Effect of film thickness on the gas-permeation characteristics of glassy polymer membranes

Films made from three glassy polymerspolysulfone, poly(2,6-dimethyl-1,4-phenylene oxide), and a commercial polyimide known as Matrimidwere prepared in thicknesses ranging from 0.4 mu m to 60 mu m, and their permeabilities to oxygen, nitrogen, and methane were monitored for more than a year. These films exhibited substantial decreases in permeability with time, because of volume relaxation that is due to physical aging, which is a reversible process. The observed permeability coefficients were originally greater than the literature values for thick, or so-called bulk films, but eventually decreased well below these values. The rate of the aging effect becomes greater the thinner the film. The implications of this observation for practical membrane gas separation processes and the selection of membrane materials based on thick film data are discussed.