Humidity-responsive color-changing radiation refrigeration material, preparation method and application thereof

Abstract

The application discloses a humidity-responsive color-changing radiation refrigeration material, which comprises a scattering layer and a rainbow photonic film arranged on the surface of the scattering layer, wherein the rainbow photonic film is a cellulose nanocrystal / polyethylene glycol composite film, and the scattering layer is a polylactic acid-silicon dioxide-aluminum oxide composite nanofiber film. The application realizes the coupling of the color mirror reflection of the CNC / PEG composite color radiation refrigeration functional layer and the broadband diffuse response of the electrospinning film reflection layer through the composite design of the top CNC / PEG rainbow photonic film and the bottom PLA / SiO2 / Al2O3 scattering layer, reduces the negative influence of moisture absorption and color development on reflectivity, and successfully combines color intelligent response and efficient radiation refrigeration. The preparation method is simple, the raw materials used are all degradable and renewable materials, the cost is low, and the application is easy to popularize and apply.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of thermal management materials, and more particularly relates to a humidity-responsive color-changing radiation refrigeration material, a preparation method therefor, and an application thereof. BACKGROUND

[0002] Daytime radiative cooling (DRC) materials as a sustainable and environmentally friendly passive refrigeration technology, the traditional design is mostly aimed at maximizing the reflection of the entire solar spectrum (including the visible light range), so such materials often have a white or mirror-like appearance. However, a white appearance is difficult to reconcile with visual aesthetics.

[0003] Common color radiation refrigerators mainly have the following strategies, such as introducing photonic crystals, nanoparticles, or introducing colorants based on optical fibers. However, the introduction of colorants may cause a decrease in refrigeration efficiency due to light absorption effects, and the color of quantum dots and dyes can easily change or fade due to oxidation or agglomeration, resulting in unstable color. The introduction of ordered photonic structures can integrate structural colors with specific visible wavelength reflection into daytime radiative cooling systems. The disadvantage of this method is that, in addition to specific reflection wavelengths, the iridescent layer usually has intrinsic transparent properties in the range of 0.25-2.5 μm (covering the ultraviolet to near-infrared band, which is also a high-energy radiation band). This property can cause some solar radiation to penetrate, and if the underlying layer lacks effective blocking, it will cause the underlying material to heat up. Therefore, a high scattering or reflection is needed below the structural color layer to promote light reflection. CN118704233A discloses a color radiation refrigeration material, which is obtained by sequentially depositing a cellulose nanocrystal upper layer with bright structural color and a long-afterglow luminescent lower layer with high reflectivity on a non-woven fabric substrate. This study not only fails to elucidate the specific influence of humidity changes on the radiation refrigeration performance, but also has a significant defect in the design of depositing long-afterglow luminescent powder on a non-woven fabric substrate. The adhesion between the powder and the substrate is insufficient, and in actual application, it is easy to fall off, directly affecting the stability and performance durability of the reflective layer. In comparison, the inorganic composite nanofiber membrane prepared by electrospinning technology is used as a scattering layer in the present application, which has strong structural integrity and fundamentally avoids the defect of powder falling off.

[0004] In addition, the environmental humidity has a significant impact on the performance of the radiation cooling material, because the cooling principle is to use the material to emit infrared radiation to the outer space through the atmospheric window (8-13 μm), thereby dissipating heat. However, the environmental humidity affects the emission characteristics of the material by changing the adsorption of moisture on the surface of the material. For example, the emissivity of the ceramic tile is as high as 0.99 in the wet state, and decreases to 0.88 after drying. This is because the presence of water can increase the roughness of the material surface and enhance the scattering effect; on the other hand, the environmental humidity can also affect the effectiveness of the atmospheric transmission window. In a humid environment, the water vapor in the atmosphere can increase the absorption of infrared radiation, resulting in a decrease in the transmittance of the atmospheric window, thereby reducing the efficiency of the radiation cooling. In addition, humidity can also indirectly affect the radiation cooling by affecting the thermal conductivity of the surrounding environment. Therefore, how to solve the impact of environmental humidity on the radiation cooling material and the efficiency loss is a difficult point in the current research of such thermal management materials. SUMMARY

[0005] The purpose of the present application is to solve the above problems, provide a humidity-responsive color-changing radiation cooling material, a preparation method and application thereof, realize the coupling of the color mirror reflection of the CNC / PEG composite color radiation cooling functional layer and the broadband diffuse response of the electrospun film reflection layer by the composite design of the top CNC / PEG iridescent photonic film and the bottom PLA / SiO2 / Al2O3 scattering layer, reduce the negative impact of moisture absorption and color development on the reflectivity, and successfully combine color intelligent response and efficient radiation cooling.

[0006] To achieve the above purpose, the present application is realized by the following technical scheme:

[0007] In a first aspect, the present application provides a humidity-responsive color-changing radiation cooling material, comprising a scattering layer and an iridescent photonic film arranged on the surface of the scattering layer, wherein the iridescent photonic film is a cellulose nanocrystal / polyethylene glycol composite film, and the scattering layer is a polylactic acid-silicon dioxide-aluminum oxide composite nanofiber film.

[0008] Further, the iridescent photonic film is deposited on the surface of the scattering layer by solution casting.

[0009] Preferably, the thickness of the scattering layer is 500 μm to 550 μm, and the thickness of the iridescent photonic film is 50 μm to 100 μm.

[0010] Preferably, the mass ratio of cellulose nanocrystals to polyethylene glycol in the iridescent photonic film is (95-75):(5-25).

[0011] Preferably, the mass ratio of polylactic acid, silicon dioxide and aluminum oxide in the scattering layer is 1:(0.05-0.5):(0.03-0.1), preferably 1:0.2:0.05.

[0012] In a second aspect, the present application further provides a preparation method of the humidity-responsive color-changing radiation refrigeration material according to the first aspect, comprising:

[0013] Preparation of polylactic acid-silica-alumina composite nanofiber membrane by electrospinning;

[0014] Preparation of cellulose nanocrystal / polyethylene glycol mixture suspension;

[0015] Uniformly casting the cellulose nanocrystal / polyethylene glycol mixture suspension on the surface of the polylactic acid-silica-alumina composite nanofiber membrane, and obtaining the humidity-responsive color-changing radiation refrigeration material after drying.

[0016] Further, the preparation of the electrospinning solution comprises:

[0017] Mixing polylactic acid, silica and alumina, adding organic solvent, and stirring until the polymer is completely dissolved and the inorganic particles are uniformly dispersed.

[0018] Further, the electrospinning solution is defoamed by ultrasonic before use.

[0019] Preferably, the parameters of the electrospinning include: voltage 5 kV ~ 15 kV, flow rate 0.1 mL / h ~ 1.5 mL / h, receiving distance 10 cm ~ 25 cm, and ambient humidity 30% ~ 80%; preferably, the parameters are: voltage 8 kV, flow rate 0.3 mL / h, receiving distance 18 cm, and ambient humidity 50% ~ 60%.

[0020] Further, the preparation of the cellulose nanocrystal / polyethylene glycol mixture suspension comprises:

[0021] Dropping the polyethylene glycol solution into the cellulose nanocrystal suspension to generate the cellulose nanocrystal / polyethylene glycol mixture suspension; wherein the solid content of the cellulose nanocrystals in the cellulose nanocrystal suspension is 1% ~ 5%.

[0022] In a third aspect, the present application further provides an application of the humidity-responsive color-changing radiation refrigeration material according to the first aspect in the preparation of a thermal management material, wherein the thermal management material includes a tent, a car cover, an outdoor tarpaulin, or an outdoor packaging material, etc.

[0023] Compared with the prior art, the present application has the following beneficial effects:

[0024] The refrigeration material has the coupling of the color mirror reflection of the CNC / PEG composite color radiation refrigeration function layer and the broadband diffusion response of the electrostatic spinning film reflection layer, reduces the negative influence of moisture absorption and color development on reflectivity, and successfully combines color intelligent response and efficient radiation refrigeration.

[0025] The present application utilizes the Bragg reflection of the chiral nematic structure of CNC to give the material bright color, avoids the absorption of sunlight by traditional colorants, and reduces the influence of sunlight on the refrigeration effect; the top color photonic film has certain sensitivity to humidity, and can produce color change according to different humidity, so as to have a humidity response intelligent color change effect; the principle is that the PEG in the photonic film absorbs moisture, causing the pitch of the CNC / PEG in the photonic film to gradually increase, and the color of the coffee ring in the center of the film gradually red shifts; when the humidity is too high, the chiral nematic structure of CNC collapses, the photonic band gap disappears, and the coating film becomes transparent; with the increase of environmental humidity, the pitch of the internal structure of the top iridescent film layer of the composite nanofiber film gradually increases, the porosity increases, and the reflectivity loss decreases, so the refrigeration effect of the composite film is actually enhanced, overcoming the defect that the efficiency of the traditional radiation refrigeration material is reduced due to the increase of environmental humidity.

[0026] The bottom nanofiber reflection layer of the present application adopts the PLA / SiO2 / Al2O3 composite nanofiber prepared by one-step electrospinning method to improve the solar spectrum reflectivity by multiple light scattering mechanisms.

[0027] The preparation method of the present application has cost advantage while realizing excellent performance, and the process is simple; the raw materials used are degradable and renewable materials, and easy to be industrialized and applied. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The reflection spectrum diagram of the iridescent film with different CNC / PEG ratios prepared in the present application embodiment one;

[0029] Figure 2 The circular dichroism spectrum diagram of the iridescent film with different CNC / PEG ratios prepared in the present application embodiment one;

[0030] Figure 3 The cross-sectional SEM diagram of the photonic film prepared in the present application embodiment one, wherein: (a) is CP 10 ; (b) is CP 11 , (c) is CP 12 ; (d) is CP 13 ; (e) is CP 14 ; (f) is CP 15 ;

[0031] Figure 4 Reflectance spectra of the iridescent fiber membranes co-assembled by pouring the white polylactic acid-silica-alumina composite nanofiber membranes prepared in Example 1 of the present application into the photonic thin films with different CNC / PEG ratios;

[0032] Figure 5 Comparison chart of the typical sample of the iridescent fiber membranes prepared in Example 1 of the present application: (a) POM chart of the photonic thin film; (b) optical photo of the photonic thin film; (c) optical photo of the iridescent fiber membrane; (d) AFM chart of the iridescent fiber membrane; (e) reflectance spectrum of the photonic thin film; (f) SEM chart of the layered structure of the iridescent fiber membrane;

[0033] Figure 6 Solar spectrum chart of the green iridescent fiber membrane CPG-PSIAL prepared in Example 1 of the present application;

[0034] Figure 7 Night cooling power of the green iridescent fiber membrane prepared in Example 1 of the present application;

[0035] Figure 8 Response test results of the iridescent fiber membranes prepared in Example 1 of the present application, in which (a) are optical photos of the CPx assembled under different environmental humidity (from left to right, CP 11 to CP 15 , respectively), (b) are ultraviolet-visible light spectra of the iridescent fiber membranes under different humidity;

[0036] Figure 9 Reflectance spectra of the photonic thin films with different CNC / PEG ratios prepared in Example 2 of the present application;

[0037] Figure 10 Reflectance spectra of the photonic thin films with different CNC / PEG ratios prepared in Example 3 of the present application;

[0038] Figure 11 Reflectance spectra of the photonic thin films with different CNC / PEG ratios prepared in Example 4 of the present application;

[0039] Figure 12 Reflectance spectrum comparison of the iridescent fiber membranes prepared in Example 1 of the present application and the color radiant refrigeration materials prepared in Comparative Examples 2 to 4;

[0040] Figure 13 Bottom real-time temperature and real-time solar radiation intensity curve comparison chart of the iridescent fiber membranes prepared in Example 1 of the present application and the color radiant refrigeration material prepared in Comparative Example 1 in the outdoor refrigeration test described in the present application. DETAILED DESCRIPTION

[0041] Traditional radiative cooling coating or film materials cannot have color because colorants absorb specific wavelengths of sunlight, so introducing color will greatly reduce the efficiency of the skin radiative cooling film material.

[0042] The optical design of the colored radiative cooling device needs to consider both the color display function and the high-efficiency heat dissipation requirement. The introduction of non-traditional colors, such as structural color (iridescent) layers, can achieve selective reflection in a specific wavelength range to present color, but the iridescent layer usually has intrinsic transparent properties in the wide spectral range of 0.25-2.5 μm (covering the ultraviolet to near-infrared wavelength range). This property will cause part of the solar radiation to penetrate, and if the underlying layer lacks effective blocking, it will cause the underlying material to absorb heat and warm up. Therefore, a high-scattering medium layer or a metal reflective substrate needs to be constructed in the lower layer of the dynamic radiative cooling (DRC) system to achieve the dual performance coupling of radiative cooling and visual color display through scattering / reflection mechanism to reflect the transmitted light to the external space twice.

[0043] The existing radiative cooling material has a significant decrease in cooling efficiency in a high-humidity environment, so the contradiction between moisture absorption, color display, and high reflectivity requirements is difficult to reconcile, resulting in performance loss.

[0044] To overcome the shortcomings and deficiencies in the prior art, the present application provides a colored radiative cooling material; the layered structure is used to achieve the coupling of color mirror reflection and wideband diffusion response, reducing the negative impact of color display on reflectivity; natural cellulose nanocrystals (CNC) are selected to produce structural color, which has significant mid-infrared radiation characteristics, low solar absorption, renewable characteristics, and excellent thermal stability. Cellulose nanocrystals can spontaneously transform from a disordered state to a left-handed cholesteric liquid crystal arrangement, thereby forming a thin film with bright structural color. The color of this film can be flexibly adjusted in the visible spectrum range by adjusting the helical period of the chiral nematic phase. Further, the hydrogen bond dynamic reorganization mechanism is used to endow the material with a cooling response under a wide range of humidity, and the internal structure is reorganized by moisture absorption, thereby providing a color-reversible iridescent film composition; the moisture-absorbing iridescent upper layer can absorb moisture in the air, reduce local humidity, and improve the efficiency of radiative cooling; at the same time, the latent heat released during the adsorption process can be dissipated through radiation. The internal structure reorganization induced by moisture absorption enhances the radiative cooling efficiency of the material in a humid environment. In addition, the synergistic effect of the nanofiber scattering layer at the bottom of the radiative cooling device and the top high-emission iridescent film can achieve continuous cooling during the day and night.

[0045] The preferred embodiments of the present application will be described in more detail below with reference to the accompanying drawings and specific examples.

[0046] The experimental methods used in the following examples are conventional methods unless otherwise specified, and the materials, reagents, etc. used are commercially available unless otherwise specified.

[0047] The raw material information used in the specific embodiments is as follows:

[0048] Cellulose nanocrystal (CNC): 200 nm ~ 400 nm, self-made in the laboratory, the preparation method is: cut medical absorbent cotton (purchased from Zhende Medical Supplies Co., Ltd.) into small pieces of 2-5 mm, treat with 5 wt% NaOH aqueous solution for 4 h, control the weight ratio of medical absorbent cotton to NaOH aqueous solution to be 1:100, and stir at room temperature; then wash the treated medical absorbent cotton thoroughly with deionized water, repeat 5 times, and wash to neutral pH, and dry the washed absorbent cotton in an oven at 60°C; under the hydrolysis condition of 64% sulfuric acid, the temperature is kept at 50°C for 2 h, and the mass ratio of treated absorbent cotton to acid is kept at 1:20; after the hydrolysis reaction is completed, 10 times of water is added for dilution and reaction stopping, and kept for several hours until the suspension is layered; the obtained suspension is centrifuged at 4000 r / min for 10 min; after the obtained centrifuged product is dialyzed to neutral in deionized water, the product is collected and freeze-dried at -60°C for 48 h to obtain cellulose nanocrystal powder.

[0049] Polyethylene glycol (PEG): Mn ~ 2000, China Pharmaceutical Shanghai Test;

[0050] Polylactic acid (PLA): Mn ~ 80000, Shanghai Mayre Biochemical Technology Co., Ltd.;

[0051] Silicon dioxide (SiO2): ~ 1 μm, Zhen Weld New Materials (Suzhou) Co., Ltd.;

[0052] Aluminum oxide (Al2O3): ~ 500 nm, Zhen Weld New Materials (Suzhou) Co., Ltd.;

[0053] Dimethyl sulfoxide (DMSO): analytical pure, Suzhou Kosarn Biological Technology Co., Ltd.;

[0054] Hexafluoroisopropanol (HFIP): 99.5%, Shanghai Maikelin Biochemical Technology Co., Ltd.

[0055] The test methods in the following examples include:

[0056] (1) Composite fiber film reflectivity test: UV-VIS-NIR spectrophotometer with integrating sphere is used to test the reflectivity of the fiber film. First, a standard white PTFE board is used to calibrate 100% reflectivity, and reflectivity higher than 100% indicates reflectivity higher than the standard white board. The test wavelength range is 0.2-2.5 μm, and the test interval is 2 nm.

[0057] (2) Emissivity characterization of composite fiber film: FTIR with an integrating sphere was used to test the emissivity of the sample film. According to Kirchhoff's law, the absorption rate of an object in thermal equilibrium is equal to the emissivity, and because the white sample has a certain thickness, the transmittance can be ignored, so the formula ε(λ) = 1 - ρ(λ) - τ(λ) can be used. Where ε(λ) is the emissivity, ρ(λ) is the reflectivity, and τ(λ) is the zero transmittance. The test wavelength range is 2.5-25 μm, and the test interval is 5 nm.

[0058] (3) Fourier infrared spectroscopy analysis: when ATR testing of the obtained sample is performed using a Thermo Scientific Nicolet iS5 infrared spectrometer, the fiber film is cut to 1 cm x 1 cm, placed on the test table, and the test hole is aligned with the test area. The scanning range is 600-4000 cm -1 , the resolution is 4 cm -1 , and the scanning is 12 times.

[0059] (4) High-resolution field emission scanning electron microscope (SEM) analysis: after the test sample is frozen and brittle fractured with liquid nitrogen, the cross section is photographed, and the cold field emission scanning electron microscope (Regulus 8230) is used to characterize the morphology of the sample. The fiber film is cut to the appropriate size and adhered to the sample table with conductive glue. After sputtering gold for 140 s in a vacuum environment, the sample is photographed.

[0060] (5) Atomic force microscope (AFM) test: the surface morphology and three-dimensional structure of the fiber film are characterized by atomic force microscope (VEECO Multimode 8).

[0061] (6) Polarizing microscope analysis (POM): a German Leica DMRX is used to analyze the sample by polarizing microscope. Under low magnification, the strong refractive property of the iridescent film and the cholesteric liquid crystal texture are observed, and under high magnification, the fingerprint texture of the cholesteric liquid crystal is observed.

[0062] (7) X-ray diffraction (XRD) analysis: the wide-angle X-ray diffraction (XRD) of the material is measured and analyzed by a Bruker BRUKER D8 instrument. The light source used in the analysis and measurement process is Cu target Kα ray radiation, λ = 1.5406 Å, voltage 40 kV, current 200 mA, scanning range 2θ = 5-90°, scanning rate 8 ° / min.

[0063] (8) Circular dichroism analysis (CD): The sample was analyzed by Jasco J-815 circular dichroism, and the CNC film sample was cut into a long strip (25 mm long and 5 mm wide). The left-handed optical properties of the cholesteric liquid crystal film were analyzed by a circular dichroism spectrometer. The CD spectrum was defined as the difference between the left-handed circularly polarized light absorption and the right-handed circularly polarized light absorption.

[0064] (9) Humidity response test: Different saturated solutions were prepared, as shown in Table 1. The sample film was placed in a sealed dry container with different humidity, and the reflectance spectrum was measured immediately after a certain period of time.

[0065] Table 1: Saturated solutions prepared in the humidity response test

[0066]

[0067] (10) Colorimetric analysis: The x, y, z of the sample film were measured by a colorimeter (Datacolor 600), and the different color coordinate values of the sample were marked in the 1931 CIE color space.

[0068] Example 1

[0069] This example provides a humidity-responsive color-changing radiative cooling material, and the preparation method thereof comprises the following steps:

[0070] S1: Preparation of organic-inorganic composite nanofiber membrane by electrospinning

[0071] At room temperature, 0.2 g of silicon dioxide, 0.05 g of aluminum oxide and 1.0 g of polylactic acid were added to a container, 10.0 g of solvent hexafluoroisopropanol was added, and stirring was carried out for 12 h to make the polymer completely dissolved and the inorganic particles uniformly dispersed, thereby preparing an electrospinning solution. Before use, defoaming was carried out for 30 min by ultrasonic.

[0072] The spinning process lasted for about 10 h, and a polylactic acid-silicon dioxide-aluminum oxide composite fiber membrane with a thickness of 0.5 mm was obtained. The electrospinning parameters were set as follows: voltage 8 kV, flow rate 0.3 mL / h, receiving distance 18 cm, and ambient humidity 50%~60%.

[0073] S2: Preparation of photonic suspension

[0074] A 5.0 wt% PEG solution was added dropwise into a CNC suspension with a solid content of 5.0%, to produce a CNC / PEG mixture suspension, wherein the mass ratio of the CNC suspension to the PEG solution was 95:5. The photonic film sample prepared from the suspension was marked as CP 11 .

[0075] In this step, a pure CNC suspension (sample marked as CP10 A suspension with a CNC / PEG ratio of 90:10 (sample labeled CP) 12 A suspension with a CNC / PEG ratio of 85:15 (sample labeled CP) 13 A suspension with a CNC / PEG ratio of 80:20 (sample labeled CP) 14 A suspension with a CNC / PEG ratio of 75:25 (sample labeled CP) 15 A suspension with a CNC / PEG ratio of 70:30 (sample labeled CP) 16 Subsequently, a certain amount of DMSO was added to the CNC / PEG suspension. After vigorous stirring of the above mixture at room temperature for 12 h, it was poured into petri dishes and allowed to slowly evaporate for about 3 days to self-assemble into an iridescent film. Based on the color of the photonic film, 95 / 5 of the CNC / PEG dosage was designated as CPB (blue), 85 / 15 as CPG (green), and 75 / 25 as CPR (red). CNC and PEG formed bright colors through self-assembly via evaporation.

[0076] The optical response of the obtained iris film (photonic thin film) in the 200-800 nm range was measured using a UV-Vis-NIR spectrometer, and its reflectance spectrum is shown in the attached figure. Figure 1 As shown. Under the same solids content CNC suspension, CP 11 To CP 15 The maximum reflection wavelength (λmax) gradually red-shifts. The chiral liquid crystal properties of the CNC / PEG iris film were analyzed using a circular dichroism spectroscopy system; its chromatogram is shown in the attached figure. Figure 2 As shown, the CD peak position of the prepared CNC / PEG iris film shifts towards longer wavelengths with increasing pitch, consistent with its Vis-NIR reflectance spectrum. Because CP 16 The prepared photonic thin film was transparent, so it was screened out in some characterization processes.

[0077] The cross-sectional morphology of photonic films with different polyethylene glycol (PEG) contents was measured by SEM to investigate the effect of the plasticizer PEG on the internal pitch of cellulose nanocrystalline photonic films, as shown in the attached figure. Figure 3 As shown, the pitch increases sequentially with increasing PEG content, which conforms to Bragg's diffraction law. These optical behaviors demonstrate that the volatile-induced self-assembly of cellulose nanocrystals forms a typical left-handed helical structure.

[0078] S3: Photonic thin film deposition

[0079] The polylactic acid-silica-alumina composite fiber membrane prepared in S1 was cut to a size of 2.0 cm x 1.5 cm, placed on a culture dish, and then 0.5 mL of the CNC / PEG mixed suspension prepared in S2 was uniformly cast on the surface of the fiber membrane. The treated composite fiber membrane was naturally dried under ambient conditions for about 2-3 days to obtain a co-assembled iridescent fiber membrane, which was denoted as CPx-PSIAL.

[0080] In this step, the CNC (CP 10 ) and different mixing ratio CNC / PEG suspensions (CP 11 -CP 16 ) were also cast on the composite nanofiber membrane obtained in step S1, respectively, to obtain iridescent fiber membranes. The reflectivity of the obtained iridescent fiber membranes was measured, and the reflectance spectra are shown in Figure 4 The iridescent fiber membrane prepared using CP 15 exhibited a relatively high reflectivity, but part of its appearance showed a transparent color.

[0081] Figure 5 The comparative diagram of the concentrated typical sample of the iridescent fiber membrane prepared in Example 1 is shown: the reflectance spectra of the composite fiber membranes prepared using blue, green and red photonic thin films were measured at normal incidence wavelength, as shown in Figure 5 (e), showing strong absorption peaks at 430 nm, 537 nm and 634 nm, respectively; the optical properties of the CNC / PEG iridescent film were tested by polarizing microscope (POM), and under orthogonal polarized light conditions, the fingerprint texture characteristic of cholesteric phase liquid crystal was observed under high magnification, as shown in Figure 5 (a); Figure 5 (b) and (c) are optical photographs of the photonic thin film prepared in S2 and the color-producing surface of the iridescent fiber membrane prepared by casting and depositing on the surface of the white fiber membrane prepared in S1, respectively, from left to right, the ratios are 95 / 5 (blue), 85 / 15 (green) and 75 / 25 (red), and with the increase of the amount of PEG, the reflection color of the photonic thin film gradually changes from purple blue to red; Figure 5 (d) shows the surface morphology of the blue, green and red iridescent fiber membranes by super deep microscope, and it is found that the surface of the thin film presents uniform nanoscale undulation, which is consistent with the self-assembly characteristics of the chiral nematic phase of CNC.

[0082] Figure 6 The solar spectrum of the green iridescent fiber membrane CPG-PSIAL prepared in this embodiment is shown in the figure, and the weighted solar waveband average reflectivity of the green iridescent fiber membrane CPG-PSIAL was measured to be 81.10%, the weighted emission rate in the full waveband was 90.90%, and in the wavelength range of 8-13 μm, it showed a weighted average emission rate of 90.48%.

[0083] Figure 7 The figure shows the real-time temperature curves of the finished iridescent fiber membranes CPx-PSIAL obtained by photonic thin film deposition with different CNC / PEG ratios prepared in this embodiment. Through simulation calculation, the net radiative cooling power of the co-assembled green iridescent composite fiber membrane CPG-PSIAL at night is approximately 105 W / m. 2 ( Figure 7 The vertical coordinate of the intersection point can be used to achieve continuous cooling day and night.

[0084] This embodiment studies the humidity response of the prepared iridescent fiber membrane by simulating different humidity levels using different saturated solutions. Figure 8 As shown in (a), CNC / PEG photonic films prepared with the same formulation under different humidity conditions show that as humidity increases, the color of the coffee ring in the center of the film gradually shifts to red. The CP was measured using a UV-Vis spectrophotometer. 12 The maximum reflection wavelength of iridescent fiber membrane under different humidity conditions, such as Figure 8 As shown in (b), as the ambient humidity gradually increases, the corresponding reflectance spectrum of the iridescent fiber membrane gradually redshifts; when the humidity is 40%, CP 12 The iridescent fiber membrane appears green, but changes color to red when placed in a 100% environment.

[0085] Example 2

[0086] This embodiment provides a humidity-responsive color-changing radiative cooling material, the preparation method of which includes the following steps:

[0087] S1: Electrospinning preparation of organic-inorganic composite nanofiber membranes

[0088] Same as Example 1.

[0089] S2: Preparation of photon suspension

[0090] A 5.0 wt% PEG solution was added dropwise to a CNC suspension with a solid content of 1.6% to produce a CNC / PEG mixture suspension, wherein the mass ratio of the CNC suspension to the PEG solution was 95:5. The photonic thin film sample prepared from this suspension was denoted as CP. 21 .

[0091] In this step, a pure CNC suspension (sample labeled CP) was also prepared. 20 A suspension with a CNC / PEG ratio of 90:10 (sample labeled CP) 22 A suspension with a CNC / PEG ratio of 85:15 (sample labeled CP) 23 A suspension with a CNC / PEG ratio of 80:20 (sample labeled CP)24 ), CNC / PEG ratio of 75:25 (sample labeled as CP 25 ); then, a certain amount of DMSO was added to the CNC / PEG suspension. After the above mixture was stirred vigorously at room temperature for 12 h, they were cast in a petri dish and slowly evaporated for about 3 days to self-assemble into the iridescent film. The optical response of the resulting photonic film at 200-800 nm was measured using a UV-vis-NIR spectrometer, and the reflectance spectrum is shown in FIG. 2. Figure 9

[0092] S3: Photonic film deposition

[0093] The same as Example One.

[0094] The weighted solar waveband average reflectance of each iridescent fiber film was measured to be 75.55%, and the weighted emissivity in the range of 8-13 μm was 80.91%.

[0095] Example 3

[0096] This example provides a humidity-responsive color-changing radiation cooling material, and the preparation method thereof includes the following steps:

[0097] S1: Preparation of organic-inorganic composite nanofiber film by electrospinning

[0098] The same as Example One.

[0099] S2: Preparation of photonic suspension

[0100] A 5.0 wt% PEG solution was dropped into a CNC suspension with a solid content of 2.5% to produce a CNC / PEG mixture suspension, wherein the mass ratio of the CNC suspension to the PEG solution was 95:5, and the photonic film sample prepared from the suspension was labeled as CP 31 .

[0101] In this step, a pure CNC suspension (sample labeled as CP 30 ), a CNC / PEG suspension with a dosage ratio of 90:10 (sample labeled as CP 32 ), a CNC / PEG suspension with a dosage ratio of 85:15 (sample labeled as CP 33 ), a CNC / PEG suspension with a dosage ratio of 80:20 (sample labeled as CP 34 ), and a CNC / PEG suspension with a dosage ratio of 75:25 (sample labeled as CP 35 ​);then, a certain amount of DMSO was added to the CNC / PEG suspension. After the above mixture was stirred vigorously at room temperature for 12 h, they were poured into a culture dish and slowly evaporated for about 3 days to self-assemble into the iridescent film. The optical response of the resulting photonic film 200-800 nm was measured using an ultraviolet (UV)-vis-NIR spectrometer, and the reflectance spectrum is shown in FIG. 4. Figure 10

[0102] S3: Photonic film deposition

[0103] The same as Example 1.

[0104] The weighted solar waveband average reflectivity of each iridescent fiber film prepared in this example was measured to be 78.91%, and the weighted emissivity in the range of 8-13 μm was 85.62%.

[0105] Example 4

[0106] This example provides a humidity-responsive color-changing radiation cooling material, and the preparation method thereof comprises the following steps:

[0107] S1: Preparation of organic-inorganic composite nanofiber film by electrospinning

[0108] The same as Example 1.

[0109] S2: Preparation of photonic suspension

[0110] A 5.0 wt% PEG solution was dropped into a CNC suspension with a solid content of 3.3% to produce a CNC / PEG mixture suspension, wherein the mass ratio of the CNC suspension to the PEG solution was 95:5, and the photonic film sample prepared from the suspension was marked as CP 41 .

[0111] In this step, a pure CNC suspension (sample marked as CP 40 ), a CNC / PEG suspension with a dosage ratio of 90:10 (sample marked as CP 42 ), a CNC / PEG suspension with a dosage ratio of 85:15 (sample marked as CP 43 ), a CNC / PEG suspension with a dosage ratio of 80:20 (sample marked as CP 44 ), and a CNC / PEG suspension with a dosage ratio of 75:25 (sample marked as CP 45 ) were prepared at the same time; then, a certain amount of DMSO was added to the CNC / PEG suspension. After the above mixture was stirred vigorously at room temperature for 12 h, they were poured into a culture dish and slowly evaporated for about 3 days to self-assemble into the iridescent film. The optical response of the resulting photonic film 200-800 nm was measured using an ultraviolet (UV)-vis-NIR spectrometer, and the reflectance spectrum is shown in FIG. 4.​Figure 11 as shown.

[0112] S3: Photonic thin film deposition

[0113] The same as Example 1.

[0114] The weighted solar waveband average reflectivity of each iridescent fiber film prepared in this example was measured to be 79.62%, and the weighted emissivity in the range of 8-13 μm was 88.75%.

[0115] According to the comparison of Figure 1 , Figure 9 , Figure 10 and Figure 11 , a relatively wide color gradient ratio was selected according to the ultraviolet reflection spectrum, and the maximum reflection peak range of the photonic thin film after blending of 5.0% CNC and 5.0% PEG with a solid content of 5.0% was 400-700 nm.

[0116] Comparative Example 1

[0117] This comparative example provides a color radiation refrigeration material, and the preparation method only includes S1 in Example 1, without depositing the cellulose nanocrystal layer (CNC layer). Other process steps are the same as Example 1. The sample is denoted as PSIAL.

[0118] Comparative Example 2

[0119] This comparative example provides a color radiation refrigeration material, and the preparation method refers to Example 1, except that the pouring amount of the CNC / PEG mixed suspension in step (3) is 0.1 mL, and other process steps are the same as Example 1.

[0120] Comparative Example 3

[0121] This comparative example provides a color radiation refrigeration material, and the preparation method refers to Example 1, except that the pouring amount of the CNC / PEG mixed suspension in step (3) is 0.3 mL, and other process steps are the same as Example 1.

[0122] Comparative Example 4

[0123] This comparative example provides a color radiation refrigeration material, and the preparation method refers to Example 1, except that the pouring amount of the CNC / PEG mixed suspension in step (3) is 0.4 mL, and other process steps are the same as Example 1.

[0124] The present application compares the solar reflectivity of the color radiation refrigeration materials prepared in Example 1 and Comparative Examples 2-4, and the reflection spectrum is as shown in Figure 12As shown in the middle, the solar reflectance of the comparative example two with the assembly liquid volume of 0.1 mL is observed to be the highest, however, at this time, the photonic film fails to completely cover the fiber film, and the appearance fails to achieve the ideal structural color effect; when the assembly liquid dosage reaches 0.5 mL, the reflectivity of the photonic film is increased instead, which is due to the gradient refractive index structure formed in the photonic crystal layer to widen the high reflection band. Therefore, the thickness of the photonic film is selected based on the basis of not affecting the refrigeration effect, so that the bright color can be assembled on the substrate, and the coverage is not easy to break.

[0125] The outdoor radiation refrigeration effect of each rainbow fiber film prepared in the example one and the colored radiation refrigeration material prepared in the comparative example one is also tested, and the real-time temperature and real-time solar radiation intensity curves of the bottom of each material are as shown in the following table. Figure 13 As shown in the table, the real-time temperature curve of the bottom of the different proportion CNC / PEG co-assembled rainbow fiber film is consistent with the change trend of the real-time solar radiation intensity, and the rainbow fiber films with different colors show different cooling effects, and the green coating composite fiber film CP 12 The average temperature difference of the PSIAL and the PSIAL (white) of the blank group comparative example 1 is the largest, followed by the red coating composite fiber film CP 14 The PSIAL, which is consistent with the solar reflectance spectrum. Compared with the PSIAL fiber film, the temperature difference between the blue coating composite fiber film and the PSIAL is the largest, reaching 4.2 ℃, the temperature difference between the red coating composite fiber film and the PSIAL is the largest, reaching 6.5 ℃, and the temperature difference between the green coating composite fiber film and the PSIAL is the largest, reaching 9.8 ℃.

[0126] In summary, the preparation and performance of three kinds of colored composite fiber films showing blue, green and red under low humidity conditions are disclosed. Tests show that the cooling of the blue coating composite fiber film after compounding can be increased by 4.2 ℃ compared with the pure bottom white fiber film, the cooling of the red coating composite fiber film can be increased by 6.5 ℃, and the cooling of the green coating composite fiber film can be increased by 9.8 ℃.

[0127] The rainbow fiber film (green coating fiber film CPG-PSIAL) prepared in the application shows a weighted reflectivity of 90.9% and a weighted emission rate of 90.48% in the full wave band, and realizes a refrigeration effect of 9.8 ℃ lower than the substrate. 2 The ideal refrigeration power at night is 105 W / m

[0128] In addition, the color composite fiber film disclosed in the present application realizes the stimulus response when the wide-range humidity (20-100% RH) changes. Due to the hygroscopic effect of PEG, the pitch of the top layer CNC photonic film expands with the increase of humidity, resulting in the red shift of its reflection wavelength. Therefore, when the environmental humidity of the material increases, the surface color gradually red shifts, and according to the outdoor temperature test results, the green and red composite fiber film has better radiation cooling effect than the blue composite fiber film. The prepared composite fiber film can further strengthen the radiation cooling effect in the transition process from low humidity to high humidity environment. At the same time, the surface color change caused by the increase of the environmental humidity of the material has intelligence, so as to realize the visual display of the environmental humidity.

[0129] The above has described the embodiments of the present application, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes are obvious to those skilled in the art without departing from the scope and technical principles of the described embodiments, and these modifications and changes should also be considered as the protection scope of the present application.

Claims

1. A humidity-responsive color-changing radiative cooling material, characterized in that, The humidity-responsive color-changing radiation cooling material comprises a scattering layer and an iridescent photonic film arranged on the surface of the scattering layer, wherein the iridescent photonic film is a cellulose nanocrystal / polyethylene glycol composite film, and the scattering layer is a polylactic acid-silica-alumina composite nanofiber film. The mass ratio of the cellulose nanocrystal to the polyethylene glycol in the iridescent photonic film is (95-75):(5-25). The mass ratio of the polylactic acid, silica and alumina in the scattering layer is 1:(0.05-0.5):(0.03-0.1).

2. The humidity-responsive, color-changing, radiative cooling material of claim 1, wherein, The iridescent photonic film is deposited on the surface of the scattering layer by solution casting.

3. The humidity-responsive, color-changing, radiative cooling material of claim 1, wherein, The thickness of the scattering layer is 500-550 μm, and the thickness of the iridescent photonic film is 50-100 μm.

4. A method of preparing the humidity-responsive, color-changing, radiative- cooling material of any one of claims 1 to 3, characterized in that, The humidity-responsive color-changing radiation cooling material comprises: The polylactic acid-silica-alumina composite nanofiber film is prepared by electrospinning. A cellulose nanocrystal / polyethylene glycol mixture suspension is prepared. The cellulose nanocrystal / polyethylene glycol mixture suspension is uniformly cast on the surface of the polylactic acid-silica-alumina composite nanofiber film, and the humidity-responsive color-changing radiation cooling material is obtained after drying.

5. The method of claim 4, wherein the humidity-responsive, color-changing, radiative cooling material is prepared by the method comprising: The preparation of the spinning solution for electrospinning comprises: The polylactic acid, silica and alumina are mixed, an organic solvent is added, and stirring is performed until the polymer is completely dissolved and the inorganic particles are uniformly dispersed.

6. The method of claim 4, wherein the humidity-responsive, color-changing, radiative cooling material is prepared by the method comprising: The parameters for electrospinning include a voltage of 5-15 kV, a flow rate of 0.1-1.5 mL / h, a receiving distance of 10-25 cm, and an ambient humidity of 30-80%.

7. The method of claim 4, wherein the humidity-responsive, color-changing, radiative cooling material is prepared by the method comprising: The preparation of the cellulose nanocrystal / polyethylene glycol mixture suspension comprises: The polyethylene glycol solution is added dropwise into the cellulose nanocrystal suspension to form the cellulose nanocrystal / polyethylene glycol mixture suspension, wherein the solid content of the cellulose nanocrystal in the cellulose nanocrystal suspension is 1-5%.

8. Use of the humidity-responsive color-changing radiation cooling material according to any one of claims 1-3 in the preparation of a heat management material, wherein the heat management material comprises a tent, a car cover, an outdoor tarpaulin or an outdoor packaging material.

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