STUDIES ON THE ORIENTATION MECHANISM OF PREFERENTIAL UNIPLANAR ORIENTATION OF (101) PLANES OF CELLULOSE II CRYSTALS

Abstract

Previous work in this series has shown that preferential orientation of (101) planes of cellulose crystals parallel to a film surface was produced by strong dehydration during coagulation and regeneration process. The object of this paper is to describe the mechanism of such orientation of (101) planes in more detail.Although the preferential uniplanar orientation of specific crystallographic planes of polyvinylacohol, cellulose, polyethyleneterephthalate and the like are familiar, but little attention has been paid to the mechanism of the orientation process. Some explanations have been given to each case in different terms; the conformational structure of molecular chains for polyvinylalcohol, the shape of crystallites for cellulose, and slipping and twinning in crystals for polyethylene respectively.In the case of cellulose it is difficult to interpret on the basis of the molecular conformation in crystals, where the larger planes of anhydrous glucose units orient themselves almost perpendicular to a film surface, in contrast to the case of polyvinylalcohol, where the planes of planar zigzag molecular chains orient themselves parallel to a film surface. Alternatively, slipping and twinning mechanism in cellulose crystals is not likely a possible one, because of the extremely strong intermolecular forces compared with polyethylene. Finally, the shape of crystallites can not also be a reasonable factor to cause the orientation, since it is observed here by x-ray analysis that cellulose crystallite is a rod with a cross-sectional dimension of about 50 by 50A, instead of a lamella or ribbon-like.Then the orientation of (101) planes must be interpreted on the basis of some other different mechanism than those mentioned above. It seems reasonable to assume that absorbed water may plasticize cellulose crystallites and make them glide to each other on their (101) surfaces, considering the fact that most of the hydroxyl groups of cellulose molecules locate on these surfaces.These assumptions may be supported by the following observation; the orientation of (101) planes take place only when swollen cellulose is compressed. If cellulose crystals deform through gliding on (101) surfaces, they undergo a rotation which brings these planes more nearly parallel to a film surface.It would be difficult to interpret the preferencial orientation of (101) planes in terms of the fringed micelle moeel, where crystallites behave possibly as cross-links embedded in an amorphous matrix. Deformation of cellulose xanthate gel wonld be interpreted in the same way: the intermolecular forces are weakened by xanthation of hydroxyl groups, and gliding or slipping in crystals would be able to take place.