Structure/permeability relationships of polyimide membranes. II

The permeability of 6FDA‐m‐PDA, 6FDA‐2,4‐DATr, 6FDA‐2,6‐DATr, and 6FDA‐3,5‐DBTF membranes to H₂, O₂, N₂, CO₂, and CH₄ was measured at 35°C in the pressure range from about 1.4 atm (20 psi) to 8.2 atm (120 psi). The permeability coefficients, \documentclass{article}\pagestyle{empty}\begin{document}${\bar P}$\end{document}, for the penetrant gases in any of the four polyimides studied decrease in the order:\documentclass{article}\pagestyle{empty}\begin{document}$\bar P({\rm H}_2 ) > \bar P({\rm O}_2 ) > \bar P({\rm N}_2 ) > \bar P({\rm CH}_4 )$\end{document}. The values of \documentclass{article}\pagestyle{empty}\begin{document}${\bar P}$\end{document} are generally independent of Δp, the pressure difference across the membranes, or decrease slightly with increasing Δp. The gas permeability of three of the polyimides increases, and their selectivity to some gas pairs decreases, in the polymer order: 6FDA‐m‐PDA < 6FDA‐2,4‐DATr < 6FDA‐2,6‐DATr. This appears to be due to an increasing mean interchain distance in these polymers. The gas permeability of 6FDA‐3,5‐DBTF is higher, and its selectivity lower, than expected from its mean interchain distance, possibly because of a higher segmental mobility. The potential usefulness of the above polyimides as membrane materials for gas separation processes is discussed.