Radiative cooling bacterial cellulose liquid crystal composite film with self-regulating solar transmittance with temperature and preparation method thereof

Abstract

The application discloses a solar transmittance temperature self-adjusting radiative cooling bacterial cellulose liquid crystal composite film and a preparation method thereof. The composite film is prepared by immersing silanized bacterial cellulose film in a prepared liquid crystal mixture and then performing field orientation and ultraviolet light irradiation curing. The whole preparation process comprises preparation of the silanized bacterial cellulose film, preparation of the liquid crystal mixture, surface spin coating and modification treatment of ITO glass, and preparation of the radiative cooling bacterial cellulose liquid crystal composite film. The radiative cooling bacterial cellulose liquid crystal composite film provided by the application has a transmittance that can be greatly adjusted in the solar spectrum range, has a high emissivity characteristic in the atmospheric window (8-13 mu m), is self-supporting and light in mass, has excellent radiative cooling and building energy saving performance, and has great application potential and market value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of building energy saving and intelligent thermal management, in particular to a solar transmittance self-adjusting radiative cooling bacterial cellulose liquid crystal composite film and a preparation method thereof. BACKGROUND

[0002] Traditional radiative cooling materials do not have the characteristic of switchable transmittance, and present a pure scattering state, which can only be used in the fields of building roofs, walls and fabrics. As the most energy-inefficient component in buildings, windows cause about 45% of energy loss. Therefore, researchers in industry and academia have explored pure transparent radiative cooling materials with high emissivity on the outside to replace traditional glass, so as to reduce the building cooling energy consumption in hot climates. However, such radiative cooling materials with high transmittance cannot adapt to variable climate environments, and have problems of unadjustable or narrow adjustable wavelength range of solar transmittance of the radiative cooling material, insufficient high infrared emissivity and poor flexibility. SUMMARY

[0003] The present application is aimed at the technical defects in the prior art, and provides a solar transmittance self-adjusting radiative cooling bacterial cellulose liquid crystal composite film.

[0004] Another object of the present application is to provide a preparation method of the radiative cooling bacterial cellulose liquid crystal composite film.

[0005] Another object of the present application is to provide an application of the radiative cooling bacterial cellulose liquid crystal composite film in a window structure of a building or a vehicle.

[0006] The technical scheme adopted to achieve the object of the present application is as follows:

[0007] The solar transmittance self-adjusting radiative cooling bacterial cellulose liquid crystal composite film comprises a PDMS encapsulating layer and a functional layer encapsulated between two PDMS encapsulating layers, the PDMS encapsulating layer is grafted with carbon-carbon double bonds by silanization, the functional layer is formed by horizontally multi-layer arranged porous bacterial cellulose and vertically arranged polymer network formed by polymerization of liquid crystal polymerizable monomers, end groups of the liquid crystal polymerizable monomers are carbon-carbon double bonds, the porous bacterial cellulose is grafted with carbon-carbon double bonds by silanization, the PDMS encapsulating layer and the porous bacterial cellulose are connected together by polymerization of the liquid crystal polymerizable monomers, and holes inside the porous bacterial cellulose are filled with small molecule liquid crystals with a smectic-chiral nematic (SmA-N * ) phase transition.

[0008] The composite film has temperature self-adapting characteristics and strong sunlight transmittance regulation ability, and presents a transparent state when the temperature is lower than the phase transition temperature (the polymer network plays an anchoring role, so that the low temperature presents a transmittance state), and when the temperature rises above the phase transition temperature, the composite film rapidly changes from a transparent state to a light scattering state. Meanwhile, the composite film has high infrared emissivity characteristics, and can achieve the effect of radiative cooling. The composite film is light in quality and has the characteristics of flexible self-supporting.

[0009] In the above technical solution, the thickness of the radiative cooling bacterial cellulose liquid crystal composite film is 80-240 μm, and the thickness of the PDMS packaging layer is 3-6 μm. The concentration of the liquid crystal polymerizable monomer and the thickness of the film are the greatest influencing factors of the film performance. The concentration of the liquid crystal polymerizable monomer is 3-6 wt.%, and the thickness of the composite film is 80-240 μm. The overall film presents excellent flexibility and lightness, and can achieve self-supporting.

[0010] The preparation method of the radiative cooling bacterial cellulose liquid crystal composite film comprises the following steps:

[0011] Step 1, preparation of a porous silanized bacterial cellulose film: a bacterial cellulose film is prepared, immersed in a deionized water-ethanol solution containing a silane coupling agent, a carbon-carbon double bond is grafted on the fiber surface through silanization (the average diameter of the cellulose before grafting the carbon-carbon double bond is 40-70 nm, and the average diameter of the cellulose after grafting the carbon-carbon double bond is 60-100 nm), washed, and freeze-dried to obtain a porous silanized bacterial cellulose film;

[0012] Step 2, preparation of a liquid crystal mixture, which comprises a small molecule liquid crystal with a smectic-chiral nematic phase transition (SmA-N * ), a liquid crystal polymerizable monomer, a chiral agent, and an ultraviolet light initiator. The end group of the liquid crystal polymerizable monomer is a carbon-carbon double bond;

[0013] Step 3, coating and modification treatment of the conductive surface of ITO conductive glass: the ITO conductive glass is cleaned, then the conductive surface is spin-coated with a PVA aqueous solution, water is evaporated, a PVA sacrificial layer is formed on the conductive surface of the ITO conductive glass, then a PDMS packaging layer is coated on the PVA sacrificial layer, and then it is placed in an air plasma machine for treatment, so that hydroxyl groups are formed on the surface of the PDMS and hydrophilicity is imparted thereto. After taking out, it is immersed in a deionized water-hydrochloric acid-hydrogen peroxide mixed solution to form hydroxyl groups on the surface of the PDMS. After taking out, it is immersed in a deionized water-ethanol solution containing a silane coupling agent to form carbon-carbon double bonds on the surface of the PDMS by silanization, and then washed to obtain the ITO conductive glass with coating modification;

[0014] Step 4, preparation of the radiative cooling bacterial cellulose liquid crystal composite film: cut the size of the silanized bacterial cellulose film into a certain size, and then immerse it in the liquid crystal mixture at 80-95℃. The liquid crystal mixture fills the pores in the silanized bacterial cellulose by capillary action. After complete immersion, two pieces of ITO conductive glass coated with the modified layer obtained in step 3 are used to clamp the film from both sides to form a liquid crystal cell, which is then cooled to room temperature. An alternating electric field is applied to the film, and after the film becomes transparent, it is irradiated with ultraviolet light to cure the internal liquid crystal polymerizable monomer. After complete curing, the liquid crystal cell is immersed in deionized water to dissolve the PVA sacrificial layer, and the film is naturally removed. After drying, the radiative cooling bacterial cellulose liquid crystal composite film is obtained.

[0015] In the above technical solution, in step 1, the strictly sterile nutrient solution and the bacterial mother liquor containing bacterial strains are injected into the culture dish, and the obtained biosynthetic bacterial cellulose film is obtained by static culture. Then, it is immersed in a 0.01-0.04wt.% sodium hydroxide solution and heated in a water bath at 70-95℃ for 8-16 hours for purification treatment to kill residual bacteria and remove the remaining nutrient solution, and then washed with deionized water to obtain a clean bacterial cellulose film. Further, the bacterial strain used is Acetobacter, Azotobacter, Rhizobium or Alcaligenes, and the nutrient solution includes yeast extract, D-mannitol, peptone, sucrose, citric acid, disodium hydrogen phosphate, potassium dihydrogen phosphate, magnesium sulfate heptahydrate and glucose.

[0016] In the above technical solution, the volume ratio of the nutrient solution to the bacterial mother liquor is (10-20):(10-20), and the culture dish has a diameter of 9-12 cm. The static culture process is as follows: the culture dish is placed in a biochemical incubator at 25-35℃, and the bacteria grow in situ for 2-4 days.

[0017] In the above technical solution, during the silanization of steps 1 and 3, the mass fraction of the silane coupling agent in the deionized water-ethanol solution is 2-4wt.%, and the deionized water-ethanol solution also contains acetic acid to adjust the pH value to weakly acidic. The volume ratio of deionized water to ethanol in the deionized water-ethanol solution is 1:1, and the immersion time for silanization is 24-48 hours.

[0018] In the above technical solution, the silane coupling agent is γ-methacryloxypropyltrimethoxysilane (KH570), vinyltrimethoxysilane (A171) or vinyltriethoxysilane (A151).

[0019] In the above technical solution, the freeze-drying process in step 1 is as follows: the sample is frozen in the refrigerator for 24-48 hours, and then freeze-dried for 24-48 hours to obtain a porous silanized bacterial cellulose film.

[0020] In the above technical solution, in step 2, the small molecule liquid crystal is one or more of the following:

[0021]

[0022] wherein R1 is an alkyl group containing 4-12 carbon atoms or an alkoxy group containing 4-10 carbon atoms, R2 is an alkyl group containing 4-8 carbon atoms, R3 is an alkyl group containing 4-8 carbon atoms, and R4 is an alkyl group containing 4-8 carbon atoms;

[0023] The liquid crystalline polymerizable monomer is one or more of the following:

[0024]

[0025] The chiral agent is:

[0026]

[0027] wherein R5 is an alkyl group containing 4-8 carbon atoms;

[0028] The ultraviolet light initiator is one or more of the following:

[0029]

[0030] The crystallization point temperature of the liquid crystal mixture is -40 to -10℃, the smectic-chiral nematic (SmA-N * ) phase transition temperature is 20-40℃, and the chiral nematic-isotropic (N * ) phase transition temperature is 70-90℃.

[0031] In the above technical solution, in step 3, the coating process of the PDMS encapsulation layer is: mixing the PDMS main agent A and the curing agent B according to a mass ratio of (5:1) to (15:1) and stirring uniformly; diluting the PDMS with n-hexane to a mass fraction of 20-30wt.%, vacuumizing at room temperature for 20-40min to remove bubbles, spin coating at a speed of 1500-2500rpm for 70-100s to form a PDMS encapsulation layer above the PVA sacrificial layer, then placing the spin coating glass into an oven and realizing the curing of the PDMS and the complete volatilization of n-hexane at 70-90℃, and obtaining a PDMS encapsulation layer of 3-6μm after 2-4 hours.

[0032] In the above technical solution, in step 3, the processing time in the air plasma machine is 5-8 min, the volume ratio of deionized water, hydrochloric acid and hydrogen peroxide in the deionized water-hydrochloric acid-hydrogen peroxide mixed solution is 5:1:1, and the soaking time in the deionized water-hydrochloric acid-hydrogen peroxide mixed solution is 5-10 min, so as to stabilize the generated hydroxyl group.

[0033] In the above technical solution, in step 4, a signal generator and a high-voltage amplifier are used in combination, and an alternating electric field is applied according to the thickness of the film.

[0034] When the ultraviolet light is irradiated and cured, the liquid crystal cell is placed at a distance of 20-30 cm from the light source under the ultraviolet light with a wavelength of 365 nm, and irradiated for 8-12 min.

[0035] The liquid crystal cell is soaked in deionized water for 24-48 hours.

[0036] Another aspect of the present application also includes the application of the radiation cooling bacterial cellulose liquid crystal composite film in the window structure of a building or a vehicle, the composite film has high infrared emissivity characteristics, and the emissivity of the composite film gradually increases with the increase of the thickness of the film, the high emissivity and the solar light modulation ability together give the film excellent radiation cooling performance, which can be applied to the window structure of a building or a vehicle to realize the effect of reducing energy consumption, and through outdoor experiments and simulation calculation and comparison with ordinary glass and low-E glass, it is verified that the radiation cooling and building energy saving characteristics are excellent.

[0037] Compared with the prior art, the present application has the following advantages:

[0038] 1. The present application proposes a radiation cooling bacterial cellulose liquid crystal composite film, which uses porous silanized bacterial cellulose as a template, fills small molecule liquid crystals with smectic phase-chiral nematic phase transition (SmA-N * ) in the template, gives the film the characteristic of switchable solar transmittance, has high emissivity in the atmospheric window (8-13 μm), is applied to the fields of radiation cooling and building energy saving, is expected to promote the rapid development of new passive radiation cooling materials, and has important research value and application prospect in the fields of building energy saving and intelligent thermal management.

[0039] 2. The solar transmittance of the radiation cooling bacterial cellulose liquid crystal composite film of the present application can be self-adjusted with the ambient temperature, and the film has excellent modulation ability in the visible-near infrared light band (380-2500 nm), which is the region where most of the solar spectrum energy exists, and this light modulation ability and high emissivity characteristics give the composite film excellent radiation cooling and building energy saving performance.

[0040] 3. The radiative cooling bacterial cellulose liquid crystal composite film of the present application can adjust the parameters such as solar light modulation ability, infrared emissivity, phase transition temperature and response speed by changing the thickness of the bacterial cellulose film, the component ratio of the liquid crystal mixture and other operations, thus having stronger environmental adaptability and broader market prospects.

[0041] 4. The radiative cooling bacterial cellulose liquid crystal composite film of the present application has excellent flexibility and self-supporting properties, is very sensitive to temperature changes in the external environment, has extremely fast response speed when reaching the phase transition temperature, and has very light overall weight, and can be applied to non-planar window structures.

[0042] 5. The radiative cooling bacterial cellulose liquid crystal composite film of the present application exhibits very excellent radiative cooling performance in outdoor experimental tests, has a very obvious cooling effect compared with ordinary glass and low-E glass; at the same time, in simulation calculations, the composite film has a significant effect of saving refrigeration energy in hot summer, which makes the composite film can be widely used in the fields of radiative cooling and building energy saving. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 It is a scanning electron microscope photograph of the cross section of the silanized bacterial cellulose film under different magnifications.

[0044] Figure 2 It is a schematic diagram of the preparation process of the radiative cooling bacterial cellulose liquid crystal composite film.

[0045] Figure 3 It is a scanning electron microscope photograph of the internal skeleton structure of the radiative cooling bacterial cellulose liquid crystal composite film.

[0046] Figure 4 It is a photograph of the radiative cooling bacterial cellulose liquid crystal composite film at different temperatures.

[0047] Figure 5 It is the solar light modulation ability and infrared emissivity curve of the composite film containing 3.5wt.% liquid crystal polymerizable monomer and 160μm.

[0048] Figure 6 It is the temperature recording curve of ordinary glass, low-E glass and the composite film in outdoor experiments. DETAILED DESCRIPTION

[0049] The present application will be further described in detail below in conjunction with specific examples. It should be understood that the specific examples described herein are only used to explain the present application and are not used to limit the present application.

[0050] Example 1

[0051] The present embodiment provides a radiative cooling bacterial cellulose liquid crystal composite film, and its flexible self-supporting performance is characterized.

[0052] The radiative cooling bacterial cellulose liquid crystal composite film is prepared by the following steps:

[0053] 1.1 Preparation of porous silanized bacterial cellulose film:

[0054] Ultrasonic dissolution of 45 g of D-mannitol, 9 g of yeast extract and 5.4 g of solid powder of proteose peptone in 1.8 L of deionized water to form a nutrient solution, pour the nutrient solution into a conical flask, and seal the bottle opening with gauze and newspaper in turn. Put the nutrient solution and clean petri dishes (Φ = 9 cm) into a vertical high-pressure steam sterilizer, sterilize for 20 min, kill bacteria, then inject 15 mL of pure nutrient solution and 15 mL of Bacillus amylovorus mother liquor into the petri dishes in a vertical flow super-clean bench. After injection, put it into a biochemical incubator and culture statically at 28℃ for 3 days. Then soak the biosynthesized bacterial cellulose film in a beaker containing 0.02wt.% sodium hydroxide solution, and put the beaker into a 90℃ water bath for 12h to kill the residual bacteria and remove the remaining nutrient solution. Then, wash with deionized water to remove residual impurities. Then, soak the clean bacterial cellulose film in an ethanol-water solution (V 去离子水 :V 乙醇 = 1:1) containing 2wt.% silane coupling agent (KH570) and 5 drops of acetic acid for 24h, and a silanization reaction occurs on the surface of the fibers, grafting carbon-carbon double bonds, to obtain a modified bacterial cellulose film. The modified bacterial cellulose film is repeatedly washed with deionized water to remove residual chemicals, and after washing, it is put into a refrigerator for 24h to freeze and ensure that the moisture inside the modified bacterial cellulose film is completely frozen. Put the frozen film into a vacuum freeze dryer for 24h to obtain a porous silanized bacterial cellulose film, and the cross-sectional scanning electron micrograph is shown in Figure 1 .

[0055] 1.2 Preparation of liquid crystal mixture:

[0056] According to the designed and calculated liquid crystal formula, 864 mg of liquid crystal small molecules, 96 mg of chiral dopant, 35 mg of liquid crystal polymerizable monomer and 5 mg of ultraviolet light initiator are weighed and poured into a brown reagent bottle, and the bottle body is completely wrapped with tin paper. Then 4g of dichloromethane is added to the bottle, and the digital ultrasonic cleaner is used for ultrasonic treatment for 30 min to dissolve the solid raw materials and form a uniform transparent liquid; the reagent bottle is placed in an oven with the opening exposed to the air, heated at 80℃ for 12h to completely evaporate the dichloromethane reagent, and after it becomes a transparent viscous liquid, it is taken out to obtain a liquid crystal mixture, which is labeled and stored.

[0057] The chemical structural formula of each component of the liquid crystal mixture used in the present embodiment is:

[0058]

[0059] The crystallization point temperature of the liquid crystal mixture is -30°C, SmA-N * The phase transition temperature is 34°C, N * The isotropic phase transition temperature is 75°C.

[0060] 1.3 Coating and modification treatment of the conductive surface of ITO conductive glass:

[0061] First, the cut ITO glass was cleaned with acetone, deionized water and anhydrous ethanol by ultrasonic cleaning, and the surface dirt was removed. A 5wt.% PVA aqueous solution was spin-coated at a speed of 2000 rpm for 90 s, and then placed in an 80°C oven for 30 min to obtain a PVA sacrificial layer with a thickness of about 2 μm. Then, the PDMS main agent A and the curing agent B (Dow Corning, USA) were mixed according to the mass ratio of 10:1, and the PDMS was diluted to 25wt.% with n-hexane reagent. Vacuum for 30 min to remove internal bubbles, then spin-coat the diluted PDMS on the PVA sacrificial layer at a speed of 2000 rpm for 90 s, then place the conductive glass in an 80°C oven for curing and completely volatilize n-hexane, and obtain an ultra-thin PDMS packaging layer after 6 h, with a thickness of about 5 μm. The ITO glass spin-coated twice was treated in an air plasma machine for 5 min to impart hydrophilicity to the PDMS, and then immediately transferred to the mixed solution (V 去离子水 :V 盐酸 :V 过氧化氢 = 5:1:1) for 10 min to stabilize the generated hydroxyl groups; then the glass was immersed in a deionized water-ethanol solution (V 去离子水 :V 乙醇 = 1:1) containing 2wt.% KH570 and 5 drops of acetic acid for 12 h to graft carbon-carbon double bonds to the surface of the PDMS; finally, the surface of the modified PDMS was cleaned with deionized water and dried for storage.

[0062] 1.4 Preparation of radiative cooling bacterial cellulose liquid crystal composite film:

[0063] First, a pre-cut porous silanized bacterial cellulose membrane was immersed in a pre-prepared liquid crystal mixture at 90°C, allowing the liquid crystal to completely fill the pores in the bacterial cellulose membrane. After the film became completely transparent, it was sandwiched between two spin-coated modified ITO glass sheets, with the conductive surfaces facing inward, forming a structure similar to a liquid crystal cell. The liquid crystal cell was then cooled to room temperature. An alternating current electric field (50Hz, square wave) was applied based on the thickness of the composite film using a signal generator and a voltage amplifier (magnification of 200x). After the liquid crystal cell became completely transparent, it was irradiated under ultraviolet light (365nm) for 10 minutes, with the film 25cm away from the light source, to initiate polymerization via ultraviolet light. Figure 2 Finally, the cured liquid crystal cell is immersed in deionized water for 48 hours to dissolve the PVA sacrificial layer. After the film naturally peels off, it is taken out, dried, and stored.

[0064] The radiation-cooled bacterial cellulose liquid crystal composite film obtained by the above preparation method has excellent flexibility and self-supporting properties, and its internal framework structure is as follows: Figure 3 As shown, it can be used in non-planar or more complex window structures, offering flexibility and lightweight design.

[0065] Example 2

[0066] This embodiment provides a radiation-cooled bacterial cellulose liquid crystal composite film and characterizes its solar transmittance and emissivity.

[0067] The radiation-cooled bacterial cellulose liquid crystal composite film is prepared by the following steps:

[0068] 2.1 Preparation of porous silanized bacterial cellulose membranes:

[0069] A 5wt.% sucrose, 1.6wt.% protein paste, 0.2wt.% citric acid, 0.2wt.% sodium phosphate dibasic, 0.3wt.% potassium phosphate monobasic and 0.03wt% magnesium sulfate heptahydrate were poured into 1.5L deionized water and ultrasonically dissolved to form a uniform nutrient solution, which was poured into a conical flask, and the mouth was sealed with gauze and newspaper in turn. The nutrient solution and clean petri dishes (Φ = 10 cm) were placed in a vertical high-pressure steam sterilizer for 25 min to kill bacteria, and then 10 mL of pure nutrient solution and 20 mL of azotobacter mother liquor were injected into the petri dishes in a vertical flow super-clean bench. After injection, it was placed in a biochemical incubator for static culture at a temperature of 30°C for 4 days. Then the biosynthesized bacterial cellulose membrane was soaked in a beaker containing 0.04wt.% sodium hydroxide solution, and the beaker was placed in a 80°C water bath for 16h to kill the residual bacteria and remove the remaining nutrient solution. Then, it was washed with deionized water to remove residual impurities. Then, the clean bacterial cellulose membrane was soaked in an ethanol-water solution (V 去离子水 :V 乙醇 = 1:1) containing 1.5wt.% silane coupling agent (KH570) and 6 drops of acetic acid for 36h, and a silanization reaction occurred on the surface of the cellulose, grafting carbon-carbon double bonds. The modified bacterial cellulose membrane was repeatedly washed with deionized water to remove residual chemicals, and after washing, it was placed in a refrigerator for 48h to freeze the water inside the bacterial cellulose membrane completely. The frozen membrane was placed in a vacuum freeze dryer for 36h to obtain a porous silanized bacterial cellulose membrane.

[0070] 2.2 Preparation of liquid crystal mixture

[0071] According to the designed and calculated liquid crystal formula, 805.5mg liquid crystal small molecules, 89.5mg chiral dopant, 100mg liquid crystal polymerizable monomer and 5mg ultraviolet light initiator were weighed and poured into a brown reagent bottle, and the bottle was completely wrapped with tin paper. Then 6g of dichloromethane was added to the bottle, and the digital ultrasonic cleaner was used for ultrasonic treatment for 20min to dissolve the solid raw materials and form a uniform transparent liquid; the reagent bottle was placed in an oven at 90°C for 8h to completely evaporate the dichloromethane reagent until it became a transparent viscous liquid, and then it was taken out to obtain a liquid crystal mixture, which was labeled and stored.

[0072] The chemical structural formula of each component of the liquid crystal mixture used in this example is:

[0073]

[0074] The crystallization point temperature of the liquid crystal mixture is -25°C, SmA-N * The phase transition temperature is 32°C, N * The isotropic phase transition temperature is 74°C.

[0075] 2.3 Coating and modification of the conductive surface of ITO conductive glass:

[0076] First, the cut ITO glass was cleaned with acetone, deionized water and anhydrous ethanol by ultrasonic cleaning, and the surface dirt was removed. A 3wt.% PVA aqueous solution was spin-coated at a speed of 1500 rpm for 100 s, and then the glass was placed in a 90°C oven for 20 min to obtain a PVA sacrificial layer with a thickness of about 1 μm. Then, PDMS precursor A and crosslinking agent B were mixed in a mass ratio of 10:1, and the PDMS was diluted to a mass fraction of 20wt.% with n-hexane reagent. Vacuum was applied for 40 min to remove internal bubbles. Then, the diluted PDMS was spin-coated on the PVA layer at a speed of 1500 rpm for 100 s. Then, the conductive glass was placed in a 90°C oven for curing and complete evaporation of n-hexane. After 4 h, an ultra-thin PDMS encapsulation layer was obtained with a thickness of about 3 μm. The ITO glass after two times of spin coating was placed in an air plasma machine for 8 min to impart hydrophilicity to the PDMS. Then, the treated glass was immediately transferred to a mixed solution (V 去离子水 :V 盐酸 :V 过氧化氢 = 4:1:1) for 5 min to stabilize the generated hydroxyl groups. Then, the glass was immersed in a deionized water-ethanol solution containing 1.5wt.% KH570 and 6 drops of acetic acid (V 去离子水 :V 乙醇 = 1:1) for 36 h to graft carbon-carbon double bonds to the PDMS surface. Finally, the surface of the modified PDMS was cleaned with deionized water and dried for storage.

[0077] 2.4 Preparation of radiation-cooled bacterial cellulose liquid crystal composite film:

[0078] First, the cut porous silanized bacterial cellulose film was immersed in a pre-prepared liquid crystal mixture at 85°C to allow the liquid crystal to completely fill the pores in the bacterial cellulose film. After the film became completely transparent, two spin-coated modified ITO glasses were used to sandwich the film with the conductive surface facing inwards, forming a structure similar to a liquid crystal cell. Then, the liquid crystal cell was cooled to room temperature. A signal generator and a voltage amplifier (amplification factor of 200) were used to apply an alternating current field (50 Hz, square wave) according to the thickness of the composite film. After the liquid crystal cell became completely transparent, it was placed under ultraviolet light (365 nm) for 15 min, with the film 30 cm away from the light source, to initiate polymerization by ultraviolet light. Finally, the cured liquid crystal cell was immersed in deionized water for 36 h to dissolve the PVA sacrificial layer. After the film naturally fell off, it was taken out and dried for storage.

[0079] 2.5 Characterization of the transmittance switchable performance and emissivity performance of the radiation-cooled bacterial cellulose liquid crystal composite film

[0080] The macroscopic states of the composite film at low temperature (30℃) and high temperature (40℃) are as follows: Figure 4 As shown in the figure. The transmittance of the composite film at low and high temperatures was characterized using a UV-Vis-NIR spectrophotometer, with a test wavelength range of 300-2500 nm. Then, the infrared emissivity of the film was measured using an infrared spectrometer, with a focus on the emissivity in the 8-13 μm wavelength range. For a composite film containing 3.5 wt.% liquid crystal polymerizable monomers and with a thickness of approximately 160 μm, its solar modulation capability and emissivity curves are shown in the figure. Figure 5 As shown, the film exhibits a solar light modulation capability of 62.65% and an infrared emissivity of 92.62%, demonstrating strong solar light modulation ability and high infrared emissivity. Furthermore, by adjusting the concentration of the liquid crystal polymerizable monomer and the film thickness parameters, the transmittance and emissivity can be flexibly adjusted.

[0081] Example 3

[0082] This embodiment provides a radiation-cooled bacterial cellulose liquid crystal composite film and applies it to the field of building energy conservation, and tests its radiation cooling performance.

[0083] The radiation-cooled bacterial cellulose liquid crystal composite film is prepared by the following steps:

[0084] 3.1 Preparation of porous silanized bacterial cellulose membranes:

[0085] 25g glucose, 5g tryptone, and 3g yeast powder were dissolved in 1L of deionized water by sonication to form a homogeneous nutrient solution. This solution was then poured into an Erlenmeyer flask, and the flask opening was sealed with gauze and then newspaper. The nutrient solution and a clean petri dish (Φ=11cm) were placed in a vertical autoclave and sterilized for 30 minutes to kill bacteria. Afterward, 20mL of pure nutrient solution and 10mL of rhizobium stock solution were injected into the petri dish in a vertical flow hood. Following injection, the dish was placed in a biochemical incubator and statically cultured at 26℃ for 3 days. The biosynthesized bacterial cellulose membrane was then immersed in a beaker containing 0.05wt.% sodium hydroxide solution and heated in a 75℃ water bath for 24 hours to kill any remaining bacteria and remove any excess nutrient solution. Finally, the membrane was washed with deionized water to remove any remaining impurities. The cleaned bacterial cellulose membrane was then soaked in an ethanol-water solution containing 3 wt.% silane coupling agent (KH570) and 3 drops of acetic acid (V). 去离子水 :V 乙醇=1:1) for 48h, the silanization reaction occurred on the surface of the cellulose, and the carbon-carbon double bond was grafted. The modified bacterial cellulose membrane was repeatedly washed with deionized water to remove residual chemicals, and after washing, it was placed in the refrigerator for 24h to freeze and ensure that the water inside the cellulose membrane was completely frozen. The frozen membrane was placed in a vacuum freeze dryer for 24h, and finally a porous silanized bacterial cellulose membrane was obtained.

[0086] 3.2 Preparation of liquid crystal mixture:

[0087] According to the designed and calculated liquid crystal formula, 1719mg of liquid crystal small molecule, 191mg of chiral dopant, 80mg of liquid crystal polymerizable monomer and 10mg of ultraviolet light initiator were weighed and poured into a brown reagent bottle, and the bottle was completely wrapped with tin paper. Then 10g of dichloromethane was added to the bottle, and the digital ultrasonic cleaner was used for ultrasonic treatment for 40min to dissolve the solid raw materials and form a uniform transparent liquid; the reagent bottle was placed in an oven with the opening exposed to heat at 85℃ for 12h to completely evaporate the dichloromethane reagent until it became a transparent viscous liquid, then it was taken out to obtain a liquid crystal mixture, which was labeled and stored.

[0088] The chemical structural formula of each component of the liquid crystal mixture used in this example is:

[0089]

[0090]

[0091] The crystallization point temperature of the liquid crystal mixture is -20℃, SmA-N * The phase transition temperature is 38℃, N * -Isotropic phase transition temperature is 82℃.

[0092] 3.3 Coating and modification treatment of the conductive surface of ITO conductive glass:

[0093] First, the cut ITO glass was cleaned with acetone, deionized water and anhydrous ethanol in sequence, and the surface dirt was removed. A PVA sacrificial layer with a thickness of about 1 μm was obtained by spin coating the glass with a 4wt.% PVA aqueous solution at a speed of 2500 rpm for 60 s and then placing the glass in an 80℃ oven for 40 min. Then, PDMS precursor A and crosslinking agent B were mixed in a mass ratio of 10:1, and the PDMS was diluted to a mass fraction of 15wt.% with n-hexane reagent. The internal bubbles were removed by vacuumizing for 20 min, and then the diluted PDMS was spin coated on the PVA layer at a speed of 2500 rpm for 60 s. The conductive glass was then placed in an 80℃ oven for curing and complete evaporation of n-hexane. After 12 h, an ultra-thin PDMS substrate was obtained as a packaging layer with a thickness of about 2 μm. The ITO glass after two spin coatings was treated in an air plasma machine for 3 min to impart hydrophilicity to the PDMS, and then the treated ITO glass was immediately transferred to a mixed solution (V 去离子水 :V 盐酸 :V 过氧化氢 = 6:1:1) for 15 min to stabilize the generated hydroxyl groups. Then, the ITO glass was immersed in a deionized water-ethanol solution (V 去离子水 :V 乙醇 = 1:1) containing 3wt.% KH570 and 3 drops of acetic acid for 48 h to graft the carbon-carbon double bond to the PDMS surface. Finally, the surface of the modified PDMS was cleaned with deionized water and dried for storage.

[0094] 3.4 Preparation of the radiative cooling bacterial cellulose liquid crystal composite film:

[0095] First, the cut porous silanized bacterial cellulose film was immersed in a pre-prepared liquid crystal mixture at 95℃, so that the liquid crystal completely filled the pores in the bacterial cellulose film. After the film became completely transparent, the film was sandwiched between two spin-coated modified ITO glasses with the conductive side facing inwards, forming a structure similar to a liquid crystal cell, and then the liquid crystal cell was cooled to room temperature. An alternating current field (50 Hz, square wave) was applied according to the thickness of the composite film using a signal generator and a voltage amplifier (amplification factor of 200). After the liquid crystal cell became completely transparent, it was placed under ultraviolet light (365 nm) for 10 min, with the film 15 cm away from the light source, and polymerization was initiated by ultraviolet light. Finally, the cured liquid crystal cell was immersed in deionized water for 48 h to dissolve the PVA sacrificial layer, and the film was taken out after natural detachment and dried for storage.

[0096] 3.5 Radiative cooling performance test of the radiative cooling bacterial cellulose liquid crystal composite film

[0097] In the summer of Tianjin, a 24-hour continuous temperature test was conducted, using three designed heat insulation boxes to test the cooling performance of ordinary glass, low-E glass and composite film. The heat insulation box is made of polystyrene foam plastic (3 cm thick) covered with an aluminum foil on the inside and outside. At noon, the cooling amplitude of the composite film in the scattering state is 16.07℃ compared with ordinary glass, and 13.64℃ compared with low-E glass Figure 6 ) At noon, the solar radiation intensity is 905W / m 2 , the relative average humidity is 32%, and the wind speed is about 0.8m / s.

[0098] 3.6 Building energy saving simulation calculation of radiative cooling bacterial cellulose liquid crystal composite film

[0099] The building energy saving performance of the composite film was tested using EnergyPlus and SketchUp software. A 12-story office building was simulated in the Turpan area of Xinjiang. Three materials were used as the window part of the building. The daily energy consumption data was recorded with a sampling interval of 1 min. The monthly energy consumption data was obtained by summing the daily energy consumption data. The composite film has better energy saving effect than ordinary glass and low-E glass from April to September. It is concluded that in the hot summer, the composite film has excellent energy saving effect compared with ordinary glass and low-E glass, which can effectively reduce the cooling energy consumption of the building and promote the realization of the grand goal of "carbon peak and carbon neutral".

[0100] The above only describes the preferred embodiments of the present application. It should be noted that for those skilled in the art, without departing from the principles of the present application, several improvements and refinements can be made, which should also be considered as the protection scope of the present application.

Claims

1. A solar light transmittance self-adjusting radiative cooling bacterial cellulose liquid crystal composite film, characterized in that, The device includes a PDMS encapsulation layer and a functional layer encapsulated between two PDMS encapsulation layers. The PDMS encapsulation layer is grafted with carbon-carbon double bonds via silanization. The functional layer is composed of horizontally arranged multilayer porous bacterial cellulose and vertically arranged polymer networks formed by polymerization of liquid crystal polymerizable monomers. The end groups of the liquid crystal polymerizable monomers are carbon-carbon double bonds. The porous bacterial cellulose is grafted with carbon-carbon double bonds via silanization. The PDMS encapsulation layer and the porous bacterial cellulose are connected together through the polymerization reaction of the liquid crystal polymerizable monomers. The pores inside the porous bacterial cellulose are filled with small molecule liquid crystals exhibiting a smectic-chiral nematic phase transition.

2. The radiative cooling bacterial cellulose liquid crystal composite film according to claim 1, wherein, The thickness of the radiation-cooled bacterial cellulose liquid crystal composite film is 80-240 μm, and the thickness of the PDMS encapsulation layer is 3-6 μm.

3. The method for preparing the radiation-cooled bacterial cellulose liquid crystal composite film as described in claim 1, characterized in that, Includes the following steps: Step 1, Preparation of porous silanized bacterial cellulose membrane: Prepare bacterial cellulose membrane by immersing it in a deionized water-ethanol solution containing silane coupling agent, grafting carbon-carbon double bonds onto the fiber surface through silanization reaction, washing, and freeze-drying to obtain porous silanized bacterial cellulose membrane. Step 2, prepare a liquid crystal mixture, wherein the liquid crystal mixture includes a small molecule liquid crystal with a smectic-chiral nematic phase transition, a liquid crystal polymerizable monomer, a chiral agent and an ultraviolet photoinitiator, wherein the end group of the liquid crystal polymerizable monomer is a carbon-carbon double bond; Step 3, Coating and modification of the conductive surface of ITO conductive glass: Clean the ITO conductive glass, then spin-coat the conductive surface with PVA aqueous solution, evaporate the water to form a PVA sacrificial layer on the conductive surface of the ITO conductive glass, then coat the PVA sacrificial layer with a PDMS encapsulation layer, and then put it into an air plasma machine for treatment to form hydroxyl groups on the PDMS surface and give it hydrophilicity. After taking it out, immerse it in a deionized water-hydrochloric acid-hydrogen peroxide mixed solution to form hydroxyl groups on the PDMS surface. After taking it out, immerse it in a deionized water-ethanol solution containing a silane coupling agent to form carbon-carbon double bonds on the PDMS surface through silanization. After washing, the coated and modified ITO conductive glass is obtained. Step 4, Preparation of radiation-cooled bacterial cellulose liquid crystal composite film: The silanized bacterial cellulose film cut to the required size is immersed in a liquid crystal mixture at 80-95℃. The liquid crystal mixture is used to fill the pores in the silanized bacterial cellulose by capillary action. After complete immersion, two pieces of coated and modified ITO conductive glass obtained in Step 3 are clamped together to form a liquid crystal cell. Then, it is cooled to room temperature, and an alternating electric field is applied to it. After the film becomes transparent, it is irradiated and cured with ultraviolet light to realize the polymerization process of the internal liquid crystal polymerizable monomers. After that, the completely cured liquid crystal cell is immersed in deionized water to dissolve the PVA sacrificial layer. The film falls off naturally and is dried to obtain the radiation-cooled bacterial cellulose liquid crystal composite film.

4. The method of claim 3, wherein the radiation-cooled bacterial cellulose liquid crystal composite film is prepared by the steps of: (a) preparing a bacterial cellulose liquid crystal composite film; (b) irradiating the bacterial cellulose liquid crystal composite film with a laser beam; and (c) cooling the bacterial cellulose liquid crystal composite film. In the step 1, the strictly sterile nutrient solution and bacterial mother liquor containing bacterial species are injected into a culture dish, and then left to culture to obtain biosynthetic bacterial cellulose film, which is then soaked in 0.01-0.04wt.% sodium hydroxide solution and purified by water bath heating at 70-95℃ for 8-16 hours to kill the residual bacteria and remove the remaining nutrient solution, and then washed with deionized water to obtain clean bacterial cellulose film, wherein the bacterial species is Acetobacter, Azotobacter, Rhizobium or Alcaligenes, and the nutrient solution comprises yeast extract, D-mannitol, proteose peptone, sucrose, citric acid, disodium hydrogen phosphate dodecahydrate, potassium dihydrogen phosphate, magnesium sulfate heptahydrate and glucose.

5. The method for preparing the radiation-cooled bacterial cellulose liquid crystal composite film as described in claim 3, characterized in that, In the silanization of the step 1 and step 3, the mass fraction of the silane coupling agent in the deionized water-ethanol solution is 2-4wt.%, and the deionized water-ethanol solution further comprises acetic acid to adjust the pH value to weak acidity, and the volume ratio of deionized water to ethanol in the deionized water-ethanol solution is 1:1, and the soaking time of silanization is 24-48 hours; The silane coupling agent is γ-methacryloxypropyltrimethoxysilane (KH570), vinyltrimethoxysilane (A171) or vinyltriethoxysilane (A151).

6. The method for preparing the radiation-cooled bacterial cellulose liquid crystal composite film as described in claim 3, characterized in that, In the step 2, the small molecule liquid crystal is one or more of the following: wherein R1 is an alkyl group containing 4-12 carbon atoms or an alkoxy group containing 4-10 carbon atoms, R2 is an alkyl group containing 4-8 carbon atoms, R3 is an alkyl group containing 4-8 carbon atoms, and R4 is an alkyl group containing 4-8 carbon atoms; The liquid crystalline polymerizable monomer is one or more of the following: The chiral agent is: wherein R5 is an alkyl group containing 4-8 carbon atoms; The ultraviolet light initiator is one or more of the following: The crystallization point temperature of the liquid crystal mixture is -40 to -10℃, the smectic-chiral nematic (SmA-N * ) phase transition temperature is 20 to 40℃, and the chiral nematic-isotropic (N * -Isotropic) phase transition temperature is 70 to 90℃.

7. The method for preparing the radiation-cooled bacterial cellulose liquid crystal composite film as described in claim 3, characterized in that, In the step 3, the coating process of the PDMS encapsulation layer is as follows: the PDMS main agent A and the curing agent B are mixed and stirred uniformly at a mass ratio of (5:1) to (15:1); the PDMS is diluted with n-hexane to a mass fraction of 20-30wt.%, vacuumized at room temperature for 20-40min to remove bubbles, spin-coated at a rotation speed of 1500-2500rpm for 70-100s to form a PDMS encapsulation layer above the PVA sacrificial layer, then the spin-coated glass is placed into an oven to realize the solidification of the PDMS and the complete volatilization of n-hexane at 70-90℃, and a PDMS encapsulation layer with a thickness of 3-6μm is obtained after 2-4 hours.

8. The method for preparing the radiation-cooled bacterial cellulose liquid crystal composite film as described in claim 3, characterized in that, In the step 3, the processing time in the air plasma machine is 5-8min, and the volume ratio of deionized water, hydrochloric acid and hydrogen peroxide in the deionized water-hydrochloric acid-hydrogen peroxide mixed solution is 5:1:1; the soaking time in the deionized water-hydrochloric acid-hydrogen peroxide mixed solution is 5-10min to stabilize the generated hydroxyl groups.

9. The method of claim 3, wherein the radiation-cooled bacterial cellulose liquid crystal composite film is prepared by the steps of: (a) preparing a bacterial cellulose liquid crystal composite film; (b) irradiating the bacterial cellulose liquid crystal composite film with a laser beam; and (c) cooling the bacterial cellulose liquid crystal composite film. In the step 4, a signal generator and a high-voltage amplifier are used to apply an alternating electric field according to the thickness of the film; When the ultraviolet light is used for irradiation and curing, the liquid crystal cell is placed at a distance of 20-30cm from the light source under the ultraviolet light with a wavelength of 365nm, and irradiated for 8-12min. The time of the liquid crystal cell soaking in deionized water is 24-48 hours.

10. Use of the radiative cooling bacterial cellulose liquid crystal composite film according to claim 1 in a window structure of a building or a vehicle.

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