Stress Relaxation in Highly Oriented Melts of Entangled Polymers*

Molecular dynamics simulations are used to study stress and conformational relaxation in entangled polymer melts deformed far from equilibrium by uniaxial extensional flow. Melts are elongated to a Hencky strain of 6 at Rouse Weissenberg numbers from 0.16-25, producing states with highly aligned chains. Then flow is ceased and the system is allowed to relax until twice the equilibrium disentanglement time. The relaxation of the stress is correlated with changes in the conformation of chains and the geometry of the tube confining them. The primitive path length of chains relaxes towards its equilibrium value on the equilibrium Rouse time and the orientation of the primitive path then relaxes on the equilibrium disentanglement time. Both results are counter to predictions of several recent models that suggest a large reduction in the entanglement density that persists for the disentanglement time, raising fundamental questions about the nature of entanglement in aligned molten polymers.