Synthetic 6FDA–ODA copolyimide membranes for gas separation and pervaporation: Functional groups and separation properties

Copolyimides were prepared from one-step polymerization of 6FDA and ODA with four diamines DBSA, DABA, DAPy and DANT as the third monomers. Polymers were characterized with GPC, FTIR, NMR, DSC and TGA. Surface free energies and membrane–water interfacial free energies were calculated from contact angles, and results indicated that DABA, DBSA and DAPy moieties helped to increase the hydrophilicity. Gas permeation was measured for N2, O2, H2, He and CO2. Linear moiety contribution method was proposed to study the moiety effects on gas selectivities. The selectivities of O2/N2, H2/N2, and He/N2 were greatly affected by the steric effects from the monomer moieties, but permeation of CO2 was controlled by its solubility in polymers as well as the interactions with the functional groups. Water permeation and dehydration of isopropanol were carried out in pervaporation processes. Concentration coefficients were proposed for feed concentration effects on permeation flux, and permeation activation energies were used to study the temperature dependence of flux. Linear moiety contribution method was applied to quantitatively compare the effects of monomer moieties. Functional groups could change the sorption and diffusion properties in pervaporation, and moiety contribution factors indicated that feed concentrations mainly affected sorption properties, whereas temperatures influenced diffusion properties.