Three vinyl functionalized imidazolium based room-temperature ionic liquids (RTILs): 1-vinyl-3-ethylimidazolium dicyanamide ([veim][dca]), 1-vinyl-3-butylimidazolium dicyanamide ([vbim][dca]) and 1-vinyl-3-heptylimidazolium dicyanamide ([vhim][dca]) were synthesized and UV-polymerized to form free standing membranes. The pure gas permeabilities of CO2 and N2 of these newly developed membranes acquired at 1 atm 35 °C increased with an increase in the number of N-alkyl group in the monomers but their ideal CO2–N2 selectivities decreased. The three vinyl functionalized monomers were blended with three free RTILs: 1-ethyl-3-methylimidazolium dicyanamide ([emim][dca]), 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][B(CN)4]) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) and subsequently subjected to UV-polymerization to form the poly(RTIL)–RTIL composite membranes. The incorporation of free RTILs not only significantly increases the CO2 permeability but also greatly improves the CO2–N2 selectivity. The best separation performance is achieved for the poly([vbim][dca])–[emim][B(CN)4] (1 : 2) and poly([vbim][dca])–[emim][dca] (1 : 2) composite membranes, which have CO2 permeabilities of 340 and 273 barrers and CO2–N2 selectivities of 42 and 53, respectively, at 1 atm 35 °C. The mixed gas permeabilities of these two composite membranes are 297 and 253.5 barrers with corresponding CO2–N2 selectivities of 38.8 and 50.6, respectively, using a 50 : 50 CO2–N2 mixed gas at 2 atm 35 °C. These two composite membranes have separation performance very close to the 2008 “Robeson Upper Bound”, suggesting their potential for industrial applications, especially for the post-combustion flue gas treatment.
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