In-depth study of the phase separation behaviour of a thermoresponsive ionic liquid and a poly(ionic liquid) in concentrated aqueous solution

The temperature-induced phase transition behaviors of a thermoresponsive ionic liquid (tributylhexylphosphonium 3-sulfopropylmethacrylate, [P4,4,4,6][MC3S]) and its polymer (poly-tributylhexylphosphonium 3-sulfopropylmethacrylate, P[P4,4,4,6][MC3S]) have been investigated using DSC, optical microscopy, temperature-variable 1H NMR, and FT-IR in combination with two-dimensional analysis methods, including perturbation correlation moving window (PCMW) and two-dimensional correlation spectroscopy (2Dcos). We found that there exists a distribution gradient of water molecules in P[P4,4,4,6][MC3S] ranging from hydrophobic backbones to hydrophilic sulfonates. Linked together by covalent bonds, P[P4,4,4,6][MC3S] would form an “urchin-like” structure, which can improve its stability and strengthen the gradient distribution of water. Finally, 2Dcos was employed to elucidate the sequential order of chemical group motions during heating. It is concluded that both [P4,4,4,6][MC3S] and P[P4,4,4,6][MC3S] experience the anionic dominated phase transition process. Moreover, the driving force for the phase transitions is shown to be the dehydration of hydrophobic ester carbonyls.