Effects of Anion and Cross-Linker on the Surface Hydrophilicity and Selective Solvent-Induced Swelling of Poly(ionic liquid) Elastomers

Poly(ionic liquid)s (PILs) have attracted great attention because they preserve conventional characteristics of polymers while having special features of ionic liquids, such as good ionic conductivity and excellent electrochemical and thermal stability. The solvent absorption behaviors and solvent-induced responsiveness of PIL elastomers, however, have been less investigated. In this work, we prepare two PILs, poly[C3mim-MA][Br] and poly[C3mim-MA][TFSI], and examine their surface hydrophilicities and solvent-induced responsiveness. Ionic liquid monomers are synthesized and then polymerized with initiators and cross-linkers via free radical polymerization, forming PIL elastomers. The anions in the monomers can be replaced using an ion exchange process to prepare PIL elastomers with different anions. The surface properties, swelling behaviors, and solvent responsiveness of the PIL elastomer films with different anions and cross-linker ratios are investigated. The surface hydrophilicities of the PIL films are characterized by water contact angle measurements; the poly[C3mim-MA][Br] elastomer films are more hydrophilic than the poly[C3mim-MA][TFSI] elastomer films. Due to the different hydrophilicities, the poly[C3mim-MA][Br] and poly[C3mim-MA][TFSI] elastomer films can be swollen selectively in water and acetone, respectively. For both elastomers, the swelling degrees are higher at lower cross-linking densities, and the volume expansions can be up to 655%. Furthermore, we investigate the solvent-vapor-induced responsiveness of bilayer films of PIL elastomers and polyimide (PI) tapes. The bilayer films can be bent selectively by exposing them to water or acetone vapors, depending on the anions.