Thermo-Responsive, Freezing-Resistant Smart Windows with

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Materials and Interfaces

Thermo-Responsive, Freezing-Resistant Smart Windows with Adjustable Transition Temperature Made from Hydroxypropyl Cellulose and Glycerol Chiaki Nakamura, Takashi Yamamoto, Kengo Manabe, Takuto Nakamura, Yasuaki Einaga, and Seimei Shiratori Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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Industrial & Engineering Chemistry Research

Thermo-Responsive, Freezing-Resistant Smart Windows with Adjustable Transition Temperature Made from Hydroxypropyl Cellulose and Glycerol Chiaki Nakamura,† Takashi Yamamoto,† Kengo Manabe,† Takuto Nakamura,† Yasuaki Einaga,† and Seimei Shiratori†,* †Center for Material Design Science, School of Integrated Design Engineering, Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan *[email protected]

ABSTRACT A thermo-responsive smart window that can switch its transmittance to control heating from sunlight is attracting great attention. Such windows made from a hydrogel of a thermo-responsive polymer such as poly(N-isopropylacrylamide) (PNIPAm) or hydroxypropyl cellulose (HPC) have been successful and can switch their transmittance at room temperature. However, such hydrogels occasionally freeze in cold places, degrading their transmittance. Thus, a thermo-responsive hydrogel which can be use in various geographical regions is desired. Here, we produced a thermo-responsive smart

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window with freezing resistance made from HPC and glycerol. We could adjust its switching temperature by simply changing the amount of added glycerol, letting us easily change it to room temperature for practical use. These smart windows showed high cyclic performance, freezing resistance, and heat shielding, demonstrating great potential. INTRODUCTION Because of climate change and the growing population, increasing energy consumption is becoming a worldwide problem. In particular, buildings account for 40% of total energy consumption, so their contribution must be reduced.1,2 The main consumers of energy in buildings are air conditioning and dimming, and especially the energy consumption of cooling system increased by 20% for a 1 C temperature increase, and predicted to increase rapidly because of global warming.3,4 So, smart windows that passively change their transmittance in response to their environment have drawn great attention.5 Many researchers have investigated smart windows made with liquid crystals, but they still consume power because their transition requires an applied voltage.6–8 Also, thermally and optically switchable liquid crystal which is used without energy consumption has been reported, but there is problem for large scale application as a window because of manufacturing cost of liquid crystal.9-11 Thus, smart windows that


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change their transmittance in response to temperature changes without energy consumption are being studied.12 Among them, smart windows that are transparent at low temperature and opaque at high temperature, which prevents the interior temperature from rising, have attracted great attention.13 Specifically, many temperature-responsive smart windows made from vanadium oxide have been reported.14–16 Vanadium oxide changes its structure when heated to 68 C, lowering its transmittance in the infrared region. However, its transition temperature is much higher than room temperature, and its transmittance at room temperature is low. To address this problem, other studies have assessed hydrogels of temperatureresponsive polymers.17–20 For example, temperature-responsive hydrogels made of poly(N-isopropylacrylamide) (PNIPAm) have very high transmittance at room temperature, and when heated to 30 C their transmittance decreases over a wide wavelength range, gives a large modulation to sunlight having intensity in a wide wavelength range from 200 nm to 2500 nm.21 They can also be fabricated easily, lending themselves to various applications. However, the fabrication approach of PNIPAm hydrogels is complicated and it is difficult to remove the odor.22 Beyond these issues, thermo-responsive smart windows have only been studied for use in the hot summer. In the winter, smart windows will freeze, decreasing their



transmittance of visible light and thus the interior temperature.23 Therefore, freezing resistance is desired.24 However, to the best of our knowledge, there are no reports of thermo-responsive, freezing-resistant smart windows. Such multifunctional windows can be used in various geographical regions, which will further decrease worldwide energy consumption. These problems may be solved by using hydroxypropyl cellulose (HPC) and glycerol. HPC has high biocompatibility and has been widely used in the medical field.25,26 Moreover, it is easier to synthesize than PNIPAm. A polymer gets its thermo-responsive properties from hydration and dehydration.27 At low temperature, it swells by taking in water, making the solution transparent. With increasing temperature, the fiber releases water and shrinks, causing scattering.18 Glycerol is used as an anti-freezing agent because it can form a strong hydrogen bond between water and glycerol, which prevents the water from freezing.28, 29 Thus, just adding glycerol should promote the dehydration reaction and adjust the transition temperature. In this study, we prepared a thermo-responsive smart window with freezing resistance by using an aqueous solution of glycerol in water. Changing the glycerol concentration can change the transition temperature of transmittance, adapting the smart window to its application. In this work, we will show that a smart window composed of HPC with a 33 wt.% glycerol mixture is the most


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effective at reducing solar transmission. Moreover, HPC with a glycerol mixture can remain as a liquid at −10 °C for 180 min. These simple-to-fabricate, multifunctional smart windows should find use in many environments. EXPERIMENTAL Materials. We fabricated the smart windows using HPC (Mw ~100,000 g/mol, SigmaAldrich, St. Louis, MO, USA) as the thermo-responsive polymer, glycerol (Wako Pure Chemical Industries, Ltd., Osaka, Japan), and ultrapure water (Aquarius GS-500.CPW, Advantec Toyo Kaisha, Ltd., Japan). Solution preparation. The glycerol concentration in the solution was adjusted to 0, 20, 33, and 50 wt.%. 0.1 g of HPC powder was added to the 20-mL glycerol mixtures, and all solutions were stirred overnight (Table S1).22 However, HPC did not dissolve in a 50 wt.% glycerol. Therefore, we used 3 samples (0, 20, 33 wt.%). Characterization. Optical spectra of the smart windows were measured with an ultraviolet–visible (UV–vis) spectrophotometer (FP-6500, JASCO Co., Tokyo, Japan). The sample temperature was adjusted by using hot-and-cold stage (INSTEC, Inc., USA). Baseline correction was done with the cavity cell inserted. The scattering size was observed with a dynamic light scattering (DLS) spectrophotometer (ELS-8000, Otsuka Electronics Co., Ltd., Osaka, Japan). Transmittance measurement of cyclic performance



was measured by a haze meter (NDH-5000, Nippon Denshoku Industries, Tokyo,Japan) with a white-light-emitting diode (5 V, 3 W). Total transmittance (TT), parallel transmittance (PT), and diffusion (DIF) were measured by haze meter. Total transmittance was defined the following equation. TT = PT + DIF Heat-shielding test. We measured the heat shielding of the smart window in the experimental system (Figure S5). We put black paper to heat inside an insulated Styrofoam box with dimensions of 6.0  6.0  6.0 cm3 and a hole in its top with dimensions of 1.0  4.0 cm2. The prepared solution was poured into an acrylic cell, which was placed under the hole. The box was exposed to an incandescent light (100 W) through the hole, and the temperature of the black paper was monitored using a thermo-camera. Freezing test. Freezing resistance was investigated in the chamber for three samples (HPC in water, HPC in the 20 wt.% glycerol mixture, and HPC in the 33 wt.% glycerol mixture). This test was conducted at 10 C and 15 C for 180 min.

RESULTS AND DISCUSSION Optical properties. Figure 1a,ce shows how the transmittance changed in the samples at various glycerol concentrations and temperatures. The cell used in this measurement


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was 2.0 mm thick. All samples decreased in transmittance with increasing temperature. HPC fibers swelled by taking in water at low temperature, but increasing the temperature drove out water from the HPC fibers and shrank them, causing scattering.30 In addition, increasing the glycerol concentration decreased the temperature of the cloud point (where the transmittance is 10% of the transparent state)31 from 50 C to 30 C. These results show that glycerol promote the release of water from the HPC fibers even at low temperature because of the strong hydrogen bonding between glycerol and water (Figure 1b). The switching temperature was highest in the HPC in water. Even 60 C was too low for this sample to completely transform, so the scattering was small (Figure 1c). Among the samples, the solar modulation was largest for HPC in the 20 wt.% glycerol mixture (Figure 1d). The HPC in the 33 wt.% glycerol mixture showed the lowest switching temperature, but here the HPC did not dissolve in glycerol, which decreased its scattering (Figure 1e). This was due to thickness of the cell. It seems the 2-mm cell was not thick enough for the 33-wt.% sample to decrease the transmittance. Therefore, thickening the cell increased the scattering. Thus, we also measured the transmittance using an acrylic cell with a thickness of only 1.0 cm (Figure S1). Figure S1c shows that HPC in the 33 wt.% glycerol mixture had the largest solar modulation.



We also performed an outdoor test at Keio University, Japan. On a summer day, we placed two smart windows (HPC with water, HPC with 33 wt.% glycerol) outside for 5 min to see how their transmittance changed. The outside temperature was 33 C. Figure 1g shows the color change of the HPC with 33 wt.% glycerol. These results show that the transition temperature can be adjusted for practical situations by adding glycerol. The cyclic performance of HPC was confirmed using capped cells (Figure 1h). HPC with the 33 wt.% glycerol mixture was heated at 60 C for 2 min in the test chamber, then cooled to 0 C for 3 min. This cycle was repeated 30 times. After 30 cycles, this sample maintained its transmittance switching, showing excellent cyclic performance for practical applications. A colorful smart window containing water can be created by mixing in a pigment that reacts with water, such as methylene blue (Figure S2). We also investigated how adding the glycerol affected scattering. The scattering size was measured by DLS. The average scattering size of HPC in water at 60 C was 122.5 nm (Figure 2a), while that of HPC in the 33 wt.% glycerol mixture was 97.3 nm (Figure 2b). HPC cannot dissolve in glycerol, and HPC reacts only with water, causing swelling and shrinking. Thus, adding glycerol did not change the scattering.


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Figure 1. Optical properties of the sample. (a) Photographs of the samples at various temperatures. (b) Schematic of the relationships between HPC, water, and glycerol. (c)



Transmittance of HPC in water at various temperature in the 2-mm-thick cell. (d) Transmittance of HPC in the 20 wt.% glycerol mixture at various temperatures. (e) Transmittance of HPC in the 33 wt.% glycerol mixture. (f) Transmittance change with increasing temperature: (light blue) HPC in water, (green) HPC in the 20 wt.% glycerol mixture, and (orange) HPC in the 33 wt.% glycerol mixture. (g) Photographs of the outdoor test for HPC in water (right) and HPC in the 33 wt.% glycerol mixture (HPC/Gly) (left). (h) Cyclic performance of the HPC with the 33 wt.% glycerol mixture.

Figure 2. Scattering intensity, showing high scattering at particle sizes of 100–150 nm. (a) HPC in water at 60 C, (b) HPC in the 33 wt.% glycerol mixture at 60 C.

Heat-shielding test. We tested heat shielding by using an incandescent light (100 W). Figure S5 shows the experimental setup. This test assessed four samples (air, pure water, HPC with pure water, and HPC with the 33 wt.% glycerol mixture) in a 1.0-cm-thick acrylic cell. Figure 3b shows how the temperature of the black paper in the box changed over time. After light exposure for 280 s, the black paper shielded by water in the cell was 5.1 C lower than that shielded by air in the cell. The water absorbed energy from the light, decreasing the heating of the paper. Adding HPC powder also improved the heat shielding. As the sample with HPC was heated by light exposure, its temperature


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increased and shrank the HPC fiber, causing scattering, so less light passed into the cell. Overall, HPC in the 33 wt.% glycerol mixture showed the most heat shielding: it reduced the heating of the paper in the insulated box by 40% compared with air in the cell. This happened because the transmittance switched at lower temperature, increasing the scattering at an earlier time.

Figure 3. The measurements of heat shielding. (a) Thermographic images of black paper in the box with various samples: cavity alone, water, HPC in water, and HPC in the 33 wt.% glycerol mixture (HPC/Gly). (b) Comparison of the temperature change of the black paper in the box: (navy) cavity cell alone, (light blue) water in cell, (yellow) HPC in water in cell, and (red) HPC in the 33 wt.% glycerol mixture. Freezing resistance. We investigated the freezing resistance of three samples, with 0, 20, and 33 wt.% glycerol. All samples were cooled in the test chamber to −10 C (Figure 4a). The sample without glycerol froze after 10 min, but the samples with glycerol did not freeze even after 180 min, because the glycerol depressed the freezing point of the water.32 Moreover, we confirmed the effect of the glycerol concentration by performing the test again at 15 C (Figure 4b). This time, the sample without glycerol froze. HPC



in the 20 wt.% glycerol mixture froze in 5 min, while HPC in the 33 wt.% glycerol did not freeze for 20 min. Thus, increasing the glycerol concentration improved the freezing resistance.

Figure 4. Freezing resistance of HPC in glycerol (Gly) mixtures of various concentrations at 10 C (a) and 15 C(b). CONCLUSION We prepared a thermo-responsive smart window with freezing resistance by using HPC as the thermo-responsive polymer and glycerol as an anti-freezing agent. By changing the glycerol concentration, we modified the switching temperature of the smart window to adapt it to practical situations. The fabricated smart window showed heat shielding, as it reduced the heating of paper in an insulation box exposed to incandescent light by 40% compared with that of the cavity cell. In addition, the smart window showed freezing resistance; it did not freeze at 10 C for 180 min. Just adding glycerol is possible to fabricate multifunction smart window by one-step method because of strong hydrogen


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bonding between water and glycerol. Overall, this smart window can be used in various environments and shows great potential for energy saving.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/XXXXXXX. Composition of various water/glycerol ratios; Optical measurement by UV-vis using the 1.0 cm-thick cell; the photographs of colorful smart window; the photographs of large-scale smart window; Response speed for smart window; the equipment of heat-shielding test; the photographs of freezing resistance performance (PDF).

AUTHOR INFORMATION

Corresponding Author

E-mail: [email protected]

Fax: +81-45-566-1602

Notes



The authors declare no competing financial interest.

ACKNOWLEDGMENTS

We are deeply grateful to Dr. Kouji Fujimoto whose comments and suggestions are valuable to our study. We appreciate the support from Dr. Yoshio Hotta, whose scrupulous comments are an enormous help. We thank Joshua Yearsley, MS, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

Author Contributions

C.N. conceived, planned and performed the experiments, analyzed the data. C.N., T.N. and K.M. wrote this paper. C.N., T.Y., and Y.E. designed the equipment. K.M. proposed parts of the experiment and sample preparation method. K.M. and T.N. provided experimental support, and contributed to the data analysis. S.S. supervised the project, provided scientific advice, and commented on the manuscript.

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