Atomic level picture of stress relaxation in polymer melts

We have been developing a physical picture on the atomic level of stress relaxation in polymer melts by means of computer simulation of the process in model systems. In this article we treat a melt of freely jointed chains, each with N = 200 bonds and with excluded-volume interactions between all nonbonded atoms, that has been subjected to an initial constant-volume uniaxial extension. We consider both the stress relaxation history sigma(t) based on atomic interactions, and the stress history sigma(e)(t; N-R) based on subdividing the chain into segments with N-R bonds each, with each segment regarded as an entropic spring. It is found that at early times sigma(t) > sigma(e)(t; N-R) for all N-R, and that, for the remainder of the simulation, there is no value of N-R for which sigma(t) = sigma(e)(t; N-R) for an extended period; by the end of the simulation sigma(t) has fallen just below the value sigma(e)(t; 50). The decay of segment orientation, (P-2(t; N-R)), and of bond orientation (P-2(t; 1)], is computed during the simulation. It is found that the decay of the atom-based stress sigma(t) is closely related to that of [P-2(t; 1)]. This result may be understood through the concept of steric shielding. The change in local structure of the polymer melt during relaxation is also studied. (C) 1998 John Wiley & Sons, Inc.