Molecular dynamics simulations of diffusion of small molecules in polymers: Effect of chain length

In order to investigate the influence of chain length on the diffusion of small molecules in polymers, molecular dynamics simulations have been carried out for two model systems. One is the short chain model which is composed of 30 n-alkane-like chains each having 20 segments. The other is the infinite length chain model, used as a model for polyethylene, which is composed of a noncyclic chain of 600 segments with no free ends. Self-diffusion coefficients of the small molecules in the two model systems are compared. It was found that the chains of shorter length facilitate faster diffusion of the small molecules as well as faster relaxation of internal rotations of the chains even for a fixed total amount of free volume. However, the self-diffusion coefficient of small molecules in the infinite length chain model was still larger than that obtained from experiment. Possible factors which give rise to the effect of chain length are discussed.