Water properties in a novel thermoswelling poly(vinyl alcohol) derivative hydrogel as studied by nuclear magnetic resonance and fourier transform infrared spectroscopy

Water properties in a novel thermoswelling hydrogel, which was prepared from poly(vinyl alcohol)-trimellitate (PVA-T) by a simple chemical cross-linking and swollen in 0.1-1.0 M Li₂SO₄ solutions, were investigated through nuclear magnetic resonance and Fourier transform infrared (FTIR) spectroscopies. The spin-spin relaxation of the water proton in the hydrogel was measured at 5-35 °C, and the results were analyzed with a two-component model to obtain a long T₂ and a short T₂ as well as their fractions (f_short = 1 - f_long). The f _short values thus obtained proved to be a linear function of the gel swelling ratio, and all of the data, except for an upper deviation at 1.0 M Li₂SO₄, were found to be on an almost same line irrespective of the temperature and the salt concentration. This dependency of f_short on the swelling degree strongly suggests that the temperature increment has an equivalent effect as that of the SO₄^2- concentration; namely, scission of inter(intra-)molecular hydrogen bonding (HB) between the COOHs on the side group must be responsible for the observed thermoswelling in the sulfate salt solutions. The upper deviation of f _short at 1.0 M from the "master line" was reasonably interpreted in terms of the salting-out effect by the concentrated sulfate anion. On the other hand, attenuated total reflection-FTIR measurements for a gel plate revealed that an appreciable dissociation of the carboxyl group occurred only in the 1.0 M Li₂SO₄ system. This finding, in turn, means that gel swelling with an increase in the salt concentration up to 0.5 M is not caused by the ionization of the gel and supports the scission of the intermolecular HB. Hydrophobic hydration around the main chain was investigated via a peak shift of the stretching vibration of -CH₂-, and the slight red shift observed only at 1.0 M suggested that the salting-out effect onto the hydrophobic hydration is rather limited and the hydration around the main chain still remained even with the significant deswelling at 1.0 M.