Microstructural parameters controlling gas permeability and permselectivity in polyimide membranes

The present paper deals with the relationships between the gas permeation properties and the microstructural parameters of amorphous polyimides. The correlation is deduced from a data compilation of about 100 polyimides, taken from the literature or synthesized and characterized in our laboratory. The microstructure of polyimides is described on different scales through density measurement, wide-angle X-ray scattering (WAXS), UV–visible spectrophotometry, dynamic mechanical thermal analysis (DMTA) and positronium annihilation lifetime spectroscopy (PALS). From these experiments, there are only three extracted probes which display strong relationships with gas transport properties. These probes are the temperature of the gamma relaxation (Tγ), the color index (λ50) and the radius (R) of the spherical free volume hole. The empirical models are based on the gas molecular diameter and on four constants for each microstructural probe. The prediction is achieved for the gas permeability coefficients of seven gas molecules and also for diffusion and sorption coefficients in some cases. The proposed fits are also consistent with the dependence of the permselectivity on the microstructural probe and gas molecule diameter.