Cellulose is one of the most fascinating organic resources, an almost inexhaustible raw material, and a key source of sustainable materials on an industrial scale in the biosphere. Natural cellulose based materials (cotton, wood, linen, hemp, etc.) have been used by our society as engineering materials for millennia and their use contin­ues today as verified by the extent of the world wide industries in building materials, paper, textiles, etc. Generally, cellulose is a fibrous, tough, water-insoluble natural polymer that plays a vital role in maintaining the structure of plant cell walls. It was first discovered and isolated by Anselme Payen58 in 1838, and since then, numerous physical and chemical prospects of cellulose have been extensively studied. As a chemical raw material, cellulose has been used for about 150 years for wide spec­trum of products and materials in daily life. Many polymer researchers are of the opinion that polymer chemistry had its origins with the characterization of cellulose. Cellulose differs in some respects from other polysaccharides produced by plants, the molecular chain being very long and consisting of one repeating unit. Cellu­lose can be characterized as a high molecular weight homopolymer of в -1,4-linked anhydro-D-glucose units in which every unit is corkscrewed 180° with respect to its neighbors, and the repeat segment is frequently taken to be a dimer of glucose, known as cellobiose (Fig. 17.3).

Naturally, it occurs in a crystalline state. From the cell walls, cellulose is isolated in microfibrils by chemical extraction. In all forms, cellulose is a very highly crys­talline, high molecular weight polymer. Because of its infusibility and insolubility, cellulose has driven the step-by-step creation of novel types of materials. Highlights were the development of cellulose esters and cellulose ethers as well as of cellulose regenerates and the discovery of the polymeric state of molecules. The very first thermoplastic polymeric material of cellulose was manufactured by Hyatt Manu — factoring Company in 1870 to make celluloid in which they had reacted cellulose with nitric acid to form cellulose nitrate. The chemical modification of cellulose on an industrial scale led to a broad range of products based on cellulose from wood. The first example was the fabrication of regenerated cellulose filaments by spinning a solution of cellulose in a mixture of copper hydroxide and aqueous ammonia.59

Natural cellulose has earned in the materials society a tremendous level of awareness that does not emerge to be yielding. The cellulose biopolymer imprima­tur such interest not only because of their unsurpassed quintessential physical and chemical properties but also because of their inherent renewability and sustainabil­ity in addition to their abundance. They have been the subject of a wide array of re­search efforts as reinforcing agents in nanocomposites due to their availability, low cost, renewability, light weight, nanoscale dimension, unique morphology and most importantly they have low environmental, animal/human health and safety risks. Currently, the isolation, characterization, and search for applications of novel forms of cellulose, variously termed crystallites, nanocrystals, whiskers, nanofibrils, and nanofibers, is generating much activity. Novel methods for their production range that begins at the highest conceptual level and works down to the details methods in­volving enzymatic/chemical/physical methodologies for their isolation from wood and forest/agricultural residues to the bottom-up production of cellulose nanofibrils from glucose by bacteria.6061 Some fungi can secrete enzymes that catalyze oxi­dation reactions of either cellulose itself or the lower molecular weight oligomers produced from the enzymatic hydrolysis of cellulose. Of these, the peroxidases can provide hydrogen peroxide for free radical attack on the C2-C3 positions of cellulose to form ‘aldehyde’ cellulose, which is very reactive and can hydrolyze to form lower molecular weight fragments while other oxidative enzymes can oxidize glucose and related oligomers to glucuronic acids. Such isolated cellulosic materials with one dimension in the nanometer range are referred to generically as nanocelluloses.46 These nanocelluloses provide important cellulose properties—such as hydrophilic — ity, wide spectrum of chemical-modification capacity, and the formation of versatile semicrystalline fiber with very large aspect ratio which is the specific features of nanoscale materials. On the basis of their dimensions, functions, and preparation methods, which in turn depend mainly on the cellulosic source and on the process­ing conditions, nanocelluloses may be classified in three main subcategories.