A study on the stability of the stress response of nonequilibrium ultrahigh molecular weight polyethylene melts during oscillatory shear flow

Dynamic shear flow and especially large-amplitude oscillatory shear have been a subject of interest to investigate limitations of theoretical approximations working under the assumption of simple shear. These studies have helped in understanding the kinematics involved in phenomena such as stick-slip and shear banding, among others. The nonequilibrium polymer melt, which transforms with time into the equilibrium state, provides a unique opportunity to investigate the influence of the thermodynamic melt state on the validity of simple shear approximations. Here, we show that during oscillatory deformation, the nonlinear rheological response of ultrahigh molecular weight polyethylene melts, having number-average molecular weight greater than one million g/mol, is strongly dependent on the entangled state of the same polymer. At sufficiently large strain amplitude, the stress response of the material departs from a periodic sinusoidal signal, with maximum stress decaying with consecutive cycles of deformation. A nonequilibrium polymer melt shows a faster decay in stress on consecutive application of oscillatory strain cycles, compared to its equilibrium state. These conclusions are supported by direct observation of the solid-liquid interface using a rheo-microscope device, where slippage appears to be the cause for the stress decay.