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[摘要]：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 (T-c) and that PIPOZ crystallizes in the polymer-rich liquid phase upon prolonged heating of the mixture at a temperature T > T-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 degrees C up to T-c. Upon prolonged heating of the phase-separated mixture at constant temperature (T-c + similar to 2 degrees 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 T-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. |
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