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 °C up to T c. Upon prolonged heating of the phase-separated mixture at constant temperature (T 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 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.

Abstract. The tacticity effects on the water-vapor sorption of the thin films of poly(N-ethylacrylamide) (PNEAm) and poly(N,N-diethylacrylamide) (PNdEAm) are investigated by the quartz crystal microbalance method and infrared spectroscopy. The quantity of sorbed water on the spin-coated films of isotactic-rich PNEAm is larger than that of atactic one. For PNdEAm, the syndiotactic-rich polymer film absorbs more water. The amide I or C=O stretching band of the polymers are monitored during water sorption on the spin-coated films. The result indicates that the water-sorption site is also influenced by the tacticity of the polymers. The annealing of these films wipes out the tacticity dependence of water sorption, indicating that the water-sorption sites in the spin-coated films are voids around C=O groups that are made by evaporation of solvent during spin-coating (so-called, the "solvent-imprint" effect). The isotactic-rich PNEAm is more influenced by the solvent-imprint effect than the syndiotactic-rich one, while an opposite tendency is found for PNdEAm.