Increasing Permittivity in Ion-Containing Polymers: Influence of Zwitterion Additives on Ion-Conduction

An effective approach to improve the ionic conductivity in ion-containing polymers is to increase the dielectric constant of the host, by adding molecules with large dipole moments. With covalently bonded cation and anion on a single molecule, zwitterions are promising candidates. Despite the high ionic conductivity observed by zwitterion addition in polymerized ionic liquids (PILs), the underlying mechanism remains misunderstood. Here, we studied the change in dielectric constant for a series of sulfonate-imidazolium-based zwitterions blended with glycol solvents using dielectric relaxation spectroscopy (DRS). Adding zwitterions drastically increases the permittivity compared with that of the pure glycol solvents. The Landau-Lifshitz mixing rule describes well the measured permittivities of the blends and was further used to extrapolate to the permittivities of pure zwitterions based on their blend behavior. The dependence of measured permittivity on zwitterion structure, composition, and temperature will be discussed in the context of ion conduction in polymeric materials. The work presented here highlights the exceptional ability of zwitterions to increase permittivity and their potential to boost conductivity of PILs by creating a more polar environment.