Progress in Characterization of Cellulose and Cellulose Esters

Cellulose is a linear, stereoregular, and semi-crystalline polysaccharide. Linear chains of cellulose connected by hydrogen bonds form the supramolecular structure of this biopolymer, consisting of elementary nanofibrils and their bundles, called microfibrils. Each nanofibril contains ordered nanocrystallites and poorly ordered nanosized non-crystalline domains. Currently, it is clear that the two main structural models, "fringed fibrils" and "fringed micelles", cannot explain many properties of cellulose and its derivatives. Therefore, a new structural model was proposed, which includes amorphous-mesomorphic non-crystalline domains and imperfect crystallites with paracrystalline layers on their surface. In this model, the crystallites and non-crystalline domains are located along the fibril and their alternation has a random character. Crystallites of various allomorphs are stable and inaccessible to most reagents, typical organic solvents, water, and diluted solutions of acids and bases. Vice versa, non-crystalline domains represent weak and accessible places of cellulose fibrils. To characterize the supramolecular structure of cellulose, it is necessary to know the sizes of crystallites, the type of crystalline allomorph, the degree of crystallinity, interplanar distances, parameters of the crystalline unit cell, and degree of lattice distortion, as well as the content, size, and packing density of paracrystalline and non-crystalline domains. Along with the supramolecular structure of cellulose, it is also important to know the structure of cellulose esters. This paper discusses novel data on structure and properties of cellulose and cellulose esters.