Local Orientational Analysis of Helical Filaments and Nematic Director

Mar 13, 2014 - Director in a Nanoscale Phase Separation Composed of Rod-Like and. Bent-Core Liquid Crystals Using Small- and Wide-Angle X‑ray...
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Local Orientational Analysis of Helical Filaments and Nematic Director in a Nanoscale Phase Separation Composed of Rod-Like and Bent-Core Liquid Crystals Using Small- and Wide-Angle X‑ray Microbeam Scattering Yoichi Takanishi,†,* Haruhiko Yao,‡ Takuya Fukasawa,§ Kenji Ema,§ Youko Ohtsuka,⊥ Yumiko Takahashi,∥ Jun Yamamoto,† Hideo Takezoe,○ and Atsuo Iida∥ †

Department of Physics, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585, Japan § Department of Physics, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan ⊥ Center of Advanced Materials Analysis, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan ∥ Photon Factory, Institute of Material Structure Science, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan ○ Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan ‡

ABSTRACT: We analyzed the local nanostructure in binary mixtures of rod- and bentshaped molecules, n-pentyl-4-cyanobiphenyl (5CB) and 1,3-phenylene bis[4-(4-noctyloxyphenyliminomethyl) benzoates] (P-8-OPIMB), respectively, using small- and wide-angle X-ray microbeam and macrobeam scattering. From the orientational X-ray scattering patterns, we concluded that the nematic director of 5CB is almost parallel to the smectic layers dominated by bent-core molecules in Bx. Moreover, we observed oriented small-angle diffraction peaks (about 300 Å), which is close to the spacing of 5−7 layers, and also consistent with the width of a helical nanofilament textures as observed by freeze−fracture transmission electron microscopy. The kinetics in B4 was also discussed based on the contact experimental method.

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B4 phase. Based on the results that showed the relaxation time is independent of the wavenumber in the small wavenumber region, we concluded that the system had a spatial confinement with characteristic length of approximately 450−600 nm, which corresponds to the size of nematic domains in the Bx phase. Hence we concluded that the nanoscale phase separation occurs in these phases of binary mixtures. Moreover, Otani et al. studied the circular dichroic enhancement from 5CB molecules in this binary system caused by the 5CB’s helical organization around the B4 helical filament.9 Clark’s group studied the structure of another confined system consisting of 8CB and NOBOW(P-8-OPIMB).10,11 Thus, the nanoscale phase separation in mixtures of rod-like and bent-core mesogenic molecules has been extensively studied as a continuing target. In this paper, we studied the local structure in the Bx phase using small- and wide-angle X-ray microbeam and macrobeam scattering measurements in order to clarify the details of the

he observation of the chiral and/or polar structures in achiral bent-core liquid crystals has attracted much attention, and many studies have been reported.1−5 Previously, we studied the binary system composed of achiral rod-like molecules (n-pentyl-4-cyanobiphenyl, 5CB) and proto-type bent-core molecules (1,3-phenylene bis[4-(4−8-alkoxyphenyliminomethyl)-benzoates], P-8-OPIMB), and found that the B4 phase appears over a wide range of mixing ratios, and extends to 95 wt % of 5CB.6 Moreover, it was also found that another chiral separated phase named Bx appears below B4 in the binary mixtures of the 5CB-rich region. Takekoshi et al. measured thermal behaviors in the 5CB-rich mixtures by high resolution AC calorimetry,7 and found that the B4−Bx phase transition is closely connected with the isotropic−nematic (Iso−N) phase transition of pure 5CB. In other words, the B4− Bx phase corresponds to the transition in the microphaseseparated system, where P-8-OPIMB stays in B4 and 5CB transitions from Iso to N. They also found that the thermal anomaly almost disappears in a 50 wt % mixture, suggesting supercritical behavior. In addition, we employed dynamic light scattering (DLS) for characterizing the binary mixtures.8 Nematic-like director fluctuation was observed only in the Bx phase but not in the © 2014 American Chemical Society

Received: October 15, 2013 Revised: March 11, 2014 Published: March 13, 2014 3998

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W multilayer monochromator and was focused on an area of 3 × 4 μm2 using a Kirkpatrick-Baez focusing system with an angular divergence of 0.5 mrad. We used a charge coupled device (CCD) camera (Hamamatsu C4880−50) coupled with an image intensifier as a two-dimensional X-ray detector.12 The sample-to-camera distance was about 16 cm and 1.68 m for wide-angle and small-angle measurement, respectively. In particular, to reduce air scattering for the small-angle measurement, the path between the sample and the detector was evacuated. Since the intensity of the wide-angle diffuse scattering was very weak, the profiles were obtained by subtracting an intensity profile in an empty glass cell from the profiles of the sample cells. Exposure time was 3000−7000 s in both the small-angle and wide-angle diffraction measurement. We also performed ultrasmall-angle X-ray scattering measurement using another synchrotron radiation source. This experiment was performed at beamline BL03XU of SPring-8 (JASRI, Hyogo).13 Mixtures were introduced into 1 mm ϕ capillary glass tubes with glass thickness of 10 μm. The incident beam was monochromated to 6.2 keV (λ = 2 Å) using a double crystal monochromator. The beam size was 80 μm vertically and 180 μm horizontally, and the angular divergences were 6 μrad vertically and 12 μrad horizontally. The minimum observable scattering wavenumber, Q, was 4.19 μm−1. We used an imaging plate (Rigaku, R-AXIS VII) as a two-dimensional Xray detector, and the camera lengths were set to 6 m. Exposure time was 20−180 s, depending on the scattering intensity from the samples. For a freeze-fracture replication liquid-crystal mixture was placed on a well-type specimen support (JEOL, EB2378). It was heated above the isotropic transition temperature for several seconds and incubated on a temperature-controlled plate at 30 °C for 5 min before being quenched into slush nitrogen. Using a freeze−fracture equipment (JEOL, JFD9010) the frozen specimen was knife fractured at −120 °C and 10−4 Pa. The fracture surface was then shadowed with platinum−carbon at an angle of 45° and carbon was deposited perpendicular to the surface for backing. The replicated specimen on the support was immersed in chloroform overnight to remove the specimen. The fragments of the replica membrane were transferred onto acetone, acetone aqueous solution, and distilled water sequentially for spreading membrane and each fragment was mounted on a 300-mesh EM grid. Micrographs were taken on 5.9 × 8.2 cm Fujifilm FG electron-microscopic films in a transmission electron microscope (JEOL, JEM-1010). Figure 2 shows the two-dimensional wide-angle X-ray scattering patterns and microphotographs at the irradiated position (identified by an arrow) in the mixtures of (a) 80 wt % 5CB, (b) 90 wt % 5CB, and the pure (c) 5CB and (d) P-8OPIMB at room temperature. Insets of the X-ray scattering profiles show the two-dimensional small-angle X-ray scattering profiles observed at the same camera length (16 cm). Since the small-angle diffraction peak positions of the 80 and 90 wt % 5CB mixtures agree with that of the B4 phase of pure P-8OPIMB, the diffraction peaks correspond to the smectic layer thickness of B4-rich domains. It was found that the B4-rich domains are partially oriented at the micrometer-scale. Wideangle diffuse scattering comes mainly from the nematic director orientation because of the rich concentration of 5CB. These diffuse peaks are also oriented, almost parallel to the B4 layer diffraction peaks. These results clearly indicate that the nematic

nanostructure. In the previous preliminary conventional X-ray diffraction measurement, the difference between the B4 and Bx phases could not be detected clearly and precisely i.e., the diffraction peak positions are almost independent of temperature and 5CB concentration.6 To our regret, the local information with respect to the molecular orientation was lost by the spatial averaging because of the macroscopic measurements. In the present study, we succeeded in performing X-ray scattering measurements from a micrometer-sized domain, and obtained important information for nanoscale phase separated structures if they are obtained. Moreover, we measured ultrasmall-angle X-ray scattering in order to obtain information for longer periodic structures in the nanoscale phase separated system. On the basis of our results from X-ray scattering and freeze-fracture transmission electron microscopy (FFTEM) experiments and previous works synthetically, we conclusively obtained the structure details of the nanoscale phase-separated Bx phase more precisely. Finally, the kinetics of helical filament reorientation in B4 is also discussed based on the contact experimental method. The achiral bent-core-shaped and rod-like compounds used in the mixture were P-8-OPIMB (Iso 173 °C B2 152 °C B3 140 °C B4)1 and n-pentyl-cyanobiphenyl (5CB, Iso 35 °C N 22.5 °C Cryst.), respectively. Chemical structures of each compound and the binary phase diagram are shown in Figure 1. Using

Figure 1. Chemical structures of P-8-OPIMB and 5CB (a) and the binary phase diagram (b).

capillary suction, mixtures with various mixing ratios were inserted into 25-μm-thick sandwich cells made of 80-μm-thick glass substrates coated with rubbed polyimide. Although the rubbed polymer was not effective for alignment, relatively uniform domains with several hundreds of micrometers were obtained by a gradual cooling process (less than −0.1 °C/min) from the isotropic phase. X-ray microbeam experiments were performed at the beamline BL-4A of the Photon Factory (Tsukuba). The optical geometry was shown in a previous paper.12 The incident beam was monochromated to 14 keV (λ = 0.88 Å) using a double Si/ 3999

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Figure 2. Two-dimensional wide-angle X-ray scattering profiles and microphotographs at the irradiated position (identified by arrows) of 80 wt % (a, a′) and 90 wt % (b, b′) 5CB mixtures at the different irradiated position, and pure 5CB (c) and P-8-OPIMB (d) at the room temperature, respectively. Two-dimensional small-angle X-ray scattering profiles observed at the same camera length are shown in insets. Red broken line indicates the B4 layer normal direction, which is parallel to the 5CB director orientation determined by wide-angle diffuse scattering.

Figure 3. Two-dimensional wide-angle X-ray scattering profiles and microphotographs at the irradiated position (identified by arrows) of 80 wt % 5CB mixtures at (a) 26 °C (Bx), (b) 46 °C by the heating process (B4), and (c) 26 °C by the cooling process from 46 °C. Twodimensional small-angle X-ray scattering profiles observed at the same camera length are shown in insets like Figure 2.

that the B4-rich anisotropic domains (helical filament structures) determined the 5CB nematic director orientation at the phase transition from B4 to Bx. Figure 4 shows two-dimensional small-angle X-ray microbeam scattering profiles in (a) 0, (b) 20, (c) 40, and (d) 60 wt % 5CB mixtures. The outer ring corresponds to the B4 smectic

director orientation of the 5CB rich region is parallel to the B4 layers of the P-8-OPIMB-rich region in the nanoscale phase separated structure. This result qualitatively agrees with the models in ref 9, i.e., 5CB molecules orient along the groove of helical nanofilaments. Next, we compared the structure in B4 with that in Bx of the mixtures. Figure 3 shows the two-dimensional wide-angle X-ray scattering profiles and microphotographs at the irradiated position (identified by an arrow) of the 80 wt % 5CB mixture at 26 °C (Bx) and 46 °C (B4). Insets of the X-ray scattering profiles show the two-dimensional small-angle X-ray scattering profiles observed at the same camera length. The small-angle diffraction profiles did not change with the phase transition from Bx to B4, indicating that the orientation of the B4 smectic layers (nanofilaments14) in the P-8-OPIMB-rich region was maintained. In contrast, the wide-angle scattering profiles changed with the phase transition. Although the wide-angle scattering profiles in Figure 3a indicated anisotropic director orientation in Bx, the wide-angle scattering intensity in Figure 3b was independent of the azimuthal angle in B4. This fact clearly indicates that the 5CB-rich region had isotropic order. The changes in the texture and the optical birefringence with the phase transition shown in the right side of each figure also originates from the change between nematic (Bx) and isotropic (B4) orders of 5CB-rich regions as observed by the wide-angle scattering. This result is consistent with the structure model obtained from our DLS measurements in a previous study,8 and also in other studies.9 Upon cooling from 46 to 26 °C, the wide-angle scattering profiles exhibited anisotropic director distribution again as shown in Figure 3c, and the director orientation reverted to the original one. This indicates that the B4 smectic layers (helical filaments) in the P-8-OPIMB-rich region remain unchanged in our cells at least below 46 °C, and

Figure 4. Two-dimensional small-angle X-ray microbeam scattering profiles in (a) 0, (b) 20, (c) 40, and (d) 60 wt % 5CB mixtures. Outer diffraction ring corresponds to the B4 smectic layer thickness (ca. 45 Å) in the P-8-OPIMB rich region, and partially oriented patterns are observed except for the pure P-8-OPIMB ((a)0 wt % 5CB mixture). 4000

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layer thickness (approximately 45 Å) in the P-8-OPIMB-rich region, and partially oriented patterns were observed for all samples except for the pure P-8-OPIMB (0 wt % 5CB mixture). Moreover, very broad peaks were observed in the small-angle region (again, with the exception of the pure P-8-OPIMB). Figure 5 shows the radial scattering intensity profiles of the

Figure 5. Radial scattering intensity profiles of the mixtures, obtained by integrating the two-dimensional intensity distribution of Figure 3; (a) 0, (b) 20, (c) 40, and (d) 60 wt % 5CB mixtures. Purple and blue arrows indicate the broad peaks corresponding to about 300 and 150 Å, respectively.

Figure 6. (a) Radial scattering intensity profiles of mixtures at the room temperature integrated from the two-dimensional intensity distribution obtained from the macroscopic ultra-small-angle X-ray scattering measurement. Black line indicates the scattering intensity obtained from the empty capillary. Inset of part a shows the radial scattering intensity profiles of 20 and 40 wt % 5CB mixtures in another capillary tubes at the room temperature. (b) Log−log plot of part a. Dotted, broken, and chain lines correspond to Q−1, Q−1.5, and Q−4 respectively.

mixtures, obtained by integrating the two-dimensional intensity distributions in Figure 4. These results indicated two broad peaks at Q values of about 0.021 and 0.042 Å−1, which correspond to a periodicity of about 300 and 150 Å, respectively. Through careful examination of Figure 4, we found that the broad peaks show anisotropic distribution, i.e., stronger along the direction parallel to the B4 smectic layer diffraction peaks. Hence, it is possible to consider that these periodicities corresponding to the broad peaks are related to the B4 layer and the anisotropy is attributed to the helical filament orientation. In order to analyze the X-ray scattering in the small-angle region further, we also performed ultrasmall-angle macrobeam X-ray scattering measurement at beamline BL03-XU of SPring8. The radial scattering intensity profiles of the mixtures at the room temperature, integrated from the two-dimensional intensity distributions, are shown in Figure 6a. In the B4 phase of the pure P-8-OPIMB, the scattering profile is the same as that in an empty capillary (black line).6 In the mixture, however, two broad but clear peaks are observed and their corresponding periodicities are qualitatively consistent with the results obtained by the microbeam experiments shown in Figures 4 and 5. Through precise analysis of the profiles, it was found that the peak positions observed in the 20 and 60 wt % 5CB mixtures were almost the same as that in the 40 and 80 wt % 5CB mixtures, respectively. Additionally, the peaks change discontinuously when the mixing ratio of 5CB changed from 40 wt % to 60 wt %. Moreover, in the 20 wt % 5CB mixture, another weak peak is also observed at Q ∼ 0.006 Å−1. This peak was also observed in the 40 wt % 5CB mixture in another capillary sample, as shown in the inset of Figure 6a. This peak corresponds to a periodicity of about 100 nm. Although the corresponding structure is unknown, it will be related with the B4 helical filaments because the peak intensity decreases with increase in the 5CB concentration.

Figure 6a shows the log−log plot of Figure 6b. The profiles are very similar to those observed in rod-like complexes.15,16 Here we also try to analyze our scattering results based on refs 15 and 16. In the 20 and 40 wt % 5CB mixtures, the scattering intensity I(Q) is proportional to Q −1 at Q values lower than 0.03 Å−1, which is characteristic of rod-like (or fibril-like) objects.15,16 On the other hand, in the 60 and 80 wt % 5CB mixtures, I(Q) ∼ Q −1.5, which is characteristic of the mass fractal aggregates, and the rod-like objects are considered to be partially collapsed with increase in the 5CB concentration.17 At Q values larger than 0.02 Å−1, I(Q) varies like Q −4, which corresponds to the Porod scattering law associated with the sharp interface. The cross over from a Q −1 to a Q −4 variation is around 0.02 Å−1, which corresponds to the filament width mentioned later. What kinds of periodic structures correspond to the broad peaks observed at the ultrasmall-angle? To answer this, it is important to compare these results and the FFTEM images of the binary mixtures of P-8-OPIMB and 5CB, as well as images reported by Hough et al.14 Figure 7 shows a FFTEM image of 50 wt % 5CB mixture. The image shows that several helical filaments are bundled, with the width of each filament about 250−300 Å, which is almost the same as the periodicity corresponding to the broad peaks in the ultrasmall-angle X-ray scattering measurement. Some unclear areas (for example, the area enclosed by a yellow dashed line in Figure 7) were observed. These are considered to be the 5CB-rich (nematic order) region. Similar images are observed in the binary system of 8CB and NOBOW(P-8-OPIMB),10 although 8CB shows the 4001

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the density contrast weakens and the small-angle scattering disappears. Let us summarize the relation between 5CB nematic director orientation and B4 helical filaments. Considering these results and our previous DLS results,8 we can estimate that in the Bx phase, 5CB nematic-rich domains with sizes of 450−600 nm are separated by B4 helical nanofilaments consisting of 5−7 smectic layers, with thickness of about 300 Å, and with the nematic director almost parallel to the B4 helical filaments. This model can explain the enhancement in CD intensity of 5CB of the binary mixture presented by Otani et al.9 Our nanophase separation model is shown schematically in Figure 8. For the

Figure 7. Freeze−fracture transmission electron micrograph showing the binary mixture of P-8-OPIMB with 50 wt % 5CB taken with a magnification of 60 000. Broken lines are eye guidelines for the direction of helical nanoflaments. The area enclosed by a yellow dashed line is considered to the 5CB-rich (nematic order) region.

smectic A phase. Moreover, this is also consistent with the microbeam X-ray scattering results that showed the direction of small-angle scattering diffuse peaks observed was almost parallel to the smectic layer diffraction peak direction. This width of 250−300 Å corresponds to 5−7 times the B4 smectic layer thickness. The broadness of the peaks may originate from (i) the distribution of filament widths or thicknesses and (ii) the short correlation length, as observed in the FFTEM images. In addition, Hough et al. presented the structure of the B4 phase of the pure bent-core molecules by high-resolution X-ray diffraction measurement.14 They concluded that the B4 phase is a nanohelical filament phase, with filament width of 25 nm (250 Å), which also supports our results. Since the periodicity appears to slightly increase with increasing 5CB ratio and since the obtained peaks are broad (fwhm ∼ 0.016 Å−1), there is a possibility that the 5CB and bent-core molecules are not completely phase separated, and that the helical filaments in the B4-rich region will be slightly swollen by 5CB, although its periodicity has a distribution. This result suggests that this binary system causes fundamentally the nanoscale phase separation, but rod-like and bent-core molecules interact with each other. Only single layer sheet can easily twist even with large widths, but bundled layer sheets with large width will need large amounts of elastic energy for twist deformation. This size (about 25−30 nm) may be the limit of the twisting deformation in bundled smectic helical filaments, which may correspond to 5−7 layers. However, one question arises: why are such small-angle peaks not observed in the pure B4 phase? We do not have a clear answer at the present stage, but the FFTEM images seem to give an idea. In the FFTEM images of the binary mixtures, helical filaments with clear boundaries were observed, which indicated clear boundaries between filaments. A similar result was reported by Clark’s group in the binary system consisting of 8CB and NOBOW(P-8-OPIMB),10 as mentioned above. On the other hand, in the FFTEM image of the B4 phase of pure P8-OPIMB,14,18 clear filament sections are not observed, and saw-blade-like textures are frequently observed. It suggests that these textures come from the closed packing of nanohelical filaments, causing the boundary between filaments and the corresponding higher-order structure to disappear. As a result,

Figure 8. Schematic image of nanophase separation of the binary system of bent-core and rod-like molecules in B4 and Bx.

origin of the relation between nematic director orientation and helical filament orientation, however, based on the microbeam X-ray scattering results shown in Figures 2 and 4, we speculate that the surface of the helical filaments has anisotropic symmetry, which induces alignment with the nematic director. From Figure 3, we concluded that B4 smectic layers in the P8-OPIMB-rich region are memorized, at least below 46 °C, and that the B4 rich anisotropic domains (helical filament structures) determine the 5CB nematic director orientation at the transition from the B4 to Bx. This memory effect suggests that the B4-rich domain is in a slow dynamic state or glassy state,19,20 working as a template, and that the surface of the nanofilament has the ability to align with the 5CB nematic director. In order to observe whether B4 or Bx texture changes occur with the thermal process or not, we observed a texture change in the contact cell between 5CB and P-8-OPIMB during a thermal cycle process. First, we held the sample at 50 °C for half a day and then cooled to 30 °C. Under these conditions, the texture in the B4 domain side hardly changed as shown in Figure 9, parts c and d. Moreover, we held the sample at 50 °C for an addition day and then cooled to 30 °C, without observing any changes, as shown in Figure 9, parts e and f. These results support the memory effect of orientation in the B4 helical nanofilament in the B4-rich region at about 50 °C, as shown in Figure 3. In contrast, this tendency changes when the 4002

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Figure 9. Microphotographs of the contact cell between pure P-8OPIMB and 5CB depending on the temperature cycling process. (a) Pure P-8-OPIMB is introduced from the right side, and the texture hardly changes up to 120 °C. (b) After cooling until 50 °C, 5CB is introduced. After half a day, the boundary hardly changes (c), but the Bx phase was observed near the boundary region at 30 °C (d). This nanoscale phase separation did not expand even after keeping at 50 °C for 1 day, as shown in parts e and f.

Figure 10. Microphotographs of a contact cell between pure P-8OPIMB and 5CB depending on the temperature cycling process. (a) Just after heating up to 80 °C from the situation shown in Figure 7f. After keeping the cell for 1 night at 80 °C, the boundary changes as shown in b, and 5CB seems to erode the B4 domain. When the sample is cooled down to 30 °C, the Bx domain is dominated in the observed region (c), and at 40 °C, chiral separated domains are observed under the depolarized condition (d). After heating up to 80 °C and keeping the sample for 10 h and cooling to 40 °C again, the pattern of chiral separated domain changes, as shown in part e.

samples are held at 80 °C. Parts a and b of Figures 10 show the microphotographs under the decrossed position of polarizers just after heating to 80 °C and after holding for 10 h, respectively. It was found that the B4 domain eroded and the boundary clearly changed. When the temperature was decreased to 30 °C, the texture in the Bx phase also clearly changed, as shown in Figure 10(c). When the temperature was increased to 80 °C again, chiral separated domain patterns with different optical rotatory power clearly changed after holding for 10 h as compared in Figure 10, parts d and e, which implies that the B4 helical nanofilament can thermally diffuse and reorient with isotropic order in the 5CB-rich region at 80 °C, but not at 50 °C. Sasaki et al. studied the thermal analysis for these binary mixtures.19 In their results, thermal anomalies occur around 376 K (103 °C) in a 70 wt % 5CB mixture. Although this temperature is slightly higher than the temperature (80 °C) we observed filament diffusion, it is qualitatively concluded that some molecular reorientation and chiral separated domain reformation with the appearance of thermal anomalies may occur. In conclusion, the local nanostructure in binary mixtures of 5CB and bent-core molecules was investigated using small- and wide-angle X-ray scattering. From the orientational X-ray diffraction patterns from the ordered microdomains, it was found that the nematic director in the 5CB-rich region is almost parallel to the smectic layers dominated by bent-core molecules in Bx. Moreover, we observed oriented small-angle diffraction peaks corresponding to longer periodicity (approximately 300 Å), which is close to the spacing of 5−7 layers, and almost consistent with the width of helicalnanofilament textures as observed by freeze−fracture transmission electron microscopy. The kinetics of helical filament reorientation in B4 was also discussed based on texture observation in contact cells.

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AUTHOR INFORMATION

Corresponding Author

*(Y.T.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. J. Watanabe for supplying bent-core molecules. This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Area “Non-equilibrium soft matter physics”, Scientific Research (A). This work was partially carried out under approval of the Photon Factory Advisory Committee (Proposal No. 08G072), and the second hutch of SPring-8 BL03XU constructed by the Consortium of Advanced Softmaterial Beamline (FSBL), with proposal number 2010A7205.

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