A polyimide, a polyimide film, and a method for producing and using the same

Abstract

The application relates to the technical field of dielectric materials, and discloses a polyimide, a polyimide film and a preparation method and application thereof, wherein the polyimide is obtained through polymerization reaction of a dianhydride monomer and a diamine monomer; the diamine monomer comprises a methylene bridging group and a phenyl group with a substituent group; and the substituent group comprises at least one methyl group. The molecular twisted conformation lock structure can not only improve the resistance performance of the polyimide, but also can doublely inhibit the intrachain and interchain charge migration of the dielectric film, so that the dielectric film still has extremely high energy storage density and energy storage efficiency at a service temperature close to a glass transition temperature.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of dielectric materials, in particular to a polyimide, a polyimide film and a preparation method and application thereof. BACKGROUND

[0002] Polymer-based dielectric materials are widely used in capacitors due to their high voltage resistance, light weight and easy processing, etc. In particular, there is an urgent need for compact capacitors that can work stably under extreme environments (150-250℃) in the fields of electric vehicles, aerospace and deep well oil and gas exploration. Therefore, dielectric polymers with excellent thermal stability and electrical insulation have become the research focus of high-temperature capacitor energy storage.

[0003] Polymers with high glass transition temperature (Tg), such as Kapton polyimide and fluorene polyester (FPE), exhibit excellent thermal stability due to their highly conjugated molecular structure. However, these conjugated structures promote electron delocalization, leading to easy electron migration in the polymer, and the electrical insulation of the polymer begins to decrease significantly at a service temperature much lower than the glass transition temperature (Tg). Therefore, there is an urgent need to provide a polymer-based electrolyte material that has excellent thermal stability and high-temperature electrical insulation performance. SUMMARY

[0004] Therefore, the present application provides a polyimide, a polyimide film and a preparation method and application thereof to solve the problem that existing dielectric polymers cannot simultaneously satisfy excellent thermal stability and high-temperature electrical insulation performance.

[0005] To solve the above technical problems, the present application adopts the following technical solutions:

[0006] In a first aspect, the present application provides a polyimide obtained by polymerization of a dianhydride monomer and a diamine monomer; the diamine monomer comprises a methylene bridging group and a phenyl group with a substituent, and the substituent comprises at least one methyl group.

[0007] Preferably, the diamine monomer comprises one or more of 4,4'-methylenebis(2,6-dimethylaniline), 4,4'-diamino-3,3'-dimethyldiphenylmethane, 4,4'-methylenebis(2-ethyl-6-methylaniline), 4,4'-methylenebis(2,6-diethylaniline) and 2,2'-dimethyl-4,4'-diaminobenzyl.

[0008] Preferably, the dianhydride monomer comprises one or more of pyromellitic dianhydride (PMDA), 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA), dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride, 4,4'-biphenyl dicarboxylic anhydride (s-BPDA), 4,4'-oxydiphthalic anhydride (ODPA), 4,4'-(4,4'-isopropylidenediphenoxyl) diphthalic anhydride (BPADA), 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), and 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA).

[0009] Preferably, the molar ratio of the dianhydride monomer to the diamine monomer is 1-1.02:1.

[0010] In a second aspect, the present application provides a polyimide film prepared from the polyimide of any one of the above.

[0011] Preferably, the polyimide film has a thickness of 5-15 μm.

[0012] In a third aspect, the present application provides a method for preparing a polyimide film, comprising the following steps:

[0013] (1) polymerizing a dianhydride monomer and a diamine monomer in an organic solvent to obtain a prepolymer solution;

[0014] (2) forming the prepolymer solution on a substrate surface and performing thermal imidization in an atmosphere containing oxygen to obtain a polyimide film.

[0015] Preferably, the organic solvent comprises one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and m-cresol.

[0016] Preferably, the dianhydride monomer and the diamine monomer account for 8-15% of the mass of the prepolymer solution.

[0017] Preferably, the thermal imidization is performed under a stepwise temperature program, which comprises the following stages: the first stage is to raise the temperature from room temperature to 60-80°C and then keep the temperature constant for 0.5-2 h, the second stage is to raise the temperature to 75-95°C and then keep the temperature constant for 0.5-2 h, the third stage is to raise the temperature to 100-120°C and then keep the temperature constant for 0.5-2 h, the fourth stage is to raise the temperature to 150-170°C and then keep the temperature constant for 0.5-2 h, the fifth stage is to raise the temperature to 210-230°C and then keep the temperature constant for 0.5-2 h, and the sixth stage is to raise the temperature to 260-320°C and then keep the temperature constant for 0.5-2 h.

[0018] In a fourth aspect, the present application provides a use of the above polyimide film in a capacitor.

[0019] The present application provides a kind of polyimide, polyimide film and its preparation method and application, compared with prior art, its beneficial effect is in at:

[0020] The present application selects the diamine monomer containing methylene bridging group and phenyl group with at least one methyl substituent, utilizes methylene and methyl substituent on benzene ring to make the conformation of polymer chain twist, thereby effectively blocking the electronic migration caused by conjugated delocalization in molecular chain, and the twist is conducive to breaking the electron cloud delocalization, reduces the transport of charge;And, the presence of methylene bridging group and methyl-substituted phenyl group can increase steric hindrance, increase the potential barrier of local motion of molecular chain at high temperature, improve glass transition temperature.In addition, the present application utilizes the crosslinking of methyl-substituted phenyl group to fix this molecular twisted conformation, constructs molecular twisted conformation lock, and the crosslinking of methyl-substituted phenyl group as a lock of molecular twisted conformation, can not only maintain the conformation of molecular twist, but also utilize the crosslinking of methyl-substituted phenyl group to induce the rearrangement of donor and acceptor structural units, thereby improving the insulation performance of polyimide at high temperature, and the crosslinking network of methyl-substituted phenyl group exists chemical crosslinking bond and physical entanglement caused by molecular twisted conformation, which helps to further improve the glass transition temperature.Therefore, the molecular twisted conformation lock structure proposed by the present application can not only improve the resistance performance of polyimide, but also doublely inhibit the intrachain and interchain charge migration of dielectric film, so that it still has very high energy storage density and energy storage efficiency at service temperature close to glass transition temperature.

[0021] The polyimide of the present application as dielectric has reduced leakage current at high temperature of 200-250 DEG C and electric field below 600 MV / m, and significantly improved energy density and energy efficiency, and the polyimide optimized by molecular twisted conformation lock shows discharge energy density of 4.3 J / cm 3 at 250 DEG C, while maintaining charge and discharge efficiency of 84.6%, and the breakdown strength is as high as 565.49 MV / m. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below, and obviously, the drawings in the following description are only embodiments of the present application, and other drawings can be obtained by the drawings provided by the person skilled in the art without creating labor.

[0023] Figure 1 Structural schematic diagram of PEI, MPEI and CMPEI;

[0024] Figure 2The mechanism diagram of PEI, MPEI and CMPEI suppressing electron migration at high temperature;

[0025] Figure 3 Fourier transform infrared spectra of PEI, MPEI, and CMPEI, and variable-temperature Fourier transform infrared spectrum of MPEI in air.

[0026] Figure 4 Thermogravimetric analysis (TGA) plots of MPEI in air and nitrogen, respectively;

[0027] Figure 5 The fluorescence emission spectra of PEI, MPEI, and CMPEI, as well as the temperature-dependent fluorescence emission spectra of MPEI and CMPEI;

[0028] Figure 6 Differential scanning calorimetry curves for PEI, MPEI, and CMPEI;

[0029] Figure 7 The dielectric spectra and dielectric temperature spectra of PEI, MPEI, and CMPEI are shown.

[0030] Figure 8 The diagram shows the energy storage performance of PEI, MPEI, and CMPEI at 200℃ and 250℃. Detailed Implementation

[0031] The present invention will be described below through specific embodiments. Those skilled in the art will understand that the specific embodiments below are merely illustrative and do not limit the scope of the invention in any way. Furthermore, in the following embodiments, unless otherwise specified, the reagents and equipment used are commercially available. If specific processing conditions and methods are not explicitly described in the later embodiments, conditions and methods known in the art can be used for processing.

[0032] In one aspect of the invention, a polyimide is provided, which is obtained by polymerization of a dianhydride monomer and a diamine monomer; the diamine monomer includes a methylene bridging group and a phenyl group having a substituent, wherein the substituent includes at least one methyl group.

[0033] The present application selects diamine monomers containing a methylene bridging group and a phenyl group with at least one methyl substituent, which skillfully utilizes the methylene and the methyl substituent on the benzene ring to twist the conformation of the polymer chain, especially at the imide, and the molecular twist mainly increases the dihedral angle of the two adjacent conjugated planes at the position of the imide ring which is beneficial to charge transport, thereby effectively blocking the electron migration caused by the conjugated delocalization in the molecular chain. This twist is beneficial to break the electron cloud delocalization and reduce the charge transport. In addition, the presence of the methylene bridging group and the methyl-substituted phenyl group can increase the steric hindrance, increase the potential barrier of local motion of the molecular chain at high temperature, and improve the glass transition temperature.

[0034] In addition, the present application uses the crosslinking of the methyl-substituted phenyl group to fix the twisted conformation of the molecule and construct a twisted conformation lock. The crosslinking of the methyl-substituted phenyl group as a lock of the twisted conformation of the molecule can not only maintain the twisted conformation of the molecule, but also induce the rearrangement of the donor and acceptor structural units by the crosslinking of the methyl-substituted phenyl group. Specifically, since the positions of the crosslinking of the methyl-substituted phenyl group are all located on the diamine unit, this breaks the aggregation state packing mode in conventional polyimides, i.e., due to electrostatic interaction, the molecular chains are packed face to face with the donor and the acceptor, but after the crosslinking of the methyl-substituted phenyl group, the arrangement mode of the donor and the donor, and the acceptor and the acceptor face to face packing is formed. This packing can significantly reduce the charge transfer between the molecular chains, thereby improving the insulation performance of the polyimide at high temperature. In addition, the crosslinking network of the methyl-substituted phenyl group exists chemical crosslinking bonds and physical entanglements caused by the twisted conformation of the molecule, which helps to further improve the glass transition temperature. Therefore, the twisted conformation lock structure proposed by the present application not only can improve the resistance performance of the polyimide, but also can double inhibit the intrachain and interchain charge migration of the dielectric thin film, so that it still has extremely high energy storage density and energy storage efficiency at a service temperature close to the glass transition temperature.

[0035] In some embodiments of the present application, the diamine monomer includes one or more of 4,4'-methylenebis(2,6-dimethylaniline), 4,4'-diamino-3,3'-dimethyldiphenylmethane, 4,4'-methylenebis(2-ethyl-6-methylaniline), 4,4'-methylenebis(2,6-diethylaniline), and 2,2'-dimethyl-4,4'-diaminobenzyl.

[0036] In some embodiments of the present application, the dianhydride monomer comprises one or more of pyromellitic dianhydride (PMDA), 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA), dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride, 4,4'-biphenyl dicarboxylic anhydride (s-BPDA), 4,4'-oxydiphthalic anhydride (ODPA), 4,4'-(4,4'-isopropylidenediphenoxy)diphthalic anhydride (BPADA), 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), and 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA).

[0037] In some embodiments of the present application, the molar ratio of the dianhydride monomer and the diamine monomer is 1-1.02:1, and can be 1.02:1, 1.01:1, and 1:1, etc.

[0038] In some embodiments of the present application, the method for preparing the polyimide comprises the following steps:

[0039] In another aspect of the present application, the present application provides a polyimide film prepared from any of the polyimides described above, wherein the thickness of the polyimide film is 5-15 μm, and can be 5 μm, 6 μm, 7 μm, 8 μm, 10 μm, 12 μm, and 15 μm, etc.

[0040] It can be understood that the thickness of the polyimide film is determined by the molding thickness, and since the thickness of the dielectric polymer film is generally required to be 5-15 μm, the thickness of the polyimide film of the present application is preferably limited to 5-15 μm, but the thickness of the polyimide film is not limited thereto, and in actual application, the film can be made thinner or thicker according to the actual situation.

[0041] In another aspect of the present application, the present application provides a method for preparing a polyimide film, comprising the following steps:

[0042] (1) polymerizing the dianhydride monomer and the diamine monomer in an organic solvent to obtain a prepolymer solution;

[0043] (2) molding the prepolymer solution on the surface of a substrate, and performing thermal imidization in an atmosphere containing oxygen to obtain a polyimide film.

[0044] The application uses the methylene bridging group of the diamine unit in the polyimide as a key influencing factor of the molecular twisted conformation, and through the thermal crosslinking of the methyl-substituted phenyl group in the diamine unit induced by heat and oxygen, the molecular twisted conformation lock is constructed by using the molecular twisted conformation and the crosslinking of the methyl-substituted phenyl group, so as to inhibit the intrachain and interchain charge transfer of the high-temperature-resistant polymer dielectric, so that the polymer dielectric still has an ultra-high energy density when it is close to its glass transition temperature, so as to achieve the purpose of optimizing the high-temperature capacitance performance of the polymer dielectric.

[0045] In the application, first, the dianhydride monomer and the diamine monomer are subjected to a polymerization reaction in an organic solvent to obtain a prepolymer solution.

[0046] In some embodiments of the application, first, the dianhydride monomer is dissolved in an organic solvent to obtain a dianhydride solution, and the diamine monomer is dissolved in the same organic solvent to obtain a diamine solution; then the diamine solution and the dianhydride solution are mixed to perform a polymerization reaction to obtain a prepolymer solution.

[0047] Or first, the diamine monomer is dissolved in an organic solvent to obtain a diamine solution, and then the dianhydride monomer is added to the diamine solution, and the same organic solvent is continuously added to perform a polymerization reaction to obtain a prepolymer solution.

[0048] The application does not have special limitations on the adding order and adding mode of the dianhydride monomer and the diamine monomer, and any of the above-mentioned modes or other conventional modes can be used. The application also does not have special limitations on the reaction conditions of the polymerization reaction, and any conventional reaction conditions can be used, for example, the reaction temperature is 0-50℃, and the reaction time is greater than 48h.

[0049] In some embodiments of the application, the organic solvent includes one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide and m-phenol. The dianhydride monomer and the diamine monomer account for 8-15% of the mass of the prepolymer solution, that is, the solid content of the prepolymer solution is 8-15%, and the rest is the organic solvent. The amount of the organic solvent can be adjusted according to the activity of the reaction monomer within the above range.

[0050] In the application, after obtaining the prepolymer solution, the prepolymer solution is formed on the surface of a substrate, and thermal imidization is performed in an atmosphere containing oxygen to obtain a polyimide film.

[0051] In some embodiments of the present application, specifically, the prepolymer solution is vacuumed to remove air bubbles, the prepolymer solution after removal of air bubbles is formed on the surface of a substrate, the substrate is dried to remove solvent, then thermal imidization is carried out in an atmosphere containing oxygen, followed by cooling to room temperature, the substrate is placed in warm water to peel off the film, the film is dried to remove water, and a polyimide film is obtained. The conditions for vacuuming, drying, cooling and peeling are not particularly limited and can be carried out by conventional methods. The forming method and the substrate are not particularly limited, and the forming method can be, for example, casting, coating, etc., and the substrate can be, for example, a glass plate, a metal plate, etc.

[0052] In some embodiments of the present application, the atmosphere containing oxygen can be an air atmosphere or an oxygen atmosphere, as long as there is more than 20% oxygen in the reaction environment, and is preferably an air atmosphere. The present application can realize crosslinking without vacuum and inert environment on the basis of the mature commercial polyimide dielectric film preparation process, effectively saving costs, and the polyimide film prepared thereby still has excellent capacitive energy storage performance when serving near its glass transition temperature (250°C).

[0053] In some embodiments of the present application, the thermal imidization is carried out under a stepwise temperature rising program, and the stepwise temperature rising program is as follows: the first stage is to rise from room temperature to 60-80°C and then keep constant temperature for 0.5-2h, the second stage is to rise to 75-95°C and then keep constant temperature for 0.5-2h, the third stage is to rise to 100-120°C and then keep constant temperature for 0.5-2h, the fourth stage is to rise to 150-170°C and then keep constant temperature for 0.5-2h, the fifth stage is to rise to 210-230°C and then keep constant temperature for 0.5-2h, and the sixth stage is to rise to 260-320°C and then keep constant temperature for 0.5-2h. It should be noted that, except for the first stage, any stage is to rise from the constant temperature of the previous stage to the corresponding temperature, for example, the second stage is to rise from 70°C to 85°C and then keep constant temperature for 0.5h, the third stage is to rise from 85°C to 110°C and then keep constant temperature for 1h, and so on.

[0054] On the basis of the preparation of commercial polyimide polymer dielectric film, the present application selects a diamine monomer containing a methylene bridging group and a phenyl group having at least one methyl substituent. The methyl-substituted phenyl group in the diamine monomer can generate free radicals and crosslink molecular chains in an oxygen-containing environment at 260-320°C, fixing the molecular twisted conformation and forming a molecular twisted conformation lock. Not only can the resistance performance of polyimide be improved, but also the intrachain and interchain charge migration of the dielectric film can be inhibited, so that it still has extremely high energy storage density and energy storage efficiency at a service temperature close to the glass transition temperature.

[0055] In another aspect of the present application, the present application provides a use of the above-mentioned polyimide film in a capacitor.

[0056] The technical solutions in the present application will be clearly and completely described below in combination with specific embodiments. The embodiments of the present application are only used as examples, and all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0057] Embodiment 1

[0058] The present embodiment provides a preparation method of a polyimide film, which has the following steps:

[0059] (1) 0.384 mmol of 4,4'-methylenebis(2,6-dimethylaniline) and 0.8 mL of N-methylpyrrolidone were sequentially added into a round-bottom flask, and stirred and dissolved at 15-30°C to obtain a diamine solution; then 0.384 mmol of 4,4'-(4,4'-isopropyl diphenyl oxy) diphthalic anhydride and 0.7 mL of N-methylpyrrolidone were sequentially added into the diamine solution, and stirred at room temperature for 48 h to obtain a prepolymer solution.

[0060] (2) The prepolymer solution was vacuumed to remove bubbles, and then the prepolymer solution after removing bubbles was coated on the surface of a glass plate, which was placed in an air atmosphere air oven, and heat imidized according to the following temperature rising program: 70°C / 1h, 85°C / 2h, 110°C / 1h, 160°C / 1h, 220°C / 1h, 260°C / 1h, 290°C / 1h, and then naturally cooled to room temperature, and the film was peeled off with warm water, and the peeled film was dried at 110°C for 24 h to obtain a polyimide film, which was recorded as CMPEI.

[0061] Comparative Example 1

[0062] The present comparative example is basically the same as Embodiment 1, and the only difference is that 4,4'-methylenebis(2,6-dimethylaniline) is replaced by 1,4-phenylenediamine.

[0063] The polyimide film prepared in the present comparative example is recorded as PEI.

[0064] Comparative Example 2

[0065] The present comparative example is basically the same as Embodiment 1, and the only difference is that the air atmosphere of the air oven in step (2) is replaced by an inert gas nitrogen atmosphere.

[0066] The polyimide film prepared in the present comparative example is recorded as MPEI.

[0067] As Figure 1The diagram shows the structural schematics of PEI, MPEI, and CMPEI. It can be seen that Example 1 yields a polyetherimide modified by molecular twisted conformation, Comparative Example 1 yields a conventional polyetherimide, and Comparative Example 2 yields a polyetherimide modified by molecular twisted conformation.

[0068] like Figure 2 The figure shows the mechanism by which PEI, MPEI, and CMPEI inhibit electron migration at high temperatures. As can be seen from the figure, compared with PEI, MPEI and CMPEI can significantly inhibit electron migration at high temperatures. MPEI inhibits electron migration due to molecular twisting, while CMPEI is hindered by molecular twisting and rearrangement of short-range donor and acceptor structural units.

[0069] like Figure 3 The figure shows the Fourier transform infrared spectra of PEI, MPEI and CMPEI (a), and the variable temperature Fourier transform infrared spectra of MPEI in air (b, c, d). It can be seen from the figure that MPEI and CMPEI are fully thermally imidized, and that benzyl groups are cross-linked under thermo-oxidative conditions. Furthermore, the cross-linking induces molecular weight rearrangement in CMPEI.

[0070] like Figure 4 The figure shows the thermogravimetric analysis (TGA) curves of MPEI in air and nitrogen. As can be seen from the figure, the thermogravimetric curve of MPEI in air shows a significant increase in weight between 250-390℃. This is because oxygen in the air induces cross-linking of benzyl groups, resulting in short-range rearrangement of the molecular chain.

[0071] like Figure 5 The figures show the fluorescence emission spectra of PEI, MPEI, and CMPEI (a), and the temperature-varying fluorescence emission spectra of MPEI (b) and CMPEI (c). Figure 5 As can be seen from graph a, compared to PEI, MPEI and CMPEI show a blue shift in fluorescence and an increase in fluorescence intensity at room temperature, indicating that intra-chain / inter-chain electron transfer in the polymer film is suppressed. From... Figure 5 As can be seen from b and 5c, the fluorescence intensity of CMPEI decreases less with increasing temperature than that of MPEI, and the fluorescence does not undergo a red shift. This indicates that the molecular twisted conformation lock present in CMPEI has a greater effect on intra- and inter-chain charge transfer.

[0072] like Figure 6 The figure shows the differential scanning calorimetry curves of PEI, MPEI and CMPEI. It can be seen from the figure that MPEI with molecular twisting design and CMPEI with molecular twisting conformation lock design have significantly improved glass transition temperature, and their electrical insulation performance is still excellent at near their glass transition temperature, and they can be used at 250℃.

[0073] like Figure 7 The figure shows the dielectric spectrum and dielectric temperature spectrum of PEI, MPEI and CMPEI. It can be seen from the figure that the individual molecular twist conformation will reduce the dielectric constant, but the molecular twist conformation lock will help to increase the dielectric constant. This is because the lock in the molecular twist conformation lock is composed of ether bonds and short aliphatic chains between molecular chains. Their introduction is equivalent to a lubricant that plays a dipole role, which helps the dipole orientation to increase its dielectric constant.

[0074] like Figure 8 The figure shows the energy storage performance of PEI, MPEI and CMPEI at 200℃ and 250℃. It can be seen from the figure that the presence of molecular twisted conformation lock is beneficial to improving the polymer's operating temperature limit and suppressing losses, thereby improving the energy storage performance of dielectric polymer films at service temperatures close to the glass transition temperature.

[0075] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A polyimide film, characterized in that, The polyimide film is obtained by polymerization of dianhydride monomers and diamine monomers; The diamine monomer is 4,4'-methylenebis(2,6-dimethylaniline), and the dianhydride monomer is 4,4'-(4,4'-isopropyldiphenoxy)phthalic anhydride; The method for preparing the polyimide film includes the following steps: (1) In an organic solvent, dianhydride monomer and diamine monomer are polymerized to obtain a precursor solution; (2) The precursor solution is formed on the surface of a substrate and thermally imidized in an oxygen-containing atmosphere to obtain a polyimide film.

2. The polyimide film according to claim 1, characterized in that, The molar ratio of the diamine monomer to the dianhydride monomer is 1-1.02:

1.

3. The polyimide film according to claim 1, characterized in that, The thickness of the polyimide film is 5-15 μm.

4. A method for preparing a polyimide film according to any one of claims 1-3, characterized in that, Includes the following steps: (1) In an organic solvent, dianhydride monomer and diamine monomer are polymerized to obtain a precursor solution; (2) The precursor solution is formed on the surface of a substrate and thermally imidized in an oxygen-containing atmosphere to obtain a polyimide film.

5. The method for preparing the polyimide film according to claim 4, characterized in that, The organic solvent includes one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide and resorcinol; The dianhydride monomer and diamine monomer account for 8-15% of the mass of the prepolymer solution.

6. The method for preparing the polyimide film according to claim 4, characterized in that, The thermal imidization is carried out under a step-by-step temperature increase program; The stepped heating program is as follows: The first stage is to raise the temperature from room temperature to 60-80℃ and then hold it at that temperature for 0.5-2 hours; the second stage is to raise the temperature to 85-95℃ and then hold it at that temperature for 0.5-2 hours; the third stage is to raise the temperature to 100-120℃ and then hold it at that temperature for 0.5-2 hours; the fourth stage is to raise the temperature to 150-170℃ and then hold it at that temperature for 0.5-2 hours; the fifth stage is to raise the temperature to 210-230℃ and then hold it at that temperature for 0.5-2 hours; and the sixth stage is to raise the temperature to 260-320℃ and then hold it at that temperature for 0.5-2 hours.

7. The use of a polyimide film according to any one of claims 1-3 or a polyimide film prepared by the method according to any one of claims 4-6 in a capacitor.

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