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We demonstrated a single-substrate polymer-stabilized blue phase liquid crystal (PSBP LC) display using a bar-coating technique. The BP temperature range and the electro-optical (EO) properties of PSBP LC cells fabricated on a single-substrate of glass, polycarbonate (PC) and polyethylene terephthalate (PET) were investigated. It was found that PSBP LC cells fabricated on the single-substrate of glass, PC, and PET exhibited a narrower BP temperature range in comparison with the conventional PSBP LC cell. This is necessary to improve the mechanical stability, the thermal stability and the EO performance of the single-substrate PSBP LC cells.
© 2013 Optical Society of America
1. Introduction
Blue phase liquid crystals (BP LCs) exhibit three-dimensional spatial periodicity and appear in a narrow temperature interval (typically less than a few kelvin) between the high-temperature isotropic phase (Iso.) and the low-temperature cholesteric phase (N*) [1–5]. The very small temperature range over which blue phases (BPs) have been thermodynamically stable limits their practical applications. Recently, Kikuchi and co-workers proposed a new type of polymer stabilization effect known as polymer-stabilized blue phase liquid crystal (PSBP LC) to increase the stability range of BPs to as much as 60 K including room temperature for potential technological applications [6]. They also showed that the temperature range of BPs can be enhanced by means of monomer/liquid crystal (LC) compositions. Since then, PSBP LC has been emerging as a next-generation display material because it offers several attractive features, such as (i) no requirement for any alignment layer [7], (ii) an optically isotropic dark state for wide and symmetric viewing angles [7,8], and (iii) sub-millisecond response time [6,9]. However, high operating voltage [10], residual birefringence, and hysteresis [11,12] are crucial issues to be resolved for a practical application to displays. To reduce operating voltages, there are three approaches, viz.: (a) to develop new host LC mixtures with a large electro-optic Kerr constant [13,14], (b) to design the electrode shape, structure, and dimension for generating strong and deep-penetrating electric fields [15–19], and (c) to optimize the monomer/LC compositions (monomer types, monomer concentration, and monomer ratio) and process [6,20,21]. Using the approaches of (a) and (c), Oo et al. and Mizunuma et al. demonstrated that lower operating voltage and reduction in hysteresis and residual birefringence can be achieved by optimizing monomer/LC compositions and matching mono-functional and di-functional monomers [22,23]. Currently, flexible flat panel display technology constitutes an eclectic research field and a potentially large industry in the future. Concerted efforts have been made in the last decade to develop flexible liquid crystal displays (LCDs) using nematic LCs and cholesteric LCs [24]. However, there are only a few initiatives in flexible and single-substrate cholesteric BP LCs and PSBP LCs, which will have attractive features such as having very fast response times and being lightweight, low power, and rugged [25,26]. In this study, we fabricated single-substrate PSBP LCDs on a rigid glass substrate as well as flexible PC and PET substrates. The purpose of this study is to report the preliminary results on the fabrication of a single-substrate PSBP LCD, its stability, and its EO properties.
2. Experimental
We prepared the BP LC mixture consisting of chiral dopant ISO-(6OBA)2 (2,5-bis-[4ʹ-(hexyloxy)-phenyl-4-carbonyl]-1,4;3,6-dianhydride-D-sorbitol, synthesized), a host LC mixture, and a reactive-mesogen/photoinitiator mixture (UCL-011-K1, DIC Co.). The BP LC mixture was composed of 7.42 wt% ISO-(6OBA)2, 42.43 wt% 5CB (Merck, Ltd.), 42.24 wt% JC-1041XX (JNC Petrochemical Co.), and 7.91 wt% UCL-011-K1. Two types of BP LC sample cells were fabricated. The type-1 BP LC sample cell is a conventional sandwich-type cell. In order to obtain an in-plane electric field, a glass substrate with inter-digitated electrodes made of indium-tin-oxide (ITO) having an electrode width of 25 μm and an electrode distance of 10 μm (Atsugi Micro Co., Ltd) without any alignment layer was used. The counterpart substrate was a bare glass substrate. The nominal cell gap is 20 μm. The type-1 cell was filled with a BP LC mixture by capillary action in the isotropic phase. The type-2 BP LC sample cell is a single-substrate cell in which a glass, PC, or PET substrate provided with ITO inter-digitated electrodes having an electrode width of 25 μm and an electrode distance of 10 μm (Atsugi Micro Co., Ltd) was used. The glass, PC, or PET substrate with ITO inter-digitated electrodes was spin-coated with polyvinyl alcohol (PVA) (2.5 wt%) without rubbing for the purpose of improving the uniformity of LC layer thickness and obtaining better wettability [27]. The BP LC film was cast in the isotropic phase on the PVA-coated glass, PC, or PET substrate using a bar coater in a nitrogen atmosphere. Here, the bar-coating direction is perpendicular to the inter-digitated electrodes as shown in Fig. 1.It should be noted that for the type-2 BP LC sample cell, no counterpart substrate was used, and the cast BP LC layer is exposed to the air. The nominal thickness of the bar-coated BP LC film is 5.7 μm, measured by the interferometric method [28]. We cooled the conventional sandwich-type cell or the single-substrate cell from the isotropic to the cholesteric phase. Then we heated the cells from the cholesteric phase to the blue phase (BP I) to measure the phase transition temperatures. BP LC textures were characterized by polarizing optical microscopy (Nikon Eclipse LV100 POL) coupled with a hot stage (TS62, Instec) and a hot-stage controller (STC200, Instec). The cells were then exposed to the ultraviolet (UV) light at a temperature slightly higher than the N*-BPI transition temperature. The UV source used is a UV lamp (deep UV, 365 nm, Ushio Spot Cure SP9-250DB). The conventional sandwich-type cell was exposed (from the electrode side) to UV light of 1.8 J/cm2 (1.5 mW/cm2 for 20 min). However, for the single-substrate cells, the UV exposure (Condition C listed in Table 1) was from the nitrogen-BP LC interface. It should be emphasized that the LC bar-coating process and the polymer stabilization were carried out in the nitrogen atmosphere since molecular oxygen is a powerful inhibitor in polymerization [29,30]. With regard to the UV exposure dosage for the single-substrate cells, four UV exposure conditions, listed in Table 1, were attempted to achieve PSBP LC. Among these four conditions, the first two conditions (A and B) did not achieve polymer stabilization, but PSBP LC was obtained by the last two conditions (C and D). However, dewetting of the cast BP LC film was observed in condition D due to the high UV intensity. It is well known that a number of factors, such as the wavelength and intensity of UV light, exposure time, and curing temperature, etc., influence the optimization of PSBP LC cells [12,22,23,31,32]. To characterize the EO properties of PSBP LC cells, a He-Ne laser (λ = 633 nm) was used as a light source, and a PSBP LC sample cell was adjusted between the crossed polarizers so that the electrode direction made an angle of 45° with respect to both their axes. The cells were driven by 1 kHz square waves. The ascending and descending voltages were performed for each sample to examine the hysteresis behavior of the cells.
Table 1. Four UV exposure conditions attempted to achieve PSBP LC for the single-substrate cells
3. Results and discussion
Figure 2 shows polarized microphotographs of the conventional sandwich-type BP LC cell between crossed polarizers before and after UV exposure. From these images shown in Fig. 2, conventional platelet texture was revealed. The phase transition temperatures of the conventional sandwich-type BP LC cell before and after UV exposure are listed in Table 2. Figure 3 depicts the voltage-dependent normalized transmittance (V-T) curves of conventional sandwich-type PSBP LC cells at 25°C. It was found that the common V-T characteristic was obtained in this sample. After phase transition temperature and V-T measurements, the counterpart glass substrate was removed, and these two measurements were again carried out to examine the effects of confining cell walls and air-BP LC interface. It was found that the conventional sandwich-type PSBP LC cells showed almost the same EO characteristics (Vmax > 80 V) and BP temperature range, ΔTBP (> 36.5°C, as shown in Table 2), before and after removing the counterpart glass substrate. Here, Vmax is defined as a voltage at the maximum transmittance of the V-T curve. These results indicated that the confining cell walls were not necessary after polymer-stabilization of BP LC as reported by Castles et al. [33]. That is, once the polymer stabilization was achieved, the effects of confining cell walls and air-BP LC interface seem to be trivial.
Fig. 2 Polarized microphotographs (reflection mode) of the conventional sandwich-type BP LC cell between crossed polarizers: (a) BPI before UV exposure, (b) BPII before UV exposure, and (c) BPI after UV exposure.
Table 2. Phase transition temperatures of the conventional sandwich-type BP LC cell before and after UV exposure
Fig. 3 V-T curves of the conventional sandwich-type PSBP LC cells before and after removing the counterpart glass substrate at 25°C. The inset shows the EO response under an applied voltage of 80 V.
Figure 4 shows polarized microphotographs of the single-glass-substrate BP LC cell between crossed polarizers before and after UV exposure. As shown in Fig. 4, a common BP texture was confirmed even when the BP LC layer was not covered by the counterpart substrate and exposed to the air. The phase transition temperatures of the single-glass-substrate BP LC cell before and after UV exposure were listed in Table 3. It was found that ΔTBP of the single-glass-substrate PSBP LC cell was ~6.6°C. Figure 5 depicts the V-T curves of the single-glass-substrate PSBP LC cell at 40°C. It was observed that the V-T curve showed Vmax at < 80 V, and the hysteresis was relatively small. From these experimental results, we deduced that it is not necessary to be a sandwich-type cell to achieve PSBP LC.
Fig. 4 Polarized microphotographs (reflection mode) of the single-glass-substrate BP LC cell between crossed polarizers: (a) BPI before UV exposure, (b) BPII before UV exposure, and (c) BPI after UV exposure.
Table 3. Phase transition temperatures of the single-glass-substrate BP LC cell before and after UV exposure
Fig. 5 V-T curves of the single-glass-substrate PSBP LC cell at 40°C. The inset shows the EO response under an applied voltage of 80 V.
Polarized microphotographs of the single-PC-substrate BP LC cell between crossed polarizers before and after UV exposure are shown in Fig. 6.The phase transition temperatures of the single-PC-substrate BP LC cell before and after UV exposure are listed in Table 4.Figure 7 illustrates the V-T curves of the single-PC-substrate PSBP LC cell at 45°C. It was observed that ΔTBP of the single-PC-substrate PSBP LC cell was ~4.6°C, and Vmax of the V-T curve was observed at > 40 V. Here, the maximum applied voltage was 50 V because the higher voltage caused the mechanical damage of the PC film probably due to the electric heating. Moreover, polarized microphotographs of the single-PET-substrate BP LC cell between crossed polarizers before and after UV exposure are shown in Fig. 8.The phase transition temperatures of the single-PET-substrate BP LC cell before and after UV exposure are listed in Table 5. Figure 9 illustrates the V-T curves of the single-PET-substrate PSBP LC cell at 40°C. It was observed that ΔTBP of the single-PET-substrate PSBP LC cell was ~5.2°C, and Vmax of the V-T curve was observed at < 80 V. These experimental results demonstrated that the structure of the BP LCD does not have to be a sandwich type. Furthermore, there is no problem with the performance of the BP LCD even when the BP LC layer is exposed to the air. It was found that ΔTBP depends on the materials of the substrate. Figure 10 shows polarized photographs of the single-PET-substrate PSBP LC cell. As shown in Fig. 10(a), the BP LC alignment is fairly robust in response to the bending stress. After removing the stress, the EO performance can be maintained, as shown in Figs. 10(b)-(d).
Fig. 6 Polarized microphotographs (reflection mode) of the single-PC-substrate BP LC cell between crossed polarizers: (a) BPI before UV exposure, (b) BPII before UV exposure, and (c) BPI after UV exposure.
Table 4. Phase transition temperatures of the single-PC-substrate BP LC cell before and after UV exposure
Fig. 7 V-T curves of the single-PC-substrate PSBP LC cell at 45°C. The inset shows the EO response under an applied voltage of 40 V.
Fig. 8 Polarized microphotographs (reflection mode) of the single-PET-substrate BP LC cell between crossed polarizers: (a) BPI before UV exposure, (b) BPII before UV exposure, and (c) BPI after UV exposure.
Table 5. Phase transition temperatures of the single-PET-substrate BP LC cell before and after UV exposure
Fig. 9 V-T curves of the single-PET-substrate PSBP LC cell at 40°C. The inset shows the EO response under an applied voltage of 80 V.
Fig. 10 Photograph of the single-PET-substrate PSBP LC cell (a) under the mechanical bending stress; polarized photographs of the single-PET-substrate PSBP LC cell between crossed polarizers (b) under no electric field, (c) under an applied voltage of 50 V, and (d) bending under an applied voltage of 50 V.
Table 6 summarizes the performance of two types of PSBP LC cells. From these results, it was found that PSBP LC cells fabricated on the single-substrate of glass, PC, and PET exhibited a narrower ΔTBP and a lower Vmax in comparison with the conventional PSBP LC cells. It is assumed that direct exposure to the single glass, PC, or PET substrate from the air-BP LC interface makes the polymerization conversion difficult and slow even in the nitrogen atmosphere, thereby reducing ΔTBP. From Tables 2-5, it was observed that the isotropic phase transition temperature of the single-PET-substrate BP LC cell considerably shifted downward after polymerization in comparison with other PSBP LC cells due to the substrate-surface-dependent polymerization [34]. Moreover, the V-T curves of the single-PET-substrate PSBP LC cell showed a very high transmittance (~35%) under no electric field due to the retardation of the PET film. In addition, the V-T curves of the conventional sandwich-type PSBP LC cell and the single-PC-substrate PSBP LC cell shown respectively in Fig. 3 and Fig. 7 exhibited anomalous behavior under low electric field owing to the LC molecular reorientation as reported in [35,36]. The relatively lower Vmax of the single-substrate cells is probably due to the shear-induced alignment and reduced thickness offered by the bar-coated film in comparison with the conventional sandwich-type cell. As a typical example, the response time of the EO performance of the single-glass-substrate PSBP LC cell was determined under an applied voltage of 80 V (16 kHz square waves). It was found to be 0.57 ms and 0.27 ms, respectively for the rise and decay processes at room temperature as shown in Fig. 11.Here, the rise time (τon) is defined as the time difference between 10% and 90% of the maximum transmittance upon on-electric field. The decay time (τoff) is defined as the time difference between 90% and 10% of the maximum transmittance upon off-electric field.
Table 6. Summary of performance of two types of PSBP LC cells
Fig. 11 Electro-optical response curves of the single-glass-substrate PSBP LC cell in the rise (a) and decay (b) processes under an applied voltage of 80 V.
4. Conclusions
We demonstrated for the first time, to the best of our knowledge, single-substrate PSBP LC cells fabricated on the rigid glass substrate as well as flexible PC and PET substrates. The single-substrate PSBP LC cell will be accompanied by the merit of an intrinsic high speed switching response. It is crucial that the molecular oxygen be purged in order to promote UV polymerization for the single-substrate PSBP LC. It was found that PSBP LC cells fabricated on the single-substrate of glass, PC, and PET exhibited a narrower BP temperature range in comparison with the conventional PSBP LC cell. This is necessary to improve the mechanical stability, the thermal stability and the EO performance of the single-substrate PSBP LC cells. Further experiments regarding the detailed EO characteristics of the single-substrate PSBP LC cells with and without bending are currently in progress. We expect that this study will open new and interesting areas of research on flexible PSBP LCD and freely suspended BP LC and PSBP LC.
Acknowledgments
This work was partly supported by an A-STEP project of Japan Science and Technology Agency (No. AS2314036B). We sincerely thank Merck Ltd., Japan; JNC Co.; and DIC Co. for providing us with the liquid crystal and UV curable liquid crystal used in this study.
References and links
1. D. C. Wright and N. D. Mermin, “Crystalline liquids: the blue phases,” Rev. Mod. Phys. 61(2), 385–432 (1989). [CrossRef]
2. S. Meiboom, J. P. Sethna, P. W. Anderson, and W. F. Brinkman, “Theory of the blue phase of cholesteric liquid crystals,” Phys. Rev. Lett. 46(18), 1216–1219 (1981). [CrossRef]
3. V. A. Belyakov and V. E. Dmitrienko, “The blue phase of liquid crystals,” Sov. Phys. Usp. 28(7), 535–562 (1985). [CrossRef]
4. P. P. Crooker, Chirality in Liquid Crystals (Springer-Verlag, 2001), Chap. 7.
5. P. G. de Gennes and J. Prost, The Physics of Liquid Crystals, 2nd ed. (Oxford University Press, 1993), Sec. 6.5.
6. H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, and T. Kajiyama, “Polymer-stabilized liquid crystal blue phases,” Nat. Mater. 1(1), 64–68 (2002). [CrossRef] [PubMed]
7. Y. Hisakado, H. Kikuchi, T. Nagamura, and T. Kajiyama, “Large electro-optic Kerr effect in polymer-stabilized liquid-crystalline blue phases,” Adv. Mater. 17(1), 96–98 (2005). [CrossRef]
8. M. S. Kim, Y. J. Lim, S. Yoon, S. W. Kang, S. H. Lee, M. Kim, and S. T. Wu, “A controllable viewing angle LCD with an optically isotropic liquid crystal,” J. Phys. D Appl. Phys. 43(14), 145502 (2010). [CrossRef]
9. K. M. Chen, S. Gauza, H. Xianyu, and S. T. Wu, “Submillisecond gray-level response time of a polymer-stabilized blue-phase liquid crystal,” J. Disp. Technol. 6(2), 49–51 (2010). [CrossRef]
10. H. Kikuchi, Y. Haseba, S. Yamamoto, T. Iwata, and H. Higuchi, “Optically isotropic nano-structured liquid crystal composites for display applications,” SID Int. Symp. Dig. Tech. Pap. 40, 578–581 (2009). [CrossRef]
11. K. M. Chen, S. Gauza, H. Xianyu, and S. T. Wu, “Hysteresis effects in blue-phase liquid crystals,” J. Disp. Technol. 6(8), 318–322 (2010). [CrossRef]
12. C. Y. Fan, C. T. Wang, T. H. Lin, F. C. Yu, T. H. Huang, C. Y. Liu, and N. Sugiura, “Hysteresis and residual birefringence free polymer-stabilized blue phase liquid crystal,” SID Int. Symp. Dig. Tech. Pap. 42, 213–215 (2011). [CrossRef]
13. L. Rao, J. Yan, S. T. Wu, S. Yamamoto, and Y. Haseba, “A large Kerr constant polymer-stabilized blue phase liquid crystal,” Appl. Phys. Lett. 98(8), 081109 (2011). [CrossRef]
14. Y. Haseba, S. Yamamoto, K. Sago, A. Takata, and H. Tobata, “Low-voltage polymer-stabilized blue-phase liquid crystals,” SID Int. Symp. Dig. Tech. Pap. 44, 254–257 (2013). [CrossRef]
15. L. Rao, Z. Ge, S. T. Wu, and S. H. Lee, “Low voltage blue-phase liquid crystal displays,” Appl. Phys. Lett. 95(23), 231101 (2009). [CrossRef]
16. Z. Ge, L. Rao, S. Gauza, and S. T. Wu, “Modeling of blue phase liquid crystal displays,” J. Disp. Technol. 5(7), 250–256 (2009). [CrossRef]
17. K. M. Chen, J. Yan, S. T. Wu, Y. P. Chang, C. C. Tsai, and J. W. Shiu, “Electrode dimension effects on blue-phase liquid crystal displays,” J. Disp. Technol. 7(7), 362–364 (2011). [CrossRef]
18. H. Lee, H. J. Park, O. J. Kwon, S. J. Yun, J. H. Park, S. Hong, and S. T. Shin, “The world’s first blue phase liquid crystal display,” SID Int. Symp. Dig. Tech. Pap. 42, 121–124 (2011).
19. D. Kubota, T. Ishitani, A. Yamashita, S. Yamagata, Y. Oe, T. Tamura, M. Ikenaga, T. Yamamoto, M. Kato, M. Nakano, R. Hatsumi, Y. Kubota, T. Murakawa, M. Hayakawa, T. Nishi, S. Seo, Y. Hirakata, S. Yamazaki, K. Okazaki, R. Sato, T. Cho, and M. Sakakura, “A new process for manufacture of low voltage, polymer-stabilized blue phase LCDs,” SID Int. Symp. Dig. Tech. Pap. 42, 125–128 (2011). [CrossRef]
20. L. Y. Wang, T. H. Huang, N. Sugiura, W. L. Liau, C. C. Han, C. J. Lung, P. L. Jung, and H. C. Lin, “Effects of liquid crystal compositions on polymer stabilized blue phase liquid crystals,” in Proceedings of the 17th International Display Workshops (Fukuoka, Japan, 2010), pp. 29–32.
21. J. Yan and S. T. Wu, “Effect of polymer concentration and composition on blue phase liquid crystals,” J. Disp. Technol. 7(9), 490–493 (2011). [CrossRef]
22. T. N. Oo, T. Mizunuma, Y. Nagano, H. Ma, Y. Ogawa, Y. Haseba, H. Higuchi, Y. Okumura, and H. Kikuchi, “Effects of monomer/liquid crystal compositions on electro-optical properties of polymer-stabilized blue phase liquid crystal,” Opt. Mater. Express 1(8), 1502–1510 (2011). [CrossRef]
23. T. Mizunuma, T. N. Oo, Y. Nagano, H. Ma, Y. Haseba, H. Higuchi, Y. Okumura, and H. Kikuchi, “Electro-optical properties of polymer-stabilized blue phase with different monomer combination and concentration,” Opt. Mater. Express 1(8), 1561–1568 (2011). [CrossRef]
24. G. P. Crawford, Flexible Flat Panel Displays (Wiley, 2005), Chaps. 1, 16, 17, and 18.
25. Z. Hussain, A. Masutani, D. Danner, F. Pleis, N. Hollfelder, G. Nelles, and P. Kilickiran, “Ultra fast polymer network blue phase liquid crystals,” J. Appl. Phys. 109(11), 114513 (2011). [CrossRef]
26. F. Castles, S. M. Morris, J. M. C. Hung, M. M. Qasim, and H. J. Coles, “Stretchable liquid crystal blue phases,” arXiv:1208.3117.
27. V. Vorflusev and S. Kumar, “Phase-separated composite films for liquid crystal displays,” Science 283(5409), 1903–1905 (1999). [CrossRef] [PubMed]
28. P. S. Drzaic, Liquid Crystal Dispersions (World Scientific, 1995) p.57.
29. C. Decker and A. D. Jenkins, “Kinetic approach of O2 inhibition in ultraviolet- and laser-induced polymerizations,” Macromolecules 18(6), 1241–1244 (1985). [CrossRef]
30. G. Odian, Principles of Polymerization (Wiley, 2004) p.259.
31. J. Yan and S. T. Wu, “Polymer-stabilized blue phase liquid crystals: a tutorial [Invited],” Opt. Mater. Express 1(8), 1527–1535 (2011). [CrossRef]
32. C. Y. Fan, H. C. Jau, T. H. Lin, F. C. Yu, T. H. Huang, C. Liu, and N. Sugiura, “Influence of polymerization temperature on hysteresis and residual birefringence of polymer stabilized blue phase LCs,” J. Disp. Technol. 7(11), 615–618 (2011). [CrossRef]
33. F. Castles, F. V. Day, S. M. Morris, D. H. Ko, D. J. Gardiner, M. M. Qasim, S. Nosheen, P. J. W. Hands, S. S. Choi, R. H. Friend, and H. J. Coles, “Blue-phase templated fabrication of three-dimensional nanostructures for photonic applications,” Nat. Mater. 11(7), 599–603 (2012). [CrossRef] [PubMed]
34. D. K. Yang, L. C. Chien, and Y. K. Fung, “Polymer-stabilized cholesteric textures,” in Liquid Crystals in Complex Geometries Formed by Polymer and Porous Networks, eds. G. P. Crawford and S. Zumer (Taylor and Francis, 1996).
35. H. Kikuchi, H. K. Jeong, and T. Kajiyama, “Electro-optical effect without hysteresis for (polymer/liquid crystal) composite films,” Mol. Cryst. Liq. Cryst. 318(1), 209–224 (1998). [CrossRef]
36. H. S. Chen, Y. H. Lin, C. H. Wu, M. Chen, and H. K. Hsu, “Hysteresis-free polymer-stabilized blue phase liquid crystals using thermal recycles,” Opt. Mater. Express 2(8), 1149–1155 (2012). [CrossRef]
