Effect of pressure on conformational dynamics in polyethylene: A molecular dynamics simulation study

Molecular dynamics simulations have been used to delineate the pressure dependence of conformational dynamics in polyethylene. Chain dynamics have been studied as a function of pressure (0-25 kbar);at 400 K and as a function of temperature (280-450 K) at an elevated pressure of 10 kbar. The volumetric glass transition is found isobarically (10 kbar) at similar to 315 K and is considerably elevated from the similar to 220 K atmospheric pressure value. On increasing pressure at constant temperature, the system displays several signatures indicative of the vitrification process that are complementary to those previously found to be associated with lowering of temperature. These include a large increase in autocorrelation function relaxation times, divergence of the relaxation times from conformational transition rates, an increase in spatial heterogeneity of the distribution of conformational transition rates over the various bonds, and an increase in self-correlation of the conformational transitions. Isobarically at high pressure (10 kbar), the variation with temperature of the relaxation times associated with the decay of the dipolar autocorrelation function is in accord with dielectric relaxation experiments. Upon lowering the temperature, the relaxation times diverge from the conformation transition rates, an effect also observed at low pressure.