DETERMINING THE MOLECULAR-WEIGHT DISTRIBUTION FROM THE STRESS-RELAXATION PROPERTIES OF A MELT

The molecular weight distributions of well-characterized polystyrenes were determined from their stress relaxation modulus as a function of time G(t) Linear viscoelastic measurements were made in the “terminal zone”. The weight fraction of chains as a function of molecular weight was determined by assuming that the shorter (relaxed) chains, which are the first to disentangle from the transient network of entanglements, act as diluent for the longer (unrelaxed) chains. Nearly monodisperse samples were found to have false bimodal distributions although the breadth of these distributions was clearly distinguishable from the samples with broader molecular weight distributions. The distributions determined for a broader distribution sample or ones with truly bimodal distributions were closer to the shapes determined by size exclusion chromatography. The unexpected results for the narrow distribution samples were attributed to the oversimplified dilution assumption stated above. © 1990, The Society of Rheology. All rights reserved.