Thermoresponsive poly(2-oxazoline) homopolymers and copolymers in aqueous solutions studied by NMR spectroscopy and dynamic light scattering

Abstract Structural changes during temperature-induced phase transition in D 2 O solutions of poly(2-ethyl-2-oxazoline) (PEOx) and P(EOx- grad -2-methyl-2-oxazoline (MOx)) copolymers were investigated by 1 H NMR methods in combination with dynamic light scattering. NMR spectra showed that in PEOx solutions on molecular level structures formed during gradual heating are preserved during subsequent cooling. Phase transition in solutions of P(EOx- grad -MOx) copolymers as revealed by NMR exists irrespective the solution is turbid or nonturbid at higher temperatures. In contrast to PEOx homopolymers it is very broad, virtually independent of the copolymer composition and molecular weight, and reversible with some hysteresis. Reduced intensities of cross-peaks in NOESY spectra of copolymer solutions at elevated temperatures are mainly due to the changed mobility of copolymer segments. In all investigated solutions two types of water, “free” and “bound” with long and very short spin–spin relaxation times T 2 , respectively, were detected in the transition region and above the transition. The existence of two T 2 components shows that exchange between “bound” and “free” water is slow regarding T 2 values.