Dissecting the Mechanism of the Heat-Induced Phase Separation and Crystallization of Poly(2-isopropyl-2-oxazoline) in Water through Vibrational Spectroscopy and Molecular Orbital Calculations

Abstract

Aqueous solutions of amphiphilic polymers often undergo a heat-induced phase separation, which is known as the lower critical solution temperature (LCST) phase transition. In the case of aqueous poly­(2-isopropyl-2-oxazoline) (PIPOZ) solutions, the phase separation is followed, upon prolonged heat treatment, by an irreversible crystallization of the polymer. Optical microscopy observation of a PIPOZ solution (60 g L^–1) in water revealed that liquid–liquid phase separation of the aqueous PIPOZ solution occurs at the cloud point (<i>T</i>_c) and that PIPOZ crystallizes in the polymer-rich liquid phase upon prolonged heating of the mixture at a temperature <i>T</i> > <i>T</i>_c. Vibrational spectroscopy combined with molecular orbital (MO) calculations and spectral measurements with model compounds were employed to monitor water/polymer interactions and changes in polymer conformation during the LCST-type phase separation. The thermally induced spectral variations suggest that the dehydration of the PIPOZ amide functions occurs gradually as the temperature is raised from 20 °C up to <i>T</i>_c. Upon prolonged heating of the phase-separated mixture at constant temperature (<i>T</i>_c + ∼2 °C), the infrared spectrum of the polymer undergoes further changes ascribed to conformational transitions of the polymer backbone. These changes, which are irreversible upon cooling the solution below <i>T</i>_c, lead to the conformation taken by the polymer in the crystalline phase. This situation facilitates crystallization of the polymer by a nucleation/growth mechanism in the polymer-rich phase, a process akin to the crystallization of proteins from solution