GAS SORPTION AND TRANSPORT IN POLYISOBUTYLENE - EQUILIBRIUM AND NONEQUILIBRIUM MOLECULAR-DYNAMICS SIMULATIONS

Molecular dynamics calculations are reported on the sorption and diffu sion of small gas molecules (He, H-2, and O2) in amorphous polyisobuty lene. An all-atom force field (with explicit hydrogen atoms) was used. In addition to the standard method of obtaining the diffusion coeffic ient 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, nonequilibri um 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 ps diffusion through polymers are studie d. We also discuss the possible presence of anomalous (non-Einstein) b ehavior of gas molecules diffusing through an amorphous polymer. Gas s olubilities in polyisobutylene are calculated by particle-insertion