Examination of Nonuniversalities in Entangled Polymer Melts during the Start-Up of Steady Shear Flow

The possibility of nonuniversality in the nonlinear rheology of polymer melts during the inception of shear flow at large strain rates has recently been questioned, and hence it is examined here using the discrete slip-link model (DSM). An expression for the Rouse relaxation time, tau(R), as a function of entanglement activity and number of Kuhn steps is found from a master curve of strain maxima, as predicted by the theory. In contrast to tube theories, this expression is then shown to collapse all entangled polymer solution and melt data to universal behavior for the maximum shear stress, tau(max)(xy), and the strain at maximum stress. The transition of these quantities from strain-rate-free values to values that scale with dimensionless strain rate as similar to(tau(R)(gamma) over dot)(0.33) is shown to correspond to primitive path stretching. Furthermore, the scaling exponents for melt (0.1-0.15) and solution (0.2-0.3) data do not show the same scaling for steady-state shear stress, tau(ss)(xy), but the melts are in agreement with DSM (0.1). There is a small amount of data for the scaling of stress at undershoot, tau(us)(xy), and strain at undershoot, which are predicted to scale as 0.1 and 0.33 for DSM, in agreement for melt data. Of the other comparisons made here with data, only the melt theory of Xie and Schweizer (XS) does almost as well in predicting the scalings. However, the magnitudes for tau(max)(xy), tau(ss)(xy), and tau(us)(xy) predicted by XS are about a factor of 2 greater compared to experiments. Moreover, XS overpredicts the steady-state shear scaling (0.3, for tau(R)(gamma)over dot> 6), which is also predicted by some coarse-grained molecular simulations and closer to what is observed in solutions. Finally, we find that DSM predicts only a very weak dependence of Rouse time on chemistry.

Automated summary

The study using the discrete slip-link model (DSM) reveals nonuniversality in the nonlinear rheology of polymer melts at large strain rates during the inception of shear flow, despite the theory predicting universal behavior for maximum shear stress and strain at maximum stress based on entanglement activity and Kuhn steps. Primitive path stretching is shown to correspond to the transition of strain-rate-free values to values that scale with dimensionless strain rate. Additionally, differences in scaling exponents for melt and solution data suggest variations in steady-state shear stress, with DSM showing agreement with melt data.