Effect of ionic liquid on mechanical properties and morphology of zwitterionic copolymer membranes

Zwitterionomers containing less than 13 mol % zwitterion functionality were synthesized using free radical copolymerization of n-butyl acrylate (nBA) and sulfobetaine monomers. X-ray scattering results revealed a two-phase morphology, which is typical of random ionomers with an ionomer peak at q ∼1.5 nm ^-1. Swelling studies in the ionic liquid (IL), 1-ethyl-3- methylimidazolium ethylsulfate (EMIm ES), showed an influence of zwitterionic structure on IL uptake. Zwitterionomer membranes were swollen to various IL contents, and the influence of IL on mechanical properties, morphology, and ionic conductivity was explored through dynamic mechanical analysis (DMA), X-ray scattering, and impedance spectroscopy, respectively. Results revealed that IL preferentially swelled the zwitterionic domains but was excluded from the matrix phase. Significant changes in mechanical properties and ionic conductivity were observed above a critical uptake of IL. Fundamental explorations of the interaction of ILs with sulfobetaine-containing copolymers may lead to the use of zwitterionomers in emerging membrane applications.