Conformational dynamics in polyethylene under isochoric conditions: A molecular dynamics simulation study

Abstract

Conformational dynamics in polyethylene (PE) have been studied via molecular dynamics simulations under isochoric conditions at four specific volumes (1.26, 1.20, 1.15 and 1.10 cm3 g−1) as a function of temperature (300–500 K). The autocorrelation function (ACF) decay for virtual perpendicular dipolar relaxation and rates of conformational transitions were monitored. The isochoric Kohlrausch–Williams–Watts (KWW) relaxation times determined from the ACFs show Vogel–Fulcher (V-F) behavior with temperature. From the pressures determined at each T, V point, constant pressure relaxation times vs temperature could also be constructed along several isobars by interpolation. The resulting temperature dependent activation energies at constant volume and at constant pressure are compared. At temperatures where the isochoric and isobaric P, V values coincide, i.e., where the ischoric and isobaric V-F curves intersect, the ratios of the constant volume activation energies to those at constant pressure are found to be ∼0.8. This high ratio can be considered as an indicator of activated barrier crossing dynamics dominating in comparison to the role of free volume. The dipolar ACF relaxation times diverge from the conformational transition rates as the temperature is lowered (under isochoric conditions) and analogously with increasing pressure (under isothermal conditions). The spatial heterogeneity of the conformational transitions increases with decreasing temperature and an increase in self-correlation of the conformational transitions ensues. These signatures of approaching vitrification are consistent with those previously found for PE under isobaric conditions. Qualitatively, decreasing the temperature at constant volume, increasing the pressure at constant temperature, and decreasing the temperature at constant pressure result in very similar behavior in the conformational dynamics.