Bond orientation decay and stress relaxation in a model polymer melt

Simulations of stress relaxation in a model polymer melt of freely-jointed chains with N = 300 bonds are performed with the use of a nonequilibrium molecular dynamics algorithm. After a deformation is applied in a short loading period, special attention is paid to the decay of bond orientation, P-2(t;1), and the relation of this quantity to the stress sigma(t) computed by the atomic virial stress formula. It is found that the ratio P-2(t;1)sigma(t) has a low value in the early glassy period and then undergoes a transition to a higher value that remains substantially constant. An explanation on the atomic level for the behavior of this ratio, which bears a close relation to the stress-optical coefficient is given. Various modes of coarse-graining the model melt are considered by subdivision of each chain into segments, each with NR bonds. A second, molecular, calculation of the stress is made for the coarse-grained melt by use of the entropic spring stress formula and denoted by sigma(e)(t;N-R). At early times sigma(t) > sigma(e)(t;N-R) for all N-R. At later times, the value of N-R for which sigma(t) = sigma(e)(t;N-R) increases from N-R = 5 to N-R = 50. In these simulations, no value of N-R is found for which sigma = sigma(e) for an extended period. Conceptual difficulties, suggested by these simulations, with the use of Rouse dynamics for the calculation of the plateau onset and plateau modulus are discussed.