Accelerated diffusion and entanglement evolution during relaxation of aligned polymer melts

The dynamics of entanglement formation and loss has long been recognized as a fundamental mechanism behind the macroscopic rheological properties of polymer melts. Models based on entanglements and a confining tube have been very successful in describing equilibrium diffusion and low rate viscous flow, but it is not yet clear how to extend these tube models to highly aligned states produced by shear or elongational flow and experiments cannot directly measure entanglements. Here we use molecular dynamics simulations to follow the motion of monomers and chains and the evolution of entanglements during relaxation from highly aligned states produced by shear and elongational flow. Polymers are modeled with the coarse-grained FENE potential with varying chain stiffness and entanglements are followed with both Primitive-Path Analysis and the Z1 code. Chain retraction occurs over the equilibrium Rouse time and chains reorient on the equilibrium disentanglement time. In sharp contrast to existing theories, the entanglement density does not decrease during chain retraction. A monotonic increase in entanglement density is observed that can be understood from the chain dynamics.