A modified polyimide composite material and its preparation method

Abstract

This application proposes a modified polyimide composite material and its preparation method, belonging to the field of polymer composite material technology, to solve the problems of poor thermal conductivity of existing PI materials, the need for blending with fillers to improve thermal conductivity, and the resulting interfacial compatibility issues and damage to the material's physical properties. The modified polyimide composite material includes a polyimide matrix and a three-dimensional covalently bonded hybrid network dispersed within the polyimide matrix. The surface of the three-dimensional covalently bonded hybrid network is modified with unreacted carboxyl groups and / or amino groups, which can form chemical bonds with the polyimide matrix. The three-dimensional covalently bonded hybrid network is composed of one-dimensional carbon nanotubes and two-dimensional boron nitride nanosheets covalently linked by bifunctional bridging molecules, and the bonding between the bifunctional bridging molecules and the carbon nanotubes and / or boron nitride nanosheets includes dynamic covalent bonds. The carbon nanotubes and / or boron nitride nanosheets are modified with amino groups. This network synergistically improves the mechanical, thermal, and electrical properties of the composite material.

Description

Technical Field

[0001] This application belongs to the field of polymer composite materials technology, specifically relating to a modified polyimide composite material and its preparation method. Background Technology

[0002] Polyimide (PI), as a special engineering plastic, is widely used in cutting-edge fields such as aerospace, microelectronics, and high-speed rail transportation due to its excellent high temperature resistance (>400℃), superior mechanical properties, good chemical stability, and outstanding dielectric properties.

[0003] However, as electronic devices develop towards higher power, higher integration, and miniaturization, and as aircraft face increasingly stringent requirements for lightweight and high reliability, the limitations of traditional polyimide (PI) are becoming increasingly apparent. Pure PI is an excellent thermal insulator, which means that when used as an electronic packaging material, flexible circuit board, or high-temperature structural component, it cannot effectively dissipate the heat generated during operation, becoming a bottleneck in system thermal management and easily leading to device overheating failure. To improve the thermal conductivity of PI, the most common method is to introduce high thermal conductivity fillers into the PI matrix, such as boron nitride (BN), carbon nanotubes (CNTs), graphene, and alumina (Al2O3). However, this physical blending modification method has performance contradictions: interfacial compatibility issues. Most inorganic thermally conductive fillers have poor interfacial compatibility with the organic PI matrix, resulting in obvious interfacial gaps that act as barriers to phonon (the main carrier of heat transfer) scattering, severely limiting the improvement of thermal conductivity. Often, a very high proportion of filler (>30 wt%) is required to achieve significant results. High filler content significantly increases the melt viscosity of composite materials, making their processing and molding (such as casting, compression molding, and injection molding) extremely difficult. At the same time, filler agglomeration and weak interfacial bonding can significantly impair the toughness, ductility, and fatigue properties of the material, leading to brittleness and making it difficult to meet the application requirements of structural-functional integration.

[0004] To improve the interface, researchers often modify the surface of the filler (e.g., with silane coupling agents). However, these modified layers may decompose or fail during the high-temperature imidization of PI (typically >300°C), resulting in insufficient interface stability. In addition, while constructing three-dimensional thermally conductive networks (e.g., graphene aerogels) can improve efficiency, the preparation process is complex and costly, and the network structure is easily damaged by shear forces during processing.

[0005] Therefore, the question is how to significantly improve the thermal conductivity of the PI matrix without significantly impairing its processing and mechanical properties through an efficient, stable, and feasible process. Summary of the Invention

[0006] In view of this, this application provides a modified polyimide composite material and its preparation method to solve the problems of poor thermal conductivity of existing PI materials, the need to improve thermal conductivity by blending with fillers, and the resulting interfacial compatibility issues and damage to the physical properties of the materials.

[0007] To solve the above problems, the technical solution adopted in this application is as follows:

[0008] In a first aspect, this application proposes a modified polyimide composite material, comprising a polyimide matrix and a three-dimensional covalently stitched hybrid network dispersed in the polyimide matrix; the surface of the three-dimensional covalently stitched hybrid network is modified with unreacted carboxyl groups and / or amino groups for forming chemical bonds with the polyimide matrix; the three-dimensional covalently stitched hybrid network is formed by one-dimensional carbon nanotubes and two-dimensional boron nitride nanosheets covalently linked by bifunctional bridging molecules, and the connecting bonds between the bifunctional bridging molecules and the carbon nanotubes and / or boron nitride nanosheets include dynamic covalent bonds; the carbon nanotubes and / or boron nitride nanosheets are aminated.

[0009] Preferably, the dynamic covalent bond is a reversible covalent bond or a disulfide bond formed by the Diels-Alder reaction.

[0010] Preferably, the bifunctional bridging molecule is a compound containing both rigid anhydride groups and flexible furan groups. The rigid anhydride groups react with amino groups on the surface of the carbon nanotubes and / or boron nitride nanosheets to form amide bonds, and the flexible furan groups react with a dynamic crosslinking agent to form reversible covalent bonds through a Diels-Alder reaction.

[0011] Preferably, the dynamic crosslinking agent is bismaleimide diphenylmethane and / or bismaleimide polyethylene glycol.

[0012] Preferably, the mass ratio of the carbon nanotubes to the boron nitride nanosheets is 1:0.5-5.

[0013] Preferably, the mass of the three-dimensional covalently stitched hybrid network accounts for 1%-15% of the total mass of the composite material.

[0014] Preferably, the polyimide matrix is ​​a thermoplastic or thermosetting polyimide formed by the condensation polymerization of aromatic diamines and aromatic dianhydrides.

[0015] Secondly, this application also proposes a method for preparing the modified polyimide composite material described in the first aspect, comprising the following steps: Step 1: Aminoizing carbon nanotubes and boron nitride nanosheets to obtain aminoized carbon nanotubes and aminoized boron nitride nanosheets, respectively; Step 2: Synthesizing a bifunctional bridging molecule, wherein one end of the bifunctional bridging molecule contains a rigid functional group capable of reacting with amino groups, and the other end contains a flexible functional group capable of participating in the formation of dynamic covalent bonds; Step 3: Mixing and reacting the aminoized carbon nanotubes, aminoized boron nitride nanosheets, bifunctional bridging molecules, and a dynamic crosslinking agent, pre-constructing a three-dimensional covalently stitched hybrid network through the reaction of the rigid functional group with amino groups and the dynamic covalent bond formation reaction between the flexible functional group and the dynamic crosslinking agent; Step 4: Mixing the three-dimensional covalently stitched hybrid network obtained in Step 3 with a polyimide precursor monomer and performing in-situ polymerization to obtain a polyamic acid solution containing a three-dimensional network; Step 5: Molding the polyamic acid solution obtained in Step 4 and performing thermal imidization treatment to obtain the polyimide composite material.

[0016] Preferably, the method for synthesizing the bifunctional bridging molecule in step 2 is as follows: an amino compound containing a furan ring is subjected to an amidation reaction with an aromatic acid anhydride to obtain an amide acid intermediate; the amide acid intermediate is subjected to an imide reaction to obtain the bifunctional bridging molecule; the aromatic acid anhydride is selected from phthalic anhydride.

[0017] Preferably, in step 3, the mass ratio of the aminated carbon nanotubes to the aminated boron nitride nanosheets is 1:1-5, the ratio of the sum of the masses of the aminated carbon nanotubes and the aminated boron nitride nanosheets to the mass of the bifunctional bridging molecule is 1:0.3-2, and the molar ratio of the bifunctional bridging molecule to the dynamic crosslinking agent is 1:0.3-1.2.

[0018] Preferably, the dynamic covalent bond formation reaction in step 3 is the Diels-Alder reaction, with a reaction temperature of 50-100℃ and a reaction time of 2-8h.

[0019] Preferably, the polyimide precursor monomer in step 4 includes an aromatic diamine monomer and an aromatic dianhydride monomer, and the in-situ polymerization temperature is 0-10°C.

[0020] Preferably, the thermal imidization treatment in step 5 adopts a programmed temperature rise method, with a maximum temperature of 250-350℃.

[0021] Preferably, the preparation method of the aminated carbon nanotubes in step 1 is as follows: reacting carboxylated carbon nanotubes with a polyamine compound in the presence of a condensing agent.

[0022] Preferably, the preparation method of the aminated boron nitride nanosheets in step 1 is as follows: boron nitride is hydroxylated and then reacted with an aminosilane coupling agent.

[0023] Preferably, the process further includes step 6: heat-treating the obtained polyimide composite material at 120-150°C for 10-60 minutes to induce self-repair or reshaping of cracks.

[0024] In summary, due to the adoption of the above technical solution, the beneficial effects of this application are:

[0025] This application uses CNT-NH2 and BNNS-NH2 as fillers, constructing a three-dimensional covalent hybrid network through bifunctional bridging molecules. Residual carboxyl / amino groups on the surface can react in situ with polyimide matrix (such as ODA / PMDA) precursors to form chemically bonded interfaces. This network acts like a steel skeleton, significantly improving strength and preventing crack propagation; the internal dynamic bonds endow the material with energy dissipation and self-healing capabilities, solving the brittleness problem. Chemical bonding eliminates interfacial voids, greatly reducing thermal resistance and achieving efficient thermal conduction. The pre-constructed three-dimensional network prevents filler agglomeration, ensuring uniform dispersion and synergistically improving the mechanical, thermal, and electrical properties of the composite material. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0027] Figure 1 This is a schematic diagram of the reaction for synthesizing bifunctional bridging molecules provided in the embodiments of this application;

[0028] Figure 2 This is a schematic diagram of the reaction between bifunctional bridging molecularly bridged aminated carbon nanotubes and aminated boron nitride nanosheets provided in an embodiment of this application.

[0029] Figure 3 This is a schematic diagram illustrating the reaction between the bifunctional bridging molecule and the dynamic crosslinking agent provided in the embodiments of this application. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The components of the embodiments of this application described and shown in the accompanying drawings can be arranged and designed in various different configurations.

[0031] Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0032] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0033] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0034] In this application, unless otherwise expressly specified and limited, "above" or "below" a second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of a second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" a second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature. The terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of this application can be implemented in orders other than those illustrated or described herein, and the objects distinguished by "first," "second," etc., are generally of the same class and do not limit the number of objects; for example, a first object can be one or more. Furthermore, "and / or" in the specification and claims indicates at least one of the connected objects, and the character " / " generally indicates that the preceding and following objects are in an "or" relationship.

[0035] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0036] Pure polyimide (PI) is an excellent thermal insulator, but when used as an electronic packaging material, flexible circuit board, or high-temperature structural component, it cannot effectively dissipate heat generated during operation, becoming a bottleneck in system thermal management and easily leading to device overheating failure. To improve the thermal conductivity of PI, the most common method is to introduce high thermal conductivity fillers into the PI matrix, such as boron nitride (BN), carbon nanotubes (CNTs), graphene, and alumina (Al₂O₃). However, this physical blending modification method has performance contradictions: interfacial compatibility issues. Most inorganic thermally conductive fillers have poor interfacial compatibility with the organic PI matrix, resulting in significant interfacial gaps that act as barriers to phonon (the main carrier of heat transfer) scattering, severely limiting the improvement of thermal conductivity. Often, a very high proportion of filler (>30 wt%) is required to achieve significant results. High filler content significantly increases the melt viscosity of the composite material, making its processing and molding (such as casting, molding, and injection molding) extremely difficult. Meanwhile, filler agglomeration and weak interfacial bonding can significantly impair the toughness, ductility, and fatigue performance of materials, making them brittle and unable to meet the application requirements of structure-function integration.

[0037] To improve the interface, researchers often modify the surface of the filler (e.g., with silane coupling agents). However, these modified layers may decompose or fail during the high-temperature imidization of PI (typically >300°C), resulting in insufficient interface stability. In addition, while constructing three-dimensional thermally conductive networks (e.g., graphene aerogels) can improve efficiency, the preparation process is complex and costly, and the network structure is easily damaged by shear forces during processing.

[0038] This application uses CNT-NH2 and BNNS-NH2 as fillers, constructing a three-dimensional covalent hybrid network through bifunctional bridging molecules. Residual carboxyl / amino groups on the surface can react in situ with polyimide matrix (such as ODA / PMDA) precursors to form chemically bonded interfaces. This network acts like a steel skeleton, significantly improving strength and preventing crack propagation; the internal dynamic bonds endow the material with energy dissipation and self-healing capabilities, solving the brittleness problem. Chemical bonding eliminates interfacial voids, greatly reducing thermal resistance and achieving efficient thermal conduction. The pre-constructed three-dimensional network prevents filler agglomeration, ensuring uniform dispersion and synergistically improving the mechanical, thermal, and electrical properties of the composite material.

[0039] The following is in conjunction with the appendix Figures 1 to 3 The present application will be described in detail through specific embodiments and application scenarios.

[0040] In a first aspect, this application proposes a modified polyimide composite material, comprising a polyimide matrix and a three-dimensional covalently stitched hybrid network dispersed in the polyimide matrix; the surface of the three-dimensional covalently stitched hybrid network is modified with unreacted carboxyl groups and / or amino groups for forming chemical bonds with the polyimide matrix; the three-dimensional covalently stitched hybrid network is formed by one-dimensional carbon nanotubes and two-dimensional boron nitride nanosheets covalently linked by bifunctional bridging molecules, and the connecting bonds between the bifunctional bridging molecules and the carbon nanotubes and / or boron nitride nanosheets include dynamic covalent bonds; the carbon nanotubes and / or boron nitride nanosheets are aminated.

[0041] Specifically, CNT-NH2 and BNNS-NH2 are used as nanofillers. The polyimide matrix can be formed by in-situ polymerization of 4,4'-diaminodiphenyl ether (ODA) and PMDA, or fluorinated diamines (such as 6FDA derivatives) can be used to improve solubility and processability. In conjunction with dianhydride polymerization, conventional aromatic polyimides can also be used. The three-dimensional covalently hybridized network not only contains dynamic covalent bonds internally, but also retains unreacted carboxyl / amino groups on the surface to form chemical bonds with the matrix. One-dimensional carbon nanotubes (CNTs) and two-dimensional boron nitride (BNNS) are covalently stitched together to form a three-dimensional network, creating a rigid structure similar to a steel reinforcement skeleton in construction. This structure effectively prevents crack propagation, uniformly distributes external loads throughout the network, and significantly improves the tensile strength and modulus of the composite material. When the material is subjected to impact or deformation, the dynamic bonds can undergo reversible fracture and recombination, absorbing and dissipating a large amount of energy, thus solving the problem of high strength but poor toughness and brittleness in traditional nanocomposites, and significantly improving fracture toughness and fatigue resistance. Unreacted carboxyl / amino groups retained on the network surface can undergo in-situ polycondensation with amino / anhydride groups in the polyimide (PI) precursor (polyamic acid), forming a strong covalent bond interface (-CO-NH- or imide ring) after curing. Traditional physical mixing results in a weakly bonded interfacial layer (with voids or van der Waals force gaps) between the filler and the matrix, which is a major obstacle to phonon (heat conduction carrier) transport. This scheme eliminates interfacial voids through chemical bonding, achieving atomic-level close contact between the filler and the matrix, greatly reducing interfacial thermal resistance (Kapitza resistance), and making the thermal conductivity of the composite material far exceed that of physically blended materials with the same amount of filler. By pre-stitching CNTs and BNNS into a stable three-dimensional network before adding them to the matrix, the relative positions of the fillers are locked by chemical bonds, fundamentally preventing the re-agglomeration of high aspect ratio CNTs and high specific surface area BNNS during subsequent high-viscosity polymerization. The surface-modified polar groups (-COOH / -NH2) enhance the compatibility of the network with polar polyamic acid solvents, ensuring that the three-dimensional network is uniformly dispersed at the molecular level in the matrix and avoiding the performance bottleneck effect caused by agglomeration. At the same time, the three-dimensional network structure itself provides a continuous transport path for phonons and electrons, avoiding the interruption of pathways caused by random dispersion of fillers, and further improving thermal / electrical conductivity.

[0042] In addition, while the bifunctional bridging molecules in the three-dimensional covalently hybridized stitched hybrid network react and crosslink with CNT-NH2 and BNNS-NH2 and the dynamic crosslinking agent, the residual unreacted carboxyl groups and / or amino groups can also form chemical bonds with the polyimide matrix, introducing a flexible crosslinkable interface buffer layer. This buffer layer simultaneously forms chemical bonds with the three-dimensional covalently hybridized stitched hybrid network and the PI matrix, constructing a bifunctional interface that can both efficiently transfer heat and dissipate stress.

[0043] Furthermore, considering the high-temperature applications and processing characteristics of polyimide (PI) composites, the dynamic covalent bonds are reversible covalent bonds or disulfide bonds formed by the Diels-Alder reaction. Offering these four bond types as options provides significant technical flexibility and adaptability to various scenarios. DA bonds are suitable for self-healing triggered at medium temperatures (100-150℃); disulfide bonds are suitable for high-temperature processing or photo-triggered repair. The most suitable dynamic bond type can be selected based on the service temperature window of the final product. Regardless of the chosen bond type, the breakage-recombination mechanism of dynamic bonds is utilized to significantly improve the toughness, fatigue resistance, and damage tolerance of the material without sacrificing the high modulus and high strength of polyimide (provided by the rigid CNT / BNNS network). Through microscopic chemical bond self-healing, the path of microcracks propagating into large cracks is blocked, significantly delaying the aging and failure process of the material, making it particularly suitable for fields with extremely high reliability requirements, such as aerospace and flexible electronics. The presence of dynamic bonds makes the interface between the three-dimensional network and the matrix no longer rigid, and can actively adapt to thermal expansion mismatch and mechanical deformation, fundamentally eliminating the interface debonding, the main failure mode of composite materials.

[0044] Furthermore, the bifunctional bridging molecule is a compound containing both rigid anhydride groups and flexible furan groups. The rigid anhydride groups react with the amino groups on the surface of the carbon nanotubes and / or boron nitride nanosheets to form amide bonds, and the flexible furan groups undergo a Diels-Alder reaction with the dynamic crosslinking agent to form reversible covalent bonds. The general structural formula of the bifunctional bridging molecule is as follows:

[0045]

[0046] Wherein, R1 is a single bond, -CH2-, -O-, -NH- or -S-; R2 is a straight-chain or branched C1-C8 alkylene chain, polyether chain or polyester chain.

[0047] This bifunctional bridging molecule is designed with two completely independent and non-interfering reaction sites (orthogonal reactions): a carboxyl group (-COOH) and a furan ring. The carboxyl group condenses with the amino group (-NH2) on the CNT / BNNS facet to form an amide bond (-CO-NH-), creating a permanent rigid anchor. The amide bond has extremely high bond energy, firmly locking the nanofiller onto the bridging molecule, forming a stable CNT-Linker-BNNS framework, ensuring that the load can be effectively transferred from the matrix to the high-strength filler. The covalent bond eliminates the physical gap (phonon scattering center) between the filler and the matrix, significantly improving the thermal conductivity of the composite material. The cyclohexane structure on the right provides a good spatial configuration, helping the carboxyl group to be exposed in a suitable position for reaction, while cyclohexane itself also provides a certain degree of rigid support. The furan ring undergoes a Diels-Alder (DA) [4+2] cycloaddition reaction with the dynamic crosslinking agent. The DA adduct formed by furan and maleimide reversibly decomposes upon heating (rDA) and regenerates upon cooling. This allows the material to self-repair cracks through heating after damage. During high-temperature processing or service, the reversible fracture of dynamic bonds allows for topological rearrangement of the network, releasing internal stress caused by the mismatch in thermal expansion coefficients and preventing interface debonding.

[0048] R2 is defined as a C1-C8 alkylene chain, polyether chain, or polyester chain. These are typical flexible segments. Between rigid carbon nanotubes (CNTs), boron nitride (BNNS), and rigid polyimide matrices, flexible R2 segments effectively absorb external impact energy and dissipate stress through conformational changes, preventing interfacial cracking caused by stress concentration. This significantly improves the fracture toughness and impact resistance of the composite material, solving the problem of brittle fracture in traditional rigid filler composites. Flexible long chains can also increase the free volume between molecules, improve processing fluidity, and prevent van der Waals aggregation of nanofillers due to close proximity. R1 is a single bond, -CH2-, -O-, -NH-, or -S-. These groups not only connect the furan ring to the flexible chain but also play an electronic regulation role. For example, -O- and -NH- have electron-donating effects, which may fine-tune the electron cloud density of the furan ring, thereby optimizing its activity (reaction rate and equilibrium constant) in the Diels-Alder (DA) reaction with maleimide. At the same time, they ensure the chemical stability of the furan ring connected to the main chain.

[0049] Furthermore, the dynamic crosslinking agent is bismaleimide diphenylmethane and / or bismaleimide polyethylene glycol. Bismaleimide diphenylmethane (BDM) contains two rigid benzene rings and a methylene bridge, resulting in a short and highly rigid molecular chain. The DA crosslinking points formed by the reaction of BDM with furan exhibit extremely high rigidity. In the dynamic network, it acts as a rigid node, effectively preventing excessive softening or creep of the material at high temperatures (but not reaching the rDA dissociation temperature), ensuring that the composite material maintains high modulus and dimensional stability at service temperatures. The aromatic structure itself possesses excellent thermal stability, significantly increasing the thermal decomposition temperature of the dynamic network, making it more compatible with the high-temperature characteristics of the polyimide matrix. The benzene ring structure has low polarity, contributing to maintaining the composite material's excellent low dielectric constant and low dielectric loss. The rigid BDM crosslinking agent synergistically with the rigid CNT / BNNS filler maximizes the tensile strength and hardness of the material. Bismaleimide-based polyethylene glycol (PEG-BMI) contains long-chain polyethylene glycol (PEG) soft segments with flexible molecular chains and an extremely low glass transition temperature. PEG segments act as flexible hinges in dynamic networks. When the material is subjected to impact or tension, the PEG segments can absorb energy through significant conformational changes, greatly improving the elongation at break and impact toughness of the composite material, thus solving the problem of high brittleness in traditional BMI resins or PI composites. The flexibility of the PEG segments increases the mobility (degrees of freedom) of the maleimide end groups, making them more likely to approach and collide with the furan rings on the bridging molecules. This lowers the temperature threshold of the Diels-Alder reaction, allowing self-healing or network construction to proceed efficiently at lower temperatures, reducing energy consumption and protecting heat-sensitive components.

[0050] Furthermore, the mass ratio of carbon nanotubes (CNTs) to boron nitride nanosheets (BNNSs) is limited to the range of 1:0.5 to 1:5. CNTs (1D): acting as "wires" or "thermal bridges," they possess an extremely high aspect ratio, enabling them to connect isolated fillers across long distances, but they are prone to aggregation and have relatively high contact resistance / thermal resistance. BNNSs (2D): acting as "planar surfaces" or "heat sinks," they have a large specific surface area and excellent in-plane thermal conductivity, but it is difficult to form continuous vertical pathways between the layers. At this ratio, CNTs act as "pillars" and "bridges" between BNNS layers, effectively preventing BNNS stacking and utilizing the high aspect ratio of CNTs to connect dispersed BNNS layers in series; simultaneously, BNNSs provide a broad attachment platform for CNTs, reducing CNT aggregation. This structure forms a dense three-dimensional interpenetrating thermal / electrical network, with the shortest phonon (heat) and electron transport paths and minimal resistance, achieving a synergistic enhancement effect of 1+1>2.

[0051] When the BNNS ratio is low (1:0.5), the CNT content is relatively high, and the network tends to form conductive pathways. This ratio is suitable for scenarios requiring electromagnetic shielding or conductive heat dissipation. When the BNNS ratio is high (1:5): a large number of two-dimensional BNNS sheets form layers of barriers in the three-dimensional network, effectively cutting off the macroscopic conductive pathways that CNTs may form (i.e., increasing the conductive percolation threshold). At this time, heat can be efficiently transferred through the covalent connection interface of CNT-BNNS (phonon transport is not affected by the insulator), but electron transport is blocked by BNNS. This allows the composite material to maintain ultra-high thermal conductivity while still having excellent electrical insulation properties (high breakdown voltage, low leakage current), solving the problem of insulation failure caused by traditional CNT addition.

[0052] If there is too much CNT (>1:0.5, i.e., the CNT ratio is too high), CNTs are prone to severe aggregation, forming stress concentration points. Excessive rigid one-dimensional structures also lead to material brittleness, a sharp increase in processing viscosity, and difficulty in dispersion. If there is too much BNNS (>1:5, i.e., the BNNS ratio is too high), excessive lamellar structures result in excessively high system viscosity. Furthermore, during three-dimensional stitching, due to a lack of sufficient bridges (CNTs), the network may become loose or discontinuous, failing to form an effective overall framework, and even exhibiting delamination. Within the range of 1:0.5-5, CNTs and BNNS can form a three-dimensional network structure with the highest space utilization and fewest defects through bifunctional bridging molecules. This ensures that the filler loading is sufficient to form pathways while retaining enough polymer matrix space to transfer the load, achieving an optimal balance between strength, toughness, and modulus.

[0053] In traditional physical blend composites, filler content typically needs to reach 20%-40% or even higher to form continuous thermal or electrical conductive pathways. High filler content can lead to insufficient matrix resin content, making the material brittle and difficult to process.

[0054] The advantage of this approach is that CNTs and BNNS are pre-constructed into a continuous three-dimensional network through chemical bonds. They no longer rely on random collisions to form pathways, but exist as an integral framework. Therefore, only a low addition amount of 1%-15% is needed to form a highly efficient phonon / electron transport channel throughout the polyimide matrix. Even with extremely low addition amounts, as long as the network is intact, it can significantly improve the thermal conductivity and mechanical modulus of the matrix, while almost completely preserving the original excellent electrical insulation and processing fluidity of PI. Within this upper limit, the network density is sufficient to provide extremely high thermal conductivity and strength, while the matrix can still fully encapsulate the network, maintaining the material's toughness. If the network content is >15%, the proportion of rigid filler is too high, which will severely restrict the movement of PI molecular chains, leading to material embrittlement, a sharp decrease in impact strength, and a tendency to generate microcracks due to stress concentration. If the network content is <1%, although the toughness is good, the reinforcement and thermal conductivity effects may not be significant, and the advantages of the three-dimensional network cannot be realized. Within the 1%-15% range, the PI matrix still dominates (85%-99%), fully leveraging its inherent high toughness and fatigue resistance; while the dispersed three-dimensional network undertakes load-bearing and heat transfer tasks. Nanofillers (especially CNTs and BNNS) have large specific surface areas, easily leading to a dramatic increase in the viscosity of the polymer melt or solution. Controlling the content of the three-dimensional network below 15% ensures that the polyamic acid (PAA) precursor solution has suitable flowability and viscosity, facilitating processing such as casting, coating, injection molding, or fiber spinning. If it exceeds 15%, the solution may become a gel or paste, difficult to form, and prone to introducing bubbles and defects. Low-content pre-constructed networks are easier to disperse macroscopically uniformly in solvents through stirring and sonication, avoiding secondary agglomeration problems at high concentrations. The low filler content of 1%-15% means minimal increase in composite density, maintaining the advantages of PI as a lightweight, high-strength material, which is crucial for weight-sensitive fields such as aerospace. As mentioned earlier, the low content combined with the barrier effect of BNNS ensures that while improving thermal conductivity, the material still maintains extremely high volume resistivity and breakdown strength, meeting high-voltage insulation requirements.

[0055] Furthermore, the polyimide matrix is ​​a thermoplastic or thermosetting polyimide formed by the condensation polymerization of aromatic diamines and aromatic dianhydrides. Aromatic diamines and dianhydrides are standard monomers for synthesizing polyamic acid (PAA) precursors. The aforementioned carboxyl / amino groups modified on the surface of the three-dimensional network can undergo perfect in-situ condensation polymerization with these monomers. The -COOH group of the network can react with the diamine; the -NH2 group of the network can react with the dianhydride. This reaction grafts the three-dimensional network onto the growing PI molecular chains in the form of chemical bonds, achieving a seamless chemical connection from the nanonetwork to the macroscopic matrix, completely eliminating interfacial defects in physical blending.

[0056] Secondly, this application also proposes a method for preparing the modified polyimide composite material described in the first aspect, comprising the following steps: Step 1: reacting carboxylated carbon nanotubes with a polyamine compound in the presence of a condensing agent to obtain aminated carbon nanotubes; first hydroxylating boron nitride, and then reacting it with an aminosilane coupling agent to obtain aminated boron nitride nanosheets.

[0057] Step 2: The amino compound containing a furan ring is subjected to an amidation reaction with an aromatic acid anhydride to obtain an amic acid intermediate; the amic acid intermediate is then subjected to an imidation reaction to obtain the bifunctional bridging molecule; wherein the structure of the amino compound containing a furan ring is as follows:

[0058]

[0059] Wherein, R1 is a single bond, -CH2-, -O-, -NH- or -S-; R2 is a straight-chain or branched C1-C8 alkylene chain, polyether chain or polyester chain; the aromatic anhydride is selected from phthalic anhydride.

[0060] Step 3: The aminated carbon nanotubes, aminated boron nitride nanosheets, bifunctional bridging molecules, and dynamic crosslinking agents are mixed and reacted. A three-dimensional covalently stitched hybrid network is pre-constructed through the reaction of rigid functional groups with amino groups and the formation of dynamic covalent bonds between flexible functional groups and the dynamic crosslinking agent. The mass ratio of aminated carbon nanotubes to aminated boron nitride nanosheets is 1:1-5; the ratio of the sum of the masses of aminated carbon nanotubes and aminated boron nitride nanosheets to the mass of the bifunctional bridging molecules is 1:0.3-2; and the molar ratio of the bifunctional bridging molecules to the dynamic crosslinking agent is 1:0.3-1.2. The dynamic covalent bond formation reaction is a Diels-Alder reaction, with a reaction temperature of 50-100℃ and a reaction time of 2-8 hours.

[0061] Step 4: Mix the three-dimensional covalently stitched hybrid network obtained in Step 3 with the polyimide precursor monomer and perform in-situ polymerization to obtain a polyamic acid solution containing a three-dimensional network; the polyimide precursor monomer includes aromatic diamine monomer and aromatic dianhydride monomer, and the in-situ polymerization temperature is 0-10℃.

[0062] Step 5: The polyamic acid solution obtained in Step 4 is molded and subjected to thermal imidization treatment to obtain the polyimide composite material; the thermal imidization treatment adopts a programmed temperature rise method, with a maximum temperature of 250-350℃.

[0063] Step 6: Heat-treat the obtained polyimide composite material at 120-150℃ for 10-60 min to allow it to self-heal or reshape cracks.

[0064] In step 1, CNTs are processed using a two-step method of carboxylation and polyamine chain extension. Compared to direct amination, introducing polyamine compounds not only introduces amino groups but also adds flexible spacer arms, reducing steric hindrance on the CNT surface and increasing the density of active sites for subsequent reactions. BNNS employs a hydroxylation + silane coupling agent strategy. Utilizing the bifunctional properties of silane coupling agents (such as APTES), one end firmly binds to the -OH group on the BNNS surface, while the other end provides highly reactive -NH2, solving the problem of high inertness and difficulty in grafting on the BNNS surface. This ensures that both CNT and BNNS surfaces possess high-density, highly reactive, and uniformly distributed amino groups, providing sufficient reaction sites for subsequent formation of dense covalent bonds with bifunctional bridging molecules, thus eliminating the weak interfacial binding layer at its source.

[0065] In step 2, a bifunctional molecule containing a furan-amino compound with a specific structure was precisely synthesized by reacting it with phthalic anhydride. This molecule has a rigid imide / carboxyl group at one end and a flexible furan unit at the other. Phthalic anhydride was chosen as the rigid source because it has a simple structure, moderate reactivity, and is easy to control in terms of molecular weight, avoiding the formation of excessively long oligomers that would lead to dispersion difficulties. The synthesized bridging molecule possesses both reactivity (carboxyl / anhydride end) and dynamic functionality (furan end), and the flexible chain in the middle can effectively buffer stress. This bifunctional bridging molecule ensures strong chemical bonding with the filler and introduces a dynamic bond precursor required for self-healing of the network.

[0066] In step 3, before adding the PI matrix, CNTs, BNNS, bridging molecules, and crosslinking agents are assembled into independent three-dimensional covalently bonded networks. This solves the problem of nanofillers being difficult to disperse and prone to aggregation in high-viscosity polymer melts / solutions. Once the network is formed, its topology is locked, and subsequent mixing is merely a physical dispersion process, ensuring absolutely uniform distribution of the filler in the matrix and the construction of continuous pathways. An appropriate ratio of filler to bridging molecules ensures that the filler surface is fully covered and connected, avoiding network breakage due to insufficient bridging molecules and preventing excessive small molecule residues from affecting matrix properties. The precise ratio of bridging molecules to crosslinking agents controls the dynamic crosslinking density. Too low a ratio results in a loose network, while too high a ratio leads to an overly brittle network and loss of dynamics. This range ensures an optimal balance between mechanical strength and dynamic exchange capacity in the network.

[0067] In step 4, the synthesis of polyamic acid (PAA) is an exothermic reaction, and high temperatures can easily lead to side reactions or early imidization precipitation. Low-temperature polymerization ensures the formation of a high-molecular-weight, high-viscosity, and homogeneous PAA solution. The low-temperature environment protects the DA dynamic bonds formed in step 3 from dissociation, maintaining the integrity of the three-dimensional network. The amino / carboxyl ends in the PAA precursor can undergo in-situ condensation polymerization with unreacted functional groups on the surface of the three-dimensional network. This transforms the three-dimensional network from a physically doped "foreign object" into a "core node" for the growth of PI molecular chains, achieving chemical copolymerization between the filler and the matrix and greatly enhancing the interfacial bonding force.

[0068] In step 5, the temperature is gradually increased from room temperature to 250-350℃. This effectively prevents surface crusting that could lead to internal bubbles. It ensures the PI matrix is ​​completely closed-loop, achieving optimal heat resistance and mechanical properties. At high temperatures (>120℃), some DA bonds may undergo reversible dissociation (rDA), giving the rigid three-dimensional network a temporary "fluidity" or "relaxation" at high temperatures. This characteristic allows the network to undergo topological rearrangement as the PI matrix shrinks, actively releasing thermal mismatch stress and avoiding microcracks caused by curing shrinkage in traditional composite materials.

[0069] In step 6, the temperature range is slightly below the rapid dissociation temperature of the DA bond, but sufficient to stimulate dynamic exchange reactions and promote chain segment movement. For microcracks generated during processing or cooling, this heat treatment promotes the recombination of broken DA bonds or causes local rearrangement of the network to fill the cracks, achieving "self-healing upon leaving the factory." Addressing the difficulty of secondary processing of thermosetting PI, this step utilizes the reversibility of dynamic bonds to enable the material to exhibit thermoplastic-like behavior under specific conditions, allowing for welding, reshaping, or recycling, greatly expanding its applications.

[0070] Example 1

[0071] 2.0 g of carboxylated CNTs were dispersed in 50 mL of DMF, and 0.5 g of EDC and 0.3 g of NHS were added for activation for 1 h. Subsequently, 2.0 g of ethylenediamine was added, and the reaction was carried out at 60 °C for 12 h. After washing and drying, CNT-NH2 was obtained.

[0072] After ball milling and exfoliating 5.0 g of hexagonal boron nitride (h-BN), it was hydroxylated by reacting with a mixture of concentrated sulfuric acid / nitric acid at 80 °C for 4 h. After washing and drying, it was dispersed in toluene, and 1.0 g of γ-aminopropyltriethoxysilane (APTES) was added. The mixture was refluxed for 24 h. After washing and drying, BNNS-NH2 was obtained.

[0073] 0.02 mol of NN-(2-aminoethyl)-2-(furan-2-yl)acetamide was dissolved in 20 mL of anhydrous NMP and cooled to 0 °C in an ice bath. 0.021 mol of phthalic anhydride powder was slowly added, and the mixture was stirred for 12 h to obtain a polyamic acid intermediate solution. Acetic anhydride (dehydrating agent) and triethylamine (catalyst) were added, and the mixture was heated to 60 °C for 4 h to induce imidization. The product was precipitated in ethanol, filtered, and vacuum dried to obtain a yellow powdery bifunctional bridging molecule.

[0074] 0.4 g CNT-NH2 and 1.2 g BNNS-NH2 were ultrasonically dispersed in 40 mL NMP. 0.64 g of bifunctional bridging molecule was added, followed by 0.3 g EDC / 0.2 g NHS. The mixture was stirred at 40°C for 6 h to form a preliminary network linked by amide bonds.

[0075] Add 0.18 g BDM and 0.14 g PEG-BMI. Heat to 70°C and stir at this temperature for 4 hours. At this point, the Diels-Alder reaction occurs, the viscosity of the system increases sharply, and a semi-solid three-dimensional covalently stitched hybrid network gel is formed.

[0076] Add the remaining NMP to the above gel to adjust the solid content and cool to 5°C. Add 10.0 g of 4,4'-diaminodiphenyl ether (ODA) and stir to dissolve for 30 min. Add 10.9 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) powder in batches, strictly controlling the temperature between 0-10°C, and stir the reaction for 12 h. A homogeneous, high-viscosity, non-agglomerated black polyamic acid (PAA) solution (containing a three-dimensional network) is obtained.

[0077] The PAA solution was cast onto a clean glass plate, and the wet film thickness was controlled using a doctor blade. The temperature program was as follows: 60℃ / 1h (solvent evaporation), 120℃ / 1h (pre-iminolation), 180℃ / 1h (deep iminolation), 250℃ / 1h (complete iminolation, at which point some DA bonds reversibly dissociate to release stress), and then naturally cooled to room temperature (dA bonds reformed during cooling, locking the network). The resulting self-supporting film (approximately 50 μm thick) was obtained by peeling.

[0078] The film was placed in an oven and heat-treated at 130°C for 30 minutes. This step stimulates dynamic bond recombination, repairs microcracks in the machining process, eliminates residual stress, and activates the self-healing function.

[0079] Example 2

[0080] The only difference between this embodiment and Embodiment 1 is that the mass ratio of CNT to BNNS is 1:0.6, that is, the amount of CNT-NH2 added is 1 g and the amount of BNNS-NH2 added is 0.6 g.

[0081] Example 3

[0082] The only difference between this embodiment and Embodiment 1 is that the mass ratio of CNT to BNNS is 1:4, that is, the amount of CNT-NH2 added is 0.32 g and the amount of BNNS-NH2 added is 1.28 g.

[0083] Example 4

[0084] The only difference between this embodiment and Example 1 is that the mass ratio of (CNT+BNNS) to the bridging molecule is 1:2, that is, the amount of CNT-NH2 added is 0.185 g, the amount of BNNS-NH2 added is 0.555 g, and the amount of bifunctional crosslinking molecule added is 1.5 g.

[0085] Example 5

[0086] The only difference between this embodiment and Example 1 is that the molar ratio of bridging molecule to crosslinking agent is as follows: the amount of CNT-NH2 added is 0.104 g, the amount of BNNS-NH2 added is 0.308 g, the amount of bifunctional crosslinking molecule added is 0.832 g, the amount of BDM added is 0.738 g, and the amount of PEG-BMI added is 0.574 g.

[0087] Example 6

[0088] The only difference between this embodiment and Example 1 is that the three-dimensional network content is 5.68%, that is, the amount of CNT-NH2 added is 0.2 g, the amount of BNNS-NH2 added is 0.6 g, the amount of bifunctional crosslinking molecule added is 0.32 g, the amount of BDM added is 0.09 g, and the amount of PEG-BMI added is 0.07 g.

[0089] Example 7

[0090] The only difference between this embodiment and Example 1 is that the three-dimensional network content is 15%, that is, the amount of CNT-NH2 added is 0.528 g, the amount of BNNS-NH2 added is 1.584 g, the amount of bifunctional crosslinking molecule added is 0.845 g, the amount of BDM added is 0.238 g, and the amount of PEG-BMI added is 0.185 g.

[0091] Example 8

[0092] The only difference between this comparative example and Example 1 is that a disulfide bond (-SS-) is used instead of a DA bond (using a disulfide-bridging molecule, without adding BMI). That is, this example does not use the DA reaction (furan-maleimide addition) or BMI (bismaleimide) as a crosslinking agent; instead, it utilizes a disulfide-bridging molecule (such as HS–R–SH or HOOC–R–S–S–R–COOH) as a dynamic crosslinking unit.

[0093] Compared to Example 1, the preparation method of Example 8 is changed as follows: the dynamic covalent bonds in the bifunctional bridging molecule are replaced with disulfide bonds (-SS-) instead of Diels-Alder bonds (DA bonds), and the original maleimide crosslinking agent is removed. The specific preparation steps are as follows: a bifunctional molecule containing disulfide bonds (1,2-di(thioglycolic acid) ethyl ester or similar) is used as the bridging molecule, dissolved in NMP solvent under nitrogen protection, and then added to a dispersion of 0.4 g CNT-NH2 and 1.2 g BNNS-NH2. The mixture is stirred at 60°C for 12 hours to form a three-dimensional network crosslinked by disulfide bonds through oxidative coupling or nucleophilic substitution. BDM and PEG-BMI are not added in this step. Subsequent steps, such as mixing with the polyimide matrix, casting, and curing heat treatment, are completely consistent with those of Example 1.

[0094] Comparative Example 1

[0095] The only difference between this embodiment and Embodiment 1 is that the nanofiller is physically blended, without bifunctional bridging molecules or dynamic bonds, and the filler is directly melt-blended with the PI matrix.

[0096] Comparative Example 2

[0097] The only difference between this comparative example and Example 1 is that the bifunctional bridging molecule has no dynamic bond chemical connection (using a furan-free rigid bridging molecule).

[0098] The specific changes in the preparation method of this comparative example are as follows: the dynamic crosslinking agents (BDM and PEG-BMI) in the reaction system are completely removed, and the bifunctional bridging molecules are replaced with rigid molecules without furan groups, such as terephthalic acid or 1,4-succinic acid. In the specific feeding steps, the amount of nanofillers (CNT-NH2 and BNNS-NH2) and the molar number of bridging molecules are kept consistent with those in Example 1, that is, the stoichiometric ratio between the fillers and bridging molecules remains unchanged. The specific reaction process is as follows: 0.4 g of CNT-NH2 and 1.2 g of BNNS-NH2 are dispersed in NMP solvent, followed by the addition of 0.64 g of rigid bridging molecules (terephthalic acid), and the reaction is stirred at 40°C for 6 hours to form a static covalent network structure through amidation or esterification. Since this system does not contain furan groups that can undergo the Diels-Alder reaction, there is no need to add BDM and PEG-BMI, two dynamic crosslinking agents containing maleimide groups, thus constructing a three-dimensional network that is only connected by rigid static bonds and does not have dynamic recombination capabilities. The subsequent steps of mixing with the polyimide matrix, casting film formation, and curing heat treatment are completely consistent with those in Example 1.

[0099] Comparative Example 3

[0100] The only difference between this comparative example and Example 1 is that unmodified raw CNTs and BNNS are used.

[0101] Comparative Example 4

[0102] The only difference between this comparative example and Example 1 is that R2 in the bridging molecule is a single bond (rigid direct connection). That is, this comparative example replaces the bridging molecule containing the flexible R2 segment in Example 1 with a bridging molecule that does not contain a flexible chain and whose R2 is a direct carbon-carbon single bond or a short rigid chain (such as: HOOC–C5H4–NH2, rigid molecules of para-aminobenzoic acid).

[0103] Compared to Example 1, the preparation method of Comparative Example 4 was changed as follows: the flexible segment (R2) connecting the functional groups at both ends of the bifunctional bridging molecule was replaced with a rigid single-bond structure, resulting in a bifunctional molecule (such as a p-aminobenzoic acid derivative) directly connected by carbon-carbon single bonds or benzene rings. The specific preparation steps were as follows: keeping all other components and amounts completely consistent, 0.64 g of the bridging molecule with a rigid single-bond structure was added to an NMP dispersion containing 0.4 g of CNT-NH2 and 1.2 g of BNNS-NH2, and stirred at 40°C for 6 hours to form a rigidly connected three-dimensional network structure through amide bonds. Since this bridging molecule does not contain furan groups, there is no need to add BDM and PEG-BMI dynamic crosslinking agents. Subsequent steps such as mixing with the polyimide matrix, casting, and curing heat treatment were completely consistent with those of Example 1.

[0104] The corresponding experimental data of Examples 1-7 above are summarized in Table 1.

[0105] Table 1. Relevant experimental data for Examples 1-7

[0106]

[0107] The self-supporting films prepared in the above embodiments and comparative examples were subjected to corresponding performance tests, and the test data are shown in Table 2.

[0108] Table 2. Performance data of Examples 1-9 and Comparative Examples 1-3

[0109]

[0110] In Example 1, the CNT to BNNS mass ratio was 1:3, achieving a high thermal conductivity of 13.2 W / m·K and 10 15 A perfect balance of ultra-high insulation. BNNS fully encapsulates the CNTs, blocking the conductive path. In Example 2, the CNT to BNNS mass ratio was 1:0.6, resulting in insufficient BNNS to completely isolate the CNTs, causing the volume resistivity to plummet to 10. 10 While the thermal conductivity slightly increases, the CNTs lose their insulating value and are unsuitable for electronic insulation applications. In Example 3, the CNT to BNNS mass ratio was 1:4, with an excess of BNNS, further improving insulation. However, due to the relatively reduced CNT content, the thermal conductivity network connectivity was slightly weaker, and the thermal conductivity dropped to 11.8, still better than traditional materials. This demonstrates that within the CNT:BNNS mass ratio range of 1:0.5-5, 1:3 is the optimal point balancing thermal conductivity and insulation.

[0111] In Example 4, the excessive bridging molecules increased the phonon scattering interface, leading to a decrease in thermal conductivity to 10.5. However, the increased flexible chains improved the elongation at break to 14.2%. In Example 5, the excessive crosslinking agent resulted in extremely high crosslinking density, causing severe embrittlement of the material (elongation of only 4.5%) and hindering dynamic bond movement, halving the self-healing efficiency to 45%. This demonstrates the necessity of a molar ratio of 1:0.3-1.2. In Example 6, the low network addition resulted in limited reinforcement and thermal conductivity (8.5 W / m·K), but good processability and toughness. In Example 7, the significantly increased actual filler content resulted in the highest strength (268 MPa) and best thermal conductivity (15.8), but also extreme brittleness (2.8%) and poor impact resistance (failure after 120 thermal shock cycles). This demonstrates the superiority of a 1%-15% content range—achieving high performance without excessively sacrificing toughness. In Comparative Example 4, without the R2 group, the elongation at break plummeted from 11.8% to 3.5% after removing the flexible chain, and the number of thermal shock cycles was significantly reduced. This demonstrates that the flexible R2 segment is key to resolving the contradiction between high strength and brittleness. In Example 8, replacing the DA bond with a disulfide bond resulted in toughness at room temperature even better than Example 1 (13.5%), and it also possessed self-healing ability. However, its high-temperature stability was slightly inferior to the DA bond system (slightly higher CTE, slightly lower thermal shock resistance), and the disulfide bond might prematurely exchange at high temperatures during PI processing. This demonstrates that the DA bond is more suitable for PI matrices used in high-temperature applications.

[0112] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0113] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0114] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0115] Furthermore, it should be noted that the scope of the methods and apparatus in the embodiments of this application is not limited to performing functions in the order shown or discussed, but may also include performing functions substantially simultaneously or in the reverse order, depending on the functions involved. For example, the described methods may be performed in a different order than described, and various steps may be added, omitted, or combined. In addition, features described with reference to certain examples may be combined in other examples.

Claims

1. A modified polyimide composite material, characterized in that, The invention includes a polyimide matrix and a three-dimensional covalently stitched hybrid network dispersed in the polyimide matrix; the surface of the three-dimensional covalently stitched hybrid network is modified with unreacted carboxyl groups and / or amino groups for forming chemical bonds with the polyimide matrix; The three-dimensional covalently stitched hybrid network is formed by covalently connecting one-dimensional carbon nanotubes and two-dimensional boron nitride nanosheets through bifunctional bridging molecules, and the connection between the bifunctional bridging molecules and the carbon nanotubes and / or boron nitride nanosheets includes dynamic covalent bonds. The carbon nanotubes and / or boron nitride nanosheets are modified with amino groups; the mass ratio of the carbon nanotubes to the boron nitride nanosheets is 1:1-5, and the ratio of the sum of the masses of the carbon nanotubes and the boron nitride nanosheets to the mass of the bifunctional bridging molecule is 1:0.3-2; the dynamic covalent bond is a reversible covalent bond or disulfide bond formed by the Diels-Alder reaction; the bifunctional bridging molecule is a compound containing both rigid anhydride groups and flexible furan groups, and the rigid anhydride groups react with the amino groups on the surface of the carbon nanotubes and / or boron nitride nanosheets to form amide bonds.

2. The polyimide composite material according to claim 1, characterized in that, The flexible furan group reacts with the dynamic crosslinking agent via a Diels-Alder reaction to form the reversible covalent bond.

3. The polyimide composite material according to claim 2, characterized in that, The dynamic crosslinking agent is bismaleimide diphenylmethane and / or bismaleimide polyethylene glycol.

4. The polyimide composite material according to any one of claims 1-3, characterized in that, The mass of the three-dimensional covalently stitched hybrid network accounts for 1%-15% of the total mass of the composite material; And / or, the polyimide matrix is ​​a thermoplastic or thermosetting polyimide formed by the condensation polymerization of aromatic diamines and aromatic dianhydrides.

5. A method for preparing the modified polyimide composite material according to any one of claims 1-4, characterized in that, Includes the following steps: Step 1: Amide modification of carbon nanotubes and boron nitride nanosheets to obtain aminated carbon nanotubes and aminated boron nitride nanosheets, respectively. Step 2: Synthesize a bifunctional bridging molecule, wherein one end of the bifunctional bridging molecule contains a rigid functional group that can react with an amino group, and the other end contains a flexible functional group that can participate in the formation of dynamic covalent bonds; Step 3: Mix and react the aminated carbon nanotubes, aminated boron nitride nanosheets, bifunctional bridging molecules and dynamic crosslinking agents. Through the reaction of the rigid functional groups with amino groups and the dynamic covalent bond formation reaction between the flexible functional groups and the dynamic crosslinking agent, a three-dimensional covalently stitched hybrid network is pre-constructed. Step 4: Mix the three-dimensional covalently stitched hybrid network obtained in Step 3 with the polyimide precursor monomer and perform in-situ polymerization to obtain a polyamic acid solution containing a three-dimensional network. Step 5: The polyamic acid solution obtained in Step 4 is molded and subjected to thermal imidization treatment to obtain the polyimide composite material.

6. The method according to claim 5, characterized in that, The method for synthesizing the bifunctional bridging molecule in step 2 is as follows: an amino compound containing a furan ring is subjected to an amidation reaction with an aromatic acid anhydride to obtain an amide acid intermediate; the amide acid intermediate is subjected to an imide reaction to obtain the bifunctional bridging molecule; the aromatic acid anhydride is selected from phthalic anhydride.

7. The method according to claim 5, characterized in that, Step 3, the molar ratio of the bifunctional bridging molecule to the dynamic crosslinking agent is 1:0.3-1.2; the dynamic covalent bond formation reaction in step 3 is the Diels-Alder reaction, the reaction temperature is 50-100℃, and the reaction time is 2-8h.

8. The method according to claim 5, characterized in that, The polyimide precursor monomers mentioned in step 4 include aromatic diamine monomers and aromatic dianhydride monomers, and the in-situ polymerization temperature is 0-10℃; And / or, the thermal imidization treatment in step 5 adopts a programmed temperature rise method, with a maximum temperature of 250-350℃; And / or, the preparation method of the aminated carbon nanotubes in step 1 is: reacting carboxylated carbon nanotubes with a polyamine compound in the presence of a condensing agent; and / or, the preparation method of the aminated boron nitride nanosheets in step 1 is: reacting boron nitride with an aminosilane coupling agent after hydroxylation.

9. The method according to claim 5, characterized in that, It also includes step 6: heat-treating the obtained polyimide composite material at 120-150℃ for 10-60 min to allow it to self-heal or reshape cracks.

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