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.

No abstract

synopsisSingle crystals of amylose V complexes with the 8, helical configuration can be obtained from aqueous solutions of amylose by using a-naphthol as a complexing agent. Morphological observations suggest that the differences in crystallization behavior among the a-naphthol complex and other complexes with alcohols are due to differences in solubility of the complexes in water. Electron diffraction studies indicate a two-dimensional tetragonal unit cell with a = b = 22.9 A. It is deduced that the space group providing a satisfactory arrangement of two helices is one of the enantiomorphs P4,bI8and P4&%. From x-ray diffraction it was found that the c axis spacing of the a-naphthol complex is equivalent to that in 6, and 7, helical amylose crystals. Consequently, the geometry of the helical configuration requires an integral number of glucose residues per turn. The true helical diameters of the n-butanol, isopropanol, and a-naphthol complexes were calculated from experimental data. The ratio was 6:7:8 and indicated that the helix of the a-naphthol complex has eight glucose residues per turn. The diversity of helical configurations in V amylose crystals is discussed.

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.

No abstract