Progress in Nanocellulose Preparation and Application

Vol.2, No.4, 2017 65

PBM·Nanocellulose Preparation & Application

1 Introduction

Nanocellulose derived from natural cellulose with a wide range of sources, low price, renewable, biodegradable. Nanocellulose possesses many desirable properties, such as its nontoxicity, good biocompatibility, high crystallinity, high transparency, high hydrophilicity, tensile strength of up to 7500 MPa, and Young’s modulus of up to 140 GPa[1]. Furthermore, nanocellulose retains the characteristics of nanomaterials; it has a high specific surface area and reactivity and produces small size and quantum tunneling effects[2]. These unique properties give rise to a wide range of applications of nanocellulose in paper and food industries, as well as in biology and medicine[3]. Nanocellulose has been developed to reinforce composites; stabilize Pickering emulsions; and to serve as antibacterial wound dressings, jelly, coconut,

Progress in Nanocellulose Preparationand Application

Abstract: Nanocellulose is a biodegradable, renewable, nonmeltable polymeric material that is insoluble in most solvents due to hydrogen bonding and crystallinity. Nanocellulose has attracted considerable attention in recent decades owing to its environmental friendliness, wide availability, good biocompatibility, high crystallinity, and high Young’s modulus. This review presents the recent achievements in preparation and applications of nanocellulose, including a discussion of the advantages and disadvantages of various preparation methods and a summary of the applications of nanocellulose in composite materials research. Finally, we examine the mounting evidence of more widespread potential applications of nanocellulose.

HaiQuan Mao, YongYang Gong*, YuanLi Liu, ShiQi Wang, LinLin Du, Chun WeiKey Laboratory of New Processing Technology for Non-ferrous Metals and Materials, Ministry

of Education, Guangxi Key Laboratory of New Energy and Building Energy Saving, College of

Materials Science and Engineering, Guilin University of Technology, Guilin, Guangxi Zhuang

Autonomous Region, 541004, China

Received: 23 July 2017; accepted: 2 September 2017.

Keywords: nanocellulose; preparation; application; nanocomposites

HaiQuan Mao, master candidate;E-mail: [email protected]

*Corresponding author: YongYang Gong, PhD; research interests: organic optoelectronic materials; E-mail: [email protected]

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paper, electronic paper, thickeners, and thixotropic agents. However, the production cost of nanocellulose remains high. Therefore, optimizing the preparation of nanocellulose and reducing its production cost is an urgent task for both domestic and foreign researchers.

Bibliometric methods were used to elucidate the p rogress in nanoce l lu lose research . The retrieval database was Web of Science, and subjects include cellulose nanofibril, cellulose nanofibers, nanofibrillated cellulose, microfibrillated cellulose, nanocrystalline cellulose, cellulose nanowhisker, cellulose nanocrystalline, nanocrystalline cellulose, and bacterial nanocellulose. Fig.1 shows the global distribution of published papers in nanocellulose research. China retains first place in terms of quantity (accounting for 33% of the total publications), followed by the United States (17.27%). Research in Sweden, Japan, Finland, Canada and France also contributed substantial publications. The number of papers published in the first half of 2017 was 560, and the annual forecast is 1120. Fig.2 demonstrates the drastic increase in publication trends in the last decade: the number of papers has increased by 100~200 per year, indicating that research on nanocellulose remains a topic of worldwide interest.

China

USASweden

Japan

Finland

Others

Fig.1 Quantitative distribution of published papers related to nanocellulose by country

In this paper, due to space limitations, we briefly discuss only the preparation and novel applications of nanocellulose. We also highlight our recent work on high-performance nanocellulose composites and explore possible future research directions in this field.

2 Nanocellulose classification and preparation

Nanocellulose is made from natural cellulose by using a series of physical or chemical processes to obtain fibrous nanomaterials with diameters ranging from several nanometers to several hundred nanometers and lengths ranging from tens of nanometers to several micrometers. Nanocellulose can be divided into five categories based on its preparation processes: cellulose nanofibrils (CNFs), cellulose nanocrystalline (CNC), microfibrillated cellulose (MFC), bacterial nanocellulose (BNC), and electrospun cellulose (ECC). The preparation methods of each type are briefly discussed below.

2.1 CNFs

CNFs, also known as nanofibrillated cellulose (NFC), are commonly prepared by mechanical treatment of cellulose materials after an oxidative process. The cellulose is treated with alkali, then oxidized via TEMPO (2,2,6,6-tetramethylpiperi-dine-1-oxyl). Finally, the treated cellulose is subjected to mechanical treatment such as high-pressure homogenization or mechanical stirring. The diameter of the CNF obtained is about 3~4 nm, and its length varies from 500 nm to several microns. CNF usually carries C6 carboxyl groups due to the TEMPO oxidation process[4-10]. Another method of preparing CNF is the Williamson etherification reaction, as shown in Fig.3.

2.2 CNC

CNC, also known as cellulose nanowhiskers (CNWs)

Fig.2 The annual numbers of published papers related to nanocellulose

1000

800

600

400

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1200

2009 2010 2011 2012 2013 2014 2015 2016

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or nanocrystalline cellulose (NCC), is mainly obtained via strong acid or enzymatic hydrolysis of its amorphous region. The morphology of CNC is rod-like, which is the shape of the crystalline component in natural cellulose. The aspect ratio of CNC is generally smaller than that of CNF, and its diameter and length vary from 5~70 nm and 100~250 nm, respectively. Since the acid or enzymatic hydrolysis removes the cellulose’s amorphous region and retains its crystalline region, the crystallinity of the CNC is very high, usually 60%~90%. CNC and CNF seem similar in micromorphology, but CNC is more rigid and has a lower aspect ratio (1/5~1/3) than CNF because CNC does not contain noncrystalline areas of cellulose. This property also leads to relatively poor elasticity, impact resistance, and flexural performance of CNC composites[11].

2.2.1 Acid hydrolysis methodThe inorganic acids used in the preparation of CNC include sulfuric, hydrochloric, and phosphoric acids. Cellulose material is first hydrolyzed with a sulfuric acid solution (58 wt%~64 wt%) at 50~60℃, followed by centrifugal washing, then dialysis with deionized water; a CNC suspension was then obtained by ultrasonic treatment. The negatively charged sulfonic acid groups on the surface of CNC prepared by sulfuric acid make it very stable in aqueous solution. Morais et al[12] used cotton as raw material for preparing cotton cellulose nanocrystals via sulfuric acid hydrolysis. Transmission electron microscopy (TEM) showed the rod-like structure of CNC prepared with sulfuric acid hydrolysis of cotton fiber. The crystallinity of prepared CNC was as high as 90.45%. The resulting cellulose nanocrystals had a Zeta potential of (-45.3±1.4) mV, indicating very good dispersion. Bai et al[13] found that

the introduction of a catalyst could greatly improve the hydrolysis rate and shorten the reaction time.

2.2.2 Enzyme hydrolysis methodIn another method of fabricating CNC, cellulase is used for enzymatic hydrolysis of amorphous cellulose. In this process, surface corrosion, peeling, fine fibrosis, and cutting may occur[14], thus reducing the polymerization degree of cellulose molecules. Hayashi et al[15] used cellulase to hydrolyze marine organisms and cellulose microciystalline, and obtained CNC. Jiang et al[16-17] ultrasonically pretreated natural cotton fiber, and then prepared the CNC with enzymatic hydrolysis. The average particle size was about 6 nm, most of which was spherical, whereas others were rod-shaped, maintaining the basic chemical structure of cellulose.

2.3 MFC

MFC is generally prepared by mechanical methods, producing a fiber-filament structure, with a diameter range of 5~60 nm, and length of 1~10 mm. MFC is flexible and its molecular structure consists of crystalline and amorphous regions. Cellulose is first extracted from raw materials, and then high-strength mechanical external force is used to destroy the plant cell wall. The fibers are gradually peeled off during the high-speed shearing process and form microfibrils (clusters). The diameter of MFC thus obtained is less than 100 nm, and its length varies from several hundred nanometers to microns. Mechanical methods include high-pressure homogenization, ball milling, freeze grinding, and high-intensity ultrasonic treatment[18-21].

2.4 BNC

BNC is mainly prepared by bacterial methods. Compared with natural plant cellulose, BNC does not

Fig.3 Flow chart of CNF preparation

NaOH

ClCH2COONa

TEMPO oxidation

Williamson etherificationn

High-pressure homogenization

Mechanical stirringCNF

HOHO

HOO

OO

OO

1' 4'2'

2'3'

3'

4'

5'5'

6'

6' 1'

OH

OH OH

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contain lignin, pectin, or hemicellulose. Typically, the crystallinity and polymerization of BNC are quite high (up to 95% and 2000~8000, respectively). BNC possesses an ultra-fine mesh structure. Each filament is composed of several nanoscale microfibers. The diameter of BNC is 20~100 nm, and its length is several microns to several tens of microns. BNC has a strong water retention capacity, and high biocompatibility, adaptability, and biodegradability[22-24].

2.5 ECC

ECC is obtained through electrostatic spinning. ECC is usually achieved by spinning a concentrated cellulose solution through a metal needle-like syringe and stably squeezing under strong electric field induction. The diameters of ECC range from tens to hundreds of nanometers[25-26].

2.6 Discussion of nanocellulose morphology

Due to the differences in sources and preparation methods, nanofibers differ in structure and performance. Fig.4 shows TEM images of nanofibers prepared by different methods, illustrating the relationship between the size of nanofibers and the source and preparation

method. The length of nanocellulose produced by sulfuric acid is shorter than that produced by mechanical and TEMPO-oxidation methods. Fig.4(e) and Fig.4(f) are based on wood cellulose, for which the length of nanofibers prepared by mechanical methods is several times higher than that by sulfuric acid. Moreover, though the preparation is the same (sulfuric acid hydrolysis), the length of cellulose nanocrystals prepared from different raw materials varies widely[27-28]. For example, Fig.4(b), Fig.4(c), Fig.4(f) are bacterial cellulose (BC), animal fiber, and wood, respectively. Although made with the same treatment method, their lengths are different. As shown in Fig.4(b), the length of the BC nanocrystals is greater than 1000 nm, indicating that BC has a very high degree of crystallinity, and a long crystal area, whereas the length of wood cellulose nanocrystals is less than 500 nm.

2.7 Preparation method discussion

The advantages and disadvantages of different nanocellulose preparation processes are summarized in Table 1. Clearly, the high energy consumption mechanical method is simple, but produces wide particle size distribution and low crystallinity. Acid

Fig.4 TEM images of bacterial-HCl(a), bacterial-sulfate(b), tunicate-sulfate(c), wood-enzymatic(d), wood-mechanically refined(e), wood-sulfate(f), and wood-TEMPO (g)

(e) (f) (g)

500 nm 500 nm 500 nm

(a) (b) (c) (d)

500 nm 500 nm 500 nm 500 nm

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hydrolysis preparation of CNC is mature, and the CNC particle size distribution is narrow, but the reaction equipment must be able to resist the corrosion of strong acid, and waste acids produced must be treated. Enzymatic hydrolysis and bacterial methods are green, with low energy consumption, no pollution, and mild production conditions, but their production efficiency is low and they have a long production cycle.

In genera l , there are many ways to prepare nanocellulose, but each method has limitations. In fact, nanocellulose production often combines several methods to use their advantages and avoid their shortcomings. At present, combinations of chemical and mechanical methods have become mainstream[29-30].

2.8 Preparation of sisal nanocellulose

Sisal is the characteristic resource of Guangxi, our research group has been studying the performance of sisal nanocellulose and its composite material. Fig.5 shows a flowchart of fabricating sisal pulp. Sisal fiber was first treated with an alkaline under high-pressure and high-temperature c o n d i t i o n s . S i s a l p u l p w a s

obtained after a bleaching step. Sisal pulp was further treated with NaOH followed by a series of steps for preparing CNF, including a ClCH2COONa Williamson etherification reaction, followed by filtration, washing, and high-speed mixing. The CNC was prepared by hydrolysis of the sisal pulp with 60 wt% H2SO4.

TEM and Atomic Force Microscope (AFM) show that CNF has a fibrous structure. The diameter distribution is narrow, the average diameter is 2.87 nm, the length is micron-scale. FT-IR confirms that the carboxyl group exists on the surface. CNC is a needle-like structure with a length less than that of CNF, a

Table 1 Advantages and disadvantages of different preparation processes

Items ProcessEnergy

consumptionYield

CNC (acid) Waste acid Moderate High

CNC (enzymatic) Green Low Moderate

CNF (TEMPO) Waste alkali Moderate High

CNF (etherification) Waste alkali Moderate High

ECC Difficult to produce Moderate Low

BCN Green Low Low

MFC Green High High

Sisal fiber High temperaturealkaline cooking

Bleaching Sisal pulp

NaOH/Na2S/H2O NaClO2/CH3COOH/H2O

Fig.5 Flow chart of the preparation of sisal pulp

Fig.6 Test results of CNF morphology

03.5

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ght/n

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1.91 nm1.59 nm

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2 3 4 5 6 7Diameter/nm

<D>=2.87 nm

(a) TEM image; (b) AFM image; (c)Height profiles corresponding to (b);(d) Histograms of measured values for (a)

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narrow diameter distribution, an average diameter of 5.04 nm, an average length of 117 nm, and a sulfonic acid group on the surface. The illustration in the upper-right corner of Fig.6(a) and Fig.7(a) are polarized photos of CNF and CNC dispersion. We can see that both show birefringence, and that the CNC liquid crystal is more obvious than that of CNF.

3 Novel applications of nanocellulose

3.1 Dispersing and stabilizing nanomaterials

At present , a thorny problem encountered by nanomaterials is aggregation, which greatly restricts their practical applications. Although intensive efforts have been made to alleviate or eliminate aggregation effects through chemical or physical methods, these approaches inevitably damage their properties. Recently, environment-friendly nanocellulose has been found to have good dispersing and stabilizing properties for nanomaterials in water solutions. Our group found nanocellulose with excellent dispersing

and stabilizing properties for carbon nanomaterials. CNF with —COO— Na+ groups can form a double layer of repulsion, and CNF can disperse stably in water, which provides the foundation for CNF co-dispersion. CNFs are amphiphilic and can form hydrophobic interactions with nanomaterials. Additionally, CNF has a large aspect ratio, its nanofiber is easy to bend and entangle with other nanomaterials. Thus, the dispersing and stabilizing properties of nanocellulose due to the synergistic effect of CNF’s adsorption and entanglement, and the electrostatic repulsion of the CNF fiber surface and nanomaterials, enable CNF to form an electrostatic double layer repulsion and the steric hindrance between nanomaterials, allowing them to be evenly and

stably dispersed in water phase.Nanocellulose has been compared with conventional

dispersants, such as, polyvinyl alcohol (PVA), hydroxyethyl cellulose (HEC) and carboxymethyl cellulose (CMC). As shown in Fig.8, CNF can effectively improve the dispersion stability of carbon black (CB), multiwalled carbon nanotubes (MWCNT). Further, dispersion stability increases with CNF content. Comparison of PVA, HEC, CNF, CMC four substances, CNF, and CMC produce good dispersion effect, storage, and centrifugal stability, whereas HEC and PVA have poor storage and centrifugal stability (Fig.9), indicating that nanocellulose with a —COO— Na+ group (Fig.10) can be used to disperse and stabilize nanomaterials.

Synergistic dispersal effects of graphene oxide (GO) and CNF have been found. Our experimental results show that 60 wt% GO can effectively disperse MWCNT, and 20 wt% CNF can effectively disperse MWCNT; however, both used together (CNF and GO content of 10% each) can effectively disperse 80%

Fig.7 Test results of CNC morphology

0 0.2 0.4 0.6 0.8 1.0

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(a)TEM image; (b) AFM image; (c) Height profiles corresponding to (b);(d) Histograms of measured values for (a)

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MWCNT (Fig.11). We believe that compared with CNF and GO monodisperse MWCNT, CNF and GO can form hydrogen bonds and a composite network system, which plays a synergistic role in the dispersion of MWCNT. This results also indicate CNF can be effectively dispersed multicomponent nanomaterials[31].

3.2 Electronic functional com-posites

Conductive polymer composites, such as polyaniline (PANI) and polypyrrole (PPy), have attracted considerable attentions in various fields, including for electric power and data storage and sensors. However, their chains are rigid, and the strong interaction between chains leads to poor solubility and film-forming properties, which greatly restricted their real applications. Fortunately, thanks to dispersing, stabilizing, and film-forming methods, nanocellulose has been a widely used matrix material for preparing flexural electronic film composites.

Fig.8 Dispersion states of CB (a, b) and MWCNT (c, d) in aqueous solutions with differing CNF content after 1 month (a, c) and centrifugation for 10 min at 12000 r/min (b, d)

Storage Centrifugation

CB

MWCNT

(a) (b)

(c) (d)

Blank Blank

Blank Blank

Fig.9 Dispersion states of CB (a, b) and MWCNT (c, d) in aqueous solutions with 50 wt% dispersant after 1 month (a, c) and centrifugation for 10 min at 12000 r/min (b, d)

MWCNT

CB

Storage Centrifugation

(a) (b)

(c) (d)

CNF CMC HEC PVA CNF CMC HEC PVA

CNF CMC HEC PVA CNF CMC HEC PVA

Fig.10 The schematic structure of CNF (a); the molecular formula of CMC (b), HEC (c), and PVA (d)

(a) (b)

(c) (d)OH

HOHO

HO

HO

HOHO

HO

HOHO

HO

HO

HOHO

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O

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O

O H

COOHCOOH

COOH

COOH

HOOC

COOHCOOH

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Yang et al reported flexible aerogel electrodes for supercapacitors fabricated by freeze-drying to combine a cold-press-process CNF, MWCNTs, and a PANI composite system after polymerized PANI onto the surface of cellulose nanofibers and carbon nanotubes by in situ polymerization. Due to the 3D porous structure of the aerogel electrodes, a high specific capacitance of 791.13 F/g was obtained at 0.2 A/g. Furthermore, the aerogel electrodes also showed excellent redox reversibility and cycling stability[32]. Wang et al reported a bendable, flexible supercapacitor based on PPy-coated core-shell BC composite networks. This composites electrode exhibited high porosity (101 m2/g). Its supercapacitor cell showed a specific capacitance of 153 F/g and an energy density of 21.22 Wh/kg at a current density of 0.2 A/g. Moreover, the PPy-TOBC supercapacitor exhibited exceptionally good cyclic stability with similar to 93% capacitance retention after 100 cycles; it also showed good bending

stability due to the mechanical failure tolerance of the nanofiber-networked electrodes[33]. Lay et al produced ternary multiphase CNF-MWCNT-PPy nanopapers with improved electrical conductivity (2.41 S/cm) and electrochemical properties (113 F/g specific capacitance), even using minimal MWCNTs[34].

Our group prepared cellulose nanocrystals@ polyaniline (CNCs@PANI) by in situ polymerization using CNC as a template and CNF to disperse and stabilize the CNCs@PANI (Fig.12), to prepare both high-conductivity and high-strength conductive films (Fig.13). When 10 wt% CNF was added, the stability of CNC@PANI/CNF dispersion increased; the dispersion and centrifuged solutions were homogeneously black, and the dispersion was stored without precipitation after 1 month. With the increase in CNF, the stability of the dispersion improved, and the conductivity of

the CNC@PANI composites was up to 166.7 S/m. This conductive dispersion has the potential to be used in conductive inks, printing sensors, flexible supercapacitors, and biomedicine[35].

Our group investigated the i n f l u e n c e o f t h e a d d i t i o n o f graphene nanosheets (GNS) on the electrochemical properties of CNF/GNS/PANI conductive film, with nanocellulose and polyaniline in opt imal propor t ion . These

Fig.11 Digital picture of CNF/GO/MWCNT dispersion before and after centrifugation

centrifugation

MWCNT MWCNT

Fig.13 Preparation flow chart of CNF and CNC@PANI

Casting Mixing

Conductive films CNC@PANI/CNF CNC@PANI

CNF

In si

tupo

lym

eriz

atio

n

Sisal pulpCNC

Sisal

Fig.12 The interaction mechanism between CNF and CNC@PANI

CNFs:

CNC@PANI

Assisted dispersed effectHydrogen interaction

Adsorbed and cntangled

CNC@PANI/CNFdispersion

High speed stirring

Centrifuged

R OH

NH

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results showed that the specific capacitance of the conductive film increased first and then decreased with increasing GNS content. The porosity of conductive film was the largest at 6% GNS, when CNF and PANI were uniformly dispersed on the surface of the GNS and were entangled with each other (which limits the agglomeration of PANI). The CNF/GNS/PANI conductive film obtained best electrochemical performance at this concentration: its specific capacitance was 342.87 F/g, which was 1.68 times of that of CNF/PANI conductive film; its electron transfer resistance was as low as 1.16 W; and its cycle stability was the best, with 78.92% of the specific capacitance maintained at a current density of 5 A/g after 1000 charge-discharge cycles[36].

We also prepared cellulose nanofiber/graphene nanosheet/polyaniline (CGP) composite films through in situ polymerization and vacuum filtration and studied the effect of the quantity of aniline on the electrochemical properties of the CGP films. We found that their specific capacitance initially increased and then decreased with increasing amounts of aniline. Among the films studied, CGP1∶15

(1∶15 is the mass rat io of GNS to aniline in the feedstock) films achieved a specific capacitance of 430 .78 F /g a t a cur ren t density of 1 A/g and a charge transfer resistance of 1.65 W. After 1000 charge-discharge cycles, 79.33% of the specific capacitance was maintained at 5 A/g (Fig.14)[37].

3.3 Adsorption material

Nanocellulose has exhibited adsorption properties for various aquatic pollutants. Maatar et al reported on

a nanocellulose aerogel based on cationic cellulose nanofibrils with surface rich in trimethylammonium chloride functional groups that was prepared by freeze drying and chemical crosslinking with an aliphatic triisocyanate. The aerogel, in the form of a rigid porous material, was shown to be an efficient adsorbent for anionic dyes and exhibited strong resistance to disintegration in water. The adsorption capacities for red dye 180, blue dye CR19, and orange dye 142 were 250, 520, and 600 mmol/g (about 160, 230, and 560 mg/g), respectively[38]. Liu et al reported that the introduction of phosphate groups onto nanocellulose significantly improved its metal sorption velocity and sorption capacity. In the case of industrial effluent from the mirror-making industry, adsorption efficiencies of Cu2+ and Fe3+ up to 99% were observed[39]. Polypyrrole@sisal nanocellulose (PPy@CNF) composites were prepared using sisal nanocellulose and pyrrole via a simple in situ chemical oxidation polymerization in our group. We investigated its adsorption properties on methyl

Fig.14 (a) cyclic voltammetry(CV) curves of CGP composite film at a scan rate of 10 mV/s; (b) CV curves of CGP 1∶15 composite films at different scan rates; (c) galvanostatic charge-

discharge curves of CGP composite films at a current density of 1 A/g; (d) specific capacitance of CGP composite films within 1~10 A/g current density range

100 200 300 400 500 600 700 800Time/s

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CPG1:20CPG1:30

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ntia

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orange; our results indicated that the best adsorption effect was 335.1 mg/g when the concentration of the pyrrole monomer was 6.0 g/L (Fig.15). The sorption kinetic data fit well with the pseudo-second-order model and Langmuir isotherm model; the maximum adsorption capacity is 369.01 mg/g on 313 K.

3.4 Other applications

Due to its excellent properties, nanocellulose has potential applicability in reinforcing conventional polymers (chitosan (CS)[40-42], PMMA[43], PVA[44-45]), biomedicine, antibacterial materials[46-47], micro/nanofilters[48-49], the paper industry[46, 50-51], etc. For example, Zhang et al found that a fabric-reinforced CS/BNC composite exhibited bacteriostatic properties against Escherichia coli and Staphylococcus aureus, and significantly improved mechanical properties, indicating that fabric-reinforced CS/BNC composite could be used for advanced wound dressings[41]. Su et al[52] investigated a Cu2O nanoparticle-functionalized cellulose-based aerogel as a high-performance visible-light photocatalyst. Flexible and transparent composites were prepared f rom BC and cas tor-oi l -based polyurethane (PU) by Barud et al. These new BC/PU composites exhibit excellent transparency (up to 90%) in the visible region and great mechanical properties, with a tensile strength of up to 69 MPa and a Young’s Modulus of up to 6 GPa[53].

4 Conclusion and prospects

As nanocellulose is a promising biodegradable

and renewable nanomaterial, its preparation and application have attracted significant attention in many fields. Although there have been many promising achievements in the laboratory, several challenges achieving industrial-scale application of nanocellulose remain. Major obstacles must be overcome for successful low-cost, large-scale, environment-friendly commercial production of nanocellulose. Consequently, more research targeting improved preparation methods is required. However, to maintain the excellent properties of nanocellulose-based composites, determinations of its compatibility and possible surface modification are also required. Finally, applications of nanocellulose to high-tech fields should be further researched, for example, its utilization in oxygen barrier electronic packaging materials, advanced optical devices, flexible wearable electronics, and biomaterials.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (51603050), the Natura l Sc ience Founda t ion o f Guangxi Autonomous Region (2016GXNSFBA380064, 2016GXNSFAA380029), the Startup Foundation for Doctors of Guilin University of Technology, and the Open Project Foundation of the Guangxi Key Laboratory of New Energy and Building Energy Saving (16-J-21-3).

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