MONOMER-LEVEL DESCRIPTION OF STRESS AND BIREFRINGENCE RELAXATION IN POLYMER MELTS

Theories for stress and birefringence relaxation in polymer melts are usually based on coarse-grained chain models in which each bead represents many mers. We here develop a physical picture of these processes on the monomer level. It makes use of an intrinsic coordinate system for each mer in which the bonds attached to that mer are fixed. It is shown, through a molecular dynamics simulation, that associated with each mer there is an intrinsic monomer stress (stresslet) that remains constant when referred to this coordinate system throughout the relaxation process and that it is through the randomization of the orientations of these stresslets that, at later times, the macroscopic stress decays. Birefringence decay is treated in a parallel manner and leads to a simple expression for C(r), the stress-optical coefficient in the rubbery regime, in terms of the monomer polarizability tensor and the intrinsic monomer stress tensor. Good agreement with experiment is found for C(r) for the present simulation of a polyethylene-like model.