A high-porosity fluorinated polyimide aerogel and its preparation method

Abstract

A method for preparing high-porosity fluorinated polyimide aerogel involves preparing a polyamic acid solution using fluorinated diamine monomer TFMB, unfluorinated aromatic diamine monomer ODA, and fluorinated dianhydride monomer 6FDA as raw materials. Triethylamine is then added to prepare a fluorinated polyamic acid salt. The fluorinated polyamic acid salt is dissolved in a mixture of water and diphenyl ether, and triethylamine and N,N-dimethylcyclohexylamine are added to prepare a polyamic acid hydrogel. The hydrogel is then freeze-dried to obtain a polyamic acid aerogel, and finally subjected to thermal imidization treatment to obtain the fluorinated polyimide aerogel. This invention produces an aerogel using three monomers, exhibiting a stable structure, high heat resistance, and good performance at high temperatures. The aerogel possesses a certain rigidity and strength, making it resistant to deformation and structural collapse under external forces. Under this process, the polyimide aerogel forms a high-porosity three-dimensional network structure, exhibiting excellent thermal insulation and sound insulation properties.

Description

Technical Field

[0001] This invention relates to the field of aerogel materials technology, specifically to a high-porosity fluorinated polyimide aerogel and its preparation method. Background Technology

[0002] Polyimide (PI) is a class of polymers containing repeating imide rings (-CO-N-CO-) in its main chain. Due to its excellent comprehensive properties, such as outstanding thermal stability, good mechanical properties, high chemical stability, and low dielectric constant, it has been widely used in many fields, including aerospace, electronics, and automotive manufacturing, and is one of the most outstanding polymer materials currently available. Traditionally, polyimides exist primarily in the form of films, fibers, or molding compounds. With the development of materials science, researchers have begun to explore novel forms and aerogels of polyimides.

[0003] With the ever-increasing demand for high-performance materials across various industries, the development of aerogels with specific functions has become a research hotspot. For example, in the aerospace field, aerogel materials are required to not only be lightweight and have thermal insulation properties, but also maintain stable performance under extreme environments; in the electronics field, aerogel materials are required to have good electrical properties and thermal stability. Polyimide aerogel is a novel aerogel material with polyimide as its framework. It is formed by first preparing a polyimide wet gel through the "sol-gel method," and then removing the liquid from the pores (while retaining air) through supercritical drying. Its structure is characterized by a nanoscale porous network with high porosity and low density. These properties endow aerogels with excellent thermal insulation performance, low sound velocity propagation characteristics, and high specific surface area, showing great application potential in fields such as thermal insulation, sound absorption and noise reduction, and catalyst supports.

[0004] Introducing fluorine atoms or fluorine-containing groups into the molecular structure of polyimides can significantly improve their properties. Fluorine atoms, with their high electronegativity, small atomic radius, and high bond energy, can impart lower dielectric constants, better chemical stability, thermal stability, and hydrophobicity to materials. Combining the high porosity and low density of aerogels with the excellent properties of fluorinated polyimides, high-porosity, low-density fluorinated polyimide aerogels have unique advantages in multiple fields. For example, in the aerospace field, their lightweight, thermally insulating, and stable properties can be used in spacecraft thermal protection systems; in electronic packaging, their low dielectric constant and good thermal stability help improve the performance and reliability of electronic devices.

[0005] However, there are many technical challenges in preparing high-porosity, low-density fluorinated polyimide aerogels. For example, during the synthesis process, it is necessary to precisely control the polymerization reaction of the fluorinated monomers to avoid the collapse of the aerogel structure due to excessively fast or uneven reactions. At the same time, maintaining high porosity and low density while ensuring that the aerogel has a certain mechanical strength during the aerogel molding process is also a major challenge. Summary of the Invention

[0006] The purpose of this invention is to provide a high-porosity fluorinated polyimide aerogel. This aerogel has high porosity, low density, and excellent mechanical strength.

[0007] Another objective of this invention is to provide a method for preparing the aforementioned polyimide aerogel. This method effectively solves the problem of poor structural stability and easy collapse of the aerogel during the preparation process.

[0008] The objective of this invention is achieved through the following technical solution:

[0009] A high-porosity fluorinated polyimide aerogel is characterized by: preparing a polyamic acid solution using fluorinated diamine monomer 2,2'-bis(trifluoromethyl)biphenyl-4,4'-diamine (TFMB), unfluorinated aromatic diamine monomer bis(4-aminophenyl) ether (ODA), and fluorinated dianhydride monomer 4,4'-(hexafluoroisopropene)phthalic anhydride (6FDA) as raw materials; adding triethylamine to the polyamic acid solution to prepare a fluorinated polyamic acid salt; dissolving the fluorinated polyamic acid salt in a mixture of water and diphenyl ether; adding triethylamine and N,N-dimethylcyclohexylamine to prepare a polyamic acid hydrogel; freeze-drying to obtain a polyamic acid aerogel; and finally, undergoing thermal imidization treatment to obtain the final product.

[0010] Furthermore, the molar ratio of TFMB, ODA and 6FDA is 0.5-0.8:0.5-0.2:1, and the solid content of the prepared polyamic acid solution is 12-20%.

[0011] Furthermore, the polar solvent is any one of DMF, DMAC, NMP, DMSO, and THF.

[0012] Furthermore, the molar ratio of the total amount of fluorinated diamine monomers and non-fluorinated aromatic diamine monomers used in the polyamic acid solution to triethylamine is 0.8~1.3:1.

[0013] Furthermore, in the process of preparing polyamic acid hydrogel, the mass ratio of water, triethylamine, N,N-dimethylcyclohexylamine, diphenyl ether and fluorinated polyamic acid salt is 10:1: 0.4~0.7:1~2:0.8-1.2.

[0014] Furthermore, the freeze-drying is carried out under conditions of 5-10 Pa and -65 to -55°C for 48 to 55 hours to obtain polyamic acid aerogel.

[0015] Furthermore, thermal imidization involves heating the polyamic acid aerogel to 90-110°C at a rate of 1-2°C / min and holding it at that temperature for 50-60 min, then heating it to 180-220°C at a rate of 3-5°C / min and holding it at that temperature for 50-60 min, and then heating it to 350-380°C at a rate of 0.5-1°C / min and holding it at that temperature for 60-80 min, causing the polyamic acid to dehydrate and condense to form polyimide, and then stabilizing it at a rate of 2-4°C / min to 25-40°C.

[0016] A method for preparing a high-porosity fluorinated polyimide aerogel is characterized by: preparing a polyamic acid solution using fluorinated diamine monomer TFMB, unfluorinated aromatic diamine monomer ODA, and fluorinated dianhydride monomer 6FDA as raw materials; adding triethylamine TEA to the polyamic acid solution to prepare a fluorinated polyamic acid salt; dissolving the fluorinated polyamic acid salt in a mixture of water and diphenyl ether DPE; adding triethylamine and N,N-dimethylcyclohexylamine DMCHA to prepare a polyamic acid hydrogel; freeze-drying to obtain a polyamic acid aerogel; and finally performing a thermal imidization treatment to obtain a fluorinated polyimide aerogel.

[0017] Furthermore, the molar ratio of TFMB, ODA and 6FDA is 0.5-0.8:0.5-0.2:1, and the solid content of the prepared polyamic acid solution is 12-20%.

[0018] Furthermore, the polar solvent is any one of DMF, DMAC, NMP, DMSO, and THF.

[0019] Furthermore, the molar ratio of the total amount of fluorinated diamine monomers and non-fluorinated aromatic diamine monomers used in the polyamic acid solution to triethylamine is 0.8~1.3:1.

[0020] Furthermore, in the process of preparing polyamic acid hydrogel, the mass ratio of water, triethylamine, N,N-dimethylcyclohexylamine, diphenyl ether and fluorinated polyamic acid salt is 10:1: 0.4~0.7:1~2:0.8~1.2.

[0021] During the preparation of fluorinated polyimide aerogels, the electron-withdrawing effect of fluorine and the steric hindrance of fluorinated groups lead to large fluctuations in the reaction rate, low crosslinking density and poor uniformity of the gel network, resulting in low structural stability. In the subsequent thermal imidization process, stress distribution imbalance causes structural collapse, and the mechanical strength of the prepared aerogel decreases significantly.

[0022] Furthermore, triethylamine reacts with polyamic acid to form a salt, and triethylamine can also act as a catalyst in the thermal imidization process, catalyzing the cyclization reaction and accelerating PAA dehydration. During the sol-gel process, triethylamine, as a linear aliphatic amine, exhibits hindered diffusion in the PAA sol, easily leading to a difference in concentration between the surface and interior layers. This causes PAA chain aggregation, resulting in poor uniformity of the gel network structure during thermal imidization, stress concentration, and a decrease in the mechanical strength of the aerogel. This invention incorporates a composite catalyst of DMCHA and triethylamine, which contains a cyclohexyl ring structure, resulting in stronger molecular rigidity and a more stable diffusion rate. This catalyst can form a synergistic effect of "rapid diffusion + uniform distribution" with triethylamine. Triethylamine rapidly neutralizes the carboxyl groups on the sol surface, promoting the slow penetration of DMCHA into the interior for uniform neutralization, reducing local aggregation, and resulting in a more uniform gel network structure. In the sol-gelation stage, a certain amount of diphenyl ether is also added. Its hydrophobic benzene ring can interact with the ethyl chain of triethylamine through van der Waals forces, reducing the local concentration gradient of triethylamine and inhibiting uneven gel shrinkage caused by solvent evaporation. The low polarity of diphenyl ether can balance the interaction between hydrophobic segments (such as fluorinated groups and aromatic rings) in polyamates and polar solvents (water), weakening the excessive aggregation of hydrophobic segments, promoting uniform entanglement of molecular chains, and forming a more regular gel network. During thermal imidization, under the regulation of diphenyl ether, the boiling point difference between triethylamine and DMCHA prolongs the catalytic cyclization reaction during imidization without destroying the porous structure of the aerogel, thus improving the uniformity of thermal stress distribution.

[0023] Furthermore, the freeze-drying is carried out under conditions of 5-10 Pa and -65 to -55°C for 48 to 55 hours to obtain polyamic acid aerogel.

[0024] Furthermore, thermal imidization involves heating the polyamic acid aerogel to 90-110°C at a rate of 1-2°C / min and holding it at that temperature for 50-60 min, then heating it to 180-220°C at a rate of 3-5°C / min and holding it at that temperature for 50-60 min, and then heating it to 350-380°C at a rate of 0.5-1°C / min and holding it at that temperature for 60-80 min, causing the polyamic acid to dehydrate and condense to form polyimide, and then stabilizing it at a rate of 2-4°C / min to 25-40°C.

[0025] A method for preparing a high-porosity fluorinated polyimide aerogel, characterized by comprising the following steps:

[0026] (a) Preparation of fluorinated polyamic acid solution:

[0027] Under a nitrogen atmosphere, fluorinated diamine monomer TFMB, unfluorinated aromatic diamine monomer ODA, and fluorinated dianhydride monomer 6FDA are dissolved in a polar solvent to obtain a pale yellow solution with a solid content of 12-20 wt%. Under a nitrogen atmosphere, the mixture is stirred for 2 hours at 0-10°C, and then stirred for 6-8 hours at 10-25°C to obtain a colorless and transparent polyamic acid solution. The molar ratio of TFBM, ODA, and 6DA is 0.5-0.8:0.5-0.2:1. The polar solvent is any one of DMF, DMAC, NMP, DMSO, and THF.

[0028] (II) Preparation of fluorinated polyamic acid salts

[0029] Triethylamine is added to a polyamic acid solution to react and generate a fluorinated polyamic acid salt. After the reaction is complete, the fluorinated polyamic acid salt is precipitated into deionized water and collected to obtain a filamentous or blocky solid. The solid fluorinated polyamic acid salt is obtained by freeze-drying at a temperature of -65 to -56°C, a pressure of 5 to 10 Pa, and a freeze-drying time of 48 to 55 h. The molar ratio of the total amount of fluorinated diamine monomer and unfluorinated aromatic diamine monomer used in the polyamic acid solution to triethylamine is 0.8 to 1.3:1.

[0030] (III) Preparation of Aerogels

[0031] (a) Using the fluorinated polyamic acid salt obtained in step (ii) as a precursor, a mixture of water and diphenyl ether is added, followed by the addition of triethylamine and N,N-dimethylcyclohexylamine. The mixture is allowed to stand for 18-24 hours for sol-gel conversion to obtain a polyamic acid hydrogel. The mass ratio of water, triethylamine, N,N-dimethylcyclohexylamine, diphenyl ether and fluorinated polyamic acid salt is 10:1:0.4-0.7:1-2:0.8-1.2.

[0032] (b) Polyamic acid hydrogel is freeze-dried to obtain polyamic acid aerogel, and finally thermal imidization is performed to obtain fluorinated polyimide aerogel. The freeze-drying is carried out at 5-10 Pa and -65~-55℃ for 48~55h to obtain polyamic acid aerogel.

[0033] (c) Thermal imidization involves heating the polyamic acid aerogel to 90-110°C at a rate of 1-2°C / min and holding it for 50-60 min, then heating it to 180-220°C at a rate of 3-5°C / min and holding it for 50-60 min, and then heating it to 350-380°C at a rate of 0.5-1°C / min and holding it for 60-80 min, causing the polyamic acid to dehydrate and condense to form polyimide, and then cooling it down to 25-40°C at a rate of 2-4°C / min.

[0034] In the thermal imidization process of this invention, the temperature is first slowly increased to a low temperature range to volatilize residual solvents and physically adsorbed water, preventing the gel skeleton from shrinking due to rapid weight loss. Then, the temperature is rapidly increased to a medium temperature range, where PAA molecular chains cyclize under the synergistic catalytic action of triethylamine and DMCHA, and the gel skeleton gradually hardens. Finally, the temperature is slowly increased to a high temperature range, where diphenyl ether effectively maintains the fluidity of the reaction system, further promoting the cyclization reaction and forming a stable polyimide network structure. This increases the porosity of the aerogel while maintaining the continuity of the skeleton, thereby improving the mechanical properties of the aerogel and removing residual stress. In this process, the catalytic time for the cyclization reaction is effectively extended by controlling the boiling point temperature difference between triethylamine and DMCHA in conjunction with specific imidization steps.

[0035] The present invention has the following technical effects:

[0036] This invention produces an aerogel using three monomers, resulting in a stable structure with high heat resistance, maintaining good performance at high temperatures. The aerogel possesses a certain degree of rigidity and strength, making it resistant to deformation and structural collapse under external forces. This process creates a high-porosity three-dimensional network structure in the polyimide aerogel, exhibiting excellent thermal and sound insulation properties. Furthermore, this polyimide aerogel demonstrates good resistance to acids, alkalis, organic solvents, and other chemicals, maintaining stable performance even in complex chemical environments. Attached Figure Description

[0037] Figure 1 Microscopic morphology images of the aerogels prepared in Example 1, Comparative Example 1, and Comparative Example 2 of this invention.

[0038] Figure 2 Thermogravimetric spectra of polyimide aerogels prepared in Examples 1, 1, and 2.

[0039] Figure 3 Comparison of density calculation results of polyimide aerogels prepared in Example 1, Comparative Example 1 and Comparative Example 2. Detailed Implementation

[0040] The present invention will be specifically described below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above description.

[0041] Example 1

[0042] A method for preparing a high-porosity fluorinated polyimide aerogel includes the following steps:

[0043] (a) Preparation of fluorinated polyamic acid solution:

[0044] Under a nitrogen atmosphere, fluorinated diamine monomer TFMB, unfluorinated aromatic diamine monomer ODA, and fluorinated dianhydride monomer 6FDA were dissolved in a polar solvent DMAC to obtain a pale yellow solution with a solid content of 16 wt%. Under a nitrogen atmosphere, the mixture was stirred at 5°C for 2 h and then at 20°C for 7 h to obtain a colorless and transparent polyamic acid solution. The molar ratio of TFBM, ODA, and 6DA was 0.6:0.6:1.

[0045] (II) Preparation of fluorinated polyamic acid salts

[0046] Triethylamine (TEA) and N,N-dimethylcyclohexylamine (DMCHA) were added to a polyamic acid solution to generate a fluorinated polyamic acid salt. After the reaction was completed, the fluorinated polyamic acid salt was precipitated into deionized water and collected to obtain filamentous or blocky solids. The solid fluorinated polyamic acid salt was obtained by freeze-drying at -60°C, at a pressure of 8 Pa, and for 50 h. The molar ratio of the total amount of fluorinated diamine monomers and unfluorinated aromatic diamine monomers used in the polyamic acid solution to triethylamine was 1:1.

[0047] (III) Preparation of Aerogels

[0048] (a) Using the fluorinated polyamic acid salt obtained in step (ii) as a precursor, a mixture of water and diphenyl ether is added, followed by the addition of triethylamine and N,N-dimethylcyclohexylamine. The polyamic acid hydrogel is obtained by sol-gel conversion after standing for 20 hours. The mass ratio of water, triethylamine, N,N-dimethylcyclohexylamine, diphenyl ether and fluorinated polyamic acid salt is 10:1:0.5:1.5:1.

[0049] (b) Polyamic acid hydrogel was freeze-dried to obtain polyamic acid aerogel, and finally thermal imidization was performed to obtain fluorinated polyimide aerogel. The freeze-drying was carried out at 6 Pa and -60°C for 48 to 55 hours to obtain polyamic acid aerogel.

[0050] (c) Thermal imidization involves heating the polyamic acid aerogel to 100°C at a rate of 1.5°C / min and holding it at that temperature for 55 min, then heating it to 200°C at a rate of 2°C / min and holding it at that temperature for 55 min, and then heating it to 360°C at a rate of 1°C / min and holding it at that temperature for 70 min, so that the polyamic acid dehydrates and condenses to form polyimide, and then cooling it down to 30°C at a rate of 3°C / min.

[0051] CK group:

[0052] The difference from Example 1 is that only triethylamine was added in step (iii) during the preparation of the sol-gel, but DPE and DMCHA were not added. The remaining steps are the same as in Example 1.

[0053] Comparative Example 1

[0054] The difference from Example 1 is that hexamethylenediamine is used instead of DMCHA in step (iii) to prepare the sol-gel process, while the rest of the steps are the same as in Example 1.

[0055] Comparative Example 2

[0056] The difference from Example 1 is that, in the preparation of the aerogel, NMP is used instead of diphenyl ether in step (a) to participate in the sol-gel conversion, while the remaining steps are the same as in Example 1.

[0057] Figure 1 The images show the microstructures of the aerogels prepared in Example 1 and the comparative examples. It can be seen that the aerogel prepared in this invention has a uniform, non-collapsed porous structure and a continuous framework structure. In contrast, the aerogels in the control group (CK) all exhibited significant collapse, failing to form a complete, well-defined porous structure and a continuous framework. In Comparative Example 1, the use of hexamethylenediamine instead of DMCHA disrupted the catalytic equilibrium, leading to an uncontrollable reaction rate and ultimately resulting in a collapsed aerogel. In Comparative Example 2, replacing diphenyl ether with NMP resulted in ineffective control of the reaction process during thermal imidization, leading to an unsatisfactory aerogel structure.

[0058] like Figure 2 As shown in the thermogravimetric spectra of each aerogel, the CK group aerogel has the lowest thermal decomposition temperature of 440℃, while the Comparative Example 1 aerogel has a thermal decomposition temperature of 490℃ and the fastest decomposition rate. The aerogels prepared in Example 1 and Comparative Example 2 have the highest thermal decomposition temperature of 570℃, but as the temperature increases, the decomposition rate of Comparative Example 2 is faster, indicating that the aerogel prepared in Example 1 has excellent thermal stability and excellent high-temperature resistance.

[0059] The mechanical properties of the aerogels prepared by the CK group, Example 1, Comparative Example 1 and Comparative Example 2, such as compressive strength and Young's modulus, as well as the thermal conductivity and water adsorption rate of the aerogels, were tested. The results are shown in Table 1.

[0060] Table 1:

[0061]

[0062] The higher the porosity and density of an aerogel, the more severe the degradation of its mechanical properties. Figure 3 As shown in the table above, the CK group had the highest density, at 345.7 mg / cm³. 3Although it has a high density, the collapse of the gel's microporous structure and discontinuous skeleton lead to numerous internal defects, significantly reducing its mechanical strength. Its compressive strength is only 1.8 MPa, and its thermal conductivity is also relatively high. In Comparative Examples 1 and 2, the density of the aerogels was lower than that of the control group (CK), while the porosity increased and the thermal conductivity decreased. However, the mechanical properties also showed a corresponding decline. The density of the aerogel in Example 1 of this invention is 61.2 mg / cm³. 3 This resulted in an aerogel structure with high porosity and low density, with a thermal conductivity as low as 0.022 W / m·K. At the same time, by maintaining a continuous skeleton, the structure and distribution of micropores were effectively regulated, which significantly improved the mechanical properties of the aerogel. The compressive strength was increased to 2.6 MPa, and the Young's modulus of compression reached 50 MPa.

[0063] Example 2

[0064] A method for preparing a high-porosity fluorinated polyimide aerogel includes the following steps:

[0065] (a) Preparation of fluorinated polyamic acid solution:

[0066] Under a nitrogen atmosphere, fluorinated diamine monomer TFMB, unfluorinated aromatic diamine monomer ODA, and fluorinated dianhydride monomer 6FDA were dissolved in the polar solvent DMF to obtain a pale yellow solution with a solid content of 12 wt%. Under a nitrogen atmosphere and at 10 °C, the mixture was stirred for 2 h and then stirred at 10 °C for 6 h to obtain a colorless and transparent polyamic acid solution. The molar ratio of TFBM, ODA, and 6DA was 0.5:0.5:1.

[0067] (II) Preparation of fluorinated polyamic acid salts

[0068] Triethylamine is added to a polyamic acid solution to react and generate a fluorinated polyamic acid salt. After the reaction is complete, the fluorinated polyamic acid salt is precipitated into deionized water and collected to obtain a filamentous or blocky solid. The solid fluorinated polyamic acid salt is obtained by freeze-drying at -65°C, at a pressure of 10 Pa, and for 48 hours. The molar ratio of the total amount of fluorinated diamine monomer and non-fluorinated aromatic diamine monomer used in the polyamic acid solution to triethylamine is 0.8:1.

[0069] (III) Preparation of Aerogels

[0070] (a) Using the fluorinated polyamic acid salt obtained in step (ii) as a precursor, a mixture of water and diphenyl ether is added, followed by the addition of triethylamine and N,N-dimethylcyclohexylamine. The polyamic acid hydrogel is obtained by sol-gel conversion after standing for 18 hours. The mass ratio of water, triethylamine, N,N-dimethylcyclohexylamine, diphenyl ether and fluorinated polyamic acid salt is 10:1:0.4:1:0.8.

[0071] (b) Polyamic acid hydrogel was freeze-dried to obtain polyamic acid aerogel, and finally thermal imidization was performed to obtain fluorinated polyimide aerogel. The freeze-drying was carried out at 5 Pa and -55 °C for 55 h to obtain polyamic acid aerogel.

[0072] (c) Thermal imidization involves heating the polyamic acid aerogel to 110°C at a rate of 1°C / min and holding it for 50 min, then heating it to 220°C at a rate of 3°C / min and holding it for 50 min, and then heating it to 380°C at a rate of 1°C / min and holding it for 60 min, so that the polyamic acid dehydrates and condenses to form polyimide, and then cooling it to 40°C at a rate of 2°C / min.

[0073] The density of the aerogel prepared in this embodiment is 62.7 mg / cm³. 3 It has a thermal conductivity of 0.024 W / m·K, a compressive strength of 2.5 MPa, and a Young's modulus of 51 MPa.

[0074] Example 3

[0075] A method for preparing a high-porosity fluorinated polyimide aerogel includes the following steps:

[0076] (a) Preparation of fluorinated polyamic acid solution:

[0077] Under a nitrogen atmosphere, fluorinated diamine monomer TFMB, unfluorinated aromatic diamine monomer ODA, and fluorinated dianhydride monomer 6FDA were dissolved in a polar solvent DMAC to obtain a pale yellow solution with a solid content of 12-20 wt%. Under a nitrogen atmosphere, the mixture was stirred at 0°C for 2 hours and then at 25°C for 8 hours to obtain a colorless and transparent polyamic acid solution. The molar ratio of TFBM, ODA, and 6DA was 0.8:0.2:1.

[0078] (II) Preparation of fluorinated polyamic acid salts

[0079] Triethylamine is added to a polyamic acid solution to react and generate a fluorinated polyamic acid salt. After the reaction is complete, the fluorinated polyamic acid salt is precipitated into deionized water and collected to obtain a filamentous or blocky solid. The solid fluorinated polyamic acid salt is obtained by freeze-drying at -56°C, at a pressure of 5 Pa, and for 55 h. The molar ratio of the total amount of fluorinated diamine monomer and non-fluorinated aromatic diamine monomer used in the polyamic acid solution to triethylamine is 1.3:1.

[0080] (III) Preparation of Aerogels

[0081] (a) Using the fluorinated polyamic acid salt obtained in step (ii) as a precursor, a mixture of water and diphenyl ether is added, followed by the addition of triethylamine and N,N-dimethylcyclohexylamine. The polyamic acid hydrogel is obtained by sol-gel conversion after standing for 24 hours. The mass ratio of water, triethylamine, N,N-dimethylcyclohexylamine, diphenyl ether and fluorinated polyamic acid salt is 10:1:0.7:2:1.2.

[0082] (b) Polyamic acid hydrogel was freeze-dried to obtain polyamic acid aerogel, and finally thermal imidized to obtain fluorinated polyimide aerogel. The freeze-drying was carried out at 10 Pa and -65 °C for 48 h to obtain polyamic acid aerogel.

[0083] (c) Thermal imidization involves heating the polyamic acid aerogel to 90°C at a rate of 2°C / min and holding it at that temperature for 60 min, then heating it to 180°C at a rate of 5°C / min and holding it at that temperature for 60 min, and then heating it to 350°C at a rate of 0.5°C / min and holding it at that temperature for 80 min, so that the polyamic acid dehydrates and condenses to form polyimide, and then cooling it down to 25°C at a rate of 4°C / min.

[0084] The density of the aerogel prepared in this embodiment is 63.6 mg / cm³. 3 It has a thermal conductivity of 0.025 W / m·K, a compressive strength of 2.8 MPa, and a Young's modulus of 47 MPa.

Claims

1. A high porosity fluoropolymer aerogel characterized by: The fluorine-containing diamine monomer 2,2'-bis(trifluoromethyl)biphenyl-4,4'-diamine TFMB and the non-fluorine-containing aromatic diamine monomer bis(4-aminophenyl)ether ODA and the fluorine-containing dianhydride monomer 4,4'-(hexafluoroisopropylidene)diphthalic anhydride 6FDA are used as raw materials to prepare a polyamic acid solution, triethylamine is added to the polyamic acid solution to prepare a fluorine-containing polyamic acid salt, the fluorine-containing polyamic acid salt is dissolved in a mixture of water and diphenyl ether, triethylamine and N,N-dimethylcyclohexylamine are further added to prepare a polyamic acid hydrogel, the polyamic acid hydrogel is freeze-dried to obtain a polyamic acid aerogel, and finally, thermal imidization treatment is performed to obtain the polyimide, wherein the thermal imidization is performed by heating the polyamic acid aerogel at a rate of 1-2 ℃ / min to 90-110 ℃, maintaining the temperature for 50-60 min, then heating at a rate of 3-5 ℃ / min to 180-220 ℃, maintaining the temperature for 50-60 min, and then heating at a rate of 0.5-1 ℃ / min to 350-380 ℃, maintaining the temperature for 60-80 min, so that the polyimide is formed by dehydration and condensation of the polyamic acid, and then the temperature is lowered to 25-40 ℃ at a rate of 2-4 ℃ / min, the molar ratio of the TFMB, ODA and 6FDA is 0.5-0.8:0.5-0.2:1, the solid content of the prepared polyamic acid solution is 12-20%, and the molar ratio of the total amount of the fluorine-containing diamine monomer and the non-fluorine-containing aromatic diamine monomer to triethylamine used in the polyamic acid solution is 0.8-1.3:1, and the mass ratio of water, triethylamine, N,N-dimethylcyclohexylamine, diphenyl ether and the fluorine-containing polyamic acid salt in the process of preparing the polyamic acid hydrogel is 10:1:0.4-0.7:0.1-0.2:0.8-1.

2.

2. A high porosity fluoropolyimide aerogel according to claim 1, wherein: The freeze-drying is performed under the conditions of 5-10 Pa and -65 to -55 ℃ for 48-55 h to obtain the polyamic acid aerogel.

3. A method of making a high porosity fluoropolyimide aerogel, characterized in that, The method comprises the following steps: (1) preparing a fluorine-containing polyamic acid solution: The fluorine-containing diamine monomer 2,2'-bis(trifluoromethyl)biphenyl-4,4'-diamine TFMB, the non-fluorine-containing aromatic diamine monomer bis(4-aminophenyl)ether ODA and the fluorine-containing dianhydride monomer 4,4'-(hexafluoroisopropylidene)diphthalic anhydride 6FDA are dissolved in a polar solvent under a nitrogen atmosphere to obtain a light yellow solution with a solid content of 12-20 wt%; the stirring is continued for 2 h at a temperature of 0-10 ℃ under a nitrogen atmosphere, and then the stirring is continued for 6-8 h at a temperature of 10-25 ℃ to obtain a colorless and transparent polyamic acid solution, the molar ratio of the TFMB, ODA and 6FDA is 0.5-0.8:0.5-0.2:1, and the polar solvent is any one of DMF, DMAC, NMP, DMSO and THF; (2) preparing a fluorine-containing polyamic acid salt adding triethylamine in the polyamic acid solution to generate fluorine-containing polyamic acid salt, and after the reaction is completed, the fluorine-containing polyamic acid salt is precipitated into deionized water and collected to obtain a filamentous or block solid, and the solid is freeze-dried to obtain a fluorine-containing polyamic acid salt solid, the freeze-drying temperature is -65~ -56℃, the pressure is 5~10Pa, the freeze-drying time is 48~55h, and the molar ratio of the sum of the fluorine-containing diamine monomer and the non-fluorine-containing aromatic diamine monomer used in the polyamic acid solution to triethylamine is 0.8~1.3:1; (Three) preparation of aerogel (a) adding the fluorine-containing polyamic acid salt prepared in step (two) into a mixed solution of water and diphenyl ether, and then adding triethylamine and N,N-dimethylcyclohexylamine, and through standing for 18~24h, a polyamic acid hydrogel is obtained through sol-gel conversion, and the mass ratio of the water, triethylamine, N,N-dimethylcyclohexylamine, diphenyl ether and fluorine-containing polyamic acid salt is 10:1:0.4~0.7:0.1~0.2:0.8~1.2; (b) the polyamic acid hydrogel is freeze-dried to obtain a polyamic acid aerogel, and finally, the fluorine-containing polyimide aerogel is obtained through thermal imidization, and the freeze-drying is carried out at 5~10Pa and -65~ -55℃ for 48~55h to obtain the polyamic acid aerogel; (c) the thermal imidization is to heat the polyamic acid aerogel to 90~110℃ at a rate of 1~2℃ / min, and then heat to 180~220℃ at a rate of 3~5℃ / min, and then heat to 350~380℃ at a rate of 0.5~1℃ / min, and then heat to 60~80min, so that the polyimide is dehydrated and condensed to form a polyimide, and then heat to 25~40℃ at a rate of 2~4℃ / min.

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