Effect of introduction of heterocyclic moieties into polymer backbone on gas transport properties of fluorinated poly(ether imide) membranes

The effects of incorporation of heterocyclic moieties into fluorinated poly(ether imide) membranes on their gas transport properties were investigated. Four novel fluorinated poly(ether imide) (PEI) membranes were prepared from four different bis(ether amine)s namely, 4,4-bis[3′-trifluromethyl-4′(4′′-aminobenzoxy)bezyl]biphenyl (BAQP); 1,4-bis[3′-trifluromethyl-4′(4′′-aminobenzoxy)bezyl] benzene (BATP); 2,6-bis[3′-trifluromethyl-4′(4′′-aminobenzoxy)bezyl]pyridine (BAPy) and 2,5-bis[3′-trifluromethyl-4′(4′′-aminobenzoxy)bezyl]thiophene (BATh), and a fluorinated dianhydride, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane (6FDA) as a common dianhydride. Gas transport properties of these membranes were measured to investigate the effect of chemical structure on gas permeation and diffusion processes over four different gases (e.g., CH4, N2, O2 and CO2) at different temperatures (e.g., 35, 45 and 55 °C) at an applied pressure of 3.5 bar. It has been found that at 35 °C the permselectivities of BAPy and BATh based polymeric membranes (6.4 and 6.6, respectively) toward O2 relative to N2 are higher in comparison to BAQP and BATP (5.5 and 5.3, respectively) containing PEI membranes. The permeability coefficient of CO2 for BAPy and BATh (51.92 and 45.31, respectively at 35 °C) based PEI membranes were observed to be much higher than BAQP and BATP based membranes (36.61 and 33.51, respectively at 35 °C) with comparable selectivity values of CO2 relative to CH4. All these membranes exhibit higher CO2/CH4 selectivity than those of common glassy polymers e.g., cellulose acetate, polysulfone and polycarbonate. The order of permeability of these gases was found as CO2 > O2 > N2 > CH4. The temperature dependency of permeation and diffusion processes enables to calculate the activation energies of the permeation and diffusion processes for these four different gases through four PEI membranes.