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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