A molecular level study of the phase transition process of hydrogen-bonding UCST polymers

Abstract

Most of the existing studies have focused on the development of the lower critical solution temperature (LCST) behavior of thermoresponsive polymers, whereas research on the upper critical solution temperature (UCST) behavior from a molecular point of view is extremely rare. In this work, a molecular level study of the UCST thermal transition of poly-N-acryloylglycinamide (PNAGA)-based polymers in water and in a water/methanol mixture (χ_m = 6.9 mol%) was performed. Turbidimetric measurements revealed the increased UCST and enlarged hysteresis of the thermal transition of the polymers with the addition of methanol. Fourier transform infrared (FTIR) spectral analysis indicated that the increased hydrophilicity among the C–H groups along the polymer chains upon heating induced the UCST-type transition of the polymers in water, while the hydrogen bonding transformation predominated the thermal transition in the mixture. Moreover, in combination with two-dimensional correlation spectroscopy (2Dcos), the structures with different types of inter/intramolecular hydrogen bonds formed among the NAGA units have been distinguished and the transformation of the amide groups in PNAGA-based polymers from CO⋯D–N hydrogen bonds to CO⋯D₂O ones during the UCST-type transition upon heating was captured.