Dispersity (Ð) in synthetic polymers is inevitable due to the statistical nature of the polymerization mechanisms and can significantly impact fundamental polymer properties. Previously, low disperse melts were targeted, however, more recently, it has been shown that melts with moderate (Ð = 1.20 -1.50) and high (Ð > 1.50) dispersity possess complementary mechanical and processing properties. Using a bead-spring polymer model, we previously presented a detailed study of the structure and dynamics of chains in entangled polymer melts that follow Schulz – Zimm molecular weight distribution. We found that mobility of test chains with N > Mw steadily increases with dispersity, due to the shorter chains contributing to early onset of disentanglement of the long chains. However, the dynamics of test chains of length N < Mw is nonmonotonic with respect to dispersity; this behavior arises from a trade-off between the increased mobility of shorter chains and the corresponding slowdown caused by the presence of longer chains. In the current study, we investigate the impact of dispersity on linear and non-linear viscoelasticity of disperse polymer melts. This is important as polymers are subject to flow (extension or shear) during processing. We perform non-equilibrium MD simulations to unravel molecular origin of the effects of Ð on melt structure, conformational evolution, rheology, entanglement network and relaxation mechanisms for polymers under deformation. We find an anisotropic behavior of probe chains in the disperse melt during relaxation, revealing that chain length distribution modulates both conformational and structural relaxation differently in the directions parallel and perpendicular to deformation. We will use these insights in the future to investigate molecular mechanisms that dictate welding at disperse polymer-polymer interfaces, as well as the strength of a thermally welded interface.
