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Magnetic Alignment of the Chiral Nematic Phase of a Cellulose Microfibril Suspension Fumiko Kimura,† Tsunehisa Kimura,*,†,‡ Moritaka Tamura,† Asako Hirai,§ Masaya Ikuno,§ and Fumitaka Horii§ Tsukuba Magnet Laboratory, National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan, Department of Applied Chemistry, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan, and Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Received October 1, 2004. In Final Form: November 21, 2004 Stable suspensions of tunicate cellulose microfibrils were prepared by acid hydrolysis of the cellulosic mantles of tunicin. They formed a chiral nematic phase above a critical concentration. External magnetic fields were applied to the chiral nematic phase in two different manners to control its phase structure. (i) Static magnetic fields ranging 1-28 T were used to align the chiral nematic axis (helical axis) in the field direction. (ii) A rotating magnetic field (5 T, 10 rpm) was applied to unwind the helices and to form a nematic phase. These phenomena were interpreted in terms of the anisotropic diamagnetic susceptibility of the cellulose microfibril. The diamagnetic susceptibility of the microfibril is smaller in the direction parallel (χ|) to the fiber axis than in the direction perpendicular (χ⊥) to the fiber axis, that is, χ| < χ⊥ < 0. Because the helical axis coincides with the direction normal (⊥) to the fiber axis, the helical axis aligned parallel to the applied field. On the other hand, the rotating magnetic field induced the uniaxial alignment of the smallest susceptibility axis, that is, χ| in the present case, and brought about unwinding of the helices.
Introduction Fibers with diamagnetic anisotropy align under static magnetic fields. Carbon fibers,1,2 carbon nanotubes,3-5 polyethylene fibers,6 cellulose fibers,7 and so forth undergo magnetic alignment. The alignment occurs so that the axis of the largest diamagnetic susceptibility lies parallel to the applied field. The alignment manner depends on the anisotropic nature of these fibers. For example, fibers with positive diamagnetic anisotropy (χa ≡ χ| - χ⊥ > 0) undergo uniaxial alignment, that is, they align with their fiber axes parallel to the applied field, while those with negative diamagnetic anisotropy (χa < 0) undergo planar alignment. Here χ| and χ⊥ are the diamagnetic susceptibilities in the directions parallel and perpendicular to the fiber axis, respectively. Carbon fibers and carbon nanotubes belong to the former, while polyethylene and cellulose fibers belong to the latter. Recently, one of the authors (T.K.) reported uniaxial alignment of the smallest diamagnetic susceptibility axis using a rotating magnetic field.8 It was shown experimentally and theoretically that a nylon fiber (χa < 0) * Corresponding author. Tel: +81 426 77 2845. Fax: +81 426 77 2821. E-mail:
[email protected]. † National Institute for Materials Science. ‡ Tokyo Metropolitan University. § Kyoto University. (1) Timbrell, V. J. Appl. Phys. 1972, 43, 4839. (2) Schmitt, Y.; Paulick, C.; Royer, F. X.; Gasser, J. G. J. Non-Cryst. Solids 1996, 139, 205-207. (3) Fujiwara, M.; Oki, E.; Hamada, M.; Tanimoto, Y.; Mukouda, I.; Shimomura, Y. J. Phys. Chem. A 2001, 105, 4383-4386. (4) Kimura, T.; Ago, H.; Tobita, M.; Ohshima, S.; Kyotani, M.; Yumura, M. Adv. Mater. 2002, 14, 1380-1383. (5) Casavant, M. J.; Walters, D. A.; Schmidt, J. J.; Smalley, R. E. J. Appl. Phys. 2003, 93, 2153-2156. (6) Kimura, T.; Yamato, M.; Koshimizu, W.; Koike, M.; Kawai, T. Langmuir 2000, 16, 858-861. (7) Sugiyama, J.; Chanzy, H.; Maret, G. Macromolecules 1992, 25, 4232-4234. (8) Kimura, T.; Yoshino, M.; Yamane, T.; Yamato, M.; Tobita, M. Langmuir 2004, 20, 5669-5672.
underwent uniaxial alignment under a rotating magnetic field. This technique is applied not only to fiber suspensions but also to many other systems possessing negative anisotropic diamagnetic susceptibilities. The magnetic alignment technique is applied not only to a simple suspension of fibers but also to a suspension having liquid crystalline structures. A number of studies on chiral nematic phases prepared with cellulosic fiber suspensions have been reported.9-11 Because χa of cellulosic fibers is negative, the chiral nematic axis (helical axis, perpendicular to the fiber axis) aligns parallel to a static magnetic field.12,13 On the other hand, a rotating magnetic field could bring about uniaxial alignment of cellulosic fibers, leading to unwinding of the helical axis. In the present paper, the magnetic alignments of a chiral nematic phase of a suspension of tunicate cellulose microfibrils are reported. Experimental Methods (1) Sample Preparation. Suspensions were prepared from tunicate cellulose (Halocynthia roretzi). The cellulose mantles were separated from the rest of the organs and were cut into small pieces. Small pieces of white mantles were prepared by repeating the following process four times: soaking in a 5% KOH solution for about 12 h followed by washing, bleaching at 80 °C for 2 h with a solution composed of 1 L of acetic acid buffer at pH of 4.8 with 3.4 g of NaClO2 added, and thorough washing. Then, the pieces were homogenized into millimeter-size fragments using a multiblender mill (Nihonseiki Kaisha Ltd., Tokyo) and mixed with 60% sulfuric acid and stirred at 50 °C for 5 h to obtain microfibrils. The acid was removed by centrifugation and (9) Revol, J.-F.; Bradford, J.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol. 1992, 14, 170. (10) Dong, X. M.; Kimura, T.; Revol, J.-F.; Gray, D. G. Langmuir 1996, 12, 2076-2082. (11) Araki, J.; Wada, S.; Kuga, S.; Okano, T. Langmuir 2000, 16, 2413-2415. (12) Revol, J.-F.; Godbout, L.; Dong, X. M.; Gray, D. G.; Chanzy, H.; Maret, G. Liq. Cryst. 1994, 16, 127-134. (13) Dong, X. M.; Gray, D. G. Langmuir 1997, 13, 3029-3034.
10.1021/la0475728 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005
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Figure 1. History of application of the 28-T magnetic field. prolonged dialysis against pure water. The sample was concentrated by evaporation, and the microfibrils were dispersed by sonication and diluted with distilled water to obtain sample suspensions. The concentration of the sample suspension was measured gravimetrically before and after evaporation of the water. The concentration of the sample suspension was 2.2 w/w%. To estimate the dimension of the microfibrils, a TEM measurement was performed. The values for length and width of microfibrils were 1-3 µm and 15-30 nm, respectively, with the aspect ratio being between 50 and 100.14 (2) Apparatus. FT-IR Spectroscopic Measurements. A droplet of the suspension was cast on an ATR crystal to form a film, and the crystal form of the microfibril was determined with a Nicolet IR spectrometer. The alignment of the suspension was characterized by polarized FT-IR transmission measurement. The suspension sample was sandwiched between two CaF2 windows that are transparent to the IR beam. Before and after the application of a 1-T magnetic field for 5 min, the dichroic spectra were recorded. The direction of the applied magnetic field was taken parallel to the polarization of the IR beam (referred to as 0°). Optical Measurements. A home-built optical apparatus was used for the determination of the optical anisotropy. The sample was poured into a UV/vis spectroscopic cell of 10-mm width, 40mm height, and 1-mm path length. The cell was set between polarizer and analyzer. The polarizer and analyzer were rotated together with parallel setup of the two polars, and the intensity of the transmitting light was measured. The wavelength of the light was 632.8 nm. Photomicrographs were taken between crossed polars with a Keyence digital VH-5000 microscope, with the cell kept upright. Static Magnetic Fields. Cryogen-free superconducting magnets of Sumitomo Heavy Industry and Japan Superconductor Technology were used to generate static magnetic fields of 1, 5, 10, and 12 T. The bore diameter was 10 or 30 cm. A home-built hybrid magnet at Tsukuba Magnet Laboratory generating a magnetic field of 28 T in a 5-cm bore was used. Because of the instrumental limitation, the history of the exposure at 28 T shown in Figure 1 was employed. Rotating Magnetic Fields. The sample was mounted on a homebuilt rotating apparatus that rotates with the vertical rotating axis in the center of a 5-T horizontal static field generated in a bore of 30-cm diameter. This gives the same effect as the rotation of the magnetic field. The sample was rotated at 10 rpm for 22.3 h.
Results and Discussion (1) Suspension before Phase Separation. The ATR IR spectra of a cast film prepared from the suspension before phase separation exhibited two bands at ca. 3275 and 706 cm-1 characteristic to cellulose Iβ.15 The bands between 1200 and 1000 cm-1 are assigned to the C-O (14) Ikuno, M.; Hirai, A.; Horii, F.; Donkai, N.; Tsuji, M. Polym. Prepr. Jpn. 2004, 53, 940. (15) Sugiyama, J.; Persson, J.; Chanzy, H. Macromolecules 1991, 24, 2461-2466.
Figure 2. Polarized FT-IR transmission spectra of the tunicate cellulose suspension before and after application of a 1-T magnetic field for 5 min.
stretching vibration. Figure 2 shows the polarized FT-IR transmission spectra of the suspension before and after the application of the magnetic field (1 T, 5 min). The transition moments corresponding to 1160, 1060, and 1035 cm-1 bands are parallel to the direction of the cellulose chain that coincides with the long axis of the cellulose microfibril. The alignment observed for the sample before the field exposure is due to the flow at sample preparation. These bands in the spectra taken immediately after a 5-min field exposure exhibit higher intensity for the 90° spectrum than for the 0° spectrum, indicating that the fiber axes of the cellulose microfibrils align perpendicular to the field direction. (2) Anisotropic Phase after Phase Separation. A well-mixed sample poured into a test tube underwent phase separation in about 10 days. In contrast, 5 days was not enough to reach the fingerprint texture. Figure 3a,b shows the photographs of the phase-separated sample taken under crossed polars. The bottom layer exhibits a fingerprint characteristic to the chiral nematic phase.9-11 The middle layer is also anisotropic, but a fingerprint texture was not observed (Figure 3b). Upon application of the horizontal field of 1 T for 5 h, the fingerprint texture in the bottom layer aligned with its helical axis being parallel to the applied field (Figure 3c). Due to the negative diamagnetic anisotropy of the cellulose microfibril, each fiber aligns perpendicular to the magnetic field. In the chiral nematic phase, the fiber axes lie perpendicular to the helical axis, and hence the helical axis aligns parallel to the applied magnetic field. To investigate the difference between the middle and bottom layers, optical measurements were carried out. The phase-separated sample contained in an optical cell (10-mm width × 40-mm height × 1-mm thickness) was exposed to the 5-T horizontal field parallel to the cell wall (a 10 mm × 4 mm surface) for 22.5 h. A home-built optical apparatus was used for the determination of the optical property. The polarizer and the analyzer were set parallel and rotated synchronously to measure the transmitting light intensity. The intensity observed by this setup is analyzed using the following equation:
I2(θ) ) I02{R2 cos4(θ - φ) + sin4(θ - φ) + (C/2)R sin2 2(θ - φ)} (1a) C ) cos(2πd∆n/λ)
(1b)
where θ is the rotation angle, and R and φ are fitting
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Figure 3. Crossed-polar images of the suspension sample with a cellulose concentration of 2.2 w/w%. (a) The sample after phase separation. Three layers are formed. (b) Magnification images of the bottom and middle layers of panel a. (c) Image taken after application of a 1-T magnetic field for 5 h. White arrows correspond to 1 mm.
Figure 4. Transmitting light intensity of the (a) bottom and (b) middle layers of the sample in Figure 3c measured as a function of the rotation angle of polarizers. The symbol b and the broken line indicate the experimental data and simulation, respectively.
parameters. The quantities λ, d, and ∆n are the wavelength of the He-Ne laser source, the film thickness, and the birefringence, respectively. By fitting (Figure 4), the retardation d∆n was determined to be 472 + mλ and 489 + m′λ nm for the middle and bottom layers, respectively, with m and m′ being integers. In the present measurement, we do not know whether they are different or not. However the intensity pattern is very similar although the appearance under the microscope is evidently different. A possible reason for the different appearance is that the helical pitch of the middle layer is too large or too small to be observed as a fingerprint texture. This could occur due to the distribution of the microfibril size, surface charge density, etc. Further study is needed to clarify the difference between these two anisotropic phases. For further analyses of the chiral nematic phase, we prepared a two-phase sample by removing the middle layer from the three-phase sample and exposed it to the magnetic field from various directions. The same optical cell (10-mm width × 40-mm height × 1-mm thickness) was used, and the observation was made through the 10-
Kimura et al.
Figure 5. Crossed-polar images of the two-phase sample exposed to a horizontal magnetic field tilted by the angles indicated in the figure, with respect to the observed cell wall surface. (a) Exposure of 12 T for 18 h, 0 T for 9 h, and 12 T for 48 h, parallel to the wall surface. (b) Exposure of 12 T for 41.5 h with tilting by 45°. (c) Exposure of 10 T normal to the wall surface for 38.5 h. A red arrow in panel a and a red circle in panel c indicate the direction of the magnetic field.
mm width × 40-mm height surface (observed cell wall surface). Before a magnetic field was applied, the sample exhibited a fingerprint texture, but its regularity was low. Figure 5a-c shows the aligned texture upon application of the horizontal field in the direction 0°, 45°, and 90° tilted with respect to the 10-mm width × 40-mm height surface, respectively. The pitch is ca. 140 µm for panel a and ca. 250 µm for panel b. A larger pitch at tilting by 45° indicates that the helical axis is also tilted with respect to the cell wall. No fingerprint texture is observed for panel c, indicating that the pitch is infinite, that is, the helical axis is perpendicular to the cell wall. Figure 6a shows the fingerprint texture obtained upon exposure to the vertical magnetic field. Here again, the helical axis aligns parallel to the applied field, and the pitch of the texture is the same as that for Figure 5a. Figure 6b shows the texture observed after a series of field exposure at 28 T whose time course is shown in Figure 1. Because the aligning rate is proportional to B2, the buildup of alignment should be quicker at 28 T than at 12 T by 5.4 times. The period of 2 h (a maximum available period for one operation at 28 T was 2 h) corresponds to 10.8 h for 12 T. Therefore, a 2-h exposure at 28 T is not enough to attain the high regularity observed in Figure 5a,b where the exposure was made at 10-12 T for over ca. 40 h. In addition, relaxation of the alignment occurs during the absence of field exposure. Figure 6c shows the disordered texture pattern due to the orientation relaxation. The sample was originally exposed to 10 T for 51.5 h and then left at 0 T for 18.6 days. Finally, the effect of a rotating magnetic field is presented. The texture before the field exposure is shown in Figure 7a. Upon application of a horizontal field of 5
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Figure 7. Effect of the rotating magnetic field on the texture. Crossed-polar images of the two-phase sample. (a) The initial texture pattern, (b) application of a 5-T static horizontal magnetic field (indicated by an arrow) for 15 h, and (c) application of a 5-T horizontal magnetic field with the vertical rotation (10 rpm) of sample b for 22.3 h with panels c1 and c2 being photographs taken under the indicated setting of the crossed polars.
Figure 6. Crossed-polar images of the two-phase sample. Panel a shows the texture alignment under a vertical magnetic field of ca. 10 T for about 50 h, panel b shows the alignment under a vertical 28-T magnetic field, and panel c shows the relaxation of alignment. Red arrows indicate the direction of the magnetic field.
T for 15 h, the alignment of the fingerprint texture was attained (Figure 7b). Then, the sample was rotated in the same magnet at 10 rpm for 22.3 h. The rotation axis was vertical. Polarizing microscope observations of the sample are shown in Figure 7 (panels c1 and c2). The fingerprint texture disappeared. To examine the alignment direction, the pair of crossed polars was rotated. When one of the polarizers coincides with the horizontal or vertical direction (Figure 7, panel c2), only very weak transmitting light was observed. In contrast, high transmittance is observed when the polarizers are tilted by 45°. These observations suggest that there is alignment in the horizontal or vertical direction. Because of the negative diamagnetic anisotropy of the individual cellulosic microfibrils composing the suspension, they tend to align uniaxially under a rotating magnetic field, with the fiber axis perpendicular to the rotating plane of the magnetic field. This tendency competes with the helical nature of the chiral nematic phase. If the effect of the applied field is strong enough, unwinding of the helices occurs. We therefore conclude that the dark view in panel c2 of Figure 7 is attributed to the alignment of the fiber axes in the
vertical direction. This induced phase could be regarded as nematic or chiral nematic with a very long pitch. Unwinding of helices in the chiral nematic phase is reported for cholesteric liquid crystal systems.16-19 In contrast to the present case, the helical axis of the chiral nematic phase of the PBLG system exhibits the smallest diamagnetic susceptibility. Therefore, the helices unwind upon exposure to the static magnetic field. Conclusions Magnetic alignment of the chiral nematic phase prepared by acid hydrolysis of the cellulosic mantles of tunicin was reported. The helical axis of the chiral nematic phase aligned in the direction of the applied static field, resulting in highly regular monodomains. Exposure to the rotating magnetic field caused unwinding of the helical axes to form nematic-like alignment. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area “Innovative utilization of strong magnetic fields” (Area 767, No. 15085207) from MEXT of Japan. LA0475728 (16) de Gennes, P. G. The Physics of Liquid Crystals; Oxford University Press: Oxford, 1974; Chapter 6. (17) Sackmann, E.; Meiboom, S.; Snyder, L. C. J. Am. Chem. Soc. 1967, 89, 5981-5982. (18) Meyer, R. B. Appl. Phys. Lett. 1969, 14, 208-209. (19) Pincus, P. J. Appl. Phys. 1970, 41, 974-979.
