*2.2.2 Properties of cellulose nanocrystals*

In addition to their nanometric size, CNC are unique biodegradable and renewable nanomaterials. Moreover, they result from a previously described optimized acid hydrolysis applied on abundant sources of cellulose and exhibit many other interesting properties. **Figure 6** summarizes the main CNC properties as well as their applications. Regarding the surface properties of CNC, they generally exhibit half-sulfate ester groups (▬SO3 ) on their surface after being treated with sulfuric acid. Even if the amount of ▬SO3 is pretty low (about 50–200 μmol g<sup>1</sup> ), these negative charges are sufficient to induce repulsive forces between nanomaterials and thus colloidal stability in aqueous media. Moreover, as presented in **Figure 6**, due to isolation process, other charged groups can be present on CNC surface, like carboxyl groups (▬COO), aldehyde groups (▬CHO), and others [23], leading to different charge properties inducing different CNC properties. Moreover, numerous hydroxyl groups (three groups in each AGU units) are at reactive surface sites for the introduction of new functional groups via hydroxyl groups' functionalization. Regarding the physical properties of CNC, they have a low density (1.606 g cm<sup>3</sup> ), a high aspect ratio length/width (e.g., varying between 10 and 30 for CNC extracted from cotton and around 70 for those extracted from tunicate [15]), and a high surface area (between 150 and 800 m<sup>2</sup> g<sup>1</sup> ). Note that all their morphological and surface properties are highly dependent on their source as well

and their annual production capacity. Note that the leader and pioneer CelluForce©

Cellulose nanocrystals are obtained by applying a chemical treatment to cellulose fibers: mild acid hydrolysis. Typically employing strong sulfuric acid H2SO4 is going to penetrate into accessible amorphous domains of cellulosic chains and dissolve them to release crystalline parts. Amorphous domains are randomly oriented and arranged inducing a lower density of these domains which are thus more vulnerable to acid hydrolysis [14] and especially to the infiltration of hydronium ions H3O<sup>+</sup> leading to hydrolytic cleavage of glycosidic bonds. In this sense, Ranby et al. were the first to prove the preparation and the presence of CNCs, the smallest cellulosic building blocks. **Figure 5** synthesizes the different steps of CNC isolation using

As previously mentioned, cellulose fibrils are exposed to a sulfuric acid hydrolysis, with defined concentration, temperature, and reaction time. Once amorphous domains are dissolved, a sonication step allows the separation between intact crystalline domains, leading to isolated CNC bearing half-sulfate ester groups on their surface. These charges come from the reaction between sulfuric acid and surface hydroxyl groups of cellulose and induce repulsive forces between negatively charged CNC leading to colloidal stability and dispersion in water [15]. While sulfuric acid is the most common acid used for cellulose fibers hydrolysis, other researches have focused on the use of other organic or mineral acids, like phosphoric acid, hydrobromic acid, or hydrochloric acid [16–19], generally leading to less stable suspension due to the lack of charges at the surface of the CNC. Moreover, the use of deep eutectic solvents is the subject of the next part of this chapter. When

*Schematic representation of sulfuric hydrolysis of cellulose fibers (This scheme was extracted from an*

*unpublished work (PhD manuscript of E. Gicquel, 2017)).*

has recently announced new strategy of efficient industrialization [11].

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

**2.2 Cellulose nanocrystals: nanomaterials with interesting properties**

*2.2.1 Isolation of cellulose nanocrystals*

sulfuric acid hydrolysis.

**Figure 5.**

**82**

added to intrinsic birefringence of CNC induce interesting optical properties. Moreover, when ordered CNC suspension is solvent evaporated and thus solidified in a self-standing film, conserved chiral nematic structure (helicoidal structures) and iridescent behavior of films are observed and monitored by CNC concentration and surface charge as well as suspension sonication [27, 28]. **Figure 7** shows explicit

*Cellulose Nanocrystals: From Classical Hydrolysis to the Use of Deep Eutectic Solvents*

and increasing interest in research and industrial field during the last decades. Although their isolation and characterization are currently well-advanced and optimized, application fields are at the center of ongoing researches, as described in the

*2.2.3 Various applications of cellulose nanocrystals and their industrialization*

As exposed in **Figure 6**, CNC found applications in various fields. Indeed, thanks to their outstanding morphological, mechanical, and rheological properties as well as their colloidal stability and high surface reactivity. All these properties added to their biodegradability and renewability make them highly interesting and innovative materials with many potential applications. **Table 2** summarizes CNC applications and corresponding exploited properties, as well as some literature

Nanocomposite field is an emerging research area which finds applications in several domains like food packaging, medical devices, and building. Renewable aspect of CNC is particularly interesting since it correlates with the development of bio-based and biodegradable polymers as mentioned in the first part of this chapter. Moreover, these same properties are just as interesting in other application fields, from coatings, electronics, filtration, and biomedical devices to energy, cosmetics, and security. Note that for applications that may enter in contact with food or human body and for any industrialization, toxicity of CNC is a key challenge to investigate. Indeed, even if cellulose is known to be a nontoxic polymer, CNC are nanomaterials—and the "nano" prefix can be frightened for media and population

*(a) Translucent gel-like CNC suspension at 15 wt% in water (extracted from 57), (b) birefringence with shear-inducing observed for an aqueous CNC suspension at 0.6 wt% in cross-polarized light (extracted from [57]), (c) solvent-casted CNC film in diffuse light, normal to the surface (on the left part) and oblique to the surface (on the right part) (extracted from [29]), and (d) schematic representation of CNC orientation in isotropic and anisotropic phases (This scheme was extracted from an unpublished work (PhD manuscript of*

All these outstanding surface and physical properties of CNC confirm their high

pictures of these rheological and liquid crystalline properties.

*DOI: http://dx.doi.org/10.5772/intechopen.89878*

following part.

references.

**Figure 7.**

**85**

*R. Bardet, 2014)).*

#### **Figure 6.**

*Main surface and physical properties of cellulose nanocrystals and inherent main applications.*

as their isolation process and conditions [8, 22]. Moreover, CNC exhibit highly interesting mechanical properties. Indeed, in addition to their high crystallinity (between 54 and 88% according to the source [24]), their high elastic modulus (≈150 50 GPa) and tensile strength (≈7.5 0.5 GPa) [25] make them interesting materials as mechanical reinforcement in polymer matrices, for example. For comparison, their mechanical properties are similar to Kevlar® fibers [26] widely used in composite field.

At low solid content (<3 wt%), due to hydrogen bonds between cellulose chains and thus between each nanocrystals, CNC water suspension is in the form of a translucent gel. Rheological properties of CNC are outstanding and concentration dependent. Indeed, at low concentration (<3 wt%), CNC suspension presents shear thinning behavior at high shear rate, and at higher concentration (>3 wt%), the suspension presents shear thinning behavior explained by the nanocrystals alignment in the flow direction at a critical shear rate [27]. Source and isolation of CNC influence these rheological properties too. Besides all these properties, CNC selforganize in ordered structure, especially to form a nematic phase. Revol et al. [28] described in the 1990s this self-organization of CNC in water suspension into stable chiral nematic phases. These last exhibit liquid crystalline properties, which when
