ACCESSIBILITY OF HYDROGEN BONDS IN CELLULOSE

Abstract

Cellulose derives most of its functional properties from its hydroxyl groups: derivatives are formed by reactions with the hydroxyl groups, when cellulose is dissolved it is usually as a complex of the hydroxyl groups with ions or polar groups, and the physical properties of cellulose in the solid state are to a large extent a function of the hydroxyl groups.It is known from infrared analysis that practically all hydroxyl groups in dry solid cellulose, both native and regenerated, are not free but engaged in hydrogen bonds of some kind. The proton donor group is always a hydroxyl group with another hydroxyl group or an oxygen group as acceptor. The accessibility of the hydrogen bonded hydroxyl groups has been measured by a combination of deuterium exchange (OH to OD groups) in heavy water and infrared analysis. Accessible OH groups are largely in amorphous (disordered) cellulose. The OH groups in crystalline (well ordered) cellulose are gradually made accessible by swelling with caustic soda solutions of increasing strength.In regenerated cellulose a large fraction (usually about 80%) of the hydroxyl groups is accessible to the exchange in heavy water. Complete accessibility is obtained with 12 to 16% aqueous caustic soda. Intrachain hydrogen bonds along the cellulose chains are more resistant than interchain bonds between adjacent chains. By certain treatment the crystallinity of regenerated cellulose can be increased substantially. It has been found that the packing density of amorphous cellulose also can be increased by these treatments. Special crystallization processes from dilute solution give cellulose crystals of high order and low accessibility.Native cellulose has a morphology of rather well defined microfibrils, containing parallel and partly crystallized cellulose chains. The accessibility is usually much lower (30 to 50%) than in regenerated accessible, we calculate an accessibility value for holocellulose from spruce of about 57%. Reported accessibilities for purified wood pulp from softwood are about 58% (8). If our measured data are accepted as given here, they would indicate that hemicellulose in the native state in wood, although X-ray amorphous, is only partially accessible to tritium exchange in tritiated water. There are, however, some difficulties in the interpretation of these measurements, and the data will be further treated elsewhere.