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Preparation of Cellulose Nanocrystallines and Their Applications in CompositeMaterials

Jinzhao Li , Zheng Li , Xupin Zhuang , Jixian Gong , Qiujin Li , Jianfei Zhang

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1293 -1310.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1293-1310 DOI: 10.7536/PC200759
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Preparation of Cellulose Nanocrystallines and Their Applications in CompositeMaterials
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Abstract

Cellulose nanocrystal(CNC) is a nano-scaled rod-like or spherical crystal isolated from cellulosic materials. CNC has shown many advantages of, for example, high strength, high specific surface area, biocompatibility, renewability and degradability. Therefore, it can be applied to the composite materials, biomedicine and environment fields. The preparation methods of CNC are detailedly summarized in this review such as acid hydrolysis, oxidation method, enzymatic hydrolysis, mechanical method, solvent methods and combined processes. Meanwhile, the advantages and shortcomings of the preparation methods are discussed. In the field of applied research, this review summarizes the research status of CNC in some popular fields such as reinforced composite materials, membrane filtration composite materials, conductive composite materials and inorganic nanocomposites. Finally, the future prospective of CNC is presented.

Contents

1 Introduction

2 Physicochemical characteristics of CNC

2.1 Size distribution and morphology

2.2 Thermal performance

2.3 Rheological properties

3 Methods to prepare CNC

3.1 Acid hydrolysis

3.2 Oxidation methods

3.3 Enzymatic hydrolysis

3.4 Mechanical methods

3.5 Solvent methods

3.6 Combined processes

4 Application of CNC in the field of composite materials

4.1 Reinforced composite materials

4.2 Membrane filtration composite materials

4.3 Conductive composite materials

4.4 Inorganic nanocomposites

5 Conclusion and outlook

Graphical abstract

Keywords

cellulose / cellulose nanocrystal / preparation / composite materials

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Jinzhao Li, Zheng Li, Xupin Zhuang, Jixian Gong, Qiujin Li, Jianfei Zhang. Preparation of Cellulose Nanocrystallines and Their Applications in CompositeMaterials. Progress in Chemistry, 2021, 33(8): 1293-1310 DOI:10.7536/PC200759

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1 Introduction

Cellulose is the most abundant and biodegradable natural macromolecular compound in nature, with an annual natural regeneration of about 180 billion tons in the world[1]. Cellulose separate from cellulose and having a one-dimensional spatial dimension in that nanometer range is cal Nano celluloses[2]. According to the size and preparation methods, Klemm et al. Classified nanocellulose into three categories: cellulose Nanocrystalline (CNC), Microfibrillated cellulose (MFC) and Bacterial cellulose (BC)[2]. cellulose nanocrystals, also known as cellulose nanowhiskers (CNW) and nanocrystalline cellulose (NCC), are composed of rod-like cellulose crystals, containing 100% cellulose and high crystallinity, ranging from 54% to 88%, depending on the source and separation process of cellulose[3][4]. CNCs are produced primarily by acid hydrolysis of cellulose fibers, with preferential hydrolysis of the amorphous or quasi-crystalline regions of the cellulose, while the crystalline regions, which are more resistant to acid attack, retain their integrity (Figure 1). Their size, morphology, and crystallinity vary depending on the source of the cellulose material and the conditions of preparation[5][5]. A microscope (transmission electron microscope, TEM; Atomic force microscope (AFM); Environmental scanning electron microscopy (E-SEM) or light scattering techniques, including small-angle neutron scattering (SANS) and dynamic light scattering (DLS), were used to characterize their morphologies[6][7][8]. Raw materials for the preparation of CNC come from a wide variety of sources, including plants, animals (tunicates), bacteria, and algae. Due to their abundant natural resources, wide availability and high content of cellulose, plants are an important source for the extraction of CNC. Due to the small particle size and large specific surface area, the surface of CNCs is covered with a large number of hydroxyl groups, which can be functionalized, including esterification, etherification, oxidation, silylation and polymer grafting[9]. In addition, CNC is an environmentally friendly natural material with high purity, high crystallinity, high Young's modulus, high strength, biocompatibility, reproducibility and degradability, which can be used in reinforced composites, conductive composites, membrane filtration composites and other fields.
In this paper, the preparation methods of CNC and the latest research and application progress in the field of composite materials in recent years are reviewed, and the shortcomings of CNC research at this stage are put forward, and finally the future research and development are prospected.

2 Physicochemical Properties of Cellulose Nanocrystal

2.1 Size distribution and shape of CNC

The size, morphology and crystallinity of CNCs vary depending on the source of the cellulosic material and the preparation conditions. Table 1 shows the size distribution of CNCs obtained from different sources of raw materials using different preparation methods[5]. In addition, since the breakage of cellulose chains occurs randomly during the acid hydrolysis, the size of the CNCs is not uniform. As can be seen from Table 1, BNC obtained from bacterial cellulose is generally larger in size, ranging from 100 to 300 nm in diameter, compared to CNC obtained from wood, because bacterial cellulose is highly crystalline and has a lower proportion of amorphous structures that need to be cleaved, resulting in larger sized nanocrystals.
Acid hydrolysis is the main method to prepare CNC. The amorphous or quasi-crystalline regions of cellulose are preferentially hydrolyzed, while the crystalline regions with higher acid resistance remain intact. Thus, after acid treatment to hydrolyze the cellulose, rod-like CNC is produced (fig. 2). The obtained CNC is similar in morphology to the original cellulose and has a higher degree of crystallinity. CNCs can also display different geometries, depending on the source of their raw materials. For example, algal cellulose membranes exhibit a rectangular structural arrangement, whereas bacterial and capsular cellulose chains both have a twisted ribbon geometry[17].

2.2 Thermal Properties

The thermal degradation of CNC or the reduction of its mechanical properties at high temperature is one of the main factors affecting its use[18]. The type and content of surface groups of CNC have an important influence on the thermal stability of CNC. Camarero et al. studied the thermal stability of cellulose nanocrystals prepared by hydrolyzing cotton cellulose with phosphoric acid, hydrochloric acid and sulfuric acid (P-CNC, H-CNC and S-CNC, respectively). The results of thermogravimetric analysis (TGA) showed that H-CNC began to decompose at about 200 ℃ and showed the highest thermal stability. The decomposition temperature of P-CNC was the same as that of H-CNC (220 ℃), but the highest decomposition temperature (325 ℃) was about 15 ~ 25 ℃ lower than that of H-CNC[19]. At 150 ℃, S-CNC begins to decompose. Voronova et al. studied the thermal stability of CNC at different pH, and the results showed that when the sulfate groups on the surface of CNC were neutralized by NaOH solution, their thermal degradation would shift to a higher temperature, indicating that the neutralization of sulfate groups by NaOH solution could improve the thermal stability of CNC[20]. At the same time, the PVA/CNC composites prepared by solution casting method were degraded at higher temperature in the presence of acidic CNC, so the acidic CNC could significantly improve the thermal stability of the composites, while the neutralized CNC had little effect on the thermal stability of the composites. Li Xiaofang et al prepared rod-like nanocellulose crystals by mixed acid. According to TG and DSC analysis results, the rod-like nanocellulose began to show a strong degradation trend at about 210 ℃[21]. The smaller the particle size is, the lower the degree of polymerization is, the larger the specific surface area is, and the higher the reaction activity is, which leads to the lower thermal stability of rod-like CNC. The thermal stability of CNC affects the molding process of nanocrystalline cellulose composites. Therefore, it is necessary to select an appropriate composite material processing method according to the properties of CNC and matrix materials, as well as the desired shape and function of the composite material.

2.3 Rheological properties

The rheological properties of CNC suspensions are mainly affected by its concentration, salinity and surface properties. Ye Daiyong et al. studied the effects of temperature and content on the viscosity of the suspension of nanocellulose whiskers prepared by acid-catalyzed hydrolysis. The results showed that the viscosity of CNC suspension increased with the increase of CNC concentration. When the CNC concentration was 0.5%, the viscosity of the suspension was about 160 times that without CNC. When the CNC concentration exceeded 2.0%, the viscosity increased rapidly[22]. When more than 3.0%, the sample becomes a white, viscous, non-flowing paste. Li Xiuwen used Ubbelohde viscometer to investigate the effects of different CNC surface charge density and NaCl content on the intrinsic viscosity of CNC suspension. The results showed that with the increase of NaCl concentration, the stability of CNC suspension decreased, and the intrinsic viscosity decreased first and then increased. The reason was that a small amount of NaCl shielded the ionic double layer on the surface of CNC, while excessive NaCl promoted the aggregation of CNC[23]. The surface properties of CNC depend on the type of acid used in the hydrolysis process (fig. 3): if prepared with HCl, the crystals are weakly charged and have poor colloidal stability; If prepared by H2SO4, the crystal has a larger negative charge, and the CNC suspension prepared by sulfuric acid has strong repellency and high colloidal stability[2]. In addition to the surface properties, the size of the dispersed crystals also affects the stability of the CNC suspension. In general, smaller nanoparticles with a low aspect ratio are more uniformly dispersed in solution.

3 Method for preparing cellulose nanocrystal

Acid hydrolysis, oxidation, enzymatic hydrolysis, mechanical method and solvent method are commonly used to prepare cellulose nanocrystals. These methods can be used alone or in combination. Table 2 lists the size and yield of CNC prepared from different raw materials by different preparation methods. It can be seen that the CNC prepared from oil palm empty fruit bundle microcrystalline cellulose by oxidation method has the highest yield of 93.0%. The CNCs prepared from microcrystalline cellulose (MCC) treated with ionic liquid have small size and narrow distribution.

3.1 Acid hydrolysis

Acid hydrolysis is an acid-induced destruction process in which heterogeneous acid hydrolysis involves the diffusion of acid molecules into the cellulose microfibril, resulting in the cleavage of glycosidic bonds in the cellulose molecular chains along the amorphous domain of the cellulose fibril, leading to the fragmentation of the hierarchical structure of the fibril bundle to form CNCs[4]. The difference in hydrolysis kinetics between the quasicrystalline and crystalline regions leads to selective cleavage of the cellulose chains[4]. The hydrogen ions released by the acid tend to penetrate the cellulose in the amorphous region and react with the oxygen in the glycosidic bond between the two anhydroglucose moieties, thereby initiating protonation of the oxygen and thus hydrolysis of the glycosidic bond in the amorphous region[4].
Hydrolysis is followed by a mechanical process, usually ultrasonic action, to disperse the CNC into a homogeneous and stable suspension. The structure, properties and phase separation behavior of CNC suspensions are closely related to the type and concentration of acid used, the hydrolysis temperature and time, and the intensity of ultrasound[2].

3.1.1 Inorganic acid hydrolysis

Sulfuric acid, hydrochloric acid, phosphoric acid and phosphotungstic acid are commonly used inorganic acids for the preparation of CNC[42]. Zianor Azrina et al. Prepared spherical CNC by ultrasound-assisted acid hydrolysis of sodium hydroxide-treated TEFBP with 64% (w/V) sulfuric acid solution in an ultrasonic bath at 45 ° C[43]. XRD analysis showed that the degree of crystallinity of CNC was as high as 80. 0%, while that of original hollow fruit bundle fiber and oil palm hollow fruit bundle pulp was 42. 0% and 73. 0%, respectively. In addition, CNC also shows higher thermal stability.
Bacterial cellulose (BC) is a kind of porous reticular nano-scale biopolymer synthesized by Acetobacter xylinum[44][45]. Compared with plant cellulose, bacterial cellulose is a kind of pure cellulose without lignin, hemicellulose and pectin, and can also be used as raw material for CNC preparation[46]. Salari et al. Prepared CNC with an average diameter of (25 ± 5) nm and an average length of (306 ± 112) nm by hydrolyzing bacterial cellulose with 50% concentrated sulfuric acid at 50 ℃ for 40 min[47]. P Pääkkönen et al. Prepared CNC by hydrolyzing bacterial cellulose aerogel with gaseous hydrochloric acid, and then carried out 2,2,6,6-tetramethylpiperidine-1-oxyl radical-mediated oxidation reaction to introduce carboxylate groups, endow it with surface charge, and make it stably disperse in solution[10]. The degree of polymerization of bacterial cellulose decreased from 2160 to 220, and the yield of bacterial cellulose nanocrystals was more than 80. 0%.The length and diameter of nanocrystals were 100 ~ 300 nm and 5 ~ 20 nm, respectively. Ramakrishnan et al. Proposed a simple and green method for the preparation of CNC by adding a small amount of sulfuric acid to a hot glycerol medium[26]. The dried medical cotton was added to a mixture containing 9% (w/w) sulfuric acid and glycerol (cotton: solvent = 1:30 w/w) and heated in an oil bath at 90 ° C for 3 H to give CNC with a yield of 56.0% and an average diameter and length of (18.4 ± 7.2) nm and (297.7 ± 98.9) nm, respectively. Glycerin acts as a plasticizing agent in the amorphous region, enabling uniform heat transfer. At the same time, under the catalysis of sulfuric acid, the amorphous region is cracked to prepare CNC. The method doe not require that use of other organic solvents, doe not require mechanical disintegration and homogenization operations, and the residual "green solvent" can be reused for several cycles aft being filtered by activated carbon.
Tuerxun et al. extracted cellulose fibers from natural rubber wood fibers (RRaw) and kenaf bast fibers (KRaw) by alkali treatment and hydrogen peroxide bleaching[48]. Then, the cellulose fibers were dispersed in 60 mL of 48% sulfuric acid and hydrolyzed at 45 ℃ with stirring for 45 min to prepare CNC successfully. After acid hydrolysis, the yields of CNC obtained from RRaw and KRaw were 27.5% and 32.5%, respectively, and the crystallinity increased from 61.2% and 54.1% to 74.3% and 73.2%, respectively. The average diameter of RRaw and KRaw was (5.14 ± 1.91) nm and (5.27 ± 2.38) nm, respectively. Cheng et al. designed a simple and rapid method to prepare carboxycellulose nanocrystals (CCNC) by one-step hydrolysis of MCC using a mixed acid solution of 8 mol/L sulfuric acid and 2 mol/L nitric acid (H2SO4/HNO3) (Fig. 4)[49]. The HNO3 is used as an oxidant to effectively convert the surface hydroxyl groups of the CNC into carboxyl groups. Meanwhile, the prepared CCNC is rod-shaped with a crystallinity of 90. 5%, a length of (186 ± 13) nm and a diameter of (9 ± 3) nm. More importantly, due to the presence of negatively charged carboxyl groups, CCNC exhibits excellent dispersibility in water and various organic solvents (ethanol, acetone, N, N-dimethylformamide (DMF), etc.). Zhang et al. prepared rod-like CNCs with an average particle size of 524 nm by one-step hydrolysis of bleached kraft bamboo pulp with very low hydrochloric acid (acid concentration of 0.1 wt%)[50]. The experimental results show that the operating pressure, the concentration of hydrochloric acid and the temperature have great influence on the yield of CNC. The yield of CNC can reach 37.1% under the conditions of 0.08 wt% hydrochloric acid, 20 MPa operating pressure and 180 ℃ hydrolysis for 60 min. Fourier transform infrared spectroscopy (FTIR) showed that the cellulose structure was retained in the pretreated CNC. Compared with the CNC prepared by the traditional concentrated sulfuric acid hydrolysis method, the CNC prepared by the very low acid hydrolysis process has higher thermal stability.
Camarero et al. prepared CNC with a yield of 76.0% ~ 80.0% by treating filter paper with phosphoric acid of different concentrations at 100 ℃ for 30 ~ 300 min[19]. Hydrolysis time and acid concentration have great influence on the morphology of CNC, and appropriate increase of hydrolysis time and acid concentration is helpful to obtain CNC with ideal size. Liu et al. Used 75% phosphotungstic acid (H3PW12O40,HPW) to catalyze the hydrolysis of bleached hardwood pulp at 90 ℃ for 30 H, and prepared rod-like CNC with a yield of 60. 0%, a diameter of 15 ~ 40 nm and a length of 600 ~ 800 nm[24]. Thermogravimetric analysis showed that the obtained CNC had higher thermal stability than CNC prepared from sulfuric acid. The concentrated HPW can be easily recovered by extraction with ether, and the recovered HPW can hydrolyze cellulose many times without loss of activity. Zhuangsenyang et al. prepared CNC by hydrolyzing mushroom bran with dilute sulfuric acid assisted by zirconium phosphate catalyst, and the obtained CNC was rod-shaped with a diameter of 10 ~ 30 nm; CNC belonged to fiber type I, and its crystallinity increased from 63. 8% to 81. 0% compared with the raw material mushroom bran[25]. In addition, the effects of ultrasonic time, temperature and the concentration of dilute sulfuric acid on the yield were investigated. The results showed that the yield of CNC was 42. 8% when the ultrasonic time was 5 H, the temperature was 75 ℃ and the concentration of dilute sulfuric acid was 12. 3%.

3.1.2 Organic acid hydrolysis

Inorganic strong acid hydrolysis usually causes problems such as equipment corrosion, large water demand, difficult acid recovery, and excessive cellulose degradation. In recent years, some researchers have used organic acids such as formic acid, oxalic acid and maleic acid to hydrolyze cellulose to prepare CNC.
Chen et al. reported that eucalyptus pulp raw material was hydrolyzed with concentrated organic acids (oxalic acid, maleic acid, p-toluenesulfonic acid), and CNC with high thermal stability and functionalization was successfully prepared. The residue that was not completely hydrolyzed could be mechanically treated to obtain cellulose nanofibrils (CNF). The reaction process is shown in Figure 5[51]. Due to the low water solubility of solid organic acids, they crystallize at lower or ambient temperatures and are easily recovered after hydrolysis. CNC prepared from eucalyptus kraft pulp bleached with oxalic acid at 50 wt% ~ 70 wt% concentration has a carboxyl group content of 0.1 ~ 0.4 mmol/G, a length of about 275 ~ 380 nm, and an initial thermal degradation temperature of 322 ℃, while CNC prepared from the same fiber material hydrolyzed with conventional concentrated sulfuric acid has an initial thermal degradation temperature of 218 ℃. Liu et al successfully prepared CNC with length of (421 ± 112) nm and diameter of (6.5 ± 2.0) nm by catalytic hydrolysis of furfural residue cellulose with 88% formic acid and a small amount of hydrochloric acid. The yield was 66.3%, which was higher than that of CNC prepared by sulfuric acid hydrolysis (34.5%). The crystallinity was 63.8% and the maximum decomposition temperature was 360 ℃[11]. In order to further reduce the concentration of formic acid and reduce the process cost, Du et al studied the preparation of CNC from bleached eucalyptus kraft pulp by hydrolysis of formic acid catalyzed by ferric chloride, which reduced the concentration of formate to 80%[52]. The results showed that the yield of CNC was 70% ~ 80%, the degree of crystallinity was high (about 75. 0%), the thermal stability was good (the maximum decomposition temperature was 355 ℃), and it had good dispersion in dimethyl sulfoxide (DMSO). The recovery rate of formic acid after the reaction is higher than 90%, and the ferric chloride can be recovered in the form of ferric hydroxide. Xu et al. Designed a one-step method to prepare CNC by hydrolyzing bleached birch kraft pulp with dilute oxalic acid (0.06 ~ 1.11 mol/L) and a small amount of hydrochloric acid at 70 ~ 100 ℃[27]. The highest yield was 85%, the width of the CNTs was 8 ~ 15 nm, the length was 200 ~ 1 200 nm, and the CNTs had high thermal stability (the maximum decomposition temperature was 350 ℃).
Lignin has higher thermal stability than cellulose or hemicellulose, and its inclusion in cellulose nanomaterials can improve thermal stability[53]. In addition, lignin can be used to adjust the polarity and hydrophilicity of cellulose[53]. Therefore, the use of lignin-containing cellulose fibers to prepare cellulose nanomaterials is expected to produce greater hydrophobicity, thereby improving the dispersibility in non-polar media and facilitating the formation of composites with hydrophobic substrates[54]. For example, Bian et al. used unbleached hardwood pulp as raw material and hydrolyzed it with 60% maleic acid at 120 ℃ for 120 min to prepare CNC containing lignin, with a diameter of 25 nm and a length of about 230 nm[28]. Although the yield is less than 6%, the higher the lignin content in CNC, the better the thermal stability and the higher the hydrophobicity, which is conducive to its application in hydrophobic matrix composites.
Xie et al. Developed a mild and effective method for preparing CNC by hydrolyzing bleached eucalyptus kraft pulp at 80 ℃ for 2 ~ 5 H with a mixture of sulfuric acid, oxalic acid and H2O in the optimal mass ratio of 1 ∶ 5 ∶ 4[29]. The yield is more than 70%, the diameter is 5 ~ 20 nm, the length is 150 ~ 400 nm, and it shows excellent dispersion stability and good thermal stability in water. In addition, compared with the typical 64% sulfuric acid hydrolysis (55 G/G CNC), the amount of sulfuric acid in the mixed acid system is greatly reduced, and the amount of sulfuric acid can be reduced to 2. 4 G/G CNC. In addition, 91% ± 2% of oxalic acid can be recovered by a simple recrystallization operation.
Compared with the inorganic acid hydrolysis, the organic acid hydrolysis has the advantages of mild conditions, less equipment corrosion and easy recovery of acid, but the organic acid has weak acidity and low reaction efficiency, so a small amount of catalyst, ultrasound and microwave can be used to accelerate the reaction rate[55].

3.2 Oxidation method

The hydroxyl groups on the surface of cellulose can be oxidized into carboxyl groups and aldehyde groups under the action of strong oxidants, and the main chain is degraded with the oxidation reaction, and the degree of polymerization is reduced[56]. Therefore, according to the reaction characteristics of cellulose, CNC can be prepared by oxidation.

3.2.1 TEMPO oxidation method

2,2,6,6-Tetramethylpiperidine-1-oxide (TEMPO) is a typical nitroxide radical of piperidine. In the presence of NaBr and NaClO, TEMPO can selectively oxidize the primary alcoholic hydroxyl group on the 6-position carbon atom (C6) of cellulose glucose sulfhydryl group to carboxyl group, and the reaction scheme is shown in Figure 6[57]. During the oxidation process, the cellulose fiber surface and some amorphous regions are gradually hydrolyzed, while the crystalline regions remain intact. With the increase of carboxyl content, the cellulose begins to disperse in the aqueous solution and release cellulose crystals[4].
Cao et al. extracted CNC with high yield (> 80.0%) and high crystallinity (69.7%) from jute fiber by using TEMPO/NaBr/NaClO system. The diameter and length of CNC were 3 ~ 10 nm and 100 ~ 200 nm, respectively[30]. Qin et al. Prepared carboxylated CNCs with a width of 5 ~ 10 nm and a length of 200 ~ 400 nm from cotton linter pulp by ultrasound-assisted TEMPO/NaBr/NaClO oxidation[58]. Rohaizu et al. Prepared cellulose nanocrystals by ultrasound-assisted TEMPO oxidation of oil palm empty fruit bundle microcrystalline cellulose and then mechanical treatment, which improved the chemical reaction rate and process efficiency, and the yield was 93%, which was about 39% higher than that of the non-assisted method[31]. The average length and width of the CNCs are 122 nm and 6 nm, respectively.

3.2.2 Periodate oxidation method

The TEMPO-mediated oxidation method still has some serious problems, such as the toxicity of the TEMPO reagent (causing environmental problems), long oxidation time, and limited oxidation of the primary alcoholic hydroxyl group at the C6 position of cellulose[4]. The researchers developed another oxidation method using periodic acid-chlorite. For example, Yang et al prepared CNC with diameter of about 13 nm and length of 120-200 nm by using softwood pulp as raw material and oxidizing with sodium periodate and sodium chlorite successively, and its crystallinity was as high as 91.0%[59]. Liu et al. First reported the method of selective separation of CNC from lignocellulose by alkaline periodate oxidation[60]. In the high concentration potassium salt with pH value of 10, the periodate ion mainly exists in the form of H2I2O104-. With the decrease of oxidation activity in alkaline solution, H2I2O104- preferentially oxidizes the disordered regions of cellulose, and alkaline environment promotes the degradation of these oxidized cellulose chains through β-alkoxy cleavage, thus forming CNC with a total yield of 40. 1% and a crystallinity of 57. 1%. The average diameter of the CNCs was between (3.7 ± 1.9) nm and (5.3 ± 2.4) nm, and the average length was between (116 ± 18.9) nm and (120.1 ± 17.3) nm. In addition, the reaction solution can be reused after the periodate is regenerated by ozone gas.

3.2.3 Ammonium persulfate oxidation

Ammonium persulfate (APS) is an oxidant with low toxicity, high water solubility and low cost. For the first time, Leung et al. Used ammonium persulfate (APS) to oxidize hemp, flax and triticale straw at 60 ℃ to prepare CNC[12]. The cross section size of CNC is 3 ~ 6 nm and the length is about 150 nm. The yield of the same raw material obtained by acid hydrolysis and bleaching is only about 5.0%, while the yield of the same raw material obtained by APS treatment is 28.0% ~ 36.0%. Because APS has the function of oxidative degradation and lignin removal, the method can directly prepare CNC from the original raw materials. Fu Junjun et al. Prepared two kinds of carboxylated cellulose nanocrystals from cotton fiber by two-step method of concentrated sulfuric acid hydrolysis-TEMPO oxidation and one-step method of APS oxidation[61]. The results showed that the CNC prepared by the two methods were rod-shaped and possessed the basic chemical structure of fibrils and cellulose type I structure. The particle size of CNC prepared by the former is mainly distributed in the range of 65.2 ~ 165.3 nm, the average particle size is 96.8 nm, and the polydispersity index is 0.166. The average particle size of the latter is 186. 7 nm, which is about twice as large as that of the CNC prepared by two-step method, and the polydispersity index is high (0. 344). This may be due to the strong hydrolysis of strong acid and the partial destruction of CNC crystals by TEMPO oxidation system, which makes the degradation of CNC crystals more serious. Wang et al. Proposed a new method for preparing CNC from cotton linter by one-pot oxidation of APS. In this method, the raw material is first expanded in APS solution and then oxidized. The reaction process is shown in Figure 7[32]. The effects of swelling time, swelling temperature and solid-liquid ratio (S-L) on the reaction were discussed. Under the optimal conditions of swelling time of 3. 0 H, swelling temperature of 25 ℃ and S-L ratio of 1 ∶ 50, the yield and crystallinity were 95. 8% and 83. 5%, respectively, and the length and diameter were (136 ± 90) nm and (10 ± 5) nm, respectively. After swelling and oxidation by APS, the crystal structure remained unchanged. In addition, swelling plays a key role in the preparation of CNC by APS oxidation, which shortens the reaction time, improves the yield of CNC and reduces the particle size.

3.3 Enzymatic hydrolysis

Cellulase is a complex enzyme, mainly composed of endoglucanase, exoglucanase and β-glucosidase, which has the ability to catalyze the hydrolysis of cellulose. These enzymes act synergistically in the process of cellulose hydrolysis, endoglucanases randomly attack and hydrolyze the amorphous regions, while exoglucanases react with the reducing or non-reducing ends of the cellulose chain to destroy the crystalline regions of the fiber[62]; β-glucosidase mainly acts directly on oligosaccharide chains and cellobiose and hydrolyzes them to glucose. However, the cellobiose formed in the reaction process can be adsorbed on the active center of the exoglucanase to avoid the complete hydrolysis of the enzyme, which provides favorable conditions for the controlled production of CNC[63]. In addition, xylanase, as a catalytic enzyme for hemicellulose hydrolysis, can cooperate with cellulase to hydrolyze cellulose to prepare CNC.
Satyamurthy et al. Prepared CNC by controlled hydrolysis of MCC with cellulase secreted by Trichoderma reesei, with a yield of 22.0%[33]. The diameter and length of the CNCs were (40.74 ± 7.59) nm and (120 ± 36.25) nm, respectively. The degree of crystallinity was 78.4%. Compared with acid hydrolysis, the fungi penetrated into the crystalline region of MCC during the culture process, resulting in a decrease in the degree of crystallinity of MCC produced by microbial hydrolysis. Chen et al. First pretreated the natural cotton fiber with dimethyl sulfoxide, sodium hydroxide or ultrasound, and then hydrolyzed it with Trichoderma viride G cellulase. The highest yield of CNC was 32.4%, and the reaction time was several days[63]. Later, they used cellulose and xylanase to prepare CNC with an average particle size of about 30 nm by enzymatic hydrolysis of pulp fiber. The particle size distribution was uniform and the dispersion was good. The yield reached the maximum under the optimal reaction conditions of 20 U/mL (the mass ratio of cellulose to xylanase was 9 ∶ 1), 50 ℃ and 5 H[64]. Compared with the single high concentration cellulase (200 U/mL), the low concentration cellulase complex (20 U/mL) was faster and more efficient. Fattahi Meyabadi et al. Prepared spherical cellulose nanocrystals with diameter less than 100 nm from waste cotton by cellulase hydrolysis and ultrasonic treatment, with a yield of less than 20.0%[65].
Tong et al. Used eucalyptus cellulose fibers as raw materials to study the effects of different complex enzymatic hydrolysis conditions on CNC morphology and the formation mechanism of controlled morphology (Fig. 8)[66]. When the ratio of cellulase to xylanase was 9 ∶ 1, the total enzyme concentration was 10 U/mL and 500 U/mL, and the hydrolysis time was 12 H and 5 H, rod-like CNC with an average length of 600 nm and a width of 30 nm and spherical CNC with an average diameter of about 40 nm were obtained, respectively. When the total enzyme concentration was as low as 10 U/mL, the product was mainly rod-shaped CNC. When the total enzyme concentration was as high as 500 U/mL, the rod-like CNC gradually disappeared with the increase of hydrolysis time, and was replaced by uniform spherical CNC after 5 H. When the concentration of cellulase is higher than that of xylanase, the shape of CNC is spherical, otherwise it is rod-shaped CNC. Therefore, the total enzyme concentration, hydrolysis time and enzyme concentration ratio could affect the morphology of CNC, and high total enzyme concentration (500 U/mL), long hydrolysis time (5 H) and high cellulase concentration were the key factors for the preparation of spherical CNC. In the complex enzyme system, the concentration of cellulase plays an important role in the formation of spherical CNC, and the concentration of cellulase higher than that of xylanase will lead to the formation of spherical CNC. At the same time, xylanase mainly increased the hygroscopicity of cellulose and improved the accessibility of cellulose to cellulase. Therefore, it is due to the synergistic effect of cellulase and xylanase that the control of CNC morphology is achieved.

3.4 Mechanical Method

Mechanical methods mainly include high-pressure homogenization, ball milling and ultrasonic treatment, which are usually combined with acid hydrolysis, oxidation and enzyme treatment, and some researchers directly use mechanical methods to prepare CNC.
For example, Li et al. Prepared rod-like CNC with diameter of 10 ~ 20 nm and length of 50 ~ 250 nm from MCC dispersion by pure physical method with high intensity ultrasound, but the yield was not more than 10.0%[34]. It was found that the crystallinity of CNC decreased with the increase of ultrasonic time. The action of ultrasound is non-selective, meaning that it can remove both amorphous and crystalline cellulose (fig. 9). Yan et al prepared nanocrystalline cellulose by ultrasonic treatment of corncob acid hydrolysis residue.Firstly, lignin in the residue was removed by sodium chlorite, and then the residual product was treated by ultrasonic (600 W) to obtain nanocrystalline with type I structure, crystallinity of 79. 7% and diameter less than 10 nm[67]. Using wood flour as raw material, Li et al prepared CNC with a length of less than 500 nm, a width of 1 ~ 9 nm and a yield of 22. 4% by a two-step method of ethanol and hydrogen peroxide solvothermal pretreatment and ultrasonic decomposition[13]. The results showed that 97% lignin and 70% hemicellulose could be separated by ethanol pretreatment alone. The residual lignin and hemicellulose were removed by further treatment with alkaline hydrogen peroxide, and high purity cellulose was obtained. Compared with the concentrated acid hydrolysis method, the CNC obtained by ultrasonic treatment has similar yield, size, morphology and crystallinity, and has better thermal stability and film forming performance.
The ball milling process is a mechanical process that relies on the energy released by the collision between the balls and the high grinding energy generated by the friction between the balls and the wall, as shown in Figure 10[68]. Mohd Amin et al. Prepared rod-like CNC with yield of 57% -76%, crystallinity of 85% -95% and initial degradation temperature of 230-263 ℃ from MCC dispersion by high energy ball milling (HEBM) method. Its morphology and aspect ratio are similar to those of CNC prepared by acid hydrolysis[69].
Gao et al. Reported a two-step synergistic extraction of CNC using solvothermal pretreatment and mechanical stripping[35]. This method avoids the use of a large amount of sulfuric acid, because there is no sulfate group, the prepared CNC has better thermal stability. The average length and width are 280 nm and 11 nm, respectively, and the yield is as high as 72. 2% ± 2.0%. Lee et al. Extracted CNC with a crystallinity of 71% -81% and a yield of 35% -67% from softwood pulp by a two-step process of short-time pretreatment by electron beam irradiation and decomposition by high-pressure homogenizer (HPH)[70]. However, these processes include unfriendly or complicated steps. Park et al. attempted a simple method of extracting CNC from the feedstock using only a high pressure homogenizer controlling the process temperature, without acid hydrolysis[36]. The obtained CNC has a rod-like shape with a width of 4-14 nm and a length of 60-320 nm, and the yield is 80.0% ± 0.5%.

3.5 Solvent method

3.5.1 Ionic liquid

Ionic liquids (ILs) are liquid salts composed of organic cations and inorganic or organic anions at temperatures below 100 ℃, which have the advantages of recyclability, high stability, low vapor pressure and low melting point[71]. Ionic liquids (ILs) are considered to be novel environmentally friendly solvents for the pretreatment and processing of lignocellulosic materials due to their unique dissolution properties, and can be used as alternative reaction media for the selective and controllable hydrolysis of cellulose to separate nanoparticles[4]. Man et al. Prepared CNC with length of 50-300 nm and diameter of 14-22 nm by hydrolyzing MCC with 1-butyl-3-methylimidazolium bisulfate ([BMIM]HSO4) ionic liquid)[72]. The results of X-ray diffraction (XRD) showed that the prepared CNC still had the cellulose type I structure, and the crystallinity was higher than that of the original MCC, but the thermal stability was significantly decreased. Mao et al. Treated MCC with aqueous solution of [BMIM]HSO4 under weak acidic conditions to prepare CNC with a yield of 48% ± 2%, the width and length of which are (3.6 ± 1.8) nm and (146.8 ± 62) nm, respectively[37]. In addition, compared with the CNC prepared by concentrated sulfuric acid, the CNC prepared by the method has higher thermal stability. On this basis, they optimized the conditions for preparing CNC with [BMIM]HSO4[73]. Using softwood, hardwood and MCC as raw materials, CNC with yields of 57. 7% ± 3.0%, 57.0% ± 2.0% and 75. 6% ± 3.0% were obtained by a two-step hydrolysis method: firstly, the cellulose materials were swelled in pure ionic liquid at room temperature for 24 H, and then an appropriate amount of deionized water was added to initiate the acid-catalyzed hydrolysis of bisulfate. The yield is close to the theoretical value and has good surface properties (sulfur content as low as 0.2%). Lazko et al. swelled cotton pulp fibers in 2% w/w 1-butyl-trimethylimidazolium chloride ( [BMIM] Cl) at 80 ℃ for 1 ~ 2 H, and then added 1 wt% ~ 4 wt% sulfuric acid to hydrolyze for 2 ~ 16 H to prepare CNC with a length of 150 ~ 350 nm and a diameter of about 20 nm. Compared with the sulfonation method, the CNC prepared by this method has a low content of sulfonic acid groups on the surface, and its thermal stability is higher than that of the CNC prepared by[38]. Tan et al., using MCC as raw material and [BMIM]HSO4 as catalyst and solvent, successfully prepared rod-like CNC with diameter of 15 ~ 20 nm, length of 70 ~ 80 nm and cellulose I structure at 90 ℃, and its crystallinity was as high as 95.8%. The reaction mechanism is shown in Figure 11[14]. Abushammala et al. reported for the first time that CNC was prepared directly from wood using 1-ethyl-3-methylimidazolium acetate ( [EMIM] [OAc]) with a yield of 44%, an aspect ratio of 65, and a crystallinity of up to 75%[74]. It was found that CNC can be prepared directly from wood because [EMIM] [OAc] only swells cellulose while dissolving lignin in situ. Miao et al. Successfully prepared hydrophobic CNC from hardwood pulp by one-pot method using tetra-n-butylammonium acetate/dimethylacetamide system (TBAA/DMAc) and acetic anhydride at a ratio of 1:9[75]. In this system, the dissolution of amorphous cellulose and the acetylation of hydroxyl groups occur simultaneously, and the reaction is shown in fig. 12. Compared with the traditional method, the CNT prepared by the system has the advantages of good thermal stability, high crystallinity, acetyl groups on the surface and good interfacial compatibility with PLA.

3.5.2 Eutectic solvent

Deep eutectic solvent (DES) is a two-component or three-component eutectic mixture composed of hydrogen bond acceptors and hydrogen bond donors with a certain stoichiometric ratio. Its concept was first proposed by Abbott et al. Its physical and chemical properties are similar to those of ionic liquids. It is a new type of green reagent[76]. Liu et al. Prepared CNC with high crystallinity (82%) and good thermal stability (initial degradation temperature is 320 ℃) by DES formed by choline chloride and oxalic acid and microwave-assisted (800 W) pretreatment of cotton pulp at 80 ℃ for 3 min, followed by high-intensity ultrasonic treatment (1200 W) for 30 min. The yield of CNC is 74.2%, the diameter is 3 ~ 25 nm, and the length is 100 ~ 350 nm[16]. Yang et al. Developed a green and recyclable FeCl3 catalyzed eutectic solvent system (F-DES)[39]. CNC with diameter of 5 ~ 20 nm and length of 50 ~ 300 nm were prepared by treating bleached eucalyptus kraft pulp with F-DES at 80 ℃ for 6 H, in which the mass ratio of oxalic acid (Oxd), choline chloride (ChCl) and FeCl3·6H2O was 4 ∶ 1 ∶ 0.2. The yield of CNC was 73%. Besides glucose and xylose, no further degradation products (such as 5-hydroxymethylfurfural, furfural) were found under these conditions, indicating that this method is much milder than the traditional inorganic acid hydrolysis method. The CNC prepared by the method has higher thermal stability than the CNC prepared by the traditional sulfuric acid hydrolysis (the initial thermal degradation temperature is over 310 deg C), and carboxyl is introduced on the surface of the CNC through esterification, so that the CNC has good dispersion stability in water. In addition, F-DES can be recovered and reused at least three times.
Wang et al prepared rod-like CNC with diameter of 50 ~ 100 nm, length of 500 ~ 800 nm and crystallinity of 77.6% from cotton by DES and high-pressure homogenization (HPH)[40]. In addition, the recoverability of DES was studied, and the solvent was successfully recovered and reused for at least three pretreatment cycles while maintaining the solvent pretreatment capacity. Although DES is a promising green solvent, high viscosity and high cost are the most obvious problems in its use, resulting in low dissolved mass concentration and slow solvent transfer operation. Diluting DES with water is an effective way to reduce the viscosity of DES system and modify DES[77,78]. Ma et al. Prepared CNC by pretreating aspen kraft pulp with highly diluted DES at 80 ℃ for 1 H and then sonicating (800 W) for 20 min[79]. It was found that the addition of large amounts of water did not decrease the hydrogen bond acidity, polarizability and solvation effect. On the contrary, water promotes the ionization of hydrogen ions and the delocalization of chloride ions, forming new hydrogen bonds of Cl-H2O ions and hydrogen bonds of oxalic -H2O, thus promoting the ionization and protonation of solvent systems.

3.6 Combined Processing

The performance and yield of CNC play a crucial role in its cost and final application, and the improvement of extraction technology and the development of combined processes by combining two or more methods mentioned above may be one of the most effective ways to improve the performance of CNC and solve the yield limitation problem[4]. For example, Cui et al. Successfully prepared rod-like cellulose from wheat straw microcrystalline cellulose by ultrasonic-assisted enzymatic hydrolysis[80]. The yield of CNC was 22. 6% when the optimal hydrolysis time was 120 H and the ultrasonic treatment time was 10 min every 12 H, while the yield of CNC without ultrasonic treatment was only 15. 8%. When the ultrasonic treatment time is 0,30 and 60 min, the width of CNC is less than 10 nm, and the length of CNC is 200-500 nm, 100-200 nm and 40-50 nm, respectively. The crystallinity of CNC samples increased from 60. 4% to 87. 5% after ultrasonic treatment, and the crystallinity of CNC samples increased with the increase of ultrasonic time.
To increase the yield of CNC, Beltramino et al. Pretreated it with cellulase and then hydrolyzed it with sulfuric acid[81]. The results showed that the yield was as high as 65% ~ 70% with a small amount of acid, which was increased by 12%. The average particle size is up to 160 nm, increased by about 35 nm; And that sulfur content of the CNC is reduce by 0.8%. Because the effect of enzymatic pretreatment depends largely on the hydrolysis conditions, they optimized the sulfuric acid hydrolysis conditions on this basis[82]. The results showed that under the optimal conditions of cellulase 10 U/G odp, hydrolysis temperature 47 ℃, hydrolysis time 25 min and sulfuric acid concentration 62%, the yield was more than 80%, which was increased by more than 10% and the hydrolysis time was shortened by 44%. Under the above optimal sulfuric acid hydrolysis conditions, they also explored the optimal conditions of enzyme pretreatment, when the enzyme dosage was 20 U/G odp and the hydrolysis time was 2 H, the total yield reached about 82%[83]. In addition, this optimization reduced the required enzyme treatment time by 90% compared to previous studies.
Lu et al. Proposed an effective method to extract CNC from bamboo pulp by mechanical activation of ball milling and hydrolysis of bamboo pulp with phosphotungstic acid[41]. More of the applied stress generated during the ball milling process is transferred to the cellulose, causing the chemical bonds in the cellulose to break and promoting the size reduction of the cellulose. At the same time, under the assistance of phosphotungstic acid catalysis, the hydrolysis degree of cellulose is enhanced, the formation of CNC is facilitated, and the reaction time is greatly shortened. Under the optimum reaction conditions of 13. 5% of phosphotungstic acid concentration, 4.7 H of reaction time and 2. 2 H of ball milling time, the highest yield of CNC was 88. 4%, the crystallinity was 79. 6%, and the length and width of CNC were 200 ~ 300 nm and 25 ~ 50 nm, respectively. Inspired by the synergistic effect of mechanochemistry, they achieved efficient cleavage and green preparation of strong hydrogen bonds between cellulose macromolecules through shear-assisted solvent pretreatment and subsequent synergistic effect of mechanochemistry, with a yield of up to 90. 8%[84]. The shear-assisted ZnCl2·3H2O pretreatment promotes the swelling of cellulose fibers and effectively cleaves the strong hydrogen bonds between cellulose chains. The synergetic effect of shear and ultrasound on cellulose formed a mechanical activation effect, which led to the dissociation of the dense network of cellulose, reduced the mass transfer resistance between fibers and in the pores of fibers, and enhanced the nano-fibrillation of cellulose, thus improving the yield. Seta et al. prepared CNC with high yield, good colloidal stability and particle size of 105.6-223.8 nm by combining ball milling pretreatment with 75% maleic acid using bamboo pulp fiber as raw material, and its yield (10.55% -24.50%) was higher than that of CNC samples prepared without ball milling pretreatment (2.80% ± 0.36%)[85]. Ball mill pretreatment can decompose and open the firm structure of bamboo pulp fibers, so that more hydroxyl groups are exposed on the surface of the fibers to esterify with maleic acid, and the way for acid molecules to enter the bamboo pulp fibers is increased, so that maleic acid molecules can easily hydrolyze cellulose, release more crystalline parts, and form more — COOH.

4 Application of Nano-cellulose Whisker in Composite Material

4.1 Reinforced composite material

Because of its excellent intrinsic properties, such as nano-size, high aspect ratio, low density (about 1.6 g/cm3), high tensile strength (about 7 GPa) and Young's modulus (about 150 GPa), CNC has attracted wide attention and is considered as one of the ideal reinforcing fillers[3][86][87].
Bai Panxing et al. Prepared CNC/polyvinyl alcohol (CNC/PVA) composite membrane by blending solution casting method. Compared with pure PVA membrane, the tensile strength of CNC/PVA composite membrane was significantly improved. When the mass fraction of CNC was 2%, the tensile strength of composite membrane was 87.6% higher than that of pure PVA membrane[88]. At the same time, the addition of CNC can increase the thermal stability of the composite membrane to a certain extent.
Liu Ye et al. Grafted poly (L-lactic acid) (PLLA) chain segments onto the surface of nano-cellulose whiskers (g-CNC) by using the hydroxyl groups on the surface of CNC to initiate the ring-opening polymerization of L-lactide[89]. Then, CNC/PLLA and g-CNC/PLLA composite films were prepared by solution casting method. The results showed that the thermal stability and hydrophilicity of the composites were improved by adding CNC. The tensile strength and fracture energy of the composite films increased firstly and then decreased with the increase of whisker content. When the mass fraction of CNC and g-CNC was 5%, the tensile strength and fracture energy of CNC/PLLA and g-CNC/PLLA composite films reached the maximum, which were 22.02 MPa, 20.01 MPa and 102.39 J/m3, 117.83 J/m3, respectively. The tensile strength and toughness of g-CNW/PLLA composite films were significantly better than those of pure PLLA and CNC/PLLA films due to the good dispersion of g-CNC in the matrix and the interfacial compatibility between g-CNC and the matrix. Zhang Jing et al. Used the same preparation method to blend CNC modified by dodecyltrimethylammonium chloride (DTAC) with polylactic acid (PLA) to prepare CNC/PLA composites[90]. The dispersion of CNC and the compatibility between CNC and PLA were improved after interfacial modification.When the content of CNC was 2%, the tensile strength of PLA increased by 70. 7%, but the elongation at break decreased.
Huan Siqi et al. prepared polystyrene (PS)/CNC nanocomposite film by electrospinning method. The tensile property of the nanocomposite film was also improved with the increase of CNC. When the CNC addition m (CNC)/m (PS) = 7/100, the maximum tensile stress was (0.4 ± 0.02) MPa, which was 5.7 times of the tensile stress of the electrospun PS nanofiber film, while the elongation at break decreased gradually[91]. This is because with the increase of CNC, the matrix PS is more closely connected with CNC through the action of surfactant Tween 80, forming an interconnected network structure to enhance its mechanical properties. In order to improve the mechanical properties of calcium alginate fibers, Ma et al prepared biocomposite fibers with CNC and its oxidized derivatives (OCNC) as reinforcing phases respectively[92]. The comparative study showed that the tensile strength of CNC and OCNC modified calcium alginate fibers increased with the increase of nanocrystal content, and reached the maximum when the content was 0. 5 wt%, and then decreased with the increase of nanocrystal content. CNC(1.32 cN·dtex-1) and the OCNC(1.34 cN·dtex-1) calcium alginate composite fibers were the same, and both were 40% greater than the measured values for the pure calcium alginate fibers (0.92 cN·dtex-1). Yang et al. studied the effect of CNC on the properties of gelatin/CNC composite films[93]. The results showed that the tensile strength of gelatin/CNC composite films increased and the elongation at break decreased with the addition of CNC. When the mass concentration of CNC was 0. 6%, the composite film showed the highest tensile strength (13. 56 ± 0.25 MPa), and the mechanical properties of gelatin/CNC composite film were significantly improved.
In order to improve the mechanical properties of chitosan-based polymer films, Akhavan-Kharazian et al. Studied the effect of CNC on the properties of polymer films based on chitosan (CS) and gelatin (GL)[94]. The results showed that the elastic modulus and elongation at break of the composite film remained almost unchanged, while the tensile strength increased slightly with the addition of 0. 3 wt% CNC. However, the addition of 1. 0 wt% CNC significantly improved the elastic modulus and tensile strength. The improvement of mechanical properties of the composites can be attributed to many factors, such as it can be considered that the hydrophilic functional groups of CS or GL can interact with the surface hydroxyl groups of CNC to form hydrogen bonds, and the high aspect ratio of CNC can improve the stress transfer efficiency, and the rigidity of CNC effectively improves the rigidity and stiffness of the polymer. Jiang et al mixed different concentrations of CNC into whey protein concentrate (WPC) matrix to prepare WPC-NCC composite films. The results showed that the mechanical properties of WPC-NCC composite films were significantly improved when CNC was blended with WPC as a filler in the protein network[95]. The tensile strength of the composite film increased from 1. 3 MPa to 3. 15 MPa with the increase of CNC content to 15 wt% of WPC. Peres et al. Studied the effect of CNC incorporation on the mechanical properties of electrospun polyacrylonitrile (PAN) nanofibers[96]. The addition of CNC increases the ultimate tensile strength, elastic modulus, and yield strength of the nanofibers regardless of the concentration. When the CNC content is 3 wt%, the sample has obviously superior mechanical properties.

4.2 Membrane filtration composite material

CNCs have been successfully used as fillers in polymer matrices, and one of the application areas for these materials is separation membranes. The introduction of CNC into the polymer matrix will directly change the hydrophilicity, pore size, mechanical strength and anti-fouling properties of the membrane. In addition, CNC is environmentally friendly and biocompatible, which makes it more widely used in membrane filtration composite materials.
Moeinzadeh et al. prepared nanocomposite ultrafiltration (UF) membranes by adding varying amounts of amino-functionalized CNC[97]. O as to improve the antifouling performance of the film to the oil deposit. The results showed that the overall porosity and hydrophilicity of the nanocomposite membranes were significantly improved by the addition of CNC, although the pore size of the nanocomposite membranes decreased. When the optimal content of CNC was 1 wt%, the water flux of the obtained nanocomposite membrane was 105.2 L/m2·h, and the oil filtration rate was 98. 2%, which were 43% and 10% higher than that of the original PSF membrane, respectively. Blending CNC with polyethersulfone (PES), Langming et al prepared nanocomposite membranes. The introduction of CNC increased the porosity of the nanocomposite membranes from 64% to 77%, and the average pore size increased from 54 nm to 81 nm. The PES ultrafiltration membrane changed from hydrophobic to hydrophilic, and the hydrophilicity gradually increased with the increase of CNC content[98].
Wu Hongyu prepared CNC/polyethersulfone composite ultrafiltration membrane by surface coating and blending modification[99]. The hydrophilicity and electronegativity of PES commercial membranes were significantly enhanced after coating with CNC dispersion. The membrane flux decreased due to the increase of mass transfer resistance of water molecules, but the removal capacity of HA by the coated membrane increased from 45. 5% to 79. 2%, and the anti-HA fouling ability of the coated membrane was stronger than that of the commercial membrane. When CNC was blended in the membrane, the porosity, pore size and molecular weight cut-off of the composite membrane increased when the concentration of CNC was lower than 2% due to the promotion of phase inversion by hydrophilic CNC, and the membrane flux increased, and the maximum flux was obtained when the concentration of CNC was 2%. The removal efficiency of hydrophobic natural organic matter (NOM) can be improved by CNC blending. Through the fouling experiment, it was found that the membrane fouling was alleviated with the increase of CNC, because the membrane fouling resistance decreased with the increase of CNC blending amount, the electronegativity enhanced the repulsion between NOM and membrane surface, and the improvement of CNC membrane hydrophilicity reduced the hydrophobic adsorption of NOM. The NOM deposited in the large pore CNC membrane is removed more effectively during the membrane hydraulic and chemical cleaning processes. Cellulose nanocrystals/polyvinylidene fluoride (CNC/PVDF) blend composite membranes were prepared by phase inversion method[100]. The results showed that the finger-like macropore structure of CNC/PVDF composite membrane increased and became larger with the increase of CNC content, and the average pore size increased from 12. 41 nm to 31. 42 nm. In the process of phase inversion, CNC spontaneously migrates to the surface of the composite membrane and preferentially combines with water molecules to form a hydration layer, which reduces the adsorption and deposition of pollutants and improves the anti-fouling performance of the membrane.
Because bacterial cellulose nanocrystals (BCNC) have a certain barrier to water vapor and oxygen, Fabra et al. Prepared thermoplastic corn starch (TPCS) nanocomposites containing BCNC by melt mixing method. When the content of BCNC is 15 wt%, the performance is the best, and the permeability of water and oxygen is reduced by 46% and 95%, respectively[101][102]. PHB-BCNC or poly (3-hydroxybutyrate) (PHB) coated nanomaterial BCNC was successfully hydrophobized by electrospinning. The multilayer structure with BCNC in both the inner layer of TPCS and the PHB coating has the largest reduction in water vapor permeability.

4.3 Conductive composite material

Conductive polymers contain a large number of conjugated chains or aromatic ring structures, the molecular chain rigidity is strong and there is a strong interaction between chains, so the solubility and film-forming property are very poor[103]. Because of its high strength, high specific area and a large number of hydrogen bonds, it is easy to form films. Coating the polymer on the surface of the CNC or in combination with the matrix reduces the percolation threshold of the composite while also increasing its strength[104].
Wu et al. Successfully prepared conductive cellulose nanocrystals (C-CNC) by in-situ chemical polymerization of pyrrole (Py) on the surface of CNC[105]. Firstly, carboxylic acid groups were grafted onto CNC by TEMPO-mediated oxidation reaction, which provided strong hydrogen bonds and enhanced the adsorption of pyrrole monomers. Then, the in-situ chemical polymerization of Py was further performed on the high aspect ratio CNC (fig. 13). The good dispersion and ideal size of CNCs facilitate the easy polymerization of polypyrrole (PPy) to form a conductive shell on the surface of each CNC. The one-dimensional structure of the CNC not only facilitates efficient transport of electrons in one controlled direction, but also provides a very low PPy/CNC permeation concentration, resulting in a conductive network. The conductivity of PPy/CNC is about 4 S/cm, and the capacitance is 248 F/G. Ravit et al. Prepared CNC-doped poly (3,4-ethylenedioxythiophene) conducting polymer (PEDOT) supercapacitor electrodes by electrochemical polymerization[106]. Compared with PEDOT, the doping of CNC into PEDOT can improve the specific capacitance and reduce the charge transfer resistance, and has good cycle stability, and the capacitance retention rate of the original capacitance is 86% after 1000 cycles.
Mukhopadhyay et al. designed a stable conducting membrane with high ion selectivity and proton conductivity by incorporating CNC into polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) matrix[107]. Since the nanocrystals contain hydroxyl functional groups, highly acidic sulfonate (—SO3H) functional groups, and an extensive intramolecular hydrogen bonding network, the membranes are endowed with excellent proton conductivity of 0.015 S/cm. In addition, CNC exhibits excellent chemical stability in harsh acidic electrolytes due to its high crystallinity (crystallinity index of approximately 86%). CNC provides excellent proton conductivity while PVDF-HFP acts as a hydrophobic matrix and provides ion selectivity and mechanical flexibility to the membrane. Therefore, the vanadium redox flow battery (VRFB) based on CNC and PVDF-HFP (CNC/PVDF-HFP) membranes has the advantages of high proton conductivity, good ion selectivity and good chemical stability. The coulombic efficiency (CE) and energy efficiency (EE) of the VRFB are 98.2% and 88.2%, respectively. At a current density of 100 mA/cm2, the VRFB has stable cycle performance with more than 650 cycles.
Inspired by the proton conduction mechanism of transmembrane proteins, Xu et al designed amino acid functionalized CNC as a proton conduction channel[108]. Proton conducting mixed matrix membranes were prepared by complexing amino acid functionalized CNC with sulfonated polysulfone. The results showed that the introduction of amino acids greatly increased the proton conductivity, and the water absorption and methanol permeation resistance of the proton-conducting mixed matrix membrane were significantly improved.

4.4 Inorganic nano composite material

When inorganic nanoparticles are dispersed in liquid medium, they tend to aggregate due to their high surface energy, while CNC has large specific surface area and excellent mechanical properties, which are easy to form films and gel.Therefore, CNC can be used as a carrier material, a matrix material or a skeleton support material to be compounded with inorganic nanoparticles to form a nano-functional material with excellent performance, and can be applied to the fields of biomedicine, catalysis, sensing and the like.

4.4.1 Antibacterial material

Silver nanoparticles (AgNPs) have excellent antibacterial properties and are often used to prepare antibacterial agents, but AgNPs are easy to aggregate in aqueous solution. Shi et al. First modified CNC with dopamine, and then reduced silver ions by polydopamine-coated CNC to generate AgNPs in situ on the surface of CNC and fix them on the surface of CNC[109]. The results showed that the prepared AgNPs-CNC had high stability, and its antibacterial activity against Gram-positive and Gram-negative bacteria was 4 times higher than that of free AgNPs, because CNC significantly improved the dispersion and stability of AgNPs, and increased the activity of nano-materials. Fan Li used CNC with its own dispersibility as a carrier to prepare silver-loaded cellulose nanocrystal solution[110]. The results showed that the silver nanoparticles were uniformly dispersed on the surface of CNC. The silver-loaded cellulose nanocrystal solution has inhibitory effect on Escherichia coli and Staphylococcus aureus. Azizi et al. Prepared CNC/ZnO nanocomposites by sol-gel method, and studied the antibacterial properties of the composites[111]. The antibacterial study showed that the antibacterial activity of CNC/ZnO nanocomposites against positive Staphylococcus was stronger than that against negative salmonella, and the antibacterial activity of nano-ZnO loaded by CNC was stronger than that of pure nano-ZnO.

4.1.2 Catalyst carrier

Due to the wide source of raw materials, eco-friendly, biodegradable, and can be used as a new type of effective catalytic support, CNC has attracted more and more attention. Zhan et al. prepared a MnFe2O4/CNC composite material with high catalytic activity for methylene blue (MB) by fixing magnetic nanoparticles MnFe2O4 on the surface of CNC through hydrothermal method[112]. The results showed that the catalytic degradation degree of MB by MnFe2O4/CNC was 99%, which was increased by more than 60% compared with the catalytic degradation degree of MB by MnFe2O4 without CNC loading.This was because the addition of CNC reduced the band gap of MnFe2O4 nanoparticles, increased the active surface area, and improved the catalytic activity. Wu et al. prepared a stable dispersion of palladium nanoparticles (PdNPs) using CNC as a reducing agent and a stabilizer, and evaluated the catalytic activity of CNC-supported PdNPs by the reduction of methylene blue and p-nitrophenol. The results showed that CNC-supported PdNTs had better catalytic activity and stability than PdNTs without CNC support[113]. Eisa et al. prepared CNC-supported catalysts of silver and gold nanoparticles[114]. In the presence of NaBH4, 4-nitrophenol can be reduced to 4-aminophenol, which shows effective catalytic activity. In addition, the prepared catalyst is easy to separate and recover, and can be reused in multiple catalytic cycles.

4.1.3 Sensor

In recent years, the application of CNC in the field of sensors has been a hot topic of continuous exploration and research. Kafy et al. Blended CNC with graphene oxide (GO) and then dried it to prepare a CNC/GO composite material that can be used as a humidity sensor. The composite material can measure the humidity range of 25% -90% at 25-45 ℃, and the sensitivity of the sensor is independent of the ambient temperature[115]. Mun et al. Synthesized TiO2/ multi-walled carbon nanotube (MWCNT) nanocomposite by hydrothermal method, and then incorporated it into the CNC matrix to develop a sensor for monitoring ammonia at room temperature. The sensor showed a large response when the gas concentration was in the range of 50-500 ppm, with a response time of 100 s and a recovery time of 50 s, respectively[116]. Sadasivuni et al. used the same method to prepare a CNC/iron oxide composite material for use as a NO2 sensor, which has a sensitivity of up to 2 ppm for NO2, high sensitivity and full recoverability at room temperature[117]. The presence of CNC significantly improves the relative resistance change between the initial resistance and the maximum value at the end of the induction period compared to iron oxide alone. Due to the formation of a local depletion region as a charge scattering site at the CNC-iron oxide interface, the response speed is fast, and the response time and recovery time are 50 s and 30 s, respectively. In addition, the response of the sensor is almost linear with the concentration of the NO2. Xu et al. assembled carboxylated CNC and positively charged polydiallyldimethylammonium chloride (PDDA) alternately on the surface of glassy carbon electrode, and then deposited poly (3,4-ethylenedioxythiophene) conductive polymer on the surface to form a sensing electrode for nitrite determination, which can detect nitrite in the range of 0.20 ~ 1.73 mmol/L[118].

5 Conclusion and prospect

As a kind of green and renewable biomaterial, cellulose nanocrystal is an environmentally friendly natural material with high purity, high crystallinity, high Young's modulus, high strength, biocompatibility and biodegradability. However, there are still some problems in the preparation of CNC: (1) The yield of CNC prepared by acid hydrolysis is high, but the amount of chemicals used is large, the reaction efficiency is low, there are a lot of impurities such as waste acid and hydrolysis by-products in the reaction system, and the waste acid and water can not be completely recycled; (2) The thermal stability of CNCs prepared by oxidation is good, but there are some problems, such as large demand for oxidant, long reaction time and high cost. (3) Enzymatic hydrolysis can not only improve the quality and purity of CNC, but also has the advantages of safety, no pollution and low energy consumption, but it has high selectivity for enzymes, strict requirements for reaction process and difficult control of process conditions. In addition, the enzymatic preparation of CNC has the disadvantages of high cost and slow hydrolysis rate. (4) The mechanical method does not need chemical reagents and has little impact on the environment, but the required equipment is special, the energy consumption is high, and the particle size distribution of the prepared CNC is wide; (5) Ionic liquids have been used to prepare CNC because of their unique properties, but ionic liquids are expensive and the cost of recovery is high; (6) The combination of various methods can improve the yield and performance of CNC, but there are still problems such as environmental pollution, cumbersome preparation process and difficult to control the degree of hydrolysis of cellulose. At present, acid hydrolysis as the main operation unit and other auxiliary methods are the mainstream methods for the preparation of CNC. In the future, the preparation methods of CNC will develop in the direction of low cost, high efficiency, fast and green sustainability.
Due to its excellent performance, CNC has achieved excellent research results in the field of composite materials. However, there are still many challenges when CNC is used to composite with organic or inorganic materials: (1) CNC has the problems of uneven dispersion and poor compatibility with polymer matrix, which hinder its further application in the field of composite materials. At present, the main solution is chemical or physical modification on the surface of CNC, but chemical modification has the problems of destroying the crystal structure and damaging the mechanical properties. Only changing the surface chemical properties of CNC, while retaining the original crystal form and maintaining the integrity of the crystal, so that it can better integrate with the matrix is a direction of future development. (2) The basic theoretical research on the binding mode of CNC and conductive materials, the interaction mechanism between two phases and the interface compatibility is still relatively simple and needs further exploration; (3) How to better control the size and distribution of metal nanoparticles on the CNT substrate to achieve the desired application will be a very meaningful development trend. Because the loading limit of different nanoparticles in CNC matrix affects its service life, the research on its recyclability or regeneration should be focused on to truly meet the requirements of sustainable development. (4) Reduce the consumption of organic solvents and other chemicals in the process of composite material preparation, develop simple, efficient, low-cost and green composite material preparation and processing processes and methods, and explore the sustainable application of CNC in the future.
At present, as a green and renewable natural material, cellulose nanocrystals have gradually developed from the laboratory stage to the industrial process. With the further basic research and application of CNC in the future, it has a good prospect in large-scale production and application.

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Funding

National Key Research and Development Project Foundation of China(2017YFB0309800)

National Key Research and Development Project Foundation of China(2016YFC0400503-02)

Tianjin Key Research and Development Project(20YFZCSN00130)

Xinjiang Autonomous Region Major Significant Project Foundation(2016A03006-3)

Tianjin Natural Science Foundation(18JCYBJC89600)

Science and Technology Guidance Project of China National Textile and Apparel Council(2017011)

Innovation Research Institute of Wolfberry Industry Co. LTD(ZNGQCX-B-2019006)

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