Characterization of Molecular Weight and Molecular Weight Distribution
The value of the viscosity-average molecular weight and its relation to the other modes vary with the polymer-solvent interaction forces in the particular measurement in which it is determined. The significance of the absolute and relative values of the various average molecular weights must be viewed in relation to polymer properties. The separation and determination steps may be carried out independently, or be combined, as in a chromatographic process with suitable calibration such as gel permeation chromatography or in an ultracentrifuge. A serious limitation of membrane osmometry is the diffusion of low-molecular-weight species through the membrane. Gel permeation chromatography separates molecules according to their size, not mass, but the technique is used primarily as an approach to measuring the molecular weight distribution. Polymer characterization relies heavily on solution properties, and certain applications involve solutions. For characterization of molecular weight, molecular weight distribution, long-chain branching, and compositional distribution, solution measurements and fractionation procedures are the methods of choice.