State of water in hydrogels and the mechanism of volume phase transitio

点击翻译按钮获取中文摘要

In the present research, we have studied the volume phase transition of N-isopropylacrylamide (NIPA) gels by Brillouin scattering, dynamic light scattering (DLS), Fourier-transformed infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR). Particular attention was payd to bound (or non-freezable) water molecules in the shrunken phase of NIPA gels.The Brillouin scattering experiment has not been applied on gels undergoing the volume phase transition so far, thus our study is the first example of such a study. It was revealed that the ultrasonic attenuation becomes maximum at the transition point, which suggests the slowing down of motions of water molecules bount to the network. The results of DLS experiment showed that the dynamics of gel network differs markedly between the swollen and the shrunken phases. The latter is more of solid-like than of liquid-like. The new mode appearing in the shrunken phase is ascribable to some side-wise motions.By means of FTIR measurement, we have observed changes in frequency of the amide-I and amide-II modes, which was induced by hydrogen-bond formation with surrounding water molecules. In the shrunken phase of this gel, it was revealed that four water molecules per NIPA monomer are contained. It is not yet clear, however, whether all these water molecules are bound to the network, or only a part of them makes a hydrogen bond at a time. NMR measurement in the shrunken phase of NIPA gel revealed only a broad peak. This shows that the network movement is highly restricted in this phase, which is consistent with the result of DLS.