SMALL-MOLECULE PENETRANT DIFFUSION IN HYDROCARBON POLYMERS AS STUDIEDBY MOLECULAR-DYNAMICS SIMULATION

Molecular dynamics simulations of simple hydrocarbon polymers have bee n extended to atactic polypropylene (aPP). The equation of state (PVT) and the diffusion of methane as a penetrant were studied. The results are compared with previous simulations on the chemically isomeric (CH 2)n polymers polyethylene (PE) and polyisobutylene (PIB). The equation of state or PVT properties are in good agreement with experiment and reproduce the fact that aPP has a melt density similar to that of PE b ut significantly lower than that for PIB. Small-molecule diffusion in aPP is found to be similar to that in Pe but much faster than that in PIB. This also is in agreement with available experimental data. The m olecular packing features that lead to these results are discussed.