Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands

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1 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 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 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 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, m.g.debije@tue.nl Phone No:

2 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

3 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.

4 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 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 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

5 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 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 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

6 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 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

7 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 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 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

8 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).

9 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) 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.

10 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 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) 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

11 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

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