*2.2. Extraction and Properties of CNC*

CNC can be often obtained from different types of lignocellulose through a top-down hydrolysis approach by combining various procedures [9,17,23,84,85]. To extract pure cellulose (PC) through the elimination of extractives, lignins and hemicelluloses, some pretreatments (chemical, physical, physicochemical, biological or their combination) of the natural source are usually required [13,34,86]. Specific treatments can be then applied to PC to produce CNC through the removing of disordered regions from pristine cellulose. The crystalline domains remain intact because of their higher resistance to the hydrolytic action, whereas the amorphous parts dispersed as chain dislocations on segments along

the cellulose fibrils are more susceptible to the hydrolysis process [19,78,82]. Afterwards, the elementary fibrils are transversely cleaved, producing short CNC with somewhat high crystallinity. Nonetheless, after this process, extra post-treatments such as solvent removing, sonication, fractionation, dialysis, centrifugation, filtration, washing, stabilization, surface modification, neutralization and drying are required to recover CNC product.

The most common hydrolysis method used to produce CNC relies on sulfuric acid, which can react with the surface hydroxyl groups of pristine cellulose through an esterification process, allowing the grafting of anionic ester groups [77,87]. This latter generates a negative electrostatic layer that covers nanocrystals, promoting the dispersion of CNC in water but reducing their thermal stability. Recently, as an alternative to sulfuric acid hydrolysis, other liquid inorganic acids such as nitric, hydrobromic, phosphoric and hydrochloric have been extensively reported [11,30]. The preparation of CNC from wood, for which the hydrolysis process causes preferential digestion of the amorphous part of cellulose while the ordered regions remain intact, is schematized in Figure 2. Both natural source and experimental conditions (acid concentration, reaction time, temperature, mass ratio, etc.) may influence the characteristic of the prepared CNC such as crystallinity, dimensional dispersity, thermal behavior, mechanical properties, density, aspect ratio and morphology. Although the hydrolysis process using mineral acids is simple and not time-consuming, certain drawbacks such as lower yield, high amount of water usage, severe environmental pollution and harsh corrosion of equipment should be overcome [30]. Therefore, to address the above issues, various recent procedures such as organic acid (oxalic, formic, etc.) hydrolysis [88], solid acid (phosphotungstic) [89], subcritical water hydrolysis [90], deep eutectic solvents [91], ionic liquids [92], oxidation [93], sonication [94], enzymatic [95] and combined approaches [5,17,31] have been applied and others continue to be developed worldwide to produce CNC with desired properties at lower costs and higher yield based on sustainability principle and environmentally friendly policy [5,13,17,31]. Nevertheless, scaling-up from laboratory to industrial scale remains one of the most important issues and considerable efforts should be made to prevail over the remaining constraints. Otherwise, some companies such as CelluForce and Alberta Innovates, among others, produce CNC at large scale [13,96].

CNC present unique features compared to the other classes of NC with the spotlight to characteristics such as physical, chemical, optical, thermal, mechanical, electrical properties [1]. CNC consist of an elongated, needle or rod-like nanoparticles. They are 4–70 nm in width and 100–6000 nm in length and aspect ratio of 5–70, as well as large surface area (150–500 m2·g<sup>−</sup>1), which allows it to be easily dispersed in water to generate a chiral nematic organization [13,16,97]. CNC also exhibit high crystallinity (50%–90%), a tensile strength of up to 7.5 GPa, a Young's modulus of ~170 GPa and a bending strength of about 10 GPa [9,13,68]. They also display good thermal stability up to 200 ◦C and can find applications in processes like thermoplastics [97]. Nevertheless, these features depend closely on the source of feedstock, extraction methods and experimental conditions, which will ultimately define their applicability [98].

It is worthy to note that the abundance of –OH or other reactive chemical groups and the high surface area to volume ratio render CNC highly reactive and easy to be functionalized [100]. Therefore, to improve their compatibility and ensure a good dispersion, CNC surface can be chemically, physically or enzymatically modified to impart stable negative or positive electrostatic charges on their surface [13,101]. Such modifications may allow tailoring the properties of the CNC-based materials depending on the intended application.

**Figure 2.** (**a**) Structural hierarchy of the cellulose fiber component from the tree to the anhydroglucose molecule. (**b**) Preparation of cellulose nanocrystals (CNC) by selective acid hydrolysis of cellulose microfibrils. Reproduced with permission from ref [99]. Licensed under a Creative Commons Attribution 3.0 International License (https://creativecommons.org/licenses/by/3.0/).
