October 11, 2013

Molecular weight distribution from rheology

The molecular weight distribution of polymers strongly influence their properties, such as tensile strength, crack resistance, and solubility. Gel permeation chromatography is a commonly used technique to measure molecular weight distribution, but relies on the ability of the polymers to be dissolved in a solvent that is readily usable in a GPC column. Additionally, the molecular weight distribution is inferred from polymer standards, which should have a similar, if not identical, repeat unit to the unknown sample for the most reliable results.

Melt rheometry can be be used to determine molecular weight distributions, using the fact that the viscosity of a polymer is strongly dependent on the molecular weight and distribution. For instance, at low shear rates, the zero shear viscosity of a polymer melt scales with the weight average molecular weight to the 3.4 power. Similarly, the dynamic viscoelastic properties of polymer melts can be used to determine the overall molecular weight distribution. To do so, a frequency sweep is conducted on the sample, providing the G' and G" as a function of frequency. Using theoretical models by Mead or Thimm, the molecular weight distribution can then be determined, assuming that the model parameters are known for the polymer in question.

Contact CPG for more information on this technique.