Primitive chain network simulations of double peaks in viscosity growth curves of densely branched pom-pom polymer melts in fast shear flows

Despite many attempts, the molecular mechanism of the nonlinear viscoelastic response of polymeric liquids in fast shear flows has not yet been fully elucidated. In this study, we examined the viscosity growth curves for a few well-characterized, nearly monodisperse, densely branched pom-pom polystyrene melts. The viscosity growth curves exhibit double peaks rather than the widely reported (for most polymer melts) single peak. To investigate the underlying molecular mechanism responsible for the observed behavior, we conducted primitive chain network (multi-chain sliplink) simulations, and found that the first and second peaks correspond to arm orientation and backbone stretch, respectively. We further observed that backbone stretch is reduced by coherent molecular tumbling at the flow start-up, and hence the second peak intensity comes out comparable to that of the orientation-induced first peak. In our sample, the number of backbone entanglements is small, so the mechanism of branchpoint withdrawal does not play a significant role.