CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201301142
Low-Temperature Properties of Polymer-Stabilised Liquid-Crystal Blue Phases Gihwan Lim,[b] Yasushi Okumura,[a] Hiroki Higuchi,[a] and Hirotsugu Kikuchi*[a] The temperature dependences of the Kerr coefficient and the response time in the electrooptical effect of polymer-stabilised blue phases (PSBPs) of liquid crystals (LCs) with various polymer concentrations are investigated in a wide temperature range including temperatures lower than room temperature. The Kerr coefficients are found to abruptly decrease at low temperature, and the response time–temperature relation obeys the Arrhenius equation. For comparison of the Kerr
effect and molecular rotation at low temperature, various physical properties such as permittivity, rotational relaxation time and dielectric relaxation strength of the PSBPs are investigated. The electrooptical response times and the dielectric relaxation times show different temperature dependences, and rotation of LC molecules in PSBPs was sufficiently active at low temperature and not strongly affected by the polymer.
1. Introduction Blue phases (BPs) are liquid-crystal (LC) phases that occur for chiral nematic liquid crystals in a narrow temperature range of typically about 2–3 K between a chiral nematic and an isotropic phase, and are optically isotropic without birefringence because of their cubic lattice structure consisting of double twisted cylinders.[1, 2] If an electric field is applied to BPs, a field-induced birefringence obeying the Kerr law is observed due to both electrostriction of the BP lattice and a reorientation of the local director of the LC within the BP lattice.[2, 3] Furthermore, BPs are known to show tuneable selective reflection of light.[4–6] However, their narrow temperature range has been a problem for their practical applications. Kikuchi et al. found that the structure of BPs could be stabilised by polymer networks without losing the electrooptical (EO) response, and the temperature range of stable BPs was successfully extended to more than 60 K.[7] These polymer-stabilised blue phases (PSBPs) have attracted attention as new-generation LC materials, because they show fast EO response (< 1 ms) and require no surface-alignment treatment in the cell due to their optical isotropy, and they have been studied as LCs for display devices.[8–17] Nevertheless, PSBPs still have some problems to be solved, such as high driving voltage, residual birefringence, and lowering of electrooptical (EO) performance at a low temperature. The strong temperature dependence of the Kerr coefficient [a] Prof. Y. Okumura, Prof. H. Higuchi, Prof. H. Kikuchi Institute for Materials Chemistry and Engineering Kyushu University 6-1 Kasuga-koen, Kasuga Fukuoka 816-8580 (Japan) E-mail: [email protected] [b] G. Lim Department of Applied Science for Electronics and Materials Kyushu University 6-1 Kasuga-koen Kasuga, Fukuoka 816-8580(Japan) Fax: (+ 81) 92 583 7789
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
and response time tEO[12–14] are emerging as profound hindrances to the application of PSBPs. From the viewpoint of practical use in diverse environments, it is important to thoroughly understand the fundamental behaviour of PSBPs in a wide temperature range, including temperatures lower than room temperature. However, low-temperature properties of PSBPs have not been well investigated so far. In this study, we investigated the temperature dependences of the Kerr coefficients and the response times of PSBPs with various polymer concentrations at low temperature. For comparison of the Kerr effect and molecular rotation at low temperature, we measured the temperature dependence of the permittivity, the rotational relaxation and the relaxation strength of PSBPs through dielectric relaxation measurements.
2. Results and Discussion 2.1. EO Properties Figure 1 shows the platelet texture of a PSBP stabilised with 8 wt % polymer (Polymer 8 wt % sample in Table 1 below) in an in-plane switching (IPS) cell and the reflection spectra of PSBPs stabilised with different monomer concentration (8–18 wt %). As shown in Figure 1 b, The Bragg reflection peaks from the (110) plane of the BP I lattice appeared within a wavelength range of 500–530 nm, and hence the resulting chiral pitches of the PSBPs were almost constant (220–230 nm). Driving voltage versus transmittance curves were recorded at various temperatures and for samples with various polymer concentrations. The electric-field-induced birefringence Dn of a PSBP in a weak electric field is well expressed by Equation (1): Dn ¼ lKE 2
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Figure 3. Arrhenius plots of response times in rising and falling processes of the EO effect in a PSBP stabilised with 8 wt % of polymer. Both processes were well fitted by the Arrhenius equation.
Figure 1. a) Platelet texture of a PSBP stabilised with 8 wt % of polymer in an IPS cell observed by polarising optical microscope. b) Bragg reflection spectra of the (110) plane in PSBPs with different polymer concentrations (8–18 wt %).
where l is the wavelength of incident light, K the Kerr coefficient and E the electric field.[8, 18] Figure 2 shows the temperature dependence of K for PSBPs with different polymer concentrations. The Kerr coefficient was found to decrease abruptly at
frozen below 280 K and the PSBP is maintained. Figure 3 shows an Arrhenius plot of the response times of PSBPs in the rising and falling processes of the EO Kerr effect at low temperature. With decreasing temperature, both response times tEO increased monotonously and the Arrhenius plot showed a linear relation. Thus, the temperature dependences of the response times of the EO Kerr effect of a non-frozen region of the PSBP obeyed the Arrhenius equation. The dynamics of reorientation of the local director had no such critical temperature as observed in the temperature dependence of the Kerr coefficient shown in Figure 2. A possible reason for the decrease in K is discussed below.
2.2. Dynamics of LC Molecular Rotation
Figure 2. Temperature dependence of Kerr coefficients of PSBPs with different polymer concentrations.
a low temperature. The magnitude of K was almost independent of temperature above 280 K, but decreased remarkably below 280 K. The decrease K at low temperature would be a serious problem in practical applications. The Kerr effect of PSBPs results mainly from reorientation of the local director in the BP lattice, because electrostriction is suppressed by the polymer network. The platelet texture of the PSBP was maintained below 280 K, that is, no phase transition occurred. Consequently, these results suggest that the local director is partially 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Dielectric relaxation measurements reveal the molecular dynamics of polar materials at the molecular level. Figure 4 shows the frequency dependence of the real part e’ and imaginary part e’’ (dielectric loss) of the permittivity of PSBP stabilized with 8 wt % of polymer. The peaks of e’’ shown in Figure 4 correspond to the dielectric relaxation in the rotation about the short axis of the LC molecule. From the peak frequency f0, the dielectric relaxation time tDR, which corresponds tothe rotational time of LC molecules about the short axis, was obtained. Figure 5 shows the Arrhenius plot of tDR. The observed relaxation time was about 100 times shorter than the EO response time, because the kinetic unit size of rotation is different. The relaxation time gradually became longer with decreasing temperature and did not show an abrupt change like the temperature dependence of K. The Arrhenius plots of tDR were non-linear (Figure 5), that is, the temperature dependence of tDR did not obey the Arrhenius equation, unlike the response times of the EO effect. Interestingly, however, they were fitted well by the Vogel–Fulcher equation t = t0 exp[B/(TT0)], in which t0 and T0 are the Vogel–Fulcher time and Vogel–Fulcher temperature, respectively. Therefore, it is reasonable to say that the kinetic modes of the EO response and the dielectric relaxation are different. Considering that the temperature corresponding to a peak in the dielectric loss at ChemPhysChem 0000, 00, 1 – 6
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Figure 6. Dependences of dielectric relaxation times of PSBPs on polymer concentration. Points at 0 wt % are those of the host nematic LC without polymer stabilisation.
at temperatures lower than the critical temperature of about 280 K for lowering of the Kerr effect. Furthermore, the dielectric relaxation strengths about their short axis, estimated from Cole–Cole plots (Figure 7), are sufficiently large even at low
Figure 4. Frequency dependence of real part e’ (a) and imaginary part e’’ (b) of the relative permittivity of PSBPs stabilised with 8 wt % of polymer, obtained by dielectric relaxation measurements.
Figure 7. Temperature dependence of dielectric relaxation strength of PSBPs with different polymer concentrations, estimated from Cole–Cole plots.
Figure 5. Arrhenius plots of dielectric relaxation times of PSBPs with various polymer concentrations measured by dielectric relaxation. Continuous solid lines are fitting curves to the Vogel–Fulcher equation.
1 kHz was 233 K, rotation of LC molecules about the short axis of the LC molecule is fast enough, that is, not frozen, even at 233 K. The three parameters, namely, the relaxation time at various temperatures shown in Figure 6, the Vogel–Fulcher time and the Vogel–Fulcher temperature, obtained by fitting to the Vogel–Fulcher equation, did not show any clear correlation with the polymer concentration. These results indicate that rotation of LC molecules about the short axis in PSBPs is unaffected by the polymer networks formed for stabilization of BP 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
temperature, although they decreased slightly with decreasing temperature. These results also clearly indicate that molecular rotational mobility in most regions of the LC is not frozen in the temperature range of 210–270 K. It is also notable that the relaxation time of the PSBP is mostly same as that of the host LC (0 wt % polymer concentration), as shown in Figure 6. Therefore, the original molecular rotational kinetics of LC molecules should be retained in the PSBPs at the molecular level in the observed temperature range. A proposed model of PSBP structure[7] in which the polymer is phase-separated from the LC ordered region and concentrated in the disclinations can explain this result well, because the LC molecules are separated from the polymers in this model. The question raised by our results is why the Kerr coefficient of PSBPs abruptly decreased at low temperature, even though the molecular rotational mobility was active and no phase transition occurred at the same temperature. It is reasonable to assume that the Kerr effect of PSBP should be dominated by the field-induced reorientation of the local director consistChemPhysChem 0000, 00, 1 – 6
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Kerr effect at low temperature, because generally x is proporing of the cooperatively ordered molecules with a common tional to a square root of K/De, where K and De are the effecorientational direction. The contribution of molecular orientative elastic constant and the dielectric anisotropy of the LC, retion of a polar molecule due to a torque exerted by an electric spectively. field, that is, Langevin-type field-induced orientation, to the To fully understand the response behaviour of PSBP in an Kerr effect should be negligible compared with the contribuapplied electric field, such hierarchical dynamics owing to the tion of the reorientation of the LC director. Generally the reorcomplicated structure of PSBP, as observed in this study, ientation of the director is strongly restrained by the anchoring should be further closely investigated. The development new effect at an interface in contact with the LC. The anchoring PSBPs that show a large and fast Kerr EO effect at low tempereffect on director reorientation in a bulk LC in an electric field ature will be the subject of a future project. is related to the electric coherence length x.[19] The x value of a simple sandwich cell filled with the host nematic LC used in this study in homogeneous alignment was measured accord3. Conclusions ing to a reported method[20] and was found to increase gradually with decreasing temperature. In a simple sandwich cell, Through measurements of the EO Kerr effect of PSBPs with the increase of the region of the LC effectively restrained by various polymer concentrations at low temperature, their localthe anchoring effect with increasing x is mild (nearly linear), director reorientation was found to abruptly decrease below because the effect is in one dimension along the normal to 280 K. With decreasing temperature, the EO response times tEO the interface. In the case of a PSBP, however, the anchoring increased monotonously and could be fitted by the Arrhenius effect of polymer networks should act more strongly on the diequation. The molecular relaxation time tDR of LC molecules in rector reorientation, because the LC in double twist cylinders PSBPs became gradually longer with decreasing temperature. in the PSBP is surrounded by three-dimensional polymer netIn addition, the temperature dependence of tDR was well fitted works, and hence the LC in PSBP is subject to anchoring efby the Vogel–Fulcher equation. Therefore, the kinetic modes of fects from three directions. The anchoring effect exerted on the EO response and the dielectric relaxation are different. The the LC should increase exponentially if the spatial dimension rotation of LC molecules about the short axis in PSBPs did not of the effect increases from 1D to 3D, in a similar way to the abruptly decrease at the temperature at which reorientation of Avrami-type growth. In BP I with a lattice constant of a, for exthe local director was frozen and is unaffected by the polymer pffiffiffi ample, the total length of seven disclinations is 4 3a in the network. The results agree well with a proposed PSBP model unit cell of volume a3. If the distance within which the director in which polymers are phase-separated from the LC ordered reorientation is restrained by the anchoring effect of polymers region and concentrated in the disclinations, where LC order is localized along the disclination lines is r, the volume of LC relower. The decrease in Kerr coefficient could be explained in pffiffiffi strained by the polymers is pr24 3a in the unit cell. If r is terms of freezing of the director due to an increase in coherlarger than a/4.7, most of the LC in PSBP should be restrained ence length, because the polymers restraining rotation of the by the anchoring effect between polymer/LC interfaces. Usualdirector through an anchoring effect are three-dimensionally ly, a is around a few hundred nanometres for BP I. Therefore, if arranged in short intervals of about 100 nm along the disclinar is about 50 nm, then it should be hard for the directors in tions in PSBP. The results obtained in this study will be useful PSBP to respond to an electric field (note that the restraining for understanding the dynamics of the electric response of effect is not constant with r but gradually decreases). On the PSBPs at low temperature. other hand, in a simple sandwich cell, if the restrained region in which the rotation of the director is suppressed by the anExperimental Section choring effect between the substrate/LC interfaces is about 50 nm in depth from each interface in a cell of 10 mm thickMaterials ness, nearly 99 % of the region is not restrained, because there Table 1 lists the constituent materials used to prepare PSBPs. As are only two flat interfaces. Therefore, it is clear from the host LC, JC1041XX (provided by JNC Co.) and 4-cyano-4’-pentylbisimple comparison above that the PSBP is very sensitive to the phenyl (5CB) were mixed in a weight ratio of 1:1. 2,5-bisanchoring effect compared to a simple sandwich LC cell. The [4’-(hexyloxy)phenyl-4-carbonyl]-1,4;3,6-dianhydride-d-sorbitol (ISOelectric coherence length is directly related to the distance of (6OBA)2) was added to each host LC as a chiral dopant to induce the restrained region. Consequently, it could be reasonable that the Kerr coefficient of PSBP Table 1. Composition of PSBPs [wt %]. remarkably decreases even though the increase in the coDA RM257 DMPAP Sample JC-1041XX:5CB (1:1) ISO(6OBA)2 herence length, that is, the rePolymer 8 wt % 84.5 7.5 4.0 4.0 0.5 strained distance, of the LC is Polymer 10 wt % 82.4 7.5 5.1 5.0 0.6 very slight. If our hypothesis is Polymer 12 wt % 80.6 7.5 6.0 6.0 0.5 Polymer 14 wt % 78.4 7.5 6.9 7.1 0.7 correct, a host LC having a small Polymer 16 wt % 76.5 7.5 8.0 8.0 0.5 K/De value at low temperature Polymer 18 wt % 74.6 7.5 9.0 9.0 0.5 could prevent freezing of the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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BP formation. The weight fraction of ISO-(6OBA)2 in the samples was adjusted to 7.5 wt % to keep the chiral pitch of BPs almost constant among the samples. Amounts of 8–18 wt % of a 1:1 mixture of dodecyl acrylate (DA; purchased from Wako) and 2-methyl-1,4-phenylene bis{4-[3-(acryloyloxy)propoxy]benzoate} (RM257; provided by Merck) monomers were mixed with the chiral nematic LC mixtures for polymer stabilization. For the photopolymerisation, 2,2-dimethoxy-2-phenylacetophenone (DMPAP; purchased from Aldrich) was used as initiator, and a UV light source (UV SP-V Spot-Cure; Ushio) as light source. Polymerisation was conducted under UV irradiation with an intensity of 2 mW cm2 for 15 min in cells that were temperature-regulated for the Figure 8. Experimental setup for electrooptical measurements on PSBPs. BP I state. After polymerisation, a polarising optical microscope (Leica, DFC420) and a UV/Vis spectrometer tions” from the Ministry of Education, Culture, Sports, Science (Jasco, MSV-350) were used to observe the optical texture of PSBP cells mounted on a temperature-controlled stage (LTS-E350, and Technology, Japan (MEXT), a Grant-in-Aid for Scientific ReLinkam). Chiral pitches of the PSBPs were estimated on the basis search (A) from the Japan Society for the Promotion of Science of reflection spectra obtained with the UV/Vis spectrometer, the re(JSPS) and a Grant-in-Aid for Scientific Research on Innovative fractive index of chiral LC mixtures and Bragg’s law under the conAreas of “Fusion Materials: Creative Development of Materials dition of q = 0, where q is the angle between the incident light and Exploration of Their Function through Molecular Control” and the scattering planes.
Electrooptical Measurements To determine the EO properties of the PSBPs, in-plane switching (IPS) cells with 12 mm cell gap were used. The width and distance of the IPS interdigitated electrode were 5 and 10 mm, respectively. The cells were filled with each chiral LC mixture in an isotropic phase by capillary action. A system composed of a function generator (WF1974, NF Corp.), a multimeter (7461A, APC Corp.) and a power amplifier (4010, NF Corp.) was used to apply an ac voltage of 1 kHz to the PSBP cells mounted on a temperature-controlled stage (LTS 300, Linkam), which was sandwiched between crossed polarisers (Figure 8). An He–Ne laser (633 nm) was used as light source, and the change in transmitted light due to electric-field-induced birefringence of PSBP was detected with a photodetector (New Focus Inc., 1621) and recorded with an oscilloscope (LeCroy, Wave runner 64Xi).
Dielectric Measurements For evaluation of molecular rotational mobility in the PSBPs, the dielectric relaxation was measured with an impedance/gain-phase analyzer SI 1260 (Solartron) and dielectric interface SI 1296 (Solartron) with temperature-controlled stage LTS 300. Sandwich cells with 10 mm gap and 10 mm square indium tin oxide (ITO) electrodes were used (E.H.C. Co.). With ITO cells, subtracting the effect of the resistance of ITO electrodes from the observed impedance is important. We initially measured precise resistances and capacitances of each empty ITO cell, and subtracted the resistance and the estimated relative permittivity from the impedance of the corresponding PSBP cell. The sample cells were heated to a temperature that resulted in an isotropic phase and kept at that temperature for 10 min. Then, the measurement temperature was decreased in steps of 10 K from the temperature showing an isotropic phase to 193 K.
(no. 2206) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). We are indebted to JNC Corporation for providing with the nematic mixture, JC-1041XX.
Keywords: blue phases · electrooptical effect · liquid crystals · molecular dynamics · polymers
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Acknowledgements This work was partially supported by the Global COE Program for “Novel Carbon Resource Sciences; Coal-Based Eco-Innova-
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D. C. Wright, N. D. Mermin, Rev. Mod. Phys. 1989, 61, 385 – 432. H. Kikuchi, Struct. Bonding 2008, 128, 99 – 117. H. S. Kitzerow, Mol. Cryst. Liq. Cryst. 1991, 202, 51 – 83. W. Cao, A. Munoz, P. Palffy-Muhoray, B. Taheri, Nat. Mater. 2002, 1, 111 – 113. H. Coles, S. Morris, Nat. Photonics 2010, 4, 676 – 685. S. Shibayama, H. Higuchi, Y. Okumura, H. Kikuchi, Adv. Funct. Mater. 2013, 23, 2387 – 2396. H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, T. Kajiyama, Nat. Mater. 2002, 1, 64 – 68. Y. Hisakado, H. Kikuchi, T. Nagamura, T. Kajiyama, Adv. Mater. 2005, 17, 96 – 98. Y. Haseba, H. Kikuchi, T. Nagamura, Adv. Mater. 2005, 17, 2311 – 2315. S. Yokoyama, S. Mashiko, H. Kikuchi, K. Uchida, T. Nakamura, Adv. Mater. 2006, 18, 48 – 51. M. Sato, A. Yoshizawa, Adv. Mater. 2007, 19, 4145 – 4148. Z. Ge, S.-T. Wu, S. S. Kim, J. W. Park, S. H. Lee, Appl. Phys. Lett. 2008, 92, 181109. Z. Ge, L. Rao, S. Gauza, S. T. Wu, J. Disp. Technol. 2009, 5, 250 – 256. L. Rao, J. Yan, S. T. Wu, S. Yamamoto, Y. Haseba, Appl. Phys. Lett. 2011, 98, 081109. H.-S. Chen, Y.-H. Lin, C.-H. Wu, M. Chen, H.-K. Hsu, Opt. Mater. Express 2012, 2, 1149 – 1155. Y.-H. Lin, H.-S. Chen, H.-C. Lin, Y.-S. Tsou, H.-K. Hsu, W.-Y. Li, Appl. Phys. Lett. 2010, 96, 113505. A. Yoshizawa, RSC Adv. 2013, 3, 25475 – 25497. P. R. Gerber, Mol. Cryst. Liq. Cryst. 1985, 116, 197 – 206. P. G. De Gennes, J. Prost, The Physics of Liquid Crystals, Oxford University Press, London, 1993, pp. 120 – 123. M. Imai, H. Naito, M. Okuda, A. Sugimura, Mol. Cryst. Liq. Cryst. 1995, 259, 37 – 46.
Received: December 2, 2013 Revised: March 5, 2014 Published online on && &&, 2014
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ARTICLES G. Lim, Y. Okumura, H. Higuchi, H. Kikuchi* && – && Low-Temperature Properties of Polymer-StabilisedLiquid-Crystal Blue Phases
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Hierarchical dynamics of polymer-stabilised blue phases are studied by means of dielectric relaxation time and electrooptical response, which showed different temperature dependences obeying the Vogel–Fulcher and Arrhenius equations (the picture shows the Arrhenius relations). The polymer concentration affects the electrooptical effect but not the rotation of the LC molecules in PSBPs.
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Low-Temperature Properties of Polymer-Stabilised Liquid-Crystal Blue Phases Gihwan Lim,[b] Yasushi Okumura,[a] Hiroki Higuchi,[a] and Hirotsugu Kikuchi*[a] The temperature dependences of the Kerr coefficient and the response time in the electrooptical effect of polymer-stabilised blue phases (PSBPs) of liquid crystals (LCs) with various polymer concentrations are investigated in a wide temperature range including temperatures lower than room temperature. The Kerr coefficients are found to abruptly decrease at low temperature, and the response time–temperature relation obeys the Arrhenius equation. For comparison of the Kerr
effect and molecular rotation at low temperature, various physical properties such as permittivity, rotational relaxation time and dielectric relaxation strength of the PSBPs are investigated. The electrooptical response times and the dielectric relaxation times show different temperature dependences, and rotation of LC molecules in PSBPs was sufficiently active at low temperature and not strongly affected by the polymer.
1. Introduction Blue phases (BPs) are liquid-crystal (LC) phases that occur for chiral nematic liquid crystals in a narrow temperature range of typically about 2–3 K between a chiral nematic and an isotropic phase, and are optically isotropic without birefringence because of their cubic lattice structure consisting of double twisted cylinders.[1, 2] If an electric field is applied to BPs, a field-induced birefringence obeying the Kerr law is observed due to both electrostriction of the BP lattice and a reorientation of the local director of the LC within the BP lattice.[2, 3] Furthermore, BPs are known to show tuneable selective reflection of light.[4–6] However, their narrow temperature range has been a problem for their practical applications. Kikuchi et al. found that the structure of BPs could be stabilised by polymer networks without losing the electrooptical (EO) response, and the temperature range of stable BPs was successfully extended to more than 60 K.[7] These polymer-stabilised blue phases (PSBPs) have attracted attention as new-generation LC materials, because they show fast EO response (< 1 ms) and require no surface-alignment treatment in the cell due to their optical isotropy, and they have been studied as LCs for display devices.[8–17] Nevertheless, PSBPs still have some problems to be solved, such as high driving voltage, residual birefringence, and lowering of electrooptical (EO) performance at a low temperature. The strong temperature dependence of the Kerr coefficient [a] Prof. Y. Okumura, Prof. H. Higuchi, Prof. H. Kikuchi Institute for Materials Chemistry and Engineering Kyushu University 6-1 Kasuga-koen, Kasuga Fukuoka 816-8580 (Japan) E-mail: [email protected] [b] G. Lim Department of Applied Science for Electronics and Materials Kyushu University 6-1 Kasuga-koen Kasuga, Fukuoka 816-8580(Japan) Fax: (+ 81) 92 583 7789
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
and response time tEO[12–14] are emerging as profound hindrances to the application of PSBPs. From the viewpoint of practical use in diverse environments, it is important to thoroughly understand the fundamental behaviour of PSBPs in a wide temperature range, including temperatures lower than room temperature. However, low-temperature properties of PSBPs have not been well investigated so far. In this study, we investigated the temperature dependences of the Kerr coefficients and the response times of PSBPs with various polymer concentrations at low temperature. For comparison of the Kerr effect and molecular rotation at low temperature, we measured the temperature dependence of the permittivity, the rotational relaxation and the relaxation strength of PSBPs through dielectric relaxation measurements.
2. Results and Discussion 2.1. EO Properties Figure 1 shows the platelet texture of a PSBP stabilised with 8 wt % polymer (Polymer 8 wt % sample in Table 1 below) in an in-plane switching (IPS) cell and the reflection spectra of PSBPs stabilised with different monomer concentration (8–18 wt %). As shown in Figure 1 b, The Bragg reflection peaks from the (110) plane of the BP I lattice appeared within a wavelength range of 500–530 nm, and hence the resulting chiral pitches of the PSBPs were almost constant (220–230 nm). Driving voltage versus transmittance curves were recorded at various temperatures and for samples with various polymer concentrations. The electric-field-induced birefringence Dn of a PSBP in a weak electric field is well expressed by Equation (1): Dn ¼ lKE 2
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Figure 3. Arrhenius plots of response times in rising and falling processes of the EO effect in a PSBP stabilised with 8 wt % of polymer. Both processes were well fitted by the Arrhenius equation.
Figure 1. a) Platelet texture of a PSBP stabilised with 8 wt % of polymer in an IPS cell observed by polarising optical microscope. b) Bragg reflection spectra of the (110) plane in PSBPs with different polymer concentrations (8–18 wt %).
where l is the wavelength of incident light, K the Kerr coefficient and E the electric field.[8, 18] Figure 2 shows the temperature dependence of K for PSBPs with different polymer concentrations. The Kerr coefficient was found to decrease abruptly at
frozen below 280 K and the PSBP is maintained. Figure 3 shows an Arrhenius plot of the response times of PSBPs in the rising and falling processes of the EO Kerr effect at low temperature. With decreasing temperature, both response times tEO increased monotonously and the Arrhenius plot showed a linear relation. Thus, the temperature dependences of the response times of the EO Kerr effect of a non-frozen region of the PSBP obeyed the Arrhenius equation. The dynamics of reorientation of the local director had no such critical temperature as observed in the temperature dependence of the Kerr coefficient shown in Figure 2. A possible reason for the decrease in K is discussed below.
2.2. Dynamics of LC Molecular Rotation
Figure 2. Temperature dependence of Kerr coefficients of PSBPs with different polymer concentrations.
a low temperature. The magnitude of K was almost independent of temperature above 280 K, but decreased remarkably below 280 K. The decrease K at low temperature would be a serious problem in practical applications. The Kerr effect of PSBPs results mainly from reorientation of the local director in the BP lattice, because electrostriction is suppressed by the polymer network. The platelet texture of the PSBP was maintained below 280 K, that is, no phase transition occurred. Consequently, these results suggest that the local director is partially 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Dielectric relaxation measurements reveal the molecular dynamics of polar materials at the molecular level. Figure 4 shows the frequency dependence of the real part e’ and imaginary part e’’ (dielectric loss) of the permittivity of PSBP stabilized with 8 wt % of polymer. The peaks of e’’ shown in Figure 4 correspond to the dielectric relaxation in the rotation about the short axis of the LC molecule. From the peak frequency f0, the dielectric relaxation time tDR, which corresponds tothe rotational time of LC molecules about the short axis, was obtained. Figure 5 shows the Arrhenius plot of tDR. The observed relaxation time was about 100 times shorter than the EO response time, because the kinetic unit size of rotation is different. The relaxation time gradually became longer with decreasing temperature and did not show an abrupt change like the temperature dependence of K. The Arrhenius plots of tDR were non-linear (Figure 5), that is, the temperature dependence of tDR did not obey the Arrhenius equation, unlike the response times of the EO effect. Interestingly, however, they were fitted well by the Vogel–Fulcher equation t = t0 exp[B/(TT0)], in which t0 and T0 are the Vogel–Fulcher time and Vogel–Fulcher temperature, respectively. Therefore, it is reasonable to say that the kinetic modes of the EO response and the dielectric relaxation are different. Considering that the temperature corresponding to a peak in the dielectric loss at ChemPhysChem 0000, 00, 1 – 6
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Figure 6. Dependences of dielectric relaxation times of PSBPs on polymer concentration. Points at 0 wt % are those of the host nematic LC without polymer stabilisation.
at temperatures lower than the critical temperature of about 280 K for lowering of the Kerr effect. Furthermore, the dielectric relaxation strengths about their short axis, estimated from Cole–Cole plots (Figure 7), are sufficiently large even at low
Figure 4. Frequency dependence of real part e’ (a) and imaginary part e’’ (b) of the relative permittivity of PSBPs stabilised with 8 wt % of polymer, obtained by dielectric relaxation measurements.
Figure 7. Temperature dependence of dielectric relaxation strength of PSBPs with different polymer concentrations, estimated from Cole–Cole plots.
Figure 5. Arrhenius plots of dielectric relaxation times of PSBPs with various polymer concentrations measured by dielectric relaxation. Continuous solid lines are fitting curves to the Vogel–Fulcher equation.
1 kHz was 233 K, rotation of LC molecules about the short axis of the LC molecule is fast enough, that is, not frozen, even at 233 K. The three parameters, namely, the relaxation time at various temperatures shown in Figure 6, the Vogel–Fulcher time and the Vogel–Fulcher temperature, obtained by fitting to the Vogel–Fulcher equation, did not show any clear correlation with the polymer concentration. These results indicate that rotation of LC molecules about the short axis in PSBPs is unaffected by the polymer networks formed for stabilization of BP 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
temperature, although they decreased slightly with decreasing temperature. These results also clearly indicate that molecular rotational mobility in most regions of the LC is not frozen in the temperature range of 210–270 K. It is also notable that the relaxation time of the PSBP is mostly same as that of the host LC (0 wt % polymer concentration), as shown in Figure 6. Therefore, the original molecular rotational kinetics of LC molecules should be retained in the PSBPs at the molecular level in the observed temperature range. A proposed model of PSBP structure[7] in which the polymer is phase-separated from the LC ordered region and concentrated in the disclinations can explain this result well, because the LC molecules are separated from the polymers in this model. The question raised by our results is why the Kerr coefficient of PSBPs abruptly decreased at low temperature, even though the molecular rotational mobility was active and no phase transition occurred at the same temperature. It is reasonable to assume that the Kerr effect of PSBP should be dominated by the field-induced reorientation of the local director consistChemPhysChem 0000, 00, 1 – 6
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Kerr effect at low temperature, because generally x is proporing of the cooperatively ordered molecules with a common tional to a square root of K/De, where K and De are the effecorientational direction. The contribution of molecular orientative elastic constant and the dielectric anisotropy of the LC, retion of a polar molecule due to a torque exerted by an electric spectively. field, that is, Langevin-type field-induced orientation, to the To fully understand the response behaviour of PSBP in an Kerr effect should be negligible compared with the contribuapplied electric field, such hierarchical dynamics owing to the tion of the reorientation of the LC director. Generally the reorcomplicated structure of PSBP, as observed in this study, ientation of the director is strongly restrained by the anchoring should be further closely investigated. The development new effect at an interface in contact with the LC. The anchoring PSBPs that show a large and fast Kerr EO effect at low tempereffect on director reorientation in a bulk LC in an electric field ature will be the subject of a future project. is related to the electric coherence length x.[19] The x value of a simple sandwich cell filled with the host nematic LC used in this study in homogeneous alignment was measured accord3. Conclusions ing to a reported method[20] and was found to increase gradually with decreasing temperature. In a simple sandwich cell, Through measurements of the EO Kerr effect of PSBPs with the increase of the region of the LC effectively restrained by various polymer concentrations at low temperature, their localthe anchoring effect with increasing x is mild (nearly linear), director reorientation was found to abruptly decrease below because the effect is in one dimension along the normal to 280 K. With decreasing temperature, the EO response times tEO the interface. In the case of a PSBP, however, the anchoring increased monotonously and could be fitted by the Arrhenius effect of polymer networks should act more strongly on the diequation. The molecular relaxation time tDR of LC molecules in rector reorientation, because the LC in double twist cylinders PSBPs became gradually longer with decreasing temperature. in the PSBP is surrounded by three-dimensional polymer netIn addition, the temperature dependence of tDR was well fitted works, and hence the LC in PSBP is subject to anchoring efby the Vogel–Fulcher equation. Therefore, the kinetic modes of fects from three directions. The anchoring effect exerted on the EO response and the dielectric relaxation are different. The the LC should increase exponentially if the spatial dimension rotation of LC molecules about the short axis in PSBPs did not of the effect increases from 1D to 3D, in a similar way to the abruptly decrease at the temperature at which reorientation of Avrami-type growth. In BP I with a lattice constant of a, for exthe local director was frozen and is unaffected by the polymer pffiffiffi ample, the total length of seven disclinations is 4 3a in the network. The results agree well with a proposed PSBP model unit cell of volume a3. If the distance within which the director in which polymers are phase-separated from the LC ordered reorientation is restrained by the anchoring effect of polymers region and concentrated in the disclinations, where LC order is localized along the disclination lines is r, the volume of LC relower. The decrease in Kerr coefficient could be explained in pffiffiffi strained by the polymers is pr24 3a in the unit cell. If r is terms of freezing of the director due to an increase in coherlarger than a/4.7, most of the LC in PSBP should be restrained ence length, because the polymers restraining rotation of the by the anchoring effect between polymer/LC interfaces. Usualdirector through an anchoring effect are three-dimensionally ly, a is around a few hundred nanometres for BP I. Therefore, if arranged in short intervals of about 100 nm along the disclinar is about 50 nm, then it should be hard for the directors in tions in PSBP. The results obtained in this study will be useful PSBP to respond to an electric field (note that the restraining for understanding the dynamics of the electric response of effect is not constant with r but gradually decreases). On the PSBPs at low temperature. other hand, in a simple sandwich cell, if the restrained region in which the rotation of the director is suppressed by the anExperimental Section choring effect between the substrate/LC interfaces is about 50 nm in depth from each interface in a cell of 10 mm thickMaterials ness, nearly 99 % of the region is not restrained, because there Table 1 lists the constituent materials used to prepare PSBPs. As are only two flat interfaces. Therefore, it is clear from the host LC, JC1041XX (provided by JNC Co.) and 4-cyano-4’-pentylbisimple comparison above that the PSBP is very sensitive to the phenyl (5CB) were mixed in a weight ratio of 1:1. 2,5-bisanchoring effect compared to a simple sandwich LC cell. The [4’-(hexyloxy)phenyl-4-carbonyl]-1,4;3,6-dianhydride-d-sorbitol (ISOelectric coherence length is directly related to the distance of (6OBA)2) was added to each host LC as a chiral dopant to induce the restrained region. Consequently, it could be reasonable that the Kerr coefficient of PSBP Table 1. Composition of PSBPs [wt %]. remarkably decreases even though the increase in the coDA RM257 DMPAP Sample JC-1041XX:5CB (1:1) ISO(6OBA)2 herence length, that is, the rePolymer 8 wt % 84.5 7.5 4.0 4.0 0.5 strained distance, of the LC is Polymer 10 wt % 82.4 7.5 5.1 5.0 0.6 very slight. If our hypothesis is Polymer 12 wt % 80.6 7.5 6.0 6.0 0.5 Polymer 14 wt % 78.4 7.5 6.9 7.1 0.7 correct, a host LC having a small Polymer 16 wt % 76.5 7.5 8.0 8.0 0.5 K/De value at low temperature Polymer 18 wt % 74.6 7.5 9.0 9.0 0.5 could prevent freezing of the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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BP formation. The weight fraction of ISO-(6OBA)2 in the samples was adjusted to 7.5 wt % to keep the chiral pitch of BPs almost constant among the samples. Amounts of 8–18 wt % of a 1:1 mixture of dodecyl acrylate (DA; purchased from Wako) and 2-methyl-1,4-phenylene bis{4-[3-(acryloyloxy)propoxy]benzoate} (RM257; provided by Merck) monomers were mixed with the chiral nematic LC mixtures for polymer stabilization. For the photopolymerisation, 2,2-dimethoxy-2-phenylacetophenone (DMPAP; purchased from Aldrich) was used as initiator, and a UV light source (UV SP-V Spot-Cure; Ushio) as light source. Polymerisation was conducted under UV irradiation with an intensity of 2 mW cm2 for 15 min in cells that were temperature-regulated for the Figure 8. Experimental setup for electrooptical measurements on PSBPs. BP I state. After polymerisation, a polarising optical microscope (Leica, DFC420) and a UV/Vis spectrometer tions” from the Ministry of Education, Culture, Sports, Science (Jasco, MSV-350) were used to observe the optical texture of PSBP cells mounted on a temperature-controlled stage (LTS-E350, and Technology, Japan (MEXT), a Grant-in-Aid for Scientific ReLinkam). Chiral pitches of the PSBPs were estimated on the basis search (A) from the Japan Society for the Promotion of Science of reflection spectra obtained with the UV/Vis spectrometer, the re(JSPS) and a Grant-in-Aid for Scientific Research on Innovative fractive index of chiral LC mixtures and Bragg’s law under the conAreas of “Fusion Materials: Creative Development of Materials dition of q = 0, where q is the angle between the incident light and Exploration of Their Function through Molecular Control” and the scattering planes.
Electrooptical Measurements To determine the EO properties of the PSBPs, in-plane switching (IPS) cells with 12 mm cell gap were used. The width and distance of the IPS interdigitated electrode were 5 and 10 mm, respectively. The cells were filled with each chiral LC mixture in an isotropic phase by capillary action. A system composed of a function generator (WF1974, NF Corp.), a multimeter (7461A, APC Corp.) and a power amplifier (4010, NF Corp.) was used to apply an ac voltage of 1 kHz to the PSBP cells mounted on a temperature-controlled stage (LTS 300, Linkam), which was sandwiched between crossed polarisers (Figure 8). An He–Ne laser (633 nm) was used as light source, and the change in transmitted light due to electric-field-induced birefringence of PSBP was detected with a photodetector (New Focus Inc., 1621) and recorded with an oscilloscope (LeCroy, Wave runner 64Xi).
Dielectric Measurements For evaluation of molecular rotational mobility in the PSBPs, the dielectric relaxation was measured with an impedance/gain-phase analyzer SI 1260 (Solartron) and dielectric interface SI 1296 (Solartron) with temperature-controlled stage LTS 300. Sandwich cells with 10 mm gap and 10 mm square indium tin oxide (ITO) electrodes were used (E.H.C. Co.). With ITO cells, subtracting the effect of the resistance of ITO electrodes from the observed impedance is important. We initially measured precise resistances and capacitances of each empty ITO cell, and subtracted the resistance and the estimated relative permittivity from the impedance of the corresponding PSBP cell. The sample cells were heated to a temperature that resulted in an isotropic phase and kept at that temperature for 10 min. Then, the measurement temperature was decreased in steps of 10 K from the temperature showing an isotropic phase to 193 K.
(no. 2206) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). We are indebted to JNC Corporation for providing with the nematic mixture, JC-1041XX.
Keywords: blue phases · electrooptical effect · liquid crystals · molecular dynamics · polymers
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Acknowledgements This work was partially supported by the Global COE Program for “Novel Carbon Resource Sciences; Coal-Based Eco-Innova-
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: December 2, 2013 Revised: March 5, 2014 Published online on && &&, 2014
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ARTICLES G. Lim, Y. Okumura, H. Higuchi, H. Kikuchi* && – && Low-Temperature Properties of Polymer-StabilisedLiquid-Crystal Blue Phases
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Hierarchical dynamics of polymer-stabilised blue phases are studied by means of dielectric relaxation time and electrooptical response, which showed different temperature dependences obeying the Vogel–Fulcher and Arrhenius equations (the picture shows the Arrhenius relations). The polymer concentration affects the electrooptical effect but not the rotation of the LC molecules in PSBPs.
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