Modified pvdf, hfp composite porous membrane and preparation method thereof

Abstract

The application belongs to the technical field of radiation refrigeration, and discloses a modified PVDF-HFP composite porous membrane and a preparation method thereof.The modified PVDF-HFP composite porous membrane comprises a substrate layer, a porous bottom layer and a dense top layer.The porous bottom layer is prepared from acetone, deionized water, a PVDF-HFP copolymer, a mixed dispersion liquid and a modified nanosheet dispersion liquid;the dense top layer is prepared from acetone, deionized water, a PVDF-HFP copolymer, synergistic microspheres and a modified nanosheet dispersion liquid;and the mixed dispersion liquid is prepared from acetone, silicon carbide, synergistic microspheres and polyvinylpyrrolidone.The introduction of the modified nanosheet dispersion liquid and the synergistic microspheres effectively improves the radiation refrigeration capacity, hydrophobicity, dimensional stability and aging resistance of the modified PVDF-HFP composite porous membrane.Therefore, the application has a wider application prospect.

Description

Technical Field

[0001] This invention belongs to the field of radiation cooling technology, specifically relating to a modified PVDF / HFP composite porous membrane and its preparation method. Background Technology

[0002] Radiative cooling materials are passive cooling materials that achieve passive cooling by reflecting sunlight and efficiently emitting infrared radiation through atmospheric windows (8-13μm band) (dissipating heat into outer space). As a zero-energy, zero-pollution passive cooling material, radiative cooling materials have been widely used in automobile roofs, vehicle exterior walls, container surfaces, photovoltaic panel surfaces, and roof and exterior wall films.

[0003] However, many problems still exist in the practical application of radiation cooling materials. For example, modified PVDF and HFP composite porous membranes, as a type of radiation cooling composite membrane, still suffer from poor daytime cooling efficiency and spectral selectivity, resulting in insufficient radiation cooling capacity; as well as poor hydrophobicity, dimensional stability, and aging resistance. Therefore, the radiation cooling capacity, hydrophobicity, dimensional stability, and aging resistance of existing modified PVDF and HFP composite porous membranes still need to be improved. Summary of the Invention

[0004] To address the shortcomings mentioned in the background section, the present invention aims to provide a modified PVDF / HFP composite porous membrane and its preparation method, thereby solving the following technical problems:

[0005] Existing modified PVDF and HFP composite porous membranes still suffer from poor radiation cooling capacity, hydrophobicity, dimensional stability, and aging resistance.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A modified PVDF / HFP composite porous membrane, wherein the modified PVDF / HFP composite porous membrane comprises a substrate layer, a porous bottom layer, and a dense top layer;

[0008] The porous substrate comprises the following raw materials in parts by weight: 110-115 parts acetone, 10-11 parts deionized water, 10-11 parts PVDF-HFP copolymer, 20-22 parts mixed dispersion, and 10-11 parts modified nanosheet dispersion.

[0009] The mixed dispersion was prepared from acetone, silicon carbide, synergistic microspheres, and polyvinylpyrrolidone.

[0010] The dense top layer comprises the following raw materials in parts by weight: 110-115 parts acetone, 10-11 parts deionized water, 10-11 parts PVDF-HFP copolymer, 0.7-0.8 parts synergistic microspheres, and 4-5 parts modified nanosheet dispersion;

[0011] The modified nanosheet dispersion is a titanium carbide MXene few-layer nanosheet dispersion that has been modified with magnesium chloride hexahydrate and ammonium fluoride and dispersed in acetone.

[0012] The enhanced microspheres are composite oxide microspheres obtained by using carbon spheres prepared from glucose as a base, modifying them sequentially with tetrabutyl titanate and zinc acetate dihydrate, and then calcining them at 500-550℃.

[0013] Preferably, the method for preparing the synergistic microspheres is as follows:

[0014] A1: Transfer the glucose aqueous solution to a high-pressure reactor and react at 180-185℃ for 6-7 hours. After cooling to room temperature, wash the mixture 3-5 times by alternating centrifugation with anhydrous ethanol and deionized water. Then disperse the precipitate in anhydrous ethanol to obtain carbon sphere ethanol dispersion.

[0015] A2: Add anhydrous ethanol 1 and acetylacetone to the carbon ball ethanol dispersion and sonicate for 30-50 min. Then, while stirring, add tetrabutyl titanate dropwise and stir for 30-40 min. Then, reflux the reaction at 70-75℃ for 6-7 h. After centrifugation and washing with anhydrous ethanol 3-5 times, disperse the precipitate in anhydrous ethanol 2 to obtain the composite microsphere dispersion.

[0016] A3: Add deionized water to the composite microsphere dispersion and stir until homogeneous. Then add zinc acetate dihydrate and hexamethylenetetramine and stir for 30-50 min. Transfer to a high-pressure reactor and react at 95-97℃ for 3-3.5 h. After cooling, centrifuge and wash the precipitate 2-3 times each with deionized water and anhydrous ethanol. After vacuum drying at 60-65℃, heat to 500-550℃ at 2℃ / min in air atmosphere and hold for 2-2.5 h. After cooling, the enhanced microspheres are obtained.

[0017] Preferably, the concentration of the glucose aqueous solution in A1 is 1-1.2 mol / L;

[0018] The concentration of the carbon sphere ethanol dispersion described in A1 is 20 g / L.

[0019] Preferably, the volume ratio of the carbon ball ethanol dispersion, anhydrous ethanol 1, acetylacetone, tetrabutyl titanate, and anhydrous ethanol 2 in A2 is 100-120:300-360:3-3.6:6-7.2:100-120.

[0020] Preferably, the mass ratio of the composite microsphere dispersion, deionized water, zinc acetate dihydrate, and hexamethylenetetramine in A3 is 80-95:300-360:1-1.2:1-1.2.

[0021] Preferably, the modified nanosheet dispersion is prepared as follows:

[0022] Under argon protection, magnesium chloride hexahydrate aqueous solution was added dropwise to the titanium carbide MXene few-layer dispersion while stirring for 30-50 min. Then, ammonium fluoride aqueous solution was added dropwise and stirred at 58-62℃ under argon atmosphere for 6-6.5 h. After cooling to room temperature, the mixture was centrifuged and washed 3-5 times with deionized water, then washed once with acetone. The precipitate was then added to acetone and ultrasonically dispersed for 15-20 min to obtain the modified nanosheet dispersion.

[0023] Preferably, the volume ratio of the titanium carbide MXene few-layer dispersion, the magnesium chloride hexahydrate aqueous solution, the ammonium fluoride aqueous solution, and acetone is 120-160:75-100:75-100:60-80.

[0024] The concentration of the titanium carbide MXene few-layer dispersion is 2.5 g / L;

[0025] The concentration of the magnesium chloride hexahydrate aqueous solution is 2 g / L;

[0026] The concentration of the ammonium fluoride aqueous solution is 1.5 g / L.

[0027] Preferably, the method for preparing the mixed dispersion is as follows:

[0028] Silicon carbide, enhanced microspheres, and polyvinylpyrrolidone were added to acetone and ultrasonically dispersed for 30-50 minutes to obtain a mixed dispersion.

[0029] Preferably, the mass ratio of acetone, silicon carbide, synergistic microspheres, and polyvinylpyrrolidone is 20-25:0.9:1.2:0.3;

[0030] The silicon carbide has a particle size of 0.5 μm.

[0031] A method for preparing a modified PVDF / HFP composite porous membrane includes the following steps:

[0032] S1: Mix acetone and deionized water, add PVDF-HFP copolymer and stir at 63-67℃ for 1-2 hours. Then add mixed dispersion and modified nanosheet dispersion and stir at 63-67℃ for 3-4 hours. After cooling, let stand for 2-3 hours, then filter with a 0.45μm filter membrane. Then coat evenly on a transparent PET substrate and dry at 25-35℃ and 65%-75% relative humidity for 24-26 hours to form a 30-40μm thick porous bottom layer.

[0033] S2: After mixing acetone and deionized water, add PVDF-HFP copolymer and stir at 63-67℃ for 1-2 hours. Then add synergistic microspheres and modified nanosheet dispersion and stir at 40-45℃ for 4-5 hours. After cooling, let stand for 4-6 hours, then filter with a 0.45μm filter membrane, and then uniformly coat it on the surface of the porous bottom layer. Finally, dry at 25-35℃ and relative humidity of 35%-45% for 24-26 hours to form a dense top layer with a thickness of 10-15μm, thus obtaining a modified PVDF and HFP composite porous membrane.

[0034] The beneficial effects of this invention are:

[0035] This invention provides a modified PVDF and HFP composite porous membrane and its preparation method. The invention effectively improves the radiation cooling capacity, hydrophobicity, dimensional stability and aging resistance of the modified PVDF and HFP composite porous membrane through the following method.

[0036] (1) In the present invention, both zinc oxide and titanium dioxide in the synergistic microspheres are wide bandgap semiconductors, with almost no absorption in the main solar radiation region and the ability to reflect most of the incident sunlight, reducing the heat absorption of the film by solar radiation; the lattice vibration of zinc oxide and the mid-infrared vibration mode of titanium dioxide have strong emission peaks in the 8-13μm atmospheric transparency window, which can efficiently dissipate the heat inside the film to the outer space in the form of infrared radiation, greatly improving the overall emissivity of the composite film in the 8-13μm range; thus simultaneously optimizing the two key links of reflected heat absorption and radiative heat dissipation, significantly improving the radiative cooling capacity of the composite film. The synergistic microspheres significantly improve the net heat dissipation power and maximum cooling range of the composite film by increasing the radiative heat dissipation and reducing the solar heat absorption. When the synergistic microspheres are dispersed in the polymer matrix, they can serve as physical crosslinking points, restricting the high-temperature movement of PVDF-HFP segments through the steric hindrance effect; at the same time, the interfacial interaction between the microspheres and PVDF-HFP can further enhance the rigidity of the matrix and reduce dimensional deformation at high temperatures. Zinc oxide and titanium dioxide are excellent UV absorbers, capable of absorbing most of the incident UV light and preventing the CC and CF bonds of PVDF-HFP from breaking due to UV radiation. The inorganic microspheres have high chemical stability, which can support the porous structure of the membrane and prevent the membrane from shrinking, cracking or collapsing due to polymer degradation during aging, thereby maintaining the stability of solar reflectivity.

[0037] (2) In the modified nanosheet dispersion of this invention, the MXene nanosheets themselves have a certain reflectivity to visible and near-infrared light. After modification with magnesium ions, their band structure is adjusted, reducing the selective absorption of solar radiation. At the same time, the "micro-nano scattering structure" formed by the nanosheets in the porous bottom layer can reflect the incident sunlight out of the membrane surface multiple times, further reducing the solar radiation absorption rate. The vibrational frequencies of the hydroxyl groups, fluorine groups and Mg-O bonds introduced by the modification on the MXene surface fall exactly in the 8-13 μm atmospheric window, which can efficiently emit heat from the membrane. The layered structure of MXene can provide a large number of infrared emission sites. When uniformly dispersed in the PVDF-HFP matrix, it will form a "continuous infrared emission layer", avoiding the problem of low emissivity caused by the small number of molecular vibration modes in a single polymer matrix. The interfacial interaction between MXene and PVDF-HFP can also suppress the infrared quenching effect of the polymer chain and improve the infrared emissivity. At the same time, the porous structure can also reduce the propagation loss of infrared light in the membrane, further improving the emission efficiency. Therefore, the addition of modified MXene can simultaneously increase radiative heat dissipation and reduce solar heat absorption, thereby significantly improving the maximum cooling range of the composite membrane. The fluoride ions introduced by ammonium fluoride modification can significantly reduce the surface energy of MXene; simultaneously, the interfacial compatibility between MXene and PVDF-HFP is improved due to the interaction between fluoride ions and the fluorine chains of PVDF-HFP, avoiding the decrease in contact angle caused by hydrophilic fillers. MXene can inhibit the thermal motion and expansion of PVDF-HFP molecular chains at high temperatures; magnesium and fluoride ions on the surface of modified MXene can form strong interfacial interactions with the CF chains and CO bonds of PVDF-HFP, reducing polymer chain slippage and creep at high temperatures, further limiting membrane dimensional deformation. The layered structure of MXene can block ultraviolet light through both absorption and scattering mechanisms; the strong interfacial interaction between modified MXene and PVDF-HFP can inhibit the migration of polymer chain degradation products during aging, reducing membrane surface cracking and pore collapse; simultaneously, magnesium ions can capture free radicals generated by photo-oxidative aging, delaying the degradation reaction.

[0038] (3) The porous structure of the porous bottom layer of this invention generates a strong light scattering effect, efficiently reflects the solar spectrum, and reduces the solar radiation absorption rate. At the same time, the porous structure can increase the specific surface area, providing more infrared emission sites for the PVDF-HFP matrix, synergistic microspheres, and modified MXene, thereby enhancing atmospheric window radiation. The dense structure of the dense top layer can prevent the pores of the porous bottom layer from being blocked by external dust and water vapor. Meanwhile, the synergistic microspheres and modified MXene in the top layer can further optimize the solar reflectivity based on the porous bottom layer. In addition, the PVDF-HFP matrix and filler in the dense top layer can supplement infrared emission and avoid the loss of emission efficiency caused by leakage radiation due to pores in the porous bottom layer. Therefore, the radiation cooling capacity of the multilayer structure is significantly better than that of a single porous layer or dense layer. The modified MXene and silicon carbide in the dense top layer can form a micro-nano rough structure, which, when superimposed with a hydrophobic matrix, increases the water contact angle of the top layer. Although the porous bottom layer does not directly contact water, the porous structure can buffer the water wetting pressure of the top layer and prevent a sudden drop in hydrophobicity due to local damage to the top layer. Both the dense top layer and the porous bottom layer are based on PVDF-HFP matrix, exhibiting good interfacial compatibility. The top layer constrains the thermal shrinkage and expansion of the bottom layer, while the porous structure of the bottom layer buffers the internal stress of the top layer, preventing dimensional fluctuations in a single structure. The dense top layer, acting as the outer layer, directly blocks ultraviolet light, water vapor, and oxygen, preventing ultraviolet degradation of the PVDF-HFP matrix in the porous bottom layer and preventing the oxidation and loss of the bottom layer filler. The titanium dioxide in the porous bottom layer absorbs a small amount of ultraviolet light penetrating the top layer, further protecting the polymer matrix. Modified MXene reduces the oxidative aging of the matrix by oxygen, thus significantly extending the aging resistance life.

[0039] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Detailed Implementation

[0040] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0041] In the description of this invention, it should be understood that the terms "opening", "upper", "lower", "thickness", "top", "middle", "length", "inner", "around", etc., which indicate orientation or positional relationship, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the components or elements referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting this invention.

[0042] Unless otherwise specified, the following information pertains to some of the raw materials used in the following embodiments and comparative examples of this invention:

[0043] Titanium carbide MXene few-layer dispersion was purchased from Xi'an Qiyue Biotechnology Co., Ltd., product number: 2353252; PVDF-HFP copolymer was purchased from Dongguan Taotao Plastic Raw Materials Co., Ltd.

[0044] Example 1: A method for preparing a modified PVDF / HFP composite porous membrane is as follows:

[0045] S1: Transfer 120 mL of 1 mol / L glucose aqueous solution to a high-pressure reactor and place the reactor in a 180°C drying oven for 6 h. After cooling to room temperature, wash the mixture three times by alternating centrifugation with anhydrous ethanol and deionized water. Then disperse the precipitate in anhydrous ethanol to obtain a carbon ball ethanol dispersion with a concentration of 20 g / L.

[0046] S2: Add 300 mL of anhydrous ethanol and 3 mL of acetylacetone to 100 mL of carbon sphere ethanol dispersion and sonicate for 30 min. Then, while stirring, add 6 mL of tetrabutyl titanate at 1 drop / s and stir for 30 min. Then, reflux the reaction at 70 °C for 6 h. After centrifugation and washing with anhydrous ethanol 3-5 times, disperse the precipitate in 100 mL of anhydrous ethanol to obtain composite microsphere dispersion.

[0047] S3: Add 300 mL of deionized water to 80 g of composite microsphere dispersion and stir evenly. Then add 1 g of zinc acetate dihydrate and 1 g of hexamethylenetetramine and stir for 30 min. Then transfer to a high-pressure reactor and react at 95 °C for 3 h. After cooling, centrifuge and wash the precipitate twice with deionized water and anhydrous ethanol. After vacuum drying at 60 °C, heat to 500 °C at 2 °C / min in air atmosphere and keep at 500 °C for 2 h. After cooling, the enhanced microspheres are obtained.

[0048] S4: Under argon protection, while stirring, 75 mL of 2 g / L magnesium chloride hexahydrate aqueous solution was added dropwise to 120 mL of titanium carbide MXene few-layer dispersion and stirred for 30 min. Then, 75 mL of 1.5 g / L ammonium fluoride aqueous solution was added dropwise and stirred at 58 °C under argon atmosphere for 6 h. After cooling to room temperature, the mixture was centrifuged and washed 3 times with deionized water, then washed once with acetone. The precipitate was then added to 60 mL of acetone and ultrasonically dispersed for 15 min to obtain modified nanosheet dispersion.

[0049] S5: Add 0.9g of silicon carbide with a particle size of 0.5μm, 1.2g of synergistic microspheres, and 0.3g of polyvinylpyrrolidone to 20g of acetone and ultrasonically disperse at 15℃ for 30min to obtain a mixed dispersion;

[0050] S6: Mix 110g of acetone and 10g of deionized water, add 10g of PVDF-HFP copolymer and stir at 63℃ for 1h. Then add 20g of mixed dispersion and 10g of modified nanosheet dispersion and stir at 63-67℃ for 3h. After cooling, let stand for 2h, then filter with a 0.45μm filter membrane. Then use a dropper to take 1.5mL and coat it evenly on a transparent PET substrate. Finally, dry at 25℃ and 65% relative humidity for 24h to form a 30μm thick porous bottom layer.

[0051] S7: Mix 110g of acetone and 10g of deionized water, add 10g of PVDF-HFP copolymer and stir at 63℃ for 1h. Then add 0.7g of synergistic microspheres and 4g of modified nanosheet dispersion and stir at 40℃ for 4h. After cooling, let stand for 4h, filter with a 0.45μm filter membrane, then use a dropper to take 1mL and coat it evenly on the surface of the porous bottom layer. Finally, dry at 25℃ and 35% relative humidity for 24h to form a 10μm thick dense top layer, and obtain the modified PVDF and HFP composite porous membrane.

[0052] Example 2: A method for preparing a modified PVDF / HFP composite porous membrane is as follows:

[0053] S1: Transfer 140 mL of 1.1 mol / L glucose aqueous solution to a high-pressure reactor and place the reactor in a 183°C drying oven for 6.5 h. After cooling to room temperature, wash the mixture four times by alternating centrifugation with anhydrous ethanol and deionized water. Then disperse the precipitate in anhydrous ethanol to obtain a carbon ball ethanol dispersion with a concentration of 20 g / L.

[0054] S2: Add 330 mL of anhydrous ethanol and 3.3 mL of acetylacetone to 110 mL of carbon sphere ethanol dispersion and sonicate for 40 min. Then, while stirring, add 6.6 mL of tetrabutyl titanate at 1 drop / s and stir for 35 min. Then, reflux the reaction at 73 °C for 6.5 h. After centrifugation and washing 4 times with anhydrous ethanol, disperse the precipitate in 110 mL of anhydrous ethanol to obtain the composite microsphere dispersion.

[0055] S3: Add 330 mL of deionized water to 87.5 g of composite microsphere dispersion and stir evenly. Then add 1.1 g of zinc acetate dihydrate and 1.1 g of hexamethylenetetramine and stir for 40 min. Then transfer to a high-pressure reactor and react at 96 °C for 3.3 h. After cooling, centrifuge and wash the precipitate twice with deionized water and anhydrous ethanol. After vacuum drying at 63 °C, heat to 530 °C at 2 °C / min in air atmosphere and keep at that temperature for 2.3 h. After cooling, the enhanced microspheres are obtained.

[0056] S4: Under argon protection, while stirring, 87 mL of 2 g / L magnesium chloride hexahydrate aqueous solution was added dropwise to 140 mL of titanium carbide MXene few-layer dispersion and stirred for 40 min. Then, 87 mL of 1.5 g / L ammonium fluoride aqueous solution was added dropwise and stirred at 60 °C under argon atmosphere for 6.3 h. After cooling to room temperature, the mixture was centrifuged and washed 4 times with deionized water, then washed once with acetone. The precipitate was then added to 70 mL of acetone and ultrasonically dispersed for 18 min to obtain modified nanosheet dispersion.

[0057] S5: Add 0.9g of silicon carbide with a particle size of 0.5μm, 1.2g of synergistic microspheres, and 0.3g of polyvinylpyrrolidone to 22.5g of acetone and ultrasonically disperse at 18℃ for 40min to obtain a mixed dispersion;

[0058] S6: Mix 112.5g of acetone and 10.5g of deionized water, add 10.5g of PVDF-HFP copolymer and stir at 65℃ for 1.5h. Then add 21g of mixed dispersion and 10.5g of modified nanosheet dispersion and stir at 65℃ for 3.5h. After cooling, let stand for 2.5h, filter with a 0.45μm filter membrane, then use a dropper to take 1.6mL and coat it evenly on a transparent PET substrate. Finally, dry at 30℃ and 70% relative humidity for 25h to form a 35μm thick porous bottom layer.

[0059] S7: Mix 112.5g of acetone and 10.5g of deionized water, then add 10.5g of PVDF-HFP copolymer and stir at 65℃ for 1.5h. Then add 0.75g of synergistic microspheres and 4.5g of modified nanosheet dispersion and stir at 43℃ for 4.5h. After cooling, let stand for 5h, then filter with a 0.45μm filter membrane. Then use a dropper to take 1.1mL and coat it evenly on the surface of the porous bottom layer. Finally, dry at 30℃ and 40% relative humidity for 25h to form a 13μm thick dense top layer, thus obtaining a modified PVDF and HFP composite porous membrane.

[0060] Example 3: A method for preparing a modified PVDF / HFP composite porous membrane is as follows:

[0061] S1: Transfer 160 mL of 1.2 mol / L glucose aqueous solution to a high-pressure reactor and place the reactor in a 185°C drying oven for 7 h. After cooling to room temperature, wash the mixture 5 times by alternating centrifugation with anhydrous ethanol and deionized water. Then disperse the precipitate in anhydrous ethanol to obtain a carbon ball ethanol dispersion with a concentration of 20 g / L.

[0062] S2: Add 360 mL of anhydrous ethanol and 3.6 mL of acetylacetone to 120 mL of carbon sphere ethanol dispersion and sonicate for 50 min. Then, while stirring, add 7.2 mL of tetrabutyl titanate at 1 drop / s and stir for 40 min. Then, reflux the reaction at 75 °C for 7 h. After centrifugation and washing 5 times with anhydrous ethanol, disperse the precipitate in 120 mL of anhydrous ethanol to obtain the composite microsphere dispersion.

[0063] S3: Add 360 mL of deionized water to 95 g of composite microsphere dispersion and stir evenly. Then add 1.2 g of zinc acetate dihydrate and 1.2 g of hexamethylenetetramine and stir for 50 min. Then transfer to a high-pressure reactor and react at 97 °C for 3.5 h. After cooling, centrifuge and wash the precipitate three times each with deionized water and anhydrous ethanol. After vacuum drying at 65 °C, heat to 550 °C at 2 °C / min in air atmosphere and keep at that temperature for 2.5 h. After cooling, the enhanced microspheres are obtained.

[0064] S4: Under argon protection, while stirring, 100 mL of 2 g / L magnesium chloride hexahydrate aqueous solution was added dropwise to 160 mL of 2.5 g / L titanium carbide MXene few-layer dispersion and stirred for 50 min. Then, 100 mL of 1.5 g / L ammonium fluoride aqueous solution was added dropwise and stirred at 62 °C under argon atmosphere for 6.5 h. After cooling to room temperature, the mixture was centrifuged and washed 5 times with deionized water, then washed once with acetone. The precipitate was then added to 80 mL of acetone and ultrasonically dispersed for 20 min to obtain the modified nanosheet dispersion.

[0065] S5: Add 0.9g of silicon carbide with a particle size of 0.5μm, 1.2g of synergistic microspheres, and 0.3g of polyvinylpyrrolidone to 25g of acetone and ultrasonically disperse at 20℃ for 50min to obtain a mixed dispersion;

[0066] S6: Mix 120g of acetone and 11g of deionized water, add 11g of PVDF-HFP copolymer and stir at 67℃ for 2h. Then add 22g of mixed dispersion and 11g of modified nanosheet dispersion and stir at 67℃ for 4h. After cooling, let stand for 3h, filter with a 0.45μm filter membrane, then use a dropper to take 1.7mL and coat it evenly on a transparent PET substrate. Finally, dry at 35℃ and 75% relative humidity for 26h to form a 40μm thick porous bottom layer.

[0067] S7: Mix 115g of acetone and 11g of deionized water, add 11g of PVDF-HFP copolymer and stir at 67℃ for 2h. Then add 0.8g of synergistic microspheres and 5g of modified nanosheet dispersion and stir at 45℃ for 5h. After cooling, let stand for 6h, filter with a 0.45μm filter membrane, then use a dropper to take 1.2mL and coat it evenly on the surface of the porous bottom layer. Finally, dry at 35℃ and 45% relative humidity for 26h to form a 15μm thick dense top layer, thus obtaining a modified PVDF and HFP composite porous membrane.

[0068] Comparative Example 1:

[0069] Compared with Example 1, this comparative example only did not add "enhancing microspheres" in the preparation processes of S5 and S7. All other steps and parameters were the same, and will not be repeated here. The final result was a modified PVDF and HFP composite porous membrane.

[0070] Comparative Example 2:

[0071] Compared with Example 1, this comparative example only omits the addition of "modified nanosheet dispersion" in the preparation processes of S6 and S7. All other steps and parameters are the same, and will not be repeated here. The final result is a modified PVDF and HFP composite porous membrane.

[0072] Comparative Example 3:

[0073] Compared with Example 1, this comparative example only did not have a "dense top layer" set on the surface of the "porous bottom layer" prepared in S6. All other steps and parameters were the same, and will not be repeated here. Finally, a modified PVDF and HFP composite porous membrane was obtained.

[0074] Comparative Example 4:

[0075] Compared with Example 1, this comparative example only omits the "porous bottom layer" between the "dense top layer" prepared in S7 and the transparent PET substrate layer. All other steps and parameters are the same, and will not be repeated here. The final result is a modified PVDF and HFP composite porous membrane.

[0076] Performance testing:

[0077] Measurement of radiative cooling capacity:

[0078] Solar spectral reflectance: Diffuse reflectance spectra were measured using a high-precision UV-Vis-NIR spectrophotometer (Shimadzu UV-3600i Plus, Japan) with an integrating sphere attachment, and the average solar reflectance (%) of the modified PVDF and HFP composite porous membranes prepared in Examples 1-3 and Comparative Examples 1-4 of this invention in the wavelength range of 250-2500 nm was calculated. The results are shown in Table 1.

[0079] Atmospheric window emissivity: The infrared spectral emissivity in the 8-13 μm atmospheric window band was measured using a Fourier transform infrared spectrometer with a gold-plated integrating sphere (ThermoScientific, model Nicolet iS50, USA), and the atmospheric window emissivity (%) of the modified PVDF and HFP composite porous membranes prepared in Examples 1-3 and Comparative Examples 1-4 of this invention at the 8-13 μm atmospheric window was calculated. The measurement results are shown in Table 1.

[0080] Maximum cooling range: Referring to GB / T 39715-2021 standard, modified PVDF and HFP composite porous membranes with a size of 10cm×10cm, prepared in Examples 1-3 and Comparative Examples 1-4 of this invention, were tightly bonded to a foam board and then placed together with a reference board of the same size (a black aluminum plate with an emissivity of 0.94 and a solar absorptivity of 0.91) under an irradiance of 1000W / m². 2 The sample chamber (a box with an open top and polyurethane foam on the sides and bottom) was used to monitor the surface temperature change simultaneously using an infrared thermal imager and a temperature sensor (placed 1m outside the sample chamber). Under the conditions of 25℃, 40% relative humidity and 0.2m / s wind speed, the maximum difference between the sample surface temperature and the ambient temperature, i.e. the maximum cooling amplitude (℃), was recorded. The measurement results are shown in Table 1.

[0081] Table 1: Detection results of radiative cooling capacity of Examples 1-3 and Comparative Examples 1-4

[0082]

[0083] Measurement of water contact angle:

[0084] Referring to GB / T 30693-2014 standard, the water contact angle (°) of the dense top layer (comparative example 3, porous bottom layer) of the modified PVDF and HFP composite porous membranes prepared in Examples 1-3 and Comparative Examples 1-4 of this invention was measured using a contact angle tester (Xiamen Chongda Intelligent Technology Co., Ltd., model JGW-360D). The measurement results are shown in Table 2.

[0085] Determination of dimensional stability:

[0086] Referring to GB / T 1036-2008 standard, the sum of the length and width changes (%) of the modified PVDF and HFP composite porous membranes prepared in Examples 1-3 and Comparative Examples 1-4 of this invention after being treated at 80℃ for 24h and then cooled to the point of reheating was measured. The results are shown in Table 2.

[0087] Determination of aging resistance:

[0088] Referring to GB / T 16422.2-2014 standard, the modified PVDF / HFP composite porous membranes with a size of 10cm × 10cm prepared in Examples 1-3 and Comparative Examples 1-4 of this invention were tested after irradiation at 550W / m². 2 The solar spectral reflectance retention rate (%) after aging treatment with wavelength of 300-400nm, temperature of 60℃, and relative humidity of 85% for 1000h (a cycle of 102min of light exposure + 18min of water spraying) is shown in Table 2.

[0089] Table 2: Performance test results of Examples 1-3 and Comparative Examples 1-4

[0090]

[0091] Data Analysis:

[0092] As can be seen from Tables 1 and 2, the modified PVDF and HFP composite porous membranes prepared in the embodiments of the present invention have excellent radiative cooling capacity, hydrophobicity, dimensional stability, and aging resistance.

[0093] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0094] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A modified PVDF / HFP composite porous membrane, characterized in that, The modified PVDF and HFP composite porous membrane includes a base layer, a porous bottom layer, and a dense top layer; The porous substrate comprises the following raw materials in parts by weight: 110-115 parts acetone, 10-11 parts deionized water, 10-11 parts PVDF-HFP copolymer, 20-22 parts mixed dispersion, and 10-11 parts modified nanosheet dispersion. The mixed dispersion was prepared from acetone, silicon carbide, synergistic microspheres, and polyvinylpyrrolidone. The dense top layer comprises the following raw materials in parts by weight: 110-115 parts acetone, 10-11 parts deionized water, 10-11 parts PVDF-HFP copolymer, 0.7-0.8 parts synergistic microspheres, and 4-5 parts modified nanosheet dispersion; The modified nanosheet dispersion is a titanium carbide MXene few-layer nanosheet dispersion that has been modified with magnesium chloride hexahydrate and ammonium fluoride and dispersed in acetone. The enhanced microspheres are composite oxide microspheres obtained by using carbon spheres prepared from glucose as a base, modifying them sequentially with tetrabutyl titanate and zinc acetate dihydrate, and then calcining them at 500-550℃.

2. The modified PVDF / HFP composite porous membrane according to claim 1, characterized in that, The method for preparing the synergistic microspheres is as follows: A1: Transfer the glucose aqueous solution to a high-pressure reactor and react at 180-185℃ for 6-7 hours. After cooling, centrifuge and wash, then disperse the precipitate in anhydrous ethanol to obtain carbon sphere ethanol dispersion. A2: Add anhydrous ethanol 1 and acetylacetone to the carbon ball ethanol dispersion and sonicate for 30-50 min. Then, while stirring, add tetrabutyl titanate dropwise and stir for 30-40 min. Then, reflux at 70-75℃ for 6-7 h. After centrifugation and washing of the precipitate, disperse the precipitate in anhydrous ethanol 2 to obtain the composite microsphere dispersion. A3: Add deionized water to the composite microsphere dispersion and stir well. Then add zinc acetate dihydrate and hexamethylenetetramine and stir for 30-50 min. Then transfer to a high-pressure reactor and react at 95-97℃ for 3-3.5 h. After cooling, centrifuge to separate and wash the precipitate, vacuum dry, and finally heat to 500-550℃ in air atmosphere and keep at that temperature for 2-2.5 h. After cooling, the enhanced microspheres are obtained.

3. The modified PVDF / HFP composite porous membrane according to claim 2, characterized in that, The concentration of the glucose aqueous solution described in A1 is 1-1.2 mol / L; The concentration of the carbon sphere ethanol dispersion described in A1 is 20 g / L.

4. The modified PVDF / HFP composite porous membrane according to claim 2, characterized in that, The volume ratio of the carbon sphere ethanol dispersion, anhydrous ethanol 1, acetylacetone, tetrabutyl titanate, and anhydrous ethanol 2 in A2 is 100-120:300-360:3-3.6:6-7.2:100-120.

5. The modified PVDF / HFP composite porous membrane according to claim 2, characterized in that, The mass ratio of the composite microsphere dispersion, deionized water, zinc acetate dihydrate, and hexamethylenetetramine described in A3 is 80-95:300-360:1-1.2:1-1.

2.

6. The modified PVDF / HFP composite porous membrane according to claim 1, characterized in that, The modified nanosheet dispersion is prepared as follows: Under argon protection, magnesium chloride hexahydrate aqueous solution was added dropwise to the titanium carbide MXene few-layer dispersion while stirring for 30-50 min. Then, ammonium fluoride aqueous solution was added dropwise and stirred for 6-6.5 h at 58-62 °C under argon atmosphere. After cooling, the precipitate was separated by centrifugation and washed. The precipitate was then added to acetone and ultrasonically dispersed for 15-20 min to obtain the modified nanosheet dispersion.

7. The modified PVDF / HFP composite porous membrane according to claim 6, characterized in that, The volume ratio of the titanium carbide MXene few-layer dispersion, magnesium chloride hexahydrate aqueous solution, ammonium fluoride aqueous solution, and acetone is 120-160:75-100:75-100:60-80. The concentration of the titanium carbide MXene few-layer dispersion is 2.5 g / L; The concentration of the magnesium chloride hexahydrate aqueous solution is 2 g / L; The concentration of the ammonium fluoride aqueous solution is 1.5 g / L.

8. The modified PVDF / HFP composite porous membrane according to claim 1, characterized in that, The preparation method of the mixed dispersion is as follows: Silicon carbide, enhanced microspheres, and polyvinylpyrrolidone were added to acetone and ultrasonically dispersed for 30-50 minutes to obtain a mixed dispersion.

9. The modified PVDF / HFP composite porous membrane according to claim 8, characterized in that, The mass ratio of acetone, silicon carbide, synergistic microspheres, and polyvinylpyrrolidone is 20-25. 0.9：1.2：0.3； The silicon carbide has a particle size of 0.5 μm.

10. A method for preparing a modified PVDF / HFP composite porous membrane according to any one of claims 1-9, characterized in that, Includes the following steps: S1: Mix acetone and deionized water, add PVDF-HFP copolymer and stir at 63-67℃ for 1-2 hours. Then add mixed dispersion and modified nanosheet dispersion and stir at 63-67℃ for 3-4 hours. After cooling, let stand for 2-3 hours, then filter with a 0.45μm filter membrane. Then coat evenly on a transparent PET substrate and dry at 25-35℃ and 65%-75% relative humidity for 24-26 hours to form a 30-40μm thick porous bottom layer. S2: After mixing acetone and deionized water, add PVDF-HFP copolymer and stir at 63-67℃ for 1-2 hours. Then add synergistic microspheres and modified nanosheet dispersion and stir at 40-45℃ for 4-5 hours. After cooling, let stand for 4-6 hours, then filter with a 0.45μm filter membrane, and then uniformly coat it on the surface of the porous bottom layer. Finally, dry at 25-35℃ and relative humidity of 35%-45% for 24-26 hours to form a dense top layer with a thickness of 10-15μm, thus obtaining a modified PVDF and HFP composite porous membrane.

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