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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

Characterization of Polymers

Molecular weight

Prop

ertie

s

Molecular weight distribution 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

Perc

enta

ge o

f m

olec

ules

Number average, Mn

Viscosity average

Weight average

Typical distribution of molar masses for a synthetic polymer

Determination of molecular weight 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)

End group analysis 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

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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

Colligative properties: Osmotic pressure

π = Osmotic pressurec = Solute concentration g/cm3R = Gas constantT = Temperature

Gel permeation/size exclusion chromatography (GPC)

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

Gel permeation cont… 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/ timeAmou

nt o

f pol

ymer

el

uted

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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

Polymer Characterization Techniques

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◦ 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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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.