic liquid
synthesis.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 149
N-octadecyl-N’-octadecyl imidazolium iodide 3a, see Livi et al. [11].
N-octadecyl-N’-docosyl imidazolium bromide 3b
According to the general procedure, N-octadecyl-N’-docosylimidazolium bromide 3b
were carried out with Na (0.460 g, 20 mmol), imidazole (1.370 g, 20 mmol, 1 equiv.),
octadecyl iodide (7.700 g, 20 mmol, 1 equiv.) and docosyl bromide (7.790 g, 20 mmol, 1
equiv.). Evaporation of the solvent under vacuum furnished the product 3b (13.290 g) as a
beige colour powder. Yield = 94 %.
13C NMR (CDCl3) δ (ppm) 14.00 (2CH3); 20.00 (2CH2Me); 26.00; 29.00; 29.35-29.69
(CH2); 30.24; 31.91 (NCH2CH2); 50.10; (CH2N=); 50.32 (CH2N-); 121.70; 122.50 (=CN);
136.90 (N-C=N).
N-docosyl-N’-docosylimidazolium bromide 3c
According to the general procedure, N-octadecyl-N’-docosylimidazolium bromide 3c
were carried out with Na (0.920 g, 40 mmol), imidazole (2.740 g, 40 mmol, 1 equiv) and
docosyl bromide (15.580 g, 40 mmol, 1 equiv.). Evaporation of the solvent under vacuum
furnished the product 3c (19.200g) as a beige colour powder. Yield = 63 %.
13C NMR (CDCl3) δ (ppm) 13.40 (2CH3); 19.40 (2CH2Me); 26.00; 29.00; 29.35-29.69
(CH2); 30.24; 31.90 (NCH2CH2); 50.10; (CH2N); 50.32 (CH2N=); 121.70; 122.50 (=CN);
136.90 (N-C=N).
IV.1.2.4 Organic modification of montmorillonite
IV.1.2.4.1 Preparation of phosphonium-MMT
After dispersion of montmorillonite Nanofil 757 (10 g) in deionized water (1 L),
phosphonium salts (9.1 mequiv., as 0.01 M solution) were slowly added per gram of
montmorillonite under continuous stirring at 80 °C during 10 h. The products were washed free
from halide ions tested using AgNO3 solution, dried at 35°C followed by over night drying at
110°C and then pulverized to pass through 300 mesh sieve. The phosphonium-MMT was
stored in a dessicator before analysis.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 150
IV.1.2.4.2 Preparation of imidazolium-MMT
An excess of the salts (2 equiv.) based on the cation exchanged capacity (CEC = 95
mequiv./100 g) of the montmorillonite Nanofil 757 was used [13]. Montmorillonite was dried
at 110°C for 48 h and washed with distilled water (10 wt%) with vigorous stirring for 24 h to
cause the delamination of the montmorillonite. An amount of obtained imidazolium salts (2.66
g) and the montmorillonite Nanofil 757 (2 g, 1.9 x10-3 equiv.) was dispersed in 400 mL of
deionised water. This dispersion was mixed and stirred vigorously at 80°C for 17 h, followed
by filtration and repeated washing with organic solvent (acetonitrile at room temperature or hot
ethanol) to remove some eventually remaining imidazolium salts, then with deionised water
until no halide ions were detected using an aqueous silver nitrate (AgNO3) solution. The
solvent was removed by evaporation under vaccuum. Modified montmorillonite was then dried
for one night, at a suitable temperature (not higher than 80°C) which prevents from any
degradation of alkyl ammonium montmorillonite by Hofmann elimination mechanism [14-15].
The imidazolium-MMT was stored in a dessicator before analysis.
IV.1.3 Results and discussion
We report herein, a general and simple method for the synthesis of a series of organic
halide salts based on octadecyltriphenylphosphonium salts 1a-1c and N-alkyl-N’-alkyl
imidazolium salts 3a-3c was developed in (Figure IV-3). These phosphonium and imidazolium
salts with long alkyl chains are used as new surfactants in place of conventional ammonium
salts commonly used to get organophilic lamellar silicate and to assist their intercalation into
non polar polymer matrices. By tuning both the chemical nature of cation and anion, the better
cation-anion combination is analyzed to get thermal stability of modified montmorillonites
and consequently of the resulting nanocomposites.
N NH37C18 C18H37
N N
H37C18C22H45
BrIN N
C22H45 C22H45
Br
3a 3b 3c
X = I
Br
PF6
1a
1b
1c
PC18H37
X
Imidazolium 3a-3c
Phosphonium 1a-1c Figure IV-3 – Structure of the phosphonium and imidazolium ionic liquids.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 151
IV.1.3.1 Thermal stability of ionic liquids
Because of their aromatic structure, the imidazolium cations are known to display a better
thermal stability than the ammonium cations [15]. Thermogravimetric measurements were
carried out on the three types of imidazolium ionic liquids with halide anions and long alkyl
chains. The thermal decomposition of imidazolium salts C18C18Im, C18C22Im, and C22C22Im is
described in Figure IV-4.
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t (%
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0 100 200 300 400 500 600 700
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Universal V4.2E TA Instruments
Figure IV-4 – Effect of the alkyl chain length on the imidazolium ionic liquids: evolution of the weight loss as a function of temperature (TGA, DTG) of the (●) C18C22Im, (■) C22C22Im, (♦) C18C18Im
(heating rate : 20 K.min-1, under nitrogen atmosphere)
The imidazolium cation with C18-C18 alkyl ligands is thermally stable up to 320°C.
Nevertheless, a slight decrease in thermal stability of imidazolium ionic liquid is shown when
the alkyl chain length is increased. With C22-C22 alkyl chains, the weight loss takes as soon as
the thermal analysis starts even if the maximal degradation temperature is slightly higher.
Under oxidative atmosphere, Awad et al [16] also concluded that the thermal stability
decreased as the organic content of the molten salt increased. According to the literature [9],
at high temperature, i.e. 600°C, the imidazole ring is thermally resistant during the thermal
rearrangements of dialkylimidazolium ions which explains that the imidazolium ionic liquids
are not fully degraded.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 152
The chemical nature of cation, imidazolium versus phosphonium, has no really an effect
on the thermal stability. With the iodide anion, the ionic liquids based on phosphonium cation
display a degradation at the same temperature as one determined for imidazolium cation, i.e.
about 320°C, as reported in Figure IV-5.
The effect of the chemical nature of the anion, halide anions (I-), (Br-) versus fluorinated
anion (PF6-), on the thermal stability of the phosphonium ionic liquids C18P is presented in
Figure IV-5.
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igh
t (%
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0 100 200 300 400 500 600 700
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Universal V4.2E TA Instruments
Figure IV-5 – Effect of the anion chemical nature associated to phosphonium ionic liquids: evolution of the weight loss as a function of temperature (TGA, DTG) of the
(●) C18P Br-, (■) C18P I-, (♦) C18P PF6-
(heating rate : 20 K.min-1, under nitrogen atmosphere).
The combination of the anion associated with the organic cation has a significant role on
the thermal stability of ionic liquid. Indeed, the use of hexafluorophosphate anion (PF6-)
combined with the phosphonium cation provides an increase of temperature about one
hundred and forty degree celsius compared to the halide salts C18P I- and C18P Br- that are
degraded at the same temperature, about 320°C. This use of fluorinated anion in order to
enhance the thermal stability of the ionic liquids is not new. Awad et al [9] had demonstrated
that the use of anion type PF6- and BF4
- associated to imidazolium salt increased the thermal
stability of one hundred degrees. Regarding the phosphonium ionic liquid, the literature [17,
18] has already reported the lower thermal stability of ionic liquid associated to bromide
anion compared to one obtained in presence of tetrafluoroborate anion.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 153
IV.1.3.2 Thermal stability of ionic liquid modified montmorillonites
After cationic exchange with the phosphonium or imidazolium ionic liquids, two
organophilic montmorillonites were prepared with varying alkyl chain length and different
counteranions. From TGA analysis, two organic species populations have been identified the
physisorbed species onto the montmorillonite surface and the intercalated species between the
montmorillonite galleries and have already been described in a previous work [11] on the
phosphonium-modified montmorillonite (MMT-P I-) and the imidazolium-treated
montmorillonite (MMT-C18C18Im). The thermal degradation of montmorillonites modified by
the imidazolium ionic liquids is reported in Figure IV-6 as a function of the ligand alkyl chain
length and the halide anion nature associated.
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Universal V4.2E TA Instruments
Figure IV-6 – Effect of the alkyl chain length and the halide anion associated to the imidazolium cation : evolution of the weight loss as a function of temperature (TGA, DTG) of the
(●) MMT-C18C22Im, (■) MMT-C18C18Im (♦) MMT-C22C22Im (heating rate : 20 K.min-1, under nitrogen atmosphere).
Neither the alkyl chain length (C18 versus C22) or the chemical nature of the halide anion
(bromide or iodide) has an influence on the thermal decomposition of modified clays. Indeed,
the same thermal degradation process takes place going through three decomposition steps
occurring at about 310, 410 and 480°C.
However, it was previously shown that the chemical nature of the anion played an
important role on the intrinsic thermal stability of phosphonium ionic liquids.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 154
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Universal V4.2E TA Instruments
Figure IV-7 – Effect of the anion chemical nature associated to phosphonium cation: evolution of the weight loss as a function of temperature (TGA, DTG) of the
(●) MMT-P Br-, (■) MMT-P PF6- (♦) MMT-P I-
(heating rate : 20 K.min-1, nitrogen atmosphere).
Regarding the TGA analysis reported in Figure IV-7 on the phosphonium-modified
montmorillonites with bromide (Br-) and iodide (I-) conteranions, the same degradation is
observed with two main degradation peaks. The first weight loss at 330°C still corresponds to
a partial physisorption on the external surface of the platelets. On the other hand, the
degradation which is evidenced at about 500°C corresponds to the well intercalated species
between clay layers. In the case of MMT-P PF6-, the use of hexafluorophosphate anion causes
a shift measured at 80°C at higher temperatures (410°C instead of 330°C) of the degradation
peak corresponding to physisorbed species.
In conclusion, the montmorillonites modified with the imidazolium and phosphonium
ionic liquids display all both an excellent thermal stability up to nearly 500°C if the
physisorbed species are removed by washing which is a huge advantage for the processing of
polymer/clay nanocomposites at high temperatures.
IV.1.3.3 Surface Energy of ionic liquid modified montmorillonites
To evaluate the interactions able to be generated by the modified montmorillonite
towards the polymer matrix as a function of the alkyl chain length and of the anion nature, the
contact angles and surface energies were determined by the sessile drop method on pressed
powder and the values are collected in (Table IV-1).
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 155
Table IV-1 – Determination of polar and dispersive components of the surface energy on pristine and on exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders) Montmorillonite Θwater (°) ΘCH2I2 (°) γ polar
(mN.m-1) γ dispersive (mN.m-1)
γ total (mN.m-1)
MMT-Na+ 22.9 ±0.9 33.6 ±0.8 30 43 73 MMT-P I- 88.9 ±0.1 49.4 ±0.6 2 35 37
MMT-P Br- 75.8 ±0.2 45.1 ±0.7 6 37 43 MMT-P PF6
- 92.2 ±0.1 48.6 ±0.7 1 35 36 MMT-C18C18Im 92.8 ±0.1 55.5 ±0.6 1 31 32 MMT-C18C22Im 80.1 ±0.2 50.8 ±0.6 5 34 39 MMT-C22C22Im 97.8 ±0.1 53.8 ±0.6 1 32 33
The both ionic liquids based on phosphonium and imidazolium salts make the
montmorillonite more hydrophobic with a surface energy similar to a polyolefin one [19]. The
organic chain length has indeed a strong effect on the surface energy. The steric hindrance of
imidazolium and phosphonium ionic liquids due to the presence of long alkyl chains causes
an important decrease of the energy surface of layered silicates. The coverage of the
hydrophilic surface by ionic liquids explains the extremely low values of polar components.
Longer the alkyl chain is, more hydrophobic the montmorillonite is. The most hydrophobic
surface is achieved with the imidazolium cation functionalized with both alkyl chains in C18
and C22. A symmetric configuration seems to be more suitable than an asymmetric one that
leads to a less hydrophobic surface. (surface energy of 39 mJ.m-² instead of 32 mJ.m-²). The
anion has also a significant influence on the surface energy. For the same cation,
phosphonium, the bromide anion leads to the highest surface energy whereas the polar
component is much lower with the iodide and fluorinated anions. In conclusion, the use of
ionic liquids as intercalating agents in the modified montmorillonites should generate a good
affinity with the polyethylene matrix and the surface properties of montmorillonite can be
tuned by the relevant choice of ionic liquid.
IV.1.3.4 PE/modified-montmorillonites nanocomposites
IV.1.3.4.1 Morphology of the nanocomposites
The distribution and dispersion of lamellar silicates in polyethylene matrix were
analyzed by transmission electron microscopy on the nanocomposites processed with 1 wt%
of imidazolium and phosphonium -treated montmorillonites. Micrographs are reported in
Figure IV-8 and Figure IV-9.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 156
• Effect of length alkyl chains
200 nm 200 nm200 nm
Figure IV-8 – TEM micrographs of PE-MMT nanocomposites: (a) PE/MMT-C18C18Im, (b) PE/MMT-C18C22Im, (c) MMT-C22C22Im
The imidazolium-treated montmorillonites are usually well distributed in the form of
isolated layers, despite the presence of small tactoïds, i.e. stacked layers. However,
differences exist depending on the alkyl chain length attached to imidazole ring. In fact,
MMT-C22C22Im is the modified montmorillonite which displays the better dispersion in the
high density polyethylene matrix. This result can be explained by the long alkyl chains (C22)
which has encouraged the hydrophobicity of the montmorillonite promising a better
compatibility between polymer matrix and clay surface.
• Effect of the chemical nature of anion
200 nm 200 nm200 nm
Figure IV-9 – TEM micrographs of PE-MMT nanocomposites: (a) PE/MMT-P I-, (b) PE/MMT-P Br-, (c) PE/MMT-P PF6
-
Previously, it was shown that the chemical nature of anion associated to the
phosphonium cation affected the surface energy of phosphonium-treated montmorillonites.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 157
The bromide anion led to a less hydrophobic montmorillonite. The final morphology of
nanocomposites processed with phosphonium modified montmorillonites depends on the
chemical nature of the anion as well. In fact, the presence of bromide anion induces a poorer
distribution of modified-montmorillonite and leads to the presence of microscale aggregates
in the high density polyethylene. The steric hindrance related to anions must play a role in the
coverage of hydroxyl groups present on the clay surface which results in a higher polar
component in the case of the halide anion (Br-).
IV.1.3.4.2 Thermal stability of the nanocomposites
Thermogravimetric analysis traces and their first derivative curve (DTG) of
imidazolium-treated montmorillonites nanocomposites are given in Figure IV-10.
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Universal V4.2E TA Instruments
Figure IV-10 – Evolution of weight loss as a function of temperature (TGA, DTG) of the (●) PE/MMT-C18C18Im, (■) PE/MMT-C22C22Im (♦) Neat PE, (○) PE/MMT-C18C22Im
(heating rate : 20 K.min-1, under nitrogen atmosphere).
A very low amount (only 1 wt%) of modified-montmorillonites with imidazolium
ionic liquids is enough to increase the thermal stability of the polyethylene matrix. Indeed, the
thermal decomposition is delayed of 10°C by the addition of imidazolium ionic liquid for the
MMT-C18C18Im and MMT-C18C22Im. A more significant improvement (+ 15°C) is observed
with imidazolium ionic liquid C22C22Im. These results are promising and can be enhanced by
using a larger amount of modified clay for designing PE-nanoclay nanocomposites.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 158
The thermal degradation of nanocomposites processed with phosphonium-treated
montmorillonites is presented in Figure IV-11.
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Universal V4.2E TA Instruments
Figure IV-11 – Evolution of weight loss as a function of temperature (TGA, DTG) of the (●) PE/MMT-P PF6
-, (■) PE/ MMT-P Br-, (○) MMT-P I-, (♦) Neat PE (heating rate : 20 K.min-1, under nitrogen atmosphere).
The addition of only 1 weight percent of phosphonium-treated montmorillonite also
induces a 10°C increase of the thermal stability of polyethylene. On the other side, the
chemical nature of the anion has no significant effect on the thermal properties of the
nanocomposite unlike to the effects observed on the modified montmorillonite, in particular
with hexafluorophosphate anion (PF6-). In fact, the thermal degradation of polyethylene filled
with MMT-P Br-, MMT-P I- or MMT-P PF6- is very similar: An increase of 10°C is observed
whatever the anion used.
IV.1.3.4.3 Mechanical properties of the nanocomposites
The uniaxial tensile properties were determined to evaluate the impact of this very
small modified montmorillonite amount on the mechanical behaviour of high density
polyethylene. The moduli and the fracture properties are reported in Table IV-2.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 159
Table IV-2 – Tensile properties of the ionic liquid-modified montmorillonites/high density polyethylene nanocomposites at room temperature (10 mm.min-1)
Sample Tensile modulus (MPa)
Strain at break (%)
Stress at break (MPa)
PE 480 18 46 PE-MMT 460 17 41
PE/MMT-C18C18Im 600 17 48 PE/MMT-C18C22Im 510 18 45 PE/MMT-C22C22Im 660 18 50
PE/MMT-P I- 620 16 50 PE/MMT-P Br- 476 15 42 PE/MMT-P PF6
- 548 17 48
The addition of only 1 wt% ionic liquid modified montmorillonite in the high density
polyethylene leads to an increase of the modulus without reducing the strain at break. This
result is very exciting because usually the introduction of fillers does not permit to improve
the compromise stiffness/fracture deformation but rather to reduce it. However, the
modification of layered silicates with ionic liquids such as imidazolium or phosphonium
functionalized with long alkyl chains is a way to enhance the compromise between stiffness
and ability to deformation.
With the imidazolium cation, the symmetric configuration with the both alkyl chains
(C18-C18 or C22-C22) is the most suitable one since a modulus increase of + 25% and + 40%
with the imidazolium treated-montmorillonites MMT-C18C18Im and MMT-C22C22Im
respectively is obtained respect to the neat matrix. In the case of MMT-C18C22Im, the slight
increase of modulus can be explained by the combination of bromide (Br-) and iodide (I-)
anions, which come respectively from (C22) and (C18) alkyl chains. The mechanical results are
in good correlation with the surface properties that demonstrate a higher hydrophobic
character with alkyl chains having the same chain length as well with the state of dispersion
of nanoplatelets within the polymer matrix.
In the case of PE/phosphonium treated-montmorillonite nanocomposites, the increase
of modulus is between 10% and 30% for only one percent of introduced fillers into the
polyethylene matrix. The lowest increase of modulus (4%) is obviously obtained with the
bromide anion (Br-) that led to the lowest hydrophobicity of the filler. On the contrary, a
better compatibility between polyethylene and MMT-P I- seems to be checked like considered
from the surface properties since a modulus increase of 35% is measured. The use of
hexafluorophosphate (PF6-) leads to an intermediate modulus with an increase of 20%.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 160
IV.1.4 Conclusions
In this work, new surfactants based on alkyltriphenyl phosphonium and dialkyl
imidazolium salts have been used to prepare highly thermally stable organophilic
montmorillonites. It was demonstrated that the use of ionic liquid modified-montmorillonites
induces an increase in the thermal stability (+15°C) and the mechanical properties of
nanocomposites. The stiffness was found to be improved without reducing the ultimate
properties such as strain and stress at break. Moreover, the ionic liquids can be tuned by a lot
of possible combinations between cation and anion that play a very important role on the
physical properties of the polymer.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 161
IV.2 Ionic Liquids as Interfacial Agents in PVDF-based nanocomposites
Different ionic liquids based on alkyltriphenylphosphonium and imidazolium-
functionalized either with two alkyl chains or with a fluorinated ligand have been synthesized
and used as interfacial agents for the layered silicates. The effect of the chemical nature of the
organic cation on the morphology and the physical properties of the polyvinylidene fluoride
(PVDF) nanocomposites has been studied. The influence of ionic liquids on polymorphic
crystalline forms, i.e. α and β phases of the polymer matrix was discussed.
IV.2.1 Introduction
For many years, research in the field of polymer/clay nanocomposites has been widely
extended [20-21]. The nanoscale of nanoplatelets leads to an increase of surface to volume
ratio generating a very huge amount of interface. The success of nanofillers dispersion into
polymer matrix implies the tailoring of interactions between fillers and matrix. The objective
is to process organic-inorganic nanomaterials for which all the polymer matrix chains are in a
confined configuration with the consequences on chain mobility and on final properties such
as thermal [1], mechanical [2] and barrier properties [5].
Polyvinylidene fluoride (PVDF) is a polymer commonly used in electronic and
chemical industries due to its excellent chemical, thermal stabilities and mechanical
properties. From a morphology view, it has several crystalline forms, i.e. α, β, γ, and
δ [22]. While the α form is the most commonly generated one; the β form is expected due to
the resulting dielectrical properties and piezoelectric applications of PVDF materials. Several
ways could be followed to obtain the β form; i) the application of a strain, ii) the use of an
electric field [23], iii) the growth from a PVDF solution, and iv) the introduction of inorganic
fillers [24-25]. For this last route, the influence of the modified clays on the mechanical
properties has been widely studied in the literature. It has been shown that the use of
Cloisite® 6A and 20A [26-27], i.e. clays modified by alkylammonium ions, promote the
formation of β phase and a significant increase in the storage modulus is obtained with fillers
contents from 1.5 to 7 wt%. Shah et al. showed that the addition of Cloisite® 30B (5 wt%)
increases Young’s modulus of 40% and strain at break of 250% [28]. However, the great
disadvantage of these ammonium salts is their poor thermal stability as degradation starts
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 162
from 180°C [8, 13, 14], which is the temperature limit for processing clay-based PVDF
nanocomposites from melt intercalation. Recently, a new alternative to conventional
ammonium emerges gradually with the ionic liquids (ILs). In particular, the imidazolium and
phosphonium salts known to possess an excellent thermal stability [11]. Patro et al. [29] used
phosphonium and pyridinium ionic liquids as intercalant agents and have achieved an increase
of 50 and 150% for Young’s modulus and the strain at break, respectively.
In this work, the interfacial interactions between the fluorinated matrix and the
lamellar silicates, i.e. montmorillonite, were tailored from a relevant selection of own-
synthesized ionic liquids. The selected ILs have either an alkyl ligand (imidazolium and
phosphonium) or a fluorinated one (imidazolium) to match the PVDF matrix nature. Then, the
key role of interfacial interactions will be argued in terms of morphology, crystallinity and
mechanical properties of nanocomposites based on layered silicates modified by these new
surfactants called ILs.
IV.2.2 Experimental
IV.2.2.1 Materials
A sodic montmorillonite, denoted Nanofil 757 (MMT), i.e. an aluminosilicate with
intercalated sodium ions, was chosen as pristine clay. This one was provided by Süd Chemie
Co. The Nanofil 757 has a cationic exchange capacity of 95 meq/100g and could be described
by the following formula Na0,65[Al,Fe]4Si8O20(OH)4. All chemicals necessary for the
synthesis of ionic liquids, i.e. triphenylphosphine (95%), imidazole (99.5%), iodooctadecyl
(95%), and all the solvents (toluene, sodium methanoate, pentane, acetonitrile, THF) were
supplied from Aldrich and used as received. 1-iodo-1H,1H,2H,2H-perfluorododecane (97%)
was purchased at ABCR Co. The poly(vinylidene fluoride) used in this study, PVDF, was
supplied from Arkema under the trade name Kynar 740 (Melt Flow: 1.1 g/10 min at 230°C).
IV.2.2.2 Synthesis of ionic liquids
IV.2.2.2.1 Synthesis of ILs with long alkyl chains
The procedures detailed for synthesizing the imidazolium and phosphonium salts were
identical to the method used by Livi et al. [11]. The synthesis of salts was checked by 13C
NMR spectroscopy collected on a Bruker AC 250 (250 MHz) spectrometer. The assignment
of 1H NMR resonance peaks is reported below.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 163
Imidazolium iodide 1H NMR (CDCl3): δ 0.75-0.90 (m, 6H, 2CH3), 1.15-1.30 (m, 64H,
32 CH2), 1.80-1.90 (m, 2H, NCH2CH2), 4.30 (t, 2H, CH2N=), 7.45 (m, 1H, H arom), 7.65 (m,
1H, H arom), 9.15 (s (b), 1H, H arom).
Phosphonium iodide 1H NMR (CDCl3): δ 0.8-0.90 (m, 3H, CH3); 1.10-1.35 (m, 28,
CH2Me); 1.50-1.70 (m, 4H, PCH2(CH2)2); 3.50-3.70 (m, 2H, PCH2); 7.70-7.90 (m, 15H, H
arom).
IV.2.2.2.2 Synthesis of imidazolium functionalized with perfluorinated chain
1 equiv. of methylimidazole (3g) and 1 equiv. of 1-Iodo-1H,1H,2H,2H-
perfluorododecane (7.3 g) were placed in a 100 mL flask under a positive nitrogen pressure,
and magnetic stirring for 72 h at 120°C in toluene (40mL). A yellow precipitate was formed.
The reaction mixture was then filtered, washed repeatedly with pentane. Most of the solvent
was removed under vacuum. The synthesis of salts was confirmed by 1H NMR. 1H NMR (CDCN): δ 2.79 (CH2-CF2); 3.79 (CH3N-); 4.45 (CH2CH2N-); 7.35 (CH); 7.42
(CH); 8.60 (N-CH=N).
IV.2.2.3 Organic modification
The montmorillonite (2 g, 1.9 meq) was dispersed in 400 mL of deionised water. The
amount of surfactant added was about 2 CEC, based on the cation exchanged capacity (CEC =
95 meq/100 g) of the MMT used [12]. This dispersion was mixed and stirred vigorously at
80°C for 6 h, followed by filtration and continuous washing at 80°C with deionised water
until no iodide ions were detected using an aqueous silver nitrate (AgNO3) solution. The
solvent was removed by evaporation under vacuum. The modified montmorillonite was then
dried for 12 hours at a temperature below 80°C. The following references were used to denote
the different types of modified montmorillonites: MMT-Na+ for the pristine montmorillonite,
MMT-P for phosphonium montmorillonite as octadecyltriphenylphosphonium iodide was
used as interfacial agent. An imidazolium montmorillonite, denoted MMT-I, was obtained as
N-octadecyl-N’-octadecylimidazolium iodide was used as intercalation agent. MMT-IC12F
denotes the montmorillonite exchanged with imidazolium functionalized with the
perfluorinated chain. The chemical structure of synthesized phosphonium and imidazolium salts
are described in Table IV-3.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 164
Table IV-3 – Pristine and ionic liquid-modified montmorillonites (MMT) Trade name
References
Intercalant
Nanofil 757
MMT-Na+
MMT-I
MMT-P
PC18H37
I
MMT-IC12F
N NC18H37
I
C18H37
N N(CH2)2(CF2)9CF3
I
H3C
IV.2.3 Results and discussion
IV.2.3.1 Characterization of ILs exchanged montmorillonites
IV.2.3.1.1 Thermal stability of ionic liquid-modified montmorillonites
The thermal stability of organically modified montmorillonites was characterized by
Thermogravimetric Analysis (TGA). Figure IV-12 displays the evolution of the weight loss as a
function of temperature performed on the three montmorillonites exchanged either with
imidazolium and phosphonium cations functionalized with long alkyl chains or with
imidazolium cation functionalized with perfluorinated chain (C12). The degradation
temperatures of physically adsorbed and intercalated species are summarized in Table IV-4.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 165
Table IV-4 – Degradation temperatures of the physisorbed and intercalated species for the treated montmorillonites (determined from the maxima of the weight loss by temperature derivative)
Sample Physisorbed temperature
(°C)
Intercalated temperature
(°C) MMT-P 330 510 MMT-I 320 420/480
MMT-IC12F 280 460
As reported in a previous study [11], imidazolium-treated and phosphonium-modified
montmorillonites, MMT-I and MMT-P, have an excellent thermal stability, which can be
useful for processing nanocomposites at high temperatures. Indeed, the organic species,
physically adsorbed on the clay surface are decomposed at around 320-330°C, whereas the
thermal degradation of the species ionically exchanged between the clay layers is delayed in a
range between 420 and 510°C. In the case of MMT-IC12F, the use of a perfluorinated chain
associated with the methylimidazole ring does not induce a better thermal stability. On the
opposite, a decrease of the thermal stability is observed with two degradation peaks at about
280°C for the physisorbed species and 460°C for the intercalated species. This phenomenon
could be attributed to the volatilization of short fluorinated chains.
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Universal V4.2E TA Instruments
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Ionic Liquids : Multifunctional agents of the polymer matrices
Page 166
IV.2.3.1.2 Structural analysis of ionic liquid-modified montmorillonites
To evaluate the organic chain organization in the galleries of the lamellar silicates, the
X-ray diffraction analysis was performed on MMT-P, MMT-I and MMT-IC12F and the
corresponding spectra are reported in Figure IV-13.
0 1 2 3 4 5 6 7 8 9 10
0
200
400
600
800
1000
1200
1400
1600
1.7 nm
4.2 nm
Inte
nsity (
u.a
)
2θ
MMT-P
MMT-I
MMT-IC12F
3.7 nm
Figure IV-13 – X-Ray diffraction spectra of ionic liquid-modified MMT:
(a)MMT-P; (b) MMT-I; (c) MMT-IC12F.
It is well known that before any treatment, the basal spacing, d001, measured on the
sodic montmorillonite is 1.2 nm [31]. After cationic exchange in water with the phosphonium
salts, the MMT-P displays a (001) diffraction peak at 2.1°2θ, corresponding to an interlayer
distance of 4.2 nm. This value could be explained by the swelling of layered silicates due to a
larger steric volume associated with the three benzyl rings and the alkyl chain. For the MMT-
I, the diffraction peak situated at 2.4°2θ is related to a distance of 3.7 nm, characteristic of a
full trans-trans conformation of the alkyl chain. On the other side, the montmorillonite
functionalized with one perfluorinated chain, MMT-IC12F shows an interlayer distance of 1.7
nm in agreement with its shorter alkyl chain length.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 167
IV.2.3.1.3 Surface energy of ionic liquid-treated montmorillonites
The contact angles and surface energy determined by the sessile drop method on
pressed clay powder are collected in Table IV-5.
Table IV-5 – Polar and dispersive components of the surface energy on pristine and exchanged montmorillonites from contact angles with water and diiodomethane (determination on pressed MMT powders)
Montmorillonite Θwater (°) ΘCH2I2 (°) γ polar (mN.m-1)
γ dispersive (mN.m-1)
γ total (mN.m-1)
MMT-Na+ 22.9 ±0.9 33.6 ±0.8 30 43 73 MMT-P 88.9 ±0.1 49.4 ±0.6 2 35 37 MMT-I 92.8 ±0.1 55.5 ±0.6 1 31 32
MMT-IC12F 83.6 ±0.2 99.4 ±0.2 14 9 23
After organic treatment in water by the imidazolium and phosphonium ionic liquids
with long alkyl chains, a significant decrease of the surface energy of organically modified
clays is obtained. The modified montmorillonites became more hydrophobic with a surface
energy characteristic of a polyolefin surface. The use of fluorinated chain, even shorter
combined with the imidazolium cation makes the montmorillonite more hydrophobic. In fact,
the surface energy of MMT-IC12F is two times lower than MMT-I and MMT-P ones close to
polytetrafluoroethylene one.
Ionic Liquids : Multifunctional agents of the polymer matrices
Page 168
IV.2.3.2 Effect of interfacial interactions on the material physical properties
The combination of the X-ray diffraction with transmission electron microscopy is
required to determine the material structuration which is tuned from the interfacial
interactions, i.e. the IL chemical nature.
IV.2.3.2.1 On the morphology of the PVDF nanocomposites
• Structural analysis of nanocomposites
WAXS analysis was used to evidence of the intercalation of PVDF chains between
platelets. The diffractograms performed on the PVDF/MMT-P, PVDF/MMT-I, and
PVDF/MMT-IC12F nanocomposites are reported in Figure IV-14.
0 1 2 3 4 5 6 7 8 9 10
0
100
200
300
400
500
600
700
800
900
1000
3.8 nm
Inte
nsity (
u.a
)
2θ
PVDF/MMT-P
PVDF/MMT-I
PVDF/MMT-IC12F
3.8 nm
Figure IV-14 – X-Ray diffraction spectra of nanocomposites: (a)PVDF/MMT-P; (b) PVDF/MMT-I; (c) PVDF/MMT-IC12F.
The X-ray spectra are very different as a function of the chemical nature of IL. Only
the fluorinated IL used for the montmorillonite modification displays a flat pattern which can
be interpreted as a well dispersed morphology, i.e. known as exfoliated morphology. On the
opposite, both ILs based on imidazolium or phosphonium cations with octadecyl chains do
not show additional swelling of nanoplatelets compared to the IL-modified nanoclays. An
intercalated structure seems to be kept.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 169
• Transmission electronic microscopy analysis (TEM) of nanocomposites
The consequence of the chemical nature of the interfacial agents on the resulting
morphology issued from the clay layers distribution has been studied by transmission
electronic microscopy. TEM micrographs of the nanocomposites prepared by melt
intercalation are presented in Figure IV-15.
1 µm 1 µm 1 µm Figure IV-15 – TEM micrographs of (a) PVDF/MMT-I, (b) PVDF/MMT-P, (c) PVDF/MMT-IC12F
TEM images performed on the PVDF/MMT-P and PVDF/MMT-I nanocomposites
reveal that nanoclays remain stacked as tactoids based on few layers homogeneously
distributed within the PVDF matrix. Nevertheless, the swelling of primary particles, i.e.
tactoïds, by the PVDF chains remains, as shown by X-ray diffraction analysis, low in the case
of phosphonium and even non-existing for imidazolium ions. In these two cases, no swelling
with PVDF chains of the tactoïds occurred. ILs, as imidazolium or phosphonium ions
functionalized with long alkyl chains, lead to the nanocomposites having an intercalated
structure.
The modification of montmorillonite with imidazolium cations functionalized with a
fluorinated chain seems to be the most suitable to induce a fine dispersion of clay layers into
the fluorinated polymer matrix. This excellent dispersion is explained by the hydrophobicity
of the modified montmorillonite higher than MMT-I and MMT-P and by a higher affinity of
interfacial agent towards matrix chains. In fact, one can expect a better compatibility of the
fluorinated ligand with the PVDF chains, which is the driving force for the later ones to
diffuse into the galleries in the molten state.
(a) (b) (c)
Ionic Liquids : Multifunctional agents of the polymer matrices
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IV.2.3.2.2 Crystallinity of PVDF based nanocomposites
• Effect of the ionic liquids on the crystallinity
Owing to the numerous crystalline phases of PVDF, an effect of interfacial agent on
the crystallinity of the fluorinated matrix could be expected. As a consequence, in a first step,
ionic liquids were added to the PVDF matrix (1wt%) using the same protocol as for the
nanocomposites. The chemical nature of the cation associated to ionic liquid, imidazolium or
phosphonium plays a significant role on the crystalline microstructure of PVDF as evidenced
by WAXD analysis (Figure IV-16).
10 15 20 25 30
0
1000
2000
3000
4000
5000
6000
2θ
(a) PVDF
0
200
400
600
800
1000
1200
1400
1600
Inte
nsit
y (
u.a
) (b) PVDF-P
0
500
1000
1500
2000
2500
(c) PVDF-I
Figure IV-16 – WAXD patterns of (a) PVDF and PVDF/ionic liquid blends (b) PVDF-P (c) PVDF-I
Chapter IV: Polymer/Layered silicates Nanocomposites
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According to the literature, for the crystallographic structure of the PVDF [29, 32], the
main diffraction peaks corresponding to α-phase are observed at 17.8, 18.4, 19.9, and 25.8°
2θ are attributed to the following reflections (100), (020), (110), and (201), respectively. The
addition of a small amount (1wt%) of imidazolium salt does not modify the X-ray spectrum of
the neat PVDF. Indeed, the diffraction peaks are related to the formation of the α phase. On
the other hand, after addition of 1wt% phosphonium salt in the PVDF matrix, a additional
peak at 20.7° 2θ is detected. This peak corresponding at (110) plane is significant of the
formation of β crystalline form.
The melting temperature (Tm) and the crystallinity ratio (%X) of the PVDF-ionic
liquid blends are reported in Table IV-6.
Table IV-6 – Differential scanning calorimetry analyses of PVDF and PVDF-ionic liquid blends, PVDF-I and PVDF-P (1wt%)
Material ∆Hm (J/g) ∆Hc (J/g) Τm (°C) Τc (°C) Xc (%) PVDF 38 42 165 140 37
PVDF-P 33 36 172 141 32 PVDF -I 38 41 165 141 37
The chemical nature of interfacial agents and the interactions between ionic liquid and
matrix predominate. As previously demonstrated by X-Ray diffraction, as imidazolium ionic
liquid is added to the PVDF matrix, the α form is kept with a melting temperature of 165°C
and the melting enthalpy is similar to the PVDF matrix one. The phosphonium ionic liquid
generates an increase in melting temperature of 7°C and a significant decrease in the
crystallinity ratio of the polymer matrix in agreement with literature reported on modified
clays/PVDF nanocomposites [29]. The higher melting point associated to the β form
compared to α phase one can be due to a higher thermal stability [33] or to a higher perfection
of crystals [34]. In any case, the phosphonium ionic liquid modifies the crystalline form of the
polyvinylidene fluoride acting as a βgene agent as described in the literature [35-36].
Ionic Liquids : Multifunctional agents of the polymer matrices
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Generally, the β form crystal is generated by applying a mechanical strain or an
electrical field, by adding of inorganic fillers, or by using of lithium salts with amounts above
2.5% in membrane applications. Nevertheless, this is the first time that a phosphonium ionic
liquid is used to generate this β form.
• Effect of ionic liquid exchanged montmorillonites on the crystallinity
The effect of the chemical nature of the organic cation of the ionic liquid combined to
the inorganic filler on the crystalline structure of PVDF was also studied. The X-Ray
diffraction analyses of nanocomposites filled with IL-modified montmorillonites are given in
Figure IV-17.
10 15 20 25 30
0100020003000400050006000
2θ
PVDF
0500
10001500200025003000
PVDF/MMT
0
500
1000
1500
2000
Inte
nsit
y (
u.a
)
PVDF/MMT-P
0500
10001500200025003000
PVDF/MMT-I
0
500
1000
1500
2000 PVDF/MMT-IC12F
Figure IV-17 – WAXD patterns of PVDF/IL-modified nanoclay (1wt%) nanocomposites
(a) PVDF/MMT-IC12F, (b) PVDF/MMT-I, (c) PVDF/MMT-P, (d) PVDF/MMT, (e) PVDF
Chapter IV: Polymer/Layered silicates Nanocomposites
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The addition of untreated montmorillonite does not change the neat matrix WAXS
pattern, i.e. the presence of pristine MMT does not disturb the crystalline microstructure,
based on α phase. According to the results reported previously for PVDF/LI blends, the
WAXS pattern of PVDF/MMT-I is similar to the PVDF one, i.e. no change in crystalline
form is induced by MMT-I. However, as the imidazolium cation is functionalized with a
fluorinated chain, a change in the crystalline form of PVDF is observed, i.e. the use of a
fluorinated imidazolium ionic liquid promotes the formation of the β phase. On the X-ray
diffraction pattern of phosphonium treated montmorillonite/PVDF nanocomposites, the
diffraction peak at 20.7° attributed to the β phase is also present. As modified clays are not
washed, physically adsorbed ionic liquid remains on the external montmorillonite platelet
surface, and could induce the formation of β-form crystal.
To confirm the effects related to phosphonium and imidazolium treated
montmorillonites on the formation of α and β phases, X-ray diffraction analysis and
differential scanning calorimetry (DSC) analysis on the nanocomposites were carried out
(Table IV-7).
Table IV-7 – Differential scanning calorimetry analysis for PVDF and nanocomposites denoted PVDF/MMT-I, PVDF/MMT-P, PVDF/MMT-IC12F (1wt%)
Material ∆Hm (J/g) ∆Hc (J/g) Τm (°C) Τc (°C) Xc (%) PVDF 38 42 165 140 37
PVDF/MMT-P 35 36 172 150 34 PVDF/MMT-I 39 41 166 148 38
PVDF/MMT-IC12F 32 33 174 149 31
These DSC analyses are in agreement with X-ray diffraction results since the use of
ionic liquids based on phosphonium or imidazolium cations functionalized by a fluorinated
chain as interfacial agents leads to lower crystallinity ratios and higher melting temperatures
associated to the formation of β phase. This result could provide a new perspective on the use
of ionic liquids in the field of membranes as β crystalline form contribute to better dielectrical
and thermal properties. Moreover, additional improvement could be achieved due to the
presence of clay nanoplatelets.
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IV.2.3.2.3 Mechanical properties of PVDF based nanocomposites
In addition to the characterization of the morphology and resulting crystallinity of
nanomaterials based on IL-modified silicate nanolayers, the mechanical properties were
analyzed by performing uniaxial tensile tests described in Table IV-8.
Table IV-8 – Tensile properties of the ionic liquid/ poly(vinylidene fluoride) blends and ionic liquid modified montmorillonites- poly(vinylidene fluoride) nanocomposites at room temperature (10 mm.min-1)
Sample Young’s modulus (MPa)
Strain at break (%)
PVDF 940 36 PVDF-P PVDF-I
760 700
30 37
PVDF-MMT 900 30 PVDF/MMT-P 750 32 PVDF/MMT-I 800
80
PVDF/MMT-I C12F 800 250
The values of the moduli and strain at break are not governed by the nature of the
crystalline forms of PVDF, i.e. α or β. In fact, the imidazolium cation has the same
plasticizing effect on the mechanical properties as it is functionalized with alkyl chains that
promotes the α form or with a fluorinated chain that promotes the β form. The phosphonium
cation that generates the β form has the same plasticizing effect and does not improve the
mechanical behavior of PVDF. The mechanical properties are mainly dependent on the
material morphology. The stiffness is less reduced and the failure properties are slightly
enhanced for the nanocomposite showing an intercalated morphology obtained with the ionic
liquids based on long alkyl chains imidazolium cation. The excellent compromise obtained
between stiffness and strength at break is achieved on the nanocomposites containing the
montmorillonite modified with perfluoroalkylimidazolium, i.e MMT-IC12F. As a
consequence, the exfoliated structure is the right morphology to get the best mechanical
behaviour.
Chapter IV: Polymer/Layered silicates Nanocomposites
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IV.2.4 Conclusions
In this work, phosphonium and imidazolium ionic liquids were used as interfacial
agents to enhance the physico-chemical interactions within the matrix and to help the lamellar
silicates intercalation in the polymer matrix. The compatibility brought by the cation is the
key parameter that governs the interactions and generates an exfoliated morphology
responsible for the excellent compromise between stiffness and failure behaviour. Thus, a
better dispersion of clay nanolayers and a plasticizing effect are observed with the use of
fluorinated imidazolium ionic liquid due to the higher compatibility of the fluorinated ligand
with the PVDF matrix. Moreover, the chemical nature of organic cation has a strong effect on
the crystalline form of PVDF. In fact, a small amount (1 wt%) of phosphonium and
fluorinated chain-functionalized imidazolium salts promotes the formation of the β phase and
opens new applications in the field of membranes for fuel cells.
Ionic Liquids : Multifunctional agents of the polymer matrices
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Conclusions of chapter IV In this chapter, many imidazolium and phosphonium ionic liquids have been
synthesized and used as modifiers agents of layered silicates.
In the first part, we have demonstrated that the use of montmorillonites modified with
imidazolim and phosphonium IL enables improved thermal stability and mechanical
properties with a good compromise between rigidity and failure strain of nanocomposites
PE/MMT.
We have also investigated the compatibility formed by the nature of the organic cation
is the main parameter controlling the physico-chemical interactions within the matrix and
contributes to a better distribution of clays in the polymer matrix. Thus, we have observed that
the addition of only 1 wt% of MMT treated with imidazolium functionalized by
perfluorinated chain leads to exfoliation of clay layers in the polymer resulting in a
plastification of the matrix. In addition, we have also demonstrated that the interfacial
chemical agents, especially fluorinated imidazolium and phosphonium ionic liquids favor the
formation of β form and open new perspectives in the field of membranes for fuell cells.
Chapter IV: Polymer/Layered silicates Nanocomposites
Page 177
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Conclusion générale
Page 179
CONCLUSION GENERALE L'objectif premier de ce travail a été de valoriser et de mettre en avant les effets
bénéfiques et les différentes possibilités proposées par les liquides ioniques dans le domaine
des polymères que ce soit en tant qu'agents renforçants, plastifiants ou encore agents
interfaciaux pour les silicates lamellaires.
Dans un premier temps, nous avons mis en évidence pour la première fois la
structuration des liquides ioniques dans une matrice polymère. Nous avons ainsi démontré que
la nature chimique du cation organique: pyridinium, imidazolium ou phosphonium ainsi que
l'effet de l'anion jouent un rôle essentiel sur les différentes structurations obtenues. En effet,
nous avons observé des morphologies différentes allant de la formation d'agrégats de clusters
ioniques en ce qui concerne l'utilisation du liquide ionique pyridinium (C18Py) à une
excellente dispersion à l'échelle du nanomètre dans le cas du phosphonium (C18P) en passant
par une morphologie co-continue pour l'imidazolium (C18C18Im). Les relations
morphologies/propriétés physiques et mécaniques ont également été établies. Ainsi, nous
avons constaté que le liquide ionique phosphonium le mieux dispersé dans la matrice fluorée
mène à une nette amélioration des propriétés mécaniques du matériau avec une augmentation
du module et de la déformation à la rupture de +160 et +190% respectivement. Les analyses
SAXS et MET ont contribué à une meilleure connaissance de la structuration du matériau
sous sollicitation.
Dans un second temps, l'utilisation des liquides ioniques comme agents intercalants en
remplacement des ammoniums conventionnels a également été discuté. Nous avons ainsi
démontré une meilleure stabilité thermique des argiles modifiées par les liquides ioniques ce
qui permet d'élargir le champ d'utilisation des silicates lamellaires dans le domaine des
nanocomposites thermoplastiques/argiles nécessitant des températures de mise en oeuvre plus
élevées. Nous avons aussi développé un procédé propre de modification des surfaces des
silicates lamellaires basé sur la combinaison CO2 supercritique-eau-liquide ionique avec pour
résultats une amélioration de la stabilité thermique des argiles organiquement modifiées par
les liquides ioniques.
Ionic Liquids : Multifunctional agents of the polymer matrices
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Dans une dernière partie, nous avons montré les effets de ces charges organiquement
modifiées sur les propriétés thermiques et mécaniques de nanocomposites préparés par
intercalation à l'état fondu. Ainsi, nous avons observé que l'influence de la longueur des
chaînes et de l'anion joue un rôle crucial sur la stabilité thermique intrinsèque des liquides
ioniques ainsi que sur les propriétés thermiques et mécaniques des polymères, notamment
dans le cas d'une polyoléfine (PEhd). Ensuite dans une matrice fluorée, le polyfluorure de
vinylidène (PVDF), nous avons démontré que l'utilisation d'un imidazolium fonctionalisé par
une chaîne perfluorée a un effet similaire à celui du cation phosphonium sur la structure
polymorphe du PVDF c'est à dire que leur utilisation en très faible quantité engendre la
formation de la phase β, favorable aux propriétés diélectriques ce qui offre de nouvelles
perspectives dans le domaine de l'énergie et en particulier dans celui des membranes de piles
à combustible. Une exfoliation des feuillets d'argiles dans la matrice polymère ainsi qu'une
augmentation de la déformation à la rupture est également obtenue dans le cas de la
montmorillonite modifiée par l'imidazolium fluoré.
Néanmoins, le coût et l'accessibilité aux liquides ioniques désirés limitent
considérablement leur utilisation dans les polymères. C'est pour ces raisons qu'il est
nécessaire d'intensifier les recherches sur les liquides ioniques afin d'apporter de la
compréhension et de prouver que les avantages des liquides ioniques sont beaucoup plus
importants que leurs inconvénients. Ce travail n'est qu'un aperçu du vrai potentiel des liquides
ioniques en science des polymères. En effet, les différentes combinaisons cations/anions ainsi
que les différentes fonctionnalisations possibles nous laissent penser qu'il est concevable en
théorie de synthétiser une infinité de liquides ioniques à propriétés spécifiques en fonction de
la matrice sélectionnée.
FOLIO ADMINISTRATIF
THESE SOUTENUE DEVANT L'INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE
LYON
NOM : LIVI DATE de SOUTENANCE : … … 2010 Prénoms : Sébastien TITRE :
IONIC LIQUIDS : MULTIFUNCTIONAL AGENTS OF THE POLYMER MATRICES NATURE : Doctorat Numéro d'ordre : 2010-ISAL- Ecole doctorale : Matériaux de Lyon Spécialité : Matériaux Polymères et Composites Cote B.I.U. - Lyon : T 50/210/19 / et bis CLASSE : RESUME : Une excellente stabilité thermique, une faible pression de vapeur saturante, une ininflammabilité, une bonne conductivité ionique ainsi que les différentes combinaisons cations/anions possibles font des liquides ioniques l'objet d'un engouement grandissant de la Recherche. De part ces avantages, les LI se présentent comme une nouvelle voie dans le domaine des polymères, et en particulier dans le milieu des nanocomposites où leur utilisation est essentiellement limitée à la fonction de surfactant des silicates lamellaires. Néanmoins, avant de pouvoir prétendre à un statut d'alternative, il est nécessaire de mettre en évidence les effets bénéfiques de leur utilisation sur les propriétés finales des matériaux polymères. Dans un premier temps, l’objectif de ce travail a été de synthétiser des liquides ioniques différents par la nature de leur cation et anion mais tous porteurs de longues chaînes alkyles afin de permettre une meilleure compatibilité avec la matrice. Ensuite, les excellentes propriétés intrinsèques des liquides ioniques ont motivé leur utilisation comme agents structurants dans une dispersion aqueuse fluorée. Ainsi, leur rôle d’agents ioniques sur la morphologie, les propriétés physiques, thermiques et mécaniques a été étudié. Dans une seconde partie, les liquides ioniques ont été utilisés comme agents intercalants des silicates lamellaires puis confrontés aux surfactants conventionnels dans le but de préparer des argiles thermiquement stables pour la préparation de nanocomposites thermoplastiques/argiles. Dans une dernière partie, une faible quantité de ces argiles organiquement modifiées ont été introduites par intercalation à l'état fondu dans deux matrices différentes afin de mettre en évidence les effets de ces nouveaux agents interfaciaux sur les propriétés finales du matériau. MOTS-CLES : Liquides ioniques ; Nanocomposites ; Silicates lamellaires ; Agents structurants ; CO2 supercritique Laboratoire (s) de recherche : Institut des Matériaux Polymères / Laboratoire des Matériaux Macromoléculaires UMR 5223 INSA de Lyon Directeurs de thèse: Jannick DUCHET-RUMEAU – Jean- François GERARD Président de jury : … Composition du jury : DUCHET-RUMEAU Jannick Professeur (INSA Lyon) – Directrice de thèse GALY Jocelyne DR CNRS (INSA Lyon) – Examinateur GANTILLON Barbara Dr (Société TEFAL) – Examinateur GERARD Jean-François Professeur (INSA Lyon) – Co-directeur de thèse PHAM Thi Nhàn Maître de Conférences (Université de Caen) – Examinateur PLUMMER John Christopher Professeur (Ecole Polytechnique Fédérale de Lausanne) – Rapporteur SEGUELA Roland DR CNRS (Université de Lille 1) – Rapporteur