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Utilization of discarded crop straw to produce cellulose nanofibrils and their assemblies

  • Corresponding author: Jinyou Lin, Linjinyou@zjlab.org.cn ; Fenggang Bian, Fenggangbian@zjlab.org.cn
  • Received Date: 2019-08-24
    Fund Project:

    National Nature Science Foundation of China 51773221

    Youth Innovation Promotion Association CAS 2017308

    National Key R & D Program of China 2018YFB0704200

  • A tremendous amount of wheat straw (WS) has been generated by wheat crops every year, while only a small percentage is being used in applications, and most get burned on the field, causing a large amount of the exhaust gas that pollutes the environment. Herein, we report on the extraction of cellulose nanofibrils (CNF) from the alkali treated WS by a combination of TEMPO-oxidation and mechanical disintegration method. The crystalline structures, thermal properties, natural charge of the CNF were examined. The resultant nano-building blocks of CNF was assembled into macroscopic cellulose materials, i.e., film, aerogel, and filament in this work. Furthermore, the morphologies and microstructues as well as other properties of these three kinds of the CNF assemblies were investigated. The obtained CNF and its assemblies showed a potential application in new materials areas. This work explored a new way to utilize the discarded WS instead of being burned.
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Utilization of discarded crop straw to produce cellulose nanofibrils and their assemblies

    Corresponding author: Jinyou Lin, Linjinyou@zjlab.org.cn
    Corresponding author: Fenggang Bian, Fenggangbian@zjlab.org.cn
  • Shanghai Synchrotron Radiation Facility of Zhangjiang Lab, Shanghai Advanced Research Institute, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China
Fund Project:  National Nature Science Foundation of China 51773221Youth Innovation Promotion Association CAS 2017308National Key R & D Program of China 2018YFB0704200

Abstract: A tremendous amount of wheat straw (WS) has been generated by wheat crops every year, while only a small percentage is being used in applications, and most get burned on the field, causing a large amount of the exhaust gas that pollutes the environment. Herein, we report on the extraction of cellulose nanofibrils (CNF) from the alkali treated WS by a combination of TEMPO-oxidation and mechanical disintegration method. The crystalline structures, thermal properties, natural charge of the CNF were examined. The resultant nano-building blocks of CNF was assembled into macroscopic cellulose materials, i.e., film, aerogel, and filament in this work. Furthermore, the morphologies and microstructues as well as other properties of these three kinds of the CNF assemblies were investigated. The obtained CNF and its assemblies showed a potential application in new materials areas. This work explored a new way to utilize the discarded WS instead of being burned.

1.   Introduction
  • The rapid development of economy not only provides human with a rich material basis, but also brings severe environmental issues. Renewable natural biopolymers that can reduce the environmental pollution are what we are hunting for. Cellulose, one of the most abundant biopolymer, has attracted increasing interest due to its renewability, low cost, and biodegradability (Klemm et al., 2005; Peng et al., 2016; Yang et al., 2019). The cellulose molecular chains consisting of repeated units of β-1, 4 linked anhydro-D-glucose units are parallel stacked by the covalent bonds and intra- and/or inter-chain hydrogen bonds (Moon et al., 2011; Zhang et al., 2013; Pang et al., 2015), which enables it to be relatively stable, high axial stiffness, and insolvable. As one of the main components of plant, cellulose provides the mechanical properties and supporting structure for cell wall, which is ordered in the form of microfibrils imbedded by lignin and hemicellulose (Morán et al., 2008). Within the cellulose microfibrils there are crystalline regions and amorphous-like regions, of which it is the former allowed to be extracted, resulting in the formation of cellulose nanocrystals (CNC) in a particle shape, while both of these two regions liberated with a high aspect ratio (5–50 nm wide and hundreds of nanometers in length) are commonly named cellulose nanofibrils (CNF) (Isogai et al., 2011; Zhang et al., 2013).

    The principle of preparation nanocelluloses is to break the inter-fibrillar hydrogen bonds by chemical treatment. Presently, several methods such as acid hydrolysis (van den Berg et al., 2007), mechanical fibrillation (Iwamoto et al., 2007), and 2, 2, 6, 6-tetramethylpyperidine-1-oxyl (TEMPO) mediated oxidation (Fukuzumi et al., 2009) have been used to prepare cellulose nanomaterials. The traditionala cid hydrolysis method is to hydrolyze amorphous regions of cellulose, remaining the crystalline regions to get the CNC due to their better resistance to acid. While the CNF consisting both of cellulose crystalline and amorphous regions can be obtained by TEMPO mediated oxidation followed by a mechanical treatment (Isogai et al., 2011). To date, the CNF with an approximate size and the crystal form is tunable by adjusting the amount of sodium hydroxide used in the pretreatment, which provides a huge prospect for its further application (Miao et al., 2016b).

    The macroscopic assemblies of the CNF with different morphological features and properties can be obtained by different post-treatments towards to the suspensions. Transparent and flexible CNF films with high Young's modulus, and low thermal expansion coefficient can be obtained by suction filtration (Fukuzumi et al., 2010), which can be used in some high-tech electronic devices in the future. The CNF aerogels with high porosity and high specific surface area can be prepared by the method of freeze-drying, which has potential applications in the field of filtration and oil-water separation (Lin et al., 2014; de France et al., 2017). The CNF suspension with uniformity and adjustable viscosity was suitable for wet spinning (Iwamoto et al., 2011). The resultant wet-spun CNF fibers shows the possible applications in structural materials and fiber reinforced composite materials.

    The crop straw and other agricultural residues, such as wheat straw (WS), rice straw, existing in the waste streams, show little inherent value. In fact, these discards represent an abundant, inexpensive, and readily available source of renewable lignocellulosic biomass. Liu et.al. (2005) have comprehensively revealed the structure and morphology of cellulose in the WS, which paved the new way to utilize this kind of biomass instead of burned for heating or combusted directly on the land. In recent years, some studies have reported the preparation of bio- or nano-composites by using materials from the WS (Alemdar and Sain, 2008; Oun and Rhim, 2016). Meanwhile, the CNF with a fiber diameter of 30–50 nm was also isolated from the WS using alkaline steam explosion followed by chemical and high shear treatment (Kaushik and Singh, 2011). More recently, the much smaller CNF with a diameter of 14 nm was obtained from the WS by mechanical process and TEMPO-mediated oxidation followed by a high-pressure homogenization (Sánchez et al., 2016).

    In China, a tremendous amount of the WS was generated by wheat crops every year, while only a low percentage is being used in applications (such as feedstock and energy production), and most of them were burned on the field, causing a large amount of the exhaust gas that pollutes the environment (Alemdar and Sain, 2008; Bian et al., 2018a). Meanwhile, increased awareness of global deforestation has raised the demand of alternatives to wood as a raw material for pulp and paper. Therefore, it is necessary to explore different methods of disposal or use of the WS for pulp and paper production as well as for producing chemical materials that would be more environmentally and economically beneficial. As shown in Fig. 1, we managed to extract the CNF from the WS and assembled the resultant nano-building blocks into macroscopic cellulose materials, i.e., film, aerogel, and filament in this work, thus exploring new way to utilize the discarded WS instead of being burned. The crystalline structures, thermal properties, natural charge of the CNF were examined. Furthermore, the morphologies and microstructures as well as other properties of these three kinds of the CNF assemblies were investigated.

    Figure 1.  Preparation process of three kinds of cellulose nanofibrils (CNF) assemblies prepared

2.   Materials and methods
  • The wheat straw (WS) was dried after being harvested in eastern China in 2017. Other starting chemicals including sodium hydroxide (NaOH, 97%), 2, 2, 6, 6-tetramethylpiperidine-1-oxyl radical (TEMPO, 98%), sodium bromide (NaBr, 99.6%), sodium hypochlorite (NaClO, 12%), and ethanol absolute were purchased from Shanghai Aladdin Chemical Regent Inc., China and used as received. All water used was purified by Milli-Q plus water purification system (Millipore Corporate, Billerica, MA).

  • The pristine WS was grinded into powder within 1 mm particle size and heated in a 3 wt% NaOH solution at 70 ℃ for 4 h, and the alkali treated powder was named as WS-3%. After completely washed with deionized water and dried in 70 ℃ for 24 h, a given mass (1.0 g) was completely dispersed in deionized water (100 g) and stirred for 0.5 h. Then, the NaBr (0.33 g) and TEMPO (0.033 g) were both added into the suspension. The reaction was initiated by the addition of a 12 wt% NaClO solution (15.0 g) and the pH value was adjusted between 10.0 and 10.2 by adding the 1 wt% NaOH solution until the pH value no longer reduced. The reaction was terminated by adding ethanol absolute (7 mL), followed by continuously stirring for several minutes. The final product (cellulose jelly) was obtained by successive centrifugations from the suspension and washed several times by using deionized water and ethanol absolute. Subsequently, the quantified fiber jelly was dispersed in 50 mL deionized water followed by a high-speed homogenization and then the CNF aqueous suspension was prepared with an expected concentration of 0.5 wt%.

  • The CNF aqueous suspension (50 mL) was vacuum filtered to remove most of the solvents (Fig. 2a). Then the semi-finished product was moved to a vacuum oven at 35 ℃ for 24 h to ensure that all water had been evaporated and no bubble or holes in the film. The thickness of film was 55 μm measured by thickness gage (AICE130, China).

    Figure 2.  Preparation of CNF assemblies: (a) film; (b) aerogel; and (c) filament

    The CNF aqueous suspension was poured into a cylindrical mold and quickly frozen in liquid nitrogen. Then the frozen liquid was put into a freeze-drier (FD-1D-80) at the temperature of –80 ℃ and the pressure of 4.0 Pa for 48 h until frozen water in the cellulose materials was sublimated completely from the solid phase to the gas phase (Fig. 2b).

    The CNF aqueous suspension was spun into an acetone coagulation bath from a 10 mL syringe (Fig. 2c), and the diameter size of the needle was 1.0 mm. Then the spun CNF fibers were moved out acetone bath and dried in an oven at 70 ℃ for 2 h. The diameter of the CNF fibers measured by a micrometer was about 50 μm.

  • The optical photographs were taken by a digital camera (Sony A580L). The morphology of WS, WS-3% and CNF assemblies were investigated by a field emission scanning electron microscopy (FE-SEM) (S4800, Japan). The cellulose nanostrip length and width was estimated from the transmission electron microscopy (TEM) image (Tecnai G2 F20 S-TWIN) which equipped with a Gatan 1 K × 1 K charge-coupled device (CCD) camera, and the sample was prepared by depositing CNF dispersion solution (~10 μL, 0.02 wt%) onto a glow-discharged carbon-coated Cu grid and put under an infrared lamp until the water totally volatilized. The Fourier transform infrared (FT-IR) spectra of WS, WS-3% and corresponding lyophilized CNF were collected using a Thermo Nicolet 6700 spectrometer (Thermo Fisher) equipped with the Smart iTR operated on the attenuated total reflectance (ATR) mode in the wave-number range of 4000–500 cm–1. Zeta potential (ξ) of 0.1 wt% CNF aqueous suspension was measured by a Zeta sizer NanoS90 (Malvern Instrument) without adjusting ionic strength.

    Synchrotron radiation wide-angle X-ray scattering (SR-WAXS) experiments were performed at BL16B1 at Shanghai Synchrotron Radiation Facility (SSRF). The wavelength was 0.124 nm and the sample-to-detector distance was 95.7 mm (corrected by CeO2 monocrystalline powder). To calculate the crystallinity index (CrI) and average crystallite size (Bhlk) of the CNF, separation of the background and overlapping peaks of the one-dimensional wide-angle X-ray scattering(WAXS) integral curves were finished by peak separation and analysis via software PeakFit (v4.12, SeaSolve Software Inc). The CrI was obtained by the following equation:

    where Acrtst is the sum of crystalline band areas belonging to CNF, and Atotal is the total area under the diffractograms. The Bhlk was determined by Scherrer equation as following (Xu et al., 2013):

    where K is the Scherrer constant (0.9 for cellulose) (Xu et al., 2012), λ is the X-ray wavelength, FWHM is the full width at half maximum of the fitting peaks, and θ is the scattering angle in radians.

    Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analysis were carried out by using a simultaneous thermal analyzer (STA 449 F3 Jupiter, Germany) in the temperature ranging from 40 ℃ to 500 ℃ with a heating rate of 10 ℃ /min in a nitrogen environment. Mechanical properties of the CNF film and filament were evaluated by a tensile test (QJ-210, Shanghai Qingji Testing Instruments Co., LT., China) at room temperature with a gage length of 20 mm and a drawing speed of 6 mm/min. The nitrogen adsorption-desorption measurements were performed on the ASAP2020-HD88 analyzer (Micromeritics Co., Ltd.) at 77 K, and the Brunner-Emmet-Teller (BET) specific surface areas were calculated from data in the relative pressure ranging from 0.10 to 0.30.

3.   Results and discussion
  • The surface of the WS is compact and smooth (Fig. 3a), covered with a layer of wax composed of saturated hydrocarbon compounds and their derivatives. The inner part of the WS is mainly consisted of cellulose, hemicellulose and lignin (Liu et al., 2005; Liu et al., 2017). Cellulose plays as the role of skeleton that provides mechanical properties. Hemicellulose is closely connected with cellulose and plays the adhesive role. Lignin is a kind of high molecular compound, which has aromatic characteristic and three-dimensional structure, affecting the performance of the WS. After the alkali treatment, the wax on the surface of the WS was dissolved and the hydrogen bonding between cellulose and hemicellulose was weakened. The saponified ester bonds between hemicellulose and lignin molecules destroyed the structure of hemicellulose and lignin. Therefore, the cellulose fibers were partially separated from the fiber bundle (Fig. 3b). However, there are still a lot of fiber bonding together (Fig. 3b), indicating that the alkali treatment with a low concentration can not completely remove hemicellulose and lignin. These results were similar to the previous study (Liu et al., 2005).

    Figure 3.  a, b: FE-SEM images of pristine and alkalized WS powder; c: photo of TEMPO-oxidized cellulose jelly from the treated WS powder; d: TEM image of CNF from cellulose jelly

    After been alkali treated, the WS was subjected to selectively TEMPO-oxidation followed by a mechanical disintegration treatment, and the jelly like hydrogel was obtained as shown in Fig. 3c. The structures of non-cellulosic materials and partial amorphous cellulose were reacted and destroyed, and most of them were dissolved, mainly remaining the cellulose components.

    The results of the TEM (Fig. 3d) revealed that the individual CNF with almost uniform widths of 5–10 nm and length of several micrometer, which is lower than that of the lignocellulosic nanofibrils generated from the WS by a combined method (Bian et al., 2018a). According to the previous literatures, the geometrical dimensions of the CNF are found to be widely various, depending on the source of the cellulosic materials and the extraction methods (Saito et al., 2006b). Typically, the geometrical characteristics of the CNF originating from different cellulose sources are shown in Table 1. The reported width is generally approximate to a few nanometers, but the length of the CNF spans a larger range, from hundreds of nanometers to several micrometer. It is worth noting that the dimensions of obtained CNF from the WS are much higher than those of plant celluloses. While comparing with the TEMPO-mediated CNFs from other native cellulosic plant materials, the width and length of the observed CNF generated from the WS were generally much larger.

    Source Length Width (nm) Reference
    Wheat straw Several micrometer 5–10 This study
    Cotton plant Several micrometer 5–10 Miao et al. (2016)
    Jute 100–2000 nm 5–10 Yu et al. (2014)
    Wood 100–500 nm 3–4 Saito et al. (2006a)
    Cotton 100–500 nm 3–5 Saito et al. (2006a)
    Tunicin >5 μm 10–20 Saito et al. (2006a)
    Bacterial >5 μm 10–20 Saito et al. (2006a)
    Tunicate ~10 μm ~20 Iwamoto et al. (2009)

    Table 1.  Length and width of CNFs from various sources obtained by TEMPO-oxidation

  • Fig. 4a shows the FT-IR spectra of the WS, WS-3% and CNF. The board band of –OH groups stretching vibrations within 3360–3330 cm–1 region and the maximum absorbance band of C–H stretching vibrations in methyl and methylene around 2900 cm–1 were observed in all spectra, indicating the principal functional groups found in lignocellulosic materials (Han et al., 2013). Four strong absorption bands at 1732 cm–1 (C=O stretching vibration of methyl ester and carboxylate groups in hemicellulose), 1512 cm–1 (C=C stretching vibration of aromatic hydrocarbon in lignin) (Rosa et al., 2012; Han et al., 2013), 1454 cm–1 (C–H and –CH2 bending vibration in lignin) (Sinha and Rout, 2009) and 1245 cm–1 (C–O stretching vibration of acetyl in hemicellulose) (Carrillo et al., 2004) in WS FT-IR spectra became unobvious in WS-3% FT-IR spectra, which almost disappeared in CNF FT-IR spectra, indicating that the majorities of hemicellulose and lignin in the WS were dissolved and removed during the processing of the CNF.

    Figure 4.  a: FT-IR results of pristine, alkalized WS and CNF; b: Zeta potential of CNF aqueous solution; c: 1D SR-WAXS integral curve; d: schematic of crystallite dimension of CNF

    In addition, the bands at 899 cm–1 (β-glucosidic linkage), 1425 cm–1 (bending vibration for C6-CH2), 1595 cm–1 (aromatic C=C vibration) and 2899 cm–1 (aliphatic saturated C–H stretching vibration in cellulose and hemicellulose) (Kasyapi et al., 2013) of the WS were shifted to 900 cm–1, 1421 cm–1, 1605 cm–1 and 2901 cm–1 of the CNF, respectively. The reason was that the original intramolecular and intermolecular hydrogen bonds were broken due to the C6 hydroxyls being oxidized to carboxylates. The higher polarity of new carboxylate groups and hydrogen bonds interactions affected the vibration modes of organic functional groups of cellulose Iβ (Bian et al., 2018b).

    Zeta potential is a measure of the stability of colloidal suspensions, reflecting the degree of repulsion between adjacent, similarly charged particles in a dispersion (Pelissari et al., 2014). The more highly negative zeta potential values of the CNF suspensions manifest the more highly converted surface C6 primary hydroxyls to carboxylates, indicating the more highly stability of the CNF suspensions. In neutral water, the CNF aqueous solution obtained from the WS exhibited negative zeta potential value of –46.1 mV (Fig. 4b). Similar values have been reported for the CNF isolated from jute (Miao et al., 2016b) and cotton plant (Miao et al., 2016a) by similar preparation methods in our previous works (from –41.0 to –50.1 mV). The oxidized carboxylates on the CNF surface provided enough surface charges to stabilize the suspension and prevented nanofibrils from aggregating. Furthermore, the stable colloidal suspension provided powerful conditions for the preparation of the CNF aerogel and CNF filament.

    Crystal information of the WS, WS-3% and CNF has been analyzed by synchrotron radiation wide-angle X-ray scattering (SR-WAXS). It can be seen from Fig. 4c, the scattering curves of WS, WS-3% and CNF are the typical scattering curves of cellulose I, and the characteristic peaks (peaks at 2θ—11.6°, 13.4°, 16.7°, 18.1° and 27.8° corresponding to I(110), I(110), I(021), I(200) and I(004), respectively) representing cellulose I can be clearly found in band resolution results of CNF scattering curve. However, there are no common scattering peaks (peaks at 2θ around 16.1° and 17.5° corresponding to the II(110) and II(021), respectively) representing cellulose II exist in all of these three curves, which indicates that the alkali treatment by using a 3% NaOH solution was unable to transform the cellulose lattice from cellulose I to cellulose II. This result is consistent with our previous conclusion on the alkali treatment of jute fiber (Yu et al., 2014).

    The crystal size and crystallinity of the WS, WS-3% and CNF were calculated, respectively, by band resolution results of SR-WAXS scattering curves. Fig. 4d shows a model for the cross section of the average crystalline region of these three kinds of cellulose microfibrils. It can be found that the cross-section dimensions of the microfibrils in the CNF is larger than those in the WS and WS-3%. The possible reason was that the cellulose chains in the WS or WS-3% were mutually combined depending on the hydrogen bonding interaction between C6 hydroxyl groups (C6-OH) in molecular chains, and the repulsion force between these cellulose chains was increased after these hydroxyl groups were selectively oxidized to carboxyl groups by TEMPO. After being washed, the carboxyl groups are presented in the form of carboxylate, as a result, the hydrogen bonding interaction is weakened and the electrostatic repulsion is further increased. Moreover, the isolated CNF are more likely to aggregate and further crystallize after the dissolution of lignin, hemicellulose and other noncellulose materials (Bian et al., 2018b). Therefore, the crystallite dimension in the CNF generated from the WS were stretched. Some of the cellulose amorphous phases, remnant lignin and hemicellulose were oxidized and removed also resulting in a higher crystallinity of the CNF than the WS and WS-3%.

  • The TGA and DSC curves of pristine WS, WS-3% and CNF films are shown in Fig. 5. All the samples showed an initial weight loss in the region of 50–110 ℃ due to the evaporation of absorbed moisture (Fig. 5a). The moisture contents of WS, WS-3%, CNF were 2.1%, 2.3%, 4.1%, respectively. The higher moisture content of the CNF was attributed to stronger hydrogen bonding interaction, which can combine a greater amount of water. Thermal decomposition (Td) point of the CNF was approximately 215 ℃, which was lower than those of the WS and WS-3% (approximately 250 ℃). The decrease of thermal degradation point was due to the change of hydroxyls on the surfaces of the CNF into the sodium carboxylate groups during TEMPO-mediated oxidation process (Fukuzumi et al., 2010). The char yields for the WS-3% and CNF at 500 ℃ is approximately to 25.2% and 26.1%, respectively, which are higher than that of WS (20.1%). This phenomenon also indicated that the other components such as lignin and hemicellulose were removed during alkali treatment.

    Figure 5.  The (a) TGA and (b) DSC thermograms of pristine, alkalized WS and CNF

    The DSC curves are shown in Fig. 5b. A broad peak observed in the temperature range of 60–150 ℃ of all samples corresponded to the heat of vaporization of absorbed moisture in these samples. According to the previous report, the hemicellulose was degraded around 220–315 ℃, cellulose was decomposed around 315–400 ℃, and the lignin was hard to decomposed (Yang et al., 2007). The two exothermic peaks around 360 ℃ of the WS and WS-3% were attributed to the decomposition of cellulose. The exothermic peaks at 238 ℃ and 314 ℃ were assigned to the co-existence of TEMPO-oxidized CNF and partially oxidized original WS microcrystalline cellulose.

  • The CNF dispersion with a concentration of 0.5 wt% was used to fabricate a CNF film by vacuum filtration using a microfilter followed by being dried in vacuum oven, for details refer to the experimental section. The prepared film showed high smooth, flexibility and remarkable transparency (Fig. 6a). The microstructure of the film was observed by SEM. The representative images are shown in Fig. 6b and 6c. The surface of the film is composed of uniform entangled nanofibrils network. These nanofibrils are closely packed with no large voids or other defects, which may contribute to the excellent mechanical properties. The fractured cross-section of the film reveals a stratified structure, bundles of nanofibrils and individual nanofibrils have being pulled-out from the layers at considerable length scales up to several tens of nanometers. As suggested by Benítez et al., 2013, the layers formation originated from concentration-induced aggregation and floc formation during late stage of the filtration. The pull-out nanofibrils on several length scales were due to the weakened interfibrillar interactions by the presence of humidity, which resulted in achieving toughness and inelastic deformation.

    Figure 6.  a: photo of CNF film; b, c: FE-SEM images of surface and cross section of CNF film; d: stress-strain curve of CNF film

    The tensile behaviors of the WS CNF film were investigated by universal tensile test. Strain-stress curve and tensile mechanical average values are shown in the inset of Fig. 6d. The glassy film displayed high mechanical properties, exhibiting the tensile strength and Young's modulus were 228.13 MPa and 21.40 GPa, respectively. The results were the highest values in comparison with the previous reports of the CNF film generated from crop straw (Aulin et al., 2012; Malho et al., 2012; Sehaqui et al., 2012; Wang et al., 2016). The WS CNF film with outstanding transparency and exceedingly mechanical properties makes it has a great potential application in electrical and optical components as well as other research areas.

  • The cylindrical freeze-dried CNF aerogel was shown in Fig. 7a. To investigate the microstructure of the aerogel, the surface and fracture cross-section were examined by SEM. As can be seen from Fig. 7b, a highly porous internal network is clearly visible in the surface of the CNF aerogel. The individual nanofibrils have been forced into sheets by the growing ice crystals in the process of freeze-drying, and these sheets are connected to form an open, highly porous structure exhibiting large pores with several micrometers both in length and width. Furthermore, the sheets show a web-like comprising numerous individual nanofibrils (inset of Fig. 7b). The typical web-like structure can be ascribed to the rapid frozen of the CNF suspension by liquid nitrogen. When the cylindrical mold was dipped into the liquid nitrogen, the liquid CNF suspension near the mold wall was rapidly cooled and frozen, and a large number of individual nanofibrils were quickly fixed that prevented their aggregation into compact sheets. These in individual nanofibrils contributed much higher specific surface area of 9.22 m2/g (Fig. 7d) to the resultant aerogel (Mulyadi et al., 2016).

    Figure 7.  a: photo of CNF-assembled aerogel; b, c: FE-SEM images of surface and cross section of aerogel; d: nitrogen adsorption and desorption isotherms of aerogel

    The morphology of the fracture cross-section is similar to that of the surface, which also shows a highly porous internal network structure (Fig. 7c). It can be seen that there were few individual nanofibrils in the presence of the sheets (Fig. 7c). During the process of frozen, the liquid CNF suspension in the core of the mold was frozen only after been put into liquid nitrogen for a period of time that have enough time to self-assemble into sheets, which also may be the reason for that the sheets accumulated in the cross-section look like more compact than the surface.

  • Wet spinning of the WS CNF suspension was successfully carried out by using the acetone as a coagulation bath. The spinning suspension was dehydrated immediately by acetone, leading the CNF to aggregate into fibrous structures. After been dried to remove the acetone, we have obtained the CNF filament with excellent flexibility (Fig. 8a). The morphology of surface and cross-section of the filament were examined by SEM as shown in Figs. 8b and 8c, respectively. At high magnifications of the filament, a rough surface and irregular cross-section (with an average diameter around 45 μm) were observed, which due to the highly mutual diffusion of acetone and water as well as the fiber-drying induced by acetone volatilization. This result was in good agreement with the filament spun from the wood cellulose nanofibers reported by Iwamoto et al. (2011). Moreover, the WS CNF could be observed on the surface under a higher magnification (inset of Fig. 8b). Fig. 8d shows the typical stress-strain curve of as-peppered filament. The table inserted in Fig. 8d lists the mechanical properties, exhibiting the CNF filament has good mechanical strength and rigidity due to these nanofibrils being arranged along the filament axis. The as-prepared single CNF can endure a counterweight of 20 g as presented in Fig. 8d.

    Figure 8.  a: photo of wet-spun filament from CNF; b, c: FE-SEM images of surface and cross section of filament; d: typical stress-strain curve of as-peppered filament

4.   Conclusions
  • In summary, we have extracted the CNF from the discarded WS via TEMPO-oxidation followed by a mechanical disintegration method and successfully obtained three types of their assemblies by using the CNF as building blocks. The alkali treatment with a low concentration of 3 wt% could partially remove the non-cellulose impurities, which enabled the CNF to be extracted from the WS fibers. The as-prepared CNF suspension was highly native charged that provided powerful conditions for the preparation of the CNF assemblies. The stable CNF suspension could be processed into film, aerogels and filament. These assemblies of the CNF obtained from discarded WS had a great promising for many potential applications, such as nanocomposities, pharmaceutical, filtration, tissue engineering, catalysts, aerogels, coatings, nanopaper, and wound dressing materials. Moreover, the utilization of the WS would solve the environmental problems caused by incineration to a certain extent.

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