## 2 Cellulose Morphology  
Cellulose, the most abundant natural glucose polymer found in the primary cell walls of all plants, is comprised of alpha-cellulose (anhydroglucose) [11; 35]. Hemi-cellulose (glucose, mannose, galactose, xylose, and arabinose--provide strength to plant cell walls), lignin (three dimensional structures of phenyl/propane units providing structural support) and extractives (organic solvent or water-soluble non-structural substances such as waxes, fats, terpen, etc.) are found alongside alpha-cellulose [36; 37; 38]. Inorganic substances, such as metallic and mineral matter, are also present in natural cellulose in plants and are measured in terms of ash content [39]. The cellulose structure consists of a cuticle, primary wall, secondary wall, and lumen (Figure 1a) [38; 40]. There are three different components to the secondary wall (S1, S2, and S3) in which the fibrils form a spiral structure with reversals at random intervals. The S1 outer layer secondary wall contains holocellulose (i.e., cellulose and hemicellulose), lignin, and pectin, while S2 and S3 do not contain lignin [41]. Natural cellulose contains fibrils within the helical secondary walls, which are themselves composed of linear cellulose \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Manufacturing Process** & \
\begin{tabular}{c} **Solvent** \\ **Process** \\ \end{tabular} & \multicolumn{2}{c}{**Advantages**} & \multicolumn{2}{c}{**Drawbacks**} \\ \hline
\begin{tabular}{c} Viscose MMCF (viscose rayon, \\ cellulose acetate, modal, \\ triacetate) \\ \end{tabular} & Derivatisation & \(\bullet\) & \(\alpha\)-cellulose feedstock & \(\bullet\) & Waste chemicals (e.g., carbon disulphide), leading to contamination of water, air, and soil, and health concerns. \\ \hline \multirow{3}{*}{L-MMCF} & Direct dissolution & \(\bullet\) & Environmentally friendly (NMMO 99\% recoverable). \(\bullet\) & More expensive to produce than viscose MMCF. \\ \cline{1-1}  & & \(\bullet\) & \(\alpha\)-cellulose feedstock & \(\bullet\) & Longer manufacturing process. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Advantages and drawbacks of MMCF manufacturing processes (reproduced from [7]). polymers. The cellulose structure contains anhydroglucose (i.e., carbohydrate) units or rings (C\({}_{6}\)H\({}_{10}\)O\({}_{5}\)), which are in a chair conformation, and these are joined together in a 1,4-\(\beta\)-glucoside linkage to form the cellulose polymer chain [5; 38; 42; 43]. Figure 1b represents the cellobiose unit as a cellulose monomer, comprising two consecutive glucose anhydride units [38]. There are three hydroxyl groups in each repeating unit of anhydroglucose, which are available for reaction (e.g., oxidative and thermal degradation). These hydroxyl groups protrude laterally from the chain, which allows for hydrogen bonding between polymer chains, creating a highly ordered, or crystalline, structure (Figure 1c) [40]. Figure 1: (**a**) Macrostructure of cellulosic cell wall organisation (reproduced from [38] with permission from TAPPI), (**b**) Cellulose polymer chain (reproduced from [38] with permission from TAPPI), (**c**) Hydrogen bonding between cellulose molecules (C1, O1, etc.) are positions of carbon and oxygen atoms; hydrogen atoms complete the valences; hydrogen bonds are shown by dotted lines) (reproduced from [40] with permission from Elsevier. The cellulose structure is composed of both crystalline and amorphous regions, with a degree of polymerisation (DP) (or number of repeat units) of 300-1500 for wood pulp, 800-10,000 for fibre crops (e.g., cotton, flax, industrial hemp), and 200-600 for regenerated cellululos (e.g., L-MMCF) [38; 43]. Typically, the greater the DP, the stronger the fibre [44]. There are four polymorph forms of cellulose, with Cellulose I (natural cellulose) and Cellulose II (mercerised or regenerated cellulose) being the most common. Cellulose I (parallel packing of cellulosic chains) can be transformed into Cellulose II (anti-parallel chain structure), but the reverse is not possible [45; 46].