Structure/permeability relationships of polyimide membranes. Applications to the separation of gas mixtures

Abstract The permeability of nine different polyimide membranes to H 2 , N 2 , O 2 , CH 4 , and CO 2 has been determined at 35°C and at applied pressures of up to 9 atm. The dianhydride monomers used for the synthesis of the polymides were PMDA and 6FDA, whereas the diamine monomers were ODA, BDAF, and p ‐PDA. The selectivities of the 6FDA polymides toward CO 2 relative to CH 4 are higher than those of the PMDA polyimides at comparable CO 2 permeabilities. Both types of polyimides exhibit significantly higher CO 2 /CH 4 selectivities than more common glassy polymers, such as cellulose acetate, polysulfone, and polycarbonate. The selectivities of the PMDA and 6FDA polyimides to O 2 relative to N 2 are of the same magnitude and generally higher than those of common glassy polymers with similar O 2 permeabilities. The polymides are more permeable to N 2 than to CH 4 , whereas the opposite is true for many other glassy polymers. Possible factors responsible for the above behavior, such as segmental mobility, mean interchain distance, and formation of charge transfer complexes, are examined. The relevance of the study to the development of more highly gas‐selective and permeable membranes for the separation of gas mixtures is also discussed.