# Fabricating Sustainable All-Cellulose Composites
## 2 Cellulose  
### Characteristics of Cellulose  
In nature, cellulose is found in the cell walls of plants, including trees, and it has a vital role in providing mechanical strength and structural support [32]. Plant-based cellulose is accompanied by hemicellulose, lignin, pectin, and other substances [33]. Besides plants, certain bacteria, algae and fungi produce cellulose [34].  
Cellulose is a linear homopolysaccharide consisting of \(\beta\)-D-glucopyranose units linked by glycosidic \(\beta\)(1-4) bonds. Each unit contains three hydroxyl groups. The long polymer chain has repeating elements, which have two anhydroglucose units (AGU) that form polymerized chain lengths of several thousand units [34]. Cellulose has an amphiphilic nature: the equatorial direction of a glucopyranose ring is hydrophilic, and the axial direction of the ring has a hydrophobic character (Figure 1). These characteristics play a significant role in hydrophilic and hydrophobic interactions between cellulose molecules and other compounds in water [35; 36; 37].  
Cellulose can be considered a semi-crystalline polymer with highly oriented crystalline domains coexisting with non-crystalline amorphous phases, which have a lower degree of order [38; 39]. The cellulose crystalline domains have four major allomorphs (I, II, III and IV) based on molecular orientation. The most common allomorph found in nature is cellulose I and the most thermodynamically stable structure is established in cellulose II [34; 40]. Cellulose I can have two sub-allomorphs, triclinic I\({}^{\alpha}\) and monoclinic I\({}^{\beta}\), which can be found alongside each other, and the ratio depends on the origin of the cellulose [34]. Cellulose I\({}^{\alpha}\) dominates in primitive organisms, such as algae and bacteria, while cellulose I\({}^{\beta}\) is found mainly in higher plants [41] and aquatic animals, such as tuicates [42]. Cellulose II can be modified irreversibly from cellulose I through alkaline treatment (mercization) or by the cellulose dissolution process (regeneration) [34]. Cellulose III is obtained through liquid ammonia (NH\({}^{3}\)), while cellulose IV is obtained through the heating of small crystallites in glycerol at 260 \({}^{\circ}\)C. Cellulose III can be reversibly formed from cellulose I, II or IV, and cellulose IV can be reversibly formed from cellulose I, II or III. The crystalline regions are strong, rigid, and quite inaccessible to water and most chemical reagents, whereas the amorphous regions are weaker and contribute to increased hydrophilicity and accessibility [42]. Physical, chemical, enzymatic or microbiological modifications of cellulose can lead to changes in its crystalline structure and result in new derivatives [43; 44].  
Cellulose has a strong capability to form intra- and intermolecular hydrogen bonds in its establishment network, and these internal hydrogen bonds hinder the free rotation of the glucopyranose rings, which increases the stiffness of the cellulose chains [45; 46]. Although cellulose has the same structural motif in each material, its degree of polymerization (DP, the number of monomer units in the polymer) and crystallinity (degree of packing order) can vary greatly [47]. DP and crystallinity depend on the origin and treatment of the raw material [34]. Insights of the physical properties and morphological structure of single polymeric chain, such as chain length or degree of crystallization, can be investigated via \(X\)-ray, optical and electron-microscope imaging, or chemical and physical analysis methods [48].  
Figure 1: Hydrophilic and hydrophobic planes of cellulose marked with the primary C6-OH (with _tg_-conformation corresponding cellulose I crystalline structure), and secondary C2-OH and C3-OH groups in glucosyl unit. Reprinted with permission from [37].