Structural changes of water in a hydrogel during dehydration

Raman spectra of poly-N,N,-dimethylacrylamide hydrogel were measured in order to investigate the mechanism of the structural changes in water and the polymer network during dehydration. The results show that the vibrational energies of the CO and the O–H stretching modes increase with the extent of dehydration, whereas that of the CH3 rocking mode decreases. The energy shifts observed in the CO stretching and CH3 rocking modes indicate that the polymer network shrinks with the dehydration and undergoes a glass transition at some point. The energy shifts of the OH stretching modes are attributed to changes in the water structure with the structural change of the polymer network. By applying a structural model of bulk water to the spectra of the OH stretching region, the local water structures in the gel and the dried glassy polymer were analyzed. The result shows that a tetragonal water structure consisting of four hydrogen bonds increases in the residual water of the dried glassy polymer, suggesting that the residual water forms a two-dimensional hydrogen-bonded network. The local water structure in a polymer might have important implications for the interpretation of properties of localized water, for instance, water in a mineral crack.