Effect of Free Cation Substituent on Gas Separation Performance of Polymer-Room-Temperature Ionic Liquid Composite Membranes

Room-temperature ionic liquid (RTIL) based monomers were photopolymerized in the presence of nonpolymerizable RTILs with various types of organic functional groups attached to the imidazolium cation. Groups employed include alkyl, ether, nitrile, fluoroalkyl, and siloxane functionalities. This straightforward method allows for a broad range of functional groups to be incorporated into poly(RTIL) matrices without the need to develop new monomers. The resultant poly(RTIL)-RTIL composites were homogeneous, optically transparent solids that exhibited no signs of phase separation, even after many months of storage. As thin films, poly(RTIL)-RTIL composites were utilized as gas separation membranes and tested for their permeabilities to CO2, N-2, and CH4. The presence of 20 mol % free RTIL within the poly(RTIL) matrix was shown to increase CO, permeability by similar to 100-250% relative to the neat poly(RTIL) membrane with no free RTIL component. The nature of the organic functional group associated with the free RTIL cation call influence both gas permeability and CO2/N-2 and CO2/CH4 selectivity.