Cellulose structural arrangements

The most common of all natural polymers is cellulose. It is ubiquitous in plant life in various molecular modifications and structural arrangements. Large quantities are found in the trunks, branches and leaves of trees as well as in... 

Finally, Muhlethaler (76) took electron micrographs of various woody tissues, before and after the lignin layer had been removed by mild treatment. The structural arrangement of the cellulose had not been altered, indicating that the cellulose was physically encased by the lignin. 

Sisson has traced the evolution of current concepts of the crystalline part of cellulose structures. The fiber diagram obtained by X-ray diffraction is now known to be produced by a series of elementary crystals, called crystallites, which have a definite arrangement with respect to the fiber axis. It is also known that the crystallites in regenerated cellulose may be oriented to varying degrees with respect to the fiber axis and that the crystallites in regenerated cellulose and mercerized cotton differ from those in native fibers. These hydrate type crystallites appear to be more reactive chemically than the native type. 

FIGURE 20-29 Cellulose structure. The plant cell wall is made up in part of cellulose molecules arranged side by side to form paracrys-talline arrays—cellulose microfibrils. Many microfibrils combine to form a cellulose fiber, seen in the scanning electron microscope as a structure 5 to 12 nm in diameter, laid down on the cell surface in several layers distinguishable by the different orientations of their fibers. 

The finer details of secondary and tertiary cellulose structure remain somewhat controversial and in any case will not concern us here. It will suffice to say that the chains align themselves side by side to form a substructure of microflbrils 35 A in diameter, and these in turn are linked together in more complex arrangements to form the main cellulose fibres. The microfibrils are believed to contain both amorphous and crystalline regions of aligned cellulose chains and the latter may adopt helical configurations. As a result of the presence of other components, differences exist between wood, cotton and synthetic cellulose fibres. 

FIGURE 10.1 Structure of cellulose. Covalent bonds are shown soUd, while hydrogen bonds are shown dotted connecting the molecule intra- and intermolecularly. The name of this compound defines the structural arrangement of the glucose monomers in the polymer. 

Natural fibers consist of aggregated cellulose chains arranged in a hierarchical structure. These elementary fibrils are composed of cellulose chains called cellulose macro fibrils [16]. Figure 6.3 shows a fransmission elecfron microscopy (TEM) image of cellulose microfibrils, or MFC [17],... 

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