Article pubs.acs.org/Biomac
Direct Fabrication of all-Cellulose Nanocomposite from Cellulose Microfibers Using Ionic Liquid-Based Nanowelding Hossein Yousefi,*,†,‡ Takashi Nishino,‡ Mehdi Faezipour,† Ghanbar Ebrahimi,† and Alireza Shakeri§ †
Department of Wood and Paper Science and Technology, Faculty of Natural Resources, University of Tehran, Karaj, Iran Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan § Department of Chemistry, Golestan University, Gorgan, Iran ‡
S Supporting Information *
ABSTRACT: All-cellulose nanocomposite was directly fabricated using nanowelding of cellulose microfibers as a starting material, in 1-butyl-3-methylimidazolium chloride (BMIMCl) as a solvent, for the first time. The average diameter of the reinforcing component (undissolved nanofibrils) in the nanocomposite made directly from cellulose microfibers (NC-microfiber) was 53 ± 16 nm. Owing to its high mechanical properties (tensile strength of 208 MPa and Young’s modulus of 20 GPa), high transparency (76% at a wavelength of 800 nm), and complete barrier to air and biodegradability, the NC-microfiber is regarded as a high multiperformance material. The NC-microfiber made directly from cellulose microfibers showed similar macro-, micro-, and nanostructures and the same properties as those made from solvent-based welding of ground cellulose nanofibers (NC-nanofiber). Omitting the step of cellulose nanofiber production makes the direct production of all-cellulose nanocomposite from cellulose microfibers easier, shorter, and cheaper than using cellulose nanofibers as starting material. The direct nanowelding of macro/micrometer-sized materials is theorized to be a fundamental approach for making nanocomposites.
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5 μm long, depending on the source of cellulose.3−7 These hierarchies are widely considered to be the basic structural unit of cellulose microfiber.3−11 There are nanoscale disconnections (gaps) within cellulose microfibers,3,12−14 as shown in Figure 1. The nanoscale gaps are naturally filled with noncrystalline cellulose, a continuous matrix of lignin and hemicelluloses. The lignin and hemicelluloses are extracted during the pulping and bleaching processes.9,10 Researchers have used these disconnections to isolate nanocellulose using several top-down approaches.4,5 We have also used these disconnections, based on the idea that cellulose solvent can deeply penetrate these gaps without a caustic dissolution of the fiber surfaces, if the solvent’s penetration-to-dissolution ratio is high enough. During the rinsing and drying processes, the selectively dissolved region between the gaps makes a weld layer among nanostructures; hence, a type of bio-based nanocomposite, called all-cellulose nanocomposite, is directly fabricated from cellulose microfibers. This study set out to verify this idea. All-cellulose composites and nanocomposites have recently emerged15,16 in which the matrix is cellulose that has been dissolved and then precipitated. The structure of the composite and nanocomposite depends on the size of the reinforcing part (undissolved cellulose). The matrix and reinforcement phases of all-cellulose composite and nanocomposite are completely
INTRODUCTION Nanocompositesmaterials made from at least one nanometersized material like nanofiber, nanowire, nanoplate, nanotubes, etc., as a starting material1are currently the subject of extensive worldwide research, as they offer many important advantages. However, most of the nanocomposites and the nanometer-sized materials are costly, time-consuming, and challenging to produce compared to those of conventional composites and micro/ macrometer-sized materials.2 Translating the advantages of nanometer-sized materials to macroscopic scales remains a challenge for a number of technical reasons, among which the most important are dispersion, distribution, incompatibility, and interaction of nanometer-sized materials in the nanocomposites.1,2 Because of this, researchers are seeking technological advancements that reduce manufacturing costs and facilitate processing, enabling the development of low-cost and scalable nanocomposites and nanometer-sized materials.1,2 Cellulose, the most abundant biopolymer in the world, has attracted much attention as a cheap, renewable, and biodegradable material with high reinforcing potential.3,4 Figure 1 shows a schematic model of the nanostructure of cellulose microfibers (diameter of 26 μm). This material is chemically composed of biosynthesized poly(β-1,4-D-anhydroglucopyranose) bound by inter- and intramolecular hydrogen bonds and organized into elementary fibrils separated by noncrystalline regions.3−7 The width of the elementary fibril is reported to vary between 3 and 7 nm.3−7 The elementary fibrils agglomerate tightly, forming a nanofibril 3−40 nm thick and up to © 2011 American Chemical Society
Received: August 18, 2011 Revised: September 21, 2011 Published: September 22, 2011 4080
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Figure 1. A schematic model of nanostructure of cellulose microfiber. The gap among nanostructures is a key point in this study. reflection was corrected for the Cu Kα doublet and an instrumental broadening. The crystallite size of cellulose was estimated by Scherrer’s equation:26
compatible with each other, allowing efficient stress transfer and adhesion at their interface/interphase. These types of composite and nanocomposite are well-known to be highly tough, fully bio-based, and fully biodegradable.15−23 There are several reports in the literature in which asproduced nanocellulose were partially dissolved and subsequently welded into all-cellulose nanocomposite (NC-nanofiber).21−23 In contrast to the studies found in the literature, we approached the direct production of all-cellulose nanocomposite using cellulose microfibers as starting material (NC-microfiber), partially dissolving the microfibers in the ionic liquid 1-butyl-3methylimidazolium chloride (BMIMCl) and welding the cellulose nanostructures. BMIMCl is well-known to be a green solvent, recyclable, highly efficient in cellulose dissolution, and to possess high thermal stability and very low vapor pressure.19,24
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(1) where D is the crystallite size, λ is the X-ray wavelength (0.15418 nm), θ is the diffraction angle for the (200) plane, and β is the corrected integral width. A field emission scanning electron microscope (FE-SEM, JSM6700F; JEOL Ltd., Japan) at an accelerating voltage of 5 kV was used to observe specimens. The specimens were dried in vacuum and coated with platinum/palladium (Pt/Pd). The diameters of 300 microfibers, ground nanofibers, and partly dissolved nanofibrils were measured on FE-SEM micrographs using AutoCAD software (Autodesk Co.). Air permeability was assessed in accordance with the TAPPI T460 standard (Gurley method) using a Gurley densometer (Gurley Precision Instruments Inc.) with 10 replicates. Commercial polyethylene and polypropylene were also tested for air permeability as controls. Optical transparency was quantitatively and qualitatively measured using a double-beam ultraviolet UV−vis spectrophotometer U-2000 (Hitachi Ltd., Japan) and a digital camera, respectively. The stress−strain curves of the specimens were measured using a tensile tester (Autograph AGS, Shimadzu Co. Japan) at room temperature. The specimens were 20 mm long and 4 mm wide. The load cell and extension rate were 1000 N and 1 mm/min, respectively. We calculated average values for the tensile strength (σ max), Young’s modulus (E), and strain at break (ε max) for eight specimens. The values were then normalized based on the native density of cellulose (1.5 g/m3) to omit the effect of density difference between specimens.
EXPERIMENTAL SECTION
Materials and Methods. Canola straw, a low-value agricultural byproduct, was used in this study. It was pulped and bleached as described elsewhere.22,23 The well-dispersed suspension of canola straw’s microfibers was filtrated by vacuuming, followed by drying at ambient temperature. It was then hot pressed at 100 °C, 2 MPa for 1 h, yielding an as-prepared microfiber sheet that was used as the starting material for making NC-microfiber. The microfiber sheets and BMIMCl (Merck and Co. Inc., Germany) were first oven-dried at 105 °C to avoid any negative effect of water on the dissolution of cellulose.19 The dried sheets were then immersed in BMIMCl at 85 °C for five dissolution times ranging from 5 min to 8 h. After the designated time, each sample was immersed in methanol (Nacalai Tesque Co., Japan) for 12 h, during which the methanol was replaced 10 times to rinse the BMIMCl. After rinsing, the sheets were cold pressed at 1 MPa, and drying was carried out under reduced pressure, resulting in NC-microfiber. NC-nanofiber was similarly produced using ground nanofibers as a control sample. The procedure of producing ground nanofibers has been described elsewhere. 22,23 The final samples were stored in a climate chamber at 60 ± 2% relative humidity and 25 °C prior to each test. Measurements. X-ray diffraction photographs were recorded on an imaging plate (IP) having a camera length of 37.5 mm. The specimen was irradiated by Cu Kα radiation, from a Rigaku RINT2000 (Rigaku Co.) at 40 kV, 20 mA, in a direction perpendicular to the specimen surface. The diffraction profile was detected using an X-ray goniometer with a symmetric reflection geometry in the range of 2θ = 10°−40° at a scanning speed of 1.2°/min. After subtracting the air scattering, the diffraction profile was curve-resolved into noncrystalline scattering and crystalline reflections using Fityk software. The apparent crystallinity was measured based on the ratio of crystalline and noncrystalline areas.18,25 The integral width of the 200
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RESULTS AND DISCUSSION Figure 2 shows the X-ray diffraction profiles of the microfiber sheet and composites prepared with partial dissolution times of 1 and 8 h using BMIMCl, together with their crystallinity and crystallite size. The diffraction profiles of the microfiber sheet showed typical peaks at 2θ = 15.2°, 16.1°, 22.6°, and 34.5° of cellulose Iβ, with a crystallinity of 69% and a crystallite size of 5.5 nm. As dissolution time increased, the noncrystalline scattering apparently increased, appearing as a broad scattering around 2θ = 18°. As partial dissolution time reached 8 h, the crystallinity and crystallite size decreased to 52% and 3 nm, respectively. These indicate that the dissolution of cellulose microfibers progressed with time and the noncrystalline cellulose was formed in the NC-microfiber. The gradual decrease in the crystallite size (from 5.5 to 3.0 nm) confirmed that solvent penetrated the gap and then partly dissolved the crystallites, as indicated in the literature.3,12−14 4081
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Figure 2. X-ray diffraction profiles of microfiber sheet and composite thereof, prepared with dissolution times of 1 and 8 h. The numbers in parentheses are crystallinity and crystallite size, respectively.
Figure 3 shows the FE-SEM micrographs of the tensilebroken surfaces of the microfiber sheet and composites prepared by partial dissolution in BMIMCl. The microfiber sheet was composed of entangled as-purified microfibers (Figure 3a). Figure 3b shows that some nanostructures (nanofibrils) appeared on the surface of the microfiber over an initial dissolution time of 5 min. It is seen that the solvent partly penetrated the gaps among the nanofibrils and began to separate them as individual cellulose nanofibrils. Here, nanofibrils with less than 100 nm thick are observed. After immersing for 0.5 h, the microfibers started to weld to each other, but some remained as individual fibers (Figure 3c). Over 2 h dissolution time, all microfibers were completely dismantled, and a unique structure had been created (Figure 3d). When we observed the disappearance of all microfibers and the creation of a fully consolidated and fine-textured structure, we were motivated to investigate the nanoscale structure of this material and the possibility of its being a nanocomposite. Based on the dissolution conditions used in this study, the microfibers were not completely dismantled over dissolution times shorter than 2 h in BMIMCl. Figure 4 shows two kinds of all-cellulose nanocomposites prepared by a direct route (NC-microfiber) and an indirect route (NC-nanofiber) at the (a) macro-, (b) micro-, and (c) nanoscales. In the direct route, all-cellulose nanocomposite was directly made from as-bleached microfibers of canola straw by partial dissolving/nanowelding. In the indirect route, the microfibers were first downsized to nanofibers by grinding, and the NC-nanofiber was then made by partially dissolving/ nanowelding of ground nanofibers in BMIMCl solvent. NCmicrofiber and NC-nanofiber had similar transparencies at the macroscale (Figure 4a), similar microfiber-free fully consolidated structures at the microscale (Figure 4b), and similar reinforcement-matrix structures at the nanoscale (Figure 4c). At the nanoscale (Figure 4c), partly dissolved nanofibrils surrounded by noncrystalline cellulose were observed in both NC-microfiber and NC-nanofiber. It is well-known that the gaps among cellulose nanostructures make a permeable path for fluids such as water and different solvents.11−14 Therefore, BMIMCl penetrated the gaps and dissolved the skin part of the nanostructures, as described above. The amount of noncrystalline phase increased as the result of the partial dissolution process. When this occurs, the cellulose chains are free to move
Figure 3. FE-SEM micrographs of tensile-broken surfaces of microfiber sheet and composites thereof, prepared by partial dissolution in BMIMCl: (a) microfiber sheet; (b) FE-SEM micrograph of microfiber surface affected by 5 min partial dissolution in BMIMCl. The partially isolated nanofibrils are seen (c) after 30 min and (d) after 2 h partial dissolution. The scale bar is 20 μm (a, c, d) and 2 μm (b).
in the solvent and entangle with other similarly dissolved chains in the other components. During the rinsing and drying processes, the chains lose their mobility: as they are brought into intimate contact, entanglement results in a weld. 27,28 This leaves a resolidified mass of entangled cellulose chains, which constitutes a nanowelded interface/interphase contact. The welding of nanostructures by noncrystalline phase has been previously applied to carbon nanotubes.30 The weld layer plays the role of a matrix, encapsulating the remained undissolved nanofibrils, filling the voids, and joining adjacent nanofibrils. Finally, NC-microfiber with a unique and fully consolidated structure can be obtained. 4082
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from 22 to 93 nm (average: 53 ± 16 nm). This average diameter is close to that of cellulose nanofibrils (40 nm) reported in the literature.3−7 The average diameter of ground nanofibers and undissolved nanofibers in the NC-nanofiber were 32 ± 10 and 25 ± 12 nm, respectively. Figure 5 demonstrates a schematic model for (a) direct and (b) indirect fabrication of all-cellulose nanocomposite from cellulose microfibers by partial dissolving/nanowelding. Light and dark green colors demonstrate noncrystalline cellulose (matrix phase) and undissolved nanofibril of cellulose I β (reinforcement phase), respectively. Orange shows the solvent, BMIMCl. The direct route (Figure 5a) started from cellulose microfibers. The as-bleached microfibers were expected to be noncollapsed with a clear cell wall and lumen in a simple model (left). The porosity of microfibers at micro- and nanoscale allows a facile penetration of BMIMCl, leading to homogeneous dissolution. 30 In the partial-dissolution step (middle), the solvent penetrated to the more accessible regions, namely, the gap among nanofibrils, which resulted in partly dissolved nanofibrils. During the rinsing and drying steps, nanowelding resulted in an all-cellulose nanocomposite with a unique and fully consolidated structure (right). In the indirect route (Figure 5b), the microfibers were first ground and turned into nanofibers. The ground nanofibers were then used as starting materials to make an all-cellulose nanocomposite through partially dissolving/welding the nanofibers. Air-permeability measurement revealed that the microfiber sheet had a structure highly permeable to air (42 ± 7 μm Pa−1 s−1), while NC-microfiber formed a complete barrier (0 μm Pa−1 s−1) like that of conventional packaging polymers (i.e., polyethylene and polypropylene that were used as control samples). The air permeability of NC-nanofiber was also 0 μm Pa−1 s−1. The superior barrier quality of NC-microfiber and NC-nanofiber is attributed to their fully consolidated structure, in which nanoscale structures were welded by noncrystalline cellulose. Figure 6 shows (a) the optical transparency and (b) the stress−strain curves of the microfiber sheet and NC-microfiber.
The average diameter of undissolved nanofibrils in the NCmicrofiber and NC-nanofiber is shown in Figure 4c. The microfibers of canola straw ranged from 10 to 60 μm thick (average: 26 ± 9 μm), which were downsized by partial dissolution to undissolved nanofibrils with diameters ranging
Figure 4. Similar structures of NC-microfiber and NC-nanofiber: (a) Photos of as-dried NC-microfiber and NC-nanofiber at macroscale. (b) FE-SEM micrographs of tensile-broken surface of NC-microfiber and NC-nanofiber at microscale. (c) Those at nanoscale. The diameter average for 300 partially dissolved nanofibril/nanofiber cores is shown.
Figure 5. Cross-sectional schematic model of (a) direct and (b) indirect fabrication of nanocomposite from cellulose microfibers by partial dissolving/nanowelding. Light green, dark green, and orange colors demonstrate noncrystalline cellulose (matrix phase), undissolved nanofibril of cellulose Iβ (reinforcement phase), and BMIMCl, respectively. 4083
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interface/interphase,16,18,31 and the smaller volume of voids, as well as the smaller amount of surface roughness16,18,31 of NC-microfiber compared to those of microfiber sheet. The NC-nanofiber sheet also showed a transparency as high as that of NC-microfiber, as shown in Figure 4a. Figure 6b shows the stress−strain curves of the microfiber sheet and the NC-microfiber. The NC-microfiber possesses far stronger mechanical properties than the microfiber sheet. The tensile strength (σ max), Young’s modulus (E), and strain at break (ε max) for the microfiber sheet were 22.6 MPa, 5 GPa, and 1.5%, respectively; for the NC-microfiber, these values gradually increased to 208 MPa, 20 GPa, and 9.8%, respectively. (The values of mechanical properties obtained by other dissolution times are listed in the Supporting Information.) In other words, the values for the NC-microfiber after a dissolution time of 8 h increased 9, 4, and 7 times, respectively, compared to those of the microfiber sheet. The solvent, BMIMCl, could effectively downsize the reinforcing component from microscale (in the microfiber sheet) to nanoscale (in the NC-microfiber). Smaller fibers resulted in greater surface area and higher strength. Additionally, the micro- and nanodisconnections among nanostructures led to very low mechanical properties for the microfiber sheet compared to those of the NCmicrofiber. In producing the NC-microfiber, the nanowelding process connected the nanostructures together and made an extended network of matrix-reinforcement shared by adjacent nanofibrils. This connectivity enhanced the structural integrity of the NC-microfiber, allowing good stress transfer through the continuous network of matrix-reinforcement. As a comparison, the allowed tensile strengths specified in standards for steel (S235, EN10025) and gray cast iron (Grade 20A, ASTM A48) are 320 and 150 MPa, respectively. The tensile strengths of wood−plastic composite (Plas TEAK Co.), chopped glass fiber (36 wt %)/unsaturated polyester composite,32 and poly(ether ether ketone)33 are reported to be 22, 117, and 92 MPa, respectively. NC-microfiber, with a tensile strength of 208 MPa, can be considered an extremely tough material. The highest σ max, E, and ε max for NC-nanofiber were 197 MPa, 19.5 GPa, and 12%, respectively, after a dissolution time of 30 min. Surprisingly, the maximum mechanical properties of nanocomposite made from microfibers as a starting material (NCmicrofiber) were similar to those of nanocomposite prepared with nanofibers as a starting material (NC-nanofiber). This similarity is reasonable given the corresponding similarity of the structures of NC-microfiber and NC-nanofiber, as shown in Figure 4. In addition, it is believed that the direct route to make all-cellulose nanocomposite is easier, cheaper, and shorter than the indirect one because the separate time-consuming and costly step of nanofiber production is omitted. Its promising properties make the NC-microfiber a highly functional material that has the potential to be used in lightweight materials, biomedical engineering, aerospace, sports equipment, and highly flexible electronic batteries and magnetic devices. The direct nanowelding of macro/micrometer-sized materials is theorized to be a fundamental approach for making nanocomposites. The key point is the creation of adequate noncrystalline regions on the surface of nanostructures to weld them using a proper nanowelding agent such as a solvent, ultrasound, or laser.
Figure 6. (a) Qualitative and quantitative optical transparency of microfiber sheet and NC-microfiber. The average thickness of sheets is 60 and 95 μm, respectively. (b) Stress−strain curves of microfiber sheet and NC-microfiber prepared with a dissolution time of 8 h.
There was a distinct difference in the transparency of the microfiber sheet and that of the NC-microfiber (Figure 6a, top), which was borne out by transparency measurements of 0.3% and 76% at wavelength of 800 nm for the microfiber sheet and NC-microfiber, respectively (Figure 6a, bottom). In other words, transparency increased 250-fold during the partial dissolving/nanowelding of the microfibers to make NCmicrofiber. As discussed, with the dissolution time less than 2 h, the microfibers were not completely dismantled; hence, the transparency of all-cellulose nanocomposite did not change considerably compared to that of microfiber sheet over this time. The high transparency can be attributed to the finer reinforcement dimension,16,31 the more effective 4084
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(9) Henriksson, M.; Berglund, L.; Isaksson, P.; Lindström, T.; Nishino, T. Biomacromolecules 2008, 9, 1579. (10) Berglund, L.; Peijs, T. MRS Bull. 2010, 35, 201. (11) Matsumura, H.; Sugiyama, J.; Glasser, W. G. J. Appl. Polym. Sci. 2000, 78, 2242. (12) Faivre, D. Nature Nanotechnol. 2010, 5, 262. (13) Nishiyama, Y.; Kim, U. J.; Kim, D. Y.; Katsumata, K. S.; May, R. P.; Langan, P. Biomacromolecules 2003, 4, 1013. (14) Isogai, T.; Saito, T.; Isogai, A. Cellulose 2011, 18, 421. (15) Nishino, T.; Matsuda, I.; Hirao, K. Macromolecules 2004, 37, 7683. (16) Gindl, W.; Keckes, J. Polymer 2005, 46, 10221. (17) Gindl, W.; Schoberl, T.; Keckes, J. Appl. Phys. A: Mater. Sci. Process. 2006, 83, 19. (18) Nishino, T.; Arimoto, N. Biomacromolecules 2006, 8, 2712. (19) Duchemin, B. J. C.; Mathew, A.; Oksman, K. Composite A 2009, 40, 2031. (20) Qi, H.; Cai, J.; Zhang, L.; Kuga, S. Biomacromolecules 2009, 10, 1597. (21) Soykeabkaew, N.; Chandeep, S.; Nishino, T.; Peijs, T. Cellulose 2009, 16, 435. (22) Yousefi, H.; Nishino, T.; Faezipour, M.; Ebrahimi, G.; Shakeri, A.; Morimone, S. Adv. Compos. Lett. 2010, 19, 190. (23) Yousefi, H.; Faezipour, M.; Nishino, T.; Shakeri, A.; Ebrahimi, G. Polym. J. 2011, 43. (24) Rogers, R. D.; Seddon, K. R. Science 2005, 302, 792. (25) Garvey, C. J.; Parker, I. H.; Simon, G. P. Macromol. Chem. Phys. 2005, 206, 1568. (26) Alexander, L. E. X-ray Diffraction Methods in Polymer Science; John Wiley & Sons: New York, 1969. (27) Troughton, M. J. Handbook of Plastics Joining: A Practical Guide; William Andrew Inc.: New York, 2008. (28) Haverhals, L. M.; Reichert, W. M.; Long, H. C. D.; Trulove, P. C. Macromol. Mater. Eng. 2010, 295, 425. (29) Changxin, C.; Yafei, Z. Nanowelded Carbon Nanotubes; Springer: New York, 2009. (30) Possidonio, S.; Fidale, L. C.; Seoud, O. A. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 134. (31) Nogi, M.; Iwamoto, S.; Nakagaito, N. A.; Yano, H. Adv. Mater. 2009, 21, 1595. (32) Mallick, P. K. In Polymer Matrix Composites; Talreja, R., Manson, J. A., Eds.; Elsevier: Amsterdam, 2001; p 322. (33) Fried, J. R. In Polymer Data Handbook; Marks, J. E., Ed.; Oxford University Press: Oxford, 2009; p 602.
CONCLUSIONS This study has verified a fundamentally different approach to produce all-cellulose nanocomposite from cellulose microfibers as a starting material using solvent-based nanowelding. This finding addresses several problems of all-cellulose nanocomposite fabrication: that it is costly, time-consuming, and challenging. A fully bio-based nanocomposite with high mechanical properties, transparency, and barrier quality, together with the potential ability to be made from very lowvalue cellulose resources, was proposed and tested in this study. Moreover, it is expected that the resulting nanocomposite will be fully biodegradable and highly recyclable. This study showed that the structure and functional properties of NC-microfiber are similar to those of NC-nanofiber. NC-microfiber is considered an economically and environmentally friendly product, the properties of which make it potentially highly useful in a large number of applications.
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ASSOCIATED CONTENT * Supporting Information The effect of dissolution time on the mechanical properties of microfiber sheet and (nano)composites. This material is available free of charge via the Internet at http://pubs.acs.org. S
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AUTHOR INFORMATION
Corresponding Author *Tel: +98 261 2249311; Fax: +98 2612249311; e-mail:
[email protected].
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ACKNOWLEDGMENTS The authors thank Prof. H. Matsuyama and Dr. Y. Ohmukai (Kobe University, Japan) for their cooperation in FE-SEM studies and analysis, Dr. M. Kotera and Ms. S. Morimune (Kobe University, Japan) for X-ray diffraction analysis, and Dr. S. Hedjazi, Mr. A. H. Heidari, and Mr. M. Ahmadi (University of Tehran, Iran) for their help in the pulping and bleaching process. The authors acknowledge the financial support by a Grant-on-Aid for Scientific Research from the Ministry of Education, Culture, Science, Sports and Technology, Japan (No. 20246100). Special Coordination Funds for Promoting Science and Technology, Creation of Innovation Centers for Advanced Interdisciplinary Research Areas (Innovative Bioproduction Kobe), MEXT, Japan, are also acknowledged. H. Y. also thanks the Iranian Ministry of Science for its financial support through a fellowship.
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