CONFORMATIONAL DYNAMICS IN BULK POLYETHYLENE - A MOLECULAR-DYNAMICS SIMULATION STUDY

Molecular dynamics (MD) simulation has been used to study chain dynamics in bulk polyethylene over a wide range of temperature. This range includes the volumetric glass transition. The latter was determined from the volume temperature relation established via constant pressure MD runs. Conformational transition rates were monitored over the temperature range and found to remain Arrhenius in nature even through the glass transition. The activation energy was found to correspond to a single torsional barrier. Correlation of transitions was monitored as well and, in common with Brownian dynamics simulations of solutions and previous bulk simulations, +/-2 neighbor correlations were found to be common. Several new types however were identified. In addition, it was found that +/-0 self correlation becomes increasingly important as temperature is lowered. In contrast to the conformational transition rates, the relaxation times for decay of the torsional angle autocorrelation function were found to display non-Arrhenius Williams-Landel-Ferry (WLF) Vogel-Fulcher temperature dependence. The disparity between the relaxation time and conformational transition behavior was traced to the spatial distribution over the bonds of the conformational jumps becoming increasingly heterogeneous as temperature decreases. This in turn is attributed to the bulk packing causing bonds to be trapped at and oscillate about torsional angles away from the torsional energy minima.