Optically bistable switching glazing achieved by memory function of

Subscriber access provided by Kaohsiung Medical University

Applications of Polymer, Composite, and Coating Materials

Optically bistable switching glazing achieved by memory function of grafted hydrogels Dowan Kim, Eunsu Lee, and Jinhwan Yoon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05818 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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

ACS Applied Materials & Interfaces

Optically bistable switching glazing achieved by memory function of grafted hydrogels Dowan Kim1, Eunsu Lee2, Jinhwan Yoon1,* 1

Department of Chemistry Education, Graduate Department of Chemical Materials, and Institute for Plastic Information and Energy Materials, Pusan National University, 2 Busandaehak-ro 63 beon-gil, Geumjeong-gu, Busan, 46241, Republic of Korea

2

Department of Chemistry, Dong-A University, 37 Nakdong-Daero 550 beon-gil, Saha-gu, Busan, 49315, Republic of Korea E-mail: [email protected]

ABSTRACT Active switching glazings driven by electrical energy have been widely used for the on-demand control of the optical transmittance of smart windows; however, continuous electrical energy consumption is necessary to maintain the optical state. In this work, to minimize the energy consumption during operation of switchable windows, we have developed an optically bistable switching glazing based on the memory function in the volume change of the hydrogels. By grafting a multi-component copolymer that has a chemical composition gradient of three different monomers onto the methyl cellulose backbone, the prepared hydrogel exhibits a smooth transition during heating and a large thermal hysteresis in the swelling behavior during cooling. Based on the novel thermal behavior of the triangular shape in volume phase transitions, an optically bistable window capable of retaining a switched state as well as stepwise activation, depending on the applied current, can be prepared. The developed bistable glazing is expected to provide energy-saving devices for on demand solar control and variation in visibility.

KEYWORDS

1 Environment ACS Paragon Plus

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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

optical bistability, switchable glazing, thermotropic hydrogels, smooth phase transition, thermal hysteresis


Page 2 of 21

Page 3 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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


1. Introduction In developed countries, it is reported that about 30–40% of the total electrical energy is consumed for the operation and rehabilitation of buildings.1 As the average global temperature is increasing at the fastest rate2, more energy will be consumed to maintain comfortable living and working conditions in buildings. Apparently, we are encountering an urgent request to reduce energy consumption, in order to prevent the greenhouse effect by reducing carbon dioxide emissions. From this viewpoint, switchable glazings for solar control3-20 have been regarded as promising energy-saving devices, as their application in buildings significantly reduces the energy consumption for cooling room temperatures by screening sunlight. Switching glazing can be categorized into passive3-11 or active types12-20. The passive switching method automatically controls the transparency of the windows without human operation by phase separation of the polymers3-5 or hydrogels6-11 in response to external environmental changes, such as in the temperature3-11 or sunlight10-11. Due to the lack of on-demand control, passive switching systems have been typically applied to fabricate skylight roofs and outdoor windows for buildings. Active switching methods rely on the modulation of the transparency induced by the color change of electrochromic materials12-16 or reorientation of the liquid crystal directors17-18 with the application of an electric field. Hybrid window systems comprised of thermotropic hydrogels placed between Joule-heatable glass were also developed for active control.19-20 While these are effective for on-demand solar control or privacy protection, continuous electrical energy consumption is necessary to maintain the optical state of the windows.12-20 For these glazing, continuous electric consumption is paradoxically needed to save the energy. Herein, to overcome this paradox, we report the development of bistable switching glazing based on the memory of the optical state, which is achieved by the large thermal hysteresis of the hydrogels. Concurrently, developed glazing can show the stepwise activation of transparency applying electrical energy. We demonstrate the stepwise control of optical 3 Environment ACS Paragon Plus


Page 4 of 21

transmittance in proportional to the applied current and the optical states are maintained without supplying continuous electric energy through the large thermal hysteresis of the developed hydrogels.

2. Experimental 2.1 Materials N,N'-diethylacrylamide

(NDEAm),

N-isopropylacrylamide

(NIPAm),

N-

acryloxysuccinimide (NAS), and 2-aminoethanethiol (AESH) were supplied by TCI (Tokyo, Japan). 2,2'-Azobisisobutyronitrile and diethyl ether were obtained from Samchun Pure Chemical (Gyeonggi-do, Korea). N-vinylpyrrolidone (VP) and all other chemicals were obtained from Sigma-Aldrich (St. Louis, MO, USA) unless otherwise indicated.

2.2 Preparation of poly(NDEAm-co-NIPAm-co-VP) (PDNV) Multi-component poly(NDEAm-co-NIPAm-co-VP) (PDNV) was synthesized by the slow addition of NIPAm and VP monomers to the NDEAm solution. First, NDEAm (953 mg, 7.5 mmol) in 20 mL N,N-dimethylformamide (DMF) with 2-aminoethanethiol (17.4 mg, 0.22 mmol) as a chain transfer agent was polymerized at 70 °C under a nitrogen atmosphere, which is initiated by 2,2-azobisisobutyronitrile (2.05 mg, 0.012 mmol). NIPAm (283 mg, 2.5 mmol) was dissolved in 10 mL of DMF, and injected to the reaction mixture over a period of 100 min by syringe pump (Legato 100, KD Scientific, USA). After injection of NIPAm, VP (266 µL, 2.5 mmol) dissolved in 10 mL of DMF was immediately added into reaction mixture at a rate of 0.1 mL/min. Then, polymerization proceeded for another 150 min. The reaction mixture was precipitated using diethyl ether, and collected by filtration. NMR spectral data for PDNV is described in Figure S1. To modify the amine end group of PDNV with the vinyl group, the collected PDNV and 1.2-molar equivalent of NAS was dissolved in 40 mL DMF and stirred at RT for 72 h. 4 Environment ACS Paragon Plus

Page 5 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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


Then, the reaction mixture was precipitated in diethyl ether and collected by filtration. After vacuum drying of the filtrate, 1.13 g of vinylated PDNV was obtained as a white powder. The number average molecular weight of 42,800 Da and dispersity of 1.38 for the obtained PDNV were determined by GPC measurement.

2.3 Fabrication of switchable windows and PDNV-g-MC hydrogels PDNV (123.3 mg, 28.8 mM), methyl cellulose (MC) (MW 14000 Da, 0.12 mg, 8.64 µM, degree of substitution: 1.5−1.9), and N,N'-methylenebis(acrylamide) (BisAA) (0.19 mg, 1.2 mM) were dissolved in 1.0 mL of distilled water. The monomer solution was degassed for removing oxygen, then 3.0 µL of N,N,N',N'-tetraethylethyleneamine (TEMED) and 6.0 µL of 10 wt% aqueous ammonium persulfate (APS) were added for free radical polymerization. The resulting solution was immediately filled by capillary force into the two glass panes separated with a 140-µm-thick spacer. The dimension of glass pane is 76 mm in wide and 52 mm in height. The gelation proceeded in a sealed chamber under a nitrogen atmosphere for 1 h. After gelation, the edges of the glass panes were sealed using butyl rubber. Then, the Peltier modules (THR-DS-CP14, Laird Technologies, MO, USA, dimension: 62 mm by 9.5 mm) were attached on the edge of the fabricated glazing and connected onto the DC power supply (ED-305, ED Laboratory, Korea). To prepare the PDNV hydrogels, 100 µL of degassed pregel solution containing PDNV (9.77 mg, 9.11 mM), BisAA (0.003 mg, 0.19 mM), MC (9.59 µg, 6.85 µM), 0.3 µL TEMED) (0.3 µL), and 0.6 µL of 10 wt % APS was filled into the two cover glasses separated with a 140-µm-thick spacer. After 1 h in the nitrogen chamber, the PDNV-g-MC hydrogel was separated from the glass cell.



2.4 Measurements 1

H NMR spectra were obtained by an MR400 DD2 spectrometer (Agilent, CA, USA)

using deuterium oxide (D2O) as a solvent. The molecular weight was determined by GPC (Waters, MA, USA, column: Shodex KF-802, KF-803) with tetrahydrofuran(THF) as a solvent. To avoid irreversible chain aggregation, few amounts of water were added into obtained PDNV before dissolving in THF.21 The optical transmittances of the windows were measured using UV-Vis spectrometer (V-550, Jasco Inc., MD, USA). The linear swelling ratios of the hydrogels were determined by tracking the fluorescent latex beads trapped in the hydrogels, as described previously.22

3. Results and discussions

Figure 1. (a) Illustration for synthetic procedure of multi-compositional gradient poly(NDEAm-co-NIPAm-co-VP). (b) GPC, and (c) NMR of products extracted from the reaction mixture throughout polymerization.


Page 6 of 21

Page 7 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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


To achieve stepwise activation of the transparent-opaque transition of hydrogels and their memory on activated optical states, the blends of copolymers with the compositional gradient are grafted with methyl cellulose (MC). Each copolymer in the blends has the different lower critical solution temperatures depending on the chemical composition. First, we prepared the blends of copolymers having the compositional gradient of N,N'diethylacrylamide (NDEAm), N-isopropylacrylamide (NIPAm), and N-vinylpyrrolidone (VP). As illustrated in Figure 1(a), NDEAm, the first monomer, is polymerized in the N,N'dimethylformamide

(DMF)

containing

2-aminoethanethiol

(AESH)

and

2,2-

azobis(isobutyronitrile) (AIBN) as a chain transfer agent and initiator for radical reaction, respectively. At this moment, the pure PNDEAm that exhibits the LCST behavior around 25 °C is obtained. Soon after initiation of first reaction, NIPAm, the second monomer, dissolved in DMF is added to the reaction mixture using a syringe pump with a slow injection rate of 0.1 mL/min for 100 min, yielding the multi-composition copolymers with a concentration gradient from NDEAm to NIPAm. The chemical composition of the obtained copolymers is continuously changed over the course of the reaction time according to the concentration of the monomers in the reaction mixture.11 Consequently, the NIPAm composition in the obtained copolymers increases as the NIPAm concentration of the reaction mixture increases. After finishing the injection of NIPAm, VP dissolved in DMF is also injected into the reaction mixture at a rate of 0.1 mL/min as a third monomer, then the VP is copolymerized with the existing monomers of NDEAm and NIPAm. As the VP concentration increases due to the continuous addition, copolymers with higher VP compositions are prepared. After the addition of VP, polymerization proceeds for another 150 min. The obtained gradient copolymer poly(NDEAm-co-NIPAm-co-VP) (PDNV) is considered to be multi-composition, with each polymer having gradient chemical compositions of the NDEAm, NIPAm, and VP.



Throughout the reaction, several products were extracted from the reaction mixture to observe the changes of molecular weight and chemical composition during the reaction using GPC and 1H NMR. As shown in Figure 1(b), the molecular weight is gradually increased with increasing reaction time, resulting in a copolymer with a low polydispersity. Finally, number average molecular weight (Mn) and dispersity of PDNV was determined as 42,800 Da and 1.38, respectively. Figure 1(c) shows the NMR spectra for the extracted products, indicating the changes in the compositions of the resulting copolymers. Based on the assignment of the relative areas among -CH2 connected to N of NDEAm, -CH- connected to NH of NIPAm, 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 NIPAm and VP, confirming the generation of multi-segmented gradient copolymers. Since prepared PDNV is multi-component including the copolymers of gradient chemical composition, the cumulative compositions of the collected samples were determined and shown in Figure 1(c).

Figure 2. (a) Transmittance profiles measured during heating and cooling processes for PDNV. (b) Synthesis scheme for crosslinked PDNV-g-MC by reaction of PDNV, MC, and BisAA. (c) FT-IR spectra for crosslinked PDNV-g-MC and PDNV, and MC. (d) 8 Environment ACS Paragon Plus

Page 8 of 21

Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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


Temperature-dependent linear swelling ratios (λf) determined during heating and cooling run. (e) DSC thermogram for crosslinked PDNV-g-MC hydrogel.

The thermal behavior of the prepared PDNV was determined by measuring the temperature-dependent transmittance at 600 nm. As shown in Figure 2(a), we found that the transmittance profile is a smooth curve over a temperature range of 18.4 °C, while the typical temperature responsive polymers3-10 show an abrupt phase transition around their LCST. This means that the prepared PDNV undergoes a continuous phase transition depending on the monomer composition. The LCST of the polymer can be tuned by adjusting the hydrophilic/hydrophobic balances, which can be achieved by the copolymerization between different monomers23-25. In our previous preparation, incorporation of relatively hydrophobic NDEAm in the polymer can shift the LCST to a lower temperature,24 while the copolymerization with the hydrophilic VP induces the LCST to increase due to the hydration of the polymer chains,25 which inhibits the aggregation of polymer chains. Because the concentration of the monomers in reaction mixture is gradually varied by the slow addition of the monomers, the blend of copolymers with a gradient composition can be prepared through one-pot synthesis. Therefore, each polymer chain exhibits different thermal behavior according to the monomer composition, as the copolymers undergo a continuous phase transition from the chain with higher NDEAm contents to that with higher VP contents with an increase in temperature. We also reveal that this continuous transition is fully reversible, and only a slight hysteresis of less than 1.5 °C is observed. Next, to enlarge the thermal hysteresis of the obtained copolymers, PDNV was grafted onto an MC backbone and crosslinked. Large thermal hysteresis can be applied to design materials that are capable of memory in the volume change.26-27 The enhanced thermal hysteresis of the hydrogels can be achieved by forming a metastable aggregate during the



heating run. In our previous study, we investigated the metastable hydrophobic junction of MC that can be formed, which can enhance the thermal hysteresis of the hydrogels.26 First, to graft the PDNV onto the MC backbone, the amino end group of the polymer is converted to a vinyl group by reacting PDNV with N-acryloxysuccinimide in DMF in Figure S2. Vinylation of PDNV enables participation in the radical cross-linking, which is essential to grafting and crosslinking of PDNV. After converting the end group, vinylated PDNV, MC (Mn: 14,000 Da), and crosslinker, N,N′-methylenebis(acrylamide) (BisAA) were dissolved in water and degassed to remove residual oxygen in water, as described in Figure 2(b). Through the radical reaction that is initiated by adding ammonium persulfate with N,N,N′,N′-tetramethylenediamine, vinyl groups of PDNV or BisAA can be reacted with hydroxyl groups on the MC backbone to form the covalent bond between the MC and PDNV or BisAA. The initiated radicals continue to propagate to vinyl groups of another PDNV or BisAA, which eventually form the crosslinked network of PDNV-g-MC. Propagated radicals can be also reacted with MC, PDNV, or BisAA. We found that the elastic gel is obtained after the reaction, confirming that the grafting and crosslinking reaction fully occurred. Grafting of PDNV onto the MC is further confirmed by the FT-IR measurements, as described in Figure 2(c). The FT-IR spectrum for the PDNV-g-MC hydrogel shows characteristic absorptions at 1055 and 1543 cm−1, which are attributed to the alkoxy group of MC and the -NH group of the PDNV, respectively. Thermal behavior of the PDNV-g-MC was determined by measuring the volume change of the resulting hydrogel as a function of temperature, as shown in Figure 2(d). The linear swelling ratio (λf) is defined as the extent to which the gel swells in each dimension in the equilibrium state when immersed in water. The λf of the PDNV-g-MC at 23 °C is 2.70; upon heating, a continuous change in the volume is observed until ~43 °C, which shows the same trend as the prepared PDNV copolymers. At 43.4 °C, λf is 1.09, which corresponds to a 10 Environment ACS Paragon Plus

Page 10 of 21

Page 11 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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


decrease of 59.6 % in λf, which corresponds to 93.4 % decrease in volume. The λf change for PDNV-g-MC during the heating run is found to be nearly linear over the wide temperature range of 20 °C, while the thermal behavior of the PDNV shows the inflection area near 35 °C. This difference results from the existence of the MC moiety in the hydrogel, which shows an LCST between 40 °C and 50 °C.28 The hydrophilic MC backbone promotes the hydration of the polymer chains, leading to a shift of LCST for the PDNV-g-MC. Consequently, by grafting the MC, a smooth and continuous volume phase transition of the hydrogels could be achieved. During cooling, the thermal behavior in the volume of PDNV-g-MC is totally different from that observed in the heating process. Even at 30 °C, the shrunken volume of PDNV-gMC is still maintained, and no more changes were observed at this temperature. This means that the re-hydration of PDNV-g-MC is hindered, leading to thermal hysteresis in the swelling behavior of the hydrogel. From 29.4 °C, we can observe the re-swelling of the hydrogel, and the initial volume is completely recovered after further cooling to 23.1 °C. These observations clearly indicate that the PDNV-g-MC hydrogel exhibits a volume phase transition of triangular shape, which is a novel thermal behavior of the hydrogels. Differential scanning calorimetry (DSC) measurement for PDNV-g-MC was carried out to further conﬁrm the enhanced thermal hysteresis, and the result is plotted in Figure 2(e). As shown in the DSC thermogram, a broad endothermic peak appears during the heating run over a wide temperature range of 23–42 °C, whereas the sharp exothermic peak is observed at 27 °C during cooling. The thermal behavior for PDNV-g-MC observed by DSC is well matched with those determined from the temperature-dependent swelling ratio. The broad endothermic pattern is attributed to the continuous phase transition from the swollen hydrated state to the shrunken dehydrated state of PDNV moiety, and the exothermic peak appears due to the rehydration of the hydrogel upon cooling. The strong molecular interactions between the MC backbones retard the rehydration of the hydrogel, leading to the increase of the 11 Environment ACS Paragon Plus


thermal hysteresis for PDNV-g-MC. We further revealed that the thermal behavior of PDNVg-MC can be customized by varying the molecular weight and chemical compositions of MC and PDNV, as described in Figure S3.

Figure 3. (a) λf changes for the PDNV-g-MC hydrogel depending on heating and cooling. (b) Proposed model for structural change of the PDNV-g-MC hydrogel depending on temperature

To confirm that the memory of the hydrogel in the volume change is reliable and reproducible for any temperature from 30 to 42 °C, we further observed the volume of the hydrogel while controlling the temperature of the medium in Figure 3(a). The initial temperature was set at 30 °C, which is above the onset temperature for erasing thermal history. At the initial temperature of 30 °C, λf for the hydrogel is 2.09, and it decreases to 1.89 upon heating to 32.5 °C. Even after returning to the initial temperature, the shrunken volume of the hydrogel is maintained. Additional heating to 37.0 °C results in further shrinking of the hydrogel to 1.49, and its volume is retained during cooling to the initial temperature. A greater decrease in volume is triggered by further heating of the medium, but the shrunken volume can be maintained due to the memory effect of the PDNV-g-MC. As expected, the volume of the hydrogel is reset by cooling to room temperature. These results indicate that the PDNV-g-MC has a thermal memory window in its volume change. The memory function in volume change of the hydrogels can be addressed by the molecular model illustrated in Figure 3(b). At low temperatures, the PDNV-g-MC chains are


Page 12 of 21

Page 13 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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


freely swollen and extended in water. With an increase in temperature, PDNV moiety gradually exhibits a volume phase transition from a swollen to a shrunken state over a wide temperature range, and the volume shrinkage is proportional to the temperature decrease in this temperature range. However, at this moment, hydrophobic junctions between the MC backbones are also formed that disturb the re-swelling of the hydrogels upon cooling.26 By excess cooling to room temperature, the hydrogels can recover their original volume through rehydration.

Figure 4. (a) Illustration for the fabrication procedure of hydrogel windows equipped with Peltier modules connected to the power supply. (b) Temperature-dependent transmittance changes determined during heating and cooling run. (c) Photographs of fabricated windows and (d) transmittance change by varying the applied current at a constant voltage of 12 V.

Based on the developed hydrogel capable of memory function on its phase transition, we aimed to fabricate optically bistable switching glazing. It is well reported that temperatureresponsive hydrogels have been used to fabricate switchable windows for solar control.6-11 Because the aggregated domains of the hydrogel networks formed above their LCST act as a scattering center for sunlight, these materials can be applied to reflect incident solar radiation. To render the switchable glazing with the hydrogels synthesized above, a 140-µm hydrogel layer was formed between two glass panes separated with spacers, as illustrated in Figure 4(a). The dimension of glass pane is 76 mm in wide and 52 mm in height. In order to change the optical transmittance through the aggregation of hydrogel networks, rather than inducing a 13 Environment ACS Paragon Plus


volume change, the crosslinking density of the hydrogels was lowered to 0.1 mol%.10 The glass panes were sealed with butyl rubber to prevent the evaporation of the medium. To control the temperature of the fabricated windows by electric energy, the Peltier modules were installed along the edge of the glass and connected on the power supply (Figure 4(a)). These modules enable heating or cooling of the glazing through the Peltier effect,28 which is a temperature-difference-induced electrified junction of two different conductors depending on the direction of the current. We confirmed that the window temperature ranging from 15.8 to 40.7 °C is well controlled by adjusting the applied voltage and current (Figure S4). The optical property of the fabricated window was determined by measuring the UVVis spectra at various temperature (Figure S5). The optical transmittances at 600 nm were monitored as a function of temperature and plotted in Figure 4(b). As seen in Figure 4(b), the transmittance change for PDNV-g-MC window shows nearly the same trend with the swelling ratio change. During heating run, the transmittance change was smoothly occurred over wide range from 23 °C to 40 °C. The opaque state was maintained until at 30 °C during cooling run. The transmittance was recovered from 29 °C, and fully transparent state was observed at 23 °C. Figure 4(c) shows the optical images of the fabricated window that are taken by varying the applied current at a constant voltage of 12 V. The fabricated glass panel is fully transparent without applying electric energy; thus, the background image is clearly visible. We measured that the glazing shows an optical transmittance above 95% at the clear state by UV/Vis spectroscopy with a wavelength of 600 nm. When a current of 0.2 A is applied, the background image become slightly cloudy due to the decrease in transmittance. As the applied current increases to 0.4 A and 0.6 A, the transmittance of the window decreases more and more. Finally, the glazing is completely opaque, and the background image is fully blocked at an applied current of 0.8 A, at which the measured transmittance is ~5%. These


Page 14 of 21

Page 15 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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


observations indicate that the optical transmittance of the glazing can be gradually controlled in a stepwise fashion by adjusting the applied current, as shown in Figure 4(d).

Figure 5. (a) Photographs of fabricated windows taken during on-off cycles. (b) Empirically determined transmittance value for hydrogel window depending on applied current and voltage.

To verify the practical operation of the switchable glazing with pulsed electric energy through the memory function of the developed hydrogels, we further observed the optical state of the window after switching off the applied current of 0.4 A at 30 oC. As seen in Figure 5(a), the optical transmittance of the window in its partially opaque state is maintained without applying electric energy, rather than recovering the transparent state. We can reset the optical transmittance of the window from the opaque state by cooling to 25 °C, which is achieved by applying a reverse bias voltage of -12 V at a current of 0.8 A to the Peltier module. Then, we applied an electric current of 0.8 A at 12 V again to reset the window, and found that the window returned to the fully opaque state. We confirmed that the full optical transmittance in the opaque state is still retained after turning off the power supply. We found that no more change in optical transmittance is observed, even after 10 days without applying any electrical energy (Figure S6). We further demonstrated the memory of the optical state for the switchable window through several cycles of applying different currents, and the result is 15 Environment ACS Paragon Plus


plotted in Figure 5(b). Without a reset process, the optical transmittance of the windows can be adjusted by the magnitude of the current in a stepwise manner, and their transmittance value is still maintained regardless of the applied current without supplying continuous electric energy. This memory function of the window can be achieved by the formation of a hydrophobic junction between the MC backbones, as discussed above. While traditional electrically switchable windows show high energy consumption for continuous activation of optical conditions, our developed windows can be activated by pulsed electrical energy and maintain the activated state without supplying further energy. We note that the switching conditions, including the working temperature of the fabricated windows, are flexible, and can be tuned for optimal performance depending on the climate conditions of the installation area or user’s demand by adjusting the chemical structure of the hydrogels.

4. Conclusion In conclusion, we prepared the novel hydrogels that can show smooth phase transitions over a wide temperature range and a large thermal hysteresis in phase transition by grafting multi-composition copolymers with methyl cellulose. Based on the distinct thermal behavior of the hydrogels, we could fabricate switchable windows enabling stepwise activation and memory in their optical transmittance. By applying electrical energy, the optical state of the windows is concisely controlled, and these states are maintained without supplying continuous eclectic energy, suggesting that the developed window can be used for on-demand control of solar transmittance and visibility with minimal energy consumption.

ASSOCIATED CONTENT Supporting Information


Page 16 of 21

Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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


NMR spectrum for PDNV, synthetic scheme for modification of the end group of PDNV from amine to vinyl, and temperature-dependent linear swelling ratio of PDNV-g-MC prepared with various MC molecular weight and chemical composition of PDNV, and temperature change of the windows equipped with a Peltier module, and photographs of the optical state for a fabricated window without supplying electrical energy, observed for 10 days. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Tel.: +82 (51) 510-2697. Email: [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.

ACKNOWLEDGEMENTS This research was supported by the Ministry of Science and ICT (MSIT) and the National Research Foundation of Korea (NRF) through the Mid-Career Researcher Fund (Strategy Research)

(2016R1E1A1A01942509)

and

Creative

Materials

(2017M3D1A1039287)


Discovery

Program


References 1.

Huovila, P., Buildings and climate change: status, challenges, and opportunities. UNEP/Earthprint: 2007.

2.

Collins, M.; An, S.-I.; Cai, W.; Ganachaud, A.; Guilyardi, E.; Jin, F.-F.; Jochum, M.; Lengaigne, M.; Power, S.; Timmermann, A., The impact of global warming on the tropical Pacific Ocean and El Niño. Nat. Geosci. 2010, 3 (6), 391-397.

3.

Raicu, A.; Wilson, H. R.; Nitz, P.; Platzer, W.; Wittwer, V.; Jahns, E., Facade systems with variable solar control using thermotropic polymer blends. Sol. Energy 2002, 72 (1), 31-42.

4.

Gong, X.; Li, J.; Chen, S.; Wen, W., Copolymer solution-based “smart window”. Appl. Phys. Lett. 2009, 95 (25), 251907.

5.

Seeboth, A.; Ruhmann, R.; Mühling, O., Thermotropic and thermochromic polymer based materials for adaptive solar control. Materials 2010, 3 (12), 5143-5168.

6.

Watanabe, H., Intelligent window using a hydrogel layer for energy efficiency. Sol. Energy Mater. Sol. Cells 1998, 54 (1), 203-211.

7.

Nitz, P.; Hartwig, H., Solar control with thermotropic layers. Sol. Energy 2005, 79 (6), 573-582.

8.

Wang, M.; Gao, Y.; Cao, C.; Chen, K.; Wen, Y.; Fang, D.; Li, L.; Guo, X., Binary solvent colloids of thermosensitive poly (n-isopropylacrylamide) microgel for smart windows. Ind. Eng. Chem. Res. 2014, 53 (48), 18462-18472.

9.

Lee, H. Y.; Cai, Y.; Bi, S.; Liang, Y. N.; Song, Y.; Hu, X. M., A Dual-Responsive Nanocomposite toward Climate-Adaptable Solar Modulation for Energy-Saving Smart Windows. ACS Appl. Mater. Interfaces 2017, 9, 6054-6063.

10. 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. 18 Environment ACS Paragon Plus

Page 18 of 21

Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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


11. Lee, E.; Kim, D.; Yoon, J., Stepwise Activation of Switchable Glazing by Compositional Gradient of Copolymers. ACS Appl. Mater. Interfaces 2016, 8 (39), 26359-26364. 12. Lampert, C. M., Electrochromic materials and devices for energy efficient windows. Sol. Energy Mater. 1984, 11 (1-2), 1-27. 13. Bange, K.; Gambke, T., Electrochromic materials for optical switching devices. Adv. Mater. 1990, 2 (1), 10-16. 14. Ko, H. C.; Kang, M.; Moon, B.; Lee, H., Enhancement of Electrochromic Contrast of Poly (3, 4‐Ethylenedioxythiophene) by Incorporating a Pendant Viologen. Adv. Mater. 2004, 16 (19), 1712-1716. 15. Deb, S. K., Opportunities and challenges in science and technology of WO 3 for electrochromic and related applications. Sol. Energy Mater. Sol. Cells 2008, 92 (2), 245258. 16. Granqvist, C. G., Electrochromics for smart windows: Oxide-based thin films and devices. Thin Solid Films 2014, 564, 1-38. 17. Gotoh, T.; Murai, H., Preparation and characteristics of new reverse mode film of polymer dispersed liquid crystal type. Appl. Phys. Lett. 1992, 60 (3), 392-394. 18. 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 (3), 329-333. 19. Gyenes, T.; Szilagyi, A.; Lohonyai, T.; Zrínyi, M., Electrically adjustable thermotropic windows based on polymer gels. Polym. Adv. Technol. 2003, 14 (11‐12), 757-762. 20. Kiruthika, S.; Kulkarni, G., Energy efficient hydrogel based smart windows with low cost transparent conducting electrodes. Sol. Energy Mater. Sol. Cells 2017, 163, 231-236. 21. Ganachaud, F.; Monteiro, M. J.; Gilbert, R. G.; Dourges, M.-A.; Thang, S. H.; Rizzardo, E., Molecular weight characterization of poly (N-isopropylacrylamide) prepared by living free-radical polymerization. Macromolecules 2000, 33 (18), 6738-6745. 19 Environment ACS Paragon Plus


22. Yoon, J.; Cai, S.; Suo, Z.; Hayward, R. C., Poroelastic swelling kinetics of thin hydrogel layers: comparison of theory and experiment. Soft Matter 2010, 6 (23), 6004-6012. 23. 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 (10), 2496-2500. 24. 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 (19), 16949-16955. 25. Kim, H.; Lee, H.; Seong, K. Y.; Lee, E.; Yang, S. Y.; Yoon, J., Visible Light‐Triggered On‐Demand Drug Release from Hybrid Hydrogels and its Application in Transdermal Patches. Adv. Healthc. Mater. 2015, 4 (14), 2071-2077. 26. Kim, D.; Kim, H.; Lee, E.; Jin, K. S.; Yoon, J., Programmable Volume Phase Transition of Hydrogels Achieved by Large Thermal Hysteresis for Static-Motion Bilayer Actuators. Chem. Mater. 2016, 28 (23), 8807-8814. 27. Zhang, K.; Feng, X.; Ye, C.; Hempenius, M. A.; Vancso, G. J., Hydrogels with a Memory: Dual-Responsive, Organometallic Poly (ionic liquid) s with Hysteretic VolumePhase Transition. J. Am. Chem. Soc. 2017, 139 (29), 10029. 28. Takahashi, M.; Shimazaki, M.; Yamamoto, J., Thermoreversible gelation and phase separation in aqueous methyl cellulose solutions. J. Polym. Sci. B Polym. Phys. 2001, 39 (1), 91-100. 29. Peltier, J. C. A., New experiments on the heat effects of electric currents, Annales de Chimie et de Physique: 1834.


Page 20 of 21

Page 21 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 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


Table of Contents Graphic


Optically bistable switching glazing achieved by memory function of

Recommend Documents