A nonequilibrium molecular dynamics simulation of the time-dependent orientational coupling between long and short chains in a bimodal polymer melt upon uniaxial stretching

Orientational coupling mediated by steric effects and relaxation in a pearl necklace model of bidisperse polymer melts is investigated by nonequilibrium molecular dynamics simulation. The melts consist of two lengths of chemically identical polymers, which ranged in size from well below to above the entanglement length. The volume fraction of the short polymer is in the range 0.1 less than or equal to phi(s) less than or equal to 0.9. After the imposition of a step strain on the melt, the relaxation of the orientational and conformational properties of both species are monitored. The relaxation properties vary as a function of polymer length and blend composition, consistent with experimental data. The degree of steric interactions is varied by changing the density of the system; as expected, slower relaxation rates result from a great;er degree of excluded volume. The orientational coupling parameter epsilon, relating the orientation of the short; polymer to that of the long, is measured and is in qualitative agreement with experiment. Finally, the stress-optical law is found to be valid during the relaxation process.