A polyimide composite aerogel and a preparation method thereof

Abstract

The application provides a polyimide composite aerogel and a preparation method thereof, and comprises the following steps: performing step-by-step polymerization reaction on flexible diamine monomers, rigid diamine monomers and dianhydride monomers in a polar aprotic solvent to obtain a polyamide acid solution; stirring and uniformly mixing alkyl trialkoxysilane, a mixed solvent, a first catalyst and a surfactant to obtain a polysiloxane solution; uniformly mixing and reacting the polysiloxane solution, the polyamide acid solution, a dehydrating agent and a second catalyst, and obtaining the polyimide composite aerogel after the obtained composite sol is sequentially subjected to aging, solvent replacement and supercritical drying. The polyimide composite aerogel prepared in the scheme has excellent flexibility, high-temperature resistance and excellent structural stability, and does not become brittle after experiencing multiple water absorption-evaporation cycles, and has a wide application prospect in the thermal protection field of extreme environments such as sweat cooling and aerospace.

Description

Technical Field

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

[0002] Polyimide aerogel combines the advantages of both polyimide and aerogel materials. Compared to traditional inorganic aerogels (such as SiO2 and Al2O3), it exhibits superior mechanical strength and good flexibility. Compared to organic polymer aerogels (such as polyurethane and cellulose aerogels), it possesses superior high-temperature resistance and thermal dimensional stability. It is a lightweight, thermally insulating, flexible, and high-temperature resistant high-performance thermal insulation material. However, in extreme high-temperature thermal protection scenarios such as aerospace, it is difficult to simultaneously improve high-temperature resistance, thermal stability, and flexibility simply by controlling the molecular structure of polyimide or introducing inorganic fillers. Furthermore, passive thermal insulation alone cannot meet the protection requirements for long-term, ultra-high heat flux densities.

[0003] Combining polyimide aerogel with evaporative cooling, which utilizes the endothermic mechanism of liquid phase change, is expected to significantly improve its reliability and thermal protection capabilities in extreme high-temperature environments. This requires the polyimide aerogel to not only be resistant to high temperatures but also to be able to quickly adsorb and evaporate moisture to utilize the latent heat of phase change, while maintaining good dimensional stability and structural integrity during the process.

[0004] However, due to the hydrophilicity of its polar groups, traditional polyimide aerogels experience enormous capillary forces during the drying or evaporation phase after absorbing water. This causes the fragile three-dimensional porous nanonetwork to collapse and aggregate irreversibly, resulting in a dramatic shrinkage of the material volume (linear shrinkage rate exceeding 40%). Consequently, the material becomes dense, brittle, and loses its thermal insulation properties. Although hydrophobic modification (such as introducing fluorinated groups or using silane coupling agents for surface modification) can improve the structural stability of polyimide aerogels in humid environments, this significantly reduces their ability to bind with water, making it difficult to achieve rapid water adsorption and evaporation. This prevents efficient utilization of the latent heat of phase change of water for sweating and cooling, thus limiting its application in high heat flux density protection scenarios.

[0005] Therefore, there is an urgent need to provide a polyimide composite aerogel and its preparation method. Summary of the Invention

[0006] This invention provides a polyimide composite aerogel and its preparation method, which can solve the problem that existing polyimide aerogels are difficult to adsorb and evaporate moisture to utilize the latent heat of phase change, while maintaining dimensional stability and structural integrity in the process.

[0007] In a first aspect, a method for preparing a polyimide composite aerogel includes the following steps: (1) A stepwise polymerization reaction of flexible diamine monomer, rigid diamine monomer and dianhydride monomer is carried out in a polar aprotic solvent to obtain a polyamic acid solution; (2) The alkyltrialkoxysilane, mixed solvent, first catalyst and surfactant are stirred and mixed to obtain a polysiloxane solution; (3) The polysiloxane solution, the polyamic acid solution, the dehydrating agent and the second catalyst are mixed and reacted. The resulting composite sol is then subjected to aging, solvent replacement and supercritical drying to obtain the polyimide composite aerogel.

[0008] Preferably, in step (1), the flexible diamine monomer is 4,4'-diaminodiphenyl ether, and the rigid diamine monomer is 9,9-bis(4-aminophenyl)fluorene; the molar ratio of the flexible diamine monomer to the rigid diamine monomer is preferably (1~2.5):1.

[0009] Preferably, the dianhydride monomer is at least one of 3,3',4,4'-biphenyltetracarboxylic dianhydride or pyromellitic dianhydride.

[0010] Preferably, the polar aprotic solvent is at least one of N,N-dimethylacetamide or N-methylpyrrolidone.

[0011] 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 dianhydride monomer is (0.7~1.0):1.

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

[0013] Preferably, in step (2), the alkyltrialkoxysilane is at least one of methyltrimethoxysilane, vinyltrimethoxysilane, or phenyltrimethoxysilane.

[0014] Preferably, in step (2), the mixed solvent is ethanol and water; wherein the volume ratio of ethanol to water is preferably (1~2):(1~2).

[0015] Preferably, in step (2), the first catalyst is acetic acid.

[0016] Preferably, in step (2), the surfactant is a polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer.

[0017] Preferably, in step (2), the volume ratio of alkyltrialkoxysilane, mixed solvent and first catalyst is (1~2):(2~4):(0.02~0.04).

[0018] Preferably, in step (2), the volume-to-mass ratio of alkyltrialkoxysilane to surfactant is (5~10):0.5mL / g.

[0019] Preferably, in step (3), the dehydrating agent is at least one of acetic anhydride or propionic anhydride; and the second catalyst is at least one of triethylamine or pyridine.

[0020] Preferably, the molar ratio of dianhydride monomer, second catalyst and dehydrating agent is 1:(7~10):(5~8).

[0021] Preferably, the volume ratio of the polyamic acid solution to the polysiloxane solution is 1:(0.02~0.04).

[0022] Preferably, in step (1), the polymerization reaction is carried out at a temperature of 20~35℃ for 12~24h.

[0023] Preferably, in step (3), a composite solvent is used for solvent replacement; wherein the composite solvent is anhydrous ethanol and N,N-dimethylacetamide.

[0024] Preferably, in step (3), 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.

[0025] More preferably, the content of anhydrous ethanol is 25% to 100% by volume percentage.

[0026] Preferably, in step (3), the aging temperature is 20~35℃ and the time is 9~12h.

[0027] Preferably, in step (3), the supercritical drying pressure is 8~10MPa, the temperature is 37~42℃, and the time is 8~10h.

[0028] Secondly, embodiments of the present invention also provide a polyimide composite aerogel, which is prepared using the preparation method described in any one of the first aspects above.

[0029] Thirdly, embodiments of the present invention also provide an application of the polyimide composite aerogel described in the second aspect above or the polyimide composite aerogel prepared by any of the preparation methods described in the first aspect above in sweating cooling heat protection.

[0030] Compared with the prior art, the present invention has at least the following beneficial effects: (1) In this invention, firstly, flexible diamine monomers, rigid diamine monomers and dianhydride monomers are polymerized in a polar aprotic solvent to form a macromolecular backbone structure with alternating flexible and rigid segments. Then, alkyltrialkoxysilane, mixed solvent, first catalyst and surfactant are stirred and mixed to form a polysiloxane solution. The formed polyamic acid solution and polysiloxane solution are then subjected to a sol-gel reaction in a dehydrating agent and catalyst system. Thus, the polyimide molecular chains coat the surface of the in-situ generated polysiloxane particles in situ, thereby forming a unique sphere-chain double network structure. This structure significantly enhances the rigidity and shrinkage resistance of the aerogel skeleton, enabling it to effectively resist capillary forces during the evaporation process after water immersion and maintain the integrity of the microstructure. The average linear shrinkage rate can be as low as about 4.5%, which is much lower than that of conventional polyimide aerogel (shrinkage rate can reach 44.9%). This results in a polyimide composite aerogel that possesses excellent flexibility, high temperature resistance, and superior structural stability. Furthermore, it does not become brittle after undergoing multiple water absorption-evaporation cycles, making it a promising candidate for applications in thermal protection in extreme environments such as sweating cooling and aerospace.

[0031] (2) The polyimide composite aerogel prepared in this invention not only has excellent evaporation size stability, but also has a simple preparation process, is easy to operate and has low cost, good formability, and the material shape can be flexibly designed according to requirements. At the same time, the volume density of the final aerogel can be precisely controlled by adjusting the solid content of the polyamic acid solution, so that it can adapt to a variety of different application environments and molding requirements, and has broad application potential. Attached Figure Description

[0032] 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.

[0033] Figure 1 These are scanning electron microscope images of a polyimide composite aerogel provided in Embodiment 2 of the present invention at different scales. Figure 2 The infrared spectrum of a polyimide composite aerogel provided in Embodiment 2 of the present invention; Figure 3 Thermogravimetric analysis diagram of a polyimide composite aerogel provided in Embodiment 2 of the present invention; Figure 4 This is a tensile stress-strain curve of a polyimide composite aerogel provided in Embodiment 2 of the present invention; Figure 5These are comparative images of the physical appearance of a polyimide composite aerogel provided in Example 2 of the present invention in its initial state and after it has completely dried following water absorption, along with a scanning electron microscope image. Figure 6 The images show a comparison of the physical appearance of the polyimide composite aerogel provided in Comparative Example 1 of this invention in its initial state and after it has completely dried following water absorption, along with a comparison using scanning electron microscopy. Detailed Implementation

[0034] 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.

[0035] As mentioned earlier, although traditional polyimide aerogels are hydrophilic, their structure is prone to collapse after absorbing water. Hydrophobic modification reduces their ability to bind with water, making it difficult to utilize the latent heat of phase change of water for sweating and cooling. This severely limits the application potential of polyimide aerogels in high heat flux density protection scenarios.

[0036] Therefore, this invention provides a method for preparing polyimide composite aerogel, the method comprising the following steps: (1) The diamine monomer and the dianhydride monomer are subjected to a stepwise polymerization reaction in a polar aprotic solvent to obtain a polyamic acid solution; (2) The alkyltrialkoxysilane, mixed solvent, first catalyst and surfactant are stirred and mixed to obtain a polysiloxane solution; (3) The polysiloxane solution, the polyamic acid solution, the dehydrating agent and the second catalyst are mixed and reacted. The resulting composite sol is then subjected to aging, solvent replacement and supercritical drying to obtain the polyimide composite aerogel.

[0037] In this embodiment of the invention, firstly, flexible diamine monomers, rigid diamine monomers, and dianhydride monomers are subjected to a stepwise polymerization reaction in a polar aprotic solvent to form a macromolecular backbone structure with alternating flexible and rigid segments. Then, alkyltrialkoxysilane, mixed solvent, first catalyst, and surfactant are stirred and mixed to form a polysiloxane solution. The formed polyamic acid solution and the polysiloxane solution are then subjected to a sol-gel reaction in a dehydrating agent and catalyst system. This allows the polyimide molecular chains to in-situ coat the surface of the in-situ generated polysiloxane particles, thereby forming a unique sphere-chain dual network structure. This structure significantly enhances the rigidity and shrinkage resistance of the aerogel skeleton, enabling it to effectively resist capillary forces during the evaporation process after water immersion, maintaining the integrity of the microstructure. The average linear shrinkage rate can be as low as about 4.5%, which is much lower than that of conventional polyimide aerogels (shrinkage rate can reach 44.9%). This results in a polyimide composite aerogel that possesses excellent flexibility, high temperature resistance, and superior structural stability. Furthermore, it does not become brittle after undergoing multiple water absorption-evaporation cycles, making it a promising candidate for applications in thermal protection in extreme environments such as sweating cooling and aerospace.

[0038] According to some preferred embodiments, in step (1), the flexible diamine monomer is 4,4'-diaminodiphenyl ether, the rigid diamine monomer is 9,9-bis(4-aminophenyl)fluorene; the dianhydride monomer is at least one of 3,3',4,4'-biphenyltetracarboxylic dianhydride or pyromellitic dianhydride; and the polar aprotic solvent is at least one of N,N-dimethylacetamide or N-methylpyrrolidone.

[0039] In this embodiment of the invention, a two-step method is used to prepare the polyamic acid solution. First, flexible diamine monomer and dianhydride monomer are added to a polar aprotic solvent at a molar ratio of (0.9~1.0):1 and a polymerization reaction is carried out under an inert gas protective atmosphere. Then, rigid diamine monomer and 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 conducive to promoting the sequential incorporation and alternating arrangement of rigid and flexible chain segments, thereby forming a rigid-flexible synergistic polyamic acid macromolecular chain structure, thereby improving the flexibility of the composite aerogel.

[0040] According to some preferred embodiments, in step (1), the molar ratio of the flexible diamine monomer to the rigid diamine monomer is preferably (1~2.5):1; (for example, it can be 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, 2.2:1 or 2.5:1); 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 dianhydride monomer is (0.7~1.0):1 (for example, it can be 0.7:1, 0.8:1, 0.9:1 or 1:1); the mass ratio of the dianhydride monomer to the polar aprotic solvent is 1:(10~20) (for example, it can be 1:10, 1:12, 1:15, 1:18 or 1:20).

[0041] In this embodiment of the invention, by precisely controlling the proportions of flexible diamine monomer, rigid diamine monomer, and 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. The rigid segments provide strong mechanical support for the skeleton, while the flexible segments impart a certain degree of flexibility to the skeleton. This is beneficial to ensure that the final composite 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.

[0042] According to some preferred embodiments, in step (2), the alkyltrialkoxysilane is at least one of methyltrimethoxysilane, vinyltrimethoxysilane, or phenyltrimethoxysilane; the mixed solvent is ethanol and water; wherein the volume ratio of ethanol to water is preferably (1~2):(1~2) (for example, it can be 1:1, 1.5:1, 2:1, 1:2, 1.5:2, or 2:2); the first catalyst is acetic acid; and the surfactant is a polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer.

[0043] In this embodiment of the invention, during the preparation of the polysiloxane solution, alkyltrialkoxysilane is first mixed with a mixed solvent composed of ethanol and water, and acetic acid as a catalyst. Then, a surfactant is added and the mixture is stirred for 10-20 minutes until the solution becomes clear. The specific type and amount of surfactant not only stabilize the polyamic acid solution and the polysiloxane solution during mixing, preventing immediate phase separation, 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 polyimide molecular chains during subsequent mixing with the polyamic acid solution, ultimately forming a unique spherical-chain composite structure. In this structure, the spherical polysiloxane particles act as rigid support nodes, significantly enhancing the overall stability of the aerogel skeleton. This not only ensures good water absorption performance of the final composite aerogel material but also effectively resists capillary forces after water absorption, preventing network structure collapse and maintaining the integrity of the composite aerogel structure.

[0044] According to some preferred embodiments, in step (2), the volume ratio of alkyltrialkoxysilane, mixed solvent, and first catalyst is (1~2):(2~4):(0.02~0.04) (for example, it can be 1:2:0.02, 1.5:2:0.02, 2:2:0.002, 1:2:0.03, 1.5:2:0.03, 2:2:0.003, 1:2:0.04, 1.5:2:0.04, 2: 2:0.004, 1:3:0.02, 1:3:0.04, 1.5:3:0.03 or 2:4:0.04); the volume ratio of alkyltrialkoxysilane to surfactant is a volume-to-mass ratio of (5~10):0.5mL / g (for example, 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).

[0045] In this embodiment of the invention, by controlling the content of alkyltrialkoxysilane, it is beneficial to form uniformly dispersed spherical particles and a stable sphere-chain composite structure, which in turn is beneficial to obtaining a composite aerogel with excellent mechanical properties. If the content of alkyltrialkoxysilane is too high, it will be difficult for alkyltrialkoxysilane to be uniformly dispersed in the system, resulting in severe agglomeration of polysiloxane particles when mixed with polyamic acid solution, thus significantly reducing the mechanical properties of the composite aerogel. Conversely, if its content is too low, it is impossible to form a sufficient number of polysiloxane spherical particles of suitable size, which is not conducive to the formation of sphere-chain composite structure, and will also lead to a decrease in the mechanical properties of the composite aerogel.

[0046] According to some preferred embodiments, in step (3), the dehydrating agent is at least one of acetic anhydride or propionic anhydride; the second catalyst is at least one of triethylamine or pyridine; the molar ratio of dianhydride monomer, second catalyst and dehydrating agent is 1:(7~10):(5~8) (for example, 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, 1:10:5, 1:10:6, 1:10:7 or 1:10:8); the volume ratio of polyamic acid solution to polysiloxane solution is 1:(0.02~0.04) (for example, it can be 1:0.02, 1:0.03 or 1:0.04).

[0047] In this embodiment of the invention, a polysiloxane solution and a polyamic acid solution are first thoroughly stirred and mixed to ensure uniform contact of the components, resulting in a homogeneous and stable polyamic acid-polysiloxane composite solution. Subsequently, an appropriate amount of dehydrating agent and catalyst are added to the composite solution to initiate an imidization reaction. The interval between the simultaneous or sequential addition of the dehydrating agent and catalyst to the reaction system does not exceed 2 minutes, ensuring simultaneous dehydration and catalysis, guaranteeing rapid reaction, and yielding a structurally stable polyimide-polysiloxane composite sol. During this process, precise control of the volume ratio of the polysiloxane solution to the polyamic acid solution helps ensure that the final composite aerogel possesses both good water absorption and mechanical support.

[0048] According to some preferred embodiments, in step (1), the temperature of the polymerization reaction is 20~35℃ (e.g., 20℃, 22℃, 25℃, 28℃, 30℃, 32℃ or 35℃), and the time is 12~24h (e.g., 12h, 14h, 16h, 18h, 20h, 22h or 24h); in step (3), a composite solvent is used for solvent replacement; wherein, the composite solvent is anhydrous ethanol and N,N-dimethylacetamide; 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%~100% by volume percentage (e.g., 25%, 50%, 75% or 100%).

[0049] According to some preferred embodiments, in step (3), the aging temperature is 20~35℃ (e.g., 20℃, 22℃, 25℃, 28℃, 30℃, 32℃ 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).

[0050] In this embodiment of the invention, after obtaining the polyimide-polysiloxane composite sol, the composite sol is poured into a mold and allowed to stand for 15-20 minutes to form a polyimide-polysiloxane composite wet gel. The wet gel is then washed with anhydrous ethanol and subsequently immersed in a composite solvent formed by anhydrous ethanol and N,N-dimethylacetamide in a specific ratio for multi-step solvent replacement. With each replacement, 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 composite aerogel. For example, the immersion can first be performed for 12 hours using a composite solvent formed by 25% anhydrous ethanol and 75% polar aprotic solvent, followed by another 12 hours using a composite solvent formed by 75% anhydrous ethanol and 25% polar aprotic solvent, and then a final 48 hours using a composite solvent formed by 100% anhydrous ethanol and 0% polar aprotic solvent.

[0051] This invention also provides a polyimide composite aerogel, which is prepared using any of the preparation methods described above.

[0052] The polyimide composite aerogel prepared in this invention not only has excellent evaporation dimensional stability, but also has a simple preparation process, is easy to operate and has low cost, good formability, and the material shape can be flexibly designed according to requirements. At the same time, the volume density of the final aerogel can be precisely controlled by adjusting the solid content of the polyamic acid solution, so that it can adapt to a variety of different application environments and molding requirements, and has broad application potential.

[0053] This invention also provides an application of the polyimide composite aerogel described above or prepared by any of the above methods in a thermal protection system.

[0054] The polyimide composite aerogel prepared in the embodiments of the present invention has good evaporation dimensional stability. Its water absorption can exceed 500% of its own weight, and the average linear shrinkage rate after water absorption and thorough drying can be stably controlled below 4.5%. Based on this excellent dimensional retention ability and moisture resistance, combined with its inherent lightweight, flexibility and high temperature resistance, it has broad application prospects in thermal protection systems. It can be used as a high-performance thermal insulation material and a flexible thermal protection material, and is suitable for the packaging and thermal management of flexible printed circuit boards and large-scale integrated circuits with extremely high requirements for dimensional accuracy and reliability.

[0055] To more clearly illustrate the technical solution and advantages of the present invention, the following examples provide a detailed description of a polyimide composite aerogel and its preparation method.

[0056] 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 dianhydride monomer (3,3',4,4'-biphenyltetracarboxylic dianhydride) were added and stirred until homogeneous. 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 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) Add 2 mL of polysiloxane solution to 100 mL of polyamic acid solution and stir to mix. Then add dehydrating agent (acetic anhydride) and second catalyst (triethylamine) and mix to react. Pour the resulting composite sol into a mold and let it stand at room temperature (25°C) for 15 min to form a composite wet gel. Wash the composite wet gel with a polar aprotic solvent (N,N-dimethylacetamide). After washing, perform solvent replacement with a composite solvent. Specifically, first, use 25% anhydrous ethanol and 75% N,N-dimethylacetamide. The aerogel was soaked in a composite solvent formed by N,N-dimethylacetamide for 12 hours, then soaked in a composite solvent formed by 75% anhydrous ethanol and 25% N,N-dimethylacetamide for 12 hours, and then soaked in 100% anhydrous ethanol for 48 hours to complete solvent replacement. Finally, the replaced wet gel was placed in a supercritical drying vessel and dried at 10 MPa and 42 °C for 10 hours to obtain polyimide composite aerogel. The molar ratio of dehydrating agent to dianhydride monomer was 8:1, and the molar ratio of dehydrating agent to second catalyst was 5:8.

[0057] Example 2: Example 2 is basically the same as Example 1, except that in step (3), the content of polysiloxane solution is 3 mL.

[0058] Example 3: Example 3 is basically the same as Example 1, except that in step (3), the content of polysiloxane solution is 4 mL.

[0059] Example 4: Example 4 is basically the same as Example 2, except that in step (2), the alkyltrialkoxysilane is the same volume of vinyltrimethoxysilane.

[0060] Example 5: Example 5 is basically the same as Example 3, except that in step (2), the alkyltrialkoxysilane is the same volume of vinyltrimethoxysilane.

[0061] Example 6: Example 6 is basically the same as Example 2, except that in step (2), the alkyltrialkoxysilane is the same volume of phenyltrimethoxysilane.

[0062] Example 7: Example 7 is basically the same as Example 3, except that in step (2), the alkyltrialkoxysilane is the same volume of phenyltrimethoxysilane.

[0063] Comparative Example 1 Comparative Example 1 is basically the same as Example 1, except that step (2) is removed, that is, the dehydrating agent (acetic anhydride) and the second catalyst (triethylamine) are directly added to 100 mL of polyamic acid solution and mixed and reacted; wherein, the molar ratio of the dehydrating agent to the dianhydride monomer is 8:1, and the molar ratio of the dehydrating agent to the second catalyst is 5:8.

[0064] The performance of the polyimide composite aerogels (hereinafter referred to as samples) provided in the examples and comparative examples was tested, and the test results are shown in Tables 1 and 2. Figures 1 to 6 As shown: The samples prepared in the above embodiments and comparative examples were subjected to performance tests. The specific test methods were as follows: density was calculated by the mass-to-volume ratio of the sample; thermal conductivity was measured at room temperature using a flat plate heat flow meter with a square sample; thermal decomposition temperature was calculated by the thermogravimetric curve when 5% mass was lost; linear shrinkage was calculated by dividing the difference between the initial size and the size after drying by the initial size before drying; and water absorption rate was calculated by dividing the difference between the mass of the sample after it was fully saturated with water and the initial mass before water absorption by the initial mass before water absorption.

[0065] Table 1 Table 2 Depend on Figure 1 As can be seen from the above, the polyimide composite aerogel prepared in Example 2 of the present invention has a hierarchical porous network structure. The polyimide molecular chains and polysiloxane nanoparticles form a continuous skeleton. The surface of the polysiloxane nanoparticles is covered with a polyimide nanonetwork, which not only endows the polyimide-polysiloxane with excellent thermal stability and flexibility, but also has good evaporation size stability. It can still maintain structural integrity and stability after multiple water absorption-evaporation cycles.

[0066] Depend on Figure 2 The infrared spectrum shows that at 1775 cm⁻¹ -1 and 1715cm -1 The characteristic peak appearing at 1373 cm⁻¹ corresponds to the C=O group. -1 The characteristic peaks appearing at 2962 cm⁻¹ correspond to CNC bonds, and this set of characteristic peaks collectively confirms the presence of a typical polyimide structure in the aerogel. Meanwhile, the peak at 2962 cm⁻¹... -1 The absorption peak appearing at 1118 cm⁻¹ originates from the CH bond and is located at 1118 cm⁻¹. -1 and 1086cm -1 A distinct Si-O-Si bond characteristic absorption peak was observed, indicating that the polysiloxane structure was successfully introduced into the polyimide aerogel.

[0067] Figure 3The thermogravimetric analysis (TGA) plots show that the thermal decomposition process of the sample mainly consists of three stages: The first stage (25–400 °C) involves only a slight decrease in sample mass, primarily due to the desorption of physically adsorbed water or residual solvents, and the volatilization of a small amount of oligomers. The second stage (400–540 °C) involves a slow pre-decomposition stage, where the polyimide (PI) backbone begins to break bonds and rearrange, accompanied by initial carbonization. Simultaneously, the surface methyl groups of the polysiloxane component begin to decompose, and residual silanols further condense and densify, increasing the network crosslinking degree. This stage is also accompanied by the decomposition of residual surfactants. The third stage (540–800 °C) involves rapid cracking of the imide rings in the sample, with a significant weight loss step (5% weight loss) appearing at 542.9 °C. At this point, the main chain chemical bonds, such as imine bonds and carbon-carbon bonds, break.

[0068] Depend on Figure 4 As shown in the tensile stress-strain curve, the stress of the polyimide composite aerogel material increases rapidly in the strain range of 0-5%, and then increases nearly linearly and stably with strain in the range of 5-25%. This indicates that at this point, the spherical-chain continuous skeleton formed by the polyimide molecular chains and polysiloxane nanoparticles begins to effectively bear the main load. When the strain exceeds about 25%, the material enters a significant strain hardening region, and the stress continues to increase monotonically, reaching the maximum tensile strength of 0.72 MPa at about 36% strain, after which fracture occurs. This confirms that the formed spherical-chain structure enables the composite aerogel to possess both excellent strength and toughness.

[0069] By comparison Figure 5 and Figure 6 The physical appearance and scanning electron microscope images of Example 2 (polyimide composite aerogel) and Comparative Example 1 (ordinary polyimide aerogel) before and after water absorption and complete drying show that the aerogel in Comparative Example 1 underwent significant shrinkage, hardening, and embrittlement after water absorption and complete drying, with an average linear shrinkage rate as high as 44.9% (as shown in Table 2). This is due to the huge capillary force generated during water evaporation, which caused severe collapse and destruction of its three-dimensional nanofiber network. In contrast, the polyimide composite aerogel obtained in Example 2 has good evaporation dimensional stability. After undergoing the same water absorption-evaporation cycle, its average linear shrinkage rate is only 4.5%, which is about 1 / 10 of that in Comparative Example 1. Moreover, its unique sphere-chain microstructure remains intact without significant changes, as can be seen from the scanning electron microscope images.

[0070] In summary, combining Tables 1 and 2 and Figures 1 to 6 As can be seen from the above, the polyimide composite aerogel prepared in the embodiments of the present invention has the advantages of being lightweight, heat-insulating, and having a high thermal decomposition temperature.

[0071] 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 polyimide composite aerogel, characterized in that, The preparation method includes the following steps: (1) A stepwise polymerization reaction of flexible diamine monomer, rigid diamine monomer and dianhydride monomer is carried out in a polar aprotic solvent to obtain a polyamic acid solution; (2) The alkyltrialkoxysilane, mixed solvent, first catalyst and surfactant are stirred and mixed to obtain a polysiloxane solution; (3) The polysiloxane solution, the polyamic acid solution, the dehydrating agent and the second catalyst are mixed and reacted. The resulting composite sol is then subjected to aging, solvent replacement and supercritical drying to obtain the polyimide composite aerogel.

2. The preparation method according to claim 1, characterized in that, In step (1), the flexible diamine monomer is 4,4'-diaminodiphenyl ether, and the rigid diamine monomer is 9,9-bis(4-aminophenyl)fluorene; the preferred molar ratio of the flexible diamine monomer to the rigid diamine monomer is (1~2.5):1; The dianhydride monomer is at least one of 3,3',4,4'-biphenyltetracarboxylic dianhydride or pyromellitic dianhydride; 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 or 2, characterized in that, 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 dianhydride monomer is (0.7~1.0):1; and / or The mass ratio of the dianhydride monomer to the polar aprotic solvent is 1:(10~20).

4. The preparation method according to claim 1, characterized in that, In step (2), the alkyltrialkoxysilane is at least one of methyltrimethoxysilane, vinyltrimethoxysilane, or phenyltrimethoxysilane; The mixed solvent is ethanol and water; wherein the volume ratio of ethanol to water is preferably (1~2):(1~2). The first catalyst is acetic acid; and / or The surfactant is a polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer.

5. The preparation method according to claim 1 or 4, characterized in that, In step (2), the volume ratio of alkyltrialkoxysilane, mixed solvent, and first 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.

6. 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 second catalyst is at least one of triethylamine or pyridine; Preferably, the molar ratio of the dianhydride monomer, the second catalyst, and the dehydrating agent is 1:(7~10):(5~8); and / or The volume ratio of the polyamic acid solution to the polysiloxane solution is 1:(0.02~0.04).

7. The preparation method according to claim 1, characterized in that, In step (1), the polymerization reaction is carried out at a temperature of 20-35°C for 12-24 hours; and / or In step (3), a composite solvent is used for solvent replacement; wherein the composite solvent is anhydrous ethanol and N,N-dimethylacetamide; Preferably, as the number of solvent replacements increases, the content gradient of anhydrous ethanol in the composite solvent increases, while the content gradient of N,N-dimethylacetamide decreases; more preferably, the content of anhydrous ethanol is 25% to 100% by volume percentage.

8. The preparation method according to claim 1, characterized in that, In step (3), 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 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 polyimide composite aerogel prepared by the preparation method according to any one of claims 1 to 8 or the polyimide composite aerogel according to claim 9 in sweating cooling heat protection.

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