Toward the dynamic phase transition mechanism of a thermoresponsive ionic liquid in the presence of different thermoresponsive polymers

The influence of two thermoresponsive polymers, poly(N-isopropylacrylamide) (PNIPAM) and poly(N-vinylcaprolactam) (PVCL), on the phase transition behavior of a thermoresponsive ionic liquid, tributylhexylphosphonium 3-sulfopropylmethacrylate ([P-4,P-4,P-4,P-6][MC3S]), was investigated. An obvious distinction was observed in the LCSTs and morphologies of [P-4,P-4,P-4,P-6][MC3S]-PNIPAM and [P-4,P-4,P-4,P-6][MC3S]-PVCL aqueous solutions, indicating their large differences in dynamic transition processes. In general, PNIPAM can "break'' the water structure of [P-4,P-4,P-4,P-6][MC3S] to decrease the transition temperature, while PVCL can "make'' the water structure to increase it. Surprisingly, [P-4,P-4,P-4,P-6][MC3S] has an unusual over-hydration behavior before dehydration while PNIPAM experiences a two-step transition process in [P-4,P-4,P-4,P-6][MC3S]-PNIPAM aqueous solution, which has never been reported so far. Further studies revealed that the formation of strong intra-/inter-molecular hydrogen bonds C=O...D-N in PNIPAM is the driving force for the LCST phenomenon of [P-4,P-4,P-4,P-6][MC3S]-PNIPAM solution, while it is the [P-4,P-4,P-4,P-6][MC3S] that dominates the phase separation of [P-4,P-4,P-4,P-6][MC3S]-PVCL solution.