Extraction of CNCs

The extraction of CNCs can be performed by strong acid hydrolysis of cellulose or purified cellulose under controlled conditions. Different common acids were used to extract CNCs such as sulfuric acid (H SO ), hydrochloric acid (HCl), and hydrobromic acid (HBr) whereas phosphoric acid (H PO ), maleic acid (HO CCHCHCO H), and hydrogen peroxide (H O) were used to a lesser extent. However, sulfuric acid is the most common acid used for acid hydrolysis of corresponding cellulose,where it reacts with hydroxyl (-OH) groups to form sulfate esters and produces stable aqueous suspension compromising thermal stabihty [6,8,59]. Apart from acid hydrolysis, many other processes have also recently been used to extract nanocrystalline cellulose from cellulose fibers such as enzymatic hydrolysis [60-62], TEMPO-mediated oxidation [63,64], hydrolysis with gaseous acid [65], and ionic liquid [66]. 

Sulfuric (H SO ) acid is most typically used because it produces highly stable aqueous suspension having negatively charged nanocrystals. Although the mechanism of add hydrolysis is not completely imderstood, it can be described through the penetration of H3O+ ions into amorphous domains of cellulosic material depending on 

Delignification using an acidified sodium chlorite solution at 7SX for 1 h and repeated four to five times until products became white. 

Filtered rinsed until residues were neutral 

---

Figure 15.7 Schematic representation of extraction of CNCs from chemically purified cellulose [70].

The research on extraction of CNCs from different sources and uniform dispersion in polymer matrices has always been an important issue in the field of bionanocomposites. The nanoscale dimensions of cellulosic fibers along with their biodegradability and... 

Crayfish nerve cord (CNC) has been used successfully as an assay for the extracts of the dinoflagellate Prorocentrum concavum (Miller et al. 1986). Three of the P. concavum extracts reduced the activity of the CNC. Purified CTX from moray eel induce spontaneous action potentials upon the node of Ranvier of frog isolated nerve fibers under current and voltage clamp conditions. This spontaneous activity Is reversible upon removal of the toxin from the external solution (Benoit et al. 1986). The postulated reason for this activity will be explained in the pharmacology section of this chapter. 

The extraction of cellulose nanociystals or CNCs (also called (nano-/ micro-)crystallites, whiskers, nanorods, and nanowires ) was first demonstrated in 1952 using acid hydrolysis of cellulose fibres. However, interest in the technique remained low until 1992, when their chiral... 

Native and surface-trimethylsilylated CNC were employed as the particulate phase in nanocomposites with a cellulose acetate butyrate matrix to improve the mechanical properties of polymers and to enhance adhesion between the particulate and matrix phase in composites [187]. Poly(oxyethylene)-based polymer electrolytes should be used above their melting temperature to display appropriate conductivity. Unfortunately, at this temperature the mechanical properties were very poor. In this regard, Azizi Samir et al. [188] evaluated the effect of CNC extracted from timicate to improve the mechanical properties of poly(oxyethylene) (PEO)-based nanocomposite electrolytes above its melting temperature. The SEM fracture surface of CNC-PEO... 

The properties of CNCs may be directly varied depending on source and extraction methods. These basic properties can be considered in two ways (1) Inherent properties and (2) suspension behavior [101]. 

Concerning the use of CNC extracted from defined species, cassava starch matrix, plasticized with an equimolar addition of glycerol and sorbitol, has been reinforced with CNC extracted from kenaf fibers, CNC did not affect composite s transparency. 

A solution of 9-phenyl-9//-purine-6-carbonitrile (0.2 g. 0.9 mmol) and N,.V-dielhylprop-l-ynamine (0.2 g, 1.8 mmol) in xylene (3 mL) was refluxed for 18 h. After cooling, the mixture was poured into excess H20, and extracted with CHC13. The extract was washed with H20, dried (Na2S04), and concentrated to dryness. The residue was chromatographed (silica gel, CHCl, then benzene/ EtOAc 20 1). The fraction eluted with benzene/ EtOAc gave the product [yield 194 mg (65 %)] as pale yellow needles after recrystallization (benz-cnc/petroleum ether) mp 157— 158 C. 

After extraction, further changes to the CNC suspension can be made by heating, sonication, and addition or removal of salts. These changes can influence the liquid crystalline phase according to the SLO theory, as explained in the discussion of liquid crystals. Furthermore, these changes can be used to control the parameters of the final structure. 

Nanocellulose in the form of NFC or CNC can be extracted by various methods. Ihis section briefly describes the processes for producing nanocellulose, their advantages and disadvantages, as well as some important issues regarding these methods. 

Figure 11.27 SEM image for CNC extracted from tunicate of poly(oxyethylene) composites (a) unfilled POE matrix and related composites filled with (b) 3 wt% and (c) 6 wt% tunicin [188].

Nanocellulose can broadly be defined as a set of particles having at least one dimension in nanoscale obtained by different mechanical/chemical, or only chemical, or acid-hydrolysis methods. According to a survey of the literature, there is no standardized nomenclature for cellulose-based nanoparticles, while various terms have been used to describe a particular set of cellulose nanoparticles. Because of inconsistency in use of these terms, there are some particle types depending on source materials of cellulose and method of extraction. However, depending on their aspect ratio, nanocelluloses have already been characterized with different terms like microfibrillated cellulose (MFC-diameter 10-100 nm) [34], nanofibrillated cellulose (NFC-diameter 4-20 nm) [35], CNCs (CNCs-diameter 3-5 nm) [34, 36], tunicate CNCs (t-CNCs-diameter 20 nm) [34, 37], algae cellulose particles (AC-diameter 20 nm) [38,39], bacterial cellulose particles (BC-diameter 6-50 nm) [40, 41]. It can commonly be called "nanocellulose,"... 

Another possibility, which is widely used for the production of bio-nanocomposites, also with resin directty obtained from biomass, is extracting cellulose nanociystals (CNC) from other biomass source or from aspecilic cellulose. This led also to original solutions, such as the fabrication of materials including a kind of hierarchical structure, hence introducing both a macrosized reinforcement and a nanosized one. In particular, in a jute and soy flour composite, with glutaraldehyde providing suitable interface adhesion, cellulose nanowhiskers have been introduced in addition or not to nanoclay (Fig. 13.5), a procedure which provided improved thermal, mechanical and dimensional stability (Iman et al., 2013). 

Cellulose nanowhiskers (CNWs), also known as cellulose nanocrystals (CNCs) or nanocrystalline cellulose, are usually produced by the acid hydrolysis of natural cellulosic material after removing nonceUulosic substances such as dewaxing, hemicelluloses, and lignin. Most CNWs show a high crystallinity index and a lower aspect ratio. Thus it is expected that CNWs extracted from tunicates exhibit a high... 

---