Synthetic 6FDA-ODA copolyimide membranes for gas separation and pervaporation: Correlation of separation properties with diamine monomers

Copolyimides were synthesized from 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 4‐aminophenyl ether (ODA) with 4‐aminophenyl sulfone (DDS), 4,4′‐methylenedianiline (MDA), 4,4′‐bis(3‐aminophenoxy)diphenyl sulfone (BADS), 4,4′‐bis(3‐aminophenoxy) benzophenone (BABP), and 2,6‐bis(3‐aminophenoxyl) benzonitrile (DABN) as the third monomer. Surface free energies and interfacial free energies were calculated for comparison of the membrane hydrophilicity. Gas permeation was carried out with N2, O2, H2, He, and CO2, and the moiety contributions to membrane selectivity were calculated. DDS and BADS moieties contribute negatively to the selectivities toward O2/N2, H2/N2, and He/N2, and the DABN moiety is favorable for improving CO2/N2 selectivity. Water permeation and dehydration of isopropanol were performed, and the linear moiety contribution method was applied to study the effects of the monomer structures on the temperature and feed concentration dependencies of the permeation flux. The steric effects of DDS and BADS moieties, as well as the interactions of BABP and DABN moieties with water, account for the differences in pervaporation properties of the membranes. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers