Core-shell structure difunctional radiation cooling fiber membrane as well as preparation method and application thereof

Abstract

The invention discloses a core-shell structure difunctional radiation cooling fiber membrane as well as a preparation method and application thereof, and belongs to the technical field of radiation cooling. The preparation method comprises the following steps: 1) dissolving dianhydride and a diacid monomer in an organic solvent for reaction to obtain polyamide acid, adding a dehydrating agent and a catalyst to obtain a mixed solution, adding the mixed solution into a non-solvent to obtain a fibrous polymer, and drying to obtain a shell polymer polyimide; 2) dissolving polyimide in an organic solvent to prepare an electrostatic spinning shell solution; dissolving a phase change material in an organic solvent to prepare an electrostatic spinning nuclear layer solution; and 3) drafting the electrostatic spinning shell layer solution and the core layer solution under 10-25kV by adopting a coaxial electrostatic spinning technology to obtain the core-shell structure difunctional radiation cooling fibrous membrane. The method solves the problems that the existing radiation cooling material is easy to violently heat and cool due to external environment factors, and the temperature fluctuation cannot be actively regulated and controlled.

Description

Technical Field

[0001] The present invention belongs to the technical field of radiative cooling, and particularly relates to a core-shell structured dual-functional radiative cooling fiber membrane, a preparation method thereof, and an application thereof. Background Art

[0002] Refrigeration is an indispensable core technology in fields such as ensuring thermal comfort, equipment stability, transportation and preservation, etc. It not only improves the quality of human life, but also promotes the progress of multiple fields such as industry, medicine, and information technology, showing extensive and profound scientific and social values. However, current traditional refrigeration technologies rely on fossil energy. It is expected that by 2050, the global refrigeration electricity demand will account for 20% of the total electricity consumption, and the direct carbon emissions are expected to account for 45% of the equivalent carbon emissions, which is not conducive to the goal of global decarbonization. In order to address this challenge, it is urgent to develop energy-saving, environmentally friendly, and high-performance cooling strategies to replace traditional cooling strategies.

[0003] Currently, researchers have produced a large number of materials with excellent radiative cooling performance by combining structural photon engineering and material property design, including biomimetic array thin films, hierarchical porous coatings, ceramics, etc. However, in practical applications, due to the complexity of environmental and weather changes, relying solely on a single radiative cooling material to reach a new thermal equilibrium may lead to drastic temperature increases and decreases due to insufficient heat insulation performance or a high heat transfer coefficient. Organisms and precision equipment have strict requirements for temperature changes, which poses a certain challenge to ensuring a thermally comfortable space. At the same time, in order to ensure a stable thermally comfortable environment, external equipment needs to be used to provide energy support, which will undoubtedly generate more energy consumption. This phenomenon is common in single-functional radiative cooling materials such as coatings, thin films, and fabrics, and is specifically manifested as a drastic fluctuation (>5 - 10 °C) in the temperature of the radiative cooler with the solar irradiance intensity in a short period of time. Therefore, it is extremely challenging to rely solely on single-functional radiative coolers to meet the requirements of thermal comfort and energy conservation.

[0004] Currently, for materials with radiative cooling performance such as traditional fiber structures, the heat continuously absorbed from sunlight and the environment through conduction and convection will cause the rapid temperature rise of the radiative cooler to reach the final thermal equilibrium. On the one hand, although its high reflection and high emission optical properties can reduce the final thermal equilibrium temperature, the temperature rise process is extremely rapid, and a large amount of heat will be transferred to the internal environment with temperature requirements in the form of heat conduction during this period, resulting in the deviation of the internal environment temperature from the original temperature. In order to avoid this phenomenon, more energy is required for refrigeration. On the other hand, when there are cloudy days, the solar irradiance intensity drops sharply, but due to the high emission characteristics of the material, heat will still be continuously emitted. At this time, the temperature of the radiative cooler will be lower than the internal environment temperature, resulting in the outward conduction of the internal environment temperature. Summary of the Invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the object of the present invention is to provide a core-shell structure dual-functional radiative cooling fiber membrane and its preparation method and application, so as to overcome the problem that the existing radiative cooling materials are prone to drastic temperature rise and fall due to external environmental factors and cannot actively regulate temperature fluctuations.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: The present invention discloses a core-shell structure dual-functional radiative cooling fiber membrane and its preparation method, including the following steps: 1) Dissolve dianhydride monomers and diamine monomers in an organic solvent to react to obtain polyamic acid, add a dehydrating agent and a catalyst to react to obtain a mixed solution, pour the mixed solution into a non-solvent to obtain a fibrous polymer, and obtain a shell polymer polyimide after drying; the mass ratio of the dianhydride and diamine monomers to the organic solvent is 1:(3 - 5), and the volume ratio of the mixed solution to the non-solvent > 1:10; 2) Dissolve polyimide in an organic solvent to prepare an electrospinning shell solution, and the mass ratio of the polyimide to the organic solvent is 1:(2 - 5); dissolve a phase change material in an organic solvent to prepare an electrospinning core solution, and the mass ratio of the phase change material to the organic solvent is (5 - 10):1; wherein, the phase change material includes one of polyethylene glycol, lauric acid and paraffin; 3) Adopt coaxial electrospinning technology to stretch the electrospinning shell solution and the core solution at a voltage of 10 - 25 kV to obtain a core-shell structure dual-functional radiative cooling fiber membrane.

[0007] In the preparation process of the above technical solution, the selected shell material and phase change material (polyethylene glycol, lauric acid or paraffin) have an important impact on the finally formed core-shell structure and radiative cooling performance. There are differences in the optical properties of different shell materials, which directly affect the solar reflectance, mid-infrared emissivity and radiative cooling efficiency of the shell. Different phase change materials have different enthalpy values and phase change temperature ranges, which directly affect the thermal shock resistance of the composite fiber material. Specifically, the greater the concentration of the shell material, the greater its viscosity, and the greater the voltage required for electrospinning. However, too large or too small a concentration will also have an adverse impact on the preparation of the fiber membrane. Because if the concentration is too large, spinning cannot be carried out, and if the concentration is too small, the formed composite droplets are unstable, and the phase change material in the core layer cannot be completely coated by the shell layer, which will affect the radiative cooling and phase change performance of the fiber membrane. In addition, the optical properties of the shell material itself are the key to determining its radiative cooling performance, but if the shell layer is too thin, its radiative cooling performance will decrease. The phase change material mainly has phase change performance. It absorbs heat from solid to liquid and releases energy from liquid to solid. Therefore, the core-shell structure is used to encapsulate the phase change material to prevent the phase change material from leaking out in the form of liquid during the phase change process. After the fiber membrane of the present invention has the phase change material, its heating and cooling processes will become smoother and will not rise and fall sharply.

[0008] In the above technical solution, the non-solvent in step 1) refers to a substance that cannot dissolve the solute but can be infinitely miscible with the solvent. Its function is to precipitate the fibrous polymer due to the decrease in solubility; the volume ratio of the mixed solution to the non-solvent needs to be greater than 1:10, and the non-solvent needs to be replaced multiple times, that is, the mixing reaction of the mixed solution and the non-solvent needs to be carried out cyclically multiple times. The non-solvent is replaced once per cycle, and the cycle is carried out once every six hours for a total of 4 cycles. The purpose is to remove the organic solvent, catalyst and dehydrating agent in the original solution. The principle is that the organic solvent will be infinitely miscible with the non-solvent, and the organic solvent will tend to penetrate into the non-solvent, eventually causing phase separation of the solute in the original solution. In addition, the coaxial electrospinning technology in step 3) must be carried out under a high voltage of 10-25 kV. The principle is to stretch the spinning solution by electrostatic force to form nanofibers. If the voltage is too small, the viscosity between the solutions cannot be overcome.

[0009] Preferably, in step 1), the molar ratio of the dianhydride to the diamine monomer is 0.8-1.2. The dianhydride monomer includes one of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, and the diamine monomer includes one of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, 2,2-bis[4-(4-aminophenoxy)benzene]hexafluoropropane and 2,2'-bis(trifluoromethyl)diaminobiphenyl.

[0010] More preferably, to gel the reaction solution and prevent rapid heat accumulation caused by the one-time addition of the dianhydride monomer, the dianhydride monomer should be added in batches.

[0011] Preferably, in step 1), the volume ratio of the dehydrating agent to the catalyst is 1:3, the dehydrating agent is acetic anhydride, and the catalyst is a pyridine solution.

[0012] Preferably, in steps 1) and 2), the organic solvent includes one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone, and the non-solvent includes one of deionized water, methanol, and ethanol.

[0013] Preferably, in step 3), the distance between the needle of the coaxial electrospinning technique and the receiving platform is 5-15 cm, the feeding rate is 0.5-2 mL / h, and the spinning temperature is 30-50 °C.

[0014] In the above technical solution, the main parameters for the fiber membrane of the present invention to form a core-shell structure are the shell layer concentration, the core layer concentration, the static voltage, and the feeding rate. Among them, the shell layer concentration determines whether it can completely cover the core layer solution, so usually the shell layer viscosity is greater than that of the core layer. The core layer feeding rate mainly affects the diameter of the core layer in the core-shell fiber structure. The faster the feeding rate, the larger the core layer diameter, the more energy can be absorbed and released, the slower the heating and cooling rates, and the more gentle the heating and cooling degrees. However, too large a feeding rate will also cause leakage of the core layer phase change material and damage to the core-shell structure.

[0015] Preferably, in step 1), the mixed solution of the dianhydride and diamine monomers and the organic solvent is fully reacted in a water bath at 0-10 °C for 5-20 h at a mechanical stirring rate of 200-500 r / min, and then fully reacted at room temperature for 15-40 h after adding the dehydrating agent and the catalyst; the drying temperature is 60-120 °C.

[0016] Preferably, in step 2), the phase change material and the spinning aid are dissolved in the organic solvent to prepare an electrospinning core layer solution, and the mass ratio of the phase change material to the spinning aid is (5-15):1; the spinning aid includes one of polyvinylpyrrolidone, polyvinyl alcohol, polyethylene oxide, and N-methylpyrrolidone.

[0017] In the above technical solution, different spinning aids have different molecular compatibilities and solution viscosities, which directly affect the stretching process of the electrostatic force during the spinning process and determine whether a strong core-shell microstructure can be formed to avoid leakage of the phase change material.

[0018] More preferably, in step 1), the volume of the non-solvent is 2-3 L.

[0019] Further preferably, the organic solvent is 3 - 8 times the total mass of the phase change material and the spinning aid.

[0020] The present invention also discloses a core - shell structured dual - functional radiative cooling fiber membrane prepared by the preparation method of the above - mentioned core - shell structured dual - functional radiative cooling fiber membrane. The core - shell structured dual - functional radiative cooling fiber membrane has a core - shell structure, and the core layer diameter is 232 nm - 786 nm.

[0021] In the above - mentioned technical solution, the core layer diameter of 232 nm - 786 nm ensures that, on the premise of not reducing the visible light reflectivity and mid - infrared emissivity of the fiber membrane, as much phase change material as possible is coated, so that it has a larger enthalpy. In the face of scenarios of sudden temperature drop and rapid temperature rise, it has a stronger ability to resist thermal shock.

[0022] Further preferably, the core layer diameter is 786 nm.

[0023] The present invention optimizes the size of the core layer structure to 786 nm, and introduces a higher content of phase change material as much as possible on the premise of ensuring radiative cooling performance and no liquid leakage. This enables it to absorb and release more heat to alleviate the temperature change of the internal environment in the face of a complex and changing external environment, effectively realizing the anti - thermal shock performance.

[0024] Further preferably, the solar reflectivity of the core - shell structured dual - functional radiative cooling fiber membrane is 90.2 - 97.5%, the mid - infrared emissivity is 82.1 - 92.7%, the thermal conductivity is 0.054 - 0.091 W·m -1 ·K -1 , and the enthalpy value is 18.2 - 124.3 J / g.

[0025] The present invention also discloses the application of the above - mentioned core - shell structured dual - functional radiative cooling fiber membrane in radiative cooling.

[0026] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a method for preparing a core-shell structured bifunctional radiative cooling fiber membrane. According to the mechanism of radiative cooling, the polyimide molecular chain is designed to improve its solar reflectivity and mid-infrared emissivity. Specifically, by introducing trifluoromethyl functional groups into diamine monomers, the generation of charge transfer complexes can be effectively inhibited, achieving effective decolorization. Due to the strong electronegativity of F, the trifluoromethyl group exhibits a strong electron-withdrawing effect, which can effectively inhibit the charge transfer ability of the benzene ring in the diamine monomer as an acceptor and a donor, reducing the formation of charge transfer complexes, thereby endowing the fiber shell layer with high solar reflectivity and high mid-infrared emissivity optical properties. Decolorization is mainly related to the visible light reflectivity and mid-infrared emissivity of the material. Different polyimides appear yellow, which causes them to absorb yellow light in sunlight in the visible light range. However, after decolorization in the present invention, the fluorinated polyimide appears white and reflects all visible light, thus reducing heat absorption. At the same time, the absorption wavelength of the C-F bond in the trifluoromethyl group coincides with the atmospheric transparent window, which can improve its mid-infrared emissivity and emit more heat into the deep space of the universe, reducing the temperature more. In addition, through the coaxial electrospinning technique, the phase change material is encapsulated inside the shell polymer polyimide, endowing the fiber membrane with the characteristic of high latent heat. When it is subjected to an external thermal shock, the heat is stored as latent heat, avoiding rapid temperature changes, and the strong core-shell structure is beneficial to enhancing its shape stability and avoiding performance degradation caused by liquid leakage. In the heat transfer of the core-shell structured bifunctional radiative cooling fiber membrane, through the characteristic of high latent heat of the phase change material, the heat absorbed from the outside is stored, the temperature of the phase change layer remains constant, and the heat is continuously conducted downward to the non-phase change layer. During this period, the high emission characteristic of the shell layer still continuously emits heat outward, which weakens the rate of heat transfer downward, and the temperature change at the bottom is small until the energy storage reaches the upper limit. When the external energy source is lost, the heat stored at the top is preferentially released, the temperature of the bottom phase change layer remains constant, reducing the temperature difference between the inside and the outside until all the heat is released, which effectively slows down the sharp temperature drop. It can be seen that the key advantage of the heat transfer of the core-shell structured bifunctional radiative cooling fiber membrane is to actively regulate temperature fluctuations through the dynamic storage / release of latent heat of phase change. In summary, the present invention cleverly utilizes the core-shell structure of nanofibers to realize the introduction of phase change materials. On the one hand, it effectively alleviates the problems of short-term overcooling and overheating of the cooler caused by external environmental factors; on the other hand, the core-shell nanofiber structure can effectively encapsulate the phase change material, preventing problems such as leakage during the phase change process. This research enriches the design inspiration and theoretical support for multifunctional and integrated radiative cooling materials, opening up new possibilities for their applications in the fields of building energy conservation, equipment stability, transportation and preservation, etc.

[0027] Furthermore, the catalyst is a pyridine solution, which promotes the occurrence of chemical imidization to convert fluorinated polyamic acid into fluorinated polyimide. Water is generated during cyclization, and acetic anhydride is a dehydrating agent. Removing water enables the reaction to proceed in the forward direction.

[0028] Furthermore, the organic solvent is selected from one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone. These substances are all highly polar aprotic solvents with strong dissolving capabilities, which can effectively dissolve dianhydride monomers, diamine monomers, and polyimide. The non-solvent is selected from one of deionized water, methanol, and ethanol because deionized water, methanol, and ethanol are all polar solvents with a large difference in solubility parameters from the organic solvent of polyamic acid, which can quickly trigger phase separation to form fibrous polymers.

[0029] Furthermore, during coaxial electrospinning, by adjusting parameters such as the spinning voltage, the distance between the needle and the receiving platform, the feeding rate, and the spinning temperature, the stretching speed and morphology of the shell solution and the core solution can be controlled, thereby controlling the core-shell fiber structure.

[0030] Furthermore, by controlling the mechanical stirring rate (200 - 500 r / min) and the low-temperature water bath (0 - 10 °C), the monomers can be uniformly dispersed and slowly polymerized at the initial stage of the reaction, reducing the occurrence of side reactions. Such conditions help the polyamic acid precursor to form a polymer chain with a uniform molecular weight distribution and regular structure, providing a basis for the subsequent dehydration and ring closure to form the polyimide shell. The design of the staged reaction (5 - 20 hours at low temperature and 15 - 40 hours at room temperature) not only ensures the full synthesis of the polyamic acid precursor but also forms a highly stable polyimide structure through dehydration and ring closure at room temperature. The selection of the drying temperature (60 - 120 °C) can effectively remove the residual solvent while avoiding the destruction of the core-shell structure of the fiber by high temperature. The moderate temperature can maintain the uniformity of the fiber diameter, thus ensuring the balance between the radiation cooling function (high reflectivity and emissivity) and the heat insulation performance.

[0031] Furthermore, the purpose of vacuum degassing is to remove the air in the spinning solution. If there are bubbles during the electrospinning process, the formed fibers will be discontinuous, affecting the fiber morphology and the final properties of the material.

[0032] Furthermore, the spinning aid is mainly used to improve the viscosity of the phase change material so that it can form fibers under the action of electrostatic force. The main reason is that the phase change material itself is as thin as water and has almost no viscosity, so it cannot form fibers. If the phase change material is directly used as the core layer solution, a stable core-shell structure cannot be formed, and even the shell layer will rupture, leading to a decline in the radiation cooling performance of the fiber membrane.

[0033] The present invention also provides a core-shell structured bifunctional radiative cooling fiber membrane prepared by the above preparation method. The core layer has a diameter of 232 nm - 786 nm. This core layer diameter ensures that, without reducing the visible light reflectivity and mid-infrared emissivity of the fiber membrane, as much phase change material as possible is coated, enabling it to have a greater enthalpy, so that the fiber membrane has a stronger ability to resist thermal shock in the face of scenarios of sudden temperature drops and rapid temperature rises.

[0034] The present invention also provides the application of the above core-shell structured bifunctional radiative cooling fiber membrane in radiative cooling. When facing an external environment with drastic temperature increases or decreases, the temperature fluctuations can be actively regulated through the shell layer structure and the core layer structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a scanning image of the core-shell structured bifunctional radiative cooling fiber membrane of the present invention; Figure 2 is an EDS image of the core-shell structured bifunctional radiative cooling fiber membrane of the present invention; Figure 3 is a transmission image of the core-shell structured bifunctional radiative cooling fiber membrane with different structures of the present invention; Figure 4a is a reflectivity spectrum of the FPL0.8 fiber membrane of the present invention in the range of 0.25 - 2.5 μm; Figure 4b is an emissivity spectrum of the FPL0.8 fiber membrane of the present invention in the range of 2.5 - 20 μm; Figure 4c is a weighted average reflectivity (0.25 - 2.5 μm) and weighted average emissivity (2.5 - 20 μm) spectrum of the FPL0.8 fiber membrane of the present invention; Figure 5 is a DSC curve of the core-shell structured bifunctional radiative cooling fiber membrane of the present invention after the first and 100th cycles; Figure 6 is the radiative cooling performance of the core-shell structured bifunctional radiative cooling fiber membrane of the present invention under simulated environment; Figure 7 is the radiative cooling performance of the core-shell structured bifunctional radiative cooling fiber membrane of the present invention under outdoor environment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0037] It should be noted that the terms "first", "second", etc. in the description, claims and above-mentioned drawings of the present invention are used to distinguish similar objects, and do not necessarily have to be used to describe a specific order or sequence. It should be understood that such data used can be interchanged under appropriate circumstances, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device comprising a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0038] The present invention will be further described in detail below in conjunction with embodiments: Embodiment 1 Step 1: First, dissolve 10.2 g of 2,2'-bis(trifluoromethyl)diaminobiphenyl in 125.3 g of N,N-dimethylacetamide solution in an ice water bath at 0 °C with a mechanical stirrer speed of 500 r / min. At the same time, introduce argon into the three-necked flask until completely dissolved. Add 14.2 g of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride in 8 batches, with an interval of 10 min each time. After the feeding is completed, continue to stir for 12 h, then add 16.8 g of acetic anhydride and 6.3 g of pyridine, and continue to stir at room temperature for 24 h to obtain a transparent viscous solution. Then slowly pour the transparent viscous solution into a beaker containing 2 L of methanol solution, and quickly stir with a glass rod during this period to obtain fibrous solids. Dry in an oven at 80 °C to obtain fibrous polymers. Then take 5 g of dry fibrous solids and dissolve them in 20 g of N,N-dimethylformamide to obtain a transparent viscous shell solution with a solid content of 20%, and store it refrigerated for later use. Dissolve 10 g of lauric acid and 1 g of N-methylpyrrolidone in 4 g of N,N-dimethylformamide at 50 °C to obtain a core solution.

[0039] Step 2: After the shell and core solutions obtained in Step 1 are respectively subjected to vacuum degassing treatment at 30 °C, connect them to an 18 / 25G coaxial spinning needle of an electrospinning machine using a hose and then carry out continuous spinning. Among them, the high-voltage spinning voltage is 19 kV, the distance between the needle and the receiving platform is 13 cm, the shell feeding rate is 0.19 mL / h, and the core feeding rate is 0.45 mL / h, and the spinning temperature is 30 °C. Under the action of electrostatic force, the composite solution is stretched, and the organic solvent quickly volatilizes to form composite nanofibers with a core-shell structure. The entire spinning lasts for 12 h.

[0040] The core layer diameter of the fiber membrane prepared in this example is 786 nm, the solar reflectance is 96.7%, the mid-infrared emissivity is 89.6%, and the thermal conductivity is 0.061 W·m -1 ·K -1 , and the enthalpy value is 114.2 J / g.

[0041] Example 2 Step 1: First, dissolve 13.7 g of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene in 83.6 g of N,N-dimethylacetamide solution in an ice water bath at 0 °C with a mechanical stirrer speed of 500 r / min. At the same time, introduce argon into the three-necked flask until it is completely dissolved. Add 7.2 g of 1,2,4,5-cyclohexanetetracarboxylic dianhydride in 6 batches, with an interval of 15 min each time. After the addition is complete, continue stirring for 12 h, then add 15.5 g of acetic anhydride and 5.9 g of pyridine, and continue stirring at room temperature for 24 h to obtain a transparent viscous solution. Then slowly pour the transparent viscous solution into a beaker containing 2 L of deionized water, and quickly stir it with a glass rod during this period to obtain fibrous solids. Dry them in an oven at 80 °C to obtain fibrous polymers. Then take 5 g of dry fibrous solids and dissolve them in 15 g of N,N-dimethylformamide to obtain a transparent viscous shell layer solution with a solid content of 25%, and store it in the refrigerator for later use. Dissolve 12 g of lauric acid and 0.8 g of N-methylpyrrolidone in 5 g of N,N-dimethylformamide at 50 °C to obtain a core layer solution.

[0042] Step 2: After vacuum degassing the shell layer and core layer solutions obtained in Step 1 at 30 °C respectively, connect them to an 18 / 25G coaxial spinning needle of an electrospinning machine using a hose and then carry out continuous spinning. Among them, the high-voltage spinning voltage is 20 kV, the distance between the needle and the receiving platform is 12 cm, the shell layer feeding rate is 0.15 mL / h, and the core layer feeding rate is 0.51 mL / h, and the spinning temperature is 30 °C. Under the action of electrostatic force, the composite solution is stretched, and the organic solvent quickly volatilizes to form composite nanofibers with a core-shell structure. The entire spinning lasts for 12 h.

[0043] The core layer diameter of the fiber membrane prepared in this example is 613 nm, the solar reflectance is 94.3%, the mid-infrared emissivity is 88.6%, and the thermal conductivity is 0.078 W·m -1 ·K -1 , and the enthalpy value is 101.5 J / g.

[0044] Example 3 Step 1: First, dissolve 16.5 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane in 83.6 g of N,N-dimethylacetamide solution in an ice water bath at 0 °C with a mechanical stirrer speed of 500 r / min. At the same time, introduce argon gas into the three-necked flask until it is completely dissolved. Add 8.7 g of 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride in batches 4 times, with an interval of 15 min each time. After the addition is complete, continue stirring for 12 h, then add 16.3 g of acetic anhydride and 6.2 g of pyridine, and continue stirring at room temperature for 24 h to obtain a transparent viscous solution. Then slowly pour the transparent viscous solution into a beaker containing 2 L of ethanol, and quickly stir with a glass rod during this process to obtain fibrous solids. Dry them in an oven at 80 °C to obtain fibrous polymers. Then take 5 g of dry fibrous solids and dissolve them in 15 g of N,N-dimethylformamide to obtain a transparent viscous shell solution with a solid content of 25%, and store it refrigerated for later use. Dissolve 9 g of lauric acid and 0.75 g of N-methylpyrrolidone in 5 g of N,N-dimethylformamide at 50 °C to obtain a core solution.

[0045] Step 2: After subjecting the shell solution and the core solution obtained in Step 1 to vacuum degassing treatment at 30 °C respectively, connect them to an 18 / 25G coaxial spinning needle of an electrospinning machine using a hose and then conduct continuous spinning. Among them, the high voltage of the spinning voltage is 16 kV, the distance between the needle and the receiving platform is 10 cm, the pushing rate of the shell layer is 0.18 mL / h, and the pushing rate of the core layer is 0.72 mL / h, and the spinning temperature is 30 °C. Under the action of electrostatic force, the composite solution is stretched, and the organic solvent quickly volatilizes to form composite nanofibers with a core-shell structure. The entire spinning process lasts for 12 h.

[0046] The core layer diameter of the fiber membrane prepared in this example is 512 nm, the solar reflectance is 91.3%, the mid-infrared emissivity is 91.1%, and the thermal conductivity is 0.066 W·m -1 ·K -1 , and the enthalpy value is 98.2 J / g.

[0047] Example 4 Step 1: First, dissolve 16.5 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane in 83.6 g of N,N-dimethylacetamide solution in an ice water bath at 0 °C with a mechanical stirrer speed of 500 r / min. Meanwhile, introduce argon gas into the three-necked flask until it is completely dissolved. Add 14.2 g of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride in 8 batches, with an interval of 10 min each time. After the addition is complete, continue stirring for 12 h, then add 18.2 g of acetic anhydride and 8.4 g of pyridine, and continue stirring at room temperature for 24 h to obtain a transparent viscous solution. Then slowly pour the transparent viscous solution into a beaker containing 3 L of methanol, and quickly stir it with a glass rod during this process to obtain fibrous solids. Dry the fibrous solids in an oven at 80 °C to obtain fibrous polymers. Then take 10 g of the dried fibrous solids and dissolve them in 30 g of N,N-dimethylformamide to obtain a transparent viscous shell solution with a solid content of 25%, and store it refrigerated for later use. Dissolve 10 g of lauric acid and 1.5 g of N-methylpyrrolidone in 5.5 g of N,N-dimethylformamide at 50 °C to obtain a core solution.

[0048] Step 2: After subjecting the shell solution and the core solution obtained in Step 1 to vacuum degassing treatment at 30 °C respectively, connect them to an 18 / 25G coaxial spinning needle of an electrospinning machine using a hose and perform continuous spinning. Among them, the high voltage of the spinning voltage is 18 kV, the distance between the needle and the receiving platform is 12 cm, the pushing rate of the shell layer is 0.2 mL / h, and the pushing rate of the core layer is 0.66 mL / h, and the spinning temperature is 30 °C. Under the action of electrostatic force, the composite solution is stretched, and the organic solvent quickly volatilizes to form composite nanofibers with a core-shell structure. The entire spinning process lasts for 12 h.

[0049] The core layer diameter of the fiber membrane prepared in this example is 402 nm, the solar reflectance is 95.1%, the mid-infrared emissivity is 85.4%, and the thermal conductivity is 0.72 W·m -1 ·K -1 , and the enthalpy value is 72.6 J / g.

[0050] Example 5 Step 1: First, dissolve 13.7 g of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene in 83.6 g of N,N-dimethylformamide solution in an ice water bath at 5 °C with a mechanical stirrer speed of 200 r / min. At the same time, introduce argon gas into the three-necked flask until it is completely dissolved. Add 7.2 g of 1,2,4,5-cyclohexanetetracarboxylic dianhydride in 6 batches, with an interval of 15 min each time. After the addition is complete, continue stirring for 5 h, then add 15.5 g of acetic anhydride and 5.9 g of pyridine, and continue stirring at room temperature for 15 h to obtain a transparent viscous solution. Then slowly pour the transparent viscous solution into a beaker containing 2 L of deionized water, and quickly stir with a glass rod during this period to obtain fibrous solids. Dry them in an oven at 60 °C to obtain fibrous polymers. Then take 5 g of the dried fibrous solids and dissolve them in 10 g of N,N-dimethylformamide to obtain a transparent viscous shell solution with a solid content of 33%, and store it in the refrigerator for later use. Dissolve 25 g of polyethylene glycol and 0.8 g of polyvinylpyrrolidone in 5 g of N,N-dimethylformamide at 50 °C to obtain a core solution.

[0051] Step 2: After vacuum degassing the shell and core solutions obtained in Step 1 at 20 °C respectively, connect them to an 18 / 25G coaxial spinning needle of an electrospinning machine using a hose and then perform continuous spinning. Among them, the high voltage of the spinning voltage is 5 kV, the distance between the needle and the receiving platform is 5 cm, the pushing rate of the shell layer is 0.15 mL / h, and the pushing rate of the core layer is 1.25 mL / h, and the spinning temperature is 30 °C. Under the action of electrostatic force, the composite solution is stretched, and the organic solvent quickly volatilizes to form composite nanofibers with a core-shell structure. The entire spinning lasts for 12 h.

[0052] The core layer diameter of the fiber membrane prepared in this example is 754 nm, the solar reflectance is 95.3%, the mid-infrared emissivity is 88.2%, and the thermal conductivity is 0.055 W·m -1 ·K -1 , and the enthalpy value is 103.9 J / g.

[0053] Example 6 Step 1: First, dissolve 13.7 g of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene in 83.6 g of N-methylpyrrolidone in an ice water bath at 10 °C with a mechanical stirrer speed of 250 r / min. Meanwhile, introduce argon gas into the three-necked flask until it is completely dissolved. Add 7.2 g of 1,2,4,5-cyclohexanetetracarboxylic dianhydride in 6 batches, with an interval of 15 min each time. After the addition is completed, continue stirring for 20 h, then add 15.5 g of acetic anhydride and 5.9 g of pyridine, and continue stirring at room temperature for 27 h to obtain a transparent viscous solution. Then slowly pour the transparent viscous solution into a beaker containing 2 L of deionized water, and quickly stir it with a glass rod during this process to obtain fibrous solids. Dry them in an oven at 90 °C to obtain fibrous polymers. Then take 5 g of the dried fibrous solids and dissolve them in 25 g of N,N-dimethylformamide to obtain a transparent viscous shell solution with a solid content of 25%, and store it in the refrigerator for later use. Dissolve 50 g of paraffin and 0.8 g of polyvinyl alcohol in 5 g of N,N-dimethylformamide at 50 °C to obtain a core solution.

[0054] Step 2: After subjecting the shell solution and the core solution obtained in Step 1 to vacuum degassing treatment at 25 °C respectively, connect them to an 18 / 25G coaxial spinning needle of an electrospinning machine using a hose and conduct continuous spinning. Among them, the high voltage of the spinning voltage is 12 kV, the distance between the needle and the receiving platform is 15 cm, the feeding rate of the shell layer is 0.15 mL / h, and the feeding rate of the core layer is 2.0 mL / h, and the spinning temperature is 30 °C. Under the action of electrostatic force, the composite solution is stretched, and the organic solvent quickly volatilizes to form composite nanofibers with a core-shell structure. The entire spinning process lasts for 12 h.

[0055] The core layer diameter of the fiber membrane prepared in this example is 766 nm, the solar reflectance is 94.8%, the mid-infrared emissivity is 88.6%, the thermal conductivity is 0.073 W·m -1 ·K -1 , and the enthalpy value is 112.9 J / g.

[0056] Example 7 Step 1: First, dissolve 13.7 g of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene in 83.6 g of N,N-dimethylformamide solution in an ice water bath at 0 °C with a mechanical stirrer speed of 500 r / min. Meanwhile, introduce argon gas into the three-necked flask until it is completely dissolved. Add 7.2 g of 1,2,4,5-cyclohexanetetracarboxylic dianhydride in 6 batches, with an interval of 15 min each time. After the addition is completed, continue stirring for 12 h, then add 15.5 g of acetic anhydride and 5.9 g of pyridine, and continue stirring at room temperature for 40 h to obtain a transparent viscous solution. Then slowly pour the transparent viscous solution into a beaker containing 2 L of deionized water, and use a glass rod to stir quickly during this process to obtain fibrous solids. Dry them in an oven at 120 °C to obtain fibrous polymers. Then take 5 g of dry fibrous solids and dissolve them in 15 g of N,N-dimethylformamide to obtain a transparent viscous shell solution with a solid content of 25%, and store it refrigerated for later use. Dissolve 35 g of paraffin and 0.8 g of polyethylene oxide in 5 g of N,N-dimethylformamide at 50 °C to obtain a core solution.

[0057] Step 2: After vacuum degassing the shell and core solutions obtained in Step 1 at 30 °C respectively, connect them to an 18 / 25G coaxial spinning needle of an electrospinning machine using a hose and perform continuous spinning. Among them, the high voltage of the spinning voltage is 20 kV, the distance between the needle and the receiving platform is 12 cm, the pushing rate of the shell layer is 0.15 mL / h, and the pushing rate of the core layer is 0.51 mL / h, and the spinning temperature is 30 °C. Under the action of electrostatic force, the composite solution is stretched, and the organic solvent quickly volatilizes to form composite nanofibers with a core-shell structure. The entire spinning lasts for 12 h.

[0058] The core layer diameter of the fiber membrane prepared in this example is 698 nm, the solar reflectance is 95.7%, the mid-infrared emissivity is 83.1%, and the thermal conductivity is 0.058 W·m -1 ·K -1 , and the enthalpy value is 112.4 J / g.

[0059] Comparative Example 1 Preparation method of ordinary polyimide fiber membrane: First, dissolve 18.4 g of 4,4'-diaminodiphenyl ether in N,N-dimethylformamide solution with a mass 4 times that of the total monomer mass. Pass argon gas and stir in a water bath at 0 °C with a mechanical stirring speed of 300 r / min until completely dissolved. Add 20.2 g of pyromellitic dianhydride in 6 batches, with each addition interval of 20 min, while maintaining the molar ratio of dianhydride to diamine monomer at 1. After the feeding is completed, continue stirring for 5 h to obtain a pale yellow transparent viscous electrospinning solution with a solid content of 20%, and store it refrigerated for later use. Connect a 22G spinning needle of an electrospinning machine and continuously spin for 8 h to form polyamic acid nanofibers. Among them, the spinning voltage is 16 kV, the distance between the needle and the receiving platform is 8 cm, the feeding rate is 0.23 mL / h, and the spinning temperature is 30 °C. Place the polyamic acid nanofibers in a vacuum oven at 120 °C to remove organic solvents and moisture. Under an argon atmosphere, first heat from 30 °C to 200 °C at a rate of 5 °C / min and hold for 1 h, then heat to 320 °C and hold for 1 h to achieve thermal imidization, and finally cool naturally to obtain an ordinary polyimide fiber membrane.

[0060] Comparative Example 2 Preparation method of single-functional radiative cooling material: First, dissolve 10.2 g of 2,2'-bis(trifluoromethyl)benzidine in 125.3 g of N,N-dimethylacetamide solution in an ice water bath at 0 °C with a mechanical stirrer speed of 500 r / min. At the same time, pass argon gas into the three-necked flask until completely dissolved. Add 14.2 g of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride in 8 batches, with each addition interval of 10 min. After the feeding is completed, continue stirring for 12 h, then add 16.8 g of acetic anhydride and 6.3 g of pyridine, and continue stirring at room temperature for 24 h to obtain a transparent viscous solution. Then slowly pour the transparent viscous solution into a beaker containing 2 L of methanol solution, and quickly stir with a glass rod during this period to obtain fibrous solids. Dry in an oven at 80 °C to obtain fibrous polymers. Then take 5 g of dry fibrous solids and dissolve them in 20 g of N,N-dimethylformamide to obtain a transparent viscous solution with a solid content of 20%, and store it refrigerated for later use. After vacuum degassing the obtained shell layer and core layer solutions at 30 °C respectively, use a 22G spinning needle of an electrospinning machine for continuous spinning. Among them, the high-voltage spinning voltage is 19 kV, the distance between the needle and the receiving platform is 13 cm, the feeding rate is 0.19 mL / h, and the spinning temperature is 30 °C. Under the action of electrostatic force, the solution is stretched, and the entire spinning lasts for 12 h, and the organic solvent quickly volatilizes to form a single-functional radiative cooling material.

[0061] As Figure 1As shown, the core-shell structured nanofibers have a uniform structure and no fiber breakage is found, proving the successful and stable preparation of the core-shell structure.

[0062] As Figure 2 shown, it can be seen from the EDS image that element F is uniformly dispersed on the surface of the fiber, proving the successful introduction of fluorine-containing functional groups.

[0063] As Figure 3 shown, as the feeding rate of the core layer increases, the diameter of the core layer gradually increases from 232 to 786 nm. However, when the feeding ratio (FPL) of the shell layer to the core layer solution is greater than 1, it can be seen that the shell layer of FPL1 cannot completely cover the core layer, and the fiber membrane with the optimal process has an FPL of 0.8.

[0064] As Figures 4a - 4c shown, the reflectivity of the fiber membrane with FPL 0.8 of the present invention in the range of 0.25 - 2.5 μm and the emissivity in the range of 2.5 - 20 μm were tested to prove the advantages of its optical properties and indirectly prove its radiative cooling performance. As Figure 4a shown, the reflectivity of the fiber membrane with FPL 0.8 is > 90% in the main energy region (0.3 - 1.5 μm) of the solar spectrum. Among them, the reflectivity in the visible light region (0.4 - 0.7 μm) is > 90%, avoiding the material from heating up due to heat absorption, and the reflectivity in the near-infrared region (0.7 - 1.5 μm) is > 90%, blocking about 50% of the thermal energy input in sunlight. This shows that the fiber membrane with FPL 0.8 has a high reflectivity to sunlight and can effectively reduce heat absorption; As Figure 4b shown, the emissivity of the fiber membrane with FPL 0.8 is as high as 95% in the 8 - 13 μm band (the transparency window of the earth's atmosphere), and the thermal energy absorbed by the material can be radiated into space in the form of infrared radiation through the atmosphere, indicating that it can efficiently radiate heat and achieve passive radiative cooling; As Figure 4c shown, the blue and red columns represent the absorption or reflection ratios in different bands respectively, indicating that the fiber membrane with FPL 0.8 almost completely reflects (95%) in the solar spectrum range and only allows very little energy to be absorbed or transmitted, which is consistent with Figure 4a the high reflectivity data.

[0065] As Figure 5 shown, 100 thermal cycles were tested. The DSC curve of the first time and the DSC curve of the 100th time almost coincide, proving that its enthalpy value does not decrease after multiple cycles, thus proving that the core-shell structure can effectively encapsulate the phase change material and prevent the leakage of the phase change material.

[0066] As Figure 6Under the same simulated sunlight conditions, the ordinary polyimide fiber membrane (PI) prepared in Comparative Example 1 has a particularly short heating and cooling time, and a very high equilibrium temperature, reaching 55 °C. However, the fiber membrane of FPL0.8 prepared in the present invention first heats up very slowly and requires a longer time to reach the equilibrium temperature. Secondly, the final equilibrium temperature is about 7 °C lower than that of PI. Moreover, after removing the simulated light source, the temperature of PI drops rapidly, but the temperature of FPL drops at a very slow rate.

[0067] The fiber membrane of FPL0.8 of the present invention and the single-functional radiative cooling material of Comparative Example 2 were used to block sunlight, and direct sunlight was irradiated on the blank group (without using any substance). Then, the temperature changes of the fiber membrane of FPL0.8, the single-functional radiative cooling material and the blank group were observed, and the following conclusions were obtained: As Figure 7 shown, the yellow background represents the sunlight intensity. As the sunlight intensity increases, the temperature of the fiber membrane of FPL0.8 of the present invention rises more slowly. At about 10:10, the weather changed violently and the sunlight intensity dropped sharply. The temperatures of the blank group and the single-functional radiative cooling material dropped sharply by more than 15 °C. This temperature difference is extremely unfriendly to both the human body and equipment. However, the temperature difference of the fiber membrane of FPL0.8 of the present invention is less than 4 °C. Thus, it can be seen that the fiber membrane prepared in the present invention can effectively resist thermal shock.

[0068] The above content is only to illustrate the technical idea of the present invention and cannot be used to limit the protection scope of the present invention. Any modification made on the basis of the technical solution according to the technical idea proposed by the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a core-shell structure dual-function radiation cooling fiber membrane, characterized in that: The following steps are involved: 1) dissolving dianhydride monomer and diamine monomer in an organic solvent to react to obtain polyamic acid, adding a dehydrating agent and a catalyst to obtain a mixed solution, adding the mixed solution to a non-solvent to obtain a fibrous polymer, and drying to obtain a shell polymer polyimide; the mass ratio of the dianhydride and diamine monomer to the organic solvent is 1: (3-5), and the volume ratio of the mixed solution to the non-solvent is >1:10; 2) dissolving polyimide in an organic solvent to prepare an electrospinning shell layer solution, wherein the mass ratio of the polyimide to the organic solvent is 1:(2-5); dissolving a phase change material in an organic solvent to prepare an electrospinning core layer solution, wherein the mass ratio of the phase change material to the organic solvent is (5-10):1; wherein the phase change material comprises one of polyethylene glycol, lauric acid and paraffin; 3) Using coaxial electrospinning technology, the electrospinning shell solution and core solution are stretched at a voltage of 10-25 kV to obtain a core-shell structured dual-functional radiation cooling fiber membrane.

2. The method for preparing a core-shell structure dual-function radiation cooling fiber membrane according to claim 1, characterized in that: In step 1), the molar ratio of the dianhydride to the diamine monomer is 0.8-1.2, the dianhydride monomer includes one of 4,4′-(hexafluoroisopropylene) diphthalic anhydride, 1,2,4,5-cyclohexanetetracarboxylic anhydride and 1,2,3,4-tetramethyl 1,2,3,4-cyclobutanetetracarboxylic anhydride; the diamine monomer includes one of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane and 2,2′-bis(trifluoromethyl)diaminobiphenyl.

3. The method for preparing a core-shell structure dual-function radiation cooling fiber membrane according to claim 1, characterized in that: In step 1), the volume ratio of the dehydrating agent to the catalyst is 1:3, the dehydrating agent is acetic anhydride, and the catalyst is pyridine solution.

4. The method for preparing a core-shell structure dual-function radiation cooling fiber membrane according to claim 1, characterized in that: In step 1) and step 2), the organic solvent includes one of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone, and the non-solvent includes one of deionized water, methanol and ethanol.

5. The method for preparing a core-shell structure dual-function radiation cooling fiber membrane according to claim 1, characterized in that: In step 3), the distance between the needle and the receiving platform of the coaxial electrospinning technology is 5~15 cm, the pushing rate is 0.5~2mL / h, and the spinning temperature is 30~50 ℃.

6. The method for preparing a core-shell structure dual-function radiation cooling fiber membrane according to claim 1, characterized in that: In step 1), a mixed solution of dianhydride monomer, diamine monomer and organic solvent is reacted in a water bath at 0-10°C for 5-20 h at a mechanical stirring rate of 200-500 r / min, and then reacted at room temperature for 15-40 h after adding a dehydrating agent and a catalyst; the drying temperature is 60-120°C.

7. The method for preparing a core-shell structure dual-function radiation cooling fiber membrane according to claim 1, characterized in that: In step 2), the phase change material and the spinning aid are dissolved in an organic solvent to prepare an electrospinning core layer solution, and the mass ratio of the phase change material to the spinning aid is (5-15):1; the spinning aid includes one of polyvinyl pyrrolidone, polyvinyl alcohol, polyethylene oxide and N-methylpyrrolidone.

8. The method for preparing a core-shell structure dual-function radiation cooling fiber membrane according to claim 1, characterized in that: In step 3), the electrospinning shell layer solution and the core layer solution are subjected to vacuum degassing treatment at 20-30°C, and then the electrospinning shell layer solution and the core layer solution are stretched by coaxial electrospinning technology.

9. A core-shell structure dual-function radiation cooling fiber membrane prepared by the method for preparing a core-shell structure dual-function radiation cooling fiber membrane according to any one of claims 1 to 8, characterized in that: The dual-function radiation cooling fiber membrane has a core-shell structure, and the diameter of the core layer is 232 nm-786 nm.

10. Use of the core-shell structure dual-function radiation cooling fiber membrane according to claim 9 in radiation cooling.

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