Ionic conductivity and assembled structures of imidazolium salt-based block copolymers with thermoresponsive segments

Abstract

Ionic liquid-based block copolymers composed of ionic (solubility tunable)-nonionic (water-soluble and thermoresponsive) segments were synthesized to explore the relationship between ionic conductivity and assembled structures. Three block copolymers, comprising poly(N-vinylimidazolium bromide) (poly(NVI-Br)) as a hydrophilic poly(ionic liquid) segment and thermoresponsive poly(N-isopropylacrylamide) (poly(NIPAM)), having different compositions, were initially prepared by RAFT polymerization. The anion-exchange reaction of the poly(NVI-Br) in the block copolymers with lithium bis(trifluoromethanesulfonyl)imide (LiNTf2) proceeded selectively to afford amphiphilic block copolymers composed of hydrophobic poly(NVI-NTf2) and hydrophilic poly(NIPAM). Resulting poly(NVI-NTf2)-b-poly(NIPAM) exhibited ionic conductivities greater than 10-3 S/cm at 90 °C and 10-4 S/cm at 25 °C, which can be tuned by the comonomer composition and addition of a molten salt. Temperature-dependent ionic conductivity and assembled structures of these block copolymers were investigated, in terms of the comonomer composition, nature of counter anion and sample preparation procedure.