Numerical simulation on the diffusion behavior of penetrant molecules in poly-<Emphasis Type="Italic">p</Emphasis>-xylylene and its chlorine substituent polymers

Models of poly-p-xylylene and its chlorine substituent polymers were established and structural optimized with Molecular Dynamics (MD) in Material Studio (MS) software. The densities and glass transition temperatures (T _g) calculated from the optimized models were consistent very well with the experimental data. The free volumes and their distributions in these polymeric models were obtained using the Atom Volumes and Surfaces tool. The results showed that the amount of free volume for each model was very small, especially for those free volume cavities of diameter larger than 2.8 Å, which made it difficult for the molecules of diameter larger than 2.8 Å penetrating in the polymer. The diffusion, solubility and permeability coefficients of some gaseous molecules in these models were also calculated based on the combined methods of MD and Great Canonical Monte Carlo (GCMC), which presented the same or 1–2 higher order of magnitude with experimental values. The trajectories of penetrant molecules diffusing in polymeric models demonstrated that the molecules transferred or moved in amorphous regions between spherical crystals with a “hopping diffusion” behavior.