Molecular Simulation Studies Of Gas Dissolved In High Permeable Membrane Diffusion Behavior

This study investigates the diffusion and solution behaviors of gases in high permeable membranes using molecular dynamics and Grand Canonical Monte Carlo (GCMC) methods together with COMPASS force field. We mainly focus on the following topics:the diffusion and solution behaviors of small gases in pure and filled with silica poly(tert-butylacetylene)[PTBA]membranes, the effect of poly(1-trimethylsiyl-1-propyne)[PTMSP]molecule structure on the gases permeation in this polymer.First of all, this study investigates the diffusion and solution behaviors of small gases in pure PTBA membrane, and also discusses the mechanism of diffusion of small molecules in polymer using free volume theory. The results show that the smaller the molecule size is, the greater the motion range is, and the larger the diffusion coefficient is. The simulated diffusion and solubility coefficients are in good agreement with the experimentals, and the relationship between the diffusion coefficients and the molecular diameters is confirmed. The obtained permeability coefficients are compared with the experimentals, and are of the same order of magnitude as that of experimentals. The simulated results show that molecular dynamics and Monte Carlo simulations are effective methods for studying the diffusion and solution of the small molecules in polymers.Then, we construct the SiO2-PTBA models, and explore the effect of the addition of SiO2 on the gases diffusion. The results show that the diffusion of small molecules in SiO2-PTBA augment to varying degrees, and increase with the concentration of SiO2. The reason lies in that the addition of SiO2 can enlarge the free volume of polymer, thus enhance the flexibility of PTBA back bone.Finally, we inquire into the influence of different PTMSP molecule structures on the gases permeation. We study some factors of double bond, the Si-atom, three methyl side groups, the single methyl and bulky trimethylsilyl side group on the diffusion coefficients of N2 and Ar on polymer. We obtain that free volume fraction is not sole reason that leads to the high diffusion coefficients, the Si atom on the backbone, methyl and double bond are also indispensable. By analyzing the energy terms of polymer, it is found that the high torsion barriers around the single bonds in the main backbone and high special electrostatic interactions are extraordinary important for the high diffusion coefficients of gases in PTMSP.This study provides an efficient method for studying the permeation of small molecules on polymer membranes and theoretical support for the fabrication, synthesize of high permeable membranes.