How do polymerized room-temperature ionic liquid membranes plasticize during high pressure CO2 permeation?

Room-temperature ionic liquids (RTILs) are a class of organic solvents that have been explored as novel media for CO2 separations. Polymerized RTILs (poly(RTILs)) can be synthesized from RTIL monomers to form dense, solid gas selective membranes. It is of interest to understand the permeation properties under single gas and mixed gas conditions at elevated pressure of CO2. The ionic nature of the polymers may result in tight arrangements between the oppositely charged ionic domains in the poly(RTIL) eventually preventing the membrane from excessive swelling and deterioration of its performance at increased pressure and/or temperature. In this work we characterize the permeation behaviour of three different poly(RTILs) at single and mixed gas pressures up to 40 bar and over a temperature range from T = 10–40 °C. We find that CO2 is an equally strong plasticizer in single gas as well as mixed gas experiments: the permeability of CO2 increases by more than 60% over a pressure range of 40 bar. Methane does not plasticize the poly(RTIL) by itself in single gas experiments, however the presence CO2 accelerates its transport by more than 250%. The plasticization effect of CO2 is fully reversible on the time scale of the diffusional processes. Even though the poly(RTILs) are glassy in nature, the plasticization behaviour is distinctly different from regular glassy polymers such as polyimides, polysulfones or polycarbonates due to the reversibility of swelling extent. Tailoring the length of the side chain of poly(RTIL) indicates that short side chains suppress plasticization and acceleration of the methane permeability up to a pressure of 10 bars CO2: longer side chains however do not. This suggests the interpretation that a molecular energy balance of ionic attraction and swelling-induced relaxation exists: however, the swelling-induced relaxations overrule the ionic interaction.