Control of Gas Separation Properties of Polymer Membranes

A gas separation polymer membrane is required to have either a small molecule permselective property or a large molecule permselective property. The former property is based on common size-sieving filtration. The latter one is, however, unusual and it was difficult to design a gas separation membrane for such applications. The transport of gases in a dense polymer membrane is thought to obey the solution-diffusion mechanism. The total permeability is a product of the solubility and the diffusivity. Hence, the permselectivity is a product of the solubility selectivity and the diffusivity selectivity. The solubility selectivity of a large molecule to a small molecule is larger than 1. In contrast, the diffusivity selectivity of a large molecule to a small molecule is always smaller than 1. The dominant factor to determine the permselectivity is the diffusivity selectivity for small molecule permselective polymer membranes and the solubility selectivity for large molecule permselective polymer membranes. The control of the polymer-gas interaction and/or the size and distribution of the fractional free volume produces a wide variety of gas separation properties in polymer membranes.