Preparation method of composite film with structural color and radiative cooling function

Abstract

The present application relates to the field of optical functional materials and thermal management technology, and particularly relates to a preparation method of a composite film with structural color and radiative cooling function. The method comprises the following steps: providing a polymer film as a base layer; integrally forming a periodic groove microstructure array on one side surface of the base layer through roll-to-roll nanoimprinting, the cross section of the groove is rectangular or trapezoidal, and the groove wall is circular arc transition; forming a metal reflection layer on the other side of the base layer through physical vapor deposition; introducing cholesteric phase material on the groove side, and filling the groove and forming a surface continuous cover layer through blade coating and self-assembly, and forming a cholesteric phase structure layer after drying and curing. The geometric confinement effect of the groove makes the cholesteric phase material form an ordered structure with limited orientation, and reduces the angle dependence of the structural color; the metal reflection layer realizes high reflection in the solar wave band; the mid-infrared molecular vibration absorption of the cholesteric phase structure layer and the mid-infrared radiation enhancement effect of the groove microstructure array make the film have high emissivity in the atmospheric window.

Description

Technical Field

[0001] This invention relates to the fields of optical functional materials and thermal management technology, and in particular to a method for preparing a composite thin film with structural color and radiation cooling functions. Background Technology

[0002] With the continuous growth of energy consumption and the increasing prominence of environmental problems, radiative cooling technology based on passive temperature control has attracted widespread attention. This technology achieves high thermal radiation in the atmospheric window band (8–13 μm) and maintains low absorption or high reflection in the solar radiation band (0.3–2.5 μm), thereby directly dissipating heat into outer space in the form of electromagnetic radiation, achieving a passive cooling effect without the need for additional energy input.

[0003] Currently, radiation cooling materials mainly include multilayer film structures, porous scattering structures, and metasurface structures based on micro / nano structures. Among them, polymer-based radiation cooling films have promising prospects in practical applications due to their advantages such as light weight, high flexibility, and ease of large-area fabrication. To improve their performance, microstructure arrays are typically introduced and combined with metal reflective layers to modulate their spectral response, thereby achieving higher solar reflectivity and infrared emissivity.

[0004] However, existing radiation cooling materials primarily aim to improve spectral selectivity, and their appearance is typically white or metallic, making it difficult to meet the color requirements of practical applications. To achieve colored radiation cooling, existing technologies have attempted to introduce color through chemical or structural colors. Chemical colors usually rely on absorption mechanisms, which can easily increase sunlight absorption, thus affecting radiation cooling performance. While structural colors can achieve color modulation through selective reflection, they still suffer from strong angle dependence when combined with radiation cooling structures, causing significant color changes with the viewing angle and affecting the usability. Furthermore, in multifunctional composite systems, achieving stable structural color output while maintaining high solar reflectivity and high infrared emissivity, and reducing the mutual interference between different functions, remains a challenge. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a composite thin film with structural color and radiation cooling functions, aiming to provide a method for preparing a composite optical thin film that combines structural color display and radiation cooling performance.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: The present invention provides a method for preparing a composite thin film with structural color and radiation cooling functions, comprising: S1: providing a polymer thin film as a substrate layer; S2: forming a periodic groove microstructure array on one side surface of the substrate layer by roll-to-roll nanoimprinting, wherein the groove microstructure array is integrally formed with the substrate layer; the cross-section of the groove is rectangular or trapezoidal, and the groove wall is a continuous curved surface or a circular arc transition structure; S3: forming a metal reflective layer on the other side surface of the substrate layer by physical vapor deposition, wherein the metal reflective layer is a continuous dense metal film with high reflectivity in the 0.3-2.5μm wavelength band; S4 ... integrally formed with the substrate layer by roll-to-roll nanoimprinting, wherein the groove wall is integrally formed with the substrate layer by roll-to-roll nanoimprinting, wherein the groove wall is integrally formed with the substrate layer by roll-to-roll nanoimprinting, wherein the groove wall is integrally formed with the substrate layer by roll-to-roll nanoimprinting, wherein the groove wall is integrally formed with the substrate layer by roll-to-roll nanoimprinting, wherein the groove wall is integrally formed with the substrate layer by roll-to-roll nanoimprinting, wherein the Cholesteric phase material is introduced on one side of the structural array. Through scraping and self-assembly, the cholesteric phase material fills the groove and forms a continuous covering layer above the groove. The groove microstructure array has a geometric confinement effect on the self-assembly of the cholesteric phase material, causing the cholesteric phase material to form an ordered arrangement with restricted orientation within the groove space, thus forming a cholesteric phase structural layer. The cholesteric phase structural layer has selective reflectivity in the visible light band. The cholesteric phase structural layer is composed of a polymer material with mid-infrared molecular vibration absorption characteristics. The mid-infrared molecular vibration absorption characteristics of the polymer material work together with the groove microstructure array to give the composite optical film a high emissivity in the atmospheric window band.

[0007] The groove depth of the groove microstructure array is 1–10 μm, the groove opening size is 3–15 μm, and the array period is 5–20 μm.

[0008] The base layer is one or more of polyethylene terephthalate, polyethylene, polycarbonate or polydimethylsiloxane.

[0009] The metal reflective layer is a silver layer, an aluminum layer, or a composite layer thereof, with a thickness of 50–300 nm.

[0010] The cholesteric phase structure layer is composed of cellulose or its derivatives, with a pitch of 150–500 nm.

[0011] After the coating in step S4, allow it to stand for 10 to 30 minutes to complete the initial orientation of the cholesteric phase material, and then proceed with drying or curing.

[0012] Drying or curing is carried out on a heating plate at 50–80°C, and the sample is covered to stabilize humidity and evaporation rate.

[0013] In step S2, the roll-to-roll nanoimprinting process has a roll pressure of 0.2 to 1.0 MPa and an imprinting speed of 0.1 to 1 m / min.

[0014] In step S4, the cholesteric phase material is an aqueous solution of hydroxypropyl cellulose with a mass fraction of 55-65 wt%; and before introducing the cholesteric phase material, the substrate layer with the grooved microstructure array is subjected to plasma treatment to improve wettability.

[0015] The prepared composite optical thin film exhibits a low angle-dependent structural color in the visible light band and has an average emissivity greater than 0.9 in the 8–13 μm band.

[0016] Compared with the prior art, this application has the following advantages: 1. This invention utilizes a specific process in step S4 to fill the grooves with cholesteric material and form a continuous overlay layer above them, creating an integrated composite structure between the cholesteric phase layer and the groove microstructure array. The cholesteric phase layer is simultaneously anchored within the grooves and continuously covers them, forming a dual combination of "embedded and overlay," effectively enhancing the adhesion strength between the cholesteric phase layer and the substrate layer, preventing delamination or detachment. The continuous surface overlay layer eliminates any unfilled areas that may exist between the grooves, ensuring the uniformity and continuity of the structural color across the entire film surface.

[0017] 2. The integral molding of the grooved microstructure array with the substrate layer eliminates the heterogeneous interface between the substrate layer and the microstructure, avoids the risks of interface reflection, scattering and delamination, improves the dimensional accuracy and replication fidelity of the microstructure array, and is beneficial to the structural consistency control in large-scale roll-to-roll processes.

[0018] 3. This invention utilizes the geometric confinement effect of a grooved microstructure array on the self-assembly of cholesteric phase materials, enabling the cholesteric phase materials to form an ordered arrangement with restricted orientation within the groove space. By allowing the material to stand for 10–30 minutes to complete the process control of the initial orientation of the cholesteric phase materials, the drift of the center wavelength of the reflection peak with the incident angle (0° to 60°) is significantly reduced, achieving true low-angle-dependent structural color.

[0019] 4. This invention achieves separate control of the solar radiation band, visible light band, and mid-infrared band through a synergistic design of a three-layer structure. In the solar radiation band, the metal reflective layer (silver layer, aluminum layer, or a composite layer thereof, thickness 50-300nm) provides ≥95% reflectivity, effectively reducing solar radiation absorption. In the visible light band, the cholesteric phase structure layer (pitch 150-500nm) selectively reflects specific wavelengths, generating structural color, while unreflected visible light is reflected by the metal reflective layer, avoiding heat absorption. In the mid-infrared band, the cellulose or its derivatives in the cholesteric phase structure layer, due to the stretching vibration of the CO / COC molecular bonds, enhance mid-infrared radiation through the morphology of the grooved microstructure array, giving the composite film an intrinsically high infrared emissivity (measured >0.9), allowing heat to dissipate into outer space through the atmospheric window in the form of thermal radiation. Attached Figure Description

[0020] Figure 1 This is a flowchart of a method for preparing a composite thin film with structural color and radiation cooling function provided in an embodiment of this application; Figure 2 This is a preparation diagram of a composite thin film preparation method with structural color and radiation cooling function provided in the embodiments of this application. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0022] In the description of the invention, it should be understood that the terms "upper," "lower," "left," "right," "front," "rear," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or relative positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Unless otherwise specified, the above-mentioned orientational descriptions can be flexibly set in practical applications, provided that the relative positional relationships shown in the accompanying drawings are satisfied.

[0023] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0024] In embodiments of the invention, the terms "comprising," "including," or any other variations thereof are intended to cover a non-exclusive inclusion, such that a process, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, article, or apparatus that includes that element.

[0025] In embodiments of the present invention, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or description. Any embodiment or design described as "exemplary" or "for example" in embodiments of the present invention should not be construed as being more preferred or advantageous than other embodiments or designs. Rather, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0026] Reference Figure 1 and Figure 2 This application provides a method for preparing a composite thin film with structural color and radiation cooling functions, comprising: S1: Provide a polymer film as a base layer.

[0027] This substrate layer serves to support the grooved microstructure array and the metal reflective layer, and provides a structural basis for the subsequent filling of cholesteric phase materials. For example, the substrate layer is selected from polymer materials with good film-forming properties, flexibility, and mechanical strength, and its thickness can be selected according to actual application requirements, preferably 25–200 μm. One surface of the substrate layer will be used to form the grooved microstructure array, and the other surface will be used to deposit the metal reflective layer. The surface smoothness and cleanliness of the substrate layer directly affect the subsequent imprinting quality and metal deposition effect.

[0028] As one possible implementation, the substrate layer is a polyethylene terephthalate (PET) film with a thickness of 50 μm, 100 μm, or 125 μm. In other embodiments of the invention, the substrate layer may be one or more of polyethylene, polycarbonate, or polydimethylsiloxane. For example, in Example 1, a 100 μm thick PET film is selected, which has high transmittance in the visible and near-infrared bands, facilitating optical alignment in subsequent imprinting processes. Before use, the film surface is ultrasonically cleaned with anhydrous ethanol and deionized water, and then dried at 60°C for later use.

[0029] S2: A periodic array of groove microstructures is formed on one side surface of the substrate layer by roll-to-roll nanoimprinting. The groove microstructure array is integrally formed with the substrate layer. The cross-section of the groove is rectangular or trapezoidal, and the groove wall is a continuous curved surface or a circular arc transition structure.

[0030] Roll-to-roll nanoimprinting is a continuous, large-area micro / nanostructure replication technology. Its typical process includes: first, preparing a master template with pre-defined groove structure parameters using techniques such as laser direct writing, electron beam lithography, or laser 3D nanoprinting; then, continuously feeding a polymer film between the imprinting roller and the template, and under specific temperature and pressure conditions, causing plastic deformation on the surface of the polymer film to form a groove structure complementary to the template's contours; finally, after demolding, a one-piece array of grooved microstructures is obtained. In this process, imprinting pressure, imprinting speed, and imprinting temperature are key parameters affecting structural fidelity.

[0031] For example, the geometric parameters of the groove microstructure array include groove depth, opening size, array period, and groove wall shape. The cross-sectional shape of the groove can be rectangular or trapezoidal. The groove wall adopts a continuous curved surface or arc transition structure to eliminate stress concentration at sharp corners, improve the quality of imprinting and demolding, and facilitate the uniform filling and orientation of subsequent cholesteric phase materials in the groove.

[0032] As one possible implementation, the master template is fabricated using a laser 3D nanoprinting system, and the template surface has a raised structure complementary to the desired groove. The structural parameters of the groove microstructure array are set as follows: groove depth is 1–10 μm, for example, 2 μm, 5 μm, or 8 μm; groove opening size is 3–15 μm, for example, 5 μm, 8 μm, or 12 μm; array period is 5–20 μm, for example, 8 μm, 12 μm, or 15 μm. For grooves with a trapezoidal cross-section, the sidewall inclination angle (the angle between the sidewall and the normal direction of the substrate) is preferably 5–30°.

[0033] The roll-to-roll nanoimprinting process conditions are as follows: roll pressure 0.2–1.0 MPa, for example 0.5 MPa or 0.8 MPa; imprinting speed 0.1–1 m / min, for example 0.3 m / min or 0.6 m / min; imprinting temperature is set according to the glass transition temperature of the polymer material, and for PET substrates, the imprinting temperature is preferably 80–120 °C. After imprinting, the substrate is cooled to below 50 °C under maintaining pressure, and then demolded to obtain a periodic groove microstructure array integrally formed with the substrate layer. Scanning electron microscopy characterization shows that the obtained groove structure is complete, the groove walls are smooth, and there are no burrs or collapse defects.

[0034] S3: A metal reflective layer is formed on the other side of the substrate by physical vapor deposition. The metal reflective layer is a continuous and dense metal film with high reflectivity in the 0.3 to 2.5 μm band.

[0035] Physical vapor deposition (PVD) is a technique that uses methods such as thermal evaporation, electron beam evaporation, or magnetron sputtering under vacuum conditions to vaporize and deposit metallic materials onto a substrate surface to form a thin film. For metallic reflective layers, the film must be dense, free of pinholes, and have strong adhesion to ensure high reflectivity (typically greater than 90%) in the solar spectrum. The selection of metallic materials should balance reflectivity and chemical stability; silver and aluminum are preferred materials due to their high reflectivity in the visible and near-infrared bands. The thickness of the metallic reflective layer needs to strike a balance between reflective performance and material cost; too thin a layer will result in insufficient reflectivity, while too thick a layer will increase cost and may introduce stress.

[0036] As one possible implementation, a direct current magnetron sputtering method is used to deposit the metal reflective layer. The sputtering target is a silver target or aluminum target with a purity of 99.99%, or a silver / aluminum composite target. Before deposition, the vacuum chamber is evacuated to a background vacuum better than 5 × 10⁻⁶. - 4 The pressure is set to Pa, then high-purity argon gas (Ar, purity 99.999%) is introduced as the working gas at a pressure of 0.3–0.8 Pa. The sputtering power density is 2–5 W / cm³. 2 The deposition rate is 0.1–0.5 nm / s. The deposition time is 5–15 minutes, resulting in a metallic reflective layer with a thickness of 50–300 nm, preferably 80 nm, 120 nm, or 200 nm. After deposition, the sheet resistance is measured using a four-point probe method to indirectly evaluate the film continuity; the reflectance spectrum is measured using a UV-Vis-NIR spectrophotometer, and the average reflectance in the 0.3–2.5 μm band can reach over 95%.

[0037] In some embodiments, the metal reflective layer may adopt a composite layer structure of silver and aluminum layers. For example, an aluminum layer (10-20 nm thick) is first deposited on the substrate as a transition layer, and then a silver layer is deposited as the main reflective layer to enhance the adhesion between the silver layer and the polymer substrate and suppress the migration of silver.

[0038] S4: Cholesteric phase material is introduced on one side of the groove microstructure array. Through scraping and self-assembly, the cholesteric phase material fills the interior of the groove and forms a continuous covering layer above the groove. The groove microstructure array has a geometric confinement effect on the self-assembly of the cholesteric phase material, so that the cholesteric phase material forms an ordered arrangement with restricted orientation within the groove space, thereby forming a cholesteric phase structure layer. The cholesteric phase structure layer has selective reflectivity in the visible light band. The cholesteric phase structure layer is composed of a polymer material with mid-infrared molecular vibration absorption characteristics. The mid-infrared molecular vibration absorption characteristics of the polymer material work together with the groove microstructure array to give the composite optical film a high emissivity in the atmospheric window band.

[0039] The cholesteric phase is a chiral structure with molecules arranged in a periodic helical pattern. The pitch (P) of this helical structure determines the central wavelength λ of selective reflection, satisfying the Bragg reflection condition: λ = n·P, where n is the average refractive index of the material. By adjusting the pitch, the reflected color can be controlled. The cholesteric phase material in the embodiments provided in this application is preferably a polymer material with mid-infrared molecular vibrational absorption characteristics, meaning its molecular structure contains chemical bonds capable of generating characteristic absorption in the 8–13 μm atmospheric window band, such as CO (carbon-oxygen single bond), COC (ether bond), OH (hydroxyl group), etc. Such materials can achieve high infrared emissivity without the need for additional infrared emitters.

[0040] Commonly used cholesteric polymer materials include cellulose and its derivatives (such as hydroxypropyl cellulose, hydroxyethyl cellulose, ethyl cellulose, etc.), as well as chiral liquid crystal polymers. Among them, hydroxypropyl cellulose (HPC) is a water-soluble cellulose ether that can self-assemble into a cholesteric liquid crystal phase in aqueous solution, and has the advantages of being green and environmentally friendly, biocompatible, and inexpensive.

[0041] The concentration of the cholesteric phase material solution directly affects the formation and self-assembly behavior of the liquid crystal phase. Too low a concentration prevents the formation of a stable cholesteric phase, while too high a concentration results in excessive viscosity, hindering coating. Therefore, the solution concentration typically needs to be controlled within a window range that allows for the formation of the cholesteric phase.

[0042] For example, a substrate layer with a grooved microstructure array is subjected to plasma treatment before the introduction of the cholesteric phase material. Plasma is the fourth state of matter, composed of high-energy electrons, ions, and free radicals. During oxygen plasma or air plasma treatment, the polymer surface is activated, introducing polar groups such as hydroxyl (-OH) and carboxyl (-COOH) groups, thereby significantly improving surface wettability. Improved wettability can promote capillary filling of the cholesteric phase solution within the grooves, avoiding the formation of bubbles or voids.

[0043] Blade coating (or doctor blading) is a common solution film formation method. A cholesteric phase material solution is quantitatively applied to the substrate surface, and the wet film thickness is controlled by the gap between the doctor blade and the substrate. The doctor blade moves relative to the substrate, spreading the solution evenly. In this invention, due to the presence of grooved microstructures on the substrate surface, the blade coating process simultaneously promotes the solution to enter the grooves and form a surface coating layer.

[0044] After the coating process is complete, the cholesteric phase material begins to self-assemble. The inner walls of the grooves anchor the orientation of the cholesteric phase molecules, restricting their free orientation and ensuring they are regularly arranged along the contour of the groove surface. This geometric confinement effect is a key mechanism for reducing the angle dependence of structural color. Sufficient settling time is required to fully achieve the initial orientation.

[0045] For example, after standing, the solvent (water for aqueous systems) is removed by drying or curing to fix the cholesteric phase structure. Drying temperature and humidity are key parameters for controlling the pitch. During the drying process, the pitch of the cholesteric phase helical structure is affected by two factors: the solvent evaporation rate and the thermal expansion of the molecular chains. When the drying temperature is higher, the thermal motion of the molecular chains intensifies, the helical structure tends to relax, and the pitch increases; at the same time, covering the sample reduces the solvent evaporation rate, allowing molecules more time to rearrange, further promoting the increase in pitch. Therefore, higher drying temperatures (e.g., 70–80°C) correspond to larger pitches and longer reflection wavelengths (red or orange); lower drying temperatures (e.g., 50–60°C) correspond to smaller pitches and shorter reflection wavelengths (purple or blue). In addition, covering the sample has two important functions: first, it stabilizes local humidity, preventing non-uniform solvent evaporation caused by surface airflow disturbance, thus avoiding the uneven color "coffee ring" effect; second, by reducing the evaporation rate, the pitch response to temperature changes becomes more significant and controllable, making it easier to precisely control the structural color by adjusting the drying temperature.

[0046] As one possible approach, hydroxypropyl cellulose (HPC, molecular weight approximately 40,000, viscosity range 2.0–2.9 mPa·s in a 2 wt.% aqueous solution at 20°C) was selected as the cholesteric phase material. HPC powder was slowly added to deionized water and stirred at room temperature for 12 hours until completely dissolved, preparing an aqueous solution with a mass fraction of 55–65 wt%, for example, 58%, 60%, or 62%. The resulting solution was centrifuged at 8000 rpm for 10 minutes to remove air bubbles and then allowed to stand. Polarizing microscopy revealed that the HPC aqueous solution within this concentration range exhibited a typical cholesteric phase fingerprint texture.

[0047] The PET film with the grooved microstructure array obtained in step S2 was placed in a plasma treatment chamber and treated with air plasma. The treatment conditions were: power 50W, air pressure 20Pa, and treatment time 1–3 minutes. After treatment, the water droplet contact angle decreased from approximately 75° of the original PET to below 20°, indicating a significant improvement in surface wettability.

[0048] The HPC solution was uniformly coated onto the plasma-treated grooved microstructure surface using a blade coating device. The gap between the blade and the substrate was set to 50–200 μm, and the coating speed was 5–30 mm / s. After coating, the solution rapidly filled the grooves under capillary action, forming a continuous pre-coating layer on the surface.

[0049] The sample is then allowed to stand at room temperature (25°C) for 10–30 minutes, for example, 15, 20, or 25 minutes. During this standing period, the helical axes of the cholesteric phase are regularly aligned along the surface contour of the groove under geometric confinement. This standing time is sufficient to complete the initial orientation of the cholesteric phase material without causing excessive solvent evaporation.

[0050] After settling, transfer the sample to a hot plate for drying. The drying temperature should be controlled between 50 and 80°C, for example, 55°C, 65°C, or 75°C. Cover the hot plate with a glass petri dish or metal cover to stabilize humidity and airflow near the sample. Dry for 30–60 minutes, until all moisture has evaporated, resulting in an immobilized cholesteric phase structure layer. After drying, the thickness of the cholesteric phase structure layer is approximately 5–20 μm, with the filling layer inside the grooves having a thickness roughly consistent with the groove depth, and the continuous overlay layer above the grooves having a thickness of 1–5 μm.

[0051] The structural color can be controlled by adjusting the drying temperature: at lower drying temperatures (e.g., 50–60°C), the resulting cholesteric phase structure layer has a shorter reflection peak wavelength, appearing purple or blue; at medium drying temperatures (e.g., 60–70°C), it appears green; and at higher drying temperatures (e.g., 70–80°C), the reflection peak wavelength increases, appearing red or orange. This is because the drying temperature affects the solvent evaporation rate and molecular rearrangement kinetics, ultimately determining the pitch of the cholesteric phase helical structure. The prepared cholesteric phase structure layer has a pitch range of 150–500 nm, corresponding to selective reflection in the visible light band (380–780 nm).

[0052] The cross-section of the film was observed using a field emission scanning electron microscope (SEM). The cholesteric phase structure layer was found to continuously fill the grooves and form a capping layer on the groove surface, with a clear interface and no delamination. The reflectance spectrum was measured using a UV-Vis-NIR spectrophotometer, revealing a reflectance peak at a specific wavelength, with the peak position red-shifting with increasing drying temperature. The emissivity in the 8–13 μm band was measured using a Fourier transform infrared spectrometer (FTIR), and the calculated average emissivity was greater than 0.9.

[0053] To further optimize the preparation effect, the embodiments of this application also provide the following preferred combinations of process parameters: Regarding the selection of the groove microstructure: the groove depth is 2–6 μm, the opening size is 5–10 μm, and the array period is 8–15 μm. This parameter range ensures that the groove provides sufficient geometric confinement without affecting the uniformity of the cholesteric phase material filling due to an excessively large aspect ratio.

[0054] Regarding the selection of the metal reflective layer: the metal reflective layer is a silver layer or a silver / aluminum composite layer, with a thickness of 80–150 nm. This thickness ensures an average reflectivity of over 96% in the 0.3–2.5 μm wavelength range.

[0055] Regarding the selection of the concentration of the cholesteric phase material: the mass fraction of the hydroxypropyl cellulose aqueous solution is 58-62%. Within this concentration range, the solution has a suitable viscosity, which can both stably maintain the cholesteric liquid crystal phase and facilitate coating and filling.

[0056] Regarding the choice of settling time: the settling time should be 15-25 minutes. If the settling time is too short, the initial orientation will be insufficient, and the improvement effect on the structural color angle dependence will not be obvious; if the settling time is too long, too much solvent will evaporate, which may lead to difficulties in subsequent color control.

[0057] Regarding the selection of drying temperature: The drying temperature should be determined based on the target color. For example, choose 55±5℃ for blue structural color; 65±5℃ for green; and 75±5℃ for red. Covering the container during the drying process is a necessary measure to ensure color uniformity.

[0058] To verify the superiority of the method provided by this invention, comparative experiments were conducted in the embodiments of this application: The composite optical film was prepared according to steps S1 to S4 above. Specific parameters: PET substrate layer thickness 100μm; groove depth 5μm, opening size 8μm, period 12μm, rectangular groove walls; the metal reflective layer is a silver layer with a thickness of 120nm; the cholesteric phase material is a 60wt% hydroxypropyl cellulose aqueous solution; plasma treatment for 2 minutes; after coating, stand for 20 minutes; drying temperature 65℃, covered, and dried for 45 minutes.

[0059] Comparative Example 1, the grooveless microstructure array, uses only a planar substrate, a metal reflective layer, and a cholesteric phase layer. Except for the use of a planar PET substrate and the grooveless microstructure array, the remaining steps are consistent with the method provided in this invention.

[0060] The PET substrate in Comparative Example 2 has a grooved microstructure array but no metal reflective layer is deposited; the remaining steps are consistent with the method provided in this invention.

[0061] Experimental results show that the method provided by this invention achieves an average reflectance of 96.2% in the solar band and an average emissivity of 0.93 in the atmospheric window band; the center wavelength of the structural green light reflection peak is 550 nm; the center wavelength shift of the reflection peak is approximately 10 nm when the incident angle changes from 0° to 60°, and there is no significant change in color observed by the naked eye regarding angle dependence. Comparative Example 1 shows an average reflectance of 95.8% in the solar band and an average emissivity of 0.92 in the atmospheric window band; the center wavelength of the structural green light reflection peak is 548 nm; the center wavelength shift of the reflection peak is approximately 100 nm when the incident angle changes from 0° to 60°; and a strong angle dependence, with the color changing from green to blue. Comparative Example 2 shows an average reflectance of less than 30% in the solar band; the center wavelength shift of the reflection peak is approximately 10 nm, and the sample temperature is higher than the ambient temperature under midday sunlight, making cooling impossible.

[0062] The above experiments demonstrate that the geometric confinement effect of the grooved microstructure array and the effective filling of the grooves by the cholesteric phase material to form a continuous, integrated structure covering the surface are key synergistic factors for simultaneously achieving low angle-dependent structural color and high radiative cooling performance.

[0063] In the description of this specification, specific features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

[0064] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a composite thin film with structural color and radiation cooling functions, characterized in that, include: S1: Provide a polymer film as a substrate layer; S2: A periodic array of grooved microstructures is formed on one side of the substrate layer using a roll-to-roll nanoimprinting process. The grooved microstructure array is integrally formed with the substrate layer. The cross-section of the groove is rectangular or trapezoidal, and the groove wall is a continuous curved surface or a circular arc transition structure. S3: A metal reflective layer is formed on the other side of the substrate layer using physical vapor deposition. The metal reflective layer is a continuous, dense metal film with high reflectivity in the 0.3–2.5 μm wavelength range. S4: A cholesteric phase material is introduced onto one side of the grooved microstructure array. The cholesteric phase material is then filled into the grooved microstructure array through a coating and self-assembly process. A continuous covering layer is formed inside and above the groove; wherein, the groove microstructure array has a geometric confinement effect on the self-assembly of the cholesteric phase material, so that the cholesteric phase material forms an ordered arrangement with restricted orientation within the groove space, thereby forming a cholesteric phase structure layer; the cholesteric phase structure layer has selective reflectivity in the visible light band; the cholesteric phase structure layer is composed of a polymer material with mid-infrared molecular vibration absorption characteristics, and the mid-infrared molecular vibration absorption characteristics of the polymer material work together with the groove microstructure array to give the composite optical film a high emissivity in the atmospheric window band.

2. The method for preparing a composite thin film with structural color and radiation cooling function according to claim 1, characterized in that, The groove depth of the groove microstructure array is 1–10 μm, the groove opening size is 3–15 μm, and the array period is 5–20 μm.

3. The method for preparing a composite thin film with structural color and radiation cooling function according to claim 1, characterized in that, The base layer is one or more of polyethylene terephthalate, polyethylene, polycarbonate, or polydimethylsiloxane.

4. The method for preparing a composite thin film with structural color and radiation cooling function according to claim 1, characterized in that, The metal reflective layer is a silver layer, an aluminum layer, or a composite layer thereof, with a thickness of 50–300 nm.

5. The method for preparing a composite thin film with structural color and radiation cooling function according to claim 1, characterized in that, The cholesteric phase structure layer is composed of cellulose or its derivatives, with a pitch of 150–500 nm.

6. The method for preparing a composite thin film with structural color and radiation cooling function according to claim 1, characterized in that, After the coating in step S4, allow it to stand for 10 to 30 minutes to complete the initial orientation of the cholesteric phase material, and then perform drying or curing treatment.

7. The method for preparing a composite thin film with structural color and radiation cooling function according to claim 6, characterized in that, The drying or curing process is carried out on a heating plate at 50–80°C, and a cover is placed over the sample to stabilize humidity and evaporation rate.

8. The method for preparing a composite thin film with structural color and radiation cooling function according to claim 1, characterized in that, In step S2, the roll-to-roll nanoimprinting process has a roll pressure of 0.2 to 1.0 MPa and an imprinting speed of 0.1 to 1 m / min.

9. The method for preparing a composite thin film with structural color and radiation cooling function according to claim 1, characterized in that, In step S4, the cholesteric phase material is an aqueous solution of hydroxypropyl cellulose with a mass fraction of 55-65 wt%; and before introducing the cholesteric phase material, the substrate layer with the grooved microstructure array is subjected to plasma treatment to improve wettability.

10. The method for preparing a composite thin film with structural color and radiation cooling function according to claim 1, characterized in that, The prepared composite optical thin film exhibits a low angle-dependent structural color in the visible light band and has an average emissivity greater than 0.9 in the 8–13 μm band.

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