Polyelectrolyte complex‐based thermochromic hydrogels containing carbonized polymer dots for smart windows with fast response, excellent solar modulation ability, and high durability

Thermochromic smart windows have gained increasing popularity in light modulation and energy management in buildings. However, the fabrication of flexible thermochromic smart windows with high luminous transmittance (Tlum), tailorable critical temperature (τc), strong solar modulation ability (ΔTsol), and long‐term durability remains a huge challenge. In this study, hydrogel‐based thermochromic smart windows are fabricated by sandwiching thermochromic hydrogels of polyallylamine hydrochloride, polyacrylic acid, and carbonized polymer dots (CPDs) complexes between two pieces of transparent substrates. Benefiting from the incorporation of nanosized CPDs, the thermochromic hydrogel has an ultrahigh Tlum of ~98.7%, a desirable τc of ~24.2 °C, a ΔTsol of ~89.3% and a rapid transition time of ~3 s from opaque state to transparent state. Moreover, the thermochromic hydrogel exhibits excellent anti‐freezing ability, tight adhesion toward various substrates, and excellent self‐healing capability. The self‐healing capability enables the fabrication of large‐area smart windows by welding multiple hydrogel pieces. The smart windows retain their original thermochromic properties after being stored under ambient conditions for at least 147 days or undergoing 10,000 uninterrupted heating/cooling cycles. The model houses with smart windows can achieve a temperature reduction of 9.2 °C, demonstrating the excellent indoor temperature modulation performance of the smart windows.

5][6][7] Among them, thermochromic smart windows are cost-effective for light modulation due to their zero artificial energy input. 3,4,8An energy-efficient thermochromic smart window with strong solar modulation ability (ΔT sol ) can keep being highly transparent at low temperatures to allow solar irradiation, while quickly turning opaque at high temperatures to effectively shield solar energy.Thermoresponsive materials with tunable optical properties are crucial to realize highly efficient temperature-triggered light management of thermochromic smart windows.3][14] Moreover, the rigid VO 2 coatings are technically difficult for the construction of flexible thermochromic windows because the repeated bending and unbending deformations can cause their exfoliation from the underlying flexible substrates.9][20] Despite the high T lum and low τ c , the PNIPAm hydrogel-based smart windows generally exhibit limited ΔT sol , which means an insufficient variation in their solar transmittance between transparent and opaque states. 18,19,21,22According to the Mie scattering theory, light can be sufficiently scattered by the particles whose diameters are comparable with the wavelengths of the incident light. 23,24As the particle size of the aggregated PNIPAm chains in hydrogels is relatively small in diameter, the PNIPAmbased hydrogels exhibit an inefficient light scattering in the IR region (780-2500 nm) of the solar spectrum. 25,26eanwhile, such thermochromic hydrogels become brittle under subzero temperatures due to the freezing of water within the hydrogels.Therefore, it is highly needed to develop new types of highly flexible thermochromic hydrogels with high T lum , adjustable τ c , strong ΔT sol, and long-term stability for the fabrication of highperformance thermochromic smart windows.
8][29] Above their LCST, the polyelectrolyte complexes dehydrate and aggregate into polyelectrolyte-rich coacervates, which precipitate out of the aqueous solution.The light scattering at the interface between the phase-separated polyelectrolyte complexes and surrounding water turns the solution from a transparent to an opaque state.The complexation of two/more kinds of complementary polyelectrolytes can generate polyelectrolyte complexes with a wide distribution of particle sizes.Therefore, the polyelectrolyte complexes in solutions can efficiently scatter light in the whole solar spectrum to achieve strong ΔT sol . 27However, the solutions of polyelectrolyte complexes are not suitable for direct use as highperformance thermochromic smart windows because it takes a relatively long time for the precipitated polyelectrolyte complexes at their opaque state to restore to their transparent state.Additionally, the possible leakage of solutions from the smart windows will reduce the long-term stability of the smart windows. 3The polyelectrolyte complexes can be conveniently processed into hydrogels, which are good candidates for the fabrication of thermochromic smart windows.1][32][33] Previous studies have revealed that nanoparticles with extremely small size and deformability can effectively enhance the diffusion of surrounding polymer chains. 34,35We proposed that introducing such kind of nanoparticles into polyelectrolyte complexes-based hydrogels would further improve their thermochromic performance by accelerating their transition rates.Carbonized polymer dots (CPDs) with carbonized rigid cores and soft shells possess good deformability and adjustable particle size (less than 10 nm), which render them ideal candidates for enhancing the performance of the thermochromic hydrogels.In this work, we showed the fabrication of hydrogel-based high-performance thermochromic smart windows by complexation of polyallylamine hydrochloride (PAH), polyacrylic acid (PAA), and CPDs in aqueous NaCl solution (denoted as PAH-PAA-CPDs).The fast dehydration/hydration of PAH-PAA-CPDs hydrogels at increased/decreased temperatures endows the thermochromic smart windows with high ΔT sol of ~89.3%, ideal τ c of ~24.2 °C, and quick transition time within ~3 s from opaque state to transparent state.Benefiting from the self-healing ability of the PAH-PAA-CPDs hydrogels, large-area smart windows can be fabricated by welding multiple pieces of hydrogel sheets.The smart windows possess excellent anti-freezing ability under subzero temperatures, tight adhesion for commonly used substrates, and outstanding durability after long-term usage.The model house equipped with the PAH-PAA-CPDs smart window achieved a 9.2 °C temperature reduction, showing a strong indoor temperature control capability.

| Fabrication of the PAH-PAA-x% CPDs thermochromic hydrogels
The aqueous PAA-CPDs solution was prepared by adding dropwise 0, 1.54, 3.08, and 4.63 mL of CPDs aqueous solution (60 mg/mL) and NaCl (23.38 g) into 200 mL of aqueous PAA solution (0.06 mol/L) under continuous stirring and then adjusting pH of the solution to 5.5 by aqueous NaOH solution (1 mol/L).The aqueous PAH solution was prepared by dissolving NaCl (23.38 g) into 200 mL of PAH aqueous solution (0.06 mol/L) under continuous stirring and adjusting the pH of the solution to 8.5 by aqueous NaOH solution (1 mol/L).Equal volume aqueous solutions of PAH and PAA-CPDs were mixed through peristaltic pumps at a flow rate of 5 mL/min under continuous stirring at room temperature.The precipitate of polyelectrolyte complexes was collected via ultracentrifugation at a rotation speed of 40,000 r/min and then compressed between two pieces of glass slides hydrophobized with 1H,1H,2H,2H-perfluorooctyltriethoxysilane at a pressure of ca. 15 kPa for 3 days.After being dried in air at room temperature and detachment from the glass slides, the as-prepared composite sheets were immersed in 2 mol/L aqueous NaCl solution for 48 h to obtain the resultant hydrogels.For simplicity, the composite hydrogels are denoted as PAH-PAA-x%CPDs, where x represents the mass fractions of CPDs in the corresponding xerogels (x = 0, 5, 10, and 15).
The PAH-PAA-10% SiO 2 hydrogel was prepared in the same method as the PAH-PAA-10%CPDs hydrogel.The mass fraction of SiO 2 in the corresponding xerogel is 10%.Following a literature method, the CPDs were synthesized via a hydrothermal reaction of citric acid and ethylenediamine. 36The transmission electron microscopy (TEM) image indicates that the CPDs have an average diameter of 6-8 nm (Figure S1). Figure 1A and Figure S2 show the preparation process of the thermochromic PAH-PAA-CPDs hydrogels.First, a given amount of aqueous solution of CPDs (60 mg/mL) was added to aqueous PAA solution (0.06 mol/L (monomer), pH 5.5) that contains 2 mol/L NaCl under continuous stirring.Subsequently, the aqueous solutions of PAH (0.06 mol/L (monomer), pH 8.5, containing 2 mol/L NaCl) and PAA-CPDs with equal volumes were mixed through peristaltic pumps under continuous stirring at room temperature.Under stirring, precipitates are immediately generated.The precipitates were collected through ultracentrifugation and then molded into sheetlike polyelectrolyte composites via two pieces of hydrophobized glass.After being completely dried in air at room temperature, the as-prepared polyelectrolyte composites were incubated in 2 mol/L aqueous NaCl solution for 48 h to obtain transparent hydrogels.For simplicity, these hydrogels are denoted as PAH-PAA-x%CPDs, where x represents the mass fraction of CPDs in the corresponding xerogels.The electrostatic and hydrogenbonding interactions in the PAH-PAA-10%CPDs hydrogel were confirmed by Fourier transform infrared (FTIR) spectroscopy (Figure S3). 31,37,38TEM image of the PAH-PAA-10%CPDs xerogel (Figure S4) confirms the uniform dispersion of CPDs in the PAH-PAA polymeric matrix.As shown in Figure 1B, the PAH-PAA-10%CPDs hydrogel with an area of 3.8 cm × 2.8 cm is highly transparent at 10 °C while quickly turning opaque within ~1 s at 35 °C, demonstrating the excellent thermochromic capability of the hydrogel.Mechanical properties of the PAH-PAA-10%CPDs hydrogel were characterized by tensile tests at a stretching speed of 50 mm/min and compression tests at a compressive speed of 2 mm/min.The hydrogel exhibits a tensile strength of ∼51 kPa and an ultrahigh-breaking strain of ~2140% (Figure 1C).In the compression test, the hydrogel displays an obvious plastic deformation in the strain range of 0%-60% (Figure S5).The soft nature and plastic deformation indicate that the PAH-PAA-10%CPDs hydrogel is mechanically compliant, which can ensure the close contact of the thermochromic hydrogel with various substrates even under large deformations.

| Thermochromic properties of the PAH-PAA-x%CPDs hydrogels
The thermochromic properties of the PAH-PAA-x%CPDs hydrogels are tailorable through adjusting CPDs contents in the corresponding hydrogels.The changes in transmittance of the PAH-PAA-x%CPDs hydrogels sandwiched between two pieces of quartz substrates as a function of temperatures in the wavelength range of 2500-300 nm were characterized by using a UV−vis−IR spectrophotometer.As shown in Figure 2A and Figure S6, the transmittance of the PAH-PAA-x%CPDs hydrogels decreases obviously with increasing environmental temperatures.Their ΔT sol in 2500-300 nm were calculated according to Equations ( 1) and (2) provided in Support Information.All the PAH-PAA-x%CPDs hydrogels possess ultrahigh ΔT sol with values higher than ~88%, confirming their strong solar heat-shielding abilities (Table S1).Meanwhile, T lum of the PAH-PAA-x% CPDs hydrogels in 780 nm-380 nm from 10 to 35 °C was calculated and presented in Figure 2B.The T lum of the PAH-PAA-x%CPDs hydrogels is higher than 98% at 10 °C, which guarantees high transparency and plenty of solar irradiation at low temperatures.Moreover, Figure 2B shows that the PAH-PAA-x%CPDs hydrogels possess different τ c .With increasing CPDs contents from 0% to 10%, the τ c of the PAH-PAA-x%CPDs hydrogels increases obviously from ~19.5 to ~24.4 °C and then keeps constant with a continuous increase of CPDs contents to 15% (Figure 2C).It is noted that all the PAH-PAA-x%CPDs hydrogels exhibit an extremely short transition time of ~1 s when switching from transparent to opaque state under heating, indicating their ultrafast thermo-response capability (Figure 2C).However, with increasing CPD contents from 0% to 15%, the transition time of the PAH-PAA-x%CPDs hydrogels undergoes a Vshape change in the cooling process.In the cooling process, the PAH-PAA-10%CPDs hydrogel possesses the highest transition rate with a transition time of ~3 s.Interestingly, the hysteresis width (Δτ c ) of the PAH-PAA-x%CPDs hydrogels, which is defined as the difference of τ c between the heating and cooling processes (Figure 2D and S7), exhibits a similar variation trend with transition time.The PAH-PAA-10%CPDs hydrogel possesses the narrowest Δτ c of ~1.9 °C (Figure 2E).These results indicate that the PAH-PAA-10%CPDs hydrogel has the most efficient thermochromic capability.Therefore, the PAH-PAA-10%CPDs hydrogels, which have a high T lum of ~98.7%, favorable τ c of ~24.2 °C, strong ΔT sol of ~89.3%, short transition time of ~3 s for cooling process and narrow Δτ c of ~1.9 °C, are further investigated for using as high-performance thermochromic smart windows.As calculated based on 1 H nuclear magnetic resonance analysis (Figure S8), the PAH-PAA-10%CPDs hydrogel has the measured monomer molar ratio of PAH to PAA being 1:0.98.Thermogravimetric analysis in Figure S9 shows that the mass ratio of NaCl in the PAH-PAA-10%CPDs xerogel is 17.4%.The loaded NaCl endows the PAH-PAA-10%CPDs hydrogel with good anti-freezing ability.Differential scanning calorimetry was employed to measure the freezing point of the PAH-PAA-10%CPDs hydrogel. 39,40As shown in Figure 2F, the PAH-PAA-10%CPDs hydrogel fabricated in water has an exothermic peak at −5.3 °C, which represents the freezing point of water entrapped within the hydrogel.The loaded NaCl can shift the exothermic peak of frozen water in the PAH-PAA-10%CPDs hydrogel to a lower temperature of −22.5 °C.The loaded NaCl within the PAH-PAA-10%CPDs hydrogel can break the hydrogen bonding interactions among water molecules and effectively prevent their freezing.The anti-freezing ability can largely extend the application of the PAH-PAA-10%CPDs hydrogel-based smart windows at subzero temperatures.Figure S10A shows that the PAH-PAA-10%CPDs-based smart window keeps its integrity at −20 °C.In contrast, the same hydrogel without NaCl became frozen at −20 °C, resulting in damage to the smart window (Figure S10B).

| Thermochromic mechanism of the PAH-PAA-10%CPDs hydrogels
The internal structures of the PAH-PAA-10%CPDs hydrogels at 10 and 35 °C were characterized by scanning electron microscope (SEM) to understand their thermochromic mechanism.The PAH-PAA-10%CPDs hydrogel was incubated at 10 and 35 °C for 8 min and then immediately immersed in liquid nitrogen to fix their structures.The SEM images in Figure 3A,B reveal that the PAH-PAA-10%CPDs hydrogel at 10 °C has a homogeneous structure with a high density of tiny pores.In contrast, the hydrogel at 35 °C has an inhomogeneous structure with large hierarchical pores of several tens of micrometers in diameter (Figures 3C,D).Such a large structural change of the hydrogel originates from the temperature-induced hydration/dehydration of the polyelectrolyte complexes.As depicted in Figure 3E, at temperatures below τ c , the polyelectrolyte complexes in the PAH-PAA-10%CPDs hydrogel are highly hydrated without obvious phase separation.Therefore, the hydrogel has a high transmittance in the entire solar spectrum with negligible light scattering (Figure 2A).When the temperature is higher than τ c , the ionic hydration between water molecules and polyelectrolyte chains is destroyed, leading to the dehydration of polyelectrolyte complexes.As a result, the polyelectrolyte complexes aggregate into hydrophobic domains and obvious phase separation occurs in the hydrogels.These micro-scaled phase-separated hydrophobic domains in hydrogels can strongly scatter visible and near-infrared light, leading to the obviously decreased solar transmittance of hydrogel above τ c . 8,26,41Such a dramatic change in transmittance endows the PAH-PAA-10%CPDs hydrogel with extremely high ΔT sol of ~89.3%, which is highly desirable for blocking a large amount of solar irradiance.The PAH-PAA-10%CPDs can maintain their hydrogel structure and thermochromic properties well after multiple heating-cooling processes because the hydrogen bonds in the PAH-PAA-10%CPDs hydrogels can ensure their structure integrity in the presence of NaCl, while the saltmediated electrostatic interactions allow the polyelectrolyte complexes to have high chain mobility.
In a control experiment, the PAH-PAA-10%CPDs hydrogel without NaCl appears opaque at room temperature.This is because the strong electrostatic interactions among PAH, PAA, and CPDs generate severely aggregated complexes of PAH, PAA, and CPDs, which strongly scatter visible light.The PAH-PAA-10%CPDs hydrogel without NaCl shows no obvious changes of transmittance when the environmental temperature gradually rises from 5 to 50 °C (Figure S11), meaning that the hydration/ dehydration of the hydrogels is severely limited by the strong electrostatic interactions among PAH, PAA, and CPDs.In contrast, the loaded NaCl in the PAH-PAA-10% CPDs hydrogels can partially break the electrostatic interactions among PAH, PAA, and CPDs, thereby facilitating chain mobility and enhancing the hydration/dehydration of the hydrogels.With increasing NaCl concentrations from 1 to 2.5 mol/L, the τ c of the PAH-PAA-10%CPDs hydrogels increases from ~19.0 to ~26.4 °C (Figure S12).This is because the increase of NaCl concentrations results in the decrease of electrostatic interactions between polyelectrolytes, generating polyelectrolyte complexes with higher hydrophilicity.Therefore, a higher temperature is needed to dehydrate the polyelectrolyte complexes, leading to a higher τ c of the PAH-PAA-10%CPDs hydrogels.The introduction of CPDs into PAH-PAA hydrogels can well tailor their thermochromic properties.The τ c of the PAH-PAA hydrogel becomes higher with increasing contents of CPDs because the hydrophilic CPDs with abundant hydroxyl, amino, and carboxylate/carboxylic acid groups on their surface can significantly enhance the hydrophilicity of the hydrogels.The PAH-PAA-10%CPDs hydrogel has a water content of 57.6%, which is higher than 42.9% of the PAH-PAA hydrogel.As a result, a higher temperature is needed to dehydrate the PAH-PAA-10%CPDs hydrogels.Therefore, the increase of CPD content in the PAH-PAA-x%CPDs hydrogels results in a higher τ c .Meanwhile, CPDs can significantly reduce the transition time of the hydrogels from an opaque to a transparent state.The CPDs with extremely small size and deformability possess much higher diffusion dynamics than the PAH and PAA chains. 34,35Upon cooling, the rapid diffusion of CPDs can drive the diffusion of PAH and PAA chains and their rehydration, thereby significantly accelerating the disassociation of aggregates in the PAH-PAA-10%CPDs hydrogels.Meanwhile, the high surface hydrophilicity of CPDs can enable rapid rehydration of the surrounding PAH and PAA chains.Consequently, in the cooling process, the PAH-PAA-10% CPDs hydrogel possesses a transition time of ~3 s, which is much shorter than that (~14 s) of the PAH-PAA hydrogel.In a control experiment, the hydrophilic and rigid SiO 2 nanoparticles with an average diameter of 6-10 nm were introduced into the PAH-PAA hydrogel (denoted as PAH-PAA-10%SiO 2 ) and its thermochromic properties were investigated (Figures S13 and S14).The PAH-PAA-10%SiO 2 hydrogel exhibits a fast transition time of ~6 s in the cooling process, indicating that the small-sized SiO 2 nanoparticles can also accelerate the transition process of the thermochromic hydrogels.However, the transition time of the PAH-PAA-10%SiO 2 hydrogel is longer than that of the PAH-PAA-10%CPDs hydrogel.These results affirmatively confirm that smallsized and deformable CPDs are crucial in enhancing the transition process of the thermochromic hydrogels.

| Durability and reliability of the PAH-PAA-10%CPDs hydrogel-based smart windows
The intimate adhesion between the PAH-PAA-10%CPDs hydrogel and the underlying substrates is critical for the thermochromic smart windows to eliminate light scattering at the substrate/hydrogel interfaces.As shown in Figure 4A, the PAH-PAA-10%CPDs hydrogel can be facilely sandwiched within a variety of substrates such as glass, polyvinyl chloride, polymethyl methacrylate (PMMA), and polycarbonate without bubbles appearing on the interface between hydrogels and these substrates.The smart windows are encapsulated with 3M VHB elastomer tapes to prevent the dehydration of the thermochromic hydrogels.Upon heating, all the thermochromic windows become opaque without separation between the hydrogels and the substrates.Due to their good mechanical compliance, the PAH-PAA-10%CPDs hydrogel can also be sandwiched in highly flexible polyethylene terephthalate (PET) substrates.Figure 4B shows that the PAH-PAA-10%CPDs hydrogel-PET windows with an area of 15 cm × 15 cm can endure repeated bending and unbending treatments at least 2000 times without fracturing or peeling off from the substrates.
Meanwhile, the PAH-PAA-10%CPDs hydrogel-PET window undergoing 2000 bending and unbending cycles can still maintain its efficient thermochromic process (Figure 4B,C).Lap-shear tests were performed to measure the adhesion strength of the PAH-PAA-10%CPDs hydrogels toward various substrates (Figure S15).The adhesion strength of the hydrogels toward all the above-mentioned substrates exceeds 50 kPa.Stretchable thermochromic smart windows can be fabricated by sandwiching the PAH-PAA-10%CPDs hydrogel between two 3M VHB elastomer tapes.The stretchable thermochromic smart window can maintain its function when being repeatedly stretched to 100% strain (Figure 4D).The optical properties of the PAH-PAA-10%CPDs hydrogel-based smart windows using different types of substrates were investigated.Figure 4E shows that all the smart windows exhibit similar T lum and ΔT sol , meaning that the thermochromic capabilities of the PAH-PAA-10%CPDs hydrogel-based smart windows are independent of the types of substrates.
Benefitting from the reversibility of the electrostatic and hydrogen-bonding interactions, the PAH-PAA-10% CPDs hydrogel exhibits efficient room-temperature selfhealing capability.The PAH-PAA-10%CPDs hydrogel was cut into two parts and then the separated parts were brought into contact at room temperature followed by incubation in 2 mol/L NaCl.After incubation for 24 h, the healed PAH-PAA-10%CPDs hydrogel was subjected to tensile tests.As shown in Figure 5A, the stress-strain curve of the healed PAH-PAA-10%CPDs hydrogel almost overlaps with that of the original sample, confirming the complete healing of the damaged hydrogel.The NaCl solution can break the electrostatic and hydrogen-bonding interactions in the surfaces of the hydrogels and largely facilitate chain mobility.Once brought to contact, polymer chains and CPDs at the contacted surfaces can diffuse toward each other and the electrostatic and hydrogen-bonding interactions can be rebuilt to heal the damaged hydrogels.Taking advantage of its self-healing capability, the PAH-PAA-10%CPDs hydrogel with a large area can be easily fabricated by welding small pieces of hydrogel together.As shown in Figure 5B, four PAH-PAA-10%CPDs hydrogel pieces with an area of 3 cm × 3 cm were welded to generate a hydrogel with an area of 6 cm × 6 cm.The joints between the hydrogels disappeared after incubating the welded samples in aqueous NaCl solution for 24 h.Therefore, the self-healing capability of the PAH-PAA-10% CPDs hydrogel provides a convenient way for the fabrication of large-area smart windows.
The long-term durability of smart windows is of paramount importance for their practical applications.The PAH-PAA-10%CPDs hydrogels sandwiched between two pieces of quartz substrates were sealed by gluing the substrates with 0.5 mm-thick 3M VHB tape. Figure 5C shows the optical properties of the smart window which was stored in an ambient environment for different days.The T lum and ΔT sol of the smart window can be maintained constant during 147 days of storage, demonstrating the excellent stability of the smart window.Furthermore, a continuous cooling-heating cycling test switched between 10 and 35 °C was conducted on the smart window to investigate its durability.Figure 5D records the T lum and ΔT sol of the smart window in every 500 cycling intervals during 10,000 heating-cooling cycles.The T lum and ΔT sol show negligible changes during 10,000 cycles of tests, indicating the ultra-durable thermochromic performance of the smart window.2][43][44][45] It can be found that the PAH-PAA-10%CPDs hydrogel with a high T lum of 98.7%, strong ΔT sol of 89.3%, desirable τ c of 24.2 °C, and fast transition time of 3 s outperforms the previously reported thermochromic hydrogels/ionogels.

| Thermal tests of model house with PAH-PAA-10%CPDs hydrogel-based smart windows
To evaluate the ability of the PAH-PAA-10%CPDs hydrogels to modulate the transmittance of solar radiation, a heat-insulating styrofoam model house was built and equipped with the PAH-PAA-10%CPDs hydrogelbased smart windows.The indoor temperature was monitored under artificial sunlight.Figure 6A shows the model house (20 cm × 20 cm × 20 cm) for the thermal tests.The smart windows were fabricated by sandwiching the PAH-PAA-10%CPDs hydrogel (0.5 mm-thick) within two pieces of PMMA substrates (10 cm × 10 cm).Meanwhile, the same model house equipped with a PMMA sandwich window with a 0.5 mm-thick air interlayer was also constructed for comparison.The indoor temperatures of these two model houses under artificial sunlight were recorded as a function of irradiation time.As shown in Figure 6B, the indoor temperature of the reference house increases rapidly from 23.8 to 45.2 °C after irradiation for 115 min, while the indoor temperature  of the house with the smart windows slowly increases to 36 °C.The large temperature difference of ~9.2 °C between these two model houses indicates that the PAH-PAA-10%CPDs hydrogel-based smart windows can efficiently regulate indoor temperatures and show excellent energy-saving performance.Furthermore, for visual observation of the difference in indoor temperatures between these two model houses, the soy waxes (melting point ~44 °C) were placed inside the model houses.The digital images in Figure 6C show the changes in the morphology of soy waxes with prolonged irradiation time.Melting of soy wax was observed after ~60 min of irradiation in the reference house (Figure 6C [ii]), while the soy wax in the house with smart windows showed no change in its shape even after irradiation for ~115 min (Figure 6C[vi]).The exposure of two houses to sunlight leads to a rapid rise of indoor temperature in the first ~40 min.For the house with smart windows, the transmittance of the windows dropped dramatically once the temperature rose above τ c , which prevented the further rapid increase of indoor temperatures.The house without smart windows cannot regulate the indoor temperature.To further evaluate the energy-saving performance of the smart windows for adaptive solar modulation in real situations, outdoor thermal tests were performed under natural sunlight irradiation in the city of Changchun (Figure S16).Two model houses were exposed to sunlight at the same time, and the real-time temperatures of indoor and outdoor environments were monitored.The temperature curves in Figure 6D show that the highest indoor temperature in the reference house is 47.7 °C at noon 1:00 PM, while the indoor temperature in the house with smart windows is only 40.1 °C.These results indicate that the PAH-PAA-10% CPDs hydrogel-based smart windows can adapt dynamically to outdoor environments and achieve excellent indoor temperature modulation performance.

| CONCLUSIONS
We have demonstrated the fabrication of the thermochromic PAH-PAA-10%CPDs hydrogels that are qualified for the construction of high-performance smart windows.Benefiting from the incorporation of CPDs, the PAH-PAA-10%CPDs hydrogels exhibit a high ΔT sol of ~89.3%, an ideal τ c of ~24.2 °C, and a quick transition time of ~3 s from opaque state to transparent state.Large-area hydrogel sheets can be conveniently fabricated by welding multiple pieces of hydrogel sheets.The PAH-PAA-10%CPDs hydrogel-based smart windows are highly reliable and durable under ambient conditions, and can work well under subzero temperatures as low as −20 °C.Compared with the reference model house, the houses with smart windows can achieve a temperature reduction of 9.2 °C, meaning that the smart windows have a strong ability to tailor indoor temperatures through light management.The thermochromic hydrogels have excellent comprehensive properties including satisfactory self-healing, adhesive, and antifreezing capabilities.The materials for the fabrication of the thermochromic PAH-PAA-10%CPDs hydrogels are easily available and the fabrication processes are technically simple to conduct.Therefore, the present work provides a cost-effective method for the massive production of hydrogel-based smart windows with excellent thermochromic performance.Given the abundant types of polymers capable of forming polymer complex-based hydrogels, we believe that thermochromic smart windows with well-tailored thermochromic properties can be fabricated through a rational combination of polymer complexes and small-sized nanoparticles, especially CPDs.

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I G U R E 1 (A) Schematic illustration of the fabrication process of the PAH-PAA-10%CPDs thermochromic hydrogels.(B) Digital images of the PAH-PAA-10%CPDs hydrogel at the cold and hot states.(C) Stress−strain curve of the PAH-PAA-10%CPDs hydrogel.

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I G U R E 2 (A) Solar transmittance for the PAH-PAA-10%CPDs hydrogel as temperature changes in the wavelength from 2500 to 300 nm.The shadow in the figure represents the solar irradiance spectrum.(B) T lum for the PAH-PAA-x%CPDs hydrogels as temperature changes in the wavelength from 780 nm to 380 nm.(C) τ c and transition time for the heating/cooling process of the PAH-PAA-x%CPDs hydrogels.(D) The temperature-dependent thermochromic changes of the PAH-PAA-10%CPDs hydrogel upon heating and cooling processes.(E) Δτ c of the PAH-PAA-x%CPDs hydrogels.(F) Differential scanning calorimetry (DSC) curves of the PAH-PAA-10%CPDs hydrogel fabricated in water and 2 mol/L NaCl.F I G U R E 3 (A-D) Cross-sectional SEM images of the freeze-dried PAH-PAA-10%CPDs hydrogel of different scales at its transparent (A and B) and opaque (C and D) states.(E) Schematic illustration of the PAH-PAA-10%CPDs hydrogel at transparent and opaque states.

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I G U R E 4 (A) Digital images of the PAH-PAA-10%CPDs hydrogels with an area of 3 cm × 3 cm sandwiched within diverse window substrates.(B, C) Digital images of the PAH-PAA-10%CPDs hydrogel sandwiched within PET substrates at their bending state (B) as well as cold and hot states after 2000 cycles of bending and unbending processes (C).(D) Digital images of the PAH-PAA-10%CPDs hydrogel sealed in 3M VHB elastomer tape at their cold (i), hot (ii), and stretching (iii) states.(E) Optical properties of the PAH-PAA-10%CPDs hydrogels sandwiched within diverse window substrates.

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I G U R E 5 (A) Stress−strain curves of the original and 24 h-healed PAH-PAA-10%CPDs hydrogels, which were previously cut into two pieces.(B) Digital images of welding four pieces of PAH-PAA-10%CPDs hydrogel into one piece.(C) Thermochromic properties of the PAH-PAA-10%CPDs hydrogel sealed in two pieces of quartz glass for 147 days.(D) Thermochromic properties of the sealed PAH-PAA-10% CPDs hydrogel during 10000 heating and cooling cycles.(E) Thermochromic properties of the PAH-PAA-10%CPDs hydrogel compared with other thermochromic hydrogels/ionogels reported in literature.

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I G U R E 6 Model house thermal tests conducted to examine the energy-saving performance of the PAH-PAA-10%CPDs smart windows.(A) Schematic of the model house for the thermal tests.(B, C) Temperature curves (B) and digital images (C) of the model houses equipped with smart windows and reference windows under artificial sunlight for different times.(D) 24 h indoor and ambient air temperature curves of the model houses equipped with smart windows and reference windows for the outdoor thermal test in Changchun.