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

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(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.