ULTRASTRUCTURAL ASPECTS OF THE ACETYLATION OF CELLULOSE

An ultrastructural study of the acetylation of cellulose was achieved by subjecting well characterized cellulose samples from Valonia cell wall and tunicin tests to homogeneous and heterogeneous acetylation. The study involved transmission electron microscopy observations on negatively stained microcrystals as well as diffraction contrast images of the cross sections of wall fragments at various stages of the reaction. These observations showed that the acetylation of crystalline cellulose proceeds by a reduction of the diameters of the crystals while their lengths are reduced to a lower extent. These results were corroborated by electron and X-ray diffraction experiments that showed that during the reaction there was a rapid decrease in the intensities of the equatorial diffraction spots of cellulose, whereas those located on the meridian or close to the meridian stayed constant. A model of acetylation of the cellulose crystal is presented. It is based on a non swelling reaction mechanism that affects only the cellulose chains located at the crystal surface. In the case of homogeneous acetylation, the partially acetylated molecules are sucked into the acetylating medium as soon as they are sufficiently soluble. In heterogeneous conditions the cellulose acetate remains insoluble and surrounds the crystalline core of unreacted cellulose.