Structure-property relationships of imidazolium-containing polymer systems: Homopolymers, block copolymers, and block copolymer/ionic liquid mixtures

Advanced solid-state polymer electrolytes for electrochemical and energy storage applications are needed to replace conventional liquid electrolytes that are unstable, flammable, and volatile. In particular, a fundamental understanding of morphology-ionic conductivity relationships is necessary to improve the ionic conductivity of ion-containing polymer systems. To this end, we investigate the structure-property relationships of homopolymer and block copolymer systems containing imidazolium-based ionic liquids (ILs). We first explore the effects of anion type and pendant alkyl chain length on the morphologies and properties of polymerized ionic liquid (PIL) homopolymers with bound cations. In both acrylate-based and vinylimidazolium PIL homopolymers, nanoscale segregation of polar and non-polar moieties is detrimental for ionic conductivity. Increasing the length of alkyl pendant groups increases ion aggregation and decreases ion mobility, while increasing the size of mobile anions decreases ionic aggregate formation and the glass transition temperature, leading to increased ionic conductivity. The incorporation of hydroxyl-terminated alkyl pendant groups in vinylimidazolium homopolymers further decreases compositional heterogeneity of polar and non-polar moieties and increases ionic conductivity by one order of magnitude. We then examine the morphology-ionic conductivity relationships of IL-containing block copolymers and show that microdomain orientation, chain length, and confinement of polymer segments strongly impact ionic conductivity. Non-ionic diblock copolymer/IL mixtures, with the same block copolymer composition, display anisotropic lamellar morphologies, and morphology factors describing the extent of anisotropy are determined from X-ray scattering data. Ionic conductivities increase when lamellar microdomains are aligned parallel to the ion transport measurement direction and with increasing molecular weight. Self-consistent field theory calculations predict a more uniform IL distribution within microdomains when molecular weight is increased, suggesting that composition and dynamic gradients are detrimental to ionic conductivity. In hydrophilic PIL copolymers, ion transport is heavily dependent on water content and monomeric sequence. The conductivity of block copolymers is ten to fifteen times greater than that of random copolymers due to the microphase separation of ion-conducting and insulating blocks and local enhancement in ion concentration. At 90% relative humidity, several PIL block copolymers exhibit ionic conductivities that exceed their analogous homopolymers, suggesting that confinement of PIL chains and water in microdomains accelerates ion transport