Molecular level investigation into selective permeability of silicon based membranes with potential use in gas separation processes

The effect of the modification of the molecular structure on the permeability coefficients of typical rubbery and glassy silane and siloxane polymers at different temperatures was experimentally investigated. It was shown that carbon dioxide had higher permeability coefficients than those of nitrogen and oxygen due to the higher affinity of the various polymers toward the gas molecules. In order to provide a detailed understanding into the effect of the molecular structure on the gas diffusion behaviour in polymers, molecular modelling of carbon dioxide diffusion in silicon based membranes was used. The polymer molecules were shown to have lower self-diffusion coefficients than the gas ones related to the small size of the gas molecules as compared to the large size of the polymeric segments, thus allowing the gas molecules to jump from one unoccupied site to another through a series of connected pores or channels within the polymeric matrix. Increasing the temperature was shown to have a proportional effect on the mean square displacement, possibly due to the increase in the kinetic energy available to the systems. At high temperatures, the glassy siloxane molecules had similar values for the mean square displacement to those of the gas molecules since the polymer in this case is in close proximity to its glass transition temperature. The presence of the alternating oxygen atoms in the main backbone of the polymeric chains led to higher values for the selfdiffusion coefficients for the siloxane polymers as compared to those of the silane polymers as a result of the change in the bond angle about the oxygen atom (∼ 144°) as compared to the tetrahedral angle (∼ 110°) about the silicon atoms.