Post on 08-Oct-2015
description
17/10/2014
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19. Spectroscopy & Characterization
of Polymers
Chain microstructure
How do you know if a vinyl polymer is isotactic, atactic etc.?
Composition?
Sequence distribution, MW distribution in copolymer
Spectroscopic methods
Molecular spectroscopy
IR NMR others
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Spectroscopy Fundamentals
General aspects of spectroscopy
Interaction of focussed EM radiation with the sample surface
Reflection, transmission, absorption and/or scattering (change in energy,
frequency)
Absorption occurs when energy of light corresponds to the atomic states of
the sample i.e. Light is only absorbed if its energy corresponds to the
energy difference between two quantum states of the sample
The speed of light, c (cm/sec), equals the
wavelength (cm), times the frequency,
hfEEE 12
cc ,
Spectroscopy Fundamentals
Light can also be described in terms of circular motion, the time period
required to go one revolution (2 radians) is
is angular frequency in radians/sec. The period of a full revolution would
equal the reciprocal of frequency, so
2period
22
2
or
period
*As frequency is no. of
cycles/unit time
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Spectroscopy Fundamentals
Spectroscopy Fundamentals
IR spectroscopy detects the molecular vibrations by the absorption of
infrared light or by the inelastic scattering of light by a molecule. It
characterizes the electromagnetic waves in terms of wavenumber which
is defined as the reciprocal of wavelength with unit cm1.
The photon energy can also be expressed in terms of wavenumber as,
c
,1
hcE
hE ph
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Origin of Molecular Vibrations:
Simple Harmonic Motion
Molecules in solids are always in vibration at their equilibrium positions and
these vibrations can be simply modelled as massless springs connecting
nuclei in a molecule.
Consider a diatomic molecule vibration by stretching or compressing the
bond (a massless spring) between two nuclei.
The vibrational energy can be calculated using
the spring model. The force F due to linear
elastic deformation of the bond is proportional
to displacement x of two nuclei from its equilibrium
distance re.
Where K is stiffness of the spring or force constant.
)1(2
2
dt
xdmmakxF
Simple Harmonic Motion
After solving the differential equation for frequency,
After substitution in eq. 1, we get
For sinusoidal function, the ois the circular frequency in radians/sec and
can be termed as the natural frequency of the oscillator.
To change the frequency into cycles/sec
m
ko
xtdt
xd
tx
ooo
o
22
2
2
cos
cos
m
ko
2
1
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Simple Harmonic Motion
Here, o is directly proportional to the stiffness of the bond, and inversely
proportional to the mass of atoms, therefore a C-C covalent bond
stretching vibration would have lower frequency than a C-H vibration for
the same stiffness of the bond.
For a simple harmonic vibration after interaction with light
The potential energy of the vibration for small
displacements
This is called simple harmonic motion if there is no
energy loss by the system.
)( errkkxF
22 )(2
1
2
1evib rrkkxE
Simple Harmonic Motion
Potential energy (E) of diatomic molecule vibration is quantized
Potential energy of harmonic motion is a parabolic function of displacement
from the equilibrium position of nuclei.
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IR Spectroscopy
Infrared spectroscopy is based on the phenomenon of infrared absorption
by molecular vibrations.
When a molecule is irradiated by electromagnetic waves within the infrared
frequency range, one particular frequency may match the natural
vibrational frequency of the molecule (vib).
Consequently, the molecular vibration will be excited by waves with the
frequency ph=vib
Ground
state
Excited/high
E state
IR Spectroscopy
Energies associated with vibrations of atoms in a molecule with respect to
one another are quantized and absorption of IR radiation give rise to
transitions between these different states.
Where is the vibration quantum number with vibration levels (0, 1,2,3,).
vib is the vibrational frequency of a molecule. The ground state
corresponds to = 0.
Since vibrating atoms are linked by bonds, it is usual to refer to the
vibrations as bond deformations such as stretching and bending.
The vibrational frequency of molecules is in the range of middle infrared
(IR) frequencies (61012 1.21014 Hz). Commonly, the vibrational
spectroscopy covers a wavenumber range from 200 to 4000 cm1
)2
1( vibvib hE
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IR Spectroscopy
Infrared modes of Polymers
The basis of the IR spectroscopy is that for a particular type of bond
deformation E and hence , depends on the type of bond involved. (E
and arte higher for N-H than C-H stretching)
Resonance occurs when the frequency of the incident light corresponds to
one of the natural frequencies of vibration of a material.
X-H stretching region (4000-2500 cm-1)
Triple-bond region (2500-2000 cm-1)
Double-bond region (2000-1500 cm-1)
Fingerprint region (1500-600 cm-1)
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Typical IR spectrum
Finger print region
Infrared modes of Polymers
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Instrumentation
IR Instrumentation
Radiation source/ IR Lamp
Focusing optics
Dispersing elements, prism, gratings
Slit
Most modem instruments use interferometers that don't need slits
A range of frequencies/IR wavelengths is incident onto your sample,
separate the light according to wavelength and measure how much light
has been absorbed at each frequency (by comparing intensities to light
from the source that hasn't passed through the sample i.e. base line
reference).
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IR Instrumentation
IR, Io I,
transmitted
1. Reference/background
run
Io I
1. Sample run