Biomacromolecules 2009, 10, 1597–1602
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Properties of Films Composed of Cellulose Nanowhiskers and a Cellulose Matrix Regenerated from Alkali/Urea Solution Haisong Qi,† Jie Cai,† Lina Zhang,*,† and Shigenori Kuga‡ Department of Chemistry, Wuhan University, Wuhan, 430072, People’s Republic of China, and Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan Received February 13, 2009; Revised Manuscript Received April 13, 2009
All-cellulose composite films were prepared, for the first time, from native cellulose nanowhiskers and cellulose matrix regenerated from aqueous NaOH-urea solvent system on the basis of their temperature-dependent solubility. The cellulose whiskers retained their needlelike morphology with mean length and diameter of 300 and 21 nm as well as native crystallinity when added to the latter solution at ambient temperature. The structure and physical properties of the nanocomposite films were characterized by scanning electron microscope, X-ray diffraction, and tensile tests. The composite films were isotropic and transparent to visible light and showed good mechanical properties as a result of the reinforcement by the whiskers. By varying the ratio of the cellulose whiskers to regenerated cellulose matrix (cellulose II), the tensile strength and elastic modulus of the nanocomposite films could be tuned to reach 124 MPa and 5 GPa, respectively. The tensile strength of the nanocomposite films could reach 157 MPa through a simple drawing process, with the calculated Hermans’ orientation parameter of 0.30. This work provided a novel pathway for the preparation of biodegradable all-cellulose nanocomposites, which are expected to be useful as biomaterials and food ingredients.
Introduction Cellulose is the most abundant natural polymer on the earth, and regaining importance as a renewable chemical resource to replace petroleum-based materials.1 Cellulose can be converted into cellulose derivatives and regenerated materials, which have been used in diverse industries including pharmaceuticals, cosmetics, foods, textiles, and so on.2 Recently, attention is increasingly devoted to biodegradable and plant-derived composites, which we designate as “green” composites.3 Natural fibers derived from bamboo,4 hemp,5 or flax6 are being added to biodegradable resins to reinforce polymer matrix and improve the mechanical properties of resulting composites.3 Utilizing natural fillers from renewable resources not only contributes to a healthy ecosystem, but also makes them economically attractive for industrial applications due to the high performance.7,8 High-strength cellulosic composites have been also obtained by self-reinforcement, embedding unidirectionally aligned ramie fibers in a matrix of regenerated cellulose. The all-cellulose composites first introduced by Nishino et al.9 had excellent mechanical properties and thermal performance. Being chemically homogeneous, self-reinforced composites are easy to recycle. Moreover, they are fully biobased and biodegradable materials.10 The preceding works9,10 on the all-cellulose composite were based on the controlled partial dissolution of cellulose by a nonaqueous cellulose solvent, LiCl/ DMAc. Recently, native cellulose whiskers have been successfully used as reinforcing fillers for both synthetic polymeric matrices11 as well as natural ones.12 Cellulose whiskers have advantages such as renewability, low cost, ready availability, high biocompatibility, and easy chemical and mechanical modification.13,14 * To whom correspondence should be addressed. Phone: +86-2787219274. Fax: +86-27-68754067. E-mail:
[email protected]. † Wuhan University. ‡ The University of Tokyo.
The reinforcing ability of the cellulose whiskers is likely to result from their large surface area and high mechanical performances. The mechanical strength of the whiskers is thought to arise from the extended-chain conformation and strong mutual association of cellulose molecules in the crystalline state.15 Thus, the cellulose whiskers have excellent mechanical properties over other reinforcing materials.16 Realization of significant improvement in material properties, however, requires good dispersion and homogeneous distribution of whiskers in the matrix.17 In our laboratory, a new cellulose solvent, 7 wt % NaOH/12 wt % urea aqueous solution precooled to -12 °C, has been developed. It can dissolve cellulose with molecular weight below 1 × 105.18-21 By using this solvent, regenerated cellulose (RC) multifilament fibers with good mechanical properties have been prepared by a wet spinning method via a pilot machine.22 Also, a series of novel transparent and photoluminescent regenerated cellulose films having homogeneous structure, excellent transparency, and high tensile strength were successfully prepared from the cellulose solution through a “green” process.23 This new pathway is a relatively low-cost, simple, and essentially nonpolluting process, in contrast to the viscose method with hazardous byproducts. Further, such cellulose films exhibit good biodegradabilities and are safe, suggesting wide applications in the biomaterial and food industrial fields. In our previous work, it has been proven that solubility of cellulose in the NaOH/urea system is strongly dependent on temperature and molecular weight of cellulose.24 By utilizing cellulose with different molecular weight and the dissolution method at low temperature, we are able to prepare regenerated cellulose film embedded with native cellulose whiskers. In the present work, cellulose self-reinforced nanocomposites films having high-strength and biodegradability were fabricated via a simple and “green” process. Their structure and properties were studied for evaluating the reinforcing effect of the cellulose whiskers. Moreover, the influence of drawing and drying
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processes on the structure and properties of the films were investigated. This finding provided important information for development of the new regenerated cellulose fibers, films, and other materials self-reinforced with cellulose whiskers.
Experimental Section Materials. The cellulose samples (cotton linter pulps) were supplied by Longma Green Cellulose Fiber Co. Ltd. (Hai’an, China), coded as C10 (nominal DP, 500) and C30 (nominal DP, 1985). All other chemical reagents were purchased from commercial sources in China and were of analytical grade. All-Cellulose Nanocomposite Films Preparation. To prepare cellulose whiskers, 80 g cellulose (C30) was dispersed in 690 mL of 30% (v/v) sulfuric acid contained in a three-necked glass flask equipped with a mechanical stirrer and a thermometer. The flask was placed into a water bath at 60 °C and stirred vigorously for 6 h. The suspension was then diluted with 2 L of distilled water, and centrifuged at 8000 rpm for 15 min. The process was repeated three times to remove sulfuric acid. The resulting suspension was dialyzed for 2 h in running water and then overnight in distilled water, until the pH reached 4. Finally, the dispersion was sonicated and stored in a refrigerator at 5 °C. A 200 g mixture of NaOH, urea, and distilled water (7:12:81 by weight) was precooled to -12.6 °C, and 8 g of cellulose (C10) was added immediately with vigorous stirring for 5 min to obtain a transparent solution. To this solution, after warming up to room temperature, 0-20 g of aqueous cellulose whisker suspension (solid content, 10%) was added and stirred for 30 min. The suspension was centrifuged at 6000 rpm for 5 min at 10 °C for removing air bubbles. The resulting nearly transparent suspension was spread on a glass plate to give a 0.25 mm thick layer, and then immersed into a coagulation bath of 5 wt % H2SO4 for 5 min at 25 °C. The resultant film was washed with running water and then deionized water. The wet films were fixed on glass plate to prevent shrinkage and, finally, were air-dried at ambient temperature for the characterization. The films containing 0, 5, 10, 15, and 20 wt % cellulose whiskers were obtained and were denoted as RC, RC-W5, RC-W10, RC-W15, and RC-W20, respectively. To study the effect of drawing, the composite films (RC-W10A) were air-dried at room temperature without fixation, with area half shrinking and thickness of 40 µm. The composite films (RC-W10B) remained at the drawing state (drawing ratio (λ) was 1.05) for 8 h to be air-dried at room temperature. Characterizations. Atomic Force Microscopy (AFM). A drop of the diluted whisker suspension was dropped onto freshly cleaved mica substrate (Digital Instruments, U.S.A.) and allowed to dry. AFM imaging was conducted with a PicoScan atomic force microscope (Molecular Imaging, U.S.A.) in contact/dynamic mode.25,26 Freshly prepared samples were mounted on the AFM stage and imaged under MAC Mode in air (relative humidity ) 40-50%, T ) ∼25 °C) using MAC lever type II probes (spring constant ) 2.8 N/m, resonant frequency ) ∼85 kHz, Molecular Imaging, U.S.A.). Scan rates were about 1 line/s. The images were rastered at 256 × 256 pixels, unfiltered, and flattened when needed. Wide-angle X-ray diffraction (WAXD) was measured with an X-ray diffractometer (D/MAX-1200, Rigaku Co., Japan). The X-ray radiation used was Ni-filtered Cu KR with a wavelength of 1.5406 Å. The voltage was set at 40 kV, and the current was set at 30 mA. The samples were mounted on a solid circular holder, and the proportional counter detector was set to collect data at a rate of 2θ ) 1° min-1 over the 2θ range from 4 to 40°. All samples were cut into particle-like size to erase the influence of the crystalline orientation. Individual crystal reflection peaks and the amorphous background were extracted by the curve-fitting process of the integrated diffraction intensity profile. Background intensity was subtracted before peak fitting. Each intensity spectrum was fitted by employing a Gaussian function as the peak profile. The crystallinity index (χc) value of cellulose samples was estimated as the ratio of the integrated area of all crystal peaks to the total integrated area (including the amorphous area).27
Figure 1. AFM topography image of cellulose whiskers after drying on a mica surface.
Two-dimensional X-ray diffraction patterns (2D WAXD) were obtained at room temperature using nickel-filtered Cu KR radiation with wavelength of 0.15418 nm, generated from a rotating anode X-ray generator (Rigaku RU-200BH) operating at 50 kV and 100 mA. A point-collimated beam was directed either orthogonal to the tangential section or on a section inclined at an angle of 17.3° to the longitudinal axis. The patterns were recorded on flat-plate imaging plates (Fuji Film BAS-IP SR 127) using an evacuated camera. Sodium fluoride (d ) 0.23166 nm) was dusted onto the surface of the wood samples to act as a calibrant. The Hermans order parameter (f) was calculated from the azimuthal profile assuming a cylindrical symmetry according to the following equation28,29
1 f ) (3〈cos2 γ〉 - 1) 2
(1)
〈cos2 γ〉 ) 1 - 2〈cos2 φ〉
(2)
〈cos2 φ〉 )
∫
π/2
0
I(φ) sin φ cos2 φdφ
∫
π/2
0
(3) I(φ) sin φdφ
Scanning electron microscopy (SEM) results were performed on a FESEM (SEM, SIRION TMP, FEI) by using an accelerating voltage of 20 KV; the samples were coated with gold to facilitate the SEM observation. Optical transmittance (Tr) of the films was measured with a UV-vis spectroscope (Shimadzu UV-160A, Japan) at the wavelength of 800 nm. The thickness of the RC films is about 20 µm. The tensile strength (σb) and elongation at break (εb) of the films were measured on a universal testing machine (CMT6503, Shenzhen SANS Test Machine Co. Ltd., Shenzhen, China) according to ISO 527-3, 1995 (E) at a speed of 5 mm · min-1. Thermal gravimetric analysis (TGA) was carried out using thermogravimetric analysis equipment (Netzsch, Germany). The films were ground into powder, and about 5 mg of the powder was placed in a platinum pan and heated from 20 to 600 °C at a rate of 10 K min-1 in air atmosphere.
Results and Discussion Morphology and Structure of Nanocomposite Films. Figure 1 shows an AFM image of the cellulose whiskers. The cellulose whiskers displayed a needlelike morphology, and their mean length (L) and diameter (D) were 300 and 21 nm, respectively. Therefore, the average aspect ratio, L/D, is about 15. Generally, the dimensions are different for cellulose whiskers prepared from different cellulose sources.30,31 The results indicated that the cellulose nanowhiskers were prepared suc-
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Figure 2. SEM images of surface (top) and cross-section (bottom) for all-cellulose nanocomposite films (b,f, RC-W5; c,g, RC-W10; d,h, RCW20) and pure regenerated cellulose films (a,e).
cessfully from cellulose having high molecular weight in 30% (v/v) sulfuric acid at 60 °C. It was noted that the NaOH/urea solvent is effective only at below -10 °C,19,24 so the cellulose nanowhiskers added at room temperature can be expected to remain as native crystal retaining their structure and needlelike shape. Thus, the films obtained can be referred to nanocomposites in which regenerated cellulose was supposed to serve as a matrix and the cellulose nanowhiskers (cellulose I) were embedded in it. Although the matrix and the filler are chemically identical, the term “composite” is considered appropriate because there were two components, which contribute to the mechanical properties differently.32 SEM images of the surfaces for all-cellulose nanocomposite films are shown in Figure 2. The pure regenerated cellulose (RC) film and the RC-W5 nanocomposite film (Figure 2a,e and b,f) exhibited a smooth surface and homogeneous cross section. It suggested that the cellulose whiskers were embedded in the regenerated cellulose matrix because of the strong interaction between cellulose I and cellulose II. However, the RC-W20 nanocomposite film gave increasingly rough surface and cross section, indicating that certain phase separation occurred here. The X-ray diffraction patterns of the RC films and the nanocomposites films are shown in Figure 3. The diffraction peaks at 2θ ) 14.8, 16.3, and 22.6° for (11j0), (110), and (200) planes are characteristic for cellulose I crystal (Figure 3a) and those at 2θ ) 12.1, 19.8, and 22.0° for (11j0), (110), and (200) planes are assigned to cellulose II crystal (Figure 3f). Figure 4 shows the fitted intensity curve and the diffraction peaks resolved from the X-ray spectrum of RC-W10. The results revealed that two kinds of crystal structure of cellulose I and II coexisted in the nanocomposite films. Interestingly, the peak of (11j0) plane was stronger than the peak of (110) and (200) plane for cellulose II, which is unusual. This indicated that the crystallites of cellulose II in these films may have substantial disparities in the dimensions for the different crystal planes compared to “normal” cellulose II.33 As shown in Figure 3, the content of cellulose I in the films increased with the initial proportion of the added cellulose whiskers from RC-W5 to RCW20. However, the overall crystallinity (χc) of the composite films was nearly invariant in the range of 40-44% (Table 1). This could be explained that the crystallinity of the nanocomposite films would increase with an increase of the cellulose whiskers content, but the increased cellulose whisker concentration could induce their aggregates and phase separation in our finding, leading to a decrease in the crystallinity. Thus, the overall crystallinity was basically the same for the films with different whisker concentrations.
Figure 3. X-ray diffraction profiles of cellulose whiskers (a), allcellulose nanocomposite films (b, RC-W20; c, RC-W15; d, RC-W10; e, RC-W5), and pure regenerated cellulose films (f).
Miscibility and Properties of Nanocomposite Films. The transparency is a useful criterion for the miscibility of the composite elements.34 The optical transmittance (Tr) of the composites at a wavelength of 800 nm is shown in Figure 5. The nanocomposite films embedded with the cellulose whiskers from 5 to 10 wt % exhibited good optical transmittance, indicating certain miscibility. With an increase in the content of the cellulose whiskers from 0 to 20 wt %, the Tr value for the composite films decreased from 87 to 49%. The decrease in the optical transmittance reflects the influence of the introduction of the whiskers on the interface structure. As mentioned above, cellulose whiskers are needlelike nanoparticles having a highly active surface that can easily aggregate when the concentration of cellulose whiskers is high. Thus, the whisker aggregates led to the scattering caused by large particles, resulting in the decrease of Tr. The results were in good agreement with those from SEM. The TGA curves of the RC and composites films are shown in Figure 6. As expected, the decomposition temperatures (Table 1) and remnant weight of the RC-W5 and RC-W10 composite films were slightly higher than that of the RC film, suggesting
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Figure 6. Temperature dependence on weight loss (TGA curve) of RC films and composite films. Figure 4. X-ray diffraction profile of sample RC-W10 and corresponding peak deconvolution process to estimate the crystallinity. (A) The experimental intensity curve, (B) the fitted intensity curve, and the diffraction peaks resolved from this (peaks 1, 4, and 5 are attributed to cellulose II, respectively; peaks 2, 3, and 6 are attributed to cellulose I, respectively), as well as the amorphous background (peak 7). Table 1. Results of Tensile Strength (σb), Young’s Modulus (E), X-ray Diffraction, and Thermal Gravimetric Analysis (TGA) with Films of All-Cellulose Nanocomposite Films Compared to Pure Regenerated Cellulose Films
materials
cellulose whiskers content (wt %)
σb (MPa)
E (MPa)
χc (%)
Td (°C)
RC RC-W5 RC-W10 RC-W15 RC-W20
0 5 10 15 20
87 112 124 118 117
3917 4975 5103 5696 5874
40 42 44 43 41
339 342 340 328 323
an increasing thermal stability. However, the thermal stability for the films with high cellulose whiskers content decreased, this was relative to phase separation as a result of the whisker aggregation. The results further support the conclusion from the Tr values. The mechanical properties of the composite materials are strongly influenced by the microstructure, and they could provide important information about the internal structure of materials. Stress-strain curves of the all-cellulose composite films and pure regenerated cellulose films are shown in Figure 7. When the cellulose whisker content increases from 0 to 10 wt %, the tensile strength (σb) increased from 87 to 124 MPa; thereafter, the tensile behavior was nearly constant (Table 1). Obviously, the incorporation of the cellulose whiskers into the
Figure 5. Optical transmittance (Tr) of all-cellulose composite films and pure regenerated cellulose films at 800 nm.
Figure 7. Stress-strain curves of all-cellulose composite films and pure regenerated cellulose films.
cellulose matrix led to the strengthening of the materials as a result of stiffness of the whiskers and the strong interactions between the cellulose matrix and the cellulose whiskers through hydrogen bonds. However, the composites containing more than 10 wt % cellulose whiskers exhibited a slight decrease in the tensile strength. Therefore, the optimal amount of cellulose whisker in the nanocopposite films should be 5-10 wt %, where a significant improvement in the thermal stability and the mechanics strength created. The increase in the cellulose whisker content led marked enhancement in Young’s modulus (E), from 3917 MPa for RC to 5674 MPa for RC-W20, whereas the elongation at break (εb) decreased slightly from 9.5 to 4.0%. The area under the stress-strain curve can be used as a measurement of the polymer toughness. The relatively large area of the RC-W5 film indicated its better toughness than others. In view of the above results, the cellulose whiskers strengthened the regenerated cellulose film, because of the role of the nanowhiskers embedded in cellulose matrix. Therefore, the selfreinforcement in the cellulose films was created in the solvent system of NaOH/urea aqueous solution. The all-cellulose films reinforced with 5-10 wt % cellulose whiskers possessed excellent mechanical and thermal properties, compared to RC. The well dispersed cellulose nanowhiskers play an important role in the improvement of the properties of the materials. Effects of Drawing on the Structure and Properties of the Films. There was random orientation of whiskers in the self-reinforced films, because no orientating force occurred during the drying process. To investigate the effect of drawing and drying process on the structure and properties of the nanocomposite films, the wet films (RC-W10) were dried through two other ways, as shown in Experimental Section, to
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Figure 8. X-ray diffraction diagram of the composite films: (a) RCW10, (b) RC-W10A, (c) RC-W10B. The X-ray beam was perpendicular to the film plane; the fiber axis is vertical.
Figure 11. Stress (σ)-strain (ε) curves of composite films dried from different process.
of the nanocopomsite films can be improved significantly after further optimization of the casting method with special attention to the machine design and the conditions for the draw process.
Figure 9. Equatorial intensity profile of the composite films obtained by exposing the X-ray beam perpendicular to the film surface. The term, “a.u.”, indicates arbitrary unit.
Figure 10. X-ray diffraction profile of azimuthal distribution of 200 reflection of RC-W10B (X-ray beam perpendicular to the film surface). The term, “a.u.”, indicates arbitrary unit.
obtain two composite films: RC-W10A and RC-W10B. Figures 8 and 9 show 2D WAXD patterns and equatorial intensity profile of the composite films under different drawing and drying conditions, respectively. All of the samples exhibited the diffraction peaks from cellulose II and cellulose I, as also indicated in the above results. Figure 10 shows X-ray diffraction profile of azimuthal distribution of 200 reflection of RC-W10B. The RC-W10B had a relatively high orientation, with the calculated Hermans’ orientation parameter being 0.30. RC-W10 and RC-W10A have no obvious orientation. Figure 11 shows the stress-strain curves of the composite films under different drawing conditions. Interestingly, the σb value of RC-W10B reached to 157 MPa, much higher than the composite films without drawing (RC-W10). Comparably, the tensile strength of RC-W10A decreased, with the increasing of the elongation at break to 8%. This could be explained that during the drawing process, the cellulose whiskers orient uniaxially along the drawing direction, leading to the enhancement of the strength of the composite films. It was noted that the mechanical properties of the nanocopomsite films could obviously be increased via a drawing process. The orientation of the RCW10B film was less than for many industrial films. It is anticipated that the orientation and the mechanical properties
Conclusion A suspension of cellulose nanowhiskers having an average length of about 300 mm and diameter around 20 nm was prepared successfully from cellulose with high molecular weight by using an aqueous sulfuric acid solution. The all-cellulose nanocomposite films could be fabricated by blending the cellulose nanowhiskers and cellulose solution in 7% NaOH/ 12% urea aqueous system precooled to -12 °C on the basis of the dependence of the cellulose solubility on the temperature and molecular weight. The all-cellulose nanocomposite films were consisted of cellulose whiskers (cellulose crystal I) and regenerated cellulose matrix, creating the self-reinforced materials. The self-reinforced films containing 5-10 wt % cellulose whiskers were miscible and exhibited excellent mechanical and thermal properties, compare to RC, as a result of the stiffness of the whiskers and strong interaction of the two components. The well-dispersed cellulose whiskers played an important role in the improvement of the properties of the cellulose materials. The biggest advantage of the new nanocomposite films was the fact that they were at the same time fully biobased, easily recyclable and biodegradable, yet reasonably strong materials. Acknowledgment. This work was supported by the National Supporting Project for Science and Technology (2006BAF02A09) and the National Natural Science Foundation of China (20674057 and 20474048). We also appreciate Prof. Masahisa Wada’s help on the XRD experiment.
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