Thermochromic dual-network hydrogel with tunable transition temperature for smart window applications

Abstract

A thermochromic device (300) includes a first substrate (302), a second substrate (304), and a thermochromic hydrogel (100) disposed between the first substrate (302) and the second substrate (304). The thermochromic hydrogel (100) includes a dual-network structure including a first polymer network having hydroxypropyl cellulose (102), a second polymer network having polyacrylamide (104) formed in-situ within the first polymer network, and an ionic additive including calcium chloride. The ionic additive is configured to modulate a transition temperature of the thermochromic hydrogel (100), such as between 15 °C and 42 °C. The thermochromic device (300) is configured for use in smart windows for vehicles or building facades to provide passive cooling by transitioning from a transparent state to an opaque state to reflect solar radiation when the transition temperature is exceeded. Near-infrared absorbing nanoparticles may be included to accelerate the transition.

Description

Attorney Docket No. 0338-800-WQ / 2024-044-02THERMOCHROMIC DUAL-NETWORK HYDROGEL WITH TUNABLE TRANSITION TEMPERATURE FOR SMART WINDOW APPLICATIONSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 776,407, filed on March 24, 2025, entitled “HYBRID THERMOCHROMIC COMPOSITES FOR SMART WINDOWS,” the disclosure of which is incorporated herein by reference in its entirety.BACKGROUND OF THE INVENTION TECHNICAL FIELD

[0002] Embodiments of the subject matter disclosed herein generally relate to a thermochromic hydrogel and a method for making a thermochromic hydrogel, and more particularly, to a hydroxypropyl cellulose (HPC) based dual-network hydrogel configured for smart passive cooling applications.DISCUSSION OF THE BACKGROUND

[0003] This background description is provided for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the0338-800-WQ / 2024-044-02description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

[0004] Solar heating through transparent or semi-transparent surfaces, such as windows in buildings and vehicles, significantly increases internal thermal loads. In particular, the accumulation of heat in parked vehicles under direct sunlight presents substantial safety risks and necessitates high energy consumption for active cooling systems. In many modern vehicle designs, especially electric vehicles, large glass roof panels are utilized to maintain vehicle height while accommodating battery systems at the base. These surfaces allow significant solar radiation to penetrate the cabin, leading to rapid overheating.

[0005] Various technologies have been developed to manage solar heat gain, including electrochromic, photochromic, and thermochromic systems. Electrochromic systems utilize an applied voltage to adjust transparency but often involve complex circuitry, external power requirements, and potential degradation of optical performance over time. Photochromic materials respond to light intensity but frequently provide insufficient control over infrared heat and may exhibit irreversible color changes.

[0006] Thermochromic hydrogels have been explored as a passive alternative due to their ability to undergo a phase transition based on ambient temperature. Conventional thermochromic hydrogels, such as those based on poly(N-isopropylacrylamide) (PNIPAm) or single-network HPC, often suffer from inherent limitations. Many such hydrogels exhibit relatively high transition temperatures,0338-800-WQ / 2024-044-02typically between 30 °C and 42 °C, which may be above the threshold required for effective thermal management and human comfort in many environments.

[0007] Furthermore, existing hydrogel systems often lack mechanical robustness and structural integrity. Single-network hydrogels can be fragile and prone to significant volume shrinkage during the hydrophilic-to-hydrophobic phase transition. This volume contraction often leads to water separation from the polymer matrix, resulting in liquid leakage and a reduction in long-term stability and reliability. Additionally, conventional hydrogel formulations may be susceptible to freezing in cold climates, which can disrupt the polymer network and limit the functional temperature range of the device. Consequently, there is a need for a thermochromic material system that provides tunable transition temperatures, enhanced mechanical stability, and improved water retention without requiring external power or complex control systems.0338-800-WQ / 2024-044-02SUMMARY OF THE INVENTION

[0008] The present document is directed to devices based on a thermochromic hydrogel and a method for forming the thermochromic hydrogel. The thermochromic hydrogel includes a dual-network structure that provides enhanced mechanical robustness and tunable optical properties for passive cooling applications, such as smart windows for buildings or vehicles.

[0009] Various embodiments of the thermochromic hydrogel include a first polymer network of HPC and a second polymer network of polyacrylamide (PAAm). The second polymer network is formed in-situ within the first polymer network to create a stable, interpenetrating dual-network structure. An ionic additive, such as calcium chloride (CaCl2), is incorporated into the dual-network structure to modulate a transition temperature of the thermochromic hydrogel. In some embodiments, the transition temperature is selectively tuned between 15 °C and 42 °C by adjusting a concentration of the ionic additive. The thermochromic hydrogel is configured to undergo a reversible phase transition from a transparent state to an opaque state when a temperature exceeds the transition temperature, thereby reflecting solar radiation.

[0010] In another embodiment, a thermochromic device includes the thermochromic hydrogel disposed between a first substrate and a second substrate. The first substrate and the second substrate may include acrylic sheets configured to encapsulate the thermochromic hydrogel to prevent water evaporation and maintain long-term stability. The thermochromic hydrogel may further include near-infrared0338-800-WQ / 2024-044-02(NIR) absorbing nanoparticles, such as cesium tungsten oxide (CS0.33WO3), dispersed within the dual-network structure to accelerate the transition from the transparent state to the opaque state and enhance solar heat gain reduction.

[0011] A method for forming the thermochromic hydrogel includes dissolving HPC and an ionic additive in a solvent to form a first solution. An acrylamide monomer (AAM) and a cross-linker are added to the first solution to form a precursor mixture. Upon adding an initiator and a catalyst, the AAM is polymerized in-situ within the first solution to form the second polymer network interspersed with the first polymer network. The resulting dual-network hydrogel may be transferred into a gap between substrates prior to the completion of polymerization to form a thermochromic device. One or more embodiments provide a passive, energyindependent solution for thermal management that addresses limitations related to mechanical fragility, volume shrinkage, and water leakage.0338-800-WQ / 2024-044-02BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0013] FIG. 1 is a schematic illustration of a synthesis process for a thermochromic hydrogel including a first polymer network and a second polymer network according to an embodiment.

[0014] FIG. 2 is a schematic illustration of a method for forming a thermochromic hydrogel according to an embodiment.

[0015] FIG. 3 is a cross-sectional view of a thermochromic device including a thermochromic hydrogel encapsulated between a first substrate and a second substrate according to an embodiment.

[0016] FIG. 4 is a view of a thermochromic device illustrating an optical transition between a transparent state at room temperature and an opaque state at an elevated temperature according to an embodiment.

[0017] FIG. 5 is a schematic representation of a phase transition mechanism of a thermochromic hydrogel relative to a lower critical solution temperature (LOST) according to an embodiment.

[0018] FIG. 6 is a graph illustrating normalized heat flux as a function of temperature for thermochromic hydrogels with varying concentrations of PAAm according to an embodiment.0338-800-WQ / 2024-044-02

[0019] FIG. 7 is a graph illustrating normalized heat flux as a function of temperature for thermochromic hydrogels with varying concentrations of CaCh according to an embodiment.

[0020] FIG. 8 is a graph illustrating Fourier transform infrared (FT-IR) spectra for a precursor solution and thermochromic hydrogels with varying polymer concentrations according to an embodiment.

[0021] FIG. 9 is a graph illustrating a transmittance of a thermochromic device as a function of wavelength relative to a solar spectrum and an atmospheric window according to an embodiment.

[0022] FIG. 10A is a graph illustrating a transmittance spectra of a thermochromic device at different temperatures and hydrogel thicknesses according to an embodiment.

[0023] FIG. 10B is a graph illustrating a luminous transmittance of a thermochromic device over multiple thermal cycles according to an embodiment.

[0024] FIG. 10C is a graph illustrating differential scanning calorimetry (DSC) curves for thermochromic hydrogels at sub-zero temperatures according to an embodiment.

[0025] FIG. 11 is a schematic illustration of an experimental setup for evaluating the passive cooling performance of a thermochromic device according to an embodiment.

[0026] FIG. 12 is a graph illustrating solar radiation transmitted through a reference window and a thermochromic smart window according to an embodiment.0338-800-WQ / 2024-044-02

[0027] FIG. 13 is a graph illustrating temperature profiles and solar radiation recorded over a multi-day outdoor test period for a thermochromic device according to an embodiment.

[0028] FIG. 14A is a graph illustrating internal air temperature reduction in a vehicle equipped with a thermochromic device according to an embodiment.

[0029] FIG. 14B is a graph illustrating temperature reduction at a front window of a vehicle equipped with a thermochromic device according to an embodiment.

[0030] FIG. 14C is a graph illustrating temperature reduction at a rear window of a vehicle equipped with a thermochromic device according to an embodiment.

[0031] FIG. 14D is a graph illustrating temperature reduction at a side window of a vehicle equipped with a thermochromic device according to an embodiment.

[0032] FIG. 15 is a schematic illustration of a synthesis process for a thermochromic hydrogel including NIR absorbing nanoparticles according to an embodiment.

[0033] FIG. 16 is a schematic illustration of a method for forming a thermochromic hydrogel including NIR absorbing nanoparticles according to an embodiment.0338-800-WQ / 2024-044-02DETAILED DESCRIPTION OF THE INVENTION

[0034] The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the scope of the disclosure. Instead, the scope of the disclosure is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a dual-network hydrogel including a first polymer network and a second polymer network. In some embodiments, the first polymer network includes HPC and the second polymer network includes PAAm, which is formed in-situ via polymerization of AAM in the presence of the first polymer network. However, the embodiments to be discussed next are not limited to this specific chemistry, but may be applied to other types of thermochromic composite systems.

[0035] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0036] In one embodiment, a dual-network hydrogel includes HPC and PAAm with integrated calcium (Ca) atoms, which may be provided by CaCh. In another embodiment, the dual-network hydrogel may further include CS0.33WO3 particles.0338-800-WQ / 2024-044-02This configuration, which may be referred to as HACsWo.12, is configured to absorb NIR light and provide enhanced thermal management. These thermochromic systems may be encapsulated within a substrate structure to facilitate passive cooling in applications such as vehicles, building facades, or greenhouses.

[0037] Various embodiments of the thermochromic systems and their associated components are now discussed in further detail.

[0038] HPC is an example of thermochromic materials that operate based on phase transitions. The thermochromism in HPC results from its hydroph ilic-to-hydrophobic transition near the LCST, resulting in a color change in the HPC film. However, hydrogels composed solely of HPC exhibit inherent softness, which may limit their suitability for applications requiring mechanical robustness. Furthermore, the hydrophilic-to-hydrophobic transition causes volume changes, leading to water separation from the polymer matrix, which may increase a risk of leakage. To address these challenges, a dual-network gel 100 is formed by incorporating PAAm into the HPC matrix.

[0039] As shown in FIG. 1 , the dual-network gel 100 includes a first polymer network of HPC 102 and a second polymer network of PAAm 104. A precursor mixture may include an AAM monomer 103, which is configured to undergo polymerization to form the second polymer network of PAAm 104. In some embodiments, the PAAm 104 is formed in-situ within the first polymer network of the HPC 102.

[0040] The dual-network gel 100 further includes an ionic additive, such as CaCI2, which provides calcium ions 106 and chloride ions 108. The PAAm 104 may0338-800-WQ / 2024-044-02be cross-linked using a cross-linker, such as N,N'-methylenebisacrylamide (MBAA) 110. The formation of the dual-network gel 100 involves the use of an initiator, such as ammonium persulfate (APS) 112, and a catalyst, such as tetramethylethylenediamine (TEMED) 114, to integrate the HPC 102 and the PAAm 104 into a single, stable network.

[0041] The incorporation of the PAAm 104 into the network of the HPC 102 enhances both the mechanical strength and water retention of the dual-network gel 100, addressing certain limitations of HPC-based systems. The structure of the dualnetworkgel 100 reinforces mechanical integrity, achieving a tensile strength of about 28 kPa, while also improving stability and limiting water leakage during prolonged use. The enhanced water retention is primarily attributed to amino groups in the PAAm 104, which form hydrogen bonds with both water molecules and hydroxyl groups of the HPC 102, effectively reducing evaporation losses. Quantitative analysis shows that the dual-network gel 100 retains 90% of its initial weight after 12 hours and 68% after 48 hours, outperforming unmodified HPC solutions.

[0042] These improvements underscore the role of the PAAm 104 in stabilizing the dual-network gel 100 and maintaining long-term functionality, making it suitable for smart window applications. Furthermore, the dual-network structure not only provides mechanical robustness but also ensures that the thermochromic properties of the HPC 102 remain intact, allowing the dual-network gel 100 to undergo reversible optical transitions in response to temperature fluctuations.

[0043] An embodiment of a process for preparing the dual-network hydrogel 100 is illustrated in FIG. 2. According to an embodiment, a first polymer, such as0338-800-WQ / 2024-044-02HPC 102, is dissolved 202 in a solvent, such as deionized (DI) water. In some embodiments, the HPC 102 is dissolved at a temperature between 50 °C and 80 °C. A concentration of the HPC 102 may be selected to be between 5 wt.% and 10 wt.% based on a total mass of the solution. The mixture may be stirred at ambient temperature for at least 12 hours to ensure the HPC 102 is completely dissolved. An ionic additive, such as CaCl2, is added 204 to the solution. The concentration of the CaCl2 may be selected to be between 2 wt.% and 15 wt.% to modulate a transition temperature of the dual-network hydrogel 100. In step 206, an AAM monomer 103 and a cross-linker, such as N,N'-methylenebisacrylamide (BIS), are added to the mixture. A concentration of the AAM monomer 103 may be between 3 wt.% and 8 wt.%. In some embodiments, the water may be between 67 wt.% to 90 wt.%. In steps 208 and 210, an initiator, such as APS 112, and a catalyst, such as TEMED 114, are added to the precursor mixture. In some embodiments, the mixture is stirred for approximately 10 minutes after adding the APS 112 and the TEMED 114 to initiate in-situ polymerization of the AAM monomer 103 to form the PAAm 104 intersped with the HPC 102 network.

[0044] The polymerization process forms the second polymer network, such as PAAm 104, interspersed within the first polymer network of the HPC 102, resulting in a meshed dual-network structure. In some embodiments, the gel 100 is transferred into a double-glazed panel 300 (see FIG. 3) that includes two acrylic sheets 302 and 304, using a syringe pump at a flow rate of approximately 2 mL / min prior to completion of the polymerization. The edge space between the two acrylic sheets 302 and 304 is then sealed with a sealing material 306 to hermetically close0338-800-WQ / 2024-044-02the gel 100 inside the panel 300. The panel 300 may be a smart window of a car or building or any other dwelling.

[0045] The dual-network gel 100 may be fully formed after standing at room temperature for approximately 30 minutes. This approach ensures a uniform hydrogel with enhanced mechanical properties and improved water retention, capable of providing reliable, temperature-responsive behavior. The resulting thermochromic hydrogel combines the phase-transition characteristics of the HPC 102 with the structural stability of the PAAm 104, creating a versatile material for energy-efficient cooling applications.

[0046] In some embodiments, the dual-network gel 100 may be susceptible to water loss at elevated temperatures due to evaporation or vaporization. To mitigate this issue, the dual-network gel 100 is integrated into the protective sandwich panel 300, as illustrated in FIG. 3. The substrates 302 and 304 serve as an effective barrier against water evaporation, preserving the hydration of the dual-network gel 100 and ensuring long-term stability under varying environmental conditions. Additionally, the substrates provide structural support and mechanical protection, safeguarding the dual-network gel 100 from physical damage during use. The integration of the substrates not only enhances performance and stability but also ensures the durability and practicality of the resulting device in real-world applications.

[0047] The optical response of this panel is illustrated in FIG. 4. As shown in the comparison, the panel 300 remains highly transparent at room temperature (the substrate supporting the gel 100 is perfectly visible as the left panel of the figure) but transitions to a white, opaque state when the dual-network gel 100 reaches its0338-800-WQ / 2024-044-02transition temperature (right panel of the figure). This change allows the panel 300 to passively reflect sunlight, minimizing solar heat gain and preventing excessive temperature increases inside a vehicle or other enclosed space. This reversible behavior is consistent with the transition mechanisms shown in FIG. 5, where the hydrophilic-to-hydrophobic transition of the HPC 102 results in light-scattering microstructures.

[0048] In an embodiment where the panel 300 is applied to a vehicle ceiling or window, the thermochromic hydrogel 100 automatically transitions to an opaque white state in response to increased temperature. This transition enables the panel 300 to block incident sunlight, thereby reducing the rate of solar heat buildup within the vehicle cabin. When the temperature decreases below a phase change threshold, the thermochromic hydrogel 100 seamlessly reverts to a transparent state. This autonomous functionality provides a passive, energy-independent cooling solution suitable for automotive and building applications without requiring external power or electrical consumption.

[0049] As shown in FIG. 5, HPC demonstrates LCST behavior — a characteristic of certain polymers that remain soluble below a critical temperature but become markedly less soluble above this threshold. In the HPC, this hydrophilic-to-hydrophobic transition not only triggers a thermochromic color change but also causes a rapid volume contraction, expelling water from the polymer matrix.However, some HPC hydrogels exhibit an LCST of 42 °C, which may be suboptimal for vehicle cabin thermal management, where passenger comfort typically involves0338-800-WQ / 2024-044-02ambient temperatures below 30 °C. Therefore, a reduction in the LCST may be used for certain vehicular applications.

[0050] The incorporation of the PAAm network introduces amide bonds that interact with hydroxyl groups in the HPC and surrounding water molecules. This interaction disrupts a hydrogen bonding network within the hydrogel, effectively lowering a transition temperature. To confirm this tunability, a mass fraction of the HPC was fixed at 5 wt.% and an AAM content was varied to observe an effect on gel properties. Introducing 3 wt.% AAM enhanced mechanical stability of the hydrogel and lowered the transition temperature to approximately 40.1 °C, as indicated by the arrow in FIG. 6. Consequently, the mass fractions of the HPC and the PAAm may be maintained at 5 wt.% and 3 wt.%, respectively.

[0051] In some embodiments, an ionic additive, such as CaCI2, may be incorporated into the hydrogel 100 to further reduce the transition temperature. The hydration capability of Ca2+ions and chloride (Cl’) ions disrupts hydrogen bonding among the polymer chains, which leads to a decrease in the phase transition temperature relative to that of a pure hydrogel (e.g., approximately 42 °C).

[0052] As illustrated in FIG. 7, DSC analysis indicates that adjusting a concentration of the CaCh may lower the transition temperature to approximately 15 °C. This provides a tunable range from approximately 42 °C to approximately 15 °C, allowing the thermochromic films 100 to be adapted for various environments. For instance, in vehicular thermal management applications, the concentration of the CaCI2 may be optimized to 10 wt.%, resulting in a transition temperature of approximately 28.4 °C. This reduction in the transition temperature enhances the0338-800-WQ / 2024-044-02suitability of the hydrogel 100 for passive cooling systems, particularly in environments where thermal regulation is preferred at activation temperatures below 30 °C. The ability to modulate the transition point facilitates compatibility with applications such as energy-efficient smart windows and automotive thermal management, where excessive heat accumulation may impact energy consumption and comfort.

[0053] To analyze the effects of AAM 103 and CaCl2 incorporation, chemical composition of the dual-network gel 100 may be evaluated using FT-IR spectroscopy, as illustrated in FIG. 8. Successful polymerization of the AAM 103 may be confirmed by a disappearance of a carbon-carbon double bond peak at 1625 cm-1following the addition of the initiator 112. As a concentration of the AAM 103 increases, for example from 3 wt.% to 5 wt.%, a hydroxyl peak may redshift from 3429 cm-1to 3376 cm-1, and a carbonyl peak at 1664 cm-1may redshift to 1650 cm-1. Such shifts may indicate formation of hydrogen bonding interactions between the PAAm 104 and the HPC 102.

[0054] Furthermore, introduction of approximately 10 wt.% CaCl2 may result in a redshift of the hydroxyl peak from 3429 cm-1to 3302 cm-1and the carbonyl peak from 1664 cm-1to 1659 cm-1, which may suggest a presence of both hydrogen bonding and ionic interactions between ions and the HPC 102.

[0055] In some embodiments, scanning electron microscopy (SEM) characterization may be utilized to evaluate a mechanism of discoloration of the dual-network gel 100. At room temperature, the dual-network gel 100 may exhibit a homogeneous structure where polymer chains are dispersed in water, which0338-800-WQ / 2024-044-02minimizes refractive index differences and allows light to pass through with minimal scattering to maintain a transparent state. Upon heating, the polymer chains may collapse and aggregate (see FIG. 5) due to decreased solubility, leading to phase separation and formation of microstructures within the dual-network gel 100. These microstructures may increase light scattering, causing the dual-network gel 100 to transition to an opaque state and enhancing an ability to reflect solar radiation for passive cooling.

[0056] In some embodiments, optical and thermal modulation characteristics of the dual-network gel 100 are evaluated by encapsulating the dual-network gel 100 between acrylic sheets 302 and 304 to form the smart window panel 300, as illustrated in FIG. 3. As shown in FIG. 9, the transparency of a double-layer acrylic plate with an air gap is approximately 0.85, which increases to 0.911 and 0.925 when the gap is filled with the dual-network gel 100 at thicknesses of 1 mm and 2 mm, respectively. This increase in transmission relative to an air-gap configuration is attributed to enhanced optical index matching. Notably, as the temperature rises above the room temperature, a visible light transmittance (Tlum) of the smart window panel 300 decreases. For a 1 mm thick layer, the Tlum may decrease from 92.7% to 48.8% at 35 °C and further to 45.6% at 45 °C. A 2 mm thick layer may exhibit a more pronounced decrease from 91.3% to 28.4% at 35 °C and to 26.2% at 45 °C. This transition to an opaque state, illustrated in FIG. 10A, is attributed to thermally induced light scattering resulting from polymer chain aggregation.

[0057] In some embodiments, solar transmittance (Tsolar) declines correspondingly. For a 1 mm thick layer, Tsolar may decrease from 79.5% to 45.7%0338-800-WQ / 2024-044-02at 35 °C and to 39.7% at 45 °C. For a 2 mm thick layer, Tsolar may decrease from 77.3% to 26.3% at 35 °C and to 24.3% at 45 °C. Based on the AM 1.5 solar spectrum, incident solar energy transmission through a 1 mm thick smart window panel 300 may be reduced from 795 W / m2in a transparent state to 457 W / m2at 35 °C and 397 W / m2at 45 °C. For a 2 mm thick window, the solar energy transmission may be reduced from 773 W / m2in a transparent state to 263 W / m2at 35 °C and 243 W / m2at 45 °C. This transition from transparency to opacity facilitates passive cooling by minimizing solar heat gain.

[0058] The response time of the dual-network gel 100 may be evaluated under laboratory conditions with an ambient temperature of 25 °C. In one example, the dual-network gel 100 transitions from a transparent state at 25 °C to a fully opaque state at 45 °C within approximately 2 minutes and 50 seconds. During a cooling process, the dual-network gel 100 may revert to the transparent state in approximately 1 minute and 40 seconds as the temperature decreases back to 25 °C.

[0059] To evaluate durability and reliability, the dual-network gel 100 may be subjected to thermal cycling. As illustrated in FIG. 10B, after 400 thermal cycles between 60 °C and room temperature, the Tlum of a 1 mm thick hydrogel may slightly increase from 39.5% to 42.7%, and a 2 mm thick hydrogel may increase from 26.0% to 28.0%, without significant degradation. Minimal transmittance changes are observed in the transparent state after multiple cycles, with a 1 mm thick layer changing from 91 .1% to 90.3% and a 2 mm thick layer changing from 92.5% to0338-800-WQ / 2024-044-0291.8%. These results confirm long-term stability for applications involving repeated thermal fluctuations.

[0060] Furthermore, the dual-network gel 100 exhibits improved freezing resistance as the concentration of CaCI2 increases, as illustrated by the DSC analysis shown in FIG. 10C. The DSC results characterize the thermal behavior and phase transitions of the dual-network gel 100 at low temperatures, specifically identifying the freezing point through heat flux measurements. In some embodiments, the incorporation of CaCI2 facilitates freezing point depression by disrupting the hydrogen bonding network of water molecules within the polymer structure.

[0061] For example, as the concentration of CaCI2 increases from 5 wt.% to 15 wt.%, a freezing point of the dual-network gel 100 may drop from -1.0 °C to -10.2 °C. This freezing point depression is attributed to ions from the CaCI2 disrupting a hydrogen bonding network of water. In some embodiments, the smart window panel 300 remains transparent after 12 hours at 0 °C, enabling functionality in cold climates.

[0062] In some embodiments, a cooling performance of a thermochromic smart window 300 is evaluated using an experimental configuration including two foam boxes. For example, as illustrated in the schematic of FIG. 11 , each foam box 1100 may have dimensions of approximately 15 cm x 15 cm x 15 cm. A black aluminum foil 1102 may be disposed at a bottom portion of each chamber 1104 to enhance absorption of solar radiation, thereby simulating solar heating conditions within a vehicle cabin. Top openings 1106 of the foam boxes may be sealed with0338-800-WQ / 2024-044-02acrylic windows 1106, where a first box includes an air gap as a reference and a second box incorporates a thermochromic hydrogel layer 100.

[0063] In an indoor test conducted at a controlled temperature of approximately 22 °C under one sun illumination from a solar simulator, temperatures within both boxes may increase over time. As shown in FIG. 12, when a 1 mm thick hydrogel layer reaches its LOST after approximately 10 minutes, transmitted energy 1202 through the thermochromic smart window decreases significantly. In one example, the solar irradiance is reduced by approximately 524 W / m2(e.g., from 809.9 W / m2in the reference box, characterized by curve 1204, to 285.9 W / m2in the smart window box), representing a reduction of approximately 64.7% relative to the control box.

[0064] To evaluate passive cooling under real-world conditions, the boxes may be placed in an outdoor environment exposed to natural sunlight.Thermocouples may be utilized to measure temperatures at different positions within the boxes, while environmental parameters such as solar radiation and ambient temperature are recorded. For instance, the experimental setup may include thermocouples positioned at a ceiling and within internal air of the boxes to monitor thermal variations.

[0065] As illustrated in FIG. 13, temperature profiles obtained during a 12-day test period demonstrate that the thermochromic smart window 300 significantly reduces peak temperatures at the ceiling compared to traditional glass, illustrating effective mitigation of solar heat gain. Internal air temperature measurements further indicate that the thermochromic smart window maintains a lower internal0338-800-WQ / 2024-044-02temperature, which may reduce a need for active cooling systems. In some embodiments, the thermochromic smart window achieves a temperature reduction of approximately 14 °C relative to a reference sample without the hydrogel. These results indicate that thermochromic smart windows may mitigate solar heating in thermally insulated spaces, such as vehicle cabins, under elevated temperatures while maintaining stability over extended periods of operation.

[0066] To demonstrate the practical cooling effect of the smart window 300 in vehicles, two identical commercial cars are utilized, each equipped with multiple windows, such as a windshield, a rear window, side windows, and a sunroof. In one embodiment, custom-fit, 1 -mm-thick hydrogel films are fabricated to serve as smart windows and are applied to the interior surfaces of each window. While thermochromic films may not be applied to a windshield in certain commercial embodiments due to driving visibility requirements, they are applied here to simulate the effect of manually adjustable sunshade films and to assess passive cooling performance.

[0067] In some embodiments, the hydrogel films 100 are encapsulated between acrylic plates 302 and 304 to ensure long-term stability, provide structural support, prevent dehydration, and protect against mechanical damage. The optical transparency of the acrylic ensures that light modulation and thermochromic behavior remain effective, contributing to consistent passive cooling performance. Additionally, encapsulation acts as a protective barrier against environmental degradation, facilitating the integration of the smart windows into commercial vehicle0338-800-WQ / 2024-044-02designs. To monitor cabin temperatures, thermocouples may be strategically placed at various locations within each vehicle.

[0068] In this embodiment, the cabin air temperature in the vehicle equipped with smart windows was reduced by 8 °C relative to the control vehicle, as illustrated in FIG. 14A. More pronounced cooling effects were observed on the window surfaces: the front window exhibited a peak temperature reduction of 12 °C (FIG. 14B), the rear window showed a 10 °C reduction (FIG. 14C), and the side windows demonstrated a 5 °C reduction (FIG. 14D). This localized cooling performance is attributed to the thermal blocking capabilities of the thermochromic hydrogel layer 100, which significantly reduces the thermal load by reflecting solar radiation upon reaching the transition temperature. These results underscore the utility of the smart window technology in mitigating heat buildup across all vehicle orientations, particularly for large-surface-area glass such as front and rear windows.

[0069] The cooling performance may be further increased with 2-mm-thick hydrogel layers to improve light-blocking properties. Given the low manufacturing costs of raw materials, these thermochromic smart windows present a cost-effective and environmentally benign solution for improving vehicle thermal management. This approach is particularly suited for electric vehicles, where passive cooling can reduce reliance on air conditioning, thereby conserving battery life and enhancing overall efficiency.

[0070] The development of the thermochromic hydrogel layer 100 represents an advancement in passive thermal management, achieved through the synergistic integration of a dual-network polymer structure and the protective architecture of the0338-800-WQ / 2024-044-02sandwich-structured panel 300. By utilizing a first polymer network of HPC for thermal responsiveness and a second polymer network of PAAm for structural reinforcement, the hydrogel layer 100 achieves high tensile strength and superior water retention. This dual-network configuration, optimized with CaCI2 to tune the transition temperature to approximately 28.4 °C, addresses historical limitations of fragility and volume shrinkage in thermochromic materials. When encapsulated within the acrylic substrates of the panel 300, the system provides a robust, maintenance-free barrier that effectively blocks up to 64.7% of incident solar radiation, translating to an internal vehicle temperature reduction of 8 °C to 12 °C under peak solar loads.

[0071] Compared to active technologies such as electrochromic films, the thermochromic hydrogel layer 100 offers a superior balance of solar modulation efficiency (53%) and operational simplicity. While maintaining a practical response time of approximately 3 minutes, the material achieves long-term thermal stability over 400 cycles without significant degradation. Furthermore, unlike many existing materials that depend on hazardous organic solvents or costly encapsulation layers, the hydrogel 100 is environmentally friendly, simple to manufacture, and highly scalable for large-area applications. The passive nature of the panel 300 eliminates the need for complex circuitry or external power sources, making it a viable and sustainable alternative for reducing energy consumption in electric vehicles and green building facades. These findings position the smart window as a promising solution for enhancing thermal comfort and advancing global sustainability goals in both the transportation and architectural sectors.0338-800-WQ / 2024-044-02

[0072] In another embodiment, a variant of the thermochromic system discussed above includes a composite hydrogel designated as HACsWo.12, which incorporates NIR absorbing nanoparticles to enhance thermal management. As illustrated in FIG. 15, the HACsWo.12 hydrogel 1500 includes a dual-network structure where a first polymer network of HPC 102 and a second polymer network of PAAm 104 are integrated with an ionic additive, such as CaCl2, and NIR-absorbing particles 109, such as CS0.33WO3 nanoparticles. The CS0.33WO3 nanoparticles 109 may have a particle size between 200 nm and 400 nm.

[0073] The PAAm network 104 is cross-linked with a cross-linker, such as MBAA. This dual-network architecture facilitates the integration of the thermoresponsive properties of the HPC, the mechanical reinforcement of the PAAm, and the strong NIR absorption of the CS0.33WO3 nanoparticles. The resulting HACsWo.12 hydrogel 1500 is configured to undergo a reversible phase transition from a transparent state to an opaque state when a temperature exceeds a transition temperature, while simultaneously providing enhanced absorption of NIR radiation to accelerate the thermochromic response and further reduce solar heat gain.

[0074] An embodiment of a method for making the HACsWo.12 hydrogel 1500 is illustrated in FIG. 16. At step 1602, the method includes dissolving HPC 102 in a solvent, such as DI water, to form a first solution. In some embodiments, the solvent is maintained at a temperature between 50 °C and 80 °C during dissolution.Following the dissolution of the HPC 102, at step 1604, the method includes adding AAM 103, a cross-linker, such as BIS, an ionic additive including CaCI2, and CS0.33WO3 nanoparticles 109 to the first solution while stirring. In some0338-800-WQ / 2024-044-02embodiments, the HPC 102 is provided at a concentration between 5 wt.% and 10 wt.%, the AAM 103 is provided at a concentration between 3 wt.% and 8 wt.%, the CaCl2 is provided at a concentration between 2 wt.% and 15 wt.%, and the CS0.33WO3 nanoparticles 109 are provided at a concentration between 0.02 wt.% and 0.5 wt.% based on a total mass of the solution.

[0075] At step 1606, an initiator, such as APS 112, is added to the mixture. Subsequently, at step 1608, a catalyst, such as TEMED 114, is added to trigger in-situ polymerization. In some embodiments, the solution is transferred into a gap between substrates, such as acrylic plates 302 and 304, using a syringe pump at a flow rate of approximately 2 mL / min prior to completion of the polymerization. The HACsW hydrogel 1500 is formed at step 1610, resulting in a stable, thermochromic composite material encapsulated within the panel 1600, which is suitable for integration into smart windows or greenhouse panels.

[0076] The terms “about” and “substantially” when used in this application mean a variation of up to 20% of the parameter characterized by these terms.

[0077] It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first object or step could be termed a second object or step, and, similarly, a second object or step could be termed a first object or step, without departing from the scope of the present disclosure. The first object or step, and the second object or step, are both, objects or steps, respectively, but they are not to be considered the same object or step.0338-800-WQ / 2024-044-02

[0078] The terminology used in the description herein is for the purpose of describing particular embodiments and is not intended to be limiting. As used in this description and the appended claims, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and / or" as used herein refers to and encompasses any possible combinations of one or more of the associated listed items. It will be further understood that the terms "includes," "including," "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Further, as used herein, the term "if" may be construed to mean "when" or "upon" or "in response to determining" or "in response to detecting," depending on the context.

[0079] The disclosed embodiments provide a thermochromic hydrogel having a dual-network structure and a thermochromic device configured to provide passive thermal regulation through reversible optical transitions. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.0338-800-WQ / 2024-044-02

[0080] Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

[0081] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

[0082] The entire content of all the publications listed herein is incorporated by reference in this patent application.[1] H. Liu, M. Jiang, S. Geng, X. Liu, Chem. Eng. J. 2024, 490, 151545.[2] J. Tao, S. Tian, B. Li, T. Ma, L. Zhou, X. Zhao, Chem. Eng. J. 2024, 482, 149079.[3] T. Ube, M. Yoshida, S. Kurihara, T. Ikeda, ACS AppL Mater. Interfaces 2024, 16, 28638.[4] G. Xu, Y. Lu, X. Zhou, N. Moloto, J. Liu, S.-Z. Kure-Chu, T. Hihara, W. Zhang, Z. Sun, Mater. Horiz. 2024, 11 , 4867.[5] Q. Jiang, M. Chen, Z. Qin, Y. Li, J. Li, H. Zhang, Chem. Eng. J. 2024, 489, 151259.[6] R. Guo, Y. Shen, Y. Chen, C. Cheng, C. Ye, S. Tang, Chem. Eng. J. 2024, 486, 150194.0338-800-WQ / 2024-044-02[7] P. Pan, Y. Liu, Z. Zhu, Y. Liu, R. Wei, Q. Wang, Q. Miao, Y. Lei, C. Guo, H. Zhang, L. Huang, L. Chen, J. Li, Int. J. Biol. Macromol. 2024, 268, 131945.[8] C. Cai, F. Chen, Z. Wei, C. Ding, Y. Chen, Y. Wang, Y. Fu, Chem. Eng. J. 2023, 476, 146668.[9] C. Cai, Y. Chen, C. Ding, Z. Wei, X. Wang, Mater. Horiz. 2024, 11, 1502.

Claims

0338-800-WQ / 2024-044-02WHAT IS CLAIMED IS:1 . A thermochromic device (300), comprising:a first substrate (302);a second substrate (304); anda thermochromic hydrogel (100) disposed between the first substrate (302) and the second substrate (304), the thermochromic hydrogel (100) comprising a dual-network structure including:a first polymer network comprising hydroxypropyl cellulose (HPC) (102); a second polymer network comprising polyacrylamide (PAAm) (104) formed in-situ within the first polymer network; andan ionic additive comprising calcium chloride (CaCl2) configured to modulate a transition temperature of the thermochromic hydrogel (100).

2. The thermochromic device of claim 1 , wherein the transition temperature is between 15 °C and 42 °C.

3. The thermochromic device of claim 1 , wherein a concentration of the CaCI2 is about 10 wt.% and the transition temperature is about 28.4 °C.

4. The thermochromic device of claim 1 , wherein the thermochromic hydrogel is configured to transition from a transparent state to an opaque state when a temperature of the thermochromic hydrogel exceeds the transition temperature.0338-800-WQ / 2024-044-025. The thermochromic device of claim 4, wherein the transparent state is characterized by a luminous transmittance of at least 90%.

6. The thermochromic device of claim 4, wherein the opaque state is configured to reflect solar radiation to provide passive cooling.

7. The thermochromic device of claim 1 , wherein the second polymer network is cross-linked with N,N'-methylenebisacrylamide (BIS).

8. The thermochromic device of claim 1 , wherein a thickness of the thermochromic hydrogel is between 1 mm and 2 mm.

9. The thermochromic device of claim 8, wherein the thermochromic hydrogel has a thickness of 1 mm and is configured to block at least 60% of incident solar radiation when in an opaque state.

10. The thermochromic device of claim 1 , wherein the first substrate and the second substrate comprise acrylic sheets configured to encapsulate the thermochromic hydrogel to prevent water evaporation.

11. The thermochromic device of claim 1 , wherein the thermochromic hydrogel is characterized by a tensile strength of about 28 kPa.0338-800-WQ / 2024-044-0212. The thermochromic device of claim 1 , wherein the thermochromic device is a smart window configured for integration into a vehicle or a building facade.

13. The thermochromic device of claim 1 , further comprising: near-infrared (NIR) absorbing nanoparticles dispersed within the dual-network structure, the NIR absorbing nanoparticles configured to accelerate a transition from a transparent state to an opaque state.

14. The thermochromic device of claim 13, wherein the NIR absorbing nanoparticles comprise cesium tungsten oxide (CS0.33WO3) nanoparticles having a particle size between 200 nm and 400 nm.

15. The thermochromic device of claim 1 , wherein the thermochromic hydrogel comprises 5 wt.% to 10 wt.% of the HPC, 3 wt.% to 8 wt.% of the PAAM, 2 wt.% to 15 wt.% of the CaCl2, and a remainder of water.

16. A method for forming a thermochromic hydrogel, the method comprising: dissolving hydroxypropyl cellulose (HPC) and an ionic additive comprising calcium chloride (CaCl2) in a solvent to form a first solution;adding an acrylamide monomer (AAM) and a cross-linker to the first solution to form a precursor mixture;adding an initiator and a catalyst to the precursor mixture; and0338-800-WQ / 2024-044-02polymerizing the AAM in-situ within the first solution to form a second polymer network comprising polyacrylamide (PAAm) interspersed with the HPC to create a dual-network hydrogel structure.

17. The method of claim 16, wherein the cross-linker comprises N,N'-methylenebisacrylamide (BIS), the initiator comprises ammonium persulfate (APS), and the catalyst comprises tetramethylethylenediamine (TEMED).

18. The method of claim 16, further comprising:transferring the precursor mixture into a gap between a first substrate (302) and a second substrate (304) prior to completing the polymerizing to form a smart window.

19. The method of claim 16, wherein a concentration of the CaCl2 is selected to be between 5 wt.% and 15 wt.% to set a transition temperature of the thermochromic hydrogel between 15 °C and 30 °C.

20. The method of claim 16, wherein the precursor mixture comprises:5 wt.% to 10 wt.% of the HPC;3 wt.% to 8 wt.% of the AAM;2 wt.% to 15 wt.% of the CaCl2; and0.02 wt.% to 0.5 wt.% of near-infrared (NIR) absorbing nanoparticles (109) comprising cesium tungsten oxide (CS0.33WO3).

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