Open Access
Add to BibSonomyAbstract
We propose a sunlight-switchable light shutter using liquid crystal/polymer composite doped with push-pull azobenzene. The proposed light shutter is switchable between the translucent and transparent states by application of an electric field or by UV irradiation. Switching by UV irradiation is based on the change of the liquid crystal (LC) clearing point by the photo-isomerization effect of push-pull azobenzene. Under sunlight, the light shutter can be switched from the translucent to the transparent state by the nematic-isotropic phase transition of the LC domains triggered by trans-cis photo-isomerization of the push-pull azobenzene molecules. When the amount of sunlight is low because of cloud cover or when there is no sunlight at sunset, the light shutter rapidly relaxes from its transparent state back to its initial translucent state by the isotropic-nematic phase transition induced by cis-trans back-isomerization of the push-pull azobenzene molecules.
© 2016 Optical Society of America
1. Introduction
Light shutter technologies that can control optical transparency have been studied extensively for developing curtain-free smart windows [1–3]. A typical light shutter is based on an electrochromic system in which electrical energy is required to switch between the transparent and translucent states. Liquid crystal/polymer composites, such as polymer dispersed liquid crystals (LCs) [4–11] and polymer networked liquid crystals (PNLCs) [12–18], have been used for information displays, as well as for light shutters. In the absence of an applied electric field, the composites display either the opaque or the translucent state due to the refractive index mismatch between the LC domains and the polymer matrix or between LC domains. When an electric field is applied between the two transparent electrodes on each substrate, LCs are aligned homeotropically along the direction of the applied field, and the composite cells become transparent.
In addition to electrically controllable composite cells, optically controllable composite cells using photosensitive materials, especially azobenzene materials, have been a subject of interest [19–21]. LCs mixed with azobenzene undergo a large change of the order parameter as a result of trans-cis photo-isomerization [22–24], which lowers the clearing point of the LC mixture over the course of the continuous UV exposure. As such, an optically controllable composite cell can be switched from the translucent to the transparent state by a nematic-isotropic phase transition. Although the cis isomer thermally relaxes to the trans form over time, thermal relaxation typically takes tens of hours. To return to the initial translucent state rapidly, a second light source with a longer wavelength is required to induce the cis-trans photo-isomerization. Slow relaxation and the requirement of two light sources make it difficult to develop practical displays or smart window application with an optically controllable composite cell.
In this study, we propose a sunlight-switchable light shutter using LCs doped with push-pull azobenzene, which is known to speed up thermal relaxation. We confirmed not only optical switching by sunlight, but also electrical switching between the translucent and transparent states by applying a vertical electric field to the proposed light shutter. A sunlight-switchable light shutter, which enables automatic transition between the translucent nematic and transparent isotropic states, depending upon the ambient conditions, is a promising approach for developing smart windows. With such a light shutter, we could clearly see outside through the window during the daytime. It could also conceal the indoor area, providing privacy, during the night. Moreover, this photo-reaction would be reversible and function with no power consumption.
2. Principle of operation
Our approach to a sunlight-switchable light shutter using LCs doped with push-pull azobenzene is illustrated in Fig. 1. In the initial translucent state, the incident light is scattered because of the refractive index mismatch between randomly oriented LC domains. The switching mechanism is based on the change of the LC order parameter induced by the photo-isomerization effect of push-pull azobenzene. The change in the order parameter lowers the clearing point of the LC over the course of continuous UV exposure [22–24]. Therefore, under sunlight irradiation, the proposed light shutter is switched from the translucent to the transparent state by nematic-isotropic phase transition of the LC domains triggered by trans-cis photo-isomerization of push-pull azobenzene molecules.
The cis-trans isomerization can occur thermally, which typically takes tens of hours. To overcome the slow relaxation, we used push-pull azobenzene molecules, which have been shown previously to speed up thermal relaxations [25, 26]. When the amount of sunlight is low because of cloud cover or when there is no sunlight at sunset, the proposed light shutter rapidly relaxes from the transparent isotropic to the translucent nematic state by cis-trans back-isomerization of the azobenzene molecules. Switching can also be activated by an applied vertical electric field, thereby aligning the LCs homeotropically along the direction of the applied field and causing the shutter to become transparent.
3. Cell fabrication
To confirm the optical and electrical switching characteristics of the proposed light shutter, we fabricated LC/polymer composite cells using 85.72 wt% of nematic LC 5CB (4′-Pentyl-4-biphenylcarbonitrile) mixed with 9.52 wt% of prepolymer BPA-DMA (bisphenol A dimethacrylate) and 4.76 wt% of push-pull azobenzene HABA (2-(4-hydroxyphenylazo)benzoic acid) (Fig. 2). The materials were mixed in a glass vial by stirring continuously for 24 h at room temperature.
In order to apply a vertical electric field for electrical switching, the top and bottom substrates contained the transparent indium-tin-oxide electrodes. The mixture was filled into a 5 μm-thick cell via capillary action and then photo-polymerized by exposure to unpolarized UV light (365 nm, mercury arc lamp Osram HBO 103 W/2) with an intensity of 50 mW/cm2 for 8 min at room temperature.
4. Experimental results and discussion
The clearing point of 5CB doped with the push-pull azobenzene HABA was measured as a function of the HABA concentration, as shown in Fig. 3. Before UV irradiation, the clearing point did not change with an increase of HABA, owing to its rod-like molecular shape [27, 28]. Once the mixture was irradiated with UV light and the trans-cis photo-isomerization took place, the bent shape of the cis-azobenzene introduced molecular disorder into the mixture [28], thereby lowering the clearing point. When exposed to UV light, the clearing point of the mixture (5 wt% HABA) changed from 35 °C to 27 °C. In other words, the phase transition of the LC mixture between nematic and isotropic states was controlled by UV light for temperatures ranging from 27 °C to 35 °C. It should be noted that the switching temperature of the proposed light shutter depends on the LCs utilized.
The electro-optical performance of the proposed light shutter was measured using a haze meter (HM-65W, Murakami Color Research Laboratory). Figure 4 shows the total transmittance, specular transmittance, and haze of the light shutter as a function of the applied voltage. The specular [diffuse] transmittance, Ts [Td], refers to the ratio of the power of the beam that emerges from a sample cell, which is parallel (within a small 2.5° range of angles) [not parallel] to a beam entering the cell, to the power carried by the beam entering the sample. The total transmittance, Tt, is the sum of the specular transmittance Ts and the diffuse transmittance Td. The haze H can be calculated as the ratio H = Td / Tt. In the initial opaque state, the measured Tt, Ts, and H of the fabricated light shutter were 81.2%, 20.9%, and 74.3%, respectively. Although the specular transmittance was low, total transmittance was high because the light shutter did not contain any material for light absorption. Specular transmittance increased and the haze decreased gradually as the applied voltage increased, while the total transmittance remained nearly the same. In the transparent state, light scattering was minimized. The measured total transmittance, specular transmittance, and haze in the transparent state were 83.6%, 81.5%, and 2.5%, respectively.
Fig. 4 Total transmittance, specular transmittance, and haze of the fabricated light shutter as function of the applied voltage.
To confirm optical switching of the fabricated light shutter, the effect of UV intensity on the specular transmittance was evaluated. The measurement was carried out at 27 °C, which is the clearing point of 5CB doped with cis-HABA, using a Mettler FP82 hot-stage and a Mettler FP90 controller. The UV intensity was varied from 5 to 15 mW/cm2. The phase transition of the LCs in the light shutter was affected by the UV intensity, as shown in Fig. 5. The transparent state was observed at all measured light intensities, and the transition time from the translucent to the transparent state decreased with increasing light intensity. The results shown in Fig. 5(a) are closely related to the amount of cis-azobenzenes in the sunlight-switchable light shutter [29]. The amount of the cis-azobenzenes produced per unit time was dependent on the intensity of the UV light. Therefore, the transition time from the translucent to the transparent state in a sunlight-switchable light shutter was dependent on the amount of the cis form of the azobenzene present as a function of the irradiation time. The transition from the transparent to the translucent state was achieved without any external power or light because the proposed light shutter contained the push-pull azobenzene, which has a fast thermal relaxation. The transition took place in about 30 s, and the transition time was not dependent on the UV intensity, as shown in Fig. 5(b).
To compare the relaxation time of push-pull azobenzene with a typical azobenzene, we fabricated a LC/polymer composite cell with a typical azobenzene. 5CB was mixed with 9.52 wt% of prepolymer BPA-DMA, and 4.76 wt% of azobenzene PABA (4-(phenylazo)benzoic acid) (Fig. 6(a))was doped into the LC mixture. Figure 6(b) shows the relaxation time of the LC/polymer composite cells. The relaxation of a composite cell with the push-pull azobenzene took place in around 30 s, whereas one with a typical azobenzene did not relax at all within the 7 h timeframe of our experiment. The fabricated composite cell with a typical azobenzene required a second light source with a longer wavelength to induce the cis-trans photo-isomerization to return to the initial state. This requirement for two light sources, along with their inherently long relaxation times, prevents such composite cells from being practically useful in optically controlled smart window applications.
Fig. 6 (a) Molecular structures of PABA (4-(phenylazo)benzoic acid). (b) Relaxation of LC/polymer composite cells using azobenzene PABA and push-pull azobenzene HABA.
The transmission spectra ranging from 400 to 700 nm of a sunlight-switchable light shutter measured using a spectrometer (MCPD 3000, Photal) are shown in Fig. 7. The filled black circle depicts the initial translucent state of the proposed light shutter. The filled blue and red circles depict the electrically and optically induced transparent states, respectively. The measured specular transmittances were 15.9%, 71.5%, and 73.2% in the translucent, electrically induced transparent, and optically induced transparent states, respectively. The optically induced isotropic state showed nearly the same specular transmittance as the electrically induced homeotropic state. In the transparent state, because of the low concentration of the polymer, the effect of refractive index mismatch between the LC in the isotropic state and the polymer is so small that it can be neglected [30]. A decrease in transmittance for short wavelengths was observed in the transparent states because the incident light with wavelength shorter than ~500 nm was absorbed by the push-pull azobenzene HABA.
Figure 8 shows the results of the outdoor switching test performed under sunlight for the fabricated sunlight-switchable light shutter and LC/polymer composite cell. The measured temperature of the fabricated cells was about 28 °C when irradiated with sunlight. At the initial state, both cells were translucent and blocked the view. When exposed to sunlight, the cell doped with the push-pull azobenzene became transparent and the background scene was clearly visible, whereas the LC/polymer composite cell remained translucent. As soon as the sunlight was blocked, the cell doped with the push-pull azobenzene returned to the initial translucent state within 30 s. All of the transitions between states were achieved without any external power or signal.
Fig. 8 Photographs of the translucent-transparent transition of the proposed light shutter (left) and a LC/polymer composite cell (right).
This reversible transition of a sunlight-switchable light shutter between transparent and translucent states could be repeated multiple times without any deterioration in performance. Figure 9 shows the specular transmittance as the cell underwent dozens of cycles of UV irradiation between 10 and 0 mW/cm2. The proposed light shutter was transparent under UV illumination and became translucent in the absence of UV illumination. The transition between the transparent and translucent states was fully reversible and repeatable.
Fig. 9 Reversible and repeatable transition of a sunlight-switchable light shutter between the transparent and translucent states.
5. Conclusion
A sunlight-switchable light shutter using a LC/polymer composite doped with push-pull azobenzene was demonstrated. The light shutter was switchable between the translucent and transparent states, not only by applying an electric field, but also by UV irradiation. Under sunlight irradiation, the light shutter switched from the translucent to transparent state by a nematic-isotropic phase transition of the LC domains triggered by trans-cis photo-isomerization of push-pull azobenzene molecules. When the intensity of sunlight was low because of cloud cover or when there was little sunlight at sunset, the light shutter rapidly relaxed from the transparent state back to its initial translucent state without a second light source with a longer wavelength. With such a light shutter, we could clearly see outside through the window during the daytime, while blocking the view of the indoor area for privacy during the night. This photo-reaction is reversible and requires no power consumption. By electrical switching of a light shutter placed at the back of a see-through display, we could choose between the see-through mode and the high-visibility mode in a display. Although the switching temperature range of the proposed light shutter is currently rather narrow, we expect that this problem can be overcome by developing new azobenzene materials. Moreover, when the temperature is out of the switching temperature range, it can be switched between the two states by applying an electric field.
Acknowledgment
This work was supported by the IT R&D program of MOTIE/KEIT [10042412, More than 60˝ Transparent Flexible Display with UD Resolution, Transparency 40% for Transparent Flexible Display in Large Area] and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2014R1A2A1A01004943).
References and links
1. D. Cupelli, F. P. Nicoletta, S. Manfredi, M. Vivacqua, P. Formoso, G. D. Filpo, and G. Chidichimo, “Self-adjusting smart windows based on polymer-dispersed liquid crystals,” Sol. Energy Mater. Sol. Cells 93(11), 2008–2012 (2009). [CrossRef]
2. T.-Y. Kim, S. M. Cho, C. S. Ah, K.-S. Suh, H. Ryu, and H. Y. Chu, “Electrochromic device for the reversible electrodeposition system,” J. Inf. Disp. 15(1), 13–17 (2014). [CrossRef]
3. D. Kim, E. Lee, H. S. Lee, and J. Yoon, “Energy efficient glazing for adaptive solar control fabricated with photothermotropic hydrogels containing graphene oxide,” Sci. Rep. 5, 7646 (2015). [CrossRef] [PubMed]
4. J. W. Doane, N. A. Vaz, B.-G. Wu, and S. Žumer, “Field controlled light scattering from nematic micorodroplets,” Appl. Phys. Lett. 48(4), 269–271 (1986). [CrossRef]
5. P. S. Drzaic, “Polymer dispersed nematic liquid crystal for large area displays and light valves,” J. Appl. Phys. 60(6), 2142–2146 (1986). [CrossRef]
6. P. P. Crooker and D. K. Yang, “Polymer-dispersed chiral liquid crystal color display,” Appl. Phys. Lett. 57(24), 2529–2531 (1990). [CrossRef]
7. V. Vorflusev and S. Kumar, “Phase-separated composite films for liquid crystal displays,” Science 283(5409), 1903–1905 (1999). [CrossRef] [PubMed]
8. P. Mach, S. J. Rodriguez, R. Notrup, P. Wiltzius, and J. A. Rogers, “Monolithically integrated, flexible display of polymer-dispersed liquid crystal driven by rubber-stamped organic thin-film transistors,” Appl. Phys. Lett. 78(23), 3592–3594 (2001). [CrossRef]
9. J. Qi, L. Li, M. D. Sarkar, and G. P. Crawford, “Nonlocal photopolymerization effect in the formation of reflective holographic polymer-dispersed liquid crystals,” J. Appl. Phys. 96(5), 2443–2450 (2004). [CrossRef]
10. W. Li, H. Cao, M. Kashima, F. Liu, Z. Cheng, Z. Yang, S. Zhu, and H. Yang, “Control of the microstructure of polymer network and effects of the microstructures on light scattering properties of UV-cured polymer-dispersed liquid crystal films,” J. Polym. Sci., B, Polym. Phys. 46(19), 2090–2099 (2008). [CrossRef]
11. S.-W. Oh, J.-M. Baek, J. Heo, and T.-H. Yoon, “Dye-doped cholesteric liquid crystal light shutter with a polymer-dispersed liquid crystal film,” Dyes Pigments 134, 36–40 (2016). [CrossRef]
12. D. J. Broer, J. Lub, and G. N. Mol, “Wide-band reflective polarizers from cholesteric polymer networks with a pitch gradient,” Nature 378(6556), 467–469 (1995). [CrossRef]
13. H. Ren and S.-T. Wu, “Anisotropic liquid crystal gels for switchable polarizers and displays,” Appl. Phys. Lett. 81(8), 1432–1434 (2002). [CrossRef]
14. H. Ren, T.-H. Lin, Y.-H. Fan, and S.-T. Wu, “In-plane switching liquid crystal gel for polarization-independent light switch,” J. Appl. Phys. 96(7), 3609–3611 (2004). [CrossRef]
15. L. T. de Haan, C. Sánchez-Somolinos, C. M. W. Bastiaansen, A. P. H. J. Schenning, and D. J. Broer, “Engineering of complex order and the macroscopic deformation of liquid crystal polymer networks,” Angew. Chem. Int. Ed. Engl. 51(50), 12469–12472 (2012). [CrossRef] [PubMed]
16. J. Heo, J.-W. Huh, and T.-H. Yoon, “Fast-switching initially-transparent liquid crystal light shutter with crossed patterned electrodes,” AIP Adv. 5(4), 047118 (2015). [CrossRef]
17. J.-W. Huh, S.-M. Ji, J. Heo, B.-H. Yu, and T.-H. Yoon, “Bistable light shutter using dye-doped cholesteric liquid crystals driven with crossed patterned electrodes,” J. Disp. Technol. 12(8), 779–783 (2016). [CrossRef]
18. D.-K. Yang and S.-T. Wu, Fundamentals of Liquid Crystal Devices (Wiley, 2006).
19. S. Kurihara, K. Masumoto, and T. Nonaka, “Optical shutter driven photochemically from anisotropic polymer network containing liquid crystalline and azobenzene molecules,” Appl. Phys. Lett. 73(2), 160–162 (1998). [CrossRef]
20. Y.-C. Liu, K.-T. Cheng, Y.-D. Chen, and A. Y.-G. Fuh, “All-optically controllable and highly efficient scattering mode light modulator based on azobenzene liquid crystals and poly(N-vinylcarbazole) films,” Opt. Express 21(15), 18492–18500 (2013). [CrossRef] [PubMed]
21. D.-Y. Kim, S.-A. Lee, H. Kim, S. Min Kim, N. Kim, and K.-U. Jeong, “An azobenzene-based photochromic liquid crystalline amphiphile for a remote-controllable light shutter,” Chem. Commun. (Camb.) 51(55), 11080–11083 (2015). [CrossRef] [PubMed]
22. J. P. Otruba III and R. G. Weiss, “Liquid crystalline solvents as mechanistic probes. 11. The syn → anti thermal isomerization mechanism of some low-bipolarity azobenzenes,” J. Org. Chem. 48(20), 3448–3453 (1983). [CrossRef]
23. Y. Kawanishi, T. Tamaki, and K. Ichimura, “Reversible photoinduced phase transition and image recording in polymer-dispersed liquid crystals,” J. Phys. D 24(5), 782–784 (1991). [CrossRef]
24. J. Bin and W. S. Oates, “A unified material description for light induced deformation in azobenzene polymers,” Sci. Rep. 5, 14654 (2015). [CrossRef] [PubMed]
25. J. García-Amorós and D. Velasco, “Recent advances towards azobenzene-based light-driven real-time information-transmitting materials,” Beilstein J. Org. Chem. 8(1), 1003–1017 (2012). [CrossRef] [PubMed]
26. S. M. Morris, M. M. Qasim, K. T. Cheng, F. Castles, D.-H. Ko, D. J. Gardiner, S. Nosheen, T. D. Wilkinson, H. J. Coles, C. Burgess, and L. Hill, “Optically activated shutter using a photo-tunable short-pitch chiral nematic liquid crystal,” Appl. Phys. Lett. 103(10), 101105 (2013). [CrossRef]
27. H. Yu and T. Kobayashi, “Photoresponsive block copolymers containing azobenzenes and other chromophores,” Molecules 15(1), 570–603 (2010). [CrossRef] [PubMed]
28. P. El-Kallassi, R. Ferrini, L. Zuppiroli, N. L. Thomas, R. Houdré, A. Berrier, S. Anand, and A. Talneau, “Optical tuning of planar photonic crystals infiltrated with organic molecules,” J. Opt. Soc. Am. B 24(9), 2165–2171 (2007). [CrossRef]
29. H.-K. Lee, A. Kanazawa, T. Shiono, T. Ikeda, T. Fujisawa, M. Aizawa, and B. Lee, “All-optically controllable polymer/liquid crystal composite films containing the azobenzene liquid crystal,” Chem. Mater. 10(5), 1402–1407 (1998). [CrossRef]
30. C.-C. Lee, The Current Trends of Optics and Photonics (Springer, 2014).
