Dynamics of linear, entangled polymeric liquids in shear flows

Abstract

We study predictions in transient and steady shearing flows of a previously proposed self-consistent reptation model, which includes chain stretching, chain-length fluctuations, segment connectivity and constraint release (CR). In an earlier paper, it was established that the model is able to capture all trends observed experimentally for viscometric flows allowing focus of the present work on the model. That is, we study in detail the physics and underlying dynamics of the model to explain the macroscopically observed rheological properties in terms of chain behavior and dynamics on the molecular level. More specifically, we discuss the effects of chain tumbling, molecular chain stretching and CR and their influence on the macroscopic stress as well as the extinction angle under various flow conditions. In particular, we find that chain tumbling causes the undershoot in extinction angle during inception of shear; chain tumbling is itself suppressed by the presence of molecular stretching; and the anticipated strong correlation between normal stress and molecular stretching is confirmed; following cessation of steady shear, it is observed that chain stretching undershoots, and a mechanism involving constraint release is suggested to explain the phenomenon; and from stresses following cessation, a previously proposed technique for estimating stretching during steady shear flows—involving a generalized damping function—is shown to be inaccurate. Also investigated is the monomer density along the chain contour which reveals information about the local chain stretching and orientation. Here, it is found that the distribution of monomers along the contour becomes non-uniform when the shear rate exceeds the inverse Rouse relaxation time. Finally, we discuss a possible violation of the stress-optic rule during start-up of steady shear flow at high shear rates.