Research Article www.acsami.org
Stepwise Activation of Switchable Glazing by Compositional Gradient of Copolymers Eunsu Lee,† Dowan Kim,† and Jinhwan Yoon* Department of Chemistry, Dong-A University, 37 Nakdong-Daero 550 beon-gil, Saha-gu, Busan, 49315, Republic of Korea S Supporting Information *
ABSTRACT: Thermotropic glazing is one of the most promising developments for adaptive solar control; however, a monotonic transparent−opaque transition limits its practical application. In this work, to render stepwise activation of the switchable glazing, we prepared multicomposition copolymers having a compositional gradient. By slow addition of the monomers in the reaction mixture during free-radical polymerization, the blend of copolymers with each polymer having different compositions of the monomers could be prepared. We found that the developed copolymers exhibit different thermal behaviors according to the monomer composition, yielding the nearly linear transmittance change over a wide temperature range due to the gradient hydrophilic−hydrophobic balances. By combining prepared copolymers with photothermal graphene oxide as a heat transducer, we demonstrated gradual solar control of the smart window in response to sunlight intensity in outdoor testing. KEYWORDS: smart windows, stepwise solar control, thermotropic hydrogels, gradient compositions, lower critical solution temperature
1. INTRODUCTION
thermotropic materials show optical switching from a transparent to opaque state near the LCST. An important constraining factor for the practical application of thermotropic glazing is the inability to control the transmittance in a stepwise manner on demand. Because of the sharp phase transition of these materials, the change in the optical properties is abrupt, such that only fully transparent or opaque states form at equilibrium. At temperatures below the LCST, the window is clear and transparent, whereas it becomes opaque above the LCST. This suggests that the modification of the LCST can adjust the temperature-dependent switching behaviors of thermotropic glazing. It has been widely reported that the LCST behavior of thermotropic polymers can be quantitatively modulated by the addition of surfactants20 or salts21 and copolymerization with hydrophilic or hydrophobic comonomers.22,23 However, past studies focused on only the shifting of the LCST, rather than the control of the overall temperature range. The thermoresponsive polymers with gradient sequence distributions have been reported to control the temperature-induced micellization of the polymers.24,25 Gradient composition of the polymers could be obtained by continuously feeding a monomer into a reaction mixture with a living cationic24,25 or atom transfer radical polymerization.26 The hydrophilic−hydrophobic balances of each copolymer were gradually controlled by manipulating the monomer composition; hence, a stepwise dehydration-induced micellization was achieved over a broader temperature range. While effectively broadening the transition
In recent decades, the climate of the world has radically changed, and the speed of change is increasing.1 Energy conservation is significant for saving the Earth’s climate, as is reducing the emitted carbon dioxide that contributes to global warming. Since energy consumption for the operation and rehabilitation of buildings comprises 30−40% of the primary energy use in developed countries,1 many energy-efficient developments for buildings have been proposed.2−20 Among them, switchable glazing materials, or so-called smart windows, have received great attention, because they can enhance the energy efficiency of a building by controlling solar transmittance.5−20 The switching of the optical properties of the glazing can also control the opacity of the windows. Glazing materials can be categorized by the switching mechanism as electrochromic, 5−10 thermochromic, 11−13 and thermotropic.14−20 Thermotropic glazing is especially interesting because it has high transparency in a clear state, self-regulating passive control, and simple installation. Thermally responsive hydrogels14−16 or polymers17−20 are mainly used to construct thermotropic glazing, in which the switching mechanism is dominated by the temperature-induced aggregation of the materials in the solvent above the lower critical solution temperature (LCST). At low temperatures, hydrogels or water-soluble polymers are homogeneously dispersed in water on a molecular level, thus minimizing light scattering. As the temperature is increased to exceed the LCST by solar radiation, phase separation between the thermotropic materials and solvent abruptly occurs. The aggregated domains act as scattering centers, affecting the reflection of incident solar radiation. This means that smart windows comprising © XXXX American Chemical Society
Received: August 11, 2016 Accepted: September 19, 2016
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DOI: 10.1021/acsami.6b10091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. (a) Schematic of synthetic approach for multicomposition poly(NIPAm-co-NDEAm-co-VP) copolymer of gradient compositions. (b) GPC, (c) NMR, and (d) temperature-dependent transmittance analysis of products extracted from the reaction mixture throughout polymerization. NDEAm injection, the continuous addition of VP (267 μL, 2.51 mmol) dissolved in 10 mL of DMF was performed at a rate of 0.1 mL/ min. After VP addition, polymerization proceeded for another 150 min. The reaction mixture was precipitated in diethyl ether and then collected by filtration. A series of copolymer blends were synthesized under identical conditions with the exception of the addition rate. To convert the amino end groups to vinyl groups, PNDV-NH2 and 1.2 mol equivalent of N-acryloxysuccinimide was dissolved in 40 mL of DMF and then stirred at 25 °C for 72 h. After the reaction, the mixture was dropped into diethyl ether and the precipitate was collected by filtration. After drying under vacuum, PNDVM was obtained as a white powder. The spectral data for PNDVM were as follows: 1H NMR (400 MHz, DMSO-d6): δ 0.92 (b, 6x + 6yH), 1.28−1.74 (b, 2x + 2y + 2zH), 1.82−2.18 (b, 2x + 2y + 2zH), 2.77 (d, 2H), 2.94 (d, 2H), 3.47 (b, 2zH), 3.69 (b, 4yH), 3.79 (b, xH), 5.84−6.19 (m, 3H) (x = degree of NIPAm polymerization, y = degree of NDEAm polymerization, z = degree of VP polymerization). 2.3. Fabrication of Switchable Glazing. PNDVM (355.1 mg, 28.8 mM) and N,N′-methylenebis(acrylamide) (BisAA; 4.9 mg, 10.6 mM) were dissolved in 3.0 mL of distilled water. After degassing the monomer solution, free-radical polymerization was initiated by adding 9.0 μL of N,N,N′,N′-tetramethylethylenediamine and 18.0 μL of 10 wt % aqueous ammonium persulfate. The initiated pregel solution was immediately loaded into the capillary channel formed by two glass panes separated with spacers of 140 μm in thickness. To covalently attach the hydrogel layer to the glass panes, the panes were treated with the adhesion-promoting [3-(methacryloxy)-propyl]trimethoxysilane. Gelation was performed in a sealed chamber under a positive pressure of nitrogen for 1 h. After formation of the hydrogel layer, the edges of the glass panes were sealed with butyl rubber to prevent water evaporation. To fabricate the sunlight-driven switchable glazing, the PNDV hydrogels incorporating GO sheets were prepared by an identical procedure, except for the composition of pregel solution. The aqueous monomer solution of 2.7 mL, containing 355.1 mg of PNDVM and 4.9 mg of BisAA, was mixed with 0.30 mL of an aqueous dispersion of
range, the technique did not permit continuous thermal transition. Herein, inspired by previous studies regarding the modification of the thermal behavior of polymers, we pursued the preparation of temperature-responsive hydrogels with wide LCST ranges, achieved by copolymers having gradient monomer compositions. Each copolymer has a different monomer composition and different LCST behavior that depends on this composition. The presence of hydrophobic comonomers is expected to decrease the LCST of the copolymer, whereas the presence of hydrophilic comonomers is expected to increase the LCST.
2. EXPERIMENTAL SECTION 2.1. Materials. N-Isopropylacrylamide (NIPAm), N,N′-diethylacrylamide (NDEAm), 2-aminoethanethiol (AESH), and N-acryloxysuccinimide were obtained from TCI (Nihonbashi-honcho, Tokyo, Japan). 2,2′-Azobis(isobutyronitrile) and diethyl ether were purchased from Samchun Pure Chemical (Pyeongtaek, Gyeonggi-do, Korea). The aqueous dispersion of graphene oxide (GO) (5 mg/mL; composition 79% carbon and 20%oxygen; flake size 0.5−5 mm) was obtained from Graphene Supermarket (Calverton, NY, USA). All other chemicals were obtained from Sigma-Aldrich (St. Louis, MO, USA) and used as received. 2.2. Preparation of Poly(NIPAm-co-NDEAm-co-VP) Macromonomer (PNDVM). Amino-terminated poly(NIPAm-co-NDEAmco-VP) (PNDV-NH2) was obtained by the slow dropping of NDEAm and N-vinylpyrrolidone (VP) onto the NIPAm solution. NIPAm (852 mg, 7.52 mmol) was dissolved in 20 mL of N,N-dimethylformamide (DMF) with the chain transfer agent of 2-aminoethanethiol (8.7 mg, 0.22 mmol), and free radical polymerization was initiated by 2,2azobis(isobutyronitrile) at 70 °C under a nitrogen atmosphere. Soon after the initiation of the polymerization, NDEAm (218 mg, 2.51 mmol) dissolved in 10 mL of DMF was added to the reaction mixture over a period of 100 min using a syringe pump (Legato 100, KD Scientific, USA) at a rate of 0.1 mL/min. Shortly after finishing the B
DOI: 10.1021/acsami.6b10091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces GO. The GO sheets were stably dispersed in the hydrogel matrixes through entrapping by the polymer chains after polymerization. The electrically controlled window was prepared with ITO substrates instead of glass panes. 2.4. Switching Test of the Fabricated Windows. For the outdoor switching test, the fabricated glass panel was attached side by side to an external window of the building. To electrically control the fabricated windows, they were connected to an electric power supply (ED-305, ED Laboratory, Korea) and voltages from 0 to 9 V were applied with a current of 0.1 A. 2.5. Measurements. The molecular weight and dispersity (Đ) of the PNDVM were determined by gel permeation chromatography (GPC; Waters, Milford, MA, USA, column Shodex KF-802, KF-803). Tetrahydrofuran and polystyrene were used as the solvent and reference. 1H NMR spectra were recorded with an MR400 DD2 (Agilent, Santa Clara, CA, USA) spectrometer using deuterium oxide (D2O) as the solvent. The transmittance as a function of temperature was measured with a UV−vis spectrometer (V-550, Jasco Inc., Easton, MD, USA). The temperature of the hydrogel layer was controlled by a bath circulator (RBC-10, Jeio Tech, Seoul, Korea) that was connected to the sample holder in the UV−vis spectrometer. The sunlight intensity was measured by an intensity meter (843-R, Newport, Irvine, CA, USA) with a thermophile sensor (919P-003-10, Newport, Irvine, CA, USA).
sample 6 was collected after termination of the reaction. The collected samples were precipitated in diethyl ether to remove unreacted monomers and then dried under vacuum. The molecular weight and composition of the collected samples were analyzed using GPC and 1H NMR. As shown in Figure 1b, the molecular weight is increased with increased reaction time, resulting in a copolymer with a low polydispersity. Figure 1c shows the NMR spectra for the collected samples, indicating the changes in the compositions of the resulting copolymers. Based on the assignment of the relative areas among −CH− connected to NH of NIPAm, −CH2 connected to N of NDEAm, and −CH2 connected to N of VP varying from 3.4 to 3.9 ppm, the compositions of the collected samples are accordingly concluded to vary by the injections of NDEAm and VP, confirming the generation of multicomposition copolymers. The cumulative compositions of the collected samples, shown in Figure 1c, indicate that the composition of the copolymers is gradually changed during the reaction with changes in the monomer composition. The transparent−opaque transitions of the collected samples were determined by measuring the optical transmittance at 600 nm as a function of temperature. As shown in Figure 1d, sample 1 shows nearly the same switching behavior as the pure PNIPAm does, exhibiting an abrupt transparent−opaque transition near the LCST of 32 °C. After the addition of NDEAm, the onset temperature of the phase transition is decreased because of the existence of the hydrophobic NDEAm incorporated copolymers. The transmittance curve for sample 2 exhibits a slightly broadened transition compared to that of pure PNIPAm. As shown in the curve for sample 3, the transmittance is decreased gradually from 25 to 32 °C, which is attributed to the gradual aggregation from the NDEAm-rich polymer to that of the pure PNIPAm. Because of the presence of hydrophobic diethyl groups near the polymer backbone, the hydration of the polymer chains is suppressed. Accordingly, polymer aggregation is promoted, leading to a decrease in the LCST. An increased concentration of NDEAm in the polymer chain decreases the LCST. For samples 4 and 5, which were collected after adding VP, the transmittance gradually decreases after 32 °C, which is attributed to the VP incorporated in the polymer chains. The hydrophilic VP promotes the hydration of the polymer chains, thereby inhibiting polymer aggregation and inducing the LCST to increase. Finally, sample 6 shows a nearly linear transmittance curve over a temperature range of 9 °C, indicating that the prepared blend of copolymer with a gradient composition undergoes a continuous phase transition from the chain with higher NDEAm contents to that with higher VP contents with an increase in temperature. The obtained copolymers show different thermal behaviors from copolymers polymerized in the premixed solution containing all of the monomers that exhibits an abrupt transition near 33 °C (Figure S1). To observe the formation of aggregates, dynamic lightscattering measurements were performed during a heating run. The temperature-dependent size profiles show high similarity to the transmittance changes, indicating that the changes in optical transmittance result from the phase transition of the polymer (Figure S2). We also investigated the relationship between the addition rates of the monomer solutions and the thermal behaviors of the resulting gradient copolymers. The addition rates of the comonomer solutions were set to 0.1, 1, 3, and 5 mL/min under conditions otherwise identical to those listed earlier. As
3. RESULTS AND DISCUSSION Our strategy for the preparation of the blends of copolymers with the compositional gradient is illustrated in Figure 1a. First, the N-isopropylacrylamide (NIPAm) is dissolved in N,Ndimethylformamide (DMF) with a chain-transfer agent of 2aminoethanethiol. Free-radical polymerization is initiated by 2,2-azobis(isobutyronitrile). Early in the polymerization, pure PNIPAm of low molecular weight is obtained. As shown in Figure 1a, soon after the initiation of the polymerization, hydrophobic N,N′-diethylacrylamide (NDEAm) dissolved in DMF is added to the reaction mixture over the course of 100 min at a constant injection rate of 0.1 mL/min using a syringe pump. This allows the formation of multicomposition copolymers with a monomer concentration gradient from PNIPAm to NDEAm. The compositions of the copolymers are influenced by the continuous change of the monomer feed in the reaction mixture caused by the addition of the comonomer. This means that copolymers with higher NDEAm compositions are obtained as reaction time increases. Shortly after finishing the NDEAm injection, a continuous addition of hydrophilic Nvinylpyrrolidone (VP) dissolved in DMF at a rate of 0.1 mL/ min to the reaction mixture is initiated to copolymerize the hydrophilic VP with the existing monomers. Consequently, the cumulative composition of VP in the resulting gradient copolymer is gradually increased during the polymerization. After the addition of VP for 100 min, polymerization is allowed to proceed for another 150 min. The obtained gradient copolymers can be considered to be multicompositions, with each polymer having different compositions of NIPAM, NDEAm, and VP. Therefore, each polymer chain is expected to exhibit different thermal behavior according to the monomer composition. In order to confirm the generation of copolymers with gradient compositions during polymerization, we extracted several products from the reaction mixture throughout the polymerization. Sample 1 was collected soon before the injection of NDEAm in NIPAm solution. Samples 2 and 3 were collected in the middle and at the end of the NDEAm injection, respectively. Samples 4 and 5 were collected in the middle and at the end of the VP injection, respectively. Finally, C
DOI: 10.1021/acsami.6b10091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. (a) Temperature-dependent transmittance and (b) its derivative curve for PNDVM prepared with various addition rates. (c) Temperaturedependent transmittance and (d) its derivative curve for PNDVM prepared with various molar ratios of NDEAm to VP.
Figure 3. (a) Illustration of fabrication of the hydrogel window. (b) Photographs of temperature-dependent transparent−opaque transition of the PNDV hydrogel window.
shown in Figure 2a, lower addition rates of comonomers induce broader thermal transitions. From the derivative curve shown in Figure 2b, the relation between the addition rate and transition temperature range is investigated. It suggests that the phasetransition temperature range of copolymers can be customized by manipulating the addition rate, as plotted in the inset of Figure 2b. We note that the rate of 0.1 mL/min is optimal for obtaining a wide transition temperature range, because no greater broadening is observed at lower rates. The thermal behavior of the resulting gradient copolymer is also affected by the composition of the monomer solutions (Figure 2c). As found in the derivative curve of transmittance in Figure 2d, the transmittance can be shifted by varying the molar ratio of NDEAm to VP. In our polymerization method, because of the chemical structure of the chain-transfer agent, amino-terminated poly(NIPAm-co-NDEAm-co-VP) (PNDV) was obtained. We attempted to prepare the PNDV macromonomer (PNDVM) by reacting PNDV with N-acryloxysuccinimide in DMF,
converting the amino end group to a vinyl group (Figure S3). End group modification enables PNDVM to act as a monomer, which can participate in the radical cross-linking process. The PNDVM exhibits nearly the same thermal behavior as PNDV does, indicating that the end groups of the gradient polymers have no influence on the transition temperatures of the copolymers (Figure S4). Based on the continuous transparent−opaque transition of the gradient copolymer hydrogel synthesized above, we fabricated a glass panel containing a 140-μm-thick thermotropic hydrogel layer. An aqueous monomer solution containing the prepared PNDVM and cross-linker was degassed, and then free-radical polymerization was initiated by the addition of ammonium persulfate. The initiated pregel solution was immediately loaded into a capillary channel formed by two glass panes separated with spacers. The edges of the glass panes were then sealed with butyl rubber to prevent water evaporation (Figure 3a). To prevent the irreversible sedimentation of the polymer aggregates, the glass panes were treated D
DOI: 10.1021/acsami.6b10091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. (a) Schematic for sunlight-induced stepwise phase transition of PNDV hydrogels containing GO. (b) Photographs of sunlight intensitydependent transmittance change of PNDV/GO hydrogel window.
mW/cm2 results in an invisible background image because of the opacity of the panel. As shown in Figure 4b, this stepwise switching process is fully reversible, thereby allowing the window to autonomously screen sunlight in a stepwise fashion. One useful application of the switchable glazing is the control of transparency to ensure the privacy of a space by moderating the visibility through the window on demand. In contrast to passive switchable windows for the solar control, an electrically controlled smart window comprising PNDV hydrogels was fabricated with indium tin oxide (ITO) glass as a substrate to achieve the active stepwise control of the visibility. A voltage was applied to initiate the Joule heating of the substrate, as illustrated in the left part of Figure 5. We confirm that the
with the adhesion promoter [3-(methacryloxy)propyl]trimethoxysilane, such that the hydrogel layer was covalently anchored to the surface of the glass during polymerization. To confirm the stepwise control of the optical properties achieved by the prepared PNDV hydrogels, we observed the optical transmittance of the fabricated glass panel as a function of temperature. As shown in Figure 3b, the fabricated glass panel is fully transparent at 25 °C, and the background image is clearly observed. With increases in the temperature, the image becomes increasingly cloudy because of the decrease in transmittance. Finally, the glass panel becomes completely opaque, blocking the background image at ∼40 °C. These observations are well matched with the temperature-dependent transmittance of the panel as measured by UV−vis spectrometry at the wavelength of 600 nm, thereby confirming the stepwise screening of light by the fabricated glass panel. We found that this transparent−opaque transition was fully reversible and repeatable. By contrast, a glass panel containing PNIPAm-only hydrogel shows an abrupt transparent−opaque transition near 32 °C (Figure S5). To verify the practical operation of the switchable solarcontrol glazing, the prototype panel was placed on an outer window all day at a constant temperature of 25 °C for outdoor testing. To fabricate the sunlight-induced switchable glazing, a composite of PNDV and exfoliated graphene oxide (GO) sheets is sandwiched between the glass panels. We have previously reported that the GO dispersed in the hydrogel generates heat by absorbing sunlight, thereby triggering the phase separation of the thermotropic hydrogel matrix.16 We showed that the increase in the temperature is proportional to the light intensity, suggesting that the temperature of the composite hydrogels can be controlled by the intensity of the sunlight. Consequently, by combining the prepared copolymer hydrogel network with exfoliated GO, a stepwise transparent− opaque transition proportional to the sunlight intensity is realized. As illustrated in Figure 4a, the transmittance of the glass panel can be controlled in a stepwise and autonomous manner depending on the sunlight intensity by the heat generated from the incorporated GO. Figure 4b shows the results of the outdoor switching test for the fabricated glass panel under constant temperature. At a low light intensity of 1.1 mW/cm2, the glass panel is transparent and the background is clearly visible. When exposed to sunlight of 12.4 mW/cm2, the glass panel becomes cloudy, partially screening the sunlight. An increase of solar intensity to 22.8
Figure 5. (top) Setup for electrically controlled PNDV hydrogel window. (bottom) Photographs of transmittance change depending on applied voltage.
temperature of the hydrogel layer is controllable by adjusting the applied voltage (Figure S6). Initially, in the “off” state, the glass panel is transparent and background image is clearly visible, as seen in Figure 5. When the applied electric energy exceeds 5.0 V, the transmittance of the fabricated glass panel is gradually decreased in proportion to the applied voltage. After application of the voltage of 9.0 V, the window becomes completely opaque with no visible background image. By switching off the current, the glass panel recovers the initial state. Even though the electrically controlled smart window is not practically proper to continuous activation of a large dimension due to the high energy consumption and slow cooling speed (2−3 min) of the Joule heating device, our demonstration suggests that the switchable glazing is feasible for the on-demand control of visibility. E
DOI: 10.1021/acsami.6b10091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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(9) Cupelli, D.; Nicoletta, F. P.; Manfredi, S.; De Filpo, G.; Chidichimo, G. Electrically Switchable Chromogenic Materials for External Glazing. Sol. Energy Mater. Sol. Cells 2009, 93, 329−333. (10) Dyer, A. L.; Bulloch, R. H.; Zhou, Y.; Kippelen, B.; Reynolds, J. R.; Zhang, F. A Vertically Integrated Solar-Powered Electrochromic Window for Energy Efficient Buildings. Adv. Mater. 2014, 26, 4895− 4900. (11) Warwick, M. E.; Ridley, I.; Binions, R. The Effect of Transition Gradient in Thermochromic Glazing Systems. Energy Build. 2014, 77, 80−90. (12) Panagopoulou, M.; Gagaoudakis, E.; Aperathitis, E.; Michail, I.; Kiriakidis, G.; Tsoukalas, D.; Raptis, Y. The Effect of Buffer Layer on the Thermochromic Properties of Undoped Radio Frequency Sputtered Vo 2 Thin Films. Thin Solid Films 2015, 594, 310−315. (13) Zhang, J.; Li, J.; Chen, P.; Rehman, F.; Jiang, Y.; Cao, M.; Zhao, Y.; Jin, H. Hydrothermal Growth of Vo2 Nanoplate Thermochromic Films on Glass with High Visible Transmittance. Sci. Rep. 2016, 6, 27898. (14) Watanabe, H. Intelligent Window Using a Hydrogel Layer for Energy Efficiency. Sol. Energy Mater. Sol. Cells 1998, 54, 203−211. (15) Seeboth, A.; Holzbauer, H. R. The Optical Behavior of Lyotropic Liquid Crystalline Polymer Gel Networks: Dependence on Temperature. Adv. Mater. 1996, 8, 408−411. (16) Kim, D.; Lee, E.; Lee, H. S.; Yoon, J. Energy Efficient Glazing for Adaptive Solar Control Fabricated with Photothermotropic Hydrogels Containing Graphene Oxide. Sci. Rep. 2015, 5, 7646. (17) Seeboth, A.; Ruhmann, R.; Mühling, O. Thermotropic and Thermochromic Polymer Based Materials for Adaptive Solar Control. Materials 2010, 3, 5143−5168. (18) Raicu, A.; Wilson, H. R.; Nitz, P.; Platzer, W.; Wittwer, V.; Jahns, E. Façade Systems with Variable Solar Control Using Thermotropic Polymer Blends. Sol. Energy 2002, 72, 31−42. (19) Park, M. J.; Char, K. Two Gel States of a Peo-Ppo-Peo Triblock Copolymer Formed by Different Mechanisms. Macromol. Rapid Commun. 2002, 23, 688−692. (20) Gong, X.; Li, J.; Chen, S.; Wen, W. Copolymer Solution-Based “Smart Window. Appl. Phys. Lett. 2009, 95, 251907. (21) Luzon, M.; Boyer, C.; Peinado, C.; Corrales, T.; Whittaker, M.; Tao, L.; Davis, T. P. Water-Soluble, Thermoresponsive, Hyperbranched Copolymers Based on Peg-Methacrylates: Synthesis, Characterization, and Lcst Behavior. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2783−2792. (22) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Effect of Comonomer Hydrophilicity and Ionization on the Lower Critical Solution Temperature of N-Isopropylacrylamide Copolymers. Macromolecules 1993, 26, 2496−2500. (23) Lee, E.; Lee, H.; Yoo, S. I.; Yoon, J. Photothermally Triggered Fast Responding Hydrogels Incorporating a Hydrophobic Moiety for Light-Controlled Microvalves. ACS Appl. Mater. Interfaces 2014, 6, 16949−16955. (24) Seno, K. I.; Tsujimoto, I.; Kanaoka, S.; Aoshima, S. Synthesis of Various Stimuli-Responsive Gradient Copolymers by Living Cationic Polymerization and Their Thermally or Solvent Induced Association Behavior. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 6444−6454. (25) Okabe, S.; Seno, K.-i.; Kanaoka, S.; Aoshima, S.; Shibayama, M. Micellization Study on Block and Gradient Copolymer Aqueous Solutions by DLS and SANS. Macromolecules 2006, 39, 1592−1597. (26) Matyjaszewski, K.; Ziegler, M. J.; Arehart, S. V.; Greszta, D.; Pakula, T. Gradient Copolymers by Atom Transfer Radical Copolymerization. J. Phys. Org. Chem. 2000, 13, 775−786.
4. CONCLUSION We have developed a facile and versatile synthesis method to prepare multicomposition copolymers with gradient monomer compositions, achieved by the slow addition of monomer solutions into the reaction mixture. We demonstrated that the obtained copolymer exhibits a volume phase transition over a wide temperature range. Based on the continuous change in transmittance of the copolymer, a switchable glazing enabling stepwise solar control as well as visibility control could be fabricated.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10091. Temperature-dependent transmittance changes of copolymers; size analysis data; synthetic scheme for PNDV macromonomer; temperature-dependent transmittance during heating for PNDV-NH2, PNDVM, cross-linked PNDV hydrogel; the transparent−opaque transition of the pure PNIPAm hydrogel window depending on temperature; PNDV hydrogel window fabricated with ITO substrate, depending on applied voltage (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +82 (51) 200-7245. E-mail:
[email protected]. Author Contributions †
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. E.S. and D.K. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Research Fund of Dong-A University. REFERENCES
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DOI: 10.1021/acsami.6b10091 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
