The molecular and crystal structure of cellulose triacetate

An x‐ray diffraction study has been made of cellulose triacetate II. Small films of highly crystalline triacetate with good fibrous orientation and a fair degree of biaxial orientation have been prepared by repeated rolling at successively higher temperatures between 160 and 215°C. The x‐ray diffraction patterns exhibited several new reflections which made it possible to determine the unit cell. It has been found that cellulose triacetate II has four cellobiose acetate residues in a pseudo‐orthorhombic cell with a = 24.5, b = 11.56, and c = 10.43 A. Pairs of these residues are related by twofold screw axes in the c or fiber‐axis direction. The paired arrangement seems to be determined by a fitting of the natural zigzagging shape of the cellulose backbone which is exaggerated by the presence of the secondary acetyl groups. The glucose residues of such a pair are parallel and inclined at about 45° with respect to the a axis. A neighboring chain pair points in the opposite direction and the median planes of the cellobiose units of two such pairs are approximately mutually perpendicular. To accommodate this type of packing, one half of the primary acetyl groups must be twisted sharply out of the median plane of the parent glucose residues. A second statistical model which basically is very similar is also considered. There do not appear to be any strong interactions between chains.