Morphology and structure of poly(vinyl alcohol) (PVA) hydrogel prepared by the "repeated freezing-and-melting" method have been investigated by X-ray diffraction, scanning electron microscopy, light-optical microscopy, and simple tension test. The PVA aqueous solution gelled highly by using this method to show rubber-like elasticity, reflecting the gel network in which the amorphous chains are physically cross-linked by the crystallites. The gel morphology was characterized by the porous structure, which was originated from the gelation of continuous PVA-rich solution phase segregated around copious ice crystal phases formed upon freezing. The high gelling ability involved in this method was closely related to the segregation mechanism.

Agarose hydrogels which showed optical anisotropy were obtained by the directional freezing of starting isotropic gels under a temperature gradient. The directional freezing caused a crystallization of many isolated ice crystal phases, leaving a honeycomb-like gel phase with a higher polymer content. The crystallographic c-axis of the ice crystals was directed to the temperature gradient. X-ray and optical analyses showed that agarose chains had a strong planar orientation along the walls'side surfaces, which were parallel to the equatorial planes of the ice crystals. Scanning electron microscopy showed that the wall consisted of a large number of sheets stacked along the wall thickness; in each sheet, agarose fibrillar structures were found to be densely aligned. With the application of repeated freezing and thawing, the anisotropy of the segregated gel phases increased.

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We succeeded in preparing the urea-polyethylene complex by the addition of a previously formed urea-paraffin complex in xylene solution of polyethylene at 120°C. This is a substitution reaction in which the guest molecules were changed from paraffin to polyethylene. The reaction mechanism is discussed here. DSC thermal analysis of the urea-polyethylene complex shows that it has a higher melting temperature, 148"C, than either urea and polyethylene. X-ray diffraction measurements show that it has the same hexagonal unit cell dimensions as those of the urea-paraffin complexes. However, scanning electron microscopic observation shows it to have significantly different morphological features from those of the urea-paraffin complexes. The hexagonal prism-like crystal of the urea-polyethylene complex is composed of a number of lamella stacked regularly along the long axis of the crystal. Moreover, it is shown that the polyethylene crystals obtained from the urea-polyethylene complex have morphological features similar to those of the complex, 277