Cellulose structural considerations

Whereas cellulose structure is now known in considerable detail [18,19], and the introduction of new solvent systems for dissolving cellulose has intensified the interest in studying point (ii), less is known about point (iii). Such knowledge, however, is a pre-requisite to control the properties of these polymers, hence increase their competitiveness as possible substitutes for synthetic polymers. 

In order to place the discussion in perspective, the next section will be devoted to a brief review of current ideas concerning the structure of cellulose, as well as to consideration of the implications of two crystallographic studies of cellobiose and 0-methylcellobioside. The response of different celluloses to acid and enzymatic hydrolysis will then be examined in light of the structural considerations. 

The realization that the shape and frequency of the infrared absorption band associated with the fundamental hydroxyl stretching-vibrations in cellulose give considerable information about the hydrogen-bonding conditions in cellulose - led to the development of refined methods for studying the detailed structure of cellulose. 

A two-phase model containing crystalline and noncrystalline domains is currently used to describe the stmctural organization of cellulose (Krassig, 1993). Further investigations also revealed the presence on the surface of crystallites a paracrystalline fraction that must be taken into consideration in an improved model of cellulose structure (loelovich et al., 2010). Statistically alternated nanocrystallites along with nanoscale noncrystalline domains are integral constituents of long and thin elementary fibrils and their bundles—microfibrils. 

Chitin, obtained mainly from the cuticle of the marine crustacean, is presently of considerable interest in industrial and medical fields,i 2 since it is a mucopolysaccharide which resembles cellulose structurally and is present in nature as much as cellulose. Deacetylation of the acetamide group at the 2-position in the acetylglucosamine unit of chitin by alkaline hydrolysis yields chitosan, a cationic polyelectrolyte. Chitosan appears more useful than chitin, having both hydroxyl and amino groups which can be modified easily. Therefore, these chitin and chitosan derivatives are finding a broad range of applications. 

The mechanical and physical properties of natural fibers vary considerably, as it is with all natural products. These properties are determined by the chemical and structural composition, which depend on the fiber type and growth circumstances. With this cellulose, the main component of all natural fibers varies from fiber to fiber. 

Cellulase enzyme complexes consist of three major types of proteins that synergistically catalyze the breakdown of a cellulosic substrate. Because the enzymes are strictly substrate-specific in their action, any change in the structure or accessibility of the substrate can have a considerable influence on the course of the hydrolysis reaction. A pretreatment method based on exposing cellulosic substrate to phosphoric acid solution [9] and addition of the nonionic... 

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