Thermochromic gels, methods of making and using the same

Abstract

This invention relates to the field of thermochromic gels, and more particularly to a thermochromic gel, its preparation method, and its application. The method employs a one-pot synthesis, with the following specific steps: Step 1) Preparation of the precursor solution: Dissolve the temperature-sensitive monomer and crosslinking agent in a solvent and stir to form a homogeneous precursor solution; Step 2) Introduction of a pore size regulator: Add a pore size regulator to the precursor solution obtained in Step 1) and stir to ensure uniform dispersion; Step 3) Initiation of the polymerization reaction: Add an initiator and a accelerator sequentially to the solution obtained in Step 2), mix thoroughly, and then inject into a polymerization mold. Under controlled temperature conditions, a free radical polymerization reaction is carried out to obtain the final product. This invention significantly optimizes the uniformity and size distribution of scattering pores generated during the high-temperature phase transition of PNIPAm hydrogel by introducing a pore size regulator and precisely controlling its molecular weight and dosage, while simultaneously achieving a synergistic effect with the pore size regulator under low-temperature polymerization conditions. This allows the pore size to be precisely controlled within the range of 200-800 nm, a scale that forms a good scattering match with the solar spectrum. The above-mentioned regulation maximizes light scattering efficiency, thereby achieving a significant and reversible switching of solar reflectivity between low and high temperature states, providing key support for obtaining excellent adaptive thermal management performance.

Description

Technical Field

[0001] This invention belongs to the field of thermochromic gels, and particularly relates to a thermochromic gel, its preparation method and application. Background Technology

[0002] With the global energy crisis and climate change becoming increasingly severe, building energy conservation has become a critical issue. Building heating and cooling energy consumption accounts for a significant proportion of global total energy consumption. Passive thermal management technologies, especially radiant cooling materials, have attracted widespread attention because they can cool without external energy input. However, traditional static coatings present a seasonal contradiction: high solar reflectivity and high infrared emissivity are needed for heat dissipation in summer, while high solar absorptivity and low infrared emissivity are needed for insulation in winter.

[0003] To address this contradiction, existing dynamic designs, such as mechanically driven louvers, microelectromechanical systems (MEMS) metasurfaces, electrochromic or fluidic devices, can provide switching functionality. However, their reliance on external power sources, complex control systems, or narrow spectral tunability limits their scalability and reliability. Furthermore, passive radiative coolers degrade in humid climates because atmospheric water vapor absorption and convective coupling inhibit radiative heat dissipation. This inherent seasonal trade-off remains a key obstacle to achieving fully passive, year-round thermal regulation.

[0004] A simple, non-external energy-required, and temperature-responsive dual-layer spectrally selective coating is provided. By adjusting the solar energy absorption / reflection function and infrared radiation characteristics to different functional layers, a spontaneous and reversible switching between summer radiative cooling and winter solar heating modes is achieved, thus solving the core contradiction that existing technologies cannot simultaneously meet the requirements of efficient passive thermal management throughout the year. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a thermochromic gel, its preparation method, and its applications. By introducing PEG as a pore size regulator, the uniformity and size distribution of scattering pores generated during the high-temperature phase transition of PNIPAm hydrogel are significantly optimized. This regulation maximizes light scattering efficiency, thereby achieving a dramatic and reversible switching of solar reflectivity from a low-temperature state (e.g., <0.4) to a high-temperature state (e.g., >0.7), which is key to obtaining excellent adaptive thermal management performance.

[0006] This invention is achieved through the following technical solution: On one hand, it provides a method for preparing thermochromic gels, which employs a one-pot synthesis method, with the specific steps as follows:

[0007] Step 1) Preparation of precursor solution: The temperature-sensitive monomer and crosslinking agent are dissolved in a solvent and stirred to form a homogeneous precursor solution; Step 2) Introduce pore size control agent: Add a pore size regulator to the precursor solution obtained in step 1) and stir to disperse it evenly; Step 3) Initiate the polymerization reaction: Initiator and accelerator are added sequentially to the solution obtained in step 2), mixed evenly, and then injected into the polymerization mold for free radical thermal polymerization under controlled temperature conditions. Alternatively, add the photoinitiator sequentially to the solution obtained in step 2), mix thoroughly, and then inject into the polymerization mold. Under UV light irradiation, carry out the photoinitiated polymerization reaction to obtain the final product.

[0008] Furthermore, the amount of the pore size regulator added accounts for 1%-10% of the total mass of the temperature-sensitive monomer.

[0009] Furthermore, the pore size regulator is selected from one or more of polyethylene glycol, polyethylene glycol diacrylate, and polyethylene glycol methacrylate; And / or, the molecular weight of the polyethylene glycol is 400-10000. Polyethylene glycol serves as a physical pore-forming template, while polyethylene glycol diacrylate and polyethylene glycol methacrylate serve as crosslinkable pore-forming templates, partially participating in the crosslinking reaction during polymerization to form chemically bonded pore wall structures, further improving the stability of the pore structure.

[0010] Further, the temperature-sensitive monomer is selected from one or more of N-isopropylacrylamide, N,N-diethylacrylamide, methyl vinyl ether, N-n-propylacrylamide, and N-vinylcaprolactam; and / or, the amount of the temperature-sensitive monomer added is 0.5-2.5 g; And / or, the crosslinking agent is N,N'-methylenebisacrylamide, N,N'-(1,2-dihydroxyethylene)bisacrylamide, pentaerythritol tetraacrylate, N,N'-ethylenebisacrylamide, N,N'-bisacryloylcysteine; and / or, the amount of the crosslinking agent added is 0.5-2.5 g; And / or, the initiator is ammonium persulfate, azobisisobutyrazoline hydrochloride; the photoinitiator is 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone, Irgacure 651 (2,2-dimethoxy-2-phenylacetophenone), 2,2-diethoxyacetophenone; and / or, the amount of the initiator added is 0.02-0.06 g; And / or, the accelerator is N,N,N',N'-tetramethylethylenediamine; and / or, the amount of the accelerator added is 5 μL-30 μL.

[0011] Another thermochromic gel prepared by the above-described method is provided, wherein the thickness of the thermochromic gel is 0.5-10 mm, preferably 1-5 mm.

[0012] Through the above technical solution, this invention introduces polyethylene glycol (PEG) as a pore size regulator and precisely controls its molecular weight (preferably 800-2000) and addition amount (preferably 5%). Simultaneously, it achieves a synergistic effect with PEG under low-temperature polymerization conditions (e.g., 4°C), thereby significantly optimizing the uniformity and size distribution of scattering pores generated during the high-temperature phase transition of the PNIPAm hydrogel. This allows the pore size to be precisely controlled within the range of 200-800 nm, a scale that forms a good scattering match with the solar spectrum (especially the visible and near-infrared bands). This control maximizes light scattering efficiency, thereby achieving a significant and reversible switching of solar reflectivity between low-temperature (≤0.4) and high-temperature (≥0.7) states, providing crucial support for obtaining excellent adaptive thermal management performance.

[0013] Finally, a spectral decoupling adaptive composite coating is provided, comprising: Flexible polymer substrate layer; A conductive network layer covering the flexible polymer substrate; A polymer protective layer covering the conductive network layer; And a thermochromic gel layer, which is laminated and adhered to the polymer protective layer.

[0014] Furthermore, the conductive network layer comprises multi-walled carbon nanotubes and silver nanowires; The silver nanowires overlap each other to form a conductive framework. The multi-walled carbon nanotubes are dispersed between the silver nanowires and bridged between adjacent silver nanowires, forming a co-diffusion conductive network composed of multi-walled carbon nanotubes and silver nanowires.

[0015] Furthermore, the diameter of the multi-walled carbon nanotubes is smaller than the diameter of the silver nanowires.

[0016] Furthermore, the multi-walled carbon nanotubes are wound or anchored to the surface of the silver nanowires to form a CNT-AgNW heterojunction structure with low contact resistance at the overlapping junctions of the silver nanowires.

[0017] Furthermore, the conductive network layer is formed on the flexible polymer substrate by spraying.

[0018] Beneficial effects The coating of this invention requires no external energy input or complex control system. Through the spontaneous response of the thermochromic layer to ambient temperature, it achieves intelligent and reversible switching of optical properties. In high-temperature environments, it automatically transitions to a scattering state to maximize solar radiation reflection, thereby achieving radiative cooling. In low-temperature environments, it reverts to a transparent state to allow direct solar radiation absorption, thus achieving passive heating. This mechanism fundamentally solves the seasonal functional conflict inherent in traditional static coatings for year-round thermal management.

[0019] This invention introduces polyethylene glycol as a pore size regulator to precisely control the phase separation behavior of poly(N-isopropylacrylamide) hydrogel at high temperatures, significantly improving the uniformity and size distribution of its microporous structure. This microstructure optimization strategy effectively enhances the light scattering efficiency of the gel in the visible-near-infrared band, enabling a significant reversible transition in solar reflectivity from below 0.5 to above 0.7 between low and high temperature states, thus providing key performance support for efficient adaptive thermal management.

[0020] The coating of this invention employs a spectral synergy design of "dynamically regulating solar radiation in the upper layer and statically managing infrared radiation in the lower layer." The upper layer dynamically adjusts the reflection and transmission behavior of the solar spectrum through a thermochromic effect; the lower layer, with its low infrared emissivity, suppresses long-wave radiation loss of indoor heat. This synergistic effect achieves optimized control of the solar spectrum and thermal radiation across different wavelengths, resulting in significantly higher thermal management efficiency than single-function coatings. In practical applications, this coating can be used on building envelopes such as exterior walls and roofs: in summer mode, it effectively reduces cooling load through high solar reflection; in winter mode, it allows for solar thermal gain while significantly suppressing indoor heat radiation loss, thus achieving all-season energy savings.

[0021] Furthermore, the infrared transparent protective layer introduced into the coating structure effectively enhances its adaptability to outdoor environments, improving weather resistance and mechanical stability. Each functional layer of the coating utilizes mature material synthesis and coating processes, possessing excellent scalability and promising engineering application prospects. In addition to building envelopes, this design can be extended to tents, outdoor equipment, and personal thermal management devices, providing a feasible technical path for achieving efficient, low-cost, and zero-energy passive temperature control. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the adaptive double-layer coating structure for spectral decoupling prepared in Example 1 of the present invention; Figure 2 Scanning electron microscope images of PNIPAm / PEG gels with different contents (0%, 2%, 5%, 8%) of polyethylene glycol prepared in Example 2 of the present invention for spectrally decoupled adaptive bilayer coatings. Figure 3The image shows optical photographs of the spectrally decoupled adaptive double-layer coating prepared in Example 1 of this invention in two states: low temperature and high temperature. It can be seen from the image that the low temperature state (25°C) is a high absorption state and the high temperature state (50°C) is a high reflection state.

[0023] Figure 4 The solar reflectance of the spectrally decoupled adaptive double-layer coating prepared in Example 1 of the present invention has a wavelength range of 300~2500 nm. It can be seen from the figure that the film has tunable reflectance in the visible and near-infrared bands. Figure 5 The mid-infrared emissivity of the PNIPAm / PEG coating prepared in Example 1 of this invention has a wavelength range of 8~14 μm. The figure shows that the film has a high emissivity in the mid-infrared. Figure 6 The figure shows the mid-infrared emissivity of the CNT / AgNW coating prepared in Example 1 of this invention, with a wavelength range of 8~14 μm. It can be seen from the figure that the film has a low emissivity in the mid-infrared. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0025] Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or as recommended by the manufacturer. Unless otherwise stated, all percentages, ratios, proportions, or parts are by weight.

[0026] Unless otherwise specified, the reagents and raw materials used in the embodiments and comparative examples of this invention are commercially available.

[0027] Example 1 A specific method for preparing an adaptive bilayer composite coating with spectral decoupling function is shown in the schematic diagram below. Figure 1 As shown.

[0028] 1. Preparation of thermochromic hydrogel layer (top layer) By introducing polyethylene glycol (PEG) as a pore size modifier and combining it with a low-temperature free radical polymerization process, a poly(N-isopropylacrylamide) (PNIPAm)-based hydrogel layer with a uniform microstructure and high optical switching contrast was prepared. The specific steps are as follows: (1) Preparation of premix: Accurately weigh 2.0 g of N-isopropylacrylamide (NIPAm) monomer and 3 mg of N,N'-methylenebisacrylamide (BIS) crosslinking agent, place them in a reaction vessel, and add 10 mL of deionized water. Place the vessel on a magnetic stirrer and stir at 900 rpm for 30 minutes to form a uniform and transparent premix.

[0029] (2) Introduction of pore size regulator: Add polyethylene glycol (PEG, number average molecular weight Mw≈1000) at 5% of the mass of NIPAm monomer to the above clear solution, i.e., 0.075 g. Continue stirring at 900 rpm for 30 minutes to ensure that the PEG is completely dissolved and uniformly dispersed in the system.

[0030] (3) Initiation-catalysis system start-up: Add 0.03 g of ammonium persulfate (APS) as a thermal initiator to the homogeneous solution obtained in the previous step, and stir at room temperature for 30 minutes to mix it evenly. Then, quickly add 20 μL of N,N,N',N'-tetramethylethylenediamine (TEMED) as a promoter, and stir rapidly for 1-2 minutes to start the polymerization reaction.

[0031] (4) Casting and Low-Temperature Polymerization: The uniformly mixed reaction solution was rapidly injected into a custom mold consisting of two glass plates and a silicone gasket, with the mold cavity thickness controlled at 3 mm. The entire mold was then transferred to a refrigerator at 4°C and allowed to stand for 10 hours. The low temperature slowed down the polymerization rate, allowing sufficient time for the polyethylene glycol pore size regulator to distribute evenly during gel network formation, thus preventing local aggregation of the pore size regulator due to excessively fast polymerization. Simultaneously, the N-isopropylacrylamide monomer exhibits optimal compatibility with polyethylene glycol at 4°C, effectively inhibiting microphase separation of polyethylene glycol during polymerization. This ensures the polymerization reaction is fully completed and a uniformly structured gel network is formed.

[0032] (5) Molding and post-processing: After the reaction is completed, the mold is removed and demolded to obtain a transparent PNIPAm / PEG composite hydrogel sheet with a certain degree of elasticity. It is then soaked in deionized water and stored at 4°C for later use to remove unreacted monomers and impurities.

[0033] 2. Preparation of the static spectral selective layer (lower layer) A composite thin film with high infrared reflectivity and strong adhesion was constructed as a static functional layer to control the loss of long-wave radiation.

[0034] (1) Preparation of conductive dispersion: Multi-walled carbon nanotubes (CNTs) and silver nanowires (AgNWs, about 60 nm in diameter and about 20 μm in length) were dispersed in anhydrous ethanol at a mass ratio of 1:2 and ultrasonically treated for 30 minutes to form a uniform and stable spraying solution.

[0035] (2) Spraying film formation: Using a spray gun with a nozzle diameter of 0.5 mm, the above dispersion is uniformly sprayed onto a pre-cleaned polyethylene (PE) flexible substrate under a nitrogen pressure of 0.2 MPa. During the spraying process, the moving speed of the spray gun is kept constant, and the spraying is repeated until a uniform and dense gray-black conductive film is formed on the substrate surface.

[0036] (3) Drying treatment: Place the sprayed sample in an oven at 60°C for 10 minutes to completely remove residual solvent and obtain CNT / AgNW composite conductive film.

[0037] (4) Application of protective layer: Prepare a 2% (w / w) solution of N,N-dimethylformamide (DMF) containing polyacrylonitrile (PAN). Apply this solution evenly to the surface of the CNT / AgNW conductive film using a spray gun to form a thin, continuous, infrared-transparent protective layer. Then dry at 60°C for 20 minutes to obtain a complete static spectral selectivity layer. This protective layer enhances the oxidation and scratch resistance of the underlying layer in outdoor environments.

[0038] The PNIPAm / PEG hydrogel layer prepared in step (1) and the CNT / AgNW / PAN static spectral selection layer prepared in step (2) are composited by physical lamination. The layers are bonded together by interfacial intermolecular forces, thus obtaining the spectral decoupling adaptive composite coating.

[0039] Example 2 The difference from Example 1 is that the thermochromic hydrogel layer was prepared using a UV-initiated free radical polymerization process: After preparing the premix and pore size adjuster according to the same proportions as in Example 1, 0.03 g of 2-hydroxy-2-methyl-1-phenylpropanone (photoinitiator 1173) was added as a photoinitiator, and the mixture was stirred and mixed evenly in the dark. The reaction solution was then poured into a mold in the dark and placed under a 365 nm ultraviolet light source (light intensity 10 mW / cm²). 2 Irradiation for 30 minutes initiates crosslinking polymerization. Subsequent post-processing and lower-layer composite processes are the same as in Example 1.

[0040] Example 3: Comparative Study on the Effect of PEG Addition Amount on Thermochromic Properties This embodiment aims to investigate the effect of the amount of pore-size modifier PEG added on the microstructure and optical switching properties of PNIPAm hydrogel, in order to clarify the PEG content range for achieving optimal thermal management performance. Except for the amount of PEG added, all raw materials and process parameters are consistent with those in Example 1.

[0041] A series of hydrogel samples with different PEG (Mw≈1000) additions were prepared. The PEG addition amount, expressed as a percentage of the NIPAm monomer mass, was set as follows: 0% (comparative example), 2%, 5%, and 8%. Each sample was characterized as follows: (1) Microstructure analysis The internal morphology of the hydrogel sample in the swollen state was observed using scanning electron microscopy (SEM), and the results are as follows: Figure 2 As shown in a-2d. Without the addition of PEG ( Figure 2 a) Pure PNIPAm hydrogels exhibit a non-uniform and open porous structure with a wide pore size distribution, some pores extending to the micrometer level, and a relatively loose pore wall structure. With increasing PEG content (2%), Figure 2 (b) The pore network begins to refine. When the PEG content reaches 5% ( Figure 2 c) The pore structure is significantly improved, exhibiting a highly uniform and dense network morphology, with pore sizes mainly concentrated in the submicron scale (approximately 200-500 nm), and the pore walls are more continuous and intact. Further increasing PEG to 8% ( Figure 2 d) The pore structure remains uniform, but the pore size is slightly reduced. The results show that the introduction of an appropriate amount of PEG can effectively regulate the microphase separation behavior of the gel during the phase transition process and induce the formation of scattering centers with controllable size and uniform distribution.

[0042] (2) Optical performance analysis The solar spectral reflectance of samples with different PEG contents was tested at high temperature (50℃, above LCST), with a wavelength range of 300-2500 nm. The changes in reflectance and total reflectance under high temperature (50℃) and low temperature (25℃) conditions for solar thermal regulation with different amounts of polyethylene glycol are shown in Table 1 below.

[0043] As shown in Table 1, the high-temperature reflectance initially increases and then stabilizes with increasing PEG content. The high-temperature reflectance of the pure PNIPAm sample (0% PEG) is approximately 0.628; at 2% PEG, the reflectance increases to approximately 0.78; at 5% PEG, the reflectance reaches its maximum value of approximately 0.817; and at 8% PEG, the reflectance remains at approximately 0.80. Combined with SEM image analysis, this is because the submicron-scale uniform pore size formed at 5% PEG content precisely matches the wavelength scale of the solar spectrum, generating a strong Mie scattering effect, thereby maximizing light scattering efficiency and achieving higher solar reflectance.

[0044] Based on the combined microstructure and optical performance data, the optimal ratio of PEG addition was determined to be 5% of the NIPAm monomer mass. The hydrogel layer prepared under this condition has the most uniform microstructure, the highest optical switching contrast, and good mechanical stability, providing core performance support for subsequent composite coatings.

[0045] The solar spectral reflectance of PEG samples with different molecular weights was measured at high temperature (50℃, above LCST) in the wavelength range of 300-2500 nm. The changes in reflectance and total reflectance under high temperature (50℃) and low temperature (25℃) conditions for solar thermal regulation at different polyethylene glycol molecular weights are shown in Table 2 below.

[0046] Table 2 shows that the molecular weight of PEG has a significant impact on the pore size control effect. When the molecular weight of PEG is too low (e.g., 800), the chain length is short, resulting in a small template size in the gel network. This leads to pore diameters less than 200 nm after elution, deviating from the scattering wavelength corresponding to the peak energy of the solar spectrum. Therefore, the improvement in high-temperature reflectivity (0.641) is limited. When the molecular weight of PEG is too high (e.g., 4000), the solubility of long-chain PEG in the monomer solution decreases, making it prone to micro-phase separation and forming excessively large aggregates. After elution, micron-sized macropores are left, and the scattering efficiency actually decreases (0.633). Only when the molecular weight of PEG is in the moderate range of 800-2000 can its chain length be uniformly dispersed in the network and form submicron-sized pores (approximately 200-800 nm) that match the wavelength of sunlight (300-2500 nm), thereby generating strong Mie scattering and achieving a significant leap in reflectivity (Δ). R Reaching 0.45-0.51), based on the existing comparison data of PEG addition amounts (0%, 2%, 5%, 8%) and molecular weights (800, 1000, 2000, 4000), when the addition amount is 5% and the molecular weight is 1000, the high-temperature reflectivity ( R H ) and switching amplitude (Δ R All reached their peak values.

[0047] Example 4: Verification of alternative thermochromic scattering layer and static layer materials To illustrate the universality of the technical solution of this invention, this embodiment uses different material systems to construct a double-layer composite coating.

[0048] 1. Preparation of thermochromic layer Following the steps of Example 1, 2 g of NIPAm monomer was replaced with an equimolar amount of N-vinylcaprolactam (NVCL) monomer to synthesize a poly(N-vinylcaprolactam) (PVCL)-based hydrogel. During polymerization, 5% (by mass) of polyethylene glycol diacrylate (PEGDA) was added as a crosslinking and pore size regulator, followed by the sequential addition of 1 mg BIS, 0.01 g APS, and 5 µL TEMED. Polymerization was carried out at 4°C for 10 hours to obtain a PVCL / PDGDA hydrogel. This hydrogel also exhibited typical thermochromic behavior: transparent at low temperatures (20°C) and transforming into a white, opaque state at high temperatures (45°C).

[0049] 2. Preparation of Static Spectral Selective Layer The CNT / AgNW mixed dispersion was replaced with a graphene oxide / copper nanowire (GO / CuNWs) mixed dispersion, dispersed in ethanol at a 1:1 mass ratio, and then ultrasonically treated before being sprayed onto a PE substrate. Subsequently, it was dried at 60°C and heat-treated at 200°C for 1 hour under nitrogen protection to reduce GO to reduced graphene oxide (rGO), forming an rGO / CuNWs composite conductive film. Testing showed that the composite film had an absorbance >0.90 in the 300-2500 nm wavelength range and an emissivity <0.25 in the 8-14 μm atmospheric window wavelength range. A PAN protective layer was then sprayed on, dried, and ready for use.

[0050] The aforementioned PVCL / PDGDA hydrogel layer was laminated with the rGO / CuNWs static layer to form a composite coating with similar functions. Test results show that this coating also achieves the function of solar spectrum modulation through low-temperature heat absorption / high-temperature reflection, demonstrating the good versatility of this invention in selecting thermochromic materials and static layer materials.

[0051] Example 5: Verification of alternative thermochromic scattering layer and static layer materials To illustrate the universality of the technical solution of this invention, this embodiment uses different material systems to construct a double-layer composite coating.

[0052] 1. Preparation of thermochromic layer Following the steps of Example 1, 1.5 g of NIPAm monomer was replaced with an equimolar amount of N-n-propylacrylamide (PNPA) monomer to synthesize a poly-N-n-propylacrylamide (PNPA)-based hydrogel. During polymerization, 8% (by mass) of polyethylene glycol methacrylate (PEGMA) was added as a crosslinking and pore size regulator, followed by the sequential addition of 2 mg BIS, 0.02 g APS, and 10 µL TEMED. Polymerization was carried out at 4°C for 10 hours to obtain the PNPA / PEGMA hydrogel. This hydrogel also exhibited typical thermochromic behavior: transparent at low temperatures (25°C) and transforming into a white, opaque state at high temperatures (50°C).

[0053] 2. Preparation of Static Spectral Selective Layer The lower layer composite process is the same as in Example 1. The aforementioned PNPA / PEGMA hydrogel layer was laminated with a CNT / AgNW / PAN static layer to form a composite coating with similar functions. Test results show that this coating also achieves the function of solar spectrum modulation through low-temperature heat absorption / high-temperature reflection, demonstrating the good versatility of this invention in selecting thermochromic materials and static layer materials.

[0054] Example 6: Verification of alternative thermochromic scattering layer and static layer materials To illustrate the universality of the technical solution of this invention, this embodiment uses different material systems to construct a double-layer composite coating.

[0055] 1. Preparation of thermochromic layer Following the steps of Example 1, 0.5 g of NIPAm monomer was replaced with an equimolar amount of N,N-diethylacrylamide (PDEA) monomer to synthesize a poly(N,N-diethylacrylamide (PDEA)-based hydrogel). During polymerization, 4% (by mass) of polyethylene glycol diacrylate (PDGDA) was added as a crosslinking and pore size regulator, followed by the sequential addition of 3 mg BIS, 0.04 g APS, and 15 µL TEMED. Polymerization was carried out at 4°C for 10 hours to obtain the PDEA / PDGDA hydrogel. This hydrogel also exhibited typical thermochromic behavior: transparent at low temperatures (20°C) and transforming into a white, opaque state at high temperatures (45°C).

[0056] 2. Preparation of Static Spectral Selective Layer The lower layer composite process is the same as in Example 1. The PDEA / PDGDA hydrogel layer and the CNT / AgNW / PAN static layer were laminated together to form a composite coating with similar functions. Test results show that this coating also achieves the function of solar spectrum modulation through low-temperature heat absorption / high-temperature reflection, demonstrating the good versatility of this invention in selecting thermochromic materials and static layer materials.

[0057] Example 7: Verification of alternative thermochromic scattering layer and static layer materials To illustrate the universality of the technical solution of this invention, this embodiment uses different material systems to construct a double-layer composite coating.

[0058] 1. Preparation of thermochromic layer Following the steps of Example 1, 2.5 g of NIPAm monomer was replaced with an equimolar amount of (methyl vinyl ether) (PMVE) monomer to synthesize a polymethyl vinyl ether (PMVE)-based hydrogel. During polymerization, 6% (by mass) of polyethylene glycol diacrylate (PDGDA) was added as a crosslinking and pore size regulator, followed by the sequential addition of 1 mg BIS, 0.05 g APS, and 5 µL LTEMED. Polymerization was carried out at 4°C for 10 hours to obtain the PMVE / PDGDA hydrogel. This hydrogel also exhibited typical thermochromic behavior: transparent at low temperatures (25°C) and transforming into a white, opaque state at high temperatures (50°C).

[0059] 2. Preparation of Static Spectral Selective Layer The lower layer composite process is the same as in Example 1. The aforementioned PMVE / PDGDA hydrogel layer was laminated with the CNT / AgNW / PAN static layer to form a composite coating with similar functions. Test results show that this coating also achieves the function of solar spectrum modulation through low-temperature heat absorption / high-temperature reflection, demonstrating the good versatility of this invention in selecting thermochromic materials and static layer materials.

[0060] Example 8: Preparation of thick composite coatings based on low-concentration monomers This embodiment aims to verify the feasibility of the technical solution of the present invention under different thicknesses and monomer concentrations.

[0061] 1. Preparation of precursor solution Accurately weigh 0.5 g of NIPAm monomer and dissolve it in 5 mL of deionized water to prepare a dilute solution with a monomer concentration of 10 wt%. Add 1 mg of BIS crosslinking agent. Add 2% (by weight of NIPAm) of polyethylene glycol diacrylate (PEGDA, Mn≈400) as a pore size adjuster. Stir the above mixture at 900 rpm for 30 minutes until completely dissolved. Add 0.01 g of APS, stir at room temperature for 30 minutes, then add 5 μL of TEMED and mix rapidly.

[0062] 2. Preparation of thick-layer hydrogels The mixed solution was rapidly injected into a custom mold with an inner cavity depth of 5 mm and placed in a 4°C refrigerator for static polymerization for 10 hours. After demolding, a transparent hydrogel block with a thickness of approximately 5 mm was obtained. Observation showed that the thick gel block had an intact structure, uniform texture, and no obvious cracks or defects, proving that the introduction of PEGDA at low concentrations helps to form a strong network structure to support a larger thickness.

[0063] 2. Preparation of Static Spectral Selective Layer The lower layer composite process is the same as in Example 1. The above-mentioned thick hydrogel was laminated with a CNT / AgNW / PAN static layer. The test results showed that the thick composite coating has a high solar absorptivity at low temperatures and can still achieve effective solar reflection switching at high temperatures. This indicates that the technical solution of the present invention has a certain tolerance for the thickness of the hydrogel layer and can be adjusted according to the actual application scenario.

[0064] Example 9: Synergistic effect of low-temperature polymerization conditions and PEG addition amount This example aims to verify the synergistic effect between low-temperature polymerization conditions (4°C) and the optimal PEG addition (5%). Three comparative experiments were set up. The basic formulation of all groups was the same as that in Example 1 (2.0 g NIPAm, 5% PEG 1000, 3 mg BIS, 0.03 g APS, 20 μL TEMED), and the polymerization time was 10 hours. Only the polymerization temperature and PEG addition were changed. Group A: Polymerize at 4℃ with 5% PEG added.

[0065] Group B: Polymerize at 25℃ with 5% PEG added.

[0066] Group C: Polymerized at 4℃, without PEG addition; Group D: 25℃, no PEG added.

[0067] The high-temperature (50℃) solar reflectance of the hydrogels obtained in each group ( R H The results of the difference in reflectance are shown in Table 3 below:

[0068] As shown in Table 3, comparing groups A and B, under the same PEG addition, low-temperature polymerization (group A) resulted in a narrower pore size distribution (200-800 nm vs 300-1200 nm) and an average pore size closer to the optimal scattering wavelength (450 nm vs 650 nm) compared to room-temperature polymerization (group B). This led to significantly higher high-temperature reflectivity (0.788 vs 0.656) and switching amplitude (0.452 vs 0.336). This demonstrates that low-temperature conditions effectively suppressed the polymerization rate, providing a sufficient kinetic window for the uniform dispersion of PEG and maximizing its role as a pore-forming template.

[0069] Comparing groups A and C, under the same low-temperature polymerization conditions, group A with 5% PEG showed a significantly refined pore size, decreasing from the micrometer level (average 1.2 μm) to the submicrometer level (average 450 nm), compared to group C without PEG. The high-temperature reflectivity also increased dramatically from 0.628 to 0.788. This indicates that the introduction of PEG is a necessary condition for the formation of effective scattering pores.

[0070] Example 10: Effect of thermochromic gel layer thickness on the performance of composite coatings This embodiment aims to verify the effect of the thickness of the thermochromic gel layer on the overall optical switching performance and mechanical stability of the composite coating. Referring to the formulation and process of Example 1, a series of PNIPAm / PEG hydrogel layers of different thicknesses were prepared by simply changing the polymerization molds of different thicknesses (1 mm, 2 mm, 3 mm, 4 mm, 5 mm), and then composited with the same CNT / AgNW / PAN static spectral selectivity layer. The performance of each composite coating was tested, and the results are shown in Table 5 below:

[0071] Table 5 shows that the composite coating exhibits the best overall performance at a thickness of 3.0 mm. It provides sufficient optical path length, achieving a high reflectivity of 0.718 at high temperatures, while maintaining good interlayer adhesion and mechanical stability. Excessive gel layer thickness introduces additional reflection / scattering mechanisms (interfacial defects, volume scattering, uneven preparation, etc.) at low temperatures, deviating from the ideal transparent behavior. Furthermore, decreased mechanical stability interferes with accurate measurements. Therefore, 3 mm is the optimal gel layer thickness for the composite coating.

[0072] Effect Example The reflectance spectra of the double-layer coating sample prepared in Example 1 at different temperatures were measured using a UV-Vis-NIR spectrophotometer (equipped with an integrating sphere accessory), with a wavelength range of 300-2500 nm. The results are as follows: Figure 4 As shown, at 25°C (low temperature state, below LCST), the PNIPAm / PEG hydrogel layer is transparent, and sunlight passing through the upper layer is strongly absorbed by the underlying black CNT / AgNW layer. At this temperature, the overall solar reflectivity of the coating is low, approximately 0.10. When the temperature rises to 50°C (high temperature state, above LCST), the hydrogel layer undergoes a phase transition, forming a dense and uniform submicron-scale scattering network, resulting in strong reflection of sunlight. The overall solar reflectivity of the coating significantly increases to approximately 0.81. This result demonstrates that the coating achieves a high-contrast, reversible optical switching from a "solar absorption state" to a "solar reflection state," with a reflectivity change as high as 0.71, providing a crucial spectral modulation basis for adaptive thermal management.

[0073] The emissivity of each functional layer of the coating in the mid-infrared band (8-14 μm) was characterized using a Fourier transform infrared spectrometer (equipped with an infrared integrating sphere). The results are as follows: Figure 5 and Figure 6 As shown.

[0074] Figure 5 The infrared emissivity of the PNIPAm / PEG hydrogel top layer at different temperatures is shown. At 25℃ (transparent state) and 50℃ (opaque state), the emissivity of the hydrogel layer remains at a high level of approximately 0.96. This stable high emissivity is mainly attributed to the abundant polar functional groups in the PNIPAm / PEG molecular network (such as C=O in amide groups, -OH in hydroxyl groups, and COC in ether bonds). These functional groups can maintain strong phonon-polaritonic resonances in the mid-infrared band, thus enabling the hydrogel layer to exhibit near-blackbody infrared radiation characteristics regardless of whether it is in the transparent or scattering state.

[0075] Figure 6 The emissivity of the CNT / AgNW static spectral selectivity layer (including the PAN protective layer) in the mid-infrared band was displayed. Throughout the entire test temperature range (25-50°C), the emissivity of this layer remained consistently low at approximately 0.15. This extremely low infrared emissivity stems from the high infrared reflectivity interface formed by the high conductivity of the CNT / AgNW composite film, which effectively suppresses long-wave radiation heat loss from the underlying layer (indoor side). Its emissivity remained almost unchanged at 50°C, confirming the excellent thermal stability of this static layer.

[0076] comprehensive Figure 4 and Figure 5 The data shows that the present invention has successfully achieved synergistic regulation of infrared radiation characteristics through a dual-layer spectral decoupling design: the top layer is responsible for maintaining high radiation heat dissipation capacity to the external environment (at high temperature), while the bottom layer is responsible for suppressing heat radiation loss to the internal space (at low temperature). The two do not interfere with each other and their functions are complementary.

[0077] The above-mentioned spectroscopic tests were performed on samples with different PEG contents in Example 3, the PVCL-based sample in Example 4, and the thick-layer sample in Example 10. The results showed that: (1) Sample of Example 3: The highest high temperature reflectance (0.817) was obtained when the PEG content was 5%, which was comparable to that of Example 1; the high temperature reflectance of the samples with 2% and 8% content were 0.784 and 0.766, respectively, which were better than the comparative example without PEG (0.628), confirming the universality of the pore size regulation effect of PEG.

[0078] (2) Sample of Example 4: The PVCL / PDGDA hydrogel layer achieved a solar reflectance of about 0.78 at high temperature, and the infrared emissivity of the bottom rGO / CuNWs layer was <0.25. The overall performance was close to that of Example 1, proving the substitutability of different material systems.

[0079] (3) Sample 10: The 5 mm thick hydrogel layer has a solar reflectance of 0.75 at high temperature, which is slightly lower than that of the 3 mm thick sample, but still maintains a high switching contrast and good absorption at low temperature, proving that the coating structure can still work effectively at different thicknesses.

[0080] In summary, the spectrally decoupled adaptive composite coating provided by this invention achieves reversible switching of high-contrast solar spectral reflectance (reflectance variation range 0.10~0.81) through the introduction of PEG-regulated thermochromic hydrogel in the upper layer; it achieves stable low infrared emissivity (~0.15) through the lower CNT / AgNW conductive composite film; and the upper hydrogel maintains high infrared emissivity (~0.96) across the entire temperature range. This "dynamic upper layer, static lower layer" spectral decoupling design enables the coating to autonomously switch between a "high solar absorption - low infrared emission" heating mode and a "high solar reflection - high infrared emission" cooling mode based on ambient temperature without relying on external energy, effectively solving the inherent contradictions of traditional static coatings in all-season thermal management. This coating has a simple preparation process, widely available materials, and strong structural adjustability, showing broad application prospects in energy-efficient buildings, outdoor equipment, and personal thermal management.

[0081] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a thermochromic gel, characterized in that, This method employs a one-pot synthesis, and the specific steps are as follows: Step 1) Preparation of precursor solution: The temperature-sensitive monomer and crosslinking agent are dissolved in a solvent and stirred to form a homogeneous precursor solution; Step 2) Introduce pore size control agent: Add a pore size regulator to the precursor solution obtained in step 1) and stir to disperse it evenly; Step 3) Initiate the polymerization reaction: Initiator and accelerator are added sequentially to the solution obtained in step 2), mixed evenly, and then injected into the polymerization mold for free radical thermal polymerization under controlled temperature conditions. Alternatively, add the photoinitiator sequentially to the solution obtained in step 2), mix thoroughly, and then inject into the polymerization mold. Under UV light irradiation, carry out the photoinitiated polymerization reaction to obtain the final product.

2. The method for preparing the thermochromic gel according to claim 1, characterized in that, The amount of the pore size modifier added is 1%-10% of the total mass of the temperature-sensitive monomer.

3. The method for preparing the thermochromic gel according to claim 1, characterized in that, The pore size regulator is selected from one or more of polyethylene glycol, polyethylene glycol diacrylate, and polyethylene glycol methacrylate; And / or, when the pore size regulator contains polyethylene glycol, the molecular weight of the polyethylene glycol is 400-10000.

4. The method for preparing the thermochromic gel according to claim 1, characterized in that, The thermosensitive monomer is selected from one or more of N-isopropylacrylamide, N,N-diethylacrylamide, methyl vinyl ether, N-n-propylacrylamide, and N-vinylcaprolactam; and / or, the amount of the thermosensitive monomer added is 0.5-2.5 g; And / or, the crosslinking agent is N,N'-methylenebisacrylamide, N,N'-(1,2-dihydroxyethylene)bisacrylamide, pentaerythritol tetraacrylate, N,N'-ethylenebisacrylamide, N,N'-bisacryloylcysteine; and / or, the amount of the crosslinking agent added is 0.5-2.5 g; And / or, the initiator is ammonium persulfate, azobisisobutyrazoline hydrochloride; the photoinitiator is 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone, Irgacure 651 (2,2-dimethoxy-2-phenylacetophenone), 2,2-diethoxyacetophenone; and / or, the amount of the initiator added is 0.02-0.06 g; And / or, the accelerator is N,N,N',N'-tetramethylethylenediamine; and / or, the amount of the accelerator added is 5 μL-30 μL.

5. A thermochromic gel prepared by the method for preparing thermochromic gel according to any one of claims 1-4, characterized in that, The thickness of the thermochromic gel is 0.5-10 mm.

6. A spectral decoupling adaptive composite coating, characterized in that, The coating includes: Flexible polymer substrate layer; A conductive network layer covering the flexible polymer substrate; A polymer protective layer covering the conductive network layer; And a thermochromic gel layer, which is laminated and adhered to the polymer protective layer.

7. The spectral decoupling adaptive composite coating according to claim 6, characterized in that, The conductive network layer comprises multi-walled carbon nanotubes and silver nanowires; The silver nanowires overlap each other to form a conductive framework. The multi-walled carbon nanotubes are dispersed between the silver nanowires and bridged between adjacent silver nanowires, forming a co-diffusion conductive network composed of multi-walled carbon nanotubes and silver nanowires.

8. The spectral decoupling adaptive composite coating according to claim 7, characterized in that, The diameter of the multi-walled carbon nanotubes is smaller than the diameter of the silver nanowires.

9. The spectral decoupling adaptive composite coating according to claim 7, characterized in that, The multi-walled carbon nanotubes are wound or anchored to the surface of the silver nanowires to form a CNT-AgNW heterojunction structure with low contact resistance at the overlapping junctions of the silver nanowires.

10. The spectral decoupling adaptive composite coating according to claim 7, characterized in that, The conductive network layer is formed on the flexible polymer substrate by spraying.

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