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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

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journal contribution
posted on 24.04.2012, 00:00 by Yukiteru Katsumoto, Aki Tsuchiizu, XingPing Qiu, Françoise M. Winnik
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 (Tc) and that PIPOZ crystallizes in the polymer-rich liquid phase upon prolonged heating of the mixture at a temperature T > Tc. 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 Tc. Upon prolonged heating of the phase-separated mixture at constant temperature (Tc + ∼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 Tc, 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.