| The diffusion behaviors and permeability of gases (O2, H2) in poly(ethylene-co-1-hexene) with different content of 1-hexene were studied by molecular dynamics and grand canonical ensemble Monte Carlo simulation. The effect of the size of the small gases and the co-monomer content on diffusion of gases and the changes of free volume were discussed. The whole simulations were processed under COMPASS force field. The calculated densities refined by NPT ensemble simulation were in good agreement with the experimental data. The diffusion coefficients were determined by NVT ensemble simulation up to 500ps times. After molecular dynamic simulation, the trajectories of the small molecules in the polymer matrix were obtained. Then the diffusion coefficients were calculated from the Einstein equation. The results showed that the diffusion coefficients of oxygen and hydrogen increase with the content of 1-hexene, which were agreed with the analysis of the interaction between the copolymer chains. It means that molecular dynamics simulation can be used as an effective method to predict the gas diffusion coefficient of small molecules in olefin Copolymers.Molecular dynamics simulations were carried out to elucidate the effect of some structural features of poly(dimethyl siloxane) on its unusually high permeabilities. The results suggest that one important structural feature in this regard is the pronounced alternation in skeletal bond angles, which makes the low-energy conformation of the chain a closed polygon. This could increase free volumes by interfering with the efficient packing of the chains in the bulk state. Also important is the relatively large value of the Si-0 bond length, which could increase chain flexibility and mobility by reducing intramolecular steric interactions.This kind of simulations offered an effective method to study the gas permeabilities of small gas in Copolymers, which will build a foundation for the manufacturing and polymerization of the Copolymers membrance.
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