Temperature dependence of the structure of bound water in dried glassy poly-N,N,-dimethylacrylamide

Raman spectroscopy was used to investigate the temperature dependence of structural changes of bound water in dried glassy poly-N,N,-dimethylacrylamide in the temperature range 286.1-329.7 K. The results show that the frequency of the O-H stretching mode of the bound water that is present in the dried glassy polymer shifts to the higher side with increasing temperature. The rate changes at around 310 K, while that for the bulk water is constant in the temperature range studied. The rates of change of the frequencies for the C=O stretching mode and CH(3) rocking mode also change at around 310 K. These results indicate a significant change in the interaction between the bound water and polymer chains at 310 K. Temperature dependence of the local structure of the bound water was analyzed by applying a structural model of bulk water to the spectra of the O-H stretching region. The result shows that the density of a tetragonal water structure consisting of four hydrogen bonds increases with increasing temperature below 310 K and begins to decrease at temperatures above 310 K. Further, estimates of the water content indicate that the evaporation rate of the bound water significantly changes at around 310 K. These results suggest that the bound water present in the dried glassy polymer can be classified as being in two states. At temperatures below 310 K, the water that forms a shell layer around the polymer chains evaporates, while at temperatures above 310 K the water that is bound to polar groups of polymer chains begins to evaporate. The structural changes of bound water might have important implications for the interpretation of properties of hydrated polymer systems, including both biological and synthetic polymers.