Determination of Molecular Weight Distributions of Diene-Containing Rubbers by Precipitation Chromatography

Abstract

Ten variables were studied to optimize fractionation of synthetic elastomers by the Baker—Williams column technique. The two most critical factors were the rate of deposition of the polymer onto the beads within the column and stabilization of polydienes against crosslinking and degradation. After optimizing the fractionation technique certain fractionation and molecular weight distribution problems were studied. A problem frequently encountered in column fractionations is reversion, i.e., a drop in viscosity near the end of the fractionation. Reversion is attributable to branching, but polymer degradation and crosslinking can also contribute. The technique described can be used to detect long chain branching in elastomers. An important characteristic of emulsion polymers is the width of the molecular weight distribution. The fractionations showed that the heterogeneity index, Mv/Mn, of SBR increased with the magnitude of the regulating index of the polymerization modifier used. The most efficient modifier on a weight basis did not necessarily give the smallest heterogeneity index, as is predicted by theory. The heterogeneity index of a polymer prepared by incremental addition of mercaptan modifier was smaller than that of the control. The fractionation results were of value in understanding the behavior of modifiers in emulsion systems. In solution polymers an initiator with a large propagation/initiation rate ratio, or the use of branching or coupling agents, broadened the molecular weight distribution. The use of the Baker—Williams column on a routine basis has been valuable in the preparation of several new commercially accepted elastomers.