Examination of non-universalities in entangled polymer melts and solutions during the startup of steady shear flow*

The possibility of non-universality during inception of shear flow at large strain rates has recently been questioned so was recently examined using the discrete slip-link model (DSM). An expression for the Rouse relaxation time as a function of entanglement activity and number of Kuhn steps was found from a master curve of strain maxima, as predicted by the  theory. DSM predicts only a very weak dependence of Rouse time on chemistry [Macromolecules 54, 8033–8042 (2021)]. This expression is shown to collapse all entangled polymer solution and melt data to universal behavior for the maximum shear stress 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 0.33 are shown to correspond to primitive path stretching. Furthermore, The scaling exponents for melt (0.1--0.15) and solution (0.2--0.3) in experimental data do not show the same scaling for steady-state shear stress, but the melts are in agreement with DSM (0.1). There is a small amount of data for the scaling of stress at undershoot, and strain at undershoot, which are predicted to scale as 0.1 and 0.33 for DSM, in agreement for melt data.