Polyimide / boron nitride nanosheet composite material with ultrahigh barrier and high thermal conductivity and preparation method of polyimide / boron nitride nanosheet composite material

Abstract

The invention discloses a polyimide / boron nitride nanosheet composite material with ultrahigh barrier and high thermal conductivity and a preparation method thereof, and polyimide is prepared by taking a diamine monomer containing an indeno [1, 2-b] fluorene-6, 12-diketone structure and aromatic dianhydride as raw materials. The indeno [1, 2-b] fluorene-6, 12-diketone structure is beneficial to closer piling of polyimide molecular chains, so that the barrier property of polyimide is improved, and the thermal expansion coefficient of polyimide is reduced. The polyimide / boron nitride nanosheet composite material is prepared on the basis of the intrinsic high-barrier polyimide containing the indeno [1, 2-b] fluorene-6, 12-diketone structure, the boron nitride nanosheet with the high length-diameter ratio can prolong the permeation path of oxygen and water vapor in the polyimide, the barrier property can be further improved, and the barrier property can be further improved. And the boron nitride nanosheet can improve the heat conductivity coefficient of polyimide, so that the polyimide / boron nitride nanosheet composite material with ultrahigh barrier and high heat conductivity coefficient is obtained.

Description

Technical Field

[0001] The present invention belongs to the field of material science and technology, and specifically relates to a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity and a preparation method thereof. Background Art

[0002] Polyimide has excellent thermal properties due to the imide ring in its main chain. It is one of the most heat-resistant varieties of industrialized engineering plastics. It has comprehensive properties that other engineering plastics cannot match, such as high mechanical strength, good high and low temperature resistance, excellent dielectric properties, good film-forming properties, etc. It can be widely used in many fields, especially in flexible displays, aerospace, microelectronics and military fields. With the rapid development of the flexible display industry, polyimide should have higher quality to meet the requirements of modern industry. Therefore, polyimide should not only have excellent thermal properties, but also have efficient barrier properties. Taking OLED devices as an example, the performance requirements of OLED devices for packaging materials include: ① Thermal performance: at least 300℃ high temperature, thermal expansion coefficient less than 10ppm / ℃; ② Barrier performance: water vapor transmission rate less than 10 -6 g / m 2 day, oxygen transmission rate is less than 10 -5 cm 3 / m 2 ·day; ③ Thermal conductivity greater than 4.5W·m -1 ·K -1 Since most polyimides can meet the thermal performance requirements, but there are few studies on polyimides that simultaneously meet good thermal performance, barrier properties and thermal conductivity, it is of great significance to develop polyimide / boron nitride nanosheet composites with both ultra-high barrier and high thermal conductivity. Polyimide has excellent thermal properties, so improving its barrier properties is the key to research. The main ways to improve the barrier properties of polyimide are to synthesize new intrinsic polyimide structures and to prepare polyimide nanocomposites. Fundamentally speaking, the two methods complement each other and work synergistically, that is, to synthesize a new intrinsic high-barrier polyimide structure, and based on this intrinsic polyimide, to prepare a polyimide material with higher barrier properties through nanocomposites. Boron nitride is often used to improve the thermal conductivity of polyimide. Doping polyimide with nanosheets with a high aspect ratio can extend the gas permeation path in the polyimide matrix, thereby further improving the barrier properties of polyimide. If the boron nitride nanosheets are simply modified and introduced into the intrinsic high-barrier polyimide matrix, it can not only improve the thermal conductivity of the polyimide, but also further improve the barrier properties of the polyimide, thereby preparing a polyimide composite material with both ultra-high barrier and high thermal conductivity. Therefore, it is necessary to carry out research on the design and synthesis of polyimide composite materials with both ultra-high barrier and high thermal conductivity based on intrinsic high-barrier polyimide. Summary of the invention

[0003] Technical problem to be solved: In view of the above technical problems, the purpose of the present invention is to disclose a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity and a preparation method thereof, wherein the polyimide is prepared using a diamine monomer containing an indeno[1,2-b]fluorene-6,12-dione structure and an aromatic dianhydride as raw materials. The indeno[1,2-b]fluorene-6,12-dione structure is conducive to a closer stacking of polyimide molecular chains, thereby improving the barrier properties of the polyimide and reducing the thermal expansion coefficient of the polyimide. Based on this type of intrinsic high-barrier polyimide containing indeno[1,2-b]fluorene-6,12-dione structure, a polyimide / boron nitride nanosheet composite material is prepared. The high aspect ratio boron nitride nanosheet can extend the permeation path of oxygen and water vapor in the polyimide, which can further improve the barrier performance. The boron nitride nanosheet can increase the thermal conductivity of the polyimide, thereby obtaining a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity.

[0004] Technical solution: A polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity, wherein the polyimide / boron nitride nanosheet composite material with both high barrier and high thermal conductivity is prepared from polyimide and boron nitride nanosheets, and the molecular structure of the polyimide is as follows: Where n is 1-10000. A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Boron nitride nanosheets are modified by using a silane coupling agent to obtain modified boron nitride nanosheets; S2. In an argon atmosphere, the modified boron nitride nanosheets are ultrasonically dispersed in a strongly polar aprotic organic solvent for 1-4 hours, a diamine containing a diketone structure and a dianhydride are added to the dispersion in a molar ratio of 1:(0.98-1.02), and the mixture is stirred and reacted at -5-25°C for 4-24 hours to obtain a homogeneous, viscous polyamic acid composite adhesive; S3. Dehydrating the polyamic acid in the polyamic acid composite adhesive containing boron nitride nanosheets to obtain a polyimide / boron nitride nanosheet composite material. Furthermore, the molecular formula of the diketone structure in the diamine is as follows: The preparation method thereof comprises the following steps: S1. 2,8-dibromo-indeno[1,2-b]fluorene-6,12-dione and p-aminophenylborate hydrochloride are sequentially added into tetrahydrofuran to dissolve; S2. Potassium carbonate solution and methyl trioctyl ammonium chloride were added sequentially, stirred magnetically and argon was passed, and the oil bath was heated to 60-80 ° C; S3. Tetrakis triphenylphosphine palladium was added and refluxed for 12-48h, and then the reaction solution was poured into ice water to precipitate; S4. Filter, wash and dry to obtain. Furthermore, in step S1, the aspect ratio of the boron nitride nanosheets is (400-2000):1. Furthermore, in step S1, the silane coupling agent is one or more of γ-aminopropyltriethoxysilane, γ-methacryloxypropyltrimethoxysilane, 3-chloropropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane or N-(2-aminoethyl)-3-aminopropyltrimethoxysilane. Furthermore, the dianhydride in step S2 includes pyromellitic dianhydride, 3,3',4,4'-diphenyl ketone tetracarboxylic dianhydride or 3,3',4,4'-biphenyltetracarboxylic dianhydride, and the polyimide synthesis reaction formula is as follows: Where R is One of them. Furthermore, in step S2, the highly polar aprotic organic solvent is one or more of N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylacetamide, and N,N-dimethylformamide. Furthermore, in step S2, the total mass of the diamine and the dianhydride is 10-40% of the mass of the glue solution. Furthermore, the dehydration method of the polyamic acid in step S3 includes thermal imidization and chemical imidization. Furthermore, the mass of the boron nitride nanosheets is 2-10% of the mass of the polyimide material. Beneficial effects: 1. In the present invention, a novel diamine monomer containing an indeno[1,2-b]fluorene-6,12-dione structure is synthesized. When introduced into polyimide, the polyimide molecular chains can be stacked more tightly and arranged more orderly, thereby giving the polyimide a smaller free volume and weakening the creep of the polyimide molecular chains, making it difficult for gas molecules to diffuse inside the polyimide, thereby improving the barrier properties of the polyimide and reducing its thermal expansion coefficient, while having no effect on the water absorption rate. 2. The diamine monomer in the present invention contains an indeno[1,2-b]fluorene-6,12-dione structure. The two carbonyl groups can give the diamine molecule stronger molecular polarity, change the stacking mode and interaction force between molecules, make it easier to form hydrogen bonds, π-π stacking and other interactions between molecules, and the fluorene dione structure has good rigid plane and symmetry, which can further improve the stability and performance of polyimide. 3. Boron nitride nanosheets have high thermal conductivity, but they are also electrically insulating. Adding them to polyimide will significantly improve the thermal conductivity of the polyimide composite material without reducing the electrical insulation of the polyimide. In addition, boron nitride nanosheets have the characteristics of a high aspect ratio, which can increase the tortuous path inside the polyimide, causing more obstacles for gas molecules to penetrate and diffuse. At the same time, they can reduce the diffusion channels of oxygen and water vapor in polyimide, extend the permeation paths of oxygen and water vapor in polyimide, and further improve the barrier properties of the composite material, thereby achieving both ultra-high barrier properties and high thermal conductivity. 4. The ultra-high barrier and high thermal conductivity polyimide / boron nitride nanosheet composite materials prepared by the present invention can be widely used in various fields, especially in the electronic information industry such as flexible display, microelectronics and electronic device packaging, and provide an excellent data basis for the design and synthesis of high-performance polyimide composite materials, which has important theoretical and practical significance. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 The NMR of Example 1 1 H spectrum; Figure 2 is the mass spectrum of Example 1; Figure 3 are infrared spectra of Examples 2-7 and Comparative Example 4, a represents Example 2, b represents Example 4, c represents Example 5, d represents 2,7-bis(4-aminophenyl)-9H-fluorene-9-one (FOPDA) in Comparative Example 4, e represents the polyimide film (PMDA-FOPDA) in Comparative Example 4, f represents Example 6, and g represents Example 7; Figure 4 This is the NMR of 2,7-bis(4-aminophenyl)-9H-fluorene-9-one (FOPDA) in Example 4. 1 H spectrum; Figure 5 This is the NMR of 2,7-bis(4-aminophenyl)-9H-fluorene-9-one (FOPDA) in Example 4. 13 C spectrum; Figure 6 It is the mass spectrum of 2,7-bis(4-aminophenyl)-9H-fluorene-9-one (FOPDA) in Comparative Example 4. DETAILED DESCRIPTION The present invention will be further described below in conjunction with embodiments, which are explanations of the present invention and are not limited to the following embodiments: Example 1 Synthesis of 2,8-Bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione 4.40g 2,8-dibromo-indeno[1,2-b]fluorene-6,12-dione and 4.3353g p-aminophenyl boron hydrochloride were added to a 500mL three-necked flask, tetrahydrofuran (THF) was used as solvent, and then 37.5mL of potassium carbonate solution with a molar concentration of 2mol / L and an appropriate amount of methyl trioctyl ammonium chloride were added. The mixture was stirred magnetically and argon was passed through. After heating to 75°C in an oil bath, 0.050g of tetratriphenylphosphine palladium was added. After reflux reaction for 24h, the reaction solution was poured into ice water, and a large amount of precipitate was precipitated. The residue was collected by suction filtration with a funnel, and the residue was washed twice with tetrahydrofuran and deionized water respectively. After drying, the product 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was obtained, and the yield was 85%. Example 2 Synthesis of polyimide PMDA-BDFPDA S1. At room temperature, 4.6452 g (0.01 mol) of 2,8-bis (4-aminophenyl) indeno [1,2-b] fluorene-6-12-dione and 42.62 g of N,N-dimethylformamide were added to a 100 ml three-necked flask, argon was introduced, and after stirring until completely dissolved, 2.1812 g (0.01 mol) of 1,2,4,5-pyromellitic dianhydride (PMDA) was added, and the reaction was continued by stirring for 6 h to obtain a homogeneous, transparent and viscous polyamic acid solution; S2. After removing bubbles from the polyamic acid solution, apply it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and heat it up as follows: heat it up from room temperature to 100°C and keep it at a constant temperature for 1 hour, heat it up from 100°C to 200°C and keep it at a constant temperature for 1 hour, heat it up from 200°C to 300°C and keep it at a constant temperature for 1 hour, heat it up from 300°C to 420°C and keep it at a constant temperature for 1 hour, and after cooling, a polyimide film (PMDA-BDFPDA) can be obtained. The molecular structure of the PMDA-BDFPDA is as follows: Example 3 The method for preparing polyimide PMDA-BDFPDA by chemical imidization comprises the following steps: S1. At room temperature, 4.6452 g (0.01 mol) of 2,8-bis (4-aminophenyl) indeno [1,2-b] fluorene-6-12-dione and 42.62 g of N,N-dimethylformamide were added to a 100 ml three-necked flask, argon was introduced, and after stirring until completely dissolved, 2.1812 g (0.01 mol) of 1,2,4,5-pyromellitic dianhydride (PMDA) was added, and the reaction was continued by stirring for 6 h to obtain a homogeneous, transparent and viscous polyamic acid solution; S2. Add 4 mL of acetic anhydride to the obtained polyamic acid solution, continue stirring, then slowly add 3 mL of triethylamine, and raise the temperature to 75°C. Continue stirring for 6 hours, then spread the solution on a clean glass plate, and then place the glass plate in a vacuum oven, evacuate, and dry at 150°C for 6 hours. After cooling, a polyimide film (PMDA-BDFPDA) can be obtained. Example 4 The synthesis of polyimide BTDA-BDFPDA comprises the following steps: S1. At room temperature, 4.6452 g (0.01 mol) of 2,8-bis (4-aminophenyl) indeno [1,2-b] fluorene-6-12-dione and 44.58 g of N,N-dimethylformamide were added to a 100 ml three-necked flask, argon was introduced, and stirred. After complete dissolution, 3.2222 g (0.01 mol) of 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) was added, and the reaction was continued with stirring for 7 h to obtain a homogeneous, transparent and viscous polyamic acid solution; S2. After removing bubbles from the polyamic acid solution, scrape it on a glass plate, then place the glass plate in a vacuum oven and evacuate it. The heating program is as follows: heat from room temperature to 100°C and keep the temperature constant for 1 hour, heat from 100°C to 200°C and keep the temperature constant for 1 hour, heat from 200°C to 300°C and keep the temperature constant for 1 hour, heat from 300°C to 430°C and keep the temperature constant for 1 hour. After cooling, the polyimide film (BTDA-BDFPDA) can be obtained. The molecular structure of the polyimide BTDA-BDFPDA is as follows: Example 5 The synthesis of polyimide BPDA-BDFPDA comprises the following steps: S1. At room temperature, 4.6452 g (0.01 mol) of 2,8-bis (4-aminophenyl) indeno [1,2-b] fluorene-6-12-dione and 30.35 g of N,N-dimethylformamide were added to a 50 ml three-necked flask, argon was introduced, and stirred. After complete dissolution, 2.9422 g (0.01 mol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) was added, and the reaction was continued with stirring for 6 h to obtain a homogeneous transparent viscous The polyamic acid solution is subjected to the following steps: removing bubbles from the polyamic acid solution and then scraping and coating it on a glass plate. The glass plate is then placed in a vacuum oven and evacuated. The heating program is as follows: heating from room temperature to 100°C and then keeping the temperature constant for 1 hour, heating from 100°C to 200°C and then keeping the temperature constant for 1 hour, heating from 200°C to 300°C and then keeping the temperature constant for 1 hour, heating from 300°C to 420°C and then keeping the temperature constant for 1 hour. After cooling, a polyimide film (BPDA-BDFPDA) is obtained. The molecular structure of the polyimide BPDA-BDFPDA is as follows: Example 6 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of modified boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Example 7 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Example 8 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Example 9 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 500:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of boron nitride nanosheets with an aspect ratio of 500:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Example 10 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 550:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of boron nitride nanosheets with an aspect ratio of 550:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Embodiment 11 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 600:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of boron nitride nanosheets with an aspect ratio of 600:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Example 12 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 650:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of boron nitride nanosheets with an aspect ratio of 650:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Embodiment 13 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.3413 g of boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Embodiment 14 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4778 g of boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Embodiment 15 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.5461 g of boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Example 16 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.6144 g of boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Embodiment 17 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.6826 g of boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Embodiment 18 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 1 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Embodiment 19 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 3 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Embodiment 20 A method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity comprises the following steps: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 4 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Comparative Example 1 The difference between this comparative example and Example 2 is that 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) is replaced by 4,4'-diaminodiphenyl ether (ODA), as follows: S1. At room temperature, under an argon atmosphere, 0.01 mol 4,4'-diaminodiphenyl ether (ODA) and 2.1812 g (0.01 mol) 1,2,4,5-pyromellitic dianhydride (PMDA) were added to 42.62 g N,N-dimethylformamide, and the reaction was continued with stirring for 6 h to obtain a homogeneous transparent viscous polyamic acid solution; S2. After removing bubbles from the polyamic acid solution, scrape it on a glass plate, then place the glass plate in a vacuum oven and evacuate it. The heating program is: heat from room temperature to 100°C and keep it constant for 1 hour, heat from 100°C to 200°C and keep it constant for 1 hour, heat from 200°C to 300°C and keep it constant for 1 hour, heat from 300°C to 400°C and keep it constant for 1 hour. After cooling, the polyimide film (PMDA-ODA) can be obtained. The molecular structure of the polyimide film (PMDA-ODA) is as follows: Comparative Example 2 The difference between this comparative example and Example 4 is that 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) is replaced by 4,4'-diaminodiphenyl ether (ODA), as follows: S1. At room temperature, under an argon atmosphere, 0.01 mol 4,4'-diaminodiphenyl ether (ODA) and 3.2222 g (0.01 mol) 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) were added to 44.58 g N,N-dimethylformamide, and the reaction was continued with stirring for 6 h to obtain a homogeneous, transparent, viscous polyamic acid solution; S2. After removing bubbles from the polyamic acid solution, scrape it on a glass plate, then place the glass plate in a vacuum oven and evacuate it. The heating program is: heat from room temperature to 100°C and keep it constant for 1 hour, heat from 100°C to 200°C and keep it constant for 1 hour, heat from 200°C to 300°C and keep it constant for 1 hour, heat from 300°C to 400°C and keep it constant for 1 hour. After cooling, the polyimide film (BTDA-ODA) can be obtained. The molecular structure of the polyimide film (BTDA-ODA) is as follows: Comparative Example 3 The difference between this comparative example and Example 5 is that 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) is replaced by 4,4'-diaminodiphenyl ether (ODA), as follows: S1. At room temperature, under an argon atmosphere, 0.01 mol of 4,4'-diaminodiphenyl ether (ODA) and 2.9422 g (0.01 mol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) were added to N,N-dimethylformamide, and the reaction was continued with stirring for 6 h to obtain a homogeneous, transparent, viscous polyamic acid solution; S2. After removing bubbles from the polyamic acid solution, scrape it on a glass plate, then place the glass plate in a vacuum oven and evacuate it. The heating program is: heat from room temperature to 100°C and keep it constant for 1 hour, heat from 100°C to 200°C and keep it constant for 1 hour, heat from 200°C to 300°C and keep it constant for 1 hour, heat from 300°C to 400°C and keep it constant for 1 hour. After cooling, the polyimide film (BPDA-ODA) can be obtained. The molecular structure of the polyimide film (BPDA-ODA) is as follows: Comparative Example 4 The difference between this comparative example and Example 2 is that 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) is replaced by 2,7-bis(4-aminophenyl)-9H-fluorene-9-one (FOPDA), as follows: S1. Synthesis of 2,7-bis(4-aminophenyl)-9H-fluorene-9-one (FOPDA): Add 10.140g 2,7-dibromofluorenone and 13.006g p-aminophenyl boron hydrochloride to a 500mL three-necked flask, add 350mL tetrahydrofuran, then add 112.5mL 2mol / L potassium carbonate solution and appropriate amount of Aliquat336, stir magnetically and pass argon, heat to 75℃ in an oil bath, add 0.150g tetrakis triphenylphosphine palladium, reflux for 24h; pour the reaction solution into water, a large amount of precipitate is precipitated, then filter with a funnel, and evaporate the solvent under reduced pressure. The product is purified by column chromatography with dichloromethane:n-hexane=2:1 as the mobile phase and silica gel as the stationary phase, collect the product and spin dry it to a reddish brown solid, dry it in a vacuum at 80℃ for 24h, and the yield is 87%. The molecular structure of the 2,7-bis(4-aminophenyl)-9H-fluorene-9-one (FOPDA) is as follows: S2. Synthesis of polyimide PMDA-FOPDA 0.3624 g of 2,7-bis(4-aminophenyl)-9H-fluorene-9-one was placed in a 50 mL round-bottom flask, 10 mL of dimethylformamide (DMF) was added to dissolve it, and then 0.2181 g of dianhydride monomer PMDA was added, with a solid content of about 6 wt %, and argon was introduced and reacted at room temperature for 6 h to obtain a homogeneous, transparent and viscous polyamic acid solution; After removing bubbles from the polyamic acid solution, the solution was scraped and coated on a glass plate. The glass plate was then placed in a vacuum oven and evacuated. The heating program was as follows: the temperature was raised from room temperature to 100°C and then kept constant at this temperature for 1 hour; the temperature was raised from 100°C to 200°C and then kept constant at this temperature for 1 hour; the temperature was raised from 200°C to 300°C and then kept constant at this temperature for 1 hour; the temperature was raised from 300°C to 400°C and then kept constant at this temperature for 1 hour. After cooling, a polyimide film (PMDA-FOPDA) was obtained. The molecular structure of the polyimide film (PMDA-FOPDA) is as follows: Depend on Figure 4 and Figure 5 It can be seen that the number of hydrogen atoms, the number of carbon atoms and the chemical shift of the synthetic substance 2,7-bis(4-aminophenyl)-9H-fluorene-9-one (FOPDA) in Comparative Example 4 are consistent with the target structure; Figure 6It can be seen that the molecular weight of the synthetic substance 2,7-bis(4-aminophenyl)-9H-fluorene-9-one (FOPDA) in Comparative Example 4 is almost the same as the molecular weight (362.14) corresponding to the target structure. Based on the above characterization structure, it can be determined that the synthetic substance in Comparative Example 4 is 2,7-bis(4-aminophenyl)-9H-fluorene-9-one (FOPDA). Comparative Example 5 The difference between this comparative example and Example 6 is that the aspect ratio of boron nitride is 2500:1, as follows: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 2500:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of boron nitride nanosheets with an aspect ratio of 2500:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Comparative Example 6 The difference between this comparative example and Example 6 is that the silane coupling agent modification is replaced by hydroxylation modification, as follows: S1. Weigh 1.0 g of boron nitride nanosheets with an aspect ratio of 700:1, add them to 10 mL of 5 mol / L sodium hydroxide solution, stir them magnetically at 80 ° C for 48 h, filter them and wash the precipitate with deionized water, repeat the operation until the pH of the filtrate is neutral, obtain the precipitate and dry it in a drying oven at 120 ° C for 24 h to obtain hydroxylated boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of hydroxylated boron nitride nanosheets were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape it on a glass plate, then place the glass plate in a vacuum oven, evacuate it, and perform gradient temperature increase. The temperature increase is controlled as follows: heat up from room temperature to 100°C and keep the temperature constant for 1 hour, heat up from 100°C to 200°C and keep the temperature constant for 1 hour, heat up from 200°C to 300°C and keep the temperature constant for 1 hour, heat up from 300°C to 420°C and keep the temperature constant for 1 hour. After cooling, the polyimide / boron nitride nanosheet composite material can be obtained. Comparative Example 7 The difference between this comparative example and Example 6 is that the gradient heating in step S2 is replaced by direct heating, as follows: S1. Dissolve 3g of aminopropyltriethoxysilane in 100mL of anhydrous ethanol and stir evenly; then add 10g of boron nitride nanosheets with an aspect ratio of 700:1 into the silane coupling agent solution, and use ultrasonic treatment for 30min to ensure that the boron nitride nanosheets are evenly dispersed; stir the reaction at 60°C for 4h to ensure that the silane coupling agent and the boron nitride nanosheets are fully reacted: after the reaction is completed, centrifuge the mixture, pour off the supernatant, wash the precipitate with 100mL of anhydrous ethanol, and repeat the washing three times to remove the unreacted silane coupling agent; dry the modified boron nitride nanosheets in a vacuum drying oven at 60°C for 12h to obtain modified boron nitride nanosheets; S2. At room temperature, in an argon atmosphere, 0.4341 g of boron nitride nanosheets with an aspect ratio of 700:1 were ultrasonically dispersed in 62.4 mL of N,N-dimethylformamide for 2 h, and then 0.01 mol of 2,8-bis(4-aminophenyl)indeno[1,2-b]fluorene-6-12-dione (BDFPDA) was added to a three-necked flask, argon was introduced, and stirred. After complete dissolution, 0.01 mol of pyromellitic dianhydride (PMDA) was added, and stirring was continued. The reaction was continued for 12 h to obtain a homogeneous, transparent and viscous polyamic acid / boron nitride nanosheet composite solution; S3. After removing bubbles from the polyamic acid / boron nitride nanosheet solution, scrape and apply it on a glass plate, then place the glass plate in a vacuum oven, evacuate the air, raise the temperature to 420°C and maintain it for 1 hour, and after cooling, a polyimide / boron nitride nanosheet composite material can be obtained. Structural characterization Depend on Figure 1 It can be seen that the number of hydrogen atoms and chemical shift of the synthetic substance in Example 1 are consistent with the target structure; Figure 2 It can be seen that the molecular weight of the synthetic substance in Example 1 is almost the same as the molecular weight (464.14) corresponding to the target structure; based on the above characterization structure, it can be determined that the synthetic substance in Example 1 is BDFPDA. The molecular structure formula of the BDFPDA is as follows: from Figure 3 It can be seen from the infrared spectra in the embodiment 2, the embodiment 4, the embodiment 5 and the comparative example 4 that the polyimide prepared has a wavelength of 1778 cm -1 and 1710cm -1 Nearby are the asymmetric and symmetric stretching vibrations of the carbonyl group, 1356 cm -1 Nearby is the stretching vibration of the imide ring, indicating that Examples 2-5 have successfully synthesized polyimide. -1 The NH bond bending vibration is at 3500-3000cm -1 In between are the characteristic absorption peaks of hydroxyl and -NH, which all indicate that the aminopropyltriethoxysilane-modified boron nitride / polyimide composite materials in Examples 6 and 7 have been successfully synthesized. Performance Testing (1) Dynamic mechanical properties (DMA) analysis The glass transition temperature (T) of PI was tested using a DMAQ850 dynamic thermal mechanical properties analyzer from TA, USA. g ), the DMA instrument was positionally calibrated before the test, and the sample was clamped into the DMA fixture according to the specification of effective size of 20×6mm (length×width), the heating rate was 5℃ / min, the test temperature range was 25-510℃, the amplitude was 20μm, and the frequency was 1Hz. (2) Static thermomechanical (TMA) analysis The dimensional stability (thermal expansion coefficient) of the polyimide film was characterized by using a TMAQ400 thermomechanical properties analyzer from TA Company of the United States. A film stretching fixture was used, the sample size (length × width) was 18 × 4.5 mm, the heating rate was 5 °C / min, and the test temperature range was 50-300 °C from room temperature. (3) Barrier properties The water vapor transmission coefficient (WVP) of polyimide film was tested using the PERMATRAN-W3 / 33 water vapor transmission rate tester from MOCON, USA. The test conditions were based on the standard ASTM F-1249, with a relative humidity of 90% and a test area of ​​5 cm. 2 According to the standard ASTM D3985, the oxygen permeability coefficient (OP) of polyimide film was tested using MOCON 2 / 21 oxygen permeability tester from MOCON Company of the United States. The test conditions were: relative humidity 0%, test area 5 cm 2 . (4) Water absorption Cut the polyimide film into 3cm×3cm in length and width respectively, put the sample to be tested into an 80℃ forced air drying oven to remove the residual organic solvent and water, and the measured mass is recorded as M0. Immerse the sample in deionized water, and measure its mass at regular intervals, and record it as M n According to the calculation formula, the water absorption rate change curve of the porous composite film at different immersion times can be obtained. Each group of samples needs to be measured five times to get the average value. The calculation formula is as follows: W(%)=(M n -M0) / M0×100% Where: W is the water absorption rate of polyimide film, M0 and M n Represent the mass of the sample before and after immersion, respectively. (5) Thermal conductivity The thermal conductivity of polyimide film was tested using a TC3000 thermal conductivity tester. The test temperature was room temperature, the fast mode was used, the test voltage was 1.2V, and the measurement time interval was 3 minutes. The calculation formula is as follows: Where: K is the thermal conductivity; q is the heat generated by the metal wire per unit length; ΔT is the temperature change of the metal wire; t is the test time. Table 1 Performance parameters of the polyimide synthesized in the examples and comparative examples As shown in Table 1, Comparative Examples 1-3 are polyimides prepared by conventional methods, Comparative Example 4 is a polyimide containing a fluorenone structure, and the moisture permeability coefficient, oxygen permeability coefficient and thermal expansion coefficient of the polyimide containing indeno[1,2-b]fluorene-6,12-dione structure prepared in Examples 2, 4 and 5 are lower than those in Comparative Examples 1-4, and the glass transition temperature is higher than that in Comparative Examples 1-4; the moisture permeability coefficient, oxygen permeability coefficient and thermal expansion coefficient of the polyimide / boron nitride nanosheet composite material in Examples 6-20 are further reduced, the glass transition temperature is further increased, and the thermal conductivity is increased by more than 3 times, indicating that the polyimide / boron nitride nanosheet composite material containing indeno[1,2-b]fluorene-6,12-dione structure has ultra-high water vapor and oxygen barrier properties, high thermal conductivity, high dimensional stability and heat resistance, the moisture permeability coefficient, oxygen permeability coefficient, thermal expansion coefficient and water absorption of Comparative Examples 5-7 are higher than those of the embodiments, and the thermal conductivity and T g All of them are lower than those in the examples, indicating that the appropriate aspect ratio and boron nitride nanosheet modification method can improve the barrier properties of the composite material, and the gradient heating can stabilize the process of imidization, relieve the thermal stress in the preparation process of the composite material, improve its thermal stability, and thus enhance the thermal conductivity of the composite material. In summary, the polyimide composite material prepared by the present invention has ultra-high barrier properties, high thermal conductivity, low thermal expansion coefficient, low water absorption and high glass transition temperature, which can meet the requirements of the application field. The above is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any technician familiar with the art can make many possible changes and modifications to the technical solution of the present invention by using the above disclosed methods and technical contents without departing from the spirit and technical solution of the present invention, or modify it into an equivalent embodiment of equivalent changes. Therefore, any simple modification, equivalent replacement, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of protection of the technical solution of the present invention.

Claims

1. A polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity, characterized in that: The polyimide / boron nitride nanosheet composite material with both high barrier and high thermal conductivity is prepared from polyimide and boron nitride nanosheets. The molecular structure of the polyimide is shown below: Where n is 1-10000.

2. A method for preparing a polyimide / boron nitride nanosheet composite material having both ultra-high barrier and high thermal conductivity, characterized in that: The following steps are involved: S1. Boron nitride nanosheets are modified by using a silane coupling agent to obtain modified boron nitride nanosheets; S2. In an argon atmosphere, the modified boron nitride nanosheets are ultrasonically dispersed in a strongly polar aprotic organic solvent for 1-4 hours, a diamine containing a diketone structure and a dianhydride are added to the dispersion in a molar ratio of 1:(0.98-1.02), and the mixture is stirred and reacted at -5-25°C for 4-24 hours to obtain a homogeneous, viscous polyamic acid composite adhesive; S3. Dehydrating the polyamic acid in the polyamic acid composite adhesive containing boron nitride nanosheets to obtain a polyimide / boron nitride nanosheet composite material.

3. The method for preparing a polyimide / boron nitride nanosheet composite material having both ultra-high barrier and high thermal conductivity according to claim 2, characterized in that: The molecular formula of the diketone structure in the diamine is as follows: The preparation method thereof comprises the following steps: S1. 2,8-dibromo-indeno[1,2-b]fluorene-6,12-dione and p-aminophenylborate hydrochloride are sequentially added into tetrahydrofuran to dissolve; S2. Potassium carbonate solution and methyl trioctyl ammonium chloride were added sequentially, stirred magnetically and argon was passed, and the oil bath was heated to 60-80 ° C; S3. Tetrakis triphenylphosphine palladium was added and refluxed for 12-48h, and then the reaction solution was poured into ice water to precipitate; S4. Filter, wash and dry to obtain.

4. The method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity according to claim 2, characterized in that: The aspect ratio of the boron nitride nanosheets in step S1 is (400-2000):

1.

5. The method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity according to claim 2, characterized in that: In step S1, the silane coupling agent is one or more of γ-aminopropyltriethoxysilane, γ-methacryloxypropyltrimethoxysilane, 3-chloropropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane or N-(2-aminoethyl)-3-aminopropyltrimethoxysilane.

6. The method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity according to claim 2, characterized in that: The dianhydride in step S2 includes pyromellitic dianhydride, 3,3',4,4'-diphenyl ketone tetracarboxylic dianhydride or 3,3',4,4'-biphenyltetracarboxylic dianhydride.

7. The method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity according to claim 2, characterized in that: In step S2, the highly polar aprotic organic solvent is one or more of N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylacetamide, and N,N-dimethylformamide.

8. The method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity according to claim 2, characterized in that: In step S2, the total mass of the diamine and the dianhydride is 10-40% of the mass of the glue solution.

9. The method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity according to claim 2, characterized in that: The dehydration method of the polyamic acid in step S3 includes thermal imidization and chemical imidization.

10. The method for preparing a polyimide / boron nitride nanosheet composite material with both ultra-high barrier and high thermal conductivity according to claim 2, characterized in that: The mass of the boron nitride nanosheets is 2-10% of the mass of the polyimide material.

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