Study of Sol-gel Transition using Various NMR Techniques

在水溶液中加入Rb^+离子形成的κ-卡拉胶凝胶在55 ℃左右随温度升高转变为溶胶。为了考察水分子在凝胶中的行为，我们用核磁共振法测定了水分子的自扩散常数。自扩散常数通过Carr-Purcell-Meiboom-Gill法和场梯度法的组合使用，使用配备PL 502场梯度单元的JEOL JNM-FX 60测量。利用已知的室温下纯水的自扩散常数对场梯度单元进行了标定，测量了纯水分子在25 ~ 75 ℃温度范围内的自扩散常数随温度的变化关系。发现观察到的值与文献中的值吻合良好。我们还测量了5wt% κ-卡拉胶水溶液的水分子自扩散常数的温度依赖性。测定了κ-卡拉胶凝胶的自扩散常数随温度的变化关系。结果表明，水分子在凝胶状态下的自扩散常数小于纯水的自扩散常数，表明水分子在凝胶网络中的平移运动受到限制。在55 ° C附近发生凝胶-溶胶转变时，自扩散常数突然增加并接近纯水的值。结果表明，转捩释放了水的平移运动自由度。当温度降低时，过渡区向低温移动，表现出滞后现象。发现数据在过渡区分散，推测暗示临界弛豫。

Kappa -Carrageenan gel which is formed by the addition of Rb^+ ion to its aqueous solution undergoes a transition into sol in the vicinity of 55^ﾟC with increasing temperature. In order to examine the behavior of water molecules in the gel, we measured the self-diffusion constant of water molecules by the nuclear magnetic resonance method.The kappa -carrageenan gel was prepared by adding 50mM RbCl to 5wt% kappa -carrageenan aqueous solution. The self-diffusion constant was measured by the combined use of the Carr-Purcell-Meiboom-Gill method and the field gradient method using JEOL JNM-FX60 equipped with a PL502 field gradient unit. The field gradient unit was calibrated using the known self-diffusion constant of pure water at room temperature.We measured the temperature dependence of the self-diffusion constant of pure water molecules in the temperature range from 25 to 75^ﾟC. The observed value was found to agree well with the value in the literature. We also measured the temperature dependence of the self-diffusion constant of water molecules of the 5wt% kappa -carrageenan aqueous solution. The values are almost the same as those of pure water.The temperature dependence of the self-diffusion constant of the kappa -carrageenan gel was measured. It was found that the self-diffusion constant of water molecules in the gel state is smaller than that of pure water, indicating the restricted translational motion of water molecules inside the gel network. In the vicinity of 55^ﾟC where the gel-to-sol transition occurs, the self diffusion constant abruptly increases and approaches the value of pure water. The results indicate that the transition releases the translational freedom of motion of water. When the temperature is lowered, the transition region shifts to lower temperature, showing the hysteresis. The data were found to scatter in the transition region, supposedly implying the critical relaxation.