Computational evidence for anomalous diffusion of small molecules in amorphous polymers

Abstract

Results of molecular dynamics simulations of oxygen molecules and helium atoms in amorphous polyisobutylene are presented. The systems are allowed to evolve for several nanoseconds. In these rather long simulations it is found that the diffusion of He and O 2 through the polymer matrix exhibits anomalous (non-Einstein) behaviour for time scales of at least 0.1 and 6 nanoseconds for He and O 2, respectively. This contrasts with previous much shorter molecular dynamics studies of gas molecules in polymers which found that Einstein diffusion sets in after a few picoseconds.