Polymerized Ionic Liquids: The Effect of Random Copolymer Composition on Ion Conduction

Abstract

Ionic conductivity in new polymerized ionic liquids is of great interest as it applies to solid-state electrolytes for electrochemical and electromechanical applications. In this study, an ionic liquid monomer was synthesized and polymerized into random copolymers and their ionic conductivity and structure were investigated as a function of copolymer composition. Both nonionic−ionic and ionic−ionic copolymers were synthesized, where the nonionic and ionic monomers were hexyl methacrylate (HMA) and a methacrylate-based imidizolium neutralized with tetrafluoroborate (BF4) or bis(trifluoromethane sulfonyl)imide (TFSI). In the nonionic−ionic copolymer, the ionic conductivity increased by over an order of magnitude with increasing HMA composition, even though the overall charge content decreased, because the addition of HMA significantly lowered the glass transition temperature. The ionic conductivity also increased by more than an order of magnitude in the ionic−ionic copolymer with increasing TFSI content, even though there was no change in the overall charge content, because substituting the larger anion TFSI for BF4 resulted in weaker ionic interactions and also significantly lowered the glass transition temperature. In both types of copolymers, the temperature dependence of the ionic conductivity was well described by both Arrhenius and Vogel−Tamman−Fulcher models. An important difference between the two classes of random copolymers was that the nonionic−ionic copolymer exhibited microphase separation in X-ray scattering that correlated with a discontinuity in the increasing ionic conductivity with increasing HMA content suggesting that structure can also play a significant role in ion transport in polymerized ionic liquids.