Effect of High Pressure Carbon Dioxide Exposure on the Permeation Properties of Asymmetric Polysulfone Membranes for Gas Separation

Abstract

In this study, the dependence of permeability on the feed pressure for glassy asymmetric polysulfone membranes has been investigated. Pure CO₂ and CH₄ were used as test gases. In pure gas experiments with CO₂, the untreated membrane showed that the carbon dioxide permeation rate increased with increasing feed pressure from one bar onwards. This is because carbon dioxide behaves as a plasticizing agent by expanding the polymer lattice and increased the frequency of molecular motions of the polymer. Moreover, high carbon dioxide concentration in the polymer film disrupts the chain packing, thereby leading to a larger free volume and enhanced rates of segmental motions. Due to swelling of the polymer matrix, the structure of the polymer matrix is loosened and consequently polymer chains become more flexible. For the treated membrane, the permeability of carbon dioxide does not increase with feed pressure but gives a steady state value over the feed pressure ranges investigated. This was in agreement with scanning electron microscopy studies, which revealed that heat treatment densified the membrane skin layer. Thus, a subsequent heat treatment to the membrane is necessary in suppressing the plasticization effect.