1,3-Substituted imidazolium bis(trifluoromethylsulfonyl)imide (Tf2N) ionic liquids (ILs) were incorporated at 40 and 60 vol % loading into a cross-linked poly(ethylene oxide) polymer network to create ion gels for carbon dioxide (CO2)-selective gas separation membranes. The ILs plasticize the poly(ethylene oxide)-based polymer to increase its gas permeability. Compared to the base polymer, with a CO2 permeability of 145 barrer and a CO2 against nitrogen (N-2) selectivity of 47, the highest CO2 permeability achieved was 530 barrer coupled with a CO2/N-2 selectivity of 31 by having 60 vol % [1-ethyl-3-methylimidazolium][Tf2N]. The extent of gas permeability improvement depends on the cations terminal substituent. Substituents that promote additional noncovalent intermolecular interactions, such as hydroxyl, benzyl, and nitrile, can reduce the gas diffusivity by reducing the polymer chain mobility. While n-alkyl, branched alkyl, and oligo(ethylene glycol) substituents can significantly increase the gas permeability, the shortest substituents (such as ethyl) were the most effective because they promote high ionic charge density.