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Molecular alignment structure of dimeric liquid crystals
Materials Science and Engineering B 120 (2005) 37–40
Molecular alignment structure of dimeric liquid crystals Hiroaki Ido a, ∗ , Hirokazu Furue a, b , Teruki Niori b , Junji Watanabe c , Jun Hatano a a
Department of Materials Science and Technology, Faculty of Industrial Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan b Japan Science and Technology Agency, Tokodai, Tsukuba, Ibaraki 300-2635, Japan c Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Abstract While an antiferroelectric liquid crystal (AFLC) with dimeric molecules which take a bent molecular structure may show the phase transition from the isotropic liquid to the chiral smectic-C AFLC (SmC∗A ) phase via the nematic (N) phase, it has not been obvious yet why the SmC∗A and N phases may coexist in the phase sequence of the dimeric AFLC medium. In this study, the molecular alignment structures of the N and the SmCA phases of an achiral dimeric LC were researched in detail with a polarized Fourier transform infrared spectroscopy (FT-IR). The domain structure with two orientational directions is formed both in the SmCA and N phase, and the occupation ratio of two types of domain does not change in the phase transition. Since their alignment structures are basically same except for the existence of layer structure, their phases may coexist in the phase sequence of the dimeric AFLC. © 2005 Elsevier B.V. All rights reserved. Keywords: Dimeric liquid crystal; Antiferroelectric liquid crystal; SmCA ; Nematic; FT-IR
1. Introduction Antiferroelectric liquid crystals (AFLCs) are attractive for LC devices because of their unique characteristics such as high-speed response and monostability [1,2]. However, it is hard to fabricate a defect-free AFLC medium owing to the phase transition from the isotropic liquid directly to the chiral smectic-C AFLC (SmC∗A ) phase not via the nematic (N) phase. Recently, it has been reported that an AFLC with dimeric molecules which take a bent molecular structure may show the N phase [3,4]. But, it has not been obvious yet why the SmC∗A and N phases may coexist in the phase sequence of the dimeric AFLC. In this study, the molecular alignment structure of both liquid–crystalline phases has been researched in detail with a polarized Fourier transform infrared spectroscopy (FT-IR). For an investigation of the molecular conformation and orientation, the polarized FTIR can be used as a powerful tool [5,6]. The orientations of the function groups with which LC molecule forms can be
∗
Corresponding author. Tel.: +81 4 7122 9685; fax: +81 4 7123 9362. E-mail address: [email protected] (H. Furue).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.02.027
cleared by measuring the polarizer or sample rotation angular dependence of the absorbance.
2. Experimentals The LC material used in this research was OB10CB. The structural formula and the phase sequence are shown in Fig. 1. This LC has not only the N phase but also the achiral SmCA phase with the so called herringbone alignment structure. The LC alignment film used was the polyimide RN-1199 (Nissan Chemical Industries) [7,8]. A solution of polyimide was spun on BaF2 substrates coated with indium-tin-oxide (ITO) and then baked. After the thermal treatment, the substrates were rubbed. Then the LC material was injected in the isotropic phase via capillary action into an empty 5 m thick cell, in which the rubbing directions were set parallel. Using a Fourier transform infrared spectrophotometer FTIR-8400 (Shimadzu), polarized FT-IR spectra were measured as a function of the sample rotation angle. The measuring geometry is illustrated in Fig. 2. The rotation angle ω is defined as zero when the polarization direction of incident infrared light coincides with the rubbing direction of
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H. Ido et al. / Materials Science and Engineering B 120 (2005) 37–40
Fig. 1. Structural formula and phase sequence of OB10CB.
Fig. 5. Sample rotation angular dependence of absorbance for the carbonyl group of OB10CB.
Fig. 2. Geometries used in polarized FT-IR measurements.
Fig. 3. Representative absorbance spectrum of OB10CB.
the cell. Two absorption peaks were investigated: one is the cyano stretching peak (2320 cm−1 ) and another is the carbonyl stretching peak (1716 cm−1 ) which characterizes the right part and the left part of the LC molecule shown in Fig. 1, respectively. The spectra were measured at 110.0 and 85.0 ◦ C where the LC medium is in the N and SmCA phase, respectively.
3. Results and discussion The representative absorbance spectrum measured with the FT-IR is shown in Fig. 3. The cyano group and the carbonyl group give well-separated absorption peaks. Fig. 4
Fig. 4. Sample rotation angular dependence of absorbance for the cyano group of OB10CB.
demonstrates the absorbance as a function of the sample rotation angle for the cyano group in the N and SmCA phases. Since the maximum absorbance is nearly at ω = 0 and the absorbance curve is almost sinusoidal, the stretching direction of the cyano group is oriented to the rubbing direction. Thus, it is found that the mesogenic part including the cyano group is rather strongly directed to the rubbing direction by the surface anchoring of alignment film and furthermore this situation remain unchanged at the phase transition between the N phase and the SmCA phase. Fig. 5 shows an example of the absorbance for the carbonyl group. Although the sinusoidal curve of absorbance with the minimum at ω = 0 should be obtained in conventional monomeric liquid crystals [5,6], the absorbance curve obtained in this study is not sinusoidal. Therefore, it is guessed that a domain structure having two alignments of the mesogenic parts including the carbonyl group may form both in the N and SmCA phases of the dimeric LC, which takes a bent molecular conformation. In order to confirm this guess, we tried to calculatingly simulate the experimental result of Fig. 5. Assuming that the absorbance per unit area as a function of ω, A(ω), is given by A(ω) = a cos2 (ω − b) + c
(1)
where a and b are the degree of absorbance and the direction of the functional group stretching, respectively, and c a minimum value of the absorbance, the total absorbance for the domain structure having two alignments of b = θ and b = −θ, in which the volumes of two types of domains are V+θ and
Fig. 6. Simulation results of sample rotation angular dependence of absorbance for the carbonyl group of OB10CB: the dots and lines show the experimental and simulation results, respectively.
H. Ido et al. / Materials Science and Engineering B 120 (2005) 37–40
39
Fig. 7. Molecular alignment structure model of dimeric LC.
V−θ , respectively, is described as Atotal = a{V+θ cos2 (ω − θ) + V−θ cos2 (ω + θ)} + C
(2)
Fig. 6 demonstrates the simulation result fit for the experimental result shown in Fig. 5. In the N phase and the SmCA phase:
absorbance curve for the carbonyl stretching depends on the volume ratio of their domains at the measuring spot. It is concluded that since the molecular alignment structure is basically same between the N and SmCA phases except for the presence of smectic layer, both the phases may coexist in the phase sequence of the dimeric liquid crystal.
2 2 AN total = 0.01765 cos (ω + π/6) + 0.01040 cos (ω − π/6)
+ 0.5990
(3)
and 2 2 ASmCA total = 0.01119 cos (ω + π/6) + 0.00720 cos (ω − π/6)
+ 0.5790
(4)
respectively. Therefore, it is confirmed that the volume ratio of the domains (b = −π/6):(b = π/6) is about 8:5 in the case shown in Fig. 5 and moreover this ratio remain unchanged at the phase transition between the N phase and the SmCA phase. Assuming that the rotations of the mesogenic parts around their respective long axes is hindered due to the molecular conformation and the mesogenic parts align parallel to the substrate surface in terms of the surface anchoring, a schematic alignment structure of the dimeric LC is illustrated in Fig. 7. The mesogenic part including the cyano group is oriented to the rubbing direction owing to the rather strong azimuthal-surface-anchoring, but on the other hand the part including the carbonyl group tilts from the rubbing direction due to the bent molecular conformation. Since there are two tilt directions in the LC cell, two types of the domains may appear. As a result, while the absorbance for the cyano stretching as a function of the sample rotation angle is invariable regardless of the measuring spots of LC cell, the
4. Conclusions The molecular alignment structure of the N and SmCA phases in the dimeric LC of OB10CB were researched in detail with the FT-IR. The mesogenic part including the cyano group is oriented to the rubbing direction owing to the rather strong azimuthal-surface-anchoring, but on the other hand the part including the carbonyl group tilts from the rubbing direction due to the bent molecular conformation. Since the molecular alignment structure of the SmCA phase is basically similar to that of the N phase except for the presence of smectic layer, both the phases may coexist in the phase sequence of the dimeric liquid crystal.
Acknowledgment We gratefully thank Mr. H. Fukuro and Mr. H. Endoh of Nissan Chem. Ind. for supplying polyimide materials.
References [1] A.D.L. Chandani, E. Gorecka, Y. Ouchi, H. Takezoe, A. Fukuda, Jpn. J. Appl. Phys. 28 (1989) L1265.
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H. Ido et al. / Materials Science and Engineering B 120 (2005) 37–40
[2] A. Fukuda, Y. Takanishi, T. Isozaki, K. Ishikawa, H. Takezoe, J. Mater. Chem. 4 (1994) 997. [3] A. Yoshizawa, N. Ise, T. Okada, Ferroelectrics 214 (1998) 75. [4] I. Nishiyama, J. Yamamoto, J.W. Goodby, H. Yokoyama, Liq. Cryst. 29 (2002) 1409. [5] K.H. Kim, K. Ishikawa, H. Takezoe, A. Fukuda, Phys. Rev. E 51 (1995) 2166.
[6] K. Miyachi, J. Matsushima, Y. Takanishi, K. Ishikawa, H. Takezoe, A. Fukuda, Phys. Rev. E 52 (1995) R2153. [7] H. Furue, Y. Iimura, Y. Miyamato, H. Endoh, H. Fukuro, S. Kobayashi, Jpn. J. Appl. Phys. 37 (1998) 3417. [8] H. Furue, T. Takahashi, S. Kobayashi, Ferroelectrics 244 (2000) 75.
Molecular alignment structure of dimeric liquid crystals Hiroaki Ido a, ∗ , Hirokazu Furue a, b , Teruki Niori b , Junji Watanabe c , Jun Hatano a a
Department of Materials Science and Technology, Faculty of Industrial Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan b Japan Science and Technology Agency, Tokodai, Tsukuba, Ibaraki 300-2635, Japan c Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Abstract While an antiferroelectric liquid crystal (AFLC) with dimeric molecules which take a bent molecular structure may show the phase transition from the isotropic liquid to the chiral smectic-C AFLC (SmC∗A ) phase via the nematic (N) phase, it has not been obvious yet why the SmC∗A and N phases may coexist in the phase sequence of the dimeric AFLC medium. In this study, the molecular alignment structures of the N and the SmCA phases of an achiral dimeric LC were researched in detail with a polarized Fourier transform infrared spectroscopy (FT-IR). The domain structure with two orientational directions is formed both in the SmCA and N phase, and the occupation ratio of two types of domain does not change in the phase transition. Since their alignment structures are basically same except for the existence of layer structure, their phases may coexist in the phase sequence of the dimeric AFLC. © 2005 Elsevier B.V. All rights reserved. Keywords: Dimeric liquid crystal; Antiferroelectric liquid crystal; SmCA ; Nematic; FT-IR
1. Introduction Antiferroelectric liquid crystals (AFLCs) are attractive for LC devices because of their unique characteristics such as high-speed response and monostability [1,2]. However, it is hard to fabricate a defect-free AFLC medium owing to the phase transition from the isotropic liquid directly to the chiral smectic-C AFLC (SmC∗A ) phase not via the nematic (N) phase. Recently, it has been reported that an AFLC with dimeric molecules which take a bent molecular structure may show the N phase [3,4]. But, it has not been obvious yet why the SmC∗A and N phases may coexist in the phase sequence of the dimeric AFLC. In this study, the molecular alignment structure of both liquid–crystalline phases has been researched in detail with a polarized Fourier transform infrared spectroscopy (FT-IR). For an investigation of the molecular conformation and orientation, the polarized FTIR can be used as a powerful tool [5,6]. The orientations of the function groups with which LC molecule forms can be
∗
Corresponding author. Tel.: +81 4 7122 9685; fax: +81 4 7123 9362. E-mail address: [email protected] (H. Furue).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.02.027
cleared by measuring the polarizer or sample rotation angular dependence of the absorbance.
2. Experimentals The LC material used in this research was OB10CB. The structural formula and the phase sequence are shown in Fig. 1. This LC has not only the N phase but also the achiral SmCA phase with the so called herringbone alignment structure. The LC alignment film used was the polyimide RN-1199 (Nissan Chemical Industries) [7,8]. A solution of polyimide was spun on BaF2 substrates coated with indium-tin-oxide (ITO) and then baked. After the thermal treatment, the substrates were rubbed. Then the LC material was injected in the isotropic phase via capillary action into an empty 5 m thick cell, in which the rubbing directions were set parallel. Using a Fourier transform infrared spectrophotometer FTIR-8400 (Shimadzu), polarized FT-IR spectra were measured as a function of the sample rotation angle. The measuring geometry is illustrated in Fig. 2. The rotation angle ω is defined as zero when the polarization direction of incident infrared light coincides with the rubbing direction of
38
H. Ido et al. / Materials Science and Engineering B 120 (2005) 37–40
Fig. 1. Structural formula and phase sequence of OB10CB.
Fig. 5. Sample rotation angular dependence of absorbance for the carbonyl group of OB10CB.
Fig. 2. Geometries used in polarized FT-IR measurements.
Fig. 3. Representative absorbance spectrum of OB10CB.
the cell. Two absorption peaks were investigated: one is the cyano stretching peak (2320 cm−1 ) and another is the carbonyl stretching peak (1716 cm−1 ) which characterizes the right part and the left part of the LC molecule shown in Fig. 1, respectively. The spectra were measured at 110.0 and 85.0 ◦ C where the LC medium is in the N and SmCA phase, respectively.
3. Results and discussion The representative absorbance spectrum measured with the FT-IR is shown in Fig. 3. The cyano group and the carbonyl group give well-separated absorption peaks. Fig. 4
Fig. 4. Sample rotation angular dependence of absorbance for the cyano group of OB10CB.
demonstrates the absorbance as a function of the sample rotation angle for the cyano group in the N and SmCA phases. Since the maximum absorbance is nearly at ω = 0 and the absorbance curve is almost sinusoidal, the stretching direction of the cyano group is oriented to the rubbing direction. Thus, it is found that the mesogenic part including the cyano group is rather strongly directed to the rubbing direction by the surface anchoring of alignment film and furthermore this situation remain unchanged at the phase transition between the N phase and the SmCA phase. Fig. 5 shows an example of the absorbance for the carbonyl group. Although the sinusoidal curve of absorbance with the minimum at ω = 0 should be obtained in conventional monomeric liquid crystals [5,6], the absorbance curve obtained in this study is not sinusoidal. Therefore, it is guessed that a domain structure having two alignments of the mesogenic parts including the carbonyl group may form both in the N and SmCA phases of the dimeric LC, which takes a bent molecular conformation. In order to confirm this guess, we tried to calculatingly simulate the experimental result of Fig. 5. Assuming that the absorbance per unit area as a function of ω, A(ω), is given by A(ω) = a cos2 (ω − b) + c
(1)
where a and b are the degree of absorbance and the direction of the functional group stretching, respectively, and c a minimum value of the absorbance, the total absorbance for the domain structure having two alignments of b = θ and b = −θ, in which the volumes of two types of domains are V+θ and
Fig. 6. Simulation results of sample rotation angular dependence of absorbance for the carbonyl group of OB10CB: the dots and lines show the experimental and simulation results, respectively.
H. Ido et al. / Materials Science and Engineering B 120 (2005) 37–40
39
Fig. 7. Molecular alignment structure model of dimeric LC.
V−θ , respectively, is described as Atotal = a{V+θ cos2 (ω − θ) + V−θ cos2 (ω + θ)} + C
(2)
Fig. 6 demonstrates the simulation result fit for the experimental result shown in Fig. 5. In the N phase and the SmCA phase:
absorbance curve for the carbonyl stretching depends on the volume ratio of their domains at the measuring spot. It is concluded that since the molecular alignment structure is basically same between the N and SmCA phases except for the presence of smectic layer, both the phases may coexist in the phase sequence of the dimeric liquid crystal.
2 2 AN total = 0.01765 cos (ω + π/6) + 0.01040 cos (ω − π/6)
+ 0.5990
(3)
and 2 2 ASmCA total = 0.01119 cos (ω + π/6) + 0.00720 cos (ω − π/6)
+ 0.5790
(4)
respectively. Therefore, it is confirmed that the volume ratio of the domains (b = −π/6):(b = π/6) is about 8:5 in the case shown in Fig. 5 and moreover this ratio remain unchanged at the phase transition between the N phase and the SmCA phase. Assuming that the rotations of the mesogenic parts around their respective long axes is hindered due to the molecular conformation and the mesogenic parts align parallel to the substrate surface in terms of the surface anchoring, a schematic alignment structure of the dimeric LC is illustrated in Fig. 7. The mesogenic part including the cyano group is oriented to the rubbing direction owing to the rather strong azimuthal-surface-anchoring, but on the other hand the part including the carbonyl group tilts from the rubbing direction due to the bent molecular conformation. Since there are two tilt directions in the LC cell, two types of the domains may appear. As a result, while the absorbance for the cyano stretching as a function of the sample rotation angle is invariable regardless of the measuring spots of LC cell, the
4. Conclusions The molecular alignment structure of the N and SmCA phases in the dimeric LC of OB10CB were researched in detail with the FT-IR. The mesogenic part including the cyano group is oriented to the rubbing direction owing to the rather strong azimuthal-surface-anchoring, but on the other hand the part including the carbonyl group tilts from the rubbing direction due to the bent molecular conformation. Since the molecular alignment structure of the SmCA phase is basically similar to that of the N phase except for the presence of smectic layer, both the phases may coexist in the phase sequence of the dimeric liquid crystal.
Acknowledgment We gratefully thank Mr. H. Fukuro and Mr. H. Endoh of Nissan Chem. Ind. for supplying polyimide materials.
References [1] A.D.L. Chandani, E. Gorecka, Y. Ouchi, H. Takezoe, A. Fukuda, Jpn. J. Appl. Phys. 28 (1989) L1265.
40
H. Ido et al. / Materials Science and Engineering B 120 (2005) 37–40
[2] A. Fukuda, Y. Takanishi, T. Isozaki, K. Ishikawa, H. Takezoe, J. Mater. Chem. 4 (1994) 997. [3] A. Yoshizawa, N. Ise, T. Okada, Ferroelectrics 214 (1998) 75. [4] I. Nishiyama, J. Yamamoto, J.W. Goodby, H. Yokoyama, Liq. Cryst. 29 (2002) 1409. [5] K.H. Kim, K. Ishikawa, H. Takezoe, A. Fukuda, Phys. Rev. E 51 (1995) 2166.
[6] K. Miyachi, J. Matsushima, Y. Takanishi, K. Ishikawa, H. Takezoe, A. Fukuda, Phys. Rev. E 52 (1995) R2153. [7] H. Furue, Y. Iimura, Y. Miyamato, H. Endoh, H. Fukuro, S. Kobayashi, Jpn. J. Appl. Phys. 37 (1998) 3417. [8] H. Furue, T. Takahashi, S. Kobayashi, Ferroelectrics 244 (2000) 75.