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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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<sup>–1</sup>) in water revealed that liquid–liquid phase separation of
the aqueous PIPOZ solution occurs at the cloud point (<i>T</i><sub>c</sub>) 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><sub>c</sub>. 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><sub>c</sub>. Upon
prolonged heating of the phase-separated mixture at constant temperature
(<i>T</i><sub>c</sub> + ∼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><sub>c</sub>, 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