| Abstract: | A most reasonable folded-chain model for the fine structure of rayons (cellulose II) has been developed through experimental work on a theoretical basis. All the materials uniformly gave a levelling-off DP of about 40, equivalent to a length around 200 Å, for cellulose molecule segments at the early stage of heterogeneous acid hydrolysis when the first minor fraction is scarcely dissolved. Measurements by small-angle and wide-angle x-ray scattering put the crystallite length from various rayon fibers at about 200 Å, without exception. A family of GPC chromatograms, furthermore, on the hydrolyzed cellulose exhibited a single peak considered to represent monodispersed materials. These data suggest that clearly divided sections exist within the microfibril along its axis in a regular manner at an interval of about 200 Å. This cannot be explained in terms of the fringed micelle model. On the other hand, the possibility that cellulose II may have a folded-chain conformation has been demonstrated. A single cellulose molecule is essentially folded back and forth in the (101) plane to form a sheetlike structure. Such a structure is the basic unit that can fit perfectly into the unit cell of cellulose II. The cellulose molecule can achieve a fairly sharp U-turn in the (101) plane, with only one glucose unit of in the half-boat conformation. A crystallite consists of a number of sheets held together by secondary forces in the (101) plane. Accordingly, crystallographically, the crystallites are closely packed at the surface of each fold at its longitudinal edges to make up the cellulose microfibril. According to our model, the oxygen atom of the glucosidic link in the fold, where acid hydrolysis would have to take place, protrudes partially from the surface of the crystallite; a pair of atoms at the folds are then facing each other and are therefore, accessible for hydrolysis. This would explain chain scission of cellulose II at these sites in hydrolysis. This folded-chain model is supported further by other experimental evidence. |
