1. Average molecular weights
 Determination of molecular weights is very important because as
the chain length is the controlling factor in determining solubility,
elasticity, processability and mechanical properties
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Molecular weight
Properties

2.  Synthetic polymers do not consist of identical
molecules
 It contains mixture of different size molecules (i.e.
different molecular weights), resulting in molecular
weight distribution
Molecular weight
Percentageof
molecules
Number average, Mn
Viscosity average
Weight average
Typical distribution of molar masses for a synthetic polymer

3.  The molecular weight of polymers can be
determined by a number of physical and chemical
methods
◦ End group analysis
◦ Measurement of colligative properties
◦ Light scattering
◦ Dilute solution viscosity
◦ Gel permeation chromatography (GPC)

4.  Determination of number average molecular weight (Mn)
 Analysis of functional groups in polymers, particularly of
those incorporated at chain ends
 Method of analysis
◦ Titrimetric method
◦ Spectrophotometric method
 Limitations/disadvantageous
◦ This can only be used when a polymer has an end gropup
◦ Less sensitive for high molecular weight polymers.
◦ Mn < 25,000

5.  Colligative property: one that is a function of the number
of solute molecules present per unit volume of solution
 Measures the number average molecular weight (Mn)
 Osmotic pressure:
◦ Depend on the number of molecules and not depend on the size
of the molecules
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π = Osmotic pressure
c = Solute concentration g/cm3
R = Gas constant
T = Temperature

6.  Prominent and widely used method for estimating molecular weight
of a polymer (i.e. Measure of molecular weight distribution)
 The separation takes place in a chromatographic column filled with
beads of rigid porous “gel”
 The pores gel contains distribution of different pore sizes and acts
as a sieve for separation polymer molecules based on molecular
size.
 A sample of dilute polymer solution is introduced into the column
 Polymer molecules diffuse into internal pores of the gel depending
on the size the molecules

7.  The larger molecules spend less time and therefore flow faster
through column.
 Different molecules are eluted from the column according to the
molecular weight, the largest emerging first
A typical gel permeation chromatogram
Retention volume/ time
Amountofpolymer
eluted

8.  Spectroscopic methods
◦ To identify the type of polymer through identifying the
functional groups
 Thermal analysis
◦ Differential scanning calorimeter (DSC);
 Measures the difference in heat flow rate between a sample
and inert reference as a function of time and temperature
 DSC measures the Tg, melting and crystallization of polymers
Thermogravimetic analysis (TGA); to measure the
composition of polymers
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9. ◦ Thermogravimetic analysis (TGA); to measure the
composition of polymers
 Measures the change in the weight of sample as a function of
temperature or time in a controlled atmosphere
 This technique characterizes polymers that exhibit weight loss or
gain due to decomposition, oxidation and dehydration
◦ Applications of TGA
 Composition of a composite material
 Thermal stability of polymer
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10.  Microscopy
◦ Optical microscopy
◦ Electron microscopy
 Improved resolution compared to optical microscopy
 This can observe the morphology of
polymer/composite
 X-ray diffraction
◦ Measures the orderly arrangement of atoms and
molecules of a material.
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