Surface States and Anomalous First-order Phase Transition in Polymer Gels

(1)凝胶体物理性质的形状依赖性。在1990年，我发现聚合物凝胶的平衡性质，如膨胀度和相变温度，很大程度上取决于样品的形状。目前的研究一直致力于阐明导致凝胶这种不寻常特征的基本机制。提出了两种合理的机制；一种是基于电离凝胶固有的异常大的表面张力，另一种是基于在凝胶化时冻结的一些结构缺陷。在不同凝胶条件下制备的许多样品上重复测量，后一种可能性被拒绝。然而，表面张力的直接测量还没有完成，因此我们不能断定第一种机制是合理的。(2)电离凝胶表面电位的测定。首先用Ag-AgCl微电极测定了离子化凝胶的表面电位。在中性凝胶中，表面电位消失，而在电离凝胶中，可以检测到几十到几百mV的表面电位。该值取决于凝胶网络中所含离子的量。这一结果支持了电化凝胶中存在电化学起源的表面张力。需要进一步的详细测量以获得更多的定量结果。(3)凝胶网络动力学不均匀性测量。最初，我试图通过激光散射来测量凝胶的表面激发，以获得表面状态的详细信息。然而，事实证明这是一个相当困难的测量，因为非常精确的光学校准是获得有意义的结果所必需的。相反，我测量了凝胶在温度跳跃时从一种平衡状态变化到另一种平衡状态时所表现出的大规模波动。根据这一结果，在体积转变附近出现了一个玻璃状的凝胶模型。

(1) Shape dependence of the bulk physical properties of gels.In 1990, I found that the equilibrium properties such as degree of swelling and phase transition temperature of polymer gels depend strongly on the shape of sample. The present research has been devoted to clarify the basic mechanism leading to this quite unusual characteristics of gels. Two plausible mechanisms are proposed ; one is based on an anomalously large surface tension inherent to ionized gels and the other is based on some structural defects which were frozen at the time of gelation. Repeated measurements on many samples prepared from different gelation conditions, the latter possibility was rejected. However, the direct measurement of the surface tension is not made yet, and thus we cannot conclude that the first mechanism is a plausible one.(2) Measurement of surface potential of ionized gels.Surface potential of ionized gels was first measured using Ag-AgCl microelectrode. It is confirmed that the surface potential Vanishes in neutral gels, whereas several tens to a few hundreds mV of the surface potential was detected in ionized gels. The value depends of the amount of ion contained in gel network. This result supports the presence of surface tension of electrochemical origin in ionized gels. Further detailed measurements are needed for more quantitative results.(3) Measurement of dynamical inhomogeneity of gel network.Initially, I tried to measure the surface excitation of gels by laser light scattering to get detailed information on the surface state. However, it turned out to be a fairly hard measurement because very precise optical alignment is essential to obtain meaningful result. Instead, I have measured a large scale fluctuations exhibited by gels when it changes from one equilibrium state to the other in response to temperature jump. From this results, a glass-like model of gels near the volume transition has emerged.