Cellulose

Cellulose is the most abundant natural polymer in the world and is known to provide physical structure and strength to the cell walls of plants (Krishnan 1975; Belton et al. 1989; Angles and Dufresne 2001; Cima et al. 1996). It has been found also in invertebrate sea animals (tunicin which contains highly crystalline cellulose) or in some bacteria secretion, in which the cellulose forms biofilms (Brown 2004) . Cellulose, (C6H10O5)n, is a linear chain polymer, assembled from several hundred to over ten thousand d-glucose units, connected through b-(1-4) glicosidic linkages. The neighbouring glucose residues are rotated with -180° one to each other and preserve the linear configuration of the molecule.

Figure 5.5 depicts the cellulose fiber and its internal structure. The intra- and intermolecular hydrogen-bonds involving the constituent polyol groups of cellulose are the driving assembling force of the cellulose fibers. The polymer chains, associated by means of hydrogen-bonds are organized in highly-ordered structures (crystalline region), which together with the amorphous regions, assemble into a microfibril. The microfibrils build up a macrofibril and several microfibrils aggregate in a cellulose fiber. This hierarchical internal organization of cellulose provides its main physico-chemical properties: a decreased reactivity, good mechanical characteristics and insolubility in common solvents including water. Cellulose, unlike starch does not gelatinize in water, but undergoes at high temperature and pressure a crystalline-to-amorphous transformation (Deguchi et al. 2006).

cellulose chains

Fig. 5.5 Schematical representation of the hierarchical internal organization of cellulose fiber cellulose chains

Fig. 5.5 Schematical representation of the hierarchical internal organization of cellulose fiber

The existence of microfibril disordered regions determines two important properties of cellulose. First, it is swelling in water, as a result of the new hydrogen bonds formed between its less ordered regions and water molecules. Secondly, it forms, by cleavage or partial acid treatment, rod-like nanocrystals - the so-called "cellulose whiskers" (de Souza Lima and Boursali 2004).

Another cellulose peculiarity is its rigid backbone and the asymmetric structure, the colloidal suspension of polysaccharide nanocrystals displaying isotropic transition to liquid crystals.

Learning from the bio-evolution processes, natural molecules could be combined with the synthetic procedures developed in materials science. The synthetic replication of some biological structures following biomimetic or bio-inspired approaches might introduce original properties of biologic structures to the artificial materials. In this context, the porous fibers of cellulose could act as suitable biotemplate for the synthesis of different materials.

The common idea was to incubate the material fibers with metal inorganic salt solutions (precipitation, sol-gel, hydrothermal methods) followed by a calcinations treatment, to remove the organic templates and to crystallize the oxides. The synthetic procedures closely replicate the morphological hierarchies of natural cellulosic substances from macroscopic to nanoscale levels. The strategies are based on the adsorption/coordination of metal-containing raw materials from solution onto/on cellulosic hydroxylated substrate surfaces, followed by subsequent heating treatments. The oxides are replicas (positive and negative) of natural cellulose fibers and are usually called "artificial fossil".

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