The Importance of Non-Universalities in Entangled Polymer Melts During the Startup of Steady Shear Flow

The importance of non-universality in inception of shear flow at large strain rates is examined using the discrete slip-link model (DSM). An expression for the Rouse relaxation time, τ_R, as a function of entanglement activity and number of Kuhn steps is found from a master curve of strain maxima. Unlike tube models τ_R is predicted by the DSM instead of added as an adjustable parameter. We find that DSM predicts only a weak dependence of τ_R on chemistry. DSM predictions are then compared to a large set of experimental data for melts and solutions, where agreement is found over three decades of Rouse-Weissenberg number, Wi_R. In contradiction with conclusions based on tube models, we find no significant non-universality in the scaling of either strain maxima or stress overshoot. Finally, our results show that the overshoot stress and strain scale as Wi_R^0.33, while the undershoot and steady-state stresses scale as Wi_R^0.1.