This section discusses the elemental composition and bonding in cellulose alongside the differ-ent crystalline structures that can be adopted by cellulose.
2.4.1 Elemental composition and bonding in cellulose
Payen, in 1837 first identified cellulose as the product that was isolated from green plants being composed of carbon, hydrogen and oxygen (Hon, 1994; O’Sullivan, 1997). Figure 2.4 shows the chair conformation structure of cellulose, which is a linear polysaccharide of D-anhydroglucopyranose (C6H10O5) as the monomer, joined byβ(1→4) glycosidic bonds (O’Sullivan, 1997; Habibi et al., 2010).
Figure 2.4: Chemical structure of cellulose (Eyley and Thielemans, 2014)
The disaccharide cellobiose has been long regarded as the repeating unit of cellulose because
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it shows theβ(1→4) linkage and a fibre repeat unit (fibre shape) of 10.3 Å, equivalent to the bond length of cellobiose unit (Zugenmaier, 2008). However, French, 2017 recently stated that β-D-anhydroglucopyranose and not cellobiose is the repeating unit of cellulose. He argued that since cellulose is synthesised by adding glucose residues and since theβ(1→4) linkage can be represented in the glucose structure, there was no reason to regard cellobiose as the repeating unit. According to French, 2017, there would be no agreement if the repeating units were to depend on the fibre shape since there are other cellulosic crystal structures with greater fibre repeat unit. Moreover, maltose, which is the disaccharide of amylose with twoα-D-glucose units, is not regarded as the repeating unit of amylose.α-D-glucose, is generally accepted to be the repeating unit of amylose and so the terminology should not be different for cellulose (French, 2017).
The degree of polymerisation (DP), represented by n, is the number of monomeric repeat units in the cellulosic chain. The C-1 glucose end group of the cellulose chain has a reducing property, while the C-4 end group has a non-reducing property (Klemm et al., 1998b). Each of theβ-D-anhydroglucopyranose units of cellulose possesses three hydroxyl groups at the C-2, C-3 and C-6 positions, arranged in equatorial positions (Hon, 1994; O’Sullivan, 1997). These hydroxyl groups serve as the reactive groups on the cellulose backbone. From a structural point of view, the hydroxyl group on C-6 behaves as a primary alcohol. This is less sterically hindered and more reactive than those on C-2 and C-3, which behave as secondary alcohols (O’Sullivan, 1997).
Cellulose is a semi-crystalline material (having both ordered and disordered regions), formed through series of strong intramolecular and intermolecular interactions among the β-D-anhydroglucopyranose units. These hydroxyl functional groups engage in intramolecular hydrogen bonding interactions and in intermolecular hydrogen bonding interactions. Thereby, conferring the cellulose chain with high strength properties and hydrophilic properties. The hydrophobic forces that exist along the axial conformation of cellulose also contribute to the strength of cellulose (Lindman et al., 2010).
2.4.2 Cellulose polymorphism
Polymorphism is the ability of a material to possess more than one crystalline structure. The six variations of cellulose crystal structures are summarised in Figure 2.5 (Wada et al., 2006;
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O’Sullivan, 1997). Cellulose I, also known as native cellulose, is designated to cellulose that has been extracted from various natural sources. In cellulose I, the chains are arranged in parallel positions in the unit cell (Medronho and Lindman, 2015; Wada et al., 2006).
Two forms of cellulose I have been successfully identified; cellulose Iα, having one chain arranged in a triclinic unit cell, and cellulose Iβ having two chains arranged in a monoclinic cell unit (Medronho and Lindman, 2015; O’Sullivan, 1997; Hon, 1994). These two sub-polymorphs of cellulose I can be found in combined proportions within their sources. While cellulose Iα is mainly identified in higher proportions in algae and in bacteria, cellulose Iβ is predominant in plants and wood. Valonia algae as an example has about 64 % cellulose Iα and 36 % cellulose Iβ (Zugenmaier, 2008). Horii et al., 1987 treated both Valonia cellulose and cotton cellulose with heated steam. They identified from13C cross polarised-magic angle spinning nuclear magnetic resonance (13C CP-MAS NMR), that this treatment converts the predominant cellulose Iα in Valonia to cellulose Iβ. However, notable changes were not observed for cotton cellulose, which is predominantly composed of Iβ, upon such treatment. Henceforth, it was concluded that cellulose Iα is thermodynamically less stable than Iβ (Horii et al., 1987; O’Sullivan, 1997; Wada et al., 2006; Zugenmaier, 2008).
The percentage proportions of cellulose Iα and cellulose Iβ in some cellulose samples, determined from13C CP-MAS NMR are summarised in Table 2.3 (Zugenmaier, 2008). Studies by Hayashi et al. (2005) revealed that the cellulose Iα proportion in Cladophora algae is more easily degraded by enzymes, leaving behind the cellulose Iβ portions. These crystalline cellulose Iβ residues were termed “shortened microcrystalline cellulose” by the authors. However, these cellulose residues are what is now generally referred to as cellulose nanocrystals (Habibi et al., 2010).
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Figure 2.5: Polymorphic transitions in cellulose showing crystalline structural coordinates. Adapted from (Zugenmaier, 2008)
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Table 2.3: Percentage compositions of Cellulose Iα and Iβ in some cellulose samples. Adap-ted from Zugenmaier, 2008
Cellulose Source Cellulose Iα (%) Cellulose Iβ (%)
Glaucocystis (Algae) 88 12
Cellulose II is a regenerated cellulose that can be obtained by dissolving native cellulose I in a suitable solvent, followed by precipitation in a non-solvent (O’Sullivan, 1997). Another method of conversion to cellulose II involves treating cellulose I in aqueous NaOH, in a process known as mercerisation (O’Sullivan, 1997; Medronho and Lindman, 2015). The crystalline structures of cellulose II and cellulose Iβ both have two cellulosic chains in their unit cells (Wada et al., 2006).
However, they vary significantly in their molecular chain arrangements and in the unique angle of the unit cell. While cellulose II molecular chains are arranged in antiparallel position in the P21space group, with the unique angleγ = 117.80◦C, cellulose Iβ molecular chains are arranged in a parallel position, having the unique angleγ = 96.5◦C.
Other cellulose crystalline forms include IIIIand IIII Iwhich are respectively produced when cellulose I and cellulose II is treated with liquid ammonia, followed by the evaporation of the same. Cellulose IVI and cellulose IVI I are formed when respective cellulose IIIIand cellulose IIII I are heated in glycerol at 260◦C (Gardiner and Sarko, 1985; O’Sullivan, 1997; Wada et al., 2006; Zugenmaier, 2008).