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

本论文采用布里渊散射、动态光散射、傅立叶变换红外光谱和核磁共振等方法研究了N-异丙基丙烯酰胺（NIPA）凝胶的体积相变。特别关注的是NIPA凝胶收缩相中的束缚（或非冻结）水分子。布里渊散射实验迄今尚未应用于经历体积相变的凝胶，因此我们的研究是此类研究的第一个例子。结果表明，在过渡点处，超声衰减达到最大值，这表明水分子的运动减慢。动态光散射实验结果表明，溶胀相和收缩相的凝胶网络动力学有明显差异。后者更像固体而不是液体。通过FTIR测量，我们观察到酰胺-I和酰胺-II模式的频率变化，这是由于与周围水分子形成氢键而引起的。在该凝胶的收缩相中，揭示了每个NIPA单体含有四个水分子。然而，目前还不清楚是所有这些水分子都结合到网络上，还是只有一部分水分子一次形成氢键。在NIPA凝胶的收缩相中的NMR测量仅揭示宽峰。这表明网络运动在此阶段受到高度限制，这与DLS的结果一致。

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.