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

The permeability of nine different polyimide membranes to H₂, N₂, O₂, CH₄, and CO₂ 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 CO₂ relative to CH₄ are higher than those of the PMDA polyimides at comparable CO₂ permeabilities. Both types of polyimides exhibit significantly higher CO₂/CH₄ selectivities than more common glassy polymers, such as cellulose acetate, polysulfone, and polycarbonate. The selectivities of the PMDA and 6FDA polyimides to O₂ relative to N₂ are of the same magnitude and generally higher than those of common glassy polymers with similar O₂ permeabilities. The polymides are more permeable to N₂ than to CH₄, 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.