Structure of Cellulose
Cellulose is a complex carbohydrate or polysaccharide consisting of D-anhydroglucopyranose units (AGU) which form a straight chain polymer. The straight cellulose chain polymer are formed by 3 different groups of AGU which are a reducing end group, a non-reducing end group and internal glucose ring.
Cellulose consists of D-anhydroglucopyranose units (AGU) linked through ?-1, 4 glycosidic bonds (Figure 2.1). 3 hydroxyl groups along cellulose chain are in equatorial position. Therefore, the formation of hydrogen bonding by hydroxyl groups are available (Kalia et al., 2011; Ng et al., 2015). Intra-chain hydrogen bonding between hydroxyl groups and oxygen of the adjoining ring molecules stabilises these linkages and results in the linear configuration of the cellulose chain (Moon et al., 2011).
The formation of intermolecular hydrogen bond is attributed by the presence of 3 hydroxyl (OH) groups on each glucopyranose unit of the cellulose chains. The strong and complex network of hydrogen bonds between the –OH groups of cellulose molecules can stabilise and arrange the cellulose chains into highly crystalline structure, resulting in the formation of slender and nearly endless crystalline rods along the microfibril axis (Khalil et al., 2014 ; Frone et al., 2011; Alemdar & Sain, 2008; Ng et al., 2015).
Certain parts of cellulose chains cannot stabilise laterally through hydrogen bonding, in the absence of hydroxyl groups and thus, form disordered amorphous nanocellulose segments bonded to cellulose crystals. The chains in the amorphous regions of the microfibrils are further apart from each other with lower density than the crystalline regions. As a result, they are available for hydrogen bonding with other molecules such as water (Kalia et al., 2011; Ng et al., 2015).
Figure 2.1: Structure of Cellulose
Regardless of its source, cellulose can be characterise as a high molecular weight homopolymer of ?- 1,4-linked anhydro-D-glucose unit, in which every unit is corkscrewed 180? with respect to its neighbours, and the repeat segments is frequently taken to be a dimer of glucose, known as cellobios, as shown in Figure 2.1. Each cellulose chain possesses a directional chemical asymmetry with respect to the termini of its molecular axis: one end is a chemically reducing functionality and the other has a pendant hydroxyl group, the nominal non-reducing end. Reducing end groups are positioned at cellulose containing free hemiacetal or aldehyde, while non-reducing end group are positioned at C4 containing free hydroxyl group. The internal glucose ring or cellobiose unit adopt 4C1 chair conformation. As a consequence, the hydroxyl groups are positioned the ring (equatorial) plane, while hydrogen atoms are in the vertical position (axial) (Borjesson & Westman, 2015).
Sources of Cellulose
Being the most abundant biopolymer on the Earth, cellulose can be found on the main constituent in the cell wall of trees, various higher order plants and several marine animals.
In plants, cellulose is synthesised through photosynthesis using carbon dioxide and water in the presence of sunlight. Plants are natural cellular hierarchical biocomposites, typically containing about 50% of cellulose.
Highly distribution of cellulose is found in higher plants, for example wood and cotton which plays an essential constituents in plant cell walls (Habibi et al., 2010). Around 36 individual cellulose molecules aggregate, forming an elementary fibrils (protofibrils), which then assemble into larger units called microfibrils, and these, in turn assemble into microfibrils. Microfibrils also known as cellulose fibres, are oriented in different layers in the cell wall (Habibi et al., 2010).
Figure 2.2: Wood hierarchical structure: from tree to cellulose (Moon et al., 2008; Dufresne, 2013)
Figure 2.3: The cellulosic fibre structure with emphasis on the cellulose microfibrils (Lavoine et al., 2012)
Plant cell walls is a complex layered structure, consisting of a thin primary wall which is encircling the 3 layers of secondary wall. Cellulose fibres are embedded in thick middle layer of secondary wall. Lignin is an amorphous polyphenolic polymer that works as a structural support in plants, which also acts as the adhesive that binds cellulose and hemicellulose together. In the cellulose fibre, cementing matrix of polysaccharide and glycoproteins encircle the cellulose’s microfibrils (Ng et al., 2015). The cell wall as shown in Figure 2.2 is composed of several layers: the middle lamella (ML), the primary wall and the outer, middle and inner layers of secondary wall. The middle lamella consists of pectin, a kind of polysaccharide that binds the neighbouring cells in the presence of high amount of lignin (Nechyporchuk et al., 2016). Cellulose presents in both crystalline and amorphous domains. As shown in Figure 2.3, in order to have a pure cellulose, the amorphous layer consisting of hemicellulose and lignin should be removed (Malucelli et al., 2017).
Tunicates or Ascidicacea are marine invertebrate animals which compete for food and space with other filter feeding animals such as mussels, scallops and clams. They are extremely fast growing creatures and can reproduce in 10 weeks.
Tunicates, also called as tunic tissue, are the only animal known to produce cellulose microfibrils. Cellulose is isolated from a thick leathery mantle during the mature phase of tunicate (Moon et al., 2011). There are over 23000 species of tunicates grow on Earth and therefore, the structure and properties of cellulose microfibrils are expected to be different between species, but with little variations (Moon et al., 2011). Tunicates are from the sessile sea creature that have nearly pure cellulose 1-? crystals which exhibit higher aspect ratio in comparison with the cellulose nanocrystals from other material (Sacui et al., 2014).
Bacteria is widespread in nature and also can produce cellulose. The bacterial cellulose (BC) is generated during the fermentation of sugars and plant carbohydrates by microorganisms takes place. Its biological formation opens up the opportunity to develop biotechnological synthesis pathways to significantly influence and control the final material features of BC.
Cellulose isolated from bacteria is free from wax, lignin, pectin and hemicellulose (Ummartyotin ; Manuspiya, 2015). Cellulose crystals are secreted extracellularly by specific bacteria; for instance, Gluconacetobacter xylinus from Acetobacter xylinum family secretes microfibrillated cellulose (Lavoine et al., 2012). Pores on bacterial surface covers the cellulose-synthesizing enzymes which produces a number of glucan chains (Klemm et al., 2018). It is believed that the bacteria-derived cellulose can demonstrate high mechanical properties, good chemical stability and are non-toxic (Trache et al., 2017).
Classification of Nanocelluloses
Generally, nanocellulose can be divided into 3 types; 1. Cellulose nanocrystal (CNCs), with other terminologies such as nanocrystalline cellulose, cellulose nanowhiskers, a rod-like cellulose crystals; 2. Cellulose nanofibrils (CNFs), with other designations such as nanofibrillated cellulose (NFC), microfibrillated cellulose (MFC), cellulose nanofibers; and 3. Bacterial cellulose (BC), also known as microbial cellulose. Nanocellulose possess a unique properties such as low density, biodegradability, high aspect ratio, high strength and stiffness.
2.3.1 Cellulose Nanocrystals (CNCs)
Cellulose Nanocrystals (CNCs), with the synonyms of cellulose nanowhiskers (CNW) or nanocrystalline cellulose (NCC), have attracted significant attention as a reinforcement material for a wide variety of polymeric matrices, including petro-based and biodegradable matrices (Favier et al., 1995; Ma et al., 2014). The investigation on elastic modulus of crystalline cellulose has been conducted since long time ago, it is either by theoretical evaluations or by experimental measurements (wave propagation, X-ray diffraction, Raman spectroscopy, and atomic force microscopy).
It is reported that the CNCs have high modulus and strength, estimated to be 140-150 GPa and 2 to 10 GPa, respectively (Takagi et al., 2013 ; Orue et al., 2017). Typically, CNCs have a rigid rod-shape with a diameter 1-100 nm and length between 10 to 100 nm (Ruiz et al., 2000; Souza Lima and Borsali 2004; Bai et al., 2009).
Source of cellulose and method of isolation is important as it determines the degree of crystallinity and size of the crystalline regions in CNCs. For instance, the cellulose derived from algae could exhibits 90% degree of crystallinity, while cellulose derived from plants are about 50% degree of crystallinity (Trache et al., 2017). These non-crystalline regions hydrolysed during extraction, forming nanocrystals in all dimensions.
2.3.2 Cellulose Nanofibrils (CNFs)
Cellulose Nanofibrils (CNFs) usually prepared by liberation of cellulose are from the constituent fiber matrix and microfibre bundles (Salas et al., 2014). Preparation of CNFs typically involves high pressure homogenisation before or after chemical treatments (Lin ; Dufresne, 2014). Both are individual and aggregated nanofibrils with soft and long chains. These microfibrils have a high aspect ratio and often exhibit a gel-like characteristic in water with pseudoplastic and thixotropic properties (Klemm et al., 2011).
A typical diameter for CNFs is between 5 to 60 nanometer (Klemm et al., 2011). During extraction of CNFs, some inter-fibrillated hydrogen bonds are broken to form fibres of micro-meter in length (non-crystalline region) and nanometer in width. Generally, CNF shows a high aspect ratio about 10 to 100 nm wide, 0.5-10 ?m in length and contain nearly 100% of cellulose (Moon et al., 2011). Controlled acid hydrolysis of native cellulose fibres can yield highly crystalline, rod-like particles through the selective degradation of the more accessible material. The dimension of CNFs largely depends on their origin (Elanthikkal et al, 2010).
2.3.3 Bacterial Cellulose (BC)
Bacterial Cellulose (BC) is obtained from secretion of various bacteria isolated from bacterial bodies and growth medium. The resultant microfibrils in bacterial cellulose are microns in length, with a large aspect ratio greater than 50 mm and morphologies is based on the type of bacteria and culturing media. Typically, microfibrils derived from Acetobacter sp. possess a rectangular cross-section of about 6 -10 nm by 30- 50 nm (Moon et al., 2011). Bacterial cellulose synthesis is currently only possible at small scale as it is costly compared to other chemical treatments of plant-based cellulose (Malucelli et al., 2017).
Isolation of Cellulose Nanocrystals (CNCs)
Cellulose Nanocrystals (CNCs) can be isolated by both chemically and mechanically destructive methods. High pressure homogenization, high–intensity ultra-sonication and stream explosion are typical mechanical treatments. Meanwhile, acid hydrolysis, enzymatic hydrolysis and oxidation method are commonly applied on cellulose to obtain nanocrystals (Jonoobi et al., 2015; Li et al., 2012; Cherian et al., 2010 ; Pentilla et al., 2013; Zhang et al., 2016).
Although the isolation of cellulose nanocrystals by mechanical ways is more environmental-friendly in the absence chemicals, the procedure is normally associated with high energy consumption and expensive (Dufresne, 2013). By changing parameters such as temperature, time and reagent’s concentration during the treatment, nanocellulose of various percentage of cellulose, hemicellulose and lignin could be obtained. The flow chart in Figure 2 involves typical pre-treatment, mercerization, delignification and purification cellulose. In the following subsections, the processes are discussed in detail.
Figure 2.4: Process of cellulose isolation of cellulose (Brinchi et al., 2013)
Pre-treatment involves removal or dissolution of lignin and extraction of hemicellulose. The lignocellulosic fibres should be milled into small sizes as shown in (Figure 2.4) in order to obtain large surface area and then washed with water to remove any impurities. Fibres containing excessive impurities may distrupt in subsequent treatment steps.
Several approaches have been proposed to obtain fibres that were less stiff and cohesive, and therefore, consequently reduced the energy consumption the production. To date, it is remarkeable to note that there are other alternative methods of pre-treatment including enzymatic, tempo-mediated oxidation, carboxymethylation and acetylation, and electrostatically induced swelling by charged group pre-treatments (Klemm et al., 2018).
2.4.2 Alkali Treatment (Mercerization)
Hemicellulose is partially solubilized from fibres in order to obtain cellulose crystals for futher processing, in which the crystallinity of the product is enhanced. In addition, residual waxes such as pectin, silica ash and natural fats from the previous treatment will be removed at this stage. Strong basic solutions (NaOH or KOH) are used in this treatment.
New approaches, such as steam explosion, have shown great results in eliminating lignin fraction. In a steam explosion process, the biomass sample is first milled and then subjected to high pressure steam for short time (20 s to 20 min) at 200-270 °C and 14-16 bar. The pressure in the digester is then reduced quickly by opening the steam and the material is exposed to normal atmospheric pressure to cause explosion which breaks down the lignocellulosic structure. Steam explosion causes the hemicelluloses and lignin from the wood to be decomposed and converted into low molecular weight fraction which can be recovered by extraction (Brinchi et al., 2013).
2.4.3 Bleaching (Delignification)
The purpose of this treatment is to remove lignin as this treatment also known as delignification. Surface lignin removal can be done using chlorine under acidic conditions (harmful for environment, yet very effective), and hydrogen peroxide (H2O2) treatment in alkaline solution (Abraham et al., 2011; Chen et al., 2011; Malucelli et al., 2017). Bleaching is often repeated several times to achieve more lignin removal.
2.4.4 Acid Hydrolysis
Acid hydrolysis breakdown the cellulose, leading to removal of the microfibrils at the defects to produce cellulose rod-like nanocrystals in pure form (Habibi et al., 2010). CNCs obtained from different acidic treatments have been observed to exhibit different characteristics, such as dispersion quality in polar or non-polar solvents and surface functionality (Dhar et al., 2016). During the hydrolysis, the hydrogen ions penetrate the cellulose chains in the amorphous regions promoting the hydrolytic cleaveage of glucosidic bonds and releasing individual crystallites after sonication (Dufresne, 2013). It is well known that controlled acid hydrolysis can lead to vary in the stability of the nanocrystal in a colloidal suspension due to the presence of different charges on the surface of the nanocrystals (Angelliar et al., 2004; Martins et al., 2015).
In some studies, they combine chemical and mechanical treatments to achieve high yield of good quality nanocellulose. CNCs are synthesised using strong acid hydrolysis on pure cellulosic material under strictly controlled condition (time, temperature, condition, agitation) and followed by sonication (mechanical process) to disperse the nanocrystals into a uniform and stable suspension.
Sulphuric acid hydrolysis introduces the anionic sulphate groups on the surface of cellulose nanocrystal, which is responsible in stabilising the cellulose nanocrystal suspension (Lu & Hsieh, 2010). The use of sulphuric acid in the chemical hydrolysis provides highly stable aqueous suspensions due to the esterification of surface hydroxyl groups to give charged sulphate groups (Ranby, 1949; Beck-Candanedo et al., 2015). Additionally, these anionic sulphate half-ester groups enhance the hydrophilicity of CNCs. In pure water, cellulose nanocrystals form a stable suspension due to electrostatic repulsion. However, aggregates tend to form when the charge density is reduced by desulphation or when the interaction is screened by addition of salt (Peddireddy et al., 2016). Repulsion among the negatively charged cellulose nanocrystals is very effective in preventing agglomeration, which allows their uniform dispersion in water, ethanol and other solvents without any additional preparation or aids (Lu & Hsieh, 2010).
2.4.5 Ammonium Persulfate (APS) Oxidation
Ammonium persulfate (APS), an oxidant with low long term toxicity, high water solubility and low cost, is favoured over its sodium and potassium counterparts.
Homogenous cellulose nanocrystals have been prepared using APS. This versatile novel one-step procedure can be applied to a variety of cellulosic biomass without the need for pre-treatments to remove non-cellulosic plant content (Lam et al, 2012). Using the APS oxidation, cellulose nanocrystals with higher crystallinity, thermal stability, transparency, water resistance and oxygen barrier properties have been reported. (Mascheroni et al., 2016 ;Zhang et al., 2016; Oun & Rhim, 2017). Furthermore, carboxylated cellulose nanocrystals are obtained from these oxidised celluloses, where by the C6 hydroxyl groups is converted to carboxyl groups during the oxidation.
When APS solution is introduced in the presence of heat, the peroxide bond of APS, the weakest bond,is cleaved to form two SO4- radical ions.
S2O82- + heat ? 2SO4.-
S2O82- + 2H2O ? 2HSO4- + H2O2
Under acidic condition (pH 1.0), hydrogen peroxide formed. Collectively, these free radicals and H2O2 are capable of penetrating the amorphous regions of cellulose and cohydrolyse the ? -1, 4 glycosidic bond of the cellulose chain in this region (Zhang et al., 2016). Both free radicals SO4- and H2O2 also open the aromatic rings of lignin to decolourise this material (Leung et al., 2011). The carboxylated ions on the surface of cellulose nanocrystals cause strong electrostatic repulsion between carboxylated cellulose nanocrystals in water (Zhang et al., 2016).