Measure of Polymer Molecular Weight and Size

Polymer molecular weight can be determined by chemical or physical methods of functional group analysis by measurement of the colligative properties, light scattering, or ultracentrifugation, or by measurement of dilute solution viscosity. All this methods, except the last are, in principle, absoulute: Molecular weight can be calculated without reference to calibration by another method. Dilute of solution viscosity, however, is not a direct measure of molecular weight. Its value lies simplicity of the technique and the fact that it can be related empirically to molecular weight for many systems.

With the exception of some types of end group analysis, all molecular weight method required solubility of the polymer, and all involve extrapolation to infinite dilution or operation in a certain solvent in which ideal solution behavior is attained. The term molar mass is preferred by some authors.

End Group Analysis
Molecular weight determination through group analysis required that the polymer contain a known number of determinable group per molecule. The long chain nature of polymers limits such groups to end groups. Thus the method is called end-group analysis. Since method of end group analysis count the number of molecules in given weight of sample. they yield the number average molecular weight for polydisperse materials. The methods become insensitive at high molecular weight, as the fraction of end groups become too small to be measured with precision. Loss of percision often occurs at molecular weight above 25,000 the limitation being as much o more due to the inability to purify sample and reagents as to lack of sensitivity in the method.