Temperature response behavior of poly(1-vinyl-3-methylimidazole dimethyl phosphate-co-N-isopropylacrylamide) in aqueous and ester solutions driven by hydrogen bonding

A series of poly(1-vinyl-3-methylimidazole dimethyl phosphate-co-N-isopropylacrylamide) oligomers was synthesized by reversible addition-fragmentation chain transfer polymerization. The co-oligomers exhibited a pH-tunable lower critical solution temperature (LCST) phase transition in aqueous solutions. By changing the pH of the aqueous solution (HCI), the hydrogen bonding between the internal molecules of the co-oligomers and water molecules was weakened, which effectively reduced the cloud-point temperature. Poly(1-vinyl-3-methylimidazole dimethyl phosphate-co-N-isopropylacrylamide) oligomers exhibit an upper critical solution temperature (UCST) type of phase transition in an ester solution, which is associated with the formation and breaking of hydrogen bonds within the co-oligomers molecules. FT-IR analysis revealed the effect of temperature on hydrogen bonding between the functional groups. Energy decomposition, Electrostatic Potential (ESP), Atoms-in-Molecules (AIM), and Interaction region indicator (IRI) analyses demonstrated the presence of hydrogen bond donors N–H and imidazole ring C–H, and hydrogen bond acceptors C = O and P = O in the co-oligomers. The sites of hydrogen bonding between the co-oligomers and the solvent molecules as well as intermolecular forces were found to be the main factors contributing to the LCST/UCST-type phase transition.