1 2 3 4 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 5 6 7 Hitesh Khandelwal, ab Roel C. G. M. Loonen, c Jan L. M. Hensen, c Albertus P. H. J. Schenning* a and Michael G. Debije* a 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 a Functional rganic Materials and Devices, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands, b Dutch Polymer Institute (DPI), P.. Box 902, 5600 AX Eindhoven, The Netherlands c Unit Building Physics and Services, Department of the Built Environment, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands Email: A.P.H.J.Schenning@tue.nl and m.g.debije@tue.nl Corresponding Author Michael G. Debije Functional rganic Materials and Devices, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands, Email: m.g.debije@tue.nl Phone No: +31-40-247-5881 28 29 30
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Abstract An infrared (IR) polymer reflector based on chiral nematic (cholesteric) liquid crystals has been fabricated which can reflect more than 60% of solar IR energy without interfering with the visible solar radiation. Simulations show that the polymer bilayer film applied to a window of a typical building can have a significant impact on the interior temperature in living and working spaces. Key words Cholesteric liquid crystals (Ch-LC), Infrared (IR) reflector, Circularly polarized (CP) light. Introduction ver the past few decades there has been a dramatic increase in the global demand for energy and this is not likely to diminish in the near future.[1] A main source of energy consumption and expense in buildings, cars, greenhouses and indoor spaces is active cooling devices used to maintain comfortable temperatures.[2] As a consequence of climate change, it is predicted that the use of heating devices will be reduced by 34% while cooling device use will increase by 74% by the year 2100.[3] The transmission of excess infrared (IR) light through glass windows requires additional energy for cooling indoor spaces. To reduce the demand on cooling systems, a number of devices have been developed that control the transparency of windows.[4] Examples include mechanical solutions such as shutters and blinds. ther solutions are more materials based,[5] such as electro- and photochromic windows that alter their coloration upon exposure to light,[6, 7] liquid crystal based switchable windows,[8, 9] and thin metallic coatings.[10] An IR reflector that is able to reject IR light without consuming any energy would be very attractive alternative solution, especially if it does not affect light in the visible region. High transparency in the visible region is preferable because solar light can be used to increase indoor illuminance levels, thereby reducing energy consumption by artificial lights, while also maximizing the positive impacts of daylight and temperature control on occupant s performance and well-being.[11, 12] Most IR-reflectors reported today are based on inorganic materials.[13-15] Recently, a switchable IR reflector based on tin-doped indium oxide nanocrystals into niobium oxide glass has been reported.[14, 15] Reflection of infrared light using alternating layers of low
61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 and high refractive index organic materials have also been reported.[16] However, fabrication of IR reflectors for office windows, for example, demands low-cost processing in which the number of layers necessary is reduced and that maintain transparency in the visible wavelength range.[13-15] Polymer based reflectors are attractive due to their ease of processing and the possibility of tailoring specific properties. In this article we present facile processing methods that use only two organic layers of a chiral nematic (cholesteric) liquid crystal to achieve maximum reflection of infrared light with transparency in the visible region. A cholesteric liquid crystal (Ch-LC) is formed by doping a nematic LC with a chiral molecule that generates a helical twist between LC layers, allowing selective reflection of a bandwidth of incident light. The wavelengths of the light reflected by Ch-LC are determined by their helical pitch.[17] A single pitched Ch-LC reflects light between λmax = nep and λmin = nop, where ne and no are the extraordinary and ordinary refractive index of aligned LCs and P is the pitch of the Ch-LC. The bandwidth of the reflected light is given by Δλ = λmax λmin = (ne-no)p = np, where n is the birefringence of the host LC.[18] For a LC mixture, the value of n is typically between 0.05 and 0.2, so the bandwidth of the light reflected by the single pitched Ch-LCs is generally relatively narrow (~ 75 nm). Because a single Ch-LC layer only reflects one handedness of incident circularly polarized (CP) light matching the twist of the helix and transmits the light of opposite handedness, total reflection by a single Ch-LC layer cannot go beyond 50%. Several methods have been demonstrated to enhance the bandwidth[19-26] and reflection limit more than 50%.[23, 27-36] Here, we extend the use of Ch-LC in the IR region, demonstrating the combination of broadband with more than 50% reflection of unpolarised light, which has not been demonstrated before. In this work we report the fabrication of pure organic-based broadband hyperreflective infrared mirror consisting of only two polymer layers. We have successfully made a device which reflects more than 90% of incoming infrared light from 700 to 1100 nm while remaining almost entirely transparent in the visible region. Infrared radiation from the sun spans the wavelengths from 700 nm to 1 mm. A simple calculation shows more than 60% of the energy of the entire infrared solar spectrum lies between 700 and 1100 nm. We have therefore focused on reflecting only this relatively small wavelength region. We also demonstrate the potential impact of these reflectors on room environments by presenting a simulation for an office room in a temperate city and show interior temperature reductions of more than 5 ºC by employing these reflectors on windows.
94 95 96 97 98 99 100 101 102 103 104 105 106 Experimental Section Materials and Sample Preparation The liquid crystals DB-162 and DB-335 were synthesized through a procedure reported earlier.[18, 19, 37] The LCs RM-257 and RM-96 were purchased from Merck, the photoinitiator Irgacure-651 and UV-absorber Tinuvin-328 were purchased from Ciba Specialty Chemicals Ltd (see Fig. 1 for chemical structures of the materials). A halfwave retarder was purchased from Edmund ptics. The following four liquid crystal mixtures were prepared to fabricate the IR reflectors: Right-handed liquid crystal mixture[18, 19, 37] Mixture 1 : DB-162/DB-335/Irgacure-651 (Weight ratio 71/28/1) Mixture 2 : DB-162/DB-335/Irgacure-651/Tinuvin-328 (Weight ratio 70/ 28/1/1) The helical twisting power (HTP) of DB-335 is found to be 4.1 μm -1 for the right-handed liquid crystal mixture. 107 108 109 110 111 Left-handed liquid crystal mixture Mixture 3 : RM-257/RM-96/Irgacure-651 (Weight ratio 64/35/1) Mixture 4 : RM-257/RM-96/Irgacure-651/Tinuvin 328 (Weight ratio 63/35/1/1) The HTP of RM-96 is found to be -3.2 μm -1 for the left-handed liquid crystal mixture. 112 113 114 115 116 117 118 119 Sample preparation Rubbed polyimide cells with gap thickness 6 and 25 µm were filled with mixtures 1, 2 and 3, 4, respectively, at 105 C by capillary action. Films 1 and 3 were photopolymerized at 105 C in presence of UV light of intensity ~7.5 mw/cm 2. Films 2 and 4 were exposed to UV light of intensity ~1.3 x 10-5 W/cm 2 for 3 and 60 min. The samples were subsequently postcured with a UV flood exposure of intensity ~7.5 mw/cm 2. The wavelength of the UV light used for polymerization of liquid crystal mixture is 365 nm for all the experiments. 120
DB-162 Cr 100 N 175 I DB-335 Cr 70 N 92 I RM-257 Cr 70 N 126 I RM-96 Cr 80 N 132 I H N N N 121 122 123 Irgacure 651 Tinuvin 328 Fig.1. Molecular structures of chemicals used for fabricating the right- and left-handed reflective Ch-LC films and their respective phases. 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 Results and Discussion Right- and left-handed Ch-LC mixtures were developed to have a reflectance peak centered around 900 nm. For the right-handed reflective film, a blend (mixture 1) of a diacrylate chiral LC dopant and a monoacrylate achiral LC was used as reported previously.[18, 19] With 28% chiral dopant, we obtained a polymer film with a reflection band centered at 875 nm and a bandwidth of 110 nm (Fig. 2a) after UV-polymerization. For the fabrication of the left-handed mixture, an achiral diacrylate LC was mixed with a chiral monoacrylate LC dopant (mixture 3). Using 35% chiral dopant, the left-handed polymer film showed a reflection band centered at 900 nm with a bandwidth of 100 nm (Fig. 2b). Due to the birefringent nature of organic-based LC materials, the bandwidth obtained from right- and left-handed mixtures (i.e. 110 and 100 nm) are too small to reflect a substantial amount of infrared light. To achieve broadband infrared reflection from 700 to 1100 nm, polymerization induced diffusion during photo-polymerization was used. A UV-intensity gradient was created by adding a UV absorbing dye (Tinuvin) to both right- and left- handed mixtures. Upon UV irradiation, the intensity gradient induces the diacrylate (bi-functional) molecules
141 142 143 144 145 Fig. 2. Transmission spectra of mixtures (a) 1 and (b) 3 after UV polymerization; mixtures (c) 2 and (d) 4 after photo-polymerization by low intensity UV (for photoinduced diffusion) followed by high intensity UV light respectively. (Insets) Photographs of the cells demonstrate the clarity of the samples. 146 147 148 149 150 151 152 153 154 155 156 157 to undergo faster polymerization at the top of the film compared to the bottom. This results in the depletion of diacrylate molecules at the top side of the cell and results in the diffusion of diacrylate molecule from bottom to top. Diffusion of this sort causes an uneven concentration distribution of chiral dopant, and thus a pitch gradient is attained throughout the thickness of the film. In the case of the right-handed mixture, the concentration of chiral dopant will be maximum at the exposed side of the film, opposite to the left-handed film, which will have the highest chiral dopant concentration at the non-exposed side. To obtain broadband infrared reflection, the right- handed mixture was illuminated with the UV light of intensity ~1.3 x 10-5 W/cm 2 at 105 C for 3 min.[18, 19] Thereafter, the sample was post cured by exposing UV light of intensity ~7.5 mw/cm 2. The bandwidth of right-handed (mixture 2) film was observed to be 268 nm (Fig. 2c). To obtain the same
158 159 160 161 162 163 164 165 166 167 168 169 170 171 enhancement in the bandwidth in left-handed mixture 4, the filled cell was exposed to UV illumination for one hour, and a broadband reflector was obtained having a bandwidth of 277 nm (Fig. 2d), comparable to the right-handed Ch-LC film. The large difference in UV illumination time is probably the result of the difference in the reactivity of the chiral molecule and/or difference in viscosity in both mixtures, and/or differences in cell thicknesses. The increase in the bandwidth of 158 and 177 nm, respectively, is due to photoinduced diffusion during photopolymerization. The films reflect a broad range of rightand left-handed CP infrared light from 700 to 1100 nm and are transparent in the visible region (Inset Fig 2(c) and (d)). To confirm the stability of the reflection band at elevated temperatures, we carried out temperature dependent transmission measurements of the polymer films. The spectra were found to be approximately the same over the temperature range 20 C to 100 C (Fig. S1 in Supplementary Information), suggesting these IR reflectors can be operated at most terrestrial locales without significantly affecting the properties of the liquid crystal polymer film. 172 173 174 175 176 177 Fig. 3. (a) Transmission spectrum of right- (film 2) and left- (film 4) handed films superimposed on each other (approach 1). (b) Transmission spectrum of the combination of halfwave plate inserted between two left-handed (film 4) Ch-LC films (approach 2). (Insets) Photographs of the cells demonstrate the clarity of the samples. 178 179 180 181 Two approaches were used to achieve the goal of nearly 100% reflection of unpolarized infrared light. In the first, right- and left-handed broadband films (mixture 2 and 4) were superimposed on each other to fabricate a polymer bilayer (Fig S2 (a) in
182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 Supplementary Information). The transmission spectrum of the bilayer film shows reflection of more than 95% of unpolarised infrared light from 700 to 1100 nm (Fig. 3a) while maintaining high transparency for visible light (Inset Fig. 3a). With this film coated over a window, more than 60% of total incident infrared light energy can be reflected and allow visible light to be transmitted. In the second approach, a halfwave birefringent polymer plate is used to achieve more than 50% reflection of unpolarised infrared light (Fig S2 (b) in Supplementary Information). Halfwave plates introduce a phase shift of between the two orthogonal plane polarized field vectors that represent right- and left-handed circularly polarized (CP) light. As a result, the right-/left-handed CP light transforms into left-/righthanded CP light, respectively. Fig. 3b shows the transmission spectrum of the halfwave plate sandwiched between two left-handed Ch-LC. The transmitted right-handed CP light through the left-handed Ch-LC (film 4) is transformed into left-handed CP light on passing through halfwave plate and reflected by left-handed Ch-LC below. Furthermore, the reflected lefthanded CP light is converted into right-handed CP light by the halfwave plate and it was transmitted through the top left-handed Ch-LC. Thus, the reflection of broadband infrared reflective left-handed Ch-LC film is now enhanced and results in more than 80% reflection without affecting the visible region (Inset Fig. 3b). By using this method, only two Ch-LC films of identical content can be used without the need of developing two different broadband Ch-LC films having opposite handed reflection. The decrease in reflection maximum when using the halfwave plate from the first approach using layered right- and left-handed cholesterics could be attributed to the inability of the halfway plate to effectively invert light-handedness over the entire incident range. In this latter approach, three layers were needed to fabricate a full infrared reflector as opposed to stacking right- and left-handed layers directly. With the increase in number of layers, scattering of the light probably increases and therefore the performance decreases. Since the position of the central reflection band of Ch-LC films is dependent on incidence angle of the light,[38] we have studied the reflectance of infrared and transparency of visible light at various viewing angles of the polymer bilayer film. As shown in Fig. 4, a blue shift of 142 nm (the right-handed, left-handed revealed 132 nm and 150 nm blue shifts, respectively, Fig. S3 in Supplementary Information)) was observed, upon changing the view angle from 0º to 60º (Inset Fig. 4).
213 214 215 216 Fig. 4. Angle dependent transmission spectrum of right- and left-handed Ch-LC polymer bilayer films superimposed on each other. (Inset) view angle of the film (Here 0 angle represents the top view of film). 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 To obtain an insight of the potential impact of our IR reflecting polymers incorporated into the building envelope, a set of dynamic building performance simulations (BPS) was performed using ESP-r.[39, 40] In this study, we evaluated the effects of the single layer right/left-handed and superimposed right- and left-handed broadband cholesteric bilayer films on indoor environmental quality, initially by comparing operative room temperatures for two identical cases, using a window with and without the cholesteric reflector. In a later phase, we assess the energy saving potential by assuming a set point temperature of 25 C for the active cooling system. The transmittance values for the reflector used in the simulation were taken from experimental data as a function of incident angle of the light. For the simulation we considered a standard, south facing office zone (3 x 4.5 x 3 m 3 ) with a window-to-wall ratio of 60%. The building model uses a typical scenario (occupancy schedule and internal heat gains) for office buildings, and has a ventilation rate as prescribed by the building standards.[41] We have assumed that the building has no solar shading system. The cholesteric reflector is applied to the inner side of exterior pane of a regular 2 x 6 mm double glazing system (U- value: 2.8 W/m 2 K). We evaluated the performance of the coating for the climates conditions of London, UK and Chicago, USA for the entire year, respectively, using reference weather data for the regions.
235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 n average, in London the peak decrease in daytime temperature with the polymer bilayer coating compared to a double glazing window on sunny days was calculated to be 4 C (Fig. 5a). The temperature difference using only a single handed coating is less than 50% of the effect with superimposed left- and right-handed coating (Fig. S4 in Supplementary Information). This is explained by the fact that transmittance in the visible range is also higher for the left- or right-handed films (~95%) than for the superimposed left- and righthanded films (~80%) coating, which has an additional impact on the total solar transmittance. The calculated temperature difference is directly proportional to the amount of solar radiation incident on the window. n the south facing facade, it is therefore more pronounced in spring and autumn than in summer due to the lower altitude of the sun. During cloudy days the expected temperature difference is smaller, but still significant. The continental climate of Chicago, USA, is known to have higher temperatures in summer and colder winters compared to the temperate conditions in London. A stronger effect on the temperature difference with the cholesteric coatings can also be found for Chicago (Fig. 5b). The results show a temperature difference up to 6 C. 250 251 252 253 254 Fig. 5. Simulated decrease in interior maximum temperature as a function of the day of the year for superimposed right-and left-handed cholesterics for an office room in (a) London, UK and (b) Chicago, USA. (Insets show the temperature decrease on the selected day of June 24th) 255 256 257 258 n comparing the results in terms of cooling demand, the superimposed coating shows the potential to reduce energy consumption for cooling by 23% and 20% in London and Chicago, respectively. It should be emphasized that all these effects take place without
259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 interfering with the visible part of the solar spectrum, in contrast to conventional types of low-solar-gain windows where temperature decrease (or energy saving potential) is achieved at the expense of less effective daylight utilization and reduction of the view to the outside, with unnatural color rendering effects. In many climates, the cholesteric bilayer has an effect not only on cooling load and overheating risk, but also influences heating energy consumption. Due to its lower solar transmittance, the positive contribution of useful solar gains in the heating season also reduces. This negative implication leads to a predicted increase in annual heating energy demand up to 21% and 40% for the case of London and Chicago, respectively. Since the cooling demand is higher in absolute terms, a net energy saving of 16% and 14% can be achieved in London and Chicago, respectively. To further improve the performance of the selectively-reflecting organic layers, a coating that is able to dynamically adapt its IR optical properties in response to variable occupants needs and weather conditions would be a promising direction for further development[9, 42, 43] and is being actively researched in our lab. Conclusions We have described the fabrication of Ch-LC polymer IR reflectors which reflect more than 60% of the total energy from solar infrared radiation while maintaining transparency to the visible region. In our approach, two opposite-handed cholesteric selective reflecting films were superimposed, yielding a polymer bilayer film with ~95% reflection of infrared light. Temperature dependent transmission studies suggest the cholesteric films are stable over the temperature range of 20 C to 100 C, and that these films could be used all over the globe without affecting the reflecting properties. These results show that it is possible to prepare efficient polymer IR reflectors using only two layers of polymer film comparable to the system employing multilayer inorganic or organic reflectors. Simulations show that when these polymer bilayers are coated over the windows of buildings or cars, a considerable amount of incident, unwanted heat could be reflected and therefore a significant amount of energy could be saved on cooling. The simulation predicts a temperature difference of up to 4.5 C and 6 C in London and Chicago respectively, by having the superimposed right- and left-handed bilayer film on the window. 289
290 291 292 Acknowledgements This research forms part of the research programme of the Dutch Polymer Institute (DPI), project 764. 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 Notes Calculated from the data given at the site of National Renewable Energy Laboratory Electronic Supplementary Information (ESI) available: Temperature and angle depended transmission spectra of right- and left- handed films, Schematic representation of the two approaches used to fabricate Infrared reflector, Impact of right- or left- handed film on the interior temperature of office room in London and Chicago. References [1] Asif M, Muneer T. Energy supply, its demand and security issues for developed and emerging economies. Renew Sust Energ Rev 2007;11:1388-413. [2] Pérez-Lombard L, rtiz J, Pout C. A review on buildings energy consumption information. Energ Buildings 2008;40:394-8. [3] Isaac M, van Vuuren DP. Modeling global residential sector energy demand for heating and air conditioning in the context of climate change. Energ Policy 2009;37:507-21. [4] Jelle BP, Hynd A, Gustavsen A, Arasteh D, Goudey H, Hart R. Fenestration of today and tomorrow: A state-of-the-art review and future research opportunities. Sol Energ Mat Sol C 2012;96:1-28. [5] Granqvist CG. Transparent conductors as solar energy materials: A panoramic review. Sol Energ Mat Sol C 2007;91:1529-98. [6] Niklasson GA, Granqvist CG. Electrochromics for smart windows: thin films of tungsten oxide and nickel oxide, and devices based on these. J Mater Chem 2007;17:127-56. [7] Jonsson A, Roos A. Visual and energy performance of switchable windows with antireflection coatings. Sol Energy 2010;84:1370-5. [8] Chen CY, Lo YL. Integration of a-si:h solar cell with novel twist nematic liquid crystal cell for adjustable brightness and enhanced power characteristics. Sol Energ Mat Sol C 2009;93:1268-75. [9] Debije MG. Solar Energy Collectors with Tunable Transmission. Adv Funct, Mater 2010;20:1498-502. [10] Lampert CM. Heat mirror coatings for energy conserving windows. Sol Energ Mater 1981;6:1-41. [11] Loftness V, Hakkinen B, Adan, Nevalainen A. Elements that contribute to healthy building design. Environ Health Persp 2007;115:965-70.
325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 [12] Aries MBC, Veitch JA, Newsham GR. Windows, view, and office characteristics predict physical and psychological discomfort. J Environ Psychol 2010;30:533-41. [13] Park Y, Roh YG, Cho C, Jeon H, Sung MG, Woo JC. GaAs-based near-infrared omnidirectional reflector. Appl Phys Lett 2003;82:2770-2. [14] Korgel BA. Materials science: Composite for smarter windows. Nature 2013;500:278-9. [15] Llordes A, Garcia G, Gazquez J, Milliron DJ. Tunable near-infrared and visible-light transmittance in nanocrystal-in-glass composites. Nature 2013;500:323-6. [16] Convertino A, Valentini A, Cingolani R. rganic multilayers as distributed Bragg reflectors. Appl Phys Lett 1999;75:322-4. [17] Meier G. Handbook of Liquid Crystals. Von H. Kelker und R. Hatz. Verlag Chemie, Weinheim 1980. XVIII, 917 S., geb. DM 420.00. Angew Chem, Int Ed 1980;92:667-8. [18] Broer DJ, Mol GN, Haaren JAMMV, Lub J. Photo-Induced Diffusion in Polymerizing Chiral-Nematic Media. Adv Mater 1999;11:573-8. [19] Broer DJ, Lub J, Mol GN. Wide-band reflective polarizers from cholesteric polymer networks with a pitch gradient. Nature 1995;378:467-9. [20] Robbie K, Brett MJ. Sculptured thin films and glancing angle deposition: Growth mechanics and applications. J Vac Sci Technol A 1997;15:1460-5. [21] Chen SH, Mastrangelo JC, Jin RJ. Glassy Liquid Crystal Films as Broadband Polarizers and Reflectors via Spatially Modulated Photoracemization. Adv Mater 1999;11:1183-6. [22] Mitov M, Boudet A, Sopéna P. From selective to wide-band light reflection: a simple thermal diffusion in a glassy cholesteric liquid crystal. Eur Phys J B 1999;8:327-30. [23] van de Witte P, Brehmer M, Lub J. LCD components obtained by patterning of chiral nematic polymer layers. J Mater Chem 1999;9:2087-94. [24] Boudet A, Binet C, Mitov M, Bourgerette C, Boucher E. Microstructure of variable pitch cholesteric films and its relationship with the optical properties. Eur Phys J E 2000;2:247-53. [25] Mitov M, Dessaud N. Going beyond the reflectance limit of cholesteric liquid crystals. Nat Mater 2006;5:361-4. [26] Guo R, Li K, Cao H, Wu X, Wang G, Cheng Z, et al. Chiral polymer networks with a broad reflection band achieved with varying temperature. Polymer 2010;51:5990-6. [27] Makow DM. Peak reflectance and color gamut of superimposed leftand right-handed cholesteric liquid crystals. Appl pt 1980;19:1274-7. [28] Mitov M, Dessaud N. Cholesteric liquid crystalline materials reflecting more than 50% of unpolarized incident light intensity. Liq Cryst 2007;34:183-93. [29] Guo J, Yang H, Li R, Ji N, Dong X, Wu H, et al. Effect of Network Concentration on the Performance of Polymer-Stabilized Cholesteric Liquid Crystals with a Double-Handed Circularly Polarized Light Reflection Band. J Phys Chem C 2009;113:16538-43. [30] Guo J, Wu H, Chen F, Zhang L, He W, Yang H, et al. Fabrication of multi-pitched photonic structure in cholesteric liquid crystals based on a polymer template with helical structure. J Mater Chem 2010;20:4094-102. [31] Agez G, Mitov M. Cholesteric Liquid Crystalline Materials with a Dual Circularly Polarized Light Reflection Band Fixed at Room Temperature. J Phys Chem B 2011;115:6421-6.
367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 [32] Guo J, Chen F, Qu Z, Yang H, Wei J. Electrothermal Switching Characteristics from a Hydrogen-Bonded Polymer Network Structure in Cholesteric Liquid Crystals with a Double- Handed Circularly Polarized Light Reflection Band. J Phys Chem B 2011;115:861-8. [33] McConney ME, Tondiglia VP, Hurtubise JM, White TJ, Bunning TJ. Photoinduced hyper-reflective cholesteric liquid crystals enabled via surface initiated photopolymerization. Chem Comm 2011;47:505-7. [34] White TJ, Cazzell SA, Freer AS, Yang D-K, Sukhomlinova L, Su L, et al. Widely Tunable, Photoinvertible Cholesteric Liquid Crystals. Adv Mater 2011;23:1389-92. [35] Xiao J, Cao H, He W, Ma Z, Geng J, Wang L, et al. Wide-band reflective polarizers from cholesteric liquid crystals with stable optical properties. J Appl Polym Sci 2007;105:2973-7. [36] Ha NY, htsuka Y, Jeong SM, Nishimura S, Suzaki G, Takanishi Y, et al. Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals. Nat Mater 2008;7:43-7. [37] Broer DJ, Lub J, Mol GN. Photo-controlled diffusion in reacting liquid crystals: A new tool for the creation of complex molecular architectures. Macromol Symp 1997;117:33-42. [38] Yan J, Chen Y, Xu D, Wu ST. Angular dependent reflections of a monodomain blue phase liquid crystal. J Appl Phys 2013;114:113106-9. [39] Clarke JA. Energy simulation in building design: xford: Butterworth-Heinemann; 2001. [40] Hensen JLM, Lamberts R. Building Performance Simulation for Design and peration: London: Spon Press; 2011. [41] ASHRAE Handbook: Fundamentals: American Society of Heating, Refrigerating, and Air-Conditioning Engineers: Atlanta, USA; 2009. [42] Loonen RCGM, Trčka M, Cóstola D, Hensen JLM. Climate adaptive building shells: State-of-the-art and future challenges. Renew Sust Energ Rev 2013;25:483-93. [43] Hoffmann S, Lee ES, Clavero C. Examination of the technical potential of near-infrared switching thermochromic windows for commercial building applications. Sol Energ Mat Sol C 2014;123:65-80. 396