Coulomb interaction-driven UCST in poly(ionic liquid) random copolymers

Abstract

Upper critical solution temperature (UCST)-type thermoresponsive phenomena is not very common in conventional hydrophobic or hydrophilic polymers in aqueous solution. The incorporation of poly(triphenyl-4-vinylbenzylphosphonium chloride) (P[VBTP][Cl]), poly(ionic liquid) (PIL) segment with either hydrophobic poly(methyl methacrylate)(PMMA)/polystyrene (PS) or hydrophilic poly(hydroxyethyl methacrylate)(PHEMA)/poly(N-isopropyl acrylamide)(PNIPAM) segments via reversible addition–fragmentation chain transfer (RAFT) copolymerization is a simple, yet versatile approach for introducing UCST property in their random copolymers. These PIL copolymers exhibit UCST-type behaviours in aqueous or in methanol solution depending on their compositions in the presence of externally added halide anion (Cl− and I−). The cause of UCST is the formation of copolymer aggregates by anion-bridging Coulomb interactions followed by their disruption upon heating. The cloud points of these copolymers can easily be tuned with respect to the copolymer compositions as well as concentrations of anions. Whereas, the random copolymer of P[VBTP][Cl] and PNIPAM segments with low (ca. 1.1–19.3 wt%) ionic PIL content show lower critical solution temperature (LCST)-type behaviour and its cloud point increases linearly with increasing ionic PIL content. More interestingly, P[VBTP][Cl]-ran-PNIPAM copolymer containing high (ca. 34.5 wt%) percentage of ionic PIL segment simultaneously show a LCST-type transition at lower temperature followed by an UCST-type transition at higher temperature in the presence of chloride ions. Tuning of both the LCST-/UCST-type cloud points is possible by varying the concentrations of copolymer and the added chloride ion.