Casting solvent effects on morphologies, gas transport properties of a novel 6FDA/PMDA–TMMDA copolyimide membrane and its derived carbon membranes

We have observed that solvents play an important role on the membrane morphology and gas separation performance of a novel 6FDA/PMDA–TMMDA copolyimide [copoly(4,4’-methylenebis(2,6-dimethyl)-2,2-bis(3,4-dicarboxylphenyl) hexafluoro propane/pyromellitic) diimide]. Copolyimide films cast from CH2Cl2 or NMP (1-methyl-2-pyrrolidinone) show amorphous morphology, while films cast from DMF (N,N-dimethylformamide) have crystalline structure which has been confirmed by polarized microscope, XRD and gas sorption tests. Gas transport properties of different morphological films are significantly different. Furthermore, we also report that gas separation performance of carbon molecular sieve membranes (CMSMs) pyrolyzed from different morphological precursors is strongly dependent on pyrolysis temperatures. Results show that the permeability of CMSMs at different pyrolysis temperatures follows the order: 550 °C > 650 °C > 800 °C and the separation factor is in the order: 800 °C > 650 °C > 550 °C. The differences between CMSMs derived from different morphological precursors decrease with an increase in pyrolysis temperatures. At low pyrolysis temperature, the CMSMs’ structure and separation performance could significantly be affected by the decomposition temperature of the precursor, however, at higher pyrolysis temperatures, the dominant factor for the CMSMs’ structure and performance is the pyrolysis temperature due to the complete degradation of the polymeric precursor.