Tailoring the micropore structure of 6FDA-based polyimide membrane for gas permselectivity studied by positron annihilation.

Two series of 6FDA-based co-polyimide membranes were synthesized by adding different diamine monomer containing special functional groups: APAF (containing OH) and DABA (containing -COOH). The ODA/APAF and ODA/DABA series were heat treated at 450 °C and 350 °C, respectively. Thermal rearrangement of polymer chain segments is confirmed in ODA/APAF membranes by infrared spectroscopy, while addition of DABA in ODA/DABA membranes causes decarboxylation crosslinking. Positron lifetime measurements reveal existence of ultramicropores in polyimides. The thermal rearrangement in ODA/APAF membranes induces remarkable increase in both size and number of ultramicropores and therefore increase in fractional free volume (FFV), but the crosslinking in ODA/DABA causes significant decrease in both size and number of ultramicropores. The increase in FFV is favorable for gas permeation, but too large size of the micropore drastically deteriorates the gas selectivity. Finally, the ODA/APAF(3:7) shows a high CO 2 permeability of 127.4 Barrer with CO 2 /CH 4 selectivity of 34.2, which surpasses the 1991 Robeson upper bound. Besides, the ODA/DABA(3:7) exhibits a H 2 permeability of 22.6 Barrer with H 2 /CH 4 selectivity of 256.8, which also exceeds the 1991 Robeson upper bound. Our results indicate that the gas permeability and selectivity of polymer membranes can be effectively adjusted by tailoring the ultramicropore structures, and positron can provide precise information about micropores. • Positron annihilation results show that the adding of APAF and DABA can change the FFV by changing both size and number of ultramicropores. • The positron annihilation results prove the increase in FFV is favorable for gas permeation, but too large size of the micropore drastically deteriorates the gas selectivity. • The positron annihilation results confirm gas selectivity depends on the difference in the diameter of the gas pairs to be separated as proposed by Robeson et al. • ODA/APAF(3:7) surpasses the 1991 CO 2 /CH 4 Robeson upper bound, while ODA/DABA(3:7) surpassing the 1991 H 2 /CH 4 Robeson upper bound. [ABSTRACT FROM AUTHOR]