Concentration dependence of diffusivities of penetrants in glassy polymer membranes

Steady‐state permeation rates for pure CO2, CH4, and O2 through two kinds of homogeneous dense membranes, copolymer of methyl methacrylate and n‐butyl acrylate and poly(methyl methacrylate), were measured at three temperatures between 20 and 40°C and upstream pressures up to 3 MPa. The logarithms of the mean permeability coefficients for CO2 in both membranes increased linearly or quadratically with increasing upstream pressure, whereas the mean permeabilities for CH4 and CO2 were independent of pressure. The pressure dependence of the mean permeability coefficient for CO2 was interpreted in terms of a modified free‐volume model of diffusion for a Henry's law population in glassy polymers.