Solid-State NMR Studies of Hydrated and Dehydrated States of Temperature-Responsive Polymers

Temperature-responsive P(MMA-co-BIEM-graft-NIPAAm) graft copolymers and a PNIPAAm homopolymer were prepared to explore their potential in a polymer-based cooling system (MMA = methyl methacrylate; BIEM = a combination of HEMA (2-hydroxyethyl methacrylate) and BIB (2-bromoisobuthyl bromide); NIPAAm and PNIPAAm = N-isopropylacrylamide and its polymer, respectively). Thermogravimetric analysis revealed that the hydration and dehydration rates of the graft copolymers were higher than those of the homopolymer. Controlled-rate thermal analysis and infrared spectroscopy suggested that the hydrogen bonding strength between water molecules and the polymer frameworks was similar in the copolymer and homopolymer. Solid-state 1H NMR relaxation measurements demonstrated that (i) the mechanism of the hydrophilic/hydrophobic switching transition in the solid state differs from the coil–globule transition observed in aqueous solution, and the release of hydrogen-bonded water molecules being closely associated with the transition; and (ii) hydrogen-bonded water in the homopolymer system promotes partial gelation, which reduces dehydration rate.