Rheo-optical studies of the response of entangled polymer solutions to step changes in shear rate

Abstract

Experimental birefringence data are reported for an entangled 4.86 wt % polystyrene solution involving a narrow molecular weight distribution, 8.42×106 g/mol polymer in tricresyl phosphate. The average number of entanglements per chain was estimated to be approximately 22. The flows considered involve step changes in shear rate from one steady value to another (including the case in which the two shear rates have the same sign and that in which the sign changes and the flow direction is reversed). Comparisons were also made for these flows with data obtained for the same polymer/solvent combination, but at 3.00 wt % where the estimated number of entanglements per chain was approximately 10. The range of shear rates was chosen to span Weissenberg numbers based on the longest Rouse time of O(1) and thus provide data that can be used to explore the effects of modest chain stretching on the dynamics. Although the long-time behavior was found to be qualitatively similar to a startup flow, the short-time dynami...