Organic electron repellent molecule grafted polyetherimide composite film as well as preparation method and application thereof

Abstract

The invention discloses an organic electron repellent molecule grafted polyetherimide composite film as well as a preparation method and application thereof, and belongs to the technical field of dielectric energy storage materials. The film takes 4-cyanopyrazole as an organic electron repelling molecule, the lowest unoccupied molecular orbital energy level of the film is higher than that of polyetherimide, the film is grafted to a polyetherimide main chain through an amido bond, and the mass ratio of 4-cyanopyrazole to polyetherimide is (5: 9995)-(15: 9985). The preparation is completed through polyetherimide dissolution, grafting reaction, defoaming coating and gradient drying, and the process is simple and convenient. The film effectively solves the problems of insufficient breakdown field strength and high loss of the traditional polyetherimide under high temperature and high field, remarkably improves the insulating property and the charge-discharge efficiency, can be used for high-temperature film capacitors, adaptive high-voltage flexible direct-current transmission converter valves, new energy vehicle motor drivers, pulse power systems and other scenes, and has important engineering application value.

Description

Technical Field

[0001] This invention relates to the field of dielectric energy storage materials technology, and in particular to a polyetherimide composite film grafted with organic electron-discarding molecules, its preparation method, and its application. Background Technology

[0002] Polymer dielectric materials are widely used in film capacitors due to their advantages such as high breakdown field strength, high charge / discharge rate, high power density, and self-healing properties. Film electrostatic capacitors using polymer films as dielectrics are key components in high-voltage flexible DC transmission converter valves, new energy vehicle motor drives, and pulse power systems.

[0003] Currently, the most commonly used dielectric in polymer film capacitors is biaxially oriented polypropylene (BOPP) film with a thickness of micrometers. BOPP film exhibits good dielectric strength and extremely low dielectric loss under operating conditions; however, its stable operating temperature is limited to an upper limit of 85°C. Exceeding this temperature leads to a sharp increase in the loss level of BOPP. With the widespread application of third-generation power semiconductor devices, the operating environment faced by film capacitors is becoming increasingly harsh. The ambient temperature around the capacitor exceeds 120°C, and it needs to withstand an electric field strength greater than 400 MV / m, significantly limiting the high-temperature application of BOPP film capacitors. Because biaxially oriented polypropylene (BOPP) film capacitors operate at lower temperatures than other components in the system under harsh conditions, they often require additional cooling devices, increasing their size and weight, and reducing system reliability.

[0004] Currently, polyetherimide (PEI) is considered an important option for high-temperature dielectrics in new film capacitors due to its excellent temperature resistance and insulation properties. However, PEI struggles to meet the requirements at high temperatures and high electric fields above 150℃ and 400MV / m, mainly due to two issues: first, the breakdown field strength of PEI is insufficient under high temperature and high electric field conditions, hindering stable operation; second, the loss level of PEI increases sharply with increasing electric field, resulting in excessively low charge and discharge efficiency. Therefore, modification of PEI is still necessary for its application as a high-temperature dielectric in film capacitors.

[0005] To improve the breakdown field strength and charge / discharge efficiency of polyetherimide films at high temperatures, various modification methods for incorporating organic compounds into polyetherimide have been developed in this field. Test results show that the performance of the composite films is improved. However, due to the weak interaction forces between polyetherimide and the blend, the blend cannot be uniformly dispersed in the polyetherimide matrix but rather in an aggregated form. This introduces an additional interface within the dielectric, resulting in lower insulation strength in the interface region and leading to the aggregation of electron carriers. This phenomenon weakens the effect of the organic compound modification method on improving the breakdown field strength and charge / discharge efficiency of the composite film at high temperatures.

[0006] Therefore, developing a thin-film dielectric that can withstand higher temperatures for polymer thin-film capacitors is an urgent problem to be solved in this field. Summary of the Invention

[0007] In view of the technical problems existing in the prior art, the present invention aims to provide a polyetherimide composite film grafted with organic electron-discarding molecules, its preparation method and application.

[0008] One objective of this invention is to provide a polyetherimide composite film grafted with an organic electron-discarding molecule, wherein the lowest unoccupied molecular orbital energy level of the organic electron-discarding molecule is higher than the lowest unoccupied molecular orbital energy level of the polyetherimide. The organic electron-discarding molecule is 4-cyanopyrazole; The 4-cyanopyrazole is grafted onto the polyetherimide via an amide bond.

[0009] The second objective of this invention is to provide a method for preparing the above-mentioned organic electron-discarding molecule grafted polyetherimide composite film, comprising the following steps: S1. Add polyetherimide to an aprotic polar solvent and stir to obtain a polyetherimide solution. S2. Add 3-amino-4-cyanopyrazole to the polyetherimide solution and stir to react to obtain a 4-cyanopyrazole-grafted polyetherimide solution. S3. After degassing the 4-cyanopyrazole-grafted polyetherimide solution, it is poured onto the carrier and dried in a gradient to obtain a carrier with a film attached. After peeling off the film, it is dried to obtain a polyetherimide composite film.

[0010] Preferably, the mass ratio of 3-amino-4-cyanopyrazole to polyetherimide is (5:9995) to (15:9985).

[0011] Preferably, the polyetherimide is dissolved at a temperature of 60-80°C for 12-24 hours in step S1; and the stirring reaction is carried out at a temperature of 60-100°C for 8-14 hours in step S2.

[0012] Preferably, the aprotic polar solvent includes one or more of N-methylpyrrolidone, dimethylacetamide, or dimethylformamide.

[0013] Preferably, the ratio of aprotic polar solvent to polyetherimide is 10 mL: (1.6 g - 2.0 g).

[0014] Preferably, the gradient drying in step S3 includes: Heat treatment at 70-90℃ for 3-5 hours, at 110-130℃ for 1-3 hours, at 140-160℃ for 1-3 hours, and at 190-210℃ under vacuum for 1-3 hours.

[0015] Preferably, the drying after peeling off the film in step S3 includes drying the film at 60~80°C for 12-36 hours.

[0016] The third objective of this invention is to provide an application of the polyetherimide composite film described above in a capacitor.

[0017] Preferably, the capacitor is a high-temperature film capacitor; Application scenarios include high-voltage flexible DC transmission converter valves, new energy vehicle motor drivers, or pulse power systems.

[0018] The beneficial effects of this invention are: In this invention, the lowest unoccupied molecular orbital (LUMO) energy level of the organic electron-rejecting molecule is higher than that of the polyetherimide. The higher the LUMO energy level, the more difficult it is for the molecule to accept electrons, and the less likely electrons are to enter the empty orbitals of the electron-rejecting molecule. Therefore, this organic electron-rejecting molecule can serve as a highly efficient electron scattering center, significantly increasing the tortuosity of the transport path of electron carriers, thereby reducing conductivity loss and suppressing leakage current.

[0019] This invention selects 4-cyanopyrazole as an effective electron scattering center. The electron-repelling properties of this molecule are synergistic with the high LUMO energy level, resulting in higher electron scattering efficiency and better charge transport suppression effect compared to other heterocyclic electron-repelling molecules.

[0020] Furthermore, 4-cyanopyrazole is grafted onto the polyetherimide backbone via amide bonds. On the one hand, this covalent bond locks electron-withdrawing molecules within the polymer matrix, completely avoiding the molecular aggregation problems that easily occur in physical blending or non-covalent bonding methods. This ensures the dielectric uniformity of the composite film and reduces the risk of electric field distortion and breakdown caused by interface defects. On the other hand, the grafting reaction partially disrupts the imide ring structure of the polyetherimide backbone, breaking the inherent charge transport channels within the molecular chain. This further suppresses the generation of leakage current from the source, forming a dual synergy with electron scattering, significantly improving the insulation performance of the film.

[0021] In summary, under high temperature and high field conditions (above 150℃ and 400MV / m), this composite film, with its high LUMO energy level and grafted structure advantages of 4-cyanopyrazole, can efficiently scatter electron carriers and suppress directional transport, resulting in a significantly improved breakdown field strength compared to traditional polyetherimide. It can stably withstand harsh electric fields. At the same time, it does not suffer from interface defects caused by molecular aggregation, and its dielectric loss does not surge at high temperatures. The charge and discharge efficiency remains at a high level, and it can be adapted to high-temperature scenarios such as high-voltage flexible DC transmission converter valves without the need for additional cooling devices, greatly improving the reliability and service stability of the equipment. Attached Figure Description

[0022] Figure 1 The reaction formula is for grafting polyetherimide with the organic electron-discarding molecule 4-cyanopyrazole; Figure 2 Comparison of differential scanning calorimetry test results of the thin films provided in Example 1, Comparative Example 1 and Comparative Example 2; Figure 3 A comparison chart of the DC breakdown strength test results of the films provided in Example 1, Comparative Example 1, and Comparative Example 2 at 150°C. Figure 4 A comparison chart of the test results of discharge energy density and charge / discharge efficiency of the films provided in Example 1, Comparative Example 1 and Comparative Example 2 at 150°C. Figure 5 A comparison chart showing the test results of the charge-discharge efficiency of the films provided in Example 1, Example 2, Example 3 and Comparative Example 1 at 150°C; Figure 6 A comparison graph showing the test results of discharge energy density and charge / discharge efficiency of the films provided in Example 2 and Comparative Example 1 at 150°C; Figure 7 The graph shows a comparison of the test results of discharge energy density and charge / discharge efficiency of the films provided in Example 3 and Comparative Example 1 at 150°C. Detailed Implementation

[0023] The present application will now be described in further detail with reference to embodiments. In the following description, certain specific details are included to provide a comprehensive understanding of the various disclosed embodiments. However, those skilled in the art will recognize that embodiments may be implemented without employing one or more of these specific details, but using other methods, components, materials, etc. Unless otherwise required by the present invention, the terms "comprising" and "including" shall be interpreted in an open-ended, inclusive sense, meaning "including but not limited to".

[0024] Throughout this specification, the phrase "a preferred embodiment" means that at least one embodiment includes a specific reference element, structure, or feature related to that embodiment. Therefore, the phrase "a preferred embodiment" appearing in different places throughout the specification does not necessarily refer to the same embodiment. Furthermore, specific elements, structures, or features may be combined in one or more embodiments in any suitable manner.

[0025] According to a first aspect of the present invention, a polyetherimide composite film grafted with an organic electron-donating molecule is provided, wherein the lowest unoccupied molecular orbital energy level of the organic electron-donating molecule is higher than the lowest unoccupied molecular orbital energy level of the polyetherimide. The organic electron-discarding molecule is 4-cyanopyrazole; The 4-cyanopyrazole is grafted onto the polyetherimide via an amide bond.

[0026] In this invention, the reaction formula for grafting polyetherimide with the organic electron-discarding molecule 4-cyanopyrazole is as follows: Figure 1 As shown.

[0027] Existing PEIs exhibit rapid electron carrier transport and aggregation at high temperatures and fields above 150℃ and 400MV / m, leading to a sharp increase in conductivity loss, decreased charge-discharge efficiency, increased leakage current, and insufficient breakdown field strength. This is essentially due to the low LUMO energy level of PEI (-2.33 eV), which allows electrons to easily enter empty molecular orbitals, forming directional transport and ultimately causing breakdown or high losses.

[0028] The LUMO energy level is the threshold of a molecule's ability to accept electrons. The higher the LUMO energy level, the greater the energy barrier that electrons need to overcome, and the more difficult it is for them to enter the empty orbitals of the molecule.

[0029] In this invention, the LUMO energy level of 4-cyanopyrazole is -0.96 eV, while that of PEI is -2.33 eV. The LUMO energy level of 4-cyanopyrazole is higher than that of PEI. When the LUMO energy level of the electron-donating molecule is higher than that of PEI, electrons cannot be captured, thus preventing electrons from accumulating on the electron-donating molecule. This forces electron carriers to repeatedly scatter between the PEI matrix and the electron-donating molecule, resulting in an extremely tortuous transport path, thereby suppressing leakage current and conductivity loss.

[0030] If the LUMO energy level of the electron-donating molecule is less than or equal to the LUMO energy level of PEI, electrons will easily enter the empty orbitals of the electron-donating molecule and accumulate. This not only fails to suppress charge transport but also becomes a weak point for breakdown, resulting in the composite film having worse performance than unmodified PEI.

[0031] Traditional heterocyclic electron-withdrawing molecules such as simple pyrazoles and pyridine derivatives lack strong electron-withdrawing groups and have weak electron-withdrawing ability. Furthermore, they lack specific active sites and cannot form stable grafts with PEI. Moreover, the LUMO energy level does not match that of PEI, making it easy to trap electrons, resulting in poor modification effects or failure to achieve the desired results.

[0032] In this invention, the molecular structure of 4-cyanopyrazole contains a cyano group (-CN), which is a strong electron-rejecting group and forms a conjugated structure with the pyrazole ring, making the electron-rejecting effect concentrated and wide-ranging, and generating a strong repulsive force on electron carriers; at the same time, the conjugated system of the pyrazole ring and the electron-rejecting effect of the cyano group superimpose, making the LUMO energy level of the molecule higher than that of PEI.

[0033] According to a second aspect of the present invention, a method for preparing a polyetherimide composite film grafted with organic electron-discarding molecules as described above is provided, comprising the following steps: S1. Add polyetherimide to an aprotic polar solvent and stir to obtain a polyetherimide solution. S2. Add 3-amino-4-cyanopyrazole to the polyetherimide solution and stir to react to obtain a 4-cyanopyrazole-grafted polyetherimide solution. S3. After degassing the 4-cyanopyrazole-grafted polyetherimide solution, it is poured onto the carrier and dried in a gradient to obtain a carrier with a film attached. After peeling off the film, it is dried to obtain a polyetherimide composite film.

[0034] In this invention, step S1 aims to fully dissolve and uniformly disperse the polyetherimide, providing a stable reaction environment for the subsequent grafting reaction. Aprotic polar solvents are compatible with the solubility characteristics of polyetherimide, and stirring prevents polyetherimide from clumping, ensuring that electron-donating molecules can fully contact the polyetherimide backbone and avoiding incomplete local grafting. Step S2 involves adding 3-amino-4-cyanopyrazole to the homogeneous solution, allowing it to react with the imide ring of the polyetherimide backbone to form amide bonds. Stirring promotes full contact of the reactants, ensuring a complete grafting reaction and preventing the aggregation of electron-donating molecules, ultimately yielding a homogeneous grafted polymer solution. Step S3 involves the formation of a functional thin film from the solution. A smooth and flat glass plate ensures uniform film thickness; solvent removal is to obtain a dense, defect-free film structure, preventing residual solvent from degrading dielectric properties; after peeling, the target composite film is obtained, suitable for the film formation requirements of capacitor applications.

[0035] In a preferred embodiment of the present invention, the mass ratio of 3-amino-4-cyanopyrazole to polyetherimide is (5:9995) to (15:9985).

[0036] In this invention, adding too low a content of 3-amino-4-cyanopyrazole is insufficient to effectively improve the performance of polyetherimide, while adding too high a content of 3-amino-4-cyanopyrazole increases the loss of the composite film at high temperatures and reduces charge-discharge efficiency. A mass ratio of 3-amino-4-cyanopyrazole to polyetherimide of 5:9995 meets the minimum requirement for the number of electron scattering centers. Only by reaching this amount can a fully covered scattering network be formed, disrupting charge transport channels by breaking a sufficient proportion of the PEI main chain imide rings, while ensuring uniform molecular dispersion in the PEI solution, avoiding performance inhomogeneity caused by ungrafted regions. If the ratio is lower than this, the breakdown field strength and charge-discharge efficiency of the composite film are not significantly different from those of unmodified PEI.

[0037] The specified mass ratio of 3-amino-4-cyanopyrazole to polyetherimide of 15:9985 avoids the increased polarization relaxation at high temperatures caused by excessive polar groups, which leads to increased dielectric loss and the inability to maintain a charge-discharge efficiency of over 90%. In addition, excessive grafting will destroy the imide ring structure of the PEI main chain, resulting in a decrease in its glass transition temperature and loss of its high-temperature resistance core advantage. It may also affect film formation due to steric hindrance. Furthermore, after the grafting sites of the PEI main chain are saturated, ungrafted free molecules will aggregate to form interface defects, becoming weak points for breakdown.

[0038] The amino group (-NH2) in the 3-amino-4-cyanopyrazole molecule reacts with the imide ring of the PEI backbone, causing 4-cyanopyrazole to be grafted onto the polyetherimide backbone via amide bonds. On the one hand, the electron-withdrawing molecule is locked in the polymer matrix through covalent bonds, completely avoiding the molecular aggregation problems that are prone to occur in physical blending or non-covalent bonding, ensuring the dielectric uniformity of the composite film and reducing the risk of electric field distortion and breakdown caused by interface defects. On the other hand, the grafting reaction partially destroys the imide ring structure of the polyetherimide backbone, interrupting the inherent charge transport channels within the molecular chain, further suppressing the generation of leakage current from the source. This, combined with electron scattering, forms a dual synergy, significantly improving the insulation performance of the film.

[0039] In a preferred embodiment of the present invention, the polyetherimide is dissolved at a temperature of 60-80°C for 12-24 hours in step S1; and the stirring reaction is carried out at a temperature of 60-100°C for 8-14 hours in step S2.

[0040] In this invention, step S1 at 60-80°C is the optimal temperature for the complete dissolution of polyetherimide in aprotic polar solvent. This avoids incomplete dissolution and clumping due to low temperatures, while also preventing damage to the solvent or the polyetherimide structure from high temperatures. The 12-24h time ensures uniform dispersion of the polyetherimide, providing a sufficient contact reaction environment for the subsequent grafting reaction. In step S2, 60-100°C meets the energy requirements for amide bond formation. Low temperatures result in incomplete reactions, while high temperatures can easily trigger side reactions or molecular chain degradation. The 8-14h time ensures sufficient directional grafting of 4-cyanopyrazole and polyetherimide, while avoiding excessive reaction that could damage the polyetherimide backbone and prevent the aggregation of free molecules.

[0041] In a preferred embodiment of the present invention, the aprotic polar solvent includes one or more of N-methylpyrrolidone, dimethylacetamide, or dimethylformamide.

[0042] In this invention, the aprotic polar solvent is, for example, N-methylpyrrolidone, dimethylacetamide, dimethylformamide, N-methylpyrrolidone and dimethylacetamide, N-methylpyrrolidone and dimethylformamide, dimethylacetamide and dimethylformamide, or a combination of N-methylpyrrolidone, dimethylacetamide and dimethylformamide.

[0043] In a preferred embodiment of the present invention, the ratio of aprotic polar solvent to polyetherimide is 10 mL: (1.6 g - 2.0 g).

[0044] In this invention, 10 mL of solvent is mixed with 1.6-2.0 g of polyetherimide to form a uniform solution with suitable viscosity. This ensures that the polyetherimide is fully dissolved and does not clump, providing a sufficient contact environment for the subsequent grafting reaction. It also avoids the problem of too much solvent leading to an overly dilute solution, resulting in a thin film with defects and time-consuming solvent removal, or too little solvent leading to an overly viscous solution. This is the optimal ratio that balances dissolution effect, reaction efficiency, and film quality.

[0045] In a preferred embodiment of the present invention, step S3, gradient drying, includes: Heat treatment at 70-90℃ for 3-5 hours, at 110-130℃ for 1-3 hours, at 140-160℃ for 1-3 hours, and at 190-210℃ under vacuum for 1-3 hours.

[0046] In this invention, heat treatment at 70-90℃ for 3-5 hours slowly removes most of the solvent to prevent rapid evaporation of the solvent at high temperatures, which could generate bubbles or pinholes and prevent defects from forming on the surface or inside the film. The residual solvent is then gradually removed by heating at 110-130℃ and 140-160℃ for 1-3 hours each, while simultaneously allowing the polymer molecular chains to slowly and regularly align, reducing residual stress and improving the film's density. A vacuum environment accelerates the evaporation of trace amounts of residual solvent, and a high temperature of 190-210℃ further stabilizes the amide bond grafting structure, preventing oxidation from affecting performance. The result is a solvent-free, structurally stable composite film suitable for high-temperature, high-field applications.

[0047] In a preferred embodiment of the present invention, step S3, gradient drying, includes: Heat treatment at 80℃ for 4 hours, at 120℃ for 2 hours, at 150℃ for 2 hours, and under vacuum at 200℃ for 2 hours.

[0048] In a preferred embodiment of the present invention, step S3, after peeling the film off the glass plate, further includes drying the film at 60-80°C for 12-36 hours.

[0049] In this invention, low-temperature drying at 60~80℃ can remove residual deionized water and trace amounts of unvolatile solvent on the film surface during peeling, avoiding increased dielectric loss due to moisture / solvent residue; the 12-36h time ensures thorough impurity removal, while making the film structure more dense and uniform, further stabilizing the graft bonding, and ensuring performance stability under high temperature and high field conditions.

[0050] In a preferred embodiment of the present invention, the thickness of the composite film is 8-12 micrometers.

[0051] In a preferred embodiment of the present invention, the method for preparing the polyetherimide composite film grafted with organic electron-discarding molecules specifically includes: First, polyetherimide is added to an aprotic polar solvent, which is selected from one or more of N-methylpyrrolidone, dimethylacetamide and dimethylformamide. The mixture is stirred at 60~80℃ for 12~24h to obtain a polyetherimide solution. Next, 3-amino-4-cyanopyrazole was added to the polyetherimide solution for reaction, and the reaction temperature and time were controlled within the range of 60~100℃ and 8~14h to obtain a 4-cyanopyrazole-grafted polyetherimide solution; the mass ratio of 3-amino-4-cyanopyrazole to polyetherimide was controlled between 5:9995 and 15:9985.

[0052] Next, the 4-cyanopyrazole-grafted polyetherimide solution was poured onto a glass plate using a doctor blade. It was then baked in an oven at 80°C for 4 hours, dried at 120°C for 2 hours, dried at 150°C for 2 hours, and dried under vacuum at 200°C for 2 hours to remove the solvent.

[0053] Finally, the glass plate covered with the dried composite dielectric film is immersed in deionized water for 30-300 seconds, and then the film is peeled off from the glass plate. The film is then dried at 60-80°C for 12-36 hours to obtain the composite film.

[0054] According to a third aspect of the invention, the application of the polyetherimide composite film as described above in a capacitor is provided.

[0055] In a preferred embodiment of the present invention, the capacitor is a high-temperature thin-film capacitor; Applications of the high-temperature thin-film capacitor include high-voltage flexible DC transmission converter valves, new energy vehicle motor drivers, or pulse power systems.

[0056] Example 1 1.6 g of polyetherimide was added to 10 ml of N-methylpyrrolidone solvent and stirred at 70 °C for 18 h to obtain a polyetherimide solution. 0.0016016 g of 3-amino-4-cyanopyrazole powder (0.1% by mass) was added to a polyetherimide solution and stirred at 80 °C for 12 h to allow the 3-amino-4-cyanopyrazole in the solution to react completely with the polyetherimide, thus obtaining a 4-cyanopyrazole-grafted polyetherimide solution. The 4-cyanopyrazole-grafted polyetherimide solution was placed in a vacuum environment at room temperature for 2 hours to eliminate air bubbles in the solution. The defoamed solution was poured onto a clean glass plate, and the solution was evenly coated onto the glass plate using a metal scraper. The glass plate covered with the solution was then placed in an oven for gradient temperature drying: 80°C for 4 hours, 120°C for 2 hours, 150°C for 2 hours, and finally under vacuum at 200°C for 2 hours.

[0057] The glass plate covered with the dried composite dielectric film was immersed in deionized water for 120 seconds. The film was then peeled off the glass plate and dried at 60°C for 24 hours to obtain a polyetherimide-grafted 4-cyanopyrazole composite dielectric film with a thickness of about 10 micrometers, denoted as PEI-g-0.1.

[0058] Example 2 The preparation process is the same as in Example 1, except that the mass of 3-amino-4-cyanopyrazole added in step 2 is 0.0008 g (mass ratio 0.05%), and the resulting film is denoted as PEI-g-0.05.

[0059] Example 3 The preparation process is the same as in Example 1, except that the mass of 3-amino-4-cyanopyrazole added in step 2 is 0.0024 g (mass ratio 0.15%), and the resulting film is denoted as PEI-g-0.15.

[0060] Comparative Example 1 The polyetherimide film provided in this comparative example is an unmodified polyetherimide film, denoted as PEI.

[0061] Comparative Example 2 In this comparative example, 4-cyanopyrazole was directly blended with polyetherimide to obtain a composite film, the resulting film being PEI-b-0.1.

[0062] Performance testing The thin films provided in Example 1, Comparative Example 1, and Comparative Example 2 were tested using differential scanning calorimetry. The analysis of the test results is as follows: Figure 2 As shown, the glass transition temperature of the pure polyetherimide film in Comparative Example 1 is 209.10 °C; the glass transition temperature of PEI-b-0.1 in the Comparative Example is 214.71 °C, but the melting peak of 4-cyanopyrazole appears prematurely at 175.05 °C; in contrast, the glass transition temperature of PEI-g-0.1 in Example 1 is 203.42 °C. The grafting reaction disrupts the rigidity of the polyetherimide molecular backbone, resulting in a lower glass transition temperature than pure polyetherimide, and the absence of an additional melting peak indicates that 4-cyanopyrazole is uniformly grafted onto the polyetherimide backbone; the comparison shows that the method provided in Example 1 does indeed cause the grafting reaction to occur.

[0063] DC breakdown strength tests were performed on the films provided in Example 1, Comparative Example 1, and Comparative Example 2. A Trek 610C high-voltage power supply was used, the test temperature was 150°C, and the voltage boost rate was 100 V / s. The Weibull distribution of the test results is shown below. Figure 3 As shown, the results indicate that the Weibull breakdown strength of PEI-g-0.1 is 1.145 times and 1.101 times that of PEI and PEI-b-0.1, respectively. This shows that the grafting modification method provided in Example 1 effectively improves the breakdown strength of polyetherimide at high temperature, and the grafting modification method is superior to the direct blending modification method.

[0064] The energy storage performance of the films provided in Example 1, Comparative Example 1, and Comparative Example 2 was tested using a Polyk CPE1901 testing device. The test temperature was 150°C, and the test frequency was 10Hz. The test results are as follows: Figure 4As shown, the results indicate that the electric field strength of PEI-g-0.1 at 90% charge-discharge efficiency is higher than that of PEI and PEI-b-0.1. This shows that the grafting modification method provided in Example 1 effectively reduces the charge-discharge loss of polyetherimide under high temperature and high field, and the grafting modification method is superior to the direct blending modification method.

[0065] Under the same test methods and conditions, the thin films obtained in Examples 2 and 3 were subjected to DC breakdown and energy storage performance tests, and the results are as follows: Figures 5-7 As shown in the comparison, although the composite films obtained in Examples 1 to 3 all have better performance than PEI films, PEI-g-0.1 exhibits the best performance. Therefore, the preferred mass ratio of 3-amino-4-cyanopyrazole to polyetherimide in the grafting reaction of this invention is 10:9990.

[0066] Based on the above performance test results, the composite film prepared by grafting 3-amino-4-cyanopyrazole onto polyetherimide in this invention exhibits significantly superior overall performance compared to unmodified polyetherimide films and traditional blended composite films. Differential scanning calorimetry (DSC) testing confirmed that 4-cyanopyrazole is uniformly grafted onto the polyetherimide backbone via amide bonds, without agglomeration, ensuring the uniformity of the film structure. DC breakdown strength testing showed that the Weibull breakdown strength of the modified film at 150℃ was 1.145 times that of the unmodified PEI and 1.101 times that of the blended film, indicating a significant improvement in insulation stability under high temperature and high field conditions. Energy storage performance testing showed that the modified film exhibited higher electric field strength at 150℃ and 90% charge-discharge efficiency, effectively addressing the problem of drastically increased losses under high temperature and high field conditions in traditional PEI. Furthermore, the comparison of different grafting ratios verified that the optimal mass ratio of 4-cyanopyrazole to polyetherimide is 10:9990. Under this ratio, the breakdown strength, charge-discharge efficiency and structural stability of the film achieve the best balance, which fully demonstrates the scientific nature and effectiveness of the grafting modification strategy of this invention and provides reliable technical support for high-performance dielectric materials for high-temperature film capacitors.

[0067] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A polyetherimide composite film grafted with organic electron-withdrawing molecules, characterized in that, The lowest unoccupied molecular orbital energy level of the organic electron-discarding molecule is higher than that of the lowest unoccupied molecular orbital energy level of the polyetherimide; The organic electron-discarding molecule is 4-cyanopyrazole; The 4-cyanopyrazole is grafted onto the polyetherimide via an amide bond.

2. A method for preparing a polyetherimide composite film grafted with organic electron-discarding molecules as described in claim 1, characterized in that, Includes the following steps: S1. Dissolve polyetherimide in an aprotic polar solvent to obtain a polyetherimide solution; S2. Add 3-amino-4-cyanopyrazole to the polyetherimide solution and stir to react to obtain a 4-cyanopyrazole-grafted polyetherimide solution. S3. After degassing the 4-cyanopyrazole-grafted polyetherimide solution, it is poured onto the carrier and dried in a gradient to obtain a carrier with a film attached. After peeling off the film, it is dried to obtain a polyetherimide composite film.

3. The preparation method according to claim 2, characterized in that, The mass ratio of 3-amino-4-cyanopyrazole to polyetherimide is (5:9995) to (15:9985).

4. The preparation method according to claim 2, characterized in that, The polyetherimide is dissolved at a temperature of 60-80°C for 12-24 hours in step S1; the stirring reaction is carried out at a temperature of 60-100°C for 8-14 hours in step S2.

5. The preparation method according to claim 2, characterized in that, The aprotic polar solvent includes one or more of N-methylpyrrolidone, dimethylacetamide, or dimethylformamide.

6. The preparation method according to claim 5, characterized in that, The ratio of the aprotic polar solvent to polyetherimide is 10 mL: (1.6 g - 2.0 g).

7. The preparation method according to claim 2, characterized in that, Step S3, the gradient drying, includes: Heat treatment at 70-90℃ for 3-5 hours, at 110-130℃ for 1-3 hours, at 140-160℃ for 1-3 hours, and at 190-210℃ under vacuum for 1-3 hours.

8. The preparation method according to claim 7, characterized in that, Step S3, which involves drying the film after peeling it off, includes drying the film at 60-80°C for 12-36 hours.

9. The application of the polyetherimide composite film according to claim 1 in a capacitor.

10. The application according to claim 9, characterized in that, The capacitor is a high-temperature thin-film capacitor; Applications of the high-temperature thin-film capacitor include high-voltage flexible DC transmission converter valves, new energy vehicle motor drivers, or pulse power systems.

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