Rheo-optical experiments and modeling studies of entangled polymer solutions in steady and transient shear flows

The goal of this work is to understand the rheology of entangled polymer solutions when subjected to nonlinear viscoelastic deformations characteristic of polymer processing applications. Experimentally, we consider two narrow molecular weight distribution entangled polymer solutions that vary only by their concentration (or number of entanglements per chain). These solutions are subjected to steady and transient shear flows, including startup from rest, double-step shear rate, reversing step shear rate, and relaxation. Data for the average orientation angle and birefringence of these materials are obtained using a phase-modulated flow birefringence apparatus. The results provide information about the degree of orientation and stretch induced by the flow. Of particular interest are the double-step shear rate results, where we observe short time scale over- and undershoots in the orientation angle when the shear rate is increased or decreased, a feature we ascribe to transient chain stretching.; We compare experimental results with predictions generated by the Doi-Edwards-Marrucci-Grizzuti (DEMG) reptation model. We also performed calculations of the model with convective constraint release and contour length fluctuation mechanisms added. These ideas have been suggested as a means of reducing the effective disengagement time and enhancing the rate of orientation relaxation. Surprisingly, the basic Doi-Edwards (DE) model without explicit chain stretching is able to qualitatively capture over- and undershoots in the orientation angle in double-step shear rate flows. When both CCR and CLF are included the model, steady state and transient orientation angle predictions are improved relative to the experimental data; however, the magnitude of the predicted birefringence is shifted to lower values as a result of the reduced orientational anisotropy, suggesting that additional physics, such as tube deformation, might be relevant to inducing residual stress for Weissenberg number based on the longest Rouse time less than O(1).