The effects of crosslinking chemistry on CO2 plasticization of polyimide gas separation membranes

To suppress undesirable plasticization effects in (CO2CH4)-C-/ separations, crosslinkable 6FDA-based copolyimides were synthesized by using 3,5-diaminobenzoic acid (DABA) as one of two diamine monomers. DABA contains a carboxylic acid group that can be used to crosslink the polymer chains with ethylene glycol and aluminum acetylacetonate. These chemistries were compared for effectiveness in suppressing CO2 plasticization on the basis of pure CO2 permeation and sorption data up to 800 psia. The time and pressure dependencies of permeation and sorption were analyzed to characterize the plasticization phenomenon and how it can be controlled by covalent crosslinking. Mixed-gas permeation data are reported up to a total feed pressure of 850 psia for the separation of 50:50 CO2/CH4 mixtures at 35 degreesC. Selectivity losses with increasing feed pressure are modeled to further understand the effects of plasticization, dual-mode sorption, gas-phase nonidealities, and bulk flow on membrane performance. Additionally, a short review of other crosslinking approaches is presented for comparison with the covalently and ionically crosslinked copolyimides. Covalent crosslinking appears most promising for plasticization suppression in practical polyimide membranes.