STRESS-RELAXATION IN A POLYMER MELT OF FREELY-ROTATING CHAINS

It has been generally assumed that the nonbonded interactions in a dense polymer melt make only an isotropic contribution to the stress. Recent computer simulations of stress relaxation in systems of idealized chain models have shown that this is not the case. In fact, the nonbonded interactions are primarily responsible for the anisotropic stress during a stress relaxation process. To check that this result is not due to the idealized character of the chain models, simulations are performed for a melt employing a more realistic model, namely that representing a freely-rotating chain. The parameters employed correspond to the freely-rotating chains (FRC) model melt studied in extensive simulations by Takeuchi and Roe. We find that the primary role of the nonbonded interactions remains unchanged. Simulations are performed at several temperature levels at constant pressure. It is found that the temperature-time equivalence principle applies to the simulated shear relaxation modulus. The required shift factor a(T) satisfies the Williams-Landel-Ferry (WLF) equation with fairly reasonable parameter values.