Mesoscopic Structures in Gel and their Evolution on Dehydration

凝胶是由交联型聚合物网络和溶剂组成的体系，因此是一个多组分体系。当组分彼此高度兼容时，聚合物网络向溶剂中扩散，以最大限度地提高混合的熵。相反，当组成不相容时，网络收缩以最小化表面能。例如，随着温度的升高，N-异丙基丙烯酰胺凝胶在36℃时体积减小。这种现象就是众所周知的体积相变。这个项目探索了脱水引起的凝胶的性质变化，以及产生收缩凝胶的另一种机制。除体积总体下降外，其他性质在脱水过程中也发生了剧烈变化，凝胶表现出类似玻璃的行为。这一点已通过粘弹性测量、拉曼散射中低频峰的观察以及非弹性中子散射实验得到证实。此外，小角X射线散射研究揭示了杂化聚合物水凝胶中独特的介观结构，这种结构在脱水时表现出微观分离。NIPA基团的溶剂亲和力没有明显变化，因为脱水是在室温下进行的。因此，我们预计，杂多聚合物凝胶的微观分离是一种普遍现象，不仅限于体积相变，而且需要一组具有竞争溶剂亲和力的平衡凝胶成分。

A gel is a system composed of a cross-linked polymer network and solvent and is therefore a multicomponent system. When the constituents are highly compatible with each other, the polymer network spreads out into the solvent to maximize the entropy of mixing. Conversely, the network shrinks to minimize the surface energy when the constituents are incompatible. For example, with increasing temperature, N-isopropylacrylamide (NIPA) gel exhibits a volume reduction at 36℃. This phenomenon is well-known as the volume phase transition. This project explores the property changes induced in a gel by dehydration and another mechanism to produce a shrunken gel. In addition to overall decrease in volume, other properties show drastic changes during dehydration and the gel exhibits a glass-like behavior. This has been confirmed with viscoelastic measurements, via the observation of a low-frequency peak in Raman scattering, and also by inelastic neutron scattering experiments. Furthermore, a small-angle X-ray scattering study reveals a distinct mesoscopic structure in a heteropolymer hydrogel, which exhibits microscopic separation upon dehydration. The solvent affinity of the NIPA group does not change appreciably because the dehydration was conducted at room temperature. Therefore, we anticipate that microscopic separation of the heteropolymer gels is a general phenomenon, which is nor restricted to the volume phase transition, and requires a balanced set of gel constituents with competing solvent affinities.