Abstract
The permeability of 6FDAโmโPDA, 6FDAโ2,4โDATr, 6FDAโ2,6โDATr, and 6FDAโ3,5โDBTF membranes to H~2~, O~2~, N~2~, CO~2~, and CH~4~ was measured at 35ยฐC 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 ฮ__p__, the pressure difference across the membranes, or decrease slightly with increasing ฮ__p__. The gas permeability of three of the polyimides increases, and their selectivity to some gas pairs decreases, in the polymer order: 6FDAโmโPDA < 6FDAโ2,4โDATr < 6FDAโ2,6โDATr. This appears to be due to an increasing mean interchain distance in these polymers. The gas permeability of 6FDAโ3,5โDBTF 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.