Double-layer polyimide composite aerogel as well as preparation method and application thereof

Abstract

The invention provides a double-layer polyimide composite aerogel and a preparation method and application thereof, and the preparation method comprises the following steps: (1) carrying out stepwise polymerization reaction on a flexible diamine monomer, a rigid diamine monomer and a high-surface-energy rigid dianhydride monomer in a polar aprotic solvent to obtain a first polyamide acid solution; (2) sequentially carrying out polymerization reaction on a flexible diamine monomer, a fluorine-containing dianhydride monomer and a high-surface-energy rigid dianhydride monomer in a polar aprotic solvent to obtain a second polyamide acid solution; (3) uniformly mixing the first polyamic acid solution and the second polyamic acid solution with a polysiloxane solution, a dehydrating agent and a first catalyst respectively for reaction to obtain first composite sol and second composite sol respectively; and (4) sequentially pouring the first composite sol and the second composite sol into a mold, and sequentially carrying out standing, aging, solvent replacement and supercritical drying to obtain the double-layer polyimide composite aerogel. The prepared double-layer polyimide composite aerogel has the advantages of being excellent in heat insulation performance, high in cooling medium transmission rate and the like.

Description

Technical Field

[0001] This invention relates to the field of aerogel materials technology, and in particular to a bilayer polyimide composite aerogel, its preparation method, and its application. Background Technology

[0002] Polyimide aerogels possess excellent high-temperature resistance, mechanical properties, chemical corrosion resistance, and radiation resistance, making them an important candidate material for next-generation high-performance lightweight thermal insulation materials. As aerospace vehicles evolve towards higher Mach numbers and longer flight times, the heat flux density of the service environment is increasing dramatically. Simply relying on the "passive insulation" of aerogel materials is often insufficient to meet the protection requirements under extreme thermal environments. Sweating cooling, through the continuous supply of a liquid cooling medium and phase change evaporation, utilizes latent heat to absorb a large amount of heat, thereby improving the system's temperature resistance, service life, and heat flux density adaptability. The stable implementation of sweating cooling depends on the formation of an effective "water absorption-transport-evaporation" coupling path in the thickness direction of the porous material, and the maintenance of a suitable gas-liquid phase change interface state, thus ensuring evaporative heat transfer efficiency and continuous liquid supply.

[0003] However, traditional polyimide aerogels face the dilemma of "uniform wettability" in sweating cooling applications: specifically, while the homogeneous hydrophilic polyimide aerogel structure is conducive to water absorption, its cooling efficiency is easily limited due to the mismatch between transport rate and evaporation demand. Furthermore, the homogeneous hydrophilic polyimide aerogel structure is prone to overwetting, resulting in the formation of a continuous liquid film on the surface, reducing the effective utilization of latent heat of vaporization, and potentially causing vapor blockage, weakening the stability and sustainability of sweating cooling. Simultaneously, during the dynamic phase transition process of sweating cooling, the repeated movement of the gas-liquid interface within the pores generates enormous capillary tension. Traditional pure organic aerogel skeletons lack sufficient stiffness and are prone to collapse, leading to performance degradation. If the polyimide aerogel is modified to be homogeneous and hydrophobic to maintain skeleton stability, although this can preserve the skeleton's stability, the enormous capillary repulsion will block the spontaneous transport of the cooling medium from the cold end to the hot end, making it difficult to achieve an effective combination of active thermal protection through sweating cooling and passive thermal insulation.

[0004] Therefore, there is an urgent need to provide a bilayer polyimide composite aerogel, its preparation method, and its application. Summary of the Invention

[0005] This invention provides a bilayer polyimide composite aerogel, its preparation method, and its application, which can solve the problem of mismatch between the supply of cooling medium and the demand for vaporization and evaporation during the sweating and cooling process of existing polyimide aerogels.

[0006] In a first aspect, a method for preparing a bilayer polyimide composite aerogel is disclosed, the method comprising the following steps: (1) A first polyamic acid solution is obtained by stepwise polymerization of flexible diamine monomer, rigid diamine monomer and high surface energy rigid dianhydride monomer in a polar aprotic solvent. (2) The flexible diamine monomer is polymerized sequentially with a fluorinated dianhydride monomer and a high surface energy rigid dianhydride monomer in a polar aprotic solvent to obtain a second polyamic acid solution; (3) The first polyamic acid solution and the second polyamic acid solution are mixed with the polysiloxane solution, the dehydrating agent and the first catalyst respectively to obtain the first composite sol and the second composite sol respectively; (4) The first composite sol and the second composite sol are poured into the mold in sequence, and after being allowed to stand, aged, replaced by solvent and dried by supercritical drying, the bilayer polyimide composite aerogel is obtained.

[0007] Preferably, in steps (1) and (2), the flexible diamine monomer is 4,4'-diaminodiphenyl ether; and the rigid diamine monomer is 9,9-bis(4-aminophenyl)fluorene.

[0008] Preferably, the high surface energy rigid dianhydride monomer is at least one of 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-oxobisphthalic anhydride, or pyromellitic dianhydride; the fluorinated dianhydride monomer is 4,4-(hexafluoroisopropyl)bisphthalic anhydride.

[0009] Preferably, the polar aprotic solvent is at least one of N,N-dimethylacetamide or N-methylpyrrolidone.

[0010] Preferably, in step (1), the molar ratio of the flexible diamine monomer to the rigid diamine monomer is (1~2.5):1.

[0011] More preferably, in step (1), the molar ratio of the sum of the moles of the flexible diamine monomer and the rigid diamine monomer to the molar ratio of the high surface energy rigid dianhydride monomer is (0.7~1.0):1.

[0012] Preferably, in step (1), the mass ratio of the high surface energy rigid dianhydride monomer to the polar aprotic solvent is 1:(10~20).

[0013] Preferably, in step (2), the molar ratio of the fluorinated dianhydride monomer and the high surface energy rigid dianhydride monomer is 1:(2.5~3.0).

[0014] Preferably, in step (2), the molar ratio of the sum of the fluorinated dianhydride monomer and the high surface energy rigid dianhydride monomer to the flexible diamine monomer is (1.0~1.2):1.

[0015] Preferably, in step (2), the mass ratio of the sum of the masses of the fluorinated dianhydride monomer and the high surface energy rigid dianhydride monomer to the mass ratio of the polar aprotic solvent is 1:(10~20).

[0016] Preferably, in step (3), the polysiloxane solution is obtained by stirring and mixing alkyltrialkoxysilane, mixed solvent, second catalyst and surfactant.

[0017] Preferably, the alkyltrialkoxysilane is at least one of methyltrimethoxysilane or vinyltrimethoxysilane.

[0018] More preferably, the mixed solvent is ethanol and water; wherein the volume ratio of ethanol to water is preferably (1~2):(1~2).

[0019] Preferably, the second catalyst is acetic acid.

[0020] Preferably, the surfactant is a polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer.

[0021] Preferably, the volume ratio of alkyltrialkoxysilane, mixed solvent and second catalyst is (1~2):(2~4):(0.02~0.04).

[0022] Preferably, the volume-to-mass ratio of alkyltrialkoxysilane to surfactant is (5~10):0.5mL / g.

[0023] Preferably, in step (3), the dehydrating agent is at least one of acetic anhydride or propionic anhydride; and the first catalyst is at least one of triethylamine or pyridine.

[0024] Preferably, the molar ratio of the first catalyst to the dehydrating agent is (7~10):(5~8).

[0025] More preferably, the molar ratio of the sum of the high surface energy rigid dianhydride monomers and the fluorinated dianhydride monomers to the first catalyst and the dehydrating agent is 1:(7~10):(5~8).

[0026] Preferably, the volume ratio of the first polyamic acid solution, the second polyamic acid solution and the polysiloxane solution is 1:(0.02~0.04).

[0027] Preferably, in step (4), the volume ratio of the first composite sol and the second composite sol is 1:(0.5~1).

[0028] Preferably, a composite solvent is used for solvent replacement; wherein the composite solvent is anhydrous ethanol and N,N-dimethylacetamide.

[0029] Preferably, the aging temperature is 20~35℃ and the time is 9~12h.

[0030] Preferably, the supercritical drying pressure is 8~10MPa, the temperature is 37~42℃, and the time is 8~10h.

[0031] Secondly, embodiments of the present invention also provide a bilayer polyimide composite aerogel, which is prepared using the preparation method described in any one of the first aspects above.

[0032] Thirdly, embodiments of the present invention also provide an application of the bilayer polyimide composite aerogel described in the second aspect above or the bilayer polyimide composite aerogel prepared by any of the preparation methods described in the first aspect above in a thermal protection system.

[0033] Compared with the prior art, the present invention has at least the following beneficial effects: (1) In this invention, firstly, a hydrophilic layer and a hydrophobic layer are prepared by polymerization reaction; wherein, the hydrophilic layer is polymerized by flexible diamine, rigid diamine and high surface energy rigid dianhydride monomers, and then reacted with polysiloxane solution through sol-gel reaction to form a ball-chain double network hydrophilic structure; the hydrophobic layer is polymerized by flexible diamine, hydrophobic dianhydride and high surface energy rigid dianhydride monomers to form a roughened polyimide aerogel dendritic hydrophobic network in a similar manner; then, the two are subjected to gelation, aging, solvent replacement and supercritical drying in a mold to form an integrated Janus structure bilayer polyimide composite aerogel, the bottom of which is a hydrophilic layer and the top of which is a hydrophobic layer, and has asymmetric wettability; and its unique ball-chain double network structure significantly enhances the rigidity and shrinkage resistance of the aerogel skeleton, so that it can effectively resist capillary forces and maintain the integrity of the microstructure during water absorption and evaporation. This results in a composite aerogel that possesses excellent mechanical support, thermal insulation, and structural stability. After multiple water absorption-evaporation cycles, its structure remains largely unchanged. In sweating cooling applications, the bottom hydrophilic layer is responsible for absorbing and transporting the cooling medium along its thickness, while the top hydrophobic layer inhibits the formation of a continuous liquid film on the surface, anchoring the gas-liquid phase change interface within the material. This achieves stable and efficient active thermal protection, with advantages such as excellent thermal insulation performance and fast cooling medium transport rate.

[0034] (2) The preparation method in this invention is simple, easy to operate and low in cost. The hydrophilicity / hydrophobicity of the aerogel can be flexibly controlled by adjusting the types of diamine and dianhydride. The macroscopic structure of the aerogel can also be changed with the help of a mold, so as to adapt to a variety of heat protection application scenarios. At the same time, the experimental verification shows that the prepared bilayer polyimide composite aerogel was tested on a 300℃ hot plate for 365s, and its cold surface temperature was only 80.5℃. The macroscopic shape of the sample remained stable after the test, and it has excellent heat insulation performance and thermal stability. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a schematic diagram of the structure of a bilayer polyimide composite aerogel provided in an embodiment of the present invention; Figure 2 This is a physical structural diagram of a bilayer polyimide composite aerogel provided in an embodiment of the present invention; Figure 3 This is a scanning electron microscope image of the hydrophobic aerogel layer in a bilayer polyimide composite aerogel provided in Embodiment 1 of the present invention; Figure 4 This is a scanning electron microscope image of the hydrophilic aerogel layer in a bilayer polyimide composite aerogel provided in Embodiment 1 of the present invention; Figure 5 The image shows the wettability test results of the hydrophobic aerogel layer in water in a bilayer polyimide composite aerogel provided in Embodiment 1 of the present invention. Figure 6 The image shows the wettability test results of the hydrophilic aerogel layer in water in a bilayer polyimide composite aerogel provided in Embodiment 1 of the present invention. Figure 7 This is a graph showing the static water contact angle test results of the hydrophobic aerogel layer in a bilayer polyimide composite aerogel provided in Embodiment 1 of the present invention. Figure 8 This is a graph showing the static water contact angle test results of the hydrophilic aerogel layer in a bilayer polyimide composite aerogel provided in Embodiment 1 of the present invention. Figure 9 The back-temperature curve of a bilayer polyimide composite aerogel provided in Embodiment 1 of the present invention after being held on a hot plate at 300°C for 365 seconds. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0038] As mentioned earlier, both homogeneous hydrophilic polyimide aerogel structures and homogeneous hydrophobic polyimide aerogel structures in the prior art have certain limitations in evaporative cooling thermal protection. The former suffers from low evaporative cooling efficiency and insufficient structural stability after water absorption, while the latter struggles to achieve rapid absorption and stable transport of the cooling medium, thus hindering the effective combination of active sweating cooling and passive insulation. To address these issues, related technologies attempt to construct asymmetric wettability or layered structures to synergistically meet the needs of water supply and interface control, such as forming hydrophilic / hydrophobic functional zones along the material thickness direction. However, the implementation methods of these structures (such as selective modification, stepwise preparation, and interlayer composites) suffer from complex processes, limited repeatability and scalability, and insufficient reliability of interlayer interface bonding. Furthermore, these problems are particularly pronounced under high-temperature and humid-heat coupling environments, further affecting the long-term service stability of the material.

[0039] Therefore, this invention provides a method for preparing a bilayer polyimide composite aerogel, the method comprising the following steps: (1) A first polyamic acid solution is obtained by stepwise polymerization of flexible diamine monomer, rigid diamine monomer and high surface energy rigid dianhydride monomer in a polar aprotic solvent. (2) The flexible diamine monomer is polymerized sequentially with a fluorinated dianhydride monomer and a high surface energy rigid dianhydride monomer in a polar aprotic solvent to obtain a second polyamic acid solution; (3) The first polyamic acid solution and the second polyamic acid solution are mixed with the polysiloxane solution, the dehydrating agent and the first catalyst respectively to obtain the first composite sol and the second composite sol respectively; (4) The first composite sol and the second composite sol are poured into the mold in sequence, and after being allowed to stand, aged, replaced by solvent and dried by supercritical drying, the bilayer polyimide composite aerogel is obtained.

[0040] In this embodiment of the invention, firstly, a hydrophilic layer and a hydrophobic layer are prepared by polymerization reactions. The hydrophilic layer is formed by polymerizing flexible diamine, rigid diamine, and high surface energy rigid dianhydride monomers, followed by a sol-gel reaction with a polysiloxane solution to form a spherical-chain dual-network hydrophilic structure. The hydrophobic layer is formed by polymerizing flexible diamine, hydrophobic dianhydride, and high surface energy rigid dianhydride monomers in a similar manner to form a roughened polyimide-like aerogel dendritic hydrophobic network. Subsequently, both layers are subjected to gelation, aging, solvent replacement, and supercritical drying in a mold to form an integrated Janus-structured bilayer polyimide composite aerogel with a hydrophilic bottom layer and a hydrophobic top layer, exhibiting asymmetric wettability. Furthermore, its unique spherical-chain dual-network structure significantly enhances the rigidity and shrinkage resistance of the aerogel skeleton, enabling it to effectively resist capillary forces and maintain the integrity of the microstructure during water absorption and evaporation. This results in a composite aerogel that possesses excellent mechanical support, thermal insulation, and structural stability. After multiple water absorption-evaporation cycles, its structure remains largely unchanged. In sweating cooling applications, the bottom hydrophilic layer is responsible for absorbing and transporting the cooling medium along its thickness, while the top hydrophobic layer inhibits the formation of a continuous liquid film on the surface, anchoring the gas-liquid phase change interface within the material. This achieves stable and efficient active thermal protection, with advantages such as excellent thermal insulation performance and fast cooling medium transport rate.

[0041] According to some preferred embodiments, in steps (1) and (2), the flexible diamine monomer is 4,4'-diaminodiphenyl ether; the rigid diamine monomer is 9,9-bis(4-aminophenyl)fluorene; the high surface energy rigid dianhydride monomer is at least one of 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-oxobisphthalic anhydride, or pyromellitic dianhydride; the fluorinated dianhydride monomer is 4,4-(hexafluoroisopropyl)bisphthalic anhydride; and the polar aprotic solvent is at least one of N,N-dimethylacetamide or N-methylpyrrolidone.

[0042] In this embodiment of the invention, a two-step method is used to prepare the first polyamic acid solution and the second polyamic acid solution. Specifically, in preparing the first polyamic acid solution, a flexible diamine monomer and a high surface energy rigid dianhydride monomer are first added to a polar aprotic solvent at a molar ratio of (0.9~1.0):1 and polymerized under an inert gas atmosphere. Then, a rigid diamine monomer and a high surface energy rigid dianhydride monomer are added to the reaction system at a molar ratio of (0.9~1.0):1 to continue the polymerization reaction. The above stepwise reaction is beneficial to promote the sequential incorporation and alternating arrangement of rigid and flexible segments, thereby forming a hydrophilic polyamic acid macromolecular chain structure with rigidity and flexibility synergy, and thus improving the flexibility of the aerogel. In preparing the second polyamic acid solution, the flexible diamine monomer and the fluorinated dianhydride monomer are first added to a polar aprotic solvent and polymerized under an inert gas atmosphere. Then, a high surface energy rigid dianhydride monomer is added to the reaction system to continue the polymerization reaction. Through the above stepwise reaction, not only is it beneficial to improve the reaction performance of the diamine monomer and the dianhydride monomer, but it is also beneficial to introduce a small amount of hydrophilic segments into the hydrophobic backbone. In this way, while maintaining the overall hydrophobic properties of the hydrophobic aerogel, its interfacial compatibility and bonding strength with the hydrophilic aerogel layer can be enhanced.

[0043] It should be noted that the temperature during the above polymerization reaction is 20~35℃ and the time is 12~24h.

[0044] According to some preferred embodiments, in step (1), the molar ratio of flexible diamine monomer to rigid diamine monomer is (1~2.5):1 (e.g., 1:1, 1.5:1, 2.0:1 or 2.5:1); the molar ratio of the sum of the moles of flexible diamine monomer and rigid diamine monomer to the molar ratio of high surface energy rigid dianhydride monomer is (0.7~1.0):1 (e.g., 0.7:1, 0.8:1, 0.9:1 or 1.0:1); the mass ratio of high surface energy rigid dianhydride monomer to polar aprotic solvent is 1:(10~20) (e.g., 1:10, 1:15 or 1:20).

[0045] In this embodiment of the invention, during the preparation of the first polyamic acid solution, by precisely controlling the proportions of flexible diamine monomer, rigid diamine monomer, and high surface energy rigid dianhydride monomer in the stepwise polymerization reaction, it is possible to ensure that flexible and rigid segments can be arranged and connected in an orderly alternation, forming a polyimide macromolecular chain skeleton that combines rigidity and flexibility. Among them, the rigid segments provide strong mechanical support for the skeleton, while the flexible segments endow the skeleton with a certain degree of flexibility. This is beneficial to ensure that the formed hydrophilic aerogel has both a low evaporation shrinkage rate and excellent high-temperature dimensional stability. However, if the content of flexible or rigid diamine monomer is too high or too low, it is not conducive to the formation of a polyimide macromolecular chain skeleton that combines rigidity and flexibility.

[0046] According to some preferred embodiments, in step (2), the molar ratio of the fluorinated dianhydride monomer and the high surface energy rigid dianhydride monomer is 1:(2.5~3.0) (for example, it can be 1:2.5, 1:2.8, 1:2.9, 1:2.97 or 1:3.0); the molar ratio of the sum of the fluorinated dianhydride monomer and the high surface energy rigid dianhydride monomer to the flexible diamine monomer is (1.0~1.2):1 (for example, it can be 1.0:1, 1.0.3:1, 1.05:1, 1.08:1, 1.1:1 or 1.2:1); the mass ratio of the sum of the fluorinated dianhydride monomer and the high surface energy rigid dianhydride monomer to the mass of the polar aprotic solvent is 1:(10~20) (for example, it can be 1:10, 1:15 or 1:20).

[0047] In this embodiment of the invention, during the preparation of the second polyamic acid solution, by precisely controlling the proportions of flexible diamine monomer, high surface energy rigid dianhydride monomer, and fluorinated dianhydride monomer in the stepwise polymerization reaction, an optimized balance between hydrophobic function and interfacial bonding performance can be achieved. An appropriate amount of fluorinated dianhydride monomer ensures sufficient hydrophobic properties in the molecular backbone, enabling the final hydrophobic aerogel layer to effectively inhibit the spreading of the cooling medium on the surface and prevent the formation of a continuous liquid film. Simultaneously, the introduction of an appropriate amount of high surface energy rigid dianhydride monomer not only enhances the reaction performance but also introduces a certain amount of hydrophilic sites onto the hydrophobic backbone. This improves the chemical compatibility between the hydrophobic layer and the underlying hydrophilic layer and also helps to enhance interlayer bonding during subsequent integrated molding, preventing interfacial separation. Through the synergistic regulation of the above components, the resulting hydrophobic aerogel layer maintains good hydrophobicity while possessing excellent structural stability and strong interlayer bonding ability, thereby ensuring the long-term stability of the Janus structure aerogel under extreme thermal environments.

[0048] In this embodiment of the invention, by precisely controlling the proportions of each component in the first polyamic acid solution and the second polyamic acid solution, and keeping the solid content of the two solutions within a similar range, while rationally selecting the type of dianhydride monomer, it is beneficial for the shrinkage rates of the two aerogel materials to tend to match in the subsequent aging stage, thereby helping to ensure the stability of the interlayer bonding performance of the bilayer composite aerogel material.

[0049] According to some preferred embodiments, in step (3), the polysiloxane solution is obtained by stirring and mixing alkyltrialkoxysilane, mixed solvent, second catalyst and surfactant to obtain polysiloxane solution; the alkyltrialkoxysilane is at least one of methyltrimethoxysilane or vinyltrimethoxysilane; the mixed solvent is ethanol and water; wherein, the volume ratio of ethanol and water is preferably (1~2):(1~2) (for example, it can be 1:1, 1:2 or 2:1); the second catalyst is acetic acid; the surfactant is polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer.

[0050] In this embodiment of the invention, a polysiloxane solution is prepared using alkyltrialkoxysilane, a mixed solvent, a second catalyst, and a surfactant. During the preparation process, the alkyltrialkoxysilane is first mixed with a mixed solvent consisting of ethanol and water, and acetic acid as a catalyst. Then, the surfactant is added and stirred for 10-20 minutes until the solution becomes clear. Specifically, the surfactant of a particular type and amount not only stabilizes the polyamic acid solution and the polysiloxane solution during mixing, preventing immediate separation of the two phases, but also delays the phase separation process, guiding the formation of an interpenetrating double network structure. Simultaneously, the surfactant acts as a template, promoting the hydrolysis and condensation of the polysiloxane precursor to form uniform spherical particles. This spherical structure facilitates effective encapsulation by the polyimide molecular chains during subsequent mixing with the polyamic acid solution, ultimately forming a unique spherical-chain composite structure. In this structure, spherical polysiloxane particles serve as rigid support nodes, which can significantly enhance the overall stability of the aerogel skeleton. This not only helps to ensure the good water absorption and hydrophobic properties of the final composite aerogel material, but also enables it to effectively resist capillary forces after water absorption, preventing the network structure from collapsing and thus maintaining the integrity of the composite aerogel structure.

[0051] According to some preferred embodiments, the volume ratio of alkyltrialkoxysilane, mixed solvent, and second catalyst is (1~2):(2~4):(0.02~0.04) (e.g., it can be 1:1:0.02, 1:3:0.02, 1:4:0.02, 2:2:0.02, 2:3:0.02, 2:4:0.02, 1:2:0.03, 1:4:0.03, 2:2:0.03, 2:3:0.04, 1:4:0.04, 2:2:0.04, or 2:4:0.04); the volume-to-mass ratio of alkyltrialkoxysilane to surfactant is (5~10):0.5mL / g (e.g., it can be 5:0.5mL / g, 6:0.5mL / g, 7:0.5mL / g, 8:0.5mL / g, 9:0.5mL / g, or 10:0.5mL / g).

[0052] In this embodiment of the invention, by controlling the content of alkyltrialkoxysilane, it is beneficial to form uniformly dispersed spherical particles. This facilitates the effective encapsulation of the polyimide molecular chains by the structure during the final preparation of hydrophilic and hydrophobic aerogels, forming a stable sphere-chain composite structure and ensuring good mechanical properties of the aerogel structure. If the content of alkyltrialkoxysilane is too high, it will be difficult for the alkyltrialkoxysilane to be uniformly dispersed in the system, leading to severe agglomeration of polysiloxane particles when mixed with polyamic acid solution, thus significantly reducing the overall mechanical properties of the aerogel. Conversely, if the content is too low, it is impossible to form a sufficient number of appropriately sized polysiloxane spherical particles, which is not conducive to the formation of an effective sphere-chain composite structure and will also lead to a decrease in the overall mechanical properties of the aerogel.

[0053] According to some preferred embodiments, in step (3), the dehydrating agent is at least one of acetic anhydride or propionic anhydride; the first catalyst is at least one of triethylamine or pyridine; the molar ratio of the first catalyst to the dehydrating agent is (7~10):(5~8) (for example, it can be 7:5, 7:6, 7:7, 7:8, 8:5, 8:6, 8:7, 8:8, 9:5, 9:6, 9:7, 9:8, 9:9, 10:5, 10:6, 10:7 or 10:8); the sum of the molar amounts of the high surface energy rigid dianhydride monomer and the fluorinated dianhydride monomer is equal to the molar ratio of the first catalyst to the dehydrating agent. The ratio is 1:(7~10):(5~8) (e.g., it can be 1:7:5, 1:7:6, 1:7:7, 1:7:8, 1:8:5, 1:8:6, 1:8:7, 1:8:8, 1:9:5, 1:9:6, 1:9:7, 1:9:8, 1:9:9, 1:10:5, 1:10:6, 1:10:7 or 1:10:8); the volume ratio of the first polyamic acid solution, the second polyamic acid solution and the polysiloxane solution is 1:(0.02~0.04) (e.g., it can be 1:0.02, 1:0.03 or 1:0.04).

[0054] In this embodiment of the invention, a polysiloxane solution is thoroughly stirred and mixed with a first polyamic acid solution and a second polyamic acid solution to ensure uniform contact of the components, thereby obtaining two homogeneous and stable polyamic acid-polysiloxane composite solutions. Subsequently, appropriate amounts of a dehydrating agent and a catalyst are added to the composite solutions to initiate an imidization reaction. The interval between the simultaneous or sequential addition of the dehydrating agent and the catalyst to the reaction system does not exceed 2 minutes, thereby enabling simultaneous dehydration and catalysis, ensuring rapid reaction, and obtaining structurally stable hydrophilic and hydrophobic polyimide-polysiloxane composite sols. Simultaneously, by precisely controlling the volume ratio of the polysiloxane solution to the polyamic acid solution, the structure and properties of the hydrophilic and hydrophobic aerogel layers can be optimized. Specifically, for the hydrophilic aerogel layer, an appropriate amount of polysiloxane solution enhances the mechanical properties of the aerogel network, effectively resisting shrinkage caused by capillary forces; for the hydrophobic aerogel layer, an appropriate amount of polysiloxane solution facilitates water vapor escape and maintains interface stability while maintaining its hydrophobic properties.

[0055] According to some preferred embodiments, in step (4), the volume ratio of the first composite sol and the second composite sol is 1:(0.5~1) (for example, it can be 1:0.5, 1:0.75 or 1:1). In this embodiment of the invention, during the preparation of the bilayer composite aerogel, a hydrophilic first composite sol is first poured into a mold and held for 2-15 minutes to allow the surface of the first composite sol to have a certain viscosity but not yet completely solidified. Then, a hydrophobic second composite sol is poured on top of the first composite sol. After static gelation, a bilayer wet gel is obtained. Following aging, solvent replacement, and drying, a bilayer polyimide composite aerogel is obtained. By controlling the volume ratio of the two composite sols, the thickness of the final hydrophilic-hydrophobic aerogel can be adjusted, thus enabling highly flexible structural design of the aerogel material. The thickness of the hydrophilic-hydrophobic aerogel layer can be optimized according to specific application scenarios (such as heat flux density, cooling medium supply conditions, etc.). (For example, increasing the thickness of the hydrophilic aerogel layer can improve the storage capacity of the cooling medium, while adjusting the thickness of the hydrophobic aerogel layer can reduce the resistance to water vapor escape). This allows for targeted optimization of the overall thermal insulation and sweating cooling performance of the composite aerogel.

[0056] It should be noted that, in the embodiments of the present invention, the mold used in the preparation process can be formed of materials such as polyethylene, polystyrene, polytetrafluoroethylene, and glass, and the shape of the mold can be a cube, cylinder, sphere, or other required shape.

[0057] According to some preferred embodiments, a composite solvent is used for solvent replacement; wherein the composite solvent is anhydrous ethanol and N,N-dimethylacetamide; the aging temperature is 20~35℃ (e.g., 20℃, 25℃, 30℃ or 35℃), and the time is 9~12h (e.g., 9h, 10h, 11h or 12h); the supercritical drying pressure is 8~10MPa (e.g., 8MPa, 9MPa or 10MPa), the temperature is 37~42℃ (e.g., 37℃, 38℃, 39℃, 40℃, 41℃ or 42℃), and the time is 8~10h (e.g., 8h, 9h or 10h).

[0058] According to some preferred embodiments, as the number of solvent replacements increases, the content gradient of anhydrous ethanol in the composite solvent increases, and the content gradient of N,N-dimethylacetamide decreases; the content of anhydrous ethanol is 25% to 100% by volume percentage (for example, it can be 25%, 50%, 75% or 100%).

[0059] In this embodiment of the invention, after obtaining a bilayer composite wet gel through static gelation and aging, the bilayer composite wet gel is washed with anhydrous ethanol, and then sequentially immersed in a composite solvent formed by anhydrous ethanol and N,N-dimethylacetamide in a specific ratio for multi-step solvent replacement. With each increase in the number of solvent replacements, the content of anhydrous ethanol in the composite solvent increases, while the content of N,N-dimethylacetamide decreases accordingly. This thoroughly removes the solvent from the pores of the wet gel, laying the foundation for obtaining a structurally intact and stable bilayer composite wet gel. For example, the immersion and replacement can first be performed for 12 hours using a composite solvent formed by 25% anhydrous ethanol and 75% polar aprotic solvent, then for 12 hours using a composite solvent formed by 75% anhydrous ethanol and 25% polar aprotic solvent, and finally for 48 hours using a composite solvent formed by 100% anhydrous ethanol and 0% polar aprotic solvent.

[0060] This invention also provides a bilayer polyimide composite aerogel, which is prepared using any of the preparation methods described above.

[0061] like Figure 1 and Figure 2As shown, the bilayer polyimide composite aerogel of this invention adopts an asymmetric structural design of "top hydrophilic and bottom hydrophilic," that is, it consists of an integrally formed bottom superhydrophilic layer and a top superhydrophobic layer in the thickness direction. Among them, the superhydrophilic layer serves as an active pumping channel for the cooling medium. With the strong capillary force generated by its high surface energy framework and microporous structure, it can quickly adsorb and transport the cooling medium (such as deionized water) upward, ensuring continuous liquid supply under high temperature and high heat flux conditions. The top superhydrophobic layer (contact angle of about 145°) serves as a gas-liquid interface control domain. It uses its low surface energy characteristics to build a physical barrier, effectively inhibiting the excessive spread of liquid water on the surface due to capillary pressure to form a continuous liquid film. This asymmetric wettability design achieves a dynamic match between the cooling medium transport flux and the phase change evaporation rate induced by surface heat flux. This overcomes the cooling failure problem caused by the inability of homogeneous hydrophobic materials to spontaneously absorb water, and avoids the problems of decreased latent heat utilization and reduced thermal resistance caused by excessive wetting and the formation of liquid films on the surface of homogeneous hydrophilic materials. The Janus structure can precisely anchor the gas-liquid phase change interface inside the aerogel or in the surface pores, greatly increasing the evaporation surface area. Thus, with its efficient sweating cooling mechanism, it significantly improves the material's temperature tolerance, service life, and critical heat flux density in practical applications.

[0062] This invention also provides an application of the above-described bilayer polyimide composite aerogel or the bilayer polyimide composite aerogel prepared by any of the above preparation methods in sweating cooling heat protection.

[0063] The bilayer polyimide composite aerogel in this invention exhibits excellent comprehensive performance due to its asymmetric wetting structure (hydrophobic on top, hydrophilic on bottom) and stable ball-chain network. Its aerogel structure itself possesses excellent passive thermal insulation properties, and it can synergize with active perspiration cooling to more efficiently cope with extreme thermal environments. Experimental tests show that after being continuously heated to a 300°C hot plate for 365 seconds, the cold surface temperature of this composite aerogel remains at 80.5°C. Based on this characteristic, it can be applied not only to thermal insulation of morphing aircraft but also to heat dissipation and protection of electronic devices such as large-scale integrated circuits.

[0064] To more clearly illustrate the technical solution and advantages of the present invention, the following examples provide a detailed description of a bilayer polyimide composite aerogel, its preparation method, and its application.

[0065] Example 1: (1) Under a nitrogen atmosphere, 100 mL of dehydrated polar aprotic solvent (N,N-dimethylacetamide) was added to a 250 mL three-necked flask, and then 0.0097 mol of flexible diamine monomer (4,4'-diaminodiphenyl ether) and 0.010 mol of high surface energy rigid dianhydride monomer (3,3',4,4'-biphenyltetracarboxylic dianhydride) were added and stirred to mix. The polymerization reaction was carried out at room temperature (25°C) for 12 h. 0.0097 mol of rigid diamine monomer (9,9-bis(4-aminophenyl)fluorene) and 0.010 mol of high surface energy rigid dianhydride monomer (3,3',4,4'-biphenyltetracarboxylic dianhydride) were added to the resulting solution, and the reaction was continued at room temperature (25°C) for 12 h to obtain the first polyamic acid solution. (2) Under a nitrogen atmosphere, 100 mL of dehydrated polar aprotic solvent (N,N-dimethylacetamide) was added to a 250 mL three-necked flask, and then 0.0173 mol of flexible diamine monomer (4,4'-diaminodiphenyl ether) and 0.0045 mol of fluorinated dianhydride monomer (4,4-(hexafluoroisopropyl) bisphthalic anhydride) were added and stirred until homogeneous. The polymerization reaction was carried out at room temperature (25°C) for 12 h. 0.0134 mol of high surface energy rigid dianhydride monomer (3,3',4,4'-biphenyltetracarboxylic dianhydride) was added to the resulting solution, and the reaction was continued at room temperature (25°C) for 12 h to obtain the second polyamic acid solution. (3) Mix 5 mL of alkyltrialkoxysilane (methyltrimethoxysilane), 10 mL of mixed solvent (deionized water and anhydrous ethanol in a volume ratio of 1:1) and 0.15 mL of the first catalyst (glacial acetic acid), then add 0.5 g of surfactant (polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer F127) and stir for 15 min until the solution is clear and transparent to obtain a polysiloxane solution; 2 mL of polysiloxane solution was stirred and mixed with 100 mL of first polyamic acid solution and 100 mL of second polyamic acid solution to obtain composite solution S1 and composite solution S2. Dehydrating agent (acetic anhydride) and first catalyst (triethylamine) were added to composite solution S1 and composite solution S2 respectively and mixed and reacted to obtain first composite sol and second composite sol respectively. In the first composite sol and the second composite sol, the molar ratio of dehydrating agent to high surface energy rigid dianhydride monomer was 8:1 (the molar ratio of the sum of the molar ratio of high surface energy rigid dianhydride monomer and fluorinated dianhydride monomer to dehydrating agent was 1:8), and the molar ratio of dehydrating agent to first catalyst was 5:8. (4) Pour 10 mL of the first composite sol into the mold, let it stand for 10 min, and then slowly pour 10 mL of the second composite sol onto the surface of the first composite sol. After standing for 15 min, a bilayer composite wet gel is obtained. The bilayer composite wet gel is washed with a polar aprotic solvent (N,N-dimethylacetamide). After washing, a composite solvent is used for solvent replacement. Specifically, the gel is first soaked in a composite solvent formed by 25% anhydrous ethanol and 75% N,N-dimethylacetamide for 12 h, then soaked in a composite solvent formed by 75% anhydrous ethanol and 25% N,N-dimethylacetamide for 12 h, and then soaked in 100% anhydrous ethanol for 48 h to complete the solvent replacement. Finally, the wet gel after replacement is placed in a supercritical drying kettle and dried at 10 MPa and 42 °C for 10 h to obtain a bilayer polyimide composite aerogel.

[0066] Example 2: Example 2 is basically the same as Example 1, except that in step (3), the amount of polysiloxane solution added is 3 mL, and the first polyamic acid solution and the second polyamic acid solution are both 100 mL, that is, the volume ratio of the first polyamic acid solution, the second polyamic acid solution and the polysiloxane solution is 1:0.03.

[0067] Example 3: Example 3 is basically the same as Example 1, except that in step (3), the amount of polysiloxane solution added is 4 mL, and the first polyamic acid solution and the second polyamic acid solution are both 100 mL, that is, the volume ratio of the first polyamic acid solution, the second polyamic acid solution and the polysiloxane solution is 0.04:1.

[0068] Example 4: Example 4 is basically the same as Example 1, except that in step (1), under a nitrogen atmosphere, 100 mL of dehydrated polar aprotic solvent (N,N-dimethylacetamide) is added to a 250 mL three-necked flask, and then 0.01385 mol of flexible diamine monomer (4,4'-diaminodiphenyl ether) and 0.010 mol of high surface energy rigid dianhydride monomer (3,3',4,4'-biphenyltetracarboxylic dianhydride) are added and stirred to mix, and the polymerization reaction is carried out at room temperature (25°C) for 12 h; 0.0055 mol of rigid diamine monomer (9,9-bis(4-aminophenyl)fluorene) and 0.010 mol of high surface energy rigid dianhydride monomer (3,3',4,4'-biphenyltetracarboxylic dianhydride) are added to the resulting solution, and the reaction is continued to be stirred at room temperature (25°C) for 12 h to obtain the first polyamic acid solution.

[0069] Example 5: Example 5 is basically the same as Example 4, except that in step (3), the amount of polysiloxane solution added is 3 mL, and the first polyamic acid solution and the second polyamic acid solution are both 100 mL, that is, the volume ratio of the first polyamic acid solution, the second polyamic acid solution and the polysiloxane solution is 1:0.03.

[0070] Example 6: Example 6 is basically the same as Example 4, except that in step (3), the amount of polysiloxane solution added is 4 mL, and the first polyamic acid solution and the second polyamic acid solution are both 100 mL, that is, the volume ratio of the first polyamic acid solution, the second polyamic acid solution and the polysiloxane solution is 1:0.04.

[0071] Comparative Example 1 (1) Under a nitrogen atmosphere, 100 mL of dehydrated polar aprotic solvent (N,N-dimethylacetamide) was added to a 250 mL three-necked flask, and then 0.0097 mol of flexible diamine monomer (4,4'-diaminodiphenyl ether) and 0.010 mol of high surface energy rigid dianhydride monomer (3,3',4,4'-biphenyltetracarboxylic dianhydride) were added and stirred to mix. The polymerization reaction was carried out at room temperature (25°C) for 12 h. 0.0097 mol of rigid diamine monomer (9,9-bis(4-aminophenyl)fluorene) and 0.010 mol of high surface energy rigid dianhydride monomer (3,3',4,4'-biphenyltetracarboxylic dianhydride) were added to the resulting solution, and the reaction was continued at room temperature (25°C) for 12 h to obtain a polyamic acid solution. (2) Mix 5 mL of alkyltrialkoxysilane (methyltrimethoxysilane), 10 mL of mixed solvent (deionized water and anhydrous ethanol in a volume ratio of 1:1) and 0.15 mL of the first catalyst (glacial acetic acid), then add 0.5 g of surfactant (polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer F127) and stir for 15 min until the solution is clear and transparent to obtain a polysiloxane solution; (3) Mix 100 mL of polyamic acid solution with 2 mL of polysiloxane solution, then add dehydrating agent (acetic anhydride) and first catalyst (triethylamine) and mix well to obtain composite sol; pour the composite sol into a mold and let it stand for 15 min to obtain composite wet gel; wash the composite wet gel with a polar aprotic solvent (N,N-dimethylacetamide), and then replace the solvent with a composite solvent. Specifically, first soak in a composite solvent formed by 25% anhydrous ethanol and 75% N,N-dimethylacetamide for 12 h, then soak in a composite solvent formed by 75% anhydrous ethanol and 25% N,N-dimethylacetamide for 12 h, and then soak in 100% anhydrous ethanol for 48 h to complete the solvent replacement. Finally, put the replaced wet gel into a supercritical drying kettle and dry it at 10 MPa and 42 °C for 10 h to obtain polyimide composite aerogel.

[0072] Comparative Example 2 (1) Under a nitrogen atmosphere, 100 mL of dehydrated polar aprotic solvent (N,N-dimethylacetamide) was added to a 250 mL three-necked flask, and then 0.0097 mol of flexible diamine monomer (4,4'-diaminodiphenyl ether) and 0.010 mol of fluorinated dianhydride monomer (4,4-(hexafluoroisopropyl) bisphthalic anhydride) were added and stirred until homogeneous. The polymerization reaction was carried out at room temperature (25 °C) for 12 h. 0.0097 mol of high surface energy rigid dianhydride monomer (3,3',4,4'-biphenyltetracarboxylic dianhydride) was added to the resulting solution, and the reaction was continued at room temperature (25 °C) for 12 h to obtain a polyamic acid solution. (2) Mix 5 mL of alkyltrialkoxysilane (methyltrimethoxysilane), 10 mL of mixed solvent (deionized water and anhydrous ethanol in a volume ratio of 1:1) and 0.15 mL of the first catalyst (glacial acetic acid), then add 0.5 g of surfactant (polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer F127) and stir for 15 min until the solution is clear and transparent to obtain a polysiloxane solution; (3) Mix 100 mL of polyamic acid solution with 2 mL of polysiloxane solution, then add dehydrating agent (acetic anhydride) and first catalyst (triethylamine) and mix well to obtain composite sol; pour the composite sol into a mold and let it stand for 15 min to obtain composite wet gel; wash the composite wet gel with a polar aprotic solvent (N,N-dimethylacetamide), and then replace the solvent with a composite solvent. Specifically, first soak in a composite solvent formed by 25% anhydrous ethanol and 75% N,N-dimethylacetamide for 12 h, then soak in a composite solvent formed by 75% anhydrous ethanol and 25% N,N-dimethylacetamide for 12 h, and then soak in 100% anhydrous ethanol for 48 h to complete the solvent replacement. Finally, put the replaced wet gel into a supercritical drying kettle and dry it at 10 MPa and 42 °C for 10 h to obtain polyimide composite aerogel.

[0073] The performance of the bilayer polyimide composite aerogel samples provided in the examples and comparative examples was tested, and the test results are shown in Table 1. Figures 3 to 8 As shown: The thermal conductivity was measured using a square sample at room temperature (25°C) by a flat plate heat flow meter method.

[0074] Table 1 Depend on Figure 3 and Figure 4 As can be seen from the above, in the bilayer polyimide composite aerogel prepared in Example 1 of this invention, the hydrophobic layer has a dendritic three-dimensional network structure similar to that of traditional polyimide aerogels; the hydrophilic layer consists of polyimide molecular chains and polysiloxane nanoparticles forming a continuous framework, possessing both flexibility and good thermal insulation properties. Wettability test results ( Figures 5 to 8 Further evidence shows that the hydrophobic layer sample can float on water for an extended period without wetting, with a water contact angle of approximately 145°, exhibiting superhydrophobicity; while the hydrophilic layer sample rapidly wets and sinks upon contact with water, with a water contact angle close to 0°, demonstrating typical superhydrophilicity and rapid permeation behavior, proving that this bilayer structure possesses significant asymmetric wetting properties. Furthermore, as... Figure 9 As shown in Table 1, after being continuously heated on a 300°C hot plate for 365 seconds, the cold surface temperature of this bilayer composite aerogel is only 80.5°C, exhibiting a low thermal conductivity and excellent thermal insulation performance. While the thermal conductivity of the aerogel materials in Comparative Examples 1 and 2 is similar to that of Example 1, they both exhibit significant drawbacks in sweating-cooling thermal protection applications. The aerogel material in Comparative Example 1 is prone to collapse under hot and humid conditions and has low latent heat of vaporization utilization; while the aerogel material in Comparative Example 2 cannot effectively transport the cooling medium, making it difficult to achieve active thermal protection through a sweating-cooling mechanism.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a bilayer polyimide composite aerogel, characterized in that, The preparation method includes the following steps: (1) A first polyamic acid solution is obtained by stepwise polymerization of flexible diamine monomer, rigid diamine monomer and high surface energy rigid dianhydride monomer in a polar aprotic solvent. (2) The flexible diamine monomer is polymerized sequentially with a fluorinated dianhydride monomer and a high surface energy rigid dianhydride monomer in a polar aprotic solvent to obtain a second polyamic acid solution; (3) The first polyamic acid solution and the second polyamic acid solution are mixed with the polysiloxane solution, the dehydrating agent and the first catalyst respectively to obtain the first composite sol and the second composite sol respectively; (4) The first composite sol and the second composite sol are poured into the mold in sequence, and after being allowed to stand, aged, replaced by solvent and dried by supercritical drying, the bilayer polyimide composite aerogel is obtained.

2. The preparation method according to claim 1, characterized in that, In steps (1) and (2), the flexible diamine monomer is 4,4'-diaminodiphenyl ether; the rigid diamine monomer is 9,9-bis(4-aminophenyl)fluorene; The high surface energy rigid dianhydride monomer is at least one of 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-oxophthalic anhydride, or pyromellitic dianhydride; the fluorinated dianhydride monomer is 4,4-(hexafluoroisopropyl)phthalic anhydride; and / or The polar aprotic solvent is at least one of N,N-dimethylacetamide or N-methylpyrrolidone.

3. The preparation method according to claim 1, characterized in that, In step (1), the molar ratio of flexible diamine monomer to rigid diamine monomer is (1~2.5):1; Preferably, the molar ratio of the sum of the flexible diamine monomer and the rigid diamine monomer to the high surface energy rigid dianhydride monomer is (0.7~1.0):1; and / or The mass ratio of high surface energy rigid dianhydride monomer to polar aprotic solvent is 1:(10~20).

4. The preparation method according to claim 1, characterized in that, In step (2), the molar ratio of the fluorinated dianhydride monomer and the high surface energy rigid dianhydride monomer is 1:(2.5~3.0). Preferably, the molar ratio of the sum of the fluorinated dianhydride monomer and the high surface energy rigid dianhydride monomer to the flexible diamine monomer is (1.0~1.2):1; and / or The mass ratio of the sum of the masses of the fluorinated dianhydride monomer and the high surface energy rigid dianhydride monomer to the mass of the polar aprotic solvent is 1:(10~20).

5. The preparation method according to claim 1, characterized in that, In step (3), the polysiloxane solution is obtained by stirring and mixing alkyltrialkoxysilane, mixed solvent, second catalyst and surfactant. Preferably, the alkyltrialkoxysilane is at least one of methyltrimethoxysilane or vinyltrimethoxysilane; More preferably, the mixed solvent is ethanol and water; wherein the volume ratio of ethanol to water is preferably (1~2):(1~2). The second catalyst is acetic acid; and / or The surfactant is a polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer.

6. The preparation method according to claim 5, characterized in that, The volume ratio of alkyltrialkoxysilane, mixed solvent, and second catalyst is (1~2):(2~4):(0.02~0.04); and / or The volume-to-mass ratio of alkyltrialkoxysilane to surfactant is (5~10):0.5mL / g.

7. The preparation method according to claim 1, characterized in that, In step (3), the dehydrating agent is at least one of acetic anhydride or propionic anhydride; the first catalyst is at least one of triethylamine or pyridine; Preferably, the molar ratio of the first catalyst to the dehydrating agent is (7~10):(5~8); More preferably, the molar ratio of the sum of the high surface energy rigid dianhydride monomers and the fluorinated dianhydride monomers to the first catalyst and the dehydrating agent is 1:(7~10):(5~8); and / or The volume ratio of the first polyamic acid solution, the second polyamic acid solution, and the polysiloxane solution is 1:(0.02~0.04).

8. The preparation method according to claim 1, characterized in that, In step (4), the volume ratio of the first composite sol to the second composite sol is 1:(0.5~1). Preferably, a composite solvent is used for solvent replacement; wherein the composite solvent is anhydrous ethanol and N,N-dimethylacetamide; The aging temperature is 20~35℃, and the time is 9~12h; and / or The supercritical drying process involves a pressure of 8-10 MPa, a temperature of 37-42°C, and a time of 8-10 hours.

9. A bilayer polyimide composite aerogel, characterized in that, It is prepared by any one of the preparation methods according to claims 1 to 8.

10. The application of the bilayer polyimide composite aerogel prepared by the preparation method according to any one of claims 1 to 8 or the bilayer polyimide composite aerogel according to claim 9 in a thermal protection system.

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