Hydroxyalkyl-Containing Imidazolium Homopolymers: Correlation of Structure with Conductivity

Abstract

Conventional free radical polymerization of (hydroxy)alkyl-containing vinylimidazolium ionic liquid monomers generated high molecular weight polymers. A study of these materials was undertaken to relate chemical composition to macromolecular properties. Varying the substituent chain length and functionality of these polymerized ionic liquids (PILs) significantly influenced numerous polymer properties. Incorporation of a hydroxyl group at the terminus of the alkyl substituent lowered the glass transition temperature ( T g ) of the PILs ∼50 °C and increased homopolymer thermal stability compared to the respective alkyl analogue. X-ray scattering demonstrated that (hydroxy)alkyl chain length influenced polymer backbone-to-backbone spacing. More importantly, X-ray scattering showed hydroxyl incorporation diminished the local compositional heterogeneity present in the alkyl-functionalized vinylimidazolium homopolymers. Solid-state NMR quantified ratios of locally ordered versus disordered components. Ionic conductivity of the imidazolium homopolymers increased over an order of magnitude upon hydroxyl group incorporation due to the hydroxyl group polarity solvating homopolymer electrostatic interactions. Effectively controlling homopolymer chemical composition enabled tuning of thermal properties and polymer morphology for improved ionic conductivity.