Molecular-dynamics simulation of diffusion of small penetrants in polymers

Molecular dynamics simulations of diffusion of a small-molecule penetrant in an amorphous polymer matrix have been carried out for the examples of methane in polyethylene (PE) and methane in polyisobutylene (PIB). Particular attention was paid to ensuring that the nonbonded potentials representing the polymer--polymer bead interactions in the matrix were adequately calibrated. Accurate representation of equation of state(P--V--T) behavior for the polymers was used as the criterion for this. The simulations were carried out over a wide range of temperatures. The results are in accord with available experimental data. The temperature variation of the diffusion coefficients PE indicates that the diffusion mechanism changes significantly over the temperature range studied. At lower temperatures, the diffusant motion is characterized by relatively long periods of quiescence interspersed with fairly large jumps. However, at higher temperatures, a broad spectrum of frequent jumps is obtained. The diffusion rates are obviously very sensitive to the free volume and hence to the molecular packing. However, the polymer chain mobility is found to play a role as well. The diffusion activation energy at higher temperature is similar to that for the decay of the torsional angle autocorrelation function for the polymer. It is also found that increasing the torsional barrier reduces the diffusion rate. In PIB where the free volume is significantly less than that in PE, the diffusion rates are much lower than those in the latter. The mechanism over the range studied resembles that of the lower temperature regime in PE.