MOLECULAR-DYNAMICS SIMULATION OF DIFFUSION OF SMALL MOLECULES IN POLYMERS .2. EFFECT OF FREE-VOLUME DISTRIBUTION

The influence of the free volume distribution on the diffusion of small molecules in polymers is studied by means of molecular dynamics simulations. A model is used which is composed of 10 O2 molecules, and 600 CH2 groups each belonging to an infinite length chain or its periodic images. A fully vibrational treatment is applied to the chains with a torsional potential and nonbonded interactions. Four model systems, each consisting of chains having the same molecular parameters except for the equilibrium skeletal bond angle, are compared at 300 K and at densities giving the same free volume fraction. The self-diffusion coefficient of oxygen strongly correlates with the average interchain separation; a tight chain packing reduces the diffusion even when the total free volume content is kept constant. The importance of the free volume distribution for diffusion was demonstrated by the good correlation obtainable in terms of the effective free volume available between the nearest neighbor chains.