H NMR Relaxation Study of Thermotropic Phase Transition in Poly(vinyl methyl ether)/D2O Solutions

Abstract

1H NMR line shapes and spin-spin relaxation times T 2 have shown that in PVME/D 2 O solutions the LCST transition results in limited mobility of most PVME units, evidently in connection with formation of compact globular-like structures. The minority mobile component, which does not take part in the phase transition, mostly consists of low-molecular-weight fraction of PVME, as shown by SEC. Measurements of spin-spin relaxation times of PVME methylene protons have shown that globular structures are more compact in dilute solutions in comparison with semidilute and concentrated solutions where globules contain a certain amount of water. A certain portion of water molecules bound at elevated temperatures in PVME globular structures in semidilute and concentrated solutions was revealed also from measurements of spin-spin and selective and nonselective spin-lattice relaxation times of residual HDO molecules. While the phase separation in dilute PVME solutions is associated with rapid dehydration of polymer chains, for semidilute and concentrated solutions T 2 measurements evidence that with time the originally bound water is slowly released from globular-like structures, and the character of the globules is changed from the spongelike to a rather compact one. From results obtained on D 2 O solutions of PVME/poly(N-isopropylmethacrylamide) (PIPMAAm) mixtures it follows that this process is significantly slower for more rigid globular-like structures of PIPMAAm in comparison with globules of the flexible PVME.