STUDIES ON THE SELECTIVE UNIPLANAR ORIENTATION OF (101) PLANES OF POLYVINYLALCOHOL CRYSTALS

Abstract

Previous studies on regenerated cellulose have led us to a conclusion that the selective uniplanar orientation of (101) planes of cellulose II crystals occurs during the coagulation and regeneration process. The purpose of this work is to investigate the similar problem, that is the selective uniplanar orientation of polyvinylalcohol crystals caused during the coagulation. It was found that the degree of orientation depends on the concentration of sulfate and coagulation temperature. Higher degree of orientation of (101) planes were observed in the films prepard in the bath of lower temperature containing much salt. Degree of preferential orientation of (101) planes was evaluated in terms of a relative parameter; where I(101) and I(10_??_) were the X-ray scattering intencities of (101) and (10_??_) planes respectively, both measured by the reflection method, and i(101) and i(10_??_) were that obtained for isotropic specimen.Polyvinylalcohol films, in which (101) planes orient parallel to thies surfaces, showed higher swelling and shrinkage in thickness than in surface area, indicating that water permeate between (101) surfaces of polyvinylalcohol crystals. This result may be explaind on the basis of the direction of hydrogen bonds formed between pairs of molecules. In addition to the above behavior of anisotropic swelling and shrinkage, the restrained crystal growth in the direction perpendicular to (101) planes gives another support to the hypothesis that (101) planes are hydrophilic and sensitive to water. From these consideretions, it may be concluded also for the orientation mechanism that absorbed water plasticizes the crystals so as to glide each other along their (101) surfaces, and consequently crystals rotate to bring their planes to parallel position of the film surface.Gliding in crystals could also takes place in another situation, where swollen polyvinylalcohol consisting of imperfect crystals is compressed, since there intermolecular hydrogen bonds would be weakened by water. This mechanism is very similar to that of the preferential orientation of (101) planes in cellulose crystals proposed in our earlier papers.