The influence of a thermoresponsive polymer on the microdynamic phase transition mechanisms of distinctly structured thermoresponsive ionic liquids

The study of a ternary solution involving a thermoresponsive polymer, a thermoresponsive ionic liquid (IL), and a solvent will not only help with interpreting their distinct phase transition behavior, but also promote the development of novel thermoresponsive systems. In this paper, we investigate the influence of a conventional thermoresponsive polymer, poly(2-isopropyl-2-oxazoline) (PIPOZ), on the phase transition behavior of two thermoresponsive ILs ([P4,4,4,6][MC3S], [P4,4,4,4][SS]) with different structures. Although the addition of PIPOZ reduces the transition temperatures of both ILs, our analyses demonstrate that there exists a large difference in the microdynamic phase transition mechanisms between [P4,4,4,6][MC3S]/PIPOZ and [P4,4,4,4][SS]/PIPOZ aqueous solutions. Both PIPOZ and [P4,4,4,6][MC3S] experience unexpectedly an unusual over-hydration process before a two-step phase transition of the mixture solution, which can be explained by the presence of a new kind of intermolecular bridging hydrogen bond (IL–water–polymer), whereas only PIPOZ undergoes dehydration around the transition temperature of the [P4,4,4,4][SS]/PIPOZ aqueous solution. Further spectral analyses reveal that both [P4,4,4,6][MC3S] and PIPOZ engage in the phase separation of the ternary solution jointly, whereas PIPOZ takes part in the phase transition of [P4,4,4,4][SS]/PIPOZ solution more independently.