Temperature dependence of gas transport in polymer melts:: Molecular dynamics simulations of CO2 in polyethylene

The temperature dependence of carbon dioxide transport in an amorphous polyethylene melt has been studied using molecular dynamics simulations. At elevated temperatures, we have found strong slope changes in the Arrhenius plots for the diffusion and the solubility coefficients of the penetrant, lowering the activation energy for diffusion and the apparent heat of solution. The thermal population of penetrant energy states was studied to explain these phenomena. At low temperatures, sparsely distributed low-energy sites in the matrix were found populated, which causes the diffusion process to be strongly activated. At high temperatures, thermally accessible sorption sites with energies higher than the aforementioned ones were observed in significantly increased numbers. Consequently, the activation zone for diffusion decreases, and the entropy of solution increases. With decreasing slope of the Arrhenius plot for diffusion, the diffusion mechanism gradually changes from "hopping diffusion" to "liquidlike" diffusion, in agreement with the observations of others on different polymer/penetrant systems.