Dynamics of light gases in rigid matrices of dense polymers

Transition-state theory was employed to study the dynamics of light gases dissolved in rigid microstructures of glassy polycarbonate and rubbery polyisobutylene modeled in atomistic detail. The gaseous molecules migrated through the polymer structures in a sequence of ‘‘hops’’ between local minima of the potential energy. The solute dynamics was characterized by three time domains: at short times, 10−12 s, the mobility was very high and the molecules traveled on a scale of 5 Å; this was followed by a domain of anomalous diffusion; at long times the tiniest molecules (He and H2) followed the Einstein diffusion law. Larger molecules (Ar, O2, and N2) did not reach the diffusive regime at the time scale of simulation (up to ca. 10−3 s) but were trapped instead in the vicinity of their initial sites without any progress in translational motion. It was concluded that the rigid-matrix approach is inadequate for studying the dynamics of light gases in dense polymers, except for He.