Crystallization of cellulose from dilute solutions

Cellulose samples having a degree of polymerization (P) of 400, 200, 100, and 50 were crystallized isothermally from dilute solutions of aqueous sodium hydroxide and from cellulose xanthate solutions. Observation of the suspension under an optical microscope revealed that in most cases various shapes of huge lamellae in the order of 10 to 50μ were formed. These lamellae could easily be disintegrated by ultrasonic cavitation and under an electron microscope they were seen as fibrous sheets. In all the cases studied, the lengths of the fibrils were equivalent to or longer than the fully extended molecular length. The huge lamellae viewed under an optical microscope are considered to be loosely packed aggregates of the fibrils. In view of the low crystallinity found in these fibrils, the fringed micelle model seems rather preferable to the extended chain crystals. Viscometric measurements showed that the induction period, where reduced viscosity decreased or did not change, was followed by the pre‐crystallization stage which was manifested by a sudden increase, and then crystallization started as revealed by a rapid decrease in the reduced viscosity. This implies that cellulose chains were stiffened by the formation of intramolecular hydrogen bonds before the intermolecular crystallization started. If the formation of the intramolecular hydrogen bonds prior to a regular ordering of the chains in a lattice was avoided, therefore, it is anticipated that cellulose crystallizes in the form of chain folded lamellae. This was actually achieved by using an aqueous solution of equal moles of sodium hydroxide and methanol as the solvent. In this case, the reduced viscosity did not increase appreciably and dropped as the crystallization started. Namely, stiffening of the cellulose chains did not occur in the pre‐crystallization stage. The electron microscopy revealed that the crystals formed were lamellar in this case. The size of the lamellae was in the range of 0.5 to 5μ, and their thickness was in the order of 80 to 120 Å. Molecular length of cellulose used being ca. 2000 Å, the chains must be folded in these crystals.