Free volume distributions in glassy polymer membranes: Comparison between molecular modeling and experiments

We compare molecular modeling results of two glassy polymer membranes and o ne rubbery polymer membrane with gas transport parameters and free-volume-r elated quantities from positronium annihilation. A simple geometric model r eveals hole size distributions of asymmetric shape. Among glassy polymers, the distribution parameters show a good correlation with average hole sizes determined by positron annihilation lifetime spectroscopy. Higher permeabi lity is measured in the glassy polymer with the higher mean value of the ho le size distribution. The permselectivity of the membranes for permanent ga ses can be interpreted in terms of the distribution broadness via free-volu me-controlled diffusion selectivity. A comparison with the rubbery polymer shows that the permeation behavior is not determined only by the free volum e concentration. The thermal fluctuations of the polymer matrix play an imp ortant role for gas transport properties.