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 CH(3) rocking mode decreases. The energy shifts observed in the C=O stretching and CH(3) 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.