In this study, we fabricated a temperature-responsive infrared reflector that adjusts to temperature changes by changing its transmittance of incident IR light. The device utilized a thermally induced change in the pitch of a cholesteric liquid crystal (CLC) to achieve near-infrared light reflection in a particular wavelength range. In addition, a polymer-stabilized cholesteric liquid crystal (PSCLC) was used as an alternative to further optimize the device performance. Polyethylene terephthalate (PET) was used as the substrate material to allow the reflector to be flexible. The light transmission performance of the reflector at different bending angles was explored, and no significant effect was found. A simulated solar device was established to study the temperature regulation effects of both CLC and PSCLC devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Energy problems are still a hot topic worldwide. Buildings account for more than one third of the total energy consumption, and this number keeps rising . More than 45% of the total solar energy is in the infrared (IR) region, in the wavelength range from 700nm to 1100nm . By controlling the infrared light transmittance without influencing the visible light transmittance, indoor temperature control can be achieved effectively, leading to energy conservation in buildings. Many materials and devices which can regulate IR light to reduce the energy consumption of air-condition and/or heating have been reported [3–5].
IR regulating devices can be divided into two categories: static and dynamic IR regulation. The first group cannot adjust its properties to environmental conditions like seasonal temperature changes, while the second group can adjust its reflectivity and transmittance according to temperature conditions of the environment. Dynamic IR regulating devices can further be divided into electrically responsive devices and temperature responsive devices. Brian et al.  proposed a device which can selectively absorb visible and near-infrared light when a suitable voltage was applied, and the application of a thin film of poly(N-isopropylacrylmide) hydrogel to thermochromic smart windows has been investigated by Zhou et al. . A large number of different materials have been explored to fabricate dynamic IR regulating devices, including thermochromic materials, phase transition materials, and aerogels. Both inorganic materials and organic materials have been used. Compared to inorganic materials, organic materials can usually be processed at lower temperature, they do not corrode, and electromagnetic waves are not be affected by it as they are non-metallic.
Cholesteric liquid crystals (CLCs) are organic materials which have been studied for this purpose for many years . With the addition of a chiral dopant, the physical state of liquid crystals can be transformed from the nematic phase into the cholesteric phase [9,10]. Liquid crystals are known to show a strong response to electric fields, temperature and/or light. In addition, the helical structure of CLCs induces Bragg’s reflection, the wavelength of which depends on the pitch of the CLCs according to Eq. (1), where P is the pitch of CLCs and n is the average reflective index.Eq. (2), ne and no are the extraordinary and ordinary refractive indices, respectively, and the reflection bandwidth is related to the pitch of the CLCs according to Eq. (3), where ∆n is the birefringence of the liquid crystal . As the properties of the reflection band are determined by the pitch of the cholesteric liquid crystal, and the pitch can be controlled by external stimuli, the reflection of CLCs is stimuli-responsive.
There are three main ways to change the IR-reflective properties of a CLC: exposure to an electric field, a specific wavelength of light (when a photoresponsive moiety is present), and a temperature change. Methods to make the reflection band appear and disappear , as well as broadening of the reflection band using electric fields [13,14] have been shown. Electrically responsive windows have very good response performance, but will consume electricity and need human input. Light also can induce a pitch change of CLC materials. Zheng et al. used an optically responsive chiral dopant to realize a light-induced pitch change of a CLC material [15–19]. But it can be very hard for the light used as the trigger to compete with the influence of solar light, so light control is not very suitable for window application. Temperature-responsive windows can also have a good response performance, and as it is an autonomous response it can save more energy and is more convenient to use. Research about thermally induced effects in liquid crystals can be found in many previous publications [20–26]. Sun et al. show the preparation and thermo-optical characteristics of a smart polymer-stabilized liquid crystal thin film based on a smectic A to chiral nematic phase transition . Tzeng et al. show the thermal influences of different concentrations of chiral dopant in liquid crystals , measuring the helical pitch in the cholesteric phase and exploring the relationship of the sensitivity of the selective reflection notch of the cholesteric phase to the thermal tuning and the concentrations of the chiral dopants.
Almost all IR-regulating windows which have been reported are based on non-flexible substrates, which means they are limited when it comes to practical applications, as the whole windows would need to be replaced when these devices are installed. In addition, it is difficult to make curved windows, which are required in certain application such as car windows. However, if a similar system can be made as a flexible film which can be attached to the surface of the windows of buildings or cars, the costs of application will be significantly lower. This paper describes the preparation of a thermo-responsive flexible infrared reflector based on CLCs and polymer stabilized liquid crystals (PSCLC). At low temperature, it will either reflect far IR light or doesn’t reflect any light, while at high temperature, it will reflect near IR light, so more heat will be blocked and the indoor temperature will be reduced. At the same time, the device is fully transparent to visible light. The materials and methods to fabricate the reflector were studied and the influences of different bending curvatures were also explored. The effect of this film on the temperature of an enclosed space was also investigated.
Two series of liquid crystal mixtures were prepared for the experiment. One mixture was composed of the commercially available liquid crystal mixture E7 and chiral dopant S-811, this mixture will hereinafter be referred to as CLC. The other consisted of E7, a variable amount of chiral dopant S-811 and liquid crystal 8CB, 5% monomer HCM009, and 0.5% photoinitiator 651, abbreviated as PSCLC. To determine the proper proportion of the mixtures, S811 and 8CB were mixed in different mass ratios in the above two series. The nematic liquid crystal mixture E7 was purchased from Merck, the liquid crystal 8CB, chiral dopant S811 and photoinitiator 651 were produced from Bayi Space LCD Co., Ltd. The monomer HCM009 was purchased from Shanghai FeiKai Photoelectric Material Co., Ltd. The chemical structures of the substances are shown in Fig. 1.
2.2 Fabrication process
A schematic representation of the flexible near-infrared reflective devices is shown in Fig. 2. The cells were made using two transparent polyethylene terephthalate (PET) films, which were coated with polyvinyl alcohol (PVA). To obtain parallel alignment of the CLC or PSCLC, the PVA-coated substrate was rubbed with a velvet cloth in one direction. The LC material was inserted into the cell by capillarity action. The cell gap was supported by the spacers of 10μm diameter. The PSCLC samples were photo polymerized using 365 nm UV light with a light intensity of 50 mW/cm2 for 300 seconds at room temperature.
2.3 Characterization and measurements
The phase transition temperatures of the mixtures were studied by Differential Scanning Calorimeter (DSC, METTLER DSC 1). The heating and cooling rates were both 2°C/min. Optical images between crossed polarizers were taken using polarized optical microscope (POM, Leica DM 2700 P). Transmittance spectra of the reflectors were measured using an Ocean Optics spectrometer 2000 PRO and a PerkinElmer UV/VIS/NIR Spectrometer Lambda 950. To obtain the transmittance spectrum changes as a function of temperature a Linkam hot stage was used. The morphology images of the polymer network in PSCLC mixture were obtained using a field emission scanning electronmicroscope in high vacuum mode at an acceleration voltage of 30 kV.
To test the effect of thermal control properties of the reflectors, we designed a simulation experiment: the temperature in a thermal insulation box exposed to simulated sunlight with reflector and without reflector was compared. The thermal insulation box was made of an aerogel blanket and insulation tape. To study the influence of bending on the performance of flexible reflectors, we measured the transmittance spectra at the center of the flexible infrared reflector at different curvatures at 26°C. The detection beam passes through the reflector from either the concave side or the convex side. The diameter of the detection beam spot is 2.5mm. We put the reflector in half-cylindrical molds with different diameters, which have an opening to allow the detection beam to go through, and force the PET substrate to bend. The diameters of the half cylindrical models are: 10cm, 5cm, 3.3cm, and 2.5cm; and the curvatures are: 0.2, 0.4, 0.6, and 0.8, respectively.
3. Results and discussion
3.1 Phase transition of CLC and PSCLC mixtures
The liquid crystal phases of the two kinds of liquid crystal mixture were studied by DSC. The CLC mixture phase transition points are shown in Fig. 3(a). It can be seen that the phase transition temperature varies with the concentration of S811 in the liquid crystal mixture. The higher the concentration of S811, the higher the temperature of the transition of the smectic to the cholesteric phase, while the temperature of the cholesteric to isotropic phase transition is decreased. For CLC liquid crystal mixtures with an S811 content of 18% by weight, the phase transition from cholesteric to isotropic was found at 45.9°C. Transition from the smectic phase into the cholesteric phase was found at 14°C. We chose this mixture for further experiments. Figure 3(b) shows the phase transition diagram of four kinds of PSCLC mixtures with different amounts of 8CB, liquid crystal monomer HCM-009, and photoinitiator 651 added to the liquid crystal mixture. 8CB was added to the mixture to ensure a good temperature responsiveness, and to not affect the mixture’s clearing point and SmA → cholesteric transition point. The temperature of the cholesteric phase to isotropic phase of four investigated PSCLC mixtures is around 47°C, which is similar to the CLC mixture we selected. For both CLC and PSCLC, the temperatures of the smectic to cholesteric phase transition are not very obvious, but they can all be found below 15°C. The PSCLC mixture with 5% 8CB was chosen for further experiments.
3.2 Thermal response of CLC and PSCLC reflectors
The reflection band changes as a function of temperature of the reflector prepared using the CLC mixture are shown in Fig. 4(a). As the temperature increased from 14.3°C to 41.4°C, the reflection center shifted to a shorter wavelength. The reflection center reached 980nm at 17.8°C, 850nm at 26°C, and 800nm at 41.1°C. The reflective band started to disappear at around 44°C due to the CLC mixture becoming isotropic and losing its reflective properties. At low temperatures, light can pass through the reflector into the environment, while some of the light in the near-infrared region is blocked by the device at high temperatures. Reflectors made with PSCLC materials appear to have better performance, as shown in Fig. 4(b). In the range of 15°C to 45°C, all the reflecting central wavelengths are in the near infrared region, and the rate of blue shift is reduced compared to the CLC-based device. The reflection center of the device is concentrated in the near-infrared band in the temperature from 15°C to 45°C. The thermal responsive of the CLC and PSCLC samples were tested again after one year’s storage in a drawer at naturally occurring temperature variations. As it kept its temperature responsiveness upon temperature change, the results shows that the performance of the samples remains the same. This indicates that this system has good reversibility.
3.3 Characterization of PSCLC device
The appearance of the reflector prepared by CLC mixture after being cooled from 45°C to a lower temperature (~14°C) is shown in Fig. 5(a). After the device cooled down, there was a very obvious scattering phenomenon visible, which seriously affected the light transmittance. This is caused by the liquid crystal transitioning from the cholesteric phase to the smectic phase, as it is difficult to for the molecules to return to a state of uniform alignment. The observed effect is scattering which mainly occurs between liquid crystal domains with different liquid crystal orientations. The device filled with a PSCLC mixture does not have this problem (Fig. 5(b)), and its transparency after cooling is very high compared to that of CLC device. Figure 5(c) to Fig. 5(f) show POM images of the PSCLC device at different temperatures. It can be seen that the cholesteric structure in the liquid crystal mixture is very clear in the region of (10~40) °C. A more detailed structure of the polymer network can be seen from the scanning electron microscope image (Fig. 5(g)). The polymer liquid crystal can control the alignment of the single domains of the free liquid crystal molecules and ensure they will all align in the same direction, even after being in the isotropic phase. The polymer-stabilized cholesteric liquid crystal system has a relatively small polymer content, and the influence of refractive index mismatch between the polymer network and the liquid crystal is negligible. Meanwhile, the domain boundary between the liquid crystals disappears due to the uniform alignment of the liquid crystal molecules controlled by the network, so that scattering does not occur even if the incident light is obliquely incident. Therefore, the PSCLC material has stable light transmittance at different temperatures.
3.4 Thermal control properties of the reflectors
To test the thermal control properties of the reflectors, we designed a thermal insulation box exposed to simulated sunlight, with reflector and without reflector, as shown in Fig. 6(a). A thermocouple (with 0.1°C accuracy) was used to measure the temperature inside the thermal insulation box, and we collected the temperature inside the thermal insulation box every 5 minutes. To study the effect of the reflector, we obtained three sets of data, corresponding to the CLC based reflector, the PSCLC based reflector, and a reference without any reflector.
The different effects inside the thermal insulation box are shown in Fig. 6(b). As time passes, the temperatures inside the box without a reflector reaches a higher level compared to the situation where a reflector was present. When the temperature was below 45°C, the slope of the curve without reflector higher than that with reflector, when the temperature exceeds 45°C. For both the CLC and PSCLC mixture, the slope of the two curves reached the same value gradually, because the phase transition(from CLC to isotropic) temperature is about 45°C, when the temperature below the phase transition point, the reflector can reflect the infrared light, preventing a lot of heat get into the thermal insulation box effectively. However, as the temperature rises to higher than 45°C, the cholesteric liquid crystal becomes isotropic, and the reflection of the device gradually disappears, so the performance of heat preservation of these two situations almost the same. According to the graph, when the time comes to 46 min, the temperature difference is about 3°C. The CLCs what we fill into the reflector is a single left-hand layer, it just reflect left-handed circular polarized light, so the maximum reflection intensity only can be 50%. In order to fabricate a 100% reflection device, we put a 1/2 wave plate between two left-handed reflection films. As shown in the interior image of Fig. 6(b), when solar light passes the first film, left-handed circularly polarized light is reflected back. The right-handed circularly polarized light passes the film and is rotated to left-handed circularly polarized light by the 1/2 wave plate and reflected back by the second left-handed reflection film. Thus, 100% reflection is achieved. The bilayer reflectors display better temperature control performance than the single ones, the temperature can be reduced by 5°C by reflection compared to the situation without a reflector present. These experimental results are based on a normal incidence. The reflection band of the CLC will shift to a shorter wavelength when the angle between incident direction and normal direction becomes larger. When the reflection band shifts to a shorter wavelength at larger incident angle, more heat will be reflected back because red light has higher radiation heat than IR light. So the devices will have better performance with an oblique incidence.
3.5 Influence of bending on the performance of flexible reflectors
To study the influence of bending on the performance of the flexible reflectors, transmittance spectra of the flexible infrared reflector at different curvatures with the detection beam passes through the reflector from either the concave side or the convex side. The curvatures are: 0.2, 0.4, 0.6, and 0.8, respectively.
The transmittance spectrum of CLC reflector under different curvatures are shown in Fig. 7(a) and Fig. 7(b). The transmitted spectrum of PSCLC reflector under different curvatures are shown in Fig. 7(c) and Fig. 7(d) respectively. According to these transmitted spectra, the reflection center of the flexural center of the reflector mainly keep in a range of 860nm to 900nm at 26°C for both PSCLC and CLC based reflectors, when bend in different curvatures. As shown in Fig. 7(e) and Fig. 7(f), the wavelength of the reflection center increases as the radius of curvature increases within a certain range. However, all reflection center wavelengths fall into the range of 850nm~910nm for both the CLC and PSCLC. There is little difference in transmitted intensity because it will be affected by different angles of incidence light and actually the reflection central have a little blue shift for Bragg reflection of different incident angles. Equation (4) can to explain this phenomenon:
We also tried to heat the bended sample with very strong light, the result shows that the reflection band of bended reflectors will blue shift when temperature rise. This indicate that the thermal response of the reflector still remains when bended. So the reflector has a high transmittance for invisible light, and it has a good thermal response. The reflector is stable in different bending curvatures.
We prepared a temperature-responsive near-infrared reflector using a cholesteric liquid crystal. PET was used as a carrier substrate of the reflector, and the flexible function of the reflective device was realized. This flexible device not only has good temperature-responsive reflection of near-infrared light, but also has good bending characteristics. To improve the light transmittance and stability of the reflector after cooling, a polymer network was used to stabilize the liquid crystal orientation. Using a solar simulation device, the reflection performance of the reflector in near-infrared light was investigated. It was found that the liquid crystal was in the cholesteric phase in the temperature range of (15~45) °C, effectively blocking light in the (800~1100) nm band. However, the reflection band-width of the PSCLC used herein is only about 150 nm, which is not good enough. So we will make broadband PSCLC reflectors by introduce pitch gradient into polymer network in the future.
National Natural Science Foundation of China (21711530647, 51561135014), Guangdong Science and Technology Department (GDSTC) (2018A050501012, 2017B020240002), Guangdong Innovative Research Team Program (2013C102), National Center for International Research on Green Optoelectronics, MOE International Laboratory for Optical Information Technologies, Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, the 111 Project and Yunnan expert workstation (No. 2017IC011).
1. L. Pérez-Lombard, J. Ortiz, and C. Pout, “A review on buildings energy consumption information,” Energy Build. 40(3), 394–398 (2008). [CrossRef]
2. V. Malshe and A. K. Bendiganavale, “Infrared Reflective Inorganic Pigments,” Recent Pat. Chem. Eng. 1(1), 67–79 (2008). [CrossRef]
5. H. Khandelwal, A. P. H. J. Schenning, and M. G. Debije, “Infrared Regulating Smart Window Based on Organic Materials,” Adv. Energy Mater. 7(14), 1602209 (2017). [CrossRef]
7. Y. Zhou, Y. Cai, X. Hu, and Y. Long, “Temperature-responsive hydrogel with ultra-large solar modulation and high luminous transmission for ‘smart window’ applications,” J. Mater. Chem. A Mater. Energy Sustain. 2(33), 13550–13555 (2014). [CrossRef]
8. T. J. White, M. E. McConney, and T. J. Bunning, “Dynamic color in stimuli-responsive cholesteric liquid crystals,” J. Mater. Chem. 20(44), 9832 (2010). [CrossRef]
9. Y. Huang, Y. Zhou, C. Doyle, and S. T. Wu, “Tuning the photonic band gap in cholesteric liquid crystals by temperature-dependent dopant solubility,” Opt. Express 14(3), 1236–1242 (2006). [CrossRef] [PubMed]
10. Z. Cheng, K. Li, F. Wang, X. Wu, J. Xiao, H. Zhang, H. Cao, and H. Yang, “A helix inversion from the temperature-dependent intramolecular chiral conflict,” Liq. Cryst. 38(5), 633–638 (2011). [CrossRef]
11. H. Khandelwal, R. C. G. M. Loonen, J. L. M. Hensen, A. P. H. J. Schenning, and M. G. Debije, “Application of broadband infrared reflector based on cholesteric liquid crystal polymer bilayer film to windows and its impact on reducing the energy consumption in buildings,” J. Mater. Chem. A Mater. Energy Sustain. 2(35), 14622 (2014). [CrossRef]
12. Y. Zhao, L. Zhang, Z. He, G. Chen, D. Wang, H. Zhang, and H. Yang, “Photoinduced polymer-stabilised chiral nematic liquid crystal films reflecting both right- and left-circularly polarised light,” Liq. Cryst. 42(8), 1120–1123 (2015). [CrossRef]
13. H. Khandelwal, M. G. Debije, T. J. White, and A. P. H. J. Schenning, “Electrically tunable infrared reflector with adjustable bandwidth broadening up to 1100 nm,” J. Mater. Chem. A Mater. Energy Sustain. 4(16), 6064–6069 (2016). [CrossRef]
14. H. Nemati, S. Liu, R. S. Zola, V. P. Tondiglia, K. M. Lee, T. White, T. Bunning, and D. K. Yang, “Mechanism of electrically induced photonic band gap broadening in polymer stabilized cholesteric liquid crystals with negative dielectric anisotropies,” Soft Matter 11(6), 1208–1213 (2015). [CrossRef] [PubMed]
15. Z. G. Zheng, Y. Li, H. K. Bisoyi, L. Wang, T. J. Bunning, and Q. Li, “Three-dimensional control of the helical axis of a chiral nematic liquid crystal by light,” Nature 531(7594), 352–356 (2016). [CrossRef] [PubMed]
16. Z. G. Zheng, B. W. Liu, L. Zhou, W. Wang, W. Hu, and D. Shen, “Wide tunable lasing in photoresponsive chiral liquid crystal emulsion,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(11), 2462–2470 (2015). [CrossRef]
17. P. Z. Sun, Z. Liu, W. Wang, L. L. Ma, D. Shen, W. Hu, Y. Lu, L. Chen, and Z. G. Zheng, “Light-reconfigured waveband-selective diffraction device enabled by micro-patterning of a photoresponsive self-organized helical superstructure,” J. Mater. Chem. C Mater. Opt. Electron. Devices 4(39), 9325–9330 (2016). [CrossRef]
18. Z. G. Zheng, C. L. Yuan, W. Hu, H. K. Bisoyi, M. J. Tang, Z. Liu, P. Z. Sun, W. Q. Yang, X. Q. Wang, D. Shen, Y. Li, F. Ye, Y. Q. Lu, G. Li, and Q. Li, “Light-Patterned Crystallographic Direction of a Self-Organized 3D Soft Photonic Crystal,” Adv. Mater. 29(42), 1703165 (2017). [CrossRef] [PubMed]
19. Z. G. Zheng, R. S. Zola, H. K. Bisoyi, L. Wang, Y. Li, T. J. Bunning, and Q. Li, “Controllable Dynamic Zigzag Pattern Formation in a Soft Helical Superstructure,” Adv. Mater. 29(30), 1701903 (2017). [CrossRef] [PubMed]
20. I. I. Gorina, S. S. Yakovenko, and M. Y. Baranovich, “New Thermal Memory Effect in CLC,” Mol. Cryst. Liq. Cryst. Inc. Nonlinear Opt. 192(1), 263–271 (1990). [CrossRef]
21. J. Li, S. Gauza, and S.-T. Wu, “Temperature effect on liquid crystal refractive indices,” J. Appl. Phys. 96(1), 19–24 (2004). [CrossRef]
22. T. N. Govindaiah, “Phase transition and thermal stability of reentrant smectic phase in mixture of liquid crystalline materials,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 625(1), 99–105 (2016). [CrossRef]
23. X. Wu, H. Cao, R. Guo, K. Li, F. Wang, and H. Yang, “Effect of cholesteric liquid crystalline elastomer with binaphthalene crosslinkings on thermal and optical properties of a liquid crystal that show smectic A-cholesteric phase transition,” Polym. Adv. Technol. 24(2), 228–235 (2013). [CrossRef]
24. J. Sun, H. Wang, L. Wang, H. Cao, H. Xie, X. Luo, J. Xiao, H. Ding, Z. Yang, and H. Yang, “Preparation and thermo-optical characteristics of a smart polymer-stabilized liquid crystal thin film based on smectic A–chiral nematic phase transition,” Smart Mater. Struct. 23(12), 125038 (2014). [CrossRef]
25. S. Y. T. Tzeng, C. N. Chen, and Y. Tzeng, “Thermal tuning band gap in cholesteric liquid crystals,” Liq. Cryst. 37(9), 1221–1224 (2010). [CrossRef]
26. Y. Wang, Z. G. Zheng, H. K. Bisoyi, K. G. Gutierrez-Cuevas, L. Wang, R. S. Zola, and Q. Li, “Thermally reversible full color selective reflection in a self-organized helical superstructure enabled by a bent-core oligomesogen exhibiting a twist-bend nematic phase,” Mater. Horiz. 3(5), 442–446 (2016). [CrossRef]