Structure and Mechanical Properties of Wet-Spun Fibers Made from

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Structure and Mechanical Properties of Wet-Spun Fibers Made from Natural Cellulose Nanofibers Shinichiro Iwamoto, Akira Isogai, and Tadahisa Iwata* Graduate School of Agricultural, Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan ABSTRACT: Cellulose nanofibers were prepared by TEMPOmediated oxidation of wood pulp and tunicate cellulose. The cellulose nanofiber suspension in water was spun into an acetone coagulation bath. The spinning rate was varied from 0.1 to 100 m/min to align the nanofibers to the spun fibers. The fibers spun from the wood nanofibers had a hollow structure at spinning rates of >10 m/min, whereas the fibers spun from tunicate nanofibers were porous. Wide-angle X-ray diffraction analysis revealed that the wood and tunicate nanofibers were aligned to the fiber direction of the spun fibers at higher spinning rates. The wood spun fibers at 100 m/min had a Young’s modulus of 23.6 GPa, tensile strength of 321 MPa, and elongation at break of 2.2%. The Young’s modulus of the wood spun fibers increased with an increase in the spinning rate because of the nanofiber orientation effect.

’ INTRODUCTION Cellulose is the most abundant biomaterial on earth. In higher plants, cellulose molecules are assembled into 3 to 4 nm wide microfibrils crystallized with the extended molecular chains. The microfibrils are used to build cell walls with hierarchical alignments. Tunicates, which are sea animals, also synthesize cellulose microfibrils of ca. 20 nm in width. Several types of cellulose are utilized widely in many applications. Cellulose derivatives, films, and fibers have been produced from cellulose molecules. Plant cell walls that are composed of cellulose are also utilized as natural fibers, pulps, papers, and wooden structures. Recently, cellulose microfibrils and their aggregated nanofibers have received much attention as a new class of cellulose for various applications because they are extremely fine natural polymeric nanofibers that are not achievable using synthetic polymers. Cellulose nanofibers are obtained through 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-mediated oxidation,1 which is a selective reaction at the C6 hydroxyl groups located on the microfibril surface. It has been known that the introduction of cationic charges onto pulp facilitates cell wall delamination.2 Because of anionic charges, Carboxylate groups generated by TEMPO oxidation accelerate dispersion of the 3 to 4 nm wide single-sized microfibrils from the oxidized wood cellulose with a supplementary mechanical treatment. Sulfuric acid hydrolysis is known to produce whisker-like microfibrils.3 Powerful fibrillation using grinding4-7 and homogenizing8-10 apparatuses was also reported to produce cellulose nanofibers. Anisotropic physical properties are expected for cellulose nanofibers, because of the alignment of extended molecular chains in the cellulose crystals. The elastic moduli of highly crystalline microfibrils in the longitudinal and transverse directions were ca. 15011 and 18-50 GPa,12 respectively. Isotropic materials such as films, gels, and composites made of cellulose nanofibers have been studied.13 However, the orientation of the cellulose nanofibers in r 2011 American Chemical Society

the materials was not considered in detail. An exception to this is the work by Kvien and Oksman14 on the orientation of cellulose whiskers in a polyvinyl alcohol matrix under a strong magnetic field. They demonstrated a higher storage modulus for the composites in the direction of fibril orientation than perpendicular to that direction. Cellulose nanofibers are not soluble but are highly dispersible in water. Thus, a cellulose nanofiber suspension in water has a cellulose solution-like viscosity and nonflocculated uniformity. This suggests that cellulose nanofiber suspensions will be suitable for spinning. In the present study, spinning of natural cellulose nanofibers was investigated for two purposes. Although spinning of composite fibers made of cellulose whiskers and matrix15,16 has been reported, this study is a first report for spinning of the cellulose nanofibers only themselves. The first purpose was to fabricate a new type of cellulose fiber using natural cellulose nanofibers. Fibers are a large application area for natural and regenerated celluloses. The natural fibers such as wood pulp, cotton, and ramie fibers are a sophisticated alignment of cellulose microfibrils arranged by nature. However, the flexibility of material design is restricted by cell wall shapes depending on the origin of the material. In contrast, regenerated cellulose fibers are made by dissolution and reconstruction of cellulose molecules. As a consequence, infinitely long and desirable fiber shapes can be obtained. A considerable difference between natural and regenerated cellulose fibers is the crystal structure: cellulose I for natural cellulose and cellulose II for regenerated cellulose. The elastic modulus of cellulose II is lower than that of cellulose I because of the loss of intermolecular hydrogen bonds.17 This implies that natural Received: December 14, 2010 Revised: January 14, 2011 Published: February 08, 2011 831

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cellulose fibers have potentially higher stiffness and strength compared with regenerated fibers. Spinning of the natural cellulose nanofibers can maintain the cellulose I crystal structure. Therefore, the spun fibers should enable flexibility in material design while retaining the physical properties of natural cellulose. The second purpose for spinning of the natural cellulose nanofibers was to control the alignment of the cellulose nanofibers. The molecular chains of many polymeric materials are easily aligned by drawing because of the high mobility of entangled molecular chains. However, the uniaxial orientation of cellulose and its nanofibers is difficult because hydrogen bonds between the hydroxyl groups reduce the mobility of molecules in the materials. It was reported that cellulose nanofibers aligned under magnetic18 and electric19 fields and shearing force conditions.20 However, knowledge about the nature of the orientation is insufficient, and convenient procedures are needed to fabricate promising nanocellulose materials. Shearing force would be the simplest method in practice for uniaxial orientation of nanofibers. During spinning of nanofibers, it is speculated that the shearing force generated from flows under several conditions aligns the cellulose nanofibers in the suspension. Moreover, the orientation of cellulose nanofibers would be fixed immediately because of a seamless process from a flowing to dehydrated suspension. In this study, cellulose nanofibers prepared from wood pulp and tunicate cellulose by TEMPO-mediated oxidation were wetspun to produce cellulose I type fibers. The cellulose nanofiber suspension in water was spun into an acetone coagulation bath. The spinning rate was set from 0.1 to 100 m/min to vary the nanofiber orientation in the spun fibers. The structures of the wet-spun fibers were investigated using scanning electron microscopy (SEM) and synchrotron X-ray diffraction. In addition, the mechanical properties of the spun fibers were determined by tensile tests.

insufficient for 100 m/min spinning of the tunicate nanofiber suspension because of its gel-like high viscosity. After the spun fibers were taken from acetone, the fibers were dried at 105 °C for 1 h under weak tension. The diameters of the wood and tunicate spun fibers as measured by a micrometer were both ca. 60 μm. Atomic Force Microscopy (AFM). The diluted wood or tunicate cellulose nanofiber suspensions in water (0.002 wt %) were cast on a cleaved mica substrate, followed by removal of the excess water with blotting paper. After drying the dispersion at room temperature, the nanofibers were observed with a scanning probe microscope system composed of SPA-300 and SN-3800 (SII Nano Technology, Japan) units and a cantilever (SI-AF-01, SII Nano Technology, Japan) in contact mode. SEM. The surfaces and cross sections of the spun fibers were observed by SEM (Hitachi S-6000, Japan). The samples were coated with osmium by sputtering for 10 s before observation. The spun fibers were fractured in liquid nitrogen to reveal the cross sections for SEM observations. Wide-Angle X-ray Diffractions. Wide-angle X-ray diffraction experiments on both the spun fibers and films made from the wood and tunicate nanofibers were carried out using beamline BL45XU with a wavelength of 0.09 nm at the SPring-8 synchrotron radiation facility. A total of 16 spun fibers were bundled together to obtain sufficient diffraction intensity. The microfibril suspensions were dried at 105 °C on PTFE Petri dishes to obtain films of ca. 100 μm thickness. Twodimensional wide-angle X-ray diffraction patterns were recorded using a charge coupled device (CCD) camera. The pixel size of the CCD camera was 108.4
108.4 μm. The camera length for measurements was 102 mm. Tensile Tests. The mechanical properties of the spun fiber monofilaments were measured with a universal testing machine (Shimadzu EZ-TEST, Japan) equipped with a 10 N load cell. The cross sections of fibers were regarded as circular in shape, and their diameters were measured by a micrometer. The tensile tests were carried out at 20 mm/min and 10 mm span length with five individual fibers for each of the spinning conditions.

’ EXPERIMENTAL SECTION Preparation of Cellulose Nanofibers. Cellulose nanofibers

’ RESULTS AND DISCUSSION

were prepared from soft wood bleached kraft pulp and tunicate was prepared by TEMPO-mediated oxidation according to Saito et al.1 The wood pulp was irradiated with ultraviolet light before the oxidation. The wood pulp and tunicate cellulose (1 g) were suspended in water (100 mL) containing TEMPO (0.1 mmol) and sodium bromide (1 mmol). The reaction was initiated by adding sodium hypochlorite (5 mmol) at room temperature. The solution was kept at pH 10 by titration with 0.5 M sodium hydroxide for 1 h. At the end of the reaction, the oxidized cellulose was filtered and then washed with distilled water. The oxidized wood pulp cellulose in water (1 wt %) was fibrillated into nanofibers by repeated high-pressure homogenization until the suspension showed high transparency. The fibrillation of oxidized tunicate cellulose was achieved in excess water by homogenizing and sonicating for a few minutes. After removing unfibrillated fractions by centrifugation at 9000g for 5 min, the tunicate cellulose nanofiber suspension was concentrated to 1 wt % by centrifugation at 40 000g for 10 min. The tunicate nanofibers used in this study were the same as fibers in the previous report.11 Wet-Spinning of the Cellulose Nanofibers. The wood and tunicate cellulose nanofiber suspensions were spun in an acetone coagulation bath from a needle (φ 0.95 mm) set on syringes (φ 6.5 and 35 mm). Different sizes of syringes were used to control the spinning rate. The syringes were pushed by a syringe pump at rates of 2.373.6 mm/min. Consequently, the spinning rates of the wood and tunicate cellulose nanofiber suspensions were controlled over the ranges of 0.1-100 and 0.1-10 m/min, respectively. The pumping load was

Morphologies of the Wood and Tunicate Cellulose Nanofibers. Figure 1 shows AFM images of the cellulose nanofibers

from wood pulp and tunicate cellulose prepared by the TEMPOmediated oxidation method. The thickness of the wood nanofibers was 3.2 ( 0.5 nm, and their length was 200-500 nm. The thickness was the same as that of previously reported nanofibers,1 indicating successful separation of single microfibrils. The length was clearly shorter than the previously reported nanofibril length (ca. 1 μm).1 The fibrillation of oxidized cellulose nanofibers at a low concentration of 0.1 wt % has been achieved by mild dispersion treatment such as by using a domestic food mixer. However, at 1 wt % concentration, the fibrillation required UV radiation and repeated high-pressure homogenization. It was assumed that the UV radiation and repeated high-pressure homogenization resulted in the decrease in nanofiber length. In the tunicate, cellulose nanofibers were reported to have parallelogram cross sectional shapes,21 and the thickness and width was 8.4 ( 0.6 and 20.3 ( 1.6 nm, respectively.11 The length was observed to be >10 μm by AFM. The tunicate nanofiber suspension had a higher viscosity than that of the wood because the aspect ratio of tunicate nanofibers (>500) was larger than that of wood nanofibers (10 m/min resulted in the spun fibers having tube-like cross sections. The hollow structures of fibers were likely to have formed by cylindrical coalescence of the sheets. The orientation of the wood cellulose nanofibers in the spun fibers was evaluated using wide-angle X-ray diffraction. Because the c-axis of cellulose crystals aligns along the fiber axis of the cellulose nanofibers, the cellulose crystal orientation represents the nanofiber orientations in the spun fibers. Figure 3 shows the X-ray diffractograms of a cast film (a) and the fibers (b-e) spun from the wood cellulose nanofibers at rates of 0.1-100 m/min. The diagram of the cast film showed ring patterns (Figure 3a), indicating a random orientation of the fiber axis of the cellulose nanofibers on the horizontal plane of the film. There were clearly three reflections assigned to the cellulose I crystal in all diffractograms, with the innermost reflections corresponding to (110) and (110) and the outer ones corresponding to (200) and (004). In contrast, the spun fibers showed equatorial arcs corresponding to (110), (110), and (200) diffractions and a meridian arc corresponding to (004) (Figure 3b-e). The results revealed that the wood cellulose nanofibers aligned along the fiber axis of the spun fibers.

Figure 2. Scanning electron micrographs of the fibers spun from wood cellulose nanofibers. The surfaces of the fibers at a spinning rate of 10 m/min observed at (a) low and (b) high magnifications and the cross sections of the fibers at spinning rates of (c) 0.1, (d) 1, (e) 10, and (f) 100 m/min.

The dehydration behavior of cellulose nanofiber suspensions was probably influenced by their carboxylate group contents. According to a report,1 the carboxylate group content of the TEMPO-oxidized tunicate cellulose (0.3 mmol in 1 g cellulose) was much lower than that of the TEMPO-oxidized wood cellulose (1.5 mmol in 1 g cellulose). The carboxylate groups on the nanofiber surfaces help to disperse the nanofibers via a mutually 833

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Table 1. Orientation Indices and Mechanical Properties of the Spun Fibers Made from the Wood and Tunicate Cellulose Nanofibers fiber wood

spinning rate (m/min) 0.1

wood wood

1 10

wood

100

tunicate

orientation index (fc)

0.1

Young’s modulus (GPa)

tensile strength (MPa)

elongation at break (%)

0.65

8.4 ( 1.2

90 ( 22

1.5 ( 0.2

0.67 0.70

11.6 ( 2.1 19.1 ( 2.6

192 ( 110 332 ( 93

2.6 ( 1.8 3.1 ( 1.1

0.72

23.6 ( 2.1

321 ( 145

2.2 ( 1.2

0.44

18.9 ( 3.2

406 ( 48

5.6 ( 0.7

tunicate

1

0.51

20.1 ( 2.9

357 ( 36

5.3 ( 1.8

tunicate

10

0.54

16.7 ( 2.1

282 ( 67

5.4 ( 2.4

cotton23

5.5-12.6

287-597

7.0-8.0

lyocell24

16

660

14-16

Figure 5. X-ray diffractograms of the cast films made from the tunicate cellulose nanofibers in (a) the through and (b) edge-on view. The schematic shows the direction of the X-ray beam.

tunicate spun fibers formed porous structures consisting of aggregated nanofiber sheets, in contrast with the hollow structure formed by the wood spun fibers. In addition, the tunicate spun fibers maintained the cylindrical shapes of the syringe needles compared with the wood spun fibers. The reason for this structural difference was presumably related to the viscosities of the suspensions. Because of the higher viscosity of the tunicate nanofiber suspension, these nanofibers moved less upon dehydration in the acetone coagulation bath than the wood nanofibers. Therefore, the aggregation of the tunicate nanofibers was restricted, resulting in the formation of porous structures and the maintenance of the cylindrical shapes. The orientation of the tunicate nanofibers in the spun fibers was analyzed by X-ray diffraction. To understand the structure of the spun fibers, we subjected cast films made from the tunicate nanofibers to X-ray analysis in the through (Figure 5a) and edgeon views (Figure 5b). In the through view (Figure 5a), the reflections corresponding to (110), (200), and (004) were present in the ring pattern. This indicated the random orientation of the fiber axis of the tunicate cellulose nanofibers. The diffractogram in the edge-on view (Figure 5b) showed a different diagram with an arc pattern of (110) reflection on the azimuth of the film thickness direction. In addition, (110) reflection was on the azimuth of the film radial direction, and (200) was strong on the middle azimuth between the thickness and radial directions. The reflection corresponding to (110) was very weak in Figure 5a. This implies that (110) was aligned along the film thickness direction, although the fiber axes of the tunicate cellulose nanofibers were oriented randomly in the horizontal plane of the films. Figure 6a-c shows X-ray fiber diagrams of the tunicate fibers spun at rates of 0.1-10 m/min. To determine the orientations of (110), (110), and (200) planes, we plotted the azimuthal profiles of these planes within the tunicate fibers spun at 10 m/min in Figure 7. The (110), (110), and (200) reflections are stressed

Figure 4. Scanning electron micrographs of the fibers spun from the tunicate cellulose nanofibers. The surfaces of the fibers observed at (a) low, (b) medium, and (c) high magnifications. The spinning rates were 10 m/min for (a) and (b) and 0.1 m/min for (c). Cross sections were the fibers spun at rates of (d) 0.1, (e) 1, and (f) 10 m/min.

To evaluate the degree of nanofiber orientation, we evaluated the orientation index (fc) of the cellulose crystals in terms of the following equation by using azimuthal breadth analysis22 fc ¼ ð180° - βc Þ = 180° βc was determined from the half-width of the azimuthal direction of the equatorial (200) reflection in the diffractograms of the spun fibers. The orientation indices are listed in Table 1. With increasing spinning rate, the orientation index also increased. This indicated that rapid spinning caused high shearing force on the cellulose nanofiber suspensions, leading to the orientation of the fiber axis of the wood nanofibers toward the spinning direction. Structures of the Fibers Spun from Tunicate Cellulose Nanofibers. The tunicate cellulose nanofibers were spun into fibers with diameters of 60 μm at spinning rates of 0.1-10 m/ min. SEM images of the surfaces of the spun fibers are shown in Figure 4a-c. The spun fibers of the tunicate nanofibers had rougher surfaces with creases along their fiber axis (Figure 4b) in comparison with those of the wood nanofibers. Furthermore, randomly oriented tunicate nanofibers were observed on the surface creases under high magnification (Figure 4c). Figure 4d-f shows cross sections of the tunicate fibers spun at rates of 0.1-10 m/min. The spinning rates did not affect the cross sectional structures. SEM observations revealed that the 834

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mechanical properties explains the increase in Young’s modulus with increasing orientation index. However, the cross sectional shapes of the wood spun fibers also depended on the spinning rates. It is difficult to understand the mechanical properties of the fibers in terms of only the nanofiber orientations. The mechanical properties of the tunicate spun fibers did not show an obvious dependence on spinning rate. The nanofiber orientation index was increased by rapid spinning, and the cross sectional shapes of the spun fibers were similar for the various spinning rates. Nanofiber orientation may contribute slightly to the mechanical properties of the tunicate spun fibers. Comparison of the elongation at break showed that the tunicate spun fibers possessed higher toughness than that of the wood. This was due to the higher aspect ratio of the tunicate nanofibers than the wood nanofibers. The mechanical properties of natural cellulose fibers, cotton,23 and regenerated cellulose fibers, lyocell,24 are listed in Table 1. Except for the wood spun fibers obtained at spinning rates of 0.1 and 1 m/min, all the spun fibers showed higher Young’s moduli and similar strengths compared with cotton. The spun fibers had higher Young’s moduli and lower strengths and elongations at break compared with lyocell. The hollow and porous structures of the spun fibers caused lower densities than lyocell, indicating higher or comparable specific strengths for the spun fibers. The small elongations at break were probably caused by lack of uniformity of the fiber structure along the fiber direction during the rapid dehydration in the acetone coagulation bath.

Figure 6. X-ray fiber diagrams of the tunicate fibers spun at rates of (a) 0.1, (b) 1, and (c) 10 m/min.

Figure 7. Azimuthal profiles of (110), (110), and (200) reflections obtained from X-ray fiber diagram of the tunicate fiber spun at rate of 10 m/min. The profiles were recorded from the equatorial azimuth.

on the equatorial azimuth in all diagrams, indicating an alignment of the tunicate nanofibers along the fiber axis of the tunicate spun fibers. In addition, (110) and (200) were stressed on the meridian and on the middle azimuth between the equatorial and meridian, respectively. This indicates the planar orientation of (110), which was also revealed by the diffractograms of the cast films (Figure 5). SEM observations of the fiber cross sections exhibited porous structures consisting of sheet-like aggregations inside the tunicate spun fibers. The sheets were assumed to be (110) layers of the tunicate nanofibers based on X-ray analysis. Therefore, it was concluded that the internal structure of the spun fibers was formed by the drawing of the aggregated sheets toward the spinning direction. The orientation of the tunicate nanofibers in the spun fibers was evaluated from calculations of (200) orientation index (fc) as for the wood spun fibers. The orientation indices are also listed in Table 1. The degree of orientation of the tunicate nanofibers along the fiber axis in the spun fibers increased with an increase in the spinning rate. Mechanical Properties of the Fibers Spun from the Wood and Tunicate Cellulose Nanofibers. The mechanical properties of the fibers spun from the wood and tunicate nanofibers are listed in Table 1. The Young’s modulus and tensile strength of the wood spun fibers increased with faster spinning rate until 10 m/min. One of the reasons for this increase in both Young’s modulus and tensile strength was because of the increase in the orientation index. The Young’s moduli of the highly crystalline cellulose nanofibers in the longitudinal and transverse directions were ca. 15011 and 18-50 GPa,12 respectively. This anisotropy in

’ CONCLUSIONS Wet spinning of wood and tunicate cellulose nanofibers prepared by TEMPO-mediated oxidation was demonstrated. The viscosity of the nanofiber suspensions was assumed to influence the structure of the spun fibers. The wood fibers spun at high spinning rate formed hollow structures, whereas the higher viscosity tunicate cellulose fiber suspension produced spun fibers with the porous structure. Increased spinning rate also increased the wood and tunicate nanofiber alignment along the fiber axis. The Young’s modulus of the wood spun fibers increased with increasing spinning rate. This indicated that the mechanical properties of the spun fibers were affected by the orientation of the cellulose nanofibers. The low density spun fibers made from wood and tunicate nanofibers showed higher Young’s moduli than natural and regenerated cellulose fibers. Spinning of the cellulose nanofibers is an effective way to orient the nanofibers because alignment of the nanofibers in the flow is fixed upon coagulation. This study has shown that the spun fibers prepared under suitable conditions to orient the cellulose nanofibers are expected to have high moldabilities and mechanical properties. ’ AUTHOR INFORMATION Corresponding Author

*Tel: þ81-3-5841-7888. Fax: þ81-3-5841-1304. E-mail: atiwata@ mail.ecc.u-tokyo.ac.jp.

’ ACKNOWLEDGMENT This study was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. 835

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