Gas sorption and transport in polyisobutylene: Equilibrium and nonequilibrium molecular dynamics simulations

Abstract

Molecular dynamics calculations are reported on the sorption and diffusion of small gas molecules (He, H2, and O2) in amorphous polyisobutylene. An all-atom force field (with explicit hydrogen atoms) was used. In addition to the standard method of obtaining the diffusion coefficient from an equilibrium calculation via the mean-square displacement we used a nonequilibrium technique that applies a fictitious external field selectively to the gas molecules. To our knowledge, nonequilibrium MD is, for the first time, applied to the problem of gas diffusion in polymers. Results of both techniques are compared. Energy profiles of the jump events underlying gas diffusion through polymers are studied. We also discuss the possible presence of anomalous (non-Einstein) behavior of gas molecules diffusing through an amorphous polymer. Gas solubilities in polyisobutylene are calculated by particle-insertion techniques.