Average molar masses

With very few exceptions, macromolecular substances exhibit considerable heterogeneity in size and mass. We are no longer dealing with well-defined molecular species, as in classical chemistry, but with macromolecules distinguished by their degree of polymerization, the number of elementary units linked together. The polymer sample is polymolecular. In the simplest case of a linear or branched synthetic homopolymer, the diversity in macromolecule size is explained by the kinetic behavior of the polymerization reaction and, in particular, by the importance of chain transfer, termination, grafting and possibly degradation reactions. If branching leads to a three-dimensional network (cross-linking), the concept of degree of polymerization and molar mass becomes indefinite.

This polymolecularity is partly responsible for the differences in physical and mechanical properties observed in samples of the same macromolecular substance, but prepared differently. To characterize a polymer sample, in addition to knowing the average values for degree of polymerization and molar mass, you need to assess the homogeneity of the substance in question, i.e. determine the molar mass distribution curve of the various constituent macromolecules. Obtaining this curve requires :

• separation of the sample into successive fractions of similar molar mass (fractionation);

• determination of the molar mass of each fraction.

The main methods for measuring polymer molar masses have not changed fundamentally over the last two decades. On the other hand, no new concepts have emerged in the field of polymer characterization to define alternative ways of visualizing mass distribution. It's just that advances in technology have, to varying degrees, affected existing characterization methods, making them more or less effective and popular.

A more recent and much more efficient technique allows, in addition to the calculation of all average molar masses, the determination of the molar mass distribution curve. This is the gel permeation chromatography (GPC) or steric exclusion chromatography (SEC) technique 4 . This...