Polymer Characterization - Molecular Mass

Molecular Mass

The molecular mass of a polymer differs from typical molecules, in that polymerization reactions produce a distribution of molecular weights and shapes. The distribution of molecular masses can be summarized by the number average molecular weight, weight average molecular weight, and polydispersity. Some of the most common methods for determining these parameters are colligative property measurements, light scattering techniques, viscometry, and size exclusion chromatography.

Gel permeation chromatography, a type of size exclusion chromatography, is an especially useful technique used to directly determine the molecular weight distribution parameters based off of the polymer's hydrodynamic volume. Gel permeation chromatography is often used in combination with Low-angle laser light scattering (LALLS) and or viscometry can be used to determine the molecular weight distribution as well as the degree of long chain branching of a polymer, provided a suitable solvent can be found.

Molar mass determination of copolymers is a much more complicated procedure. The complications arise from the effect of solvent on the homopolymers and how this can affect the copolymer morphology. Analysis of copolymers typically requires multiple characterization methods. For instance, copolymers with short chain branching such as linear low-density polyethylene (a copolymer of ethylene and a higher alkene such as hexene or octene) require the use of Analytical Temperature Rising Elution Fractionation (ATREF) techniques. These techniques can reveal how the short chain branches are distributed over the various molecular weights.