Polyimide, polyimide film and preparation method and application thereof

Abstract

The invention relates to the technical field of dielectric materials, and discloses polyimide, a polyimide film and a preparation method and application thereof, the polyimide is obtained by a 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 methylene bridging group is a phenyl group with a substituent group. The substituent group at least comprises one methyl group. The molecular twisted conformation lock structure provided by the invention not only can improve the tolerance of polyimide, but also can doubly inhibit intrachain and inter-chain 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 invention relates to the technical field of dielectric materials, and particularly to a polyimide, a polyimide film, and a preparation method and application thereof. Background Art

[0002] Polymer-based dielectric materials are widely used in capacitors due to their advantages such as high voltage resistance, light weight, and easy processing. Especially in the fields of electric vehicles, aerospace, and deep well oil and gas exploration, there is an urgent need for compact capacitors that can operate stably under extreme environments (150 - 250 °C). 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 temperatures (Tg), such as Kapton polyimide, fluorene polyester (FPE), etc., show excellent thermal stability due to their highly conjugated molecular structures. However, these conjugated structures promote electron delocalization, leading to easy electron migration in the polymer, and the electrical insulation of the polymer begins to significantly decline at service temperatures far below the glass transition temperature (Tg). Therefore, there is an urgent need to provide a polymer-based electrolyte material that simultaneously has excellent thermal stability and high-temperature electrical insulation performance. Summary of the Invention

[0004] In view of this, the present invention provides a polyimide, a polyimide film, and a preparation method and application thereof to solve the problem that existing dielectric polymers cannot simultaneously meet the requirements of excellent thermal stability and high-temperature electrical insulation performance.

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

[0006] In a first aspect, the present invention provides a polyimide, which is obtained by a polymerization reaction of a dianhydride monomer and a diamine monomer; the diamine monomer includes a methylene bridging group and a phenyl group with substituents, and the substituents include at least one methyl group.

[0007] Preferably, 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'-diaminobibenzyl.

[0008] Preferably, the dianhydride monomer includes one or more of pyromellitic dianhydride (PMDA), 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA), dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride, 4,4'-diphthalic 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).

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

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

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

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

[0013] (1) In an organic solvent, polymerize the dianhydride monomer and the diamine monomer to obtain a prepolymer solution;

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

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

[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 carried out under a stepped temperature rising program, and the stepped temperature rising program is as follows: The first stage is to raise the temperature from room temperature to 60 - 80 °C and then keep it constant for 0.5 - 2 h, the second stage is to raise the temperature to 75 - 95 °C and then keep it constant for 0.5 - 2 h, the third stage is to raise the temperature to 100 - 120 °C and then keep it constant for 0.5 - 2 h, the fourth stage is to raise the temperature to 150 - 170 °C and then keep it constant for 0.5 - 2 h, the fifth stage is to raise the temperature to 210 - 23 °C and then keep it constant for 0.5 - 2 h, and the sixth stage is to raise the temperature to 260 - 320 °C and then keep it constant for 0.5 - 2 h.

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

[0019] The present invention provides a polyimide, a polyimide film, a preparation method thereof, and an application thereof. Compared with the prior art, the present invention has the following advantages:

[0020] The present invention selects a diamine monomer containing a methylene bridging group and a phenyl group having at least one methyl substituent, and utilizes the methyl substituents on the methylene and benzene rings to distort the conformation of the polymer chain, thereby effectively blocking the electron migration caused by the conjugated delocalization in the molecular chain, and this distortion is conducive to breaking the electron cloud delocalization and reducing the transport of charge; and, the presence of the methylene bridging group and the methyl-substituted phenyl group can increase steric hindrance, increase the barrier for local motion of the molecular chain at high temperature, and improve the glass transition temperature. In addition, the present invention utilizes the cross-linking of the methyl-substituted phenyl group to fix this molecular twisted conformation, construct a molecular twisted conformation lock, and the cross-linking of the methyl-substituted phenyl group serves as a lock for the molecular twisted conformation, which can not only maintain the molecular twisted conformation, but also utilize the cross-linking of the methyl-substituted phenyl group to induce the rearrangement of the donor and acceptor structural units, thereby improving the insulating properties of the polyimide at high temperatures, and the presence of chemical cross-linking bonds in the cross-linked network of the methyl-substituted phenyl group and the physical entanglement caused by the molecular twisted conformation helps to further increase the glass transition temperature. Therefore, the molecular twisted conformation lock structure proposed in the present invention can not only improve the durability of polyimide, but also doubly inhibit the intra-chain and inter-chain charge migration of the dielectric 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.

[0021] The polyimide of the present invention, as a dielectric, has a reduced leakage current at high temperatures of 200-250°C and an electric field below 600 MV / m, and significantly improved energy density and energy efficiency. The polyimide optimized by molecular twist conformation lock shows an energy density of 4.3 J / cm at 250°C. 3 The discharge energy density is 100%, while maintaining a charge and discharge efficiency of 84.6%, and its breakdown strength is as high as 565.49MV / m. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0023] Figure 1 Schematic diagram of the structures of PEI, MPEI and CMPEI;

[0024] Figure 2Mechanism diagrams of PEI, MPEI, and CMPEI for inhibiting electron migration at high temperatures;

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

[0026] Figure 4 Thermogravimetric analysis diagrams of MPEI in air and nitrogen, respectively;

[0027] Figure 5 Fluorescence emission spectra of PEI, MPEI, and CMPEI, and variable-temperature fluorescence emission spectra of MPEI and CMPEI;

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

[0029] Figure 7 Dielectric spectra and dielectric temperature spectra of PEI, MPEI, and CMPEI;

[0030] Figure 8 Energy storage performance diagrams of PEI, MPEI, and CMPEI at 200 °C and 250 °C. Detailed implementation manners

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

[0032] In one aspect of the present invention, the present invention provides a polyimide, which is obtained by a polymerization reaction of a dianhydride monomer and a diamine monomer; the diamine monomer includes a methylene bridging group and a phenyl group with substituents, and the substituents include at least one methyl group.

[0033] The present invention selects a diamine monomer containing a methylene bridging group and a phenyl group having at least one methyl substituent, and cleverly utilizes the methylene group and the methyl substituent on the benzene ring to distort the conformation of the polymer chain. In particular, at the imide portion, the molecular distortion mainly increases the dihedral angle of two adjacent conjugated planes at the position where the imide ring is located, which is beneficial to charge transport, thereby effectively blocking electron migration caused by conjugated delocalization within the molecular chain. This distortion is beneficial to breaking the electron cloud delocalization and reducing charge transport. In addition, the presence of the methylene bridging group and the methyl-substituted phenyl group can increase steric hindrance, increase the potential barrier for local movement of the molecular chain at high temperature, and improve the glass transition temperature.

[0034] In addition, the present invention utilizes the cross-linking of methyl-substituted phenyl groups to fix this molecular twisted conformation and construct a molecular twisted conformation lock. The cross-linking of methyl-substituted phenyl groups acts as a lock for the molecular twisted conformation, which can not only maintain the molecular twisted conformation, but also utilize the cross-linking of methyl-substituted phenyl groups to induce the rearrangement of donor and acceptor structural units. Specifically, since the positions where the cross-linking of methyl-substituted phenyl groups occurs are all located on the diamine unit, this breaks the aggregation stacking mode in conventional polyimides, that is, due to electrostatic interaction, the molecular chains are stacked face to face with the donor and the acceptor, but after the cross-linking of the methyl-substituted phenyl groups, an arrangement of donors and donors, and acceptors and acceptors stacked face to face is formed. This stacking can significantly reduce the charge transfer between the molecular chains, thereby improving the insulation performance of the polyimide at high temperatures; and the presence of chemical cross-linking bonds in the cross-linked network of methyl-substituted phenyl groups and physical entanglement caused by the molecular twisted conformation helps to further increase the glass transition temperature. Therefore, the molecular twisted conformation lock structure proposed in the present invention can not only improve the durability of polyimide, but also doubly inhibit the intra-chain and inter-chain charge migration of the dielectric 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 invention, 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'-diaminobibenzyl.

[0036] In some embodiments of the present invention, the dianhydride monomer includes one or more of pyromellitic dianhydride (PMDA), 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA), dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride, 4,4'-sulfonyldiphthalic 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 invention, the molar ratio of the diamine monomer to the dianhydride monomer is 1 - 1.02:1, specifically, it can be 1.02:1, 1.01:1, 1:1, etc.

[0038] In some embodiments of the present invention, the method for preparing the polyimide is as follows: in an organic solvent, the dianhydride monomer and the diamine monomer are subjected to a polymerization reaction to obtain a prepolymer solution; the prepolymer solution is thermally imidized in an oxygen-containing atmosphere to obtain the polyimide. The conditions for the polymerization reaction and thermal imidization are the same as those in the following preparation process of the polyimide film, and will not be elaborated here.

[0039] In another aspect of the present invention, the present invention provides a polyimide film, which is made from the polyimide described in any one of the above, and the thickness of the polyimide film is 5 - 15 μm, specifically, it can be 5 μm, 6 μm, 7 μm, 8 μm, , 12 μm, 15 μm, etc.

[0040] It can be understood that the thickness of the polyimide film is determined by the forming thickness. Since the thickness requirement of the dielectric polymer film is generally 5 - 15 μm, therefore, preferably, the thickness of the polyimide film of the present invention is limited to 5 - 15 μm, but the thickness of the polyimide film is not limited thereto. In actual applications, it can be processed according to the actual situation to obtain a thinner or thicker film.

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

[0042] (1) In an organic solvent, the dianhydride monomer and the diamine monomer are subjected to a polymerization reaction to obtain a prepolymer solution;

[0043] (2) The prepolymer solution is formed on the surface of a substrate and thermally imidized in an oxygen-containing atmosphere to obtain a polyimide film.

[0044] The present invention utilizes the methylene bridging group of the diamine unit in polyimide as the key influencing factor for the molecular twisted conformation, and through thermo-oxidation induction, the phenyl group substituted by methyl in the diamine unit undergoes thermal cross-linking. Furthermore, the molecular twisted conformation and the cross-linking of the phenyl group substituted by methyl are used to construct a molecular twisted conformation lock, so as to inhibit the intra-chain and inter-chain charge transfer of the high-temperature resistant polymer dielectric. As a result, the polymer dielectric still has an ultra-high energy density when approaching its glass transition temperature, so as to achieve the purpose of optimizing the high-temperature capacitance performance of the polymer dielectric.

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

[0046] In some embodiments of the present invention, 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 and subjected to 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, then the dianhydride monomer is added to the diamine solution, and the same organic solvent is continuously added, and a polymerization reaction is carried out to obtain a prepolymer solution.

[0048] The present invention does not specifically limit the addition sequence, addition method, etc. of the dianhydride monomer and the diamine monomer, and the above-listed methods or other conventional methods can be used. The present invention also does not specifically limit the reaction conditions of the polymerization reaction, and conventional reaction conditions can be used, for example, the reaction temperature is 0-50 °C, and the reaction time is greater than 48 h.

[0049] In some embodiments of the present invention, 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 balance is the organic solvent. Specifically, the addition amount of the organic solvent can be adjusted within the above range according to the activity of the reaction monomers.

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

[0051] In some embodiments of the present invention, the prepolymer solution is vacuumed to remove air bubbles, then formed onto a substrate surface. The substrate is dried to remove the solvent, and then thermal imidization is performed in an oxygen-containing atmosphere. The film is then cooled to room temperature, and the substrate is placed in warm water to peel off the film. The film is then dried to remove moisture, thereby obtaining a polyimide film. The vacuuming, drying, cooling, and peeling conditions are not particularly limited and can be performed using conventional methods. The forming method and substrate are also not particularly limited and can be, for example, casting or coating. The substrate can be, for example, a glass plate or a metal plate.

[0052] In some embodiments of the present invention, the oxygen-containing atmosphere can be air or oxygen, as long as the reaction environment is maintained at a concentration of at least 20% oxygen, with air being preferred. This invention utilizes the proven process for preparing commercial polyimide dielectric films, achieving crosslinking without the need for a vacuum or inert environment, effectively saving costs. Furthermore, the polyimide film produced thereby maintains excellent capacitive energy storage performance when used near its glass transition temperature (250°C).

[0053] In some embodiments of the present invention, the thermal imidization is carried out under a step-by-step temperature increase program, and the step-by-step temperature increase program is as follows: the first stage is heating from room temperature to 60-80°C and then maintaining the constant temperature for 0.5-2h, the second stage is heating to 75-95°C and then maintaining the constant temperature for 0.5-2h, the third stage is heating to 100-120°C and then maintaining the constant temperature for 0.5-2h, the fourth stage is heating to 150-170°C and then maintaining the constant temperature for 0.5-2h, the fifth stage is heating to 210-230°C and then maintaining the constant temperature for 0.5-2h, and the sixth stage is heating to 260-320°C and then maintaining the constant temperature for 0.5-2h. It should be noted that, except for the first stage, any other stages are heated from the constant temperature insulation temperature of the previous stage to the corresponding temperature. For example, the second stage is heated from 70°C to 85°C and then kept at a constant temperature for 0.5h. The third stage is heated from 85°C to 110°C and then kept at a constant temperature for 1h, and so on.

[0054] The present invention selects a diamine monomer containing a methylene bridging group and a phenyl group having at least one methyl substituent on the basis of preparing a commercial polyimide polymer dielectric film. The methyl-substituted phenyl group in the diamine monomer can generate free radicals and cause molecular chain crosslinking in an oxygen-containing environment at 260-320°C, fixing the twisted conformation of the molecules to form a molecular twisted conformation lock. This not only improves the resistance of the polyimide but also dually inhibits intra-chain and inter-chain charge migration in the dielectric film, thereby ensuring that the dielectric film still has extremely high energy storage density and energy storage efficiency at a service temperature close to the glass transition temperature.

[0055] In yet another aspect of the present invention, the present invention provides an application of the above polyimide film in a capacitor.

[0056] Hereinafter, specific embodiments will be used to clearly and completely describe the technical solutions in the present invention. The embodiments of this application are only for illustration. All other embodiments obtained by those skilled in the art without creative efforts fall within the protection scope of the present invention.

[0057] Example 1

[0058] This example provides a method for preparing 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 successively added to 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 diphenoxy) diphthalic anhydride and 0.7 mL of N-methylpyrrolidone were successively added to the diamine solution, and the mixture was stirred and reacted at room temperature for 48 h to obtain a prepolymer solution.

[0060] (2) The prepolymer solution was evacuated to remove air bubbles, and then the prepolymer solution after removing air bubbles was coated on the surface of a glass plate. It was placed in a forced-air oven in an air atmosphere and subjected to thermal imidization according to the following heating program: 70 °C / 1 h, 85 °C / 2 h, 110 °C / 1 h, 160 °C / 1 h, 220 °C / 1 h, 260 °C / 1 h, 290 °C / 1 h, and then naturally cooled to room temperature. 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, denoted as CMPEI.

[0061] Comparative Example 1

[0062] This comparative example is basically the same as Example 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 this comparative example is denoted as PEI.

[0064] Comparative Example 2

[0065] This comparative example is basically the same as Example 1, and the only difference is that in step (2), the air atmosphere in the forced-air oven is replaced by an inert gas nitrogen atmosphere.

[0066] The polyimide film prepared in this comparative example is denoted as MPEI.

[0067] As Figure 1As shown, the structural schematic diagrams of PEI, MPEI, and CMPEI are presented. It can be seen that the polyetherimide obtained in Example 1 has an improved molecular twisted conformation lock, the polyetherimide obtained in Comparative Example 1 is a conventional one, and the polyetherimide obtained in Comparative Example 2 has a modified molecular twisted conformation.

[0068] As Figure 2 shown, the mechanism diagrams of PEI, MPEI, and CMPEI for inhibiting electron migration at high temperatures are presented. It can be seen from the figure that compared with PEI, MPEI and CMPEI can significantly inhibit electron migration at high temperatures. Among them, MPEI inhibits electron migration due to the molecular twisting effect, and CMPEI inhibits electron migration due to the molecular twisting effect and the rearrangement of short-range donor and acceptor structural units.

[0069] As Figure 3 shown, the Fourier transform infrared spectra of PEI, MPEI, and CMPEI (a), and the variable-temperature Fourier transform infrared spectra of MPEI in air environment (b, c, d) are presented. It can be seen from the figure that MPEI and CMPEI are completely thermally imidized, and benzyl groups crosslink under thermal oxygen conditions, and the crosslinking induces the rearrangement of the molecular weight in CMPEI.

[0070] As Figure 4 shown, the thermogravimetric analysis diagrams of MPEI in air and nitrogen are presented. It can be seen from the figure that there is a significant weight gain in the thermogravimetric curve of MPEI in air at 250 - 390 °C, which is due to the crosslinking of benzyl groups induced by oxygen in the air, resulting in the rearrangement of the short-range structure of the molecular chain.

[0071] As Figure 5 shown, the fluorescence emission spectra of PEI, MPEI, and CMPEI (a), and the variable-temperature fluorescence emission spectra of MPEI (b) and CMPEI (c) are presented. From Figure 5 a, it can be seen that compared with PEI, MPEI and CMPEI show a blue shift in fluorescence and an increase in fluorescence intensity at room temperature, indicating that the intra-chain / inter-chain electron transfer in the polymer film is inhibited. From Figure 5 b and 5c, it can be seen that the fluorescence intensity of CMPEI decays less with the increase of temperature than that of MPEI and the fluorescence does not show a red shift, indicating that the molecular twisted conformation lock existing in CMPEI has a greater effect on intra-chain and inter-chain charge transfer.

[0072] 7]As Figure 6 shown, the differential scanning calorimetry diagrams of PEI, MPEI, and CMPEI are presented. It can be seen from the figure that MPEI with molecular twisting design and CMPEI with molecular twisted conformation lock design have a significantly increased glass transition temperature, and their electrical insulation performance is still excellent near their glass transition temperature and can be used at 250 °C.

[0073] As Figure 7 shown, the dielectric spectra and dielectric temperature spectra of PEI, MPEI, and CMPEI are presented. It can be seen from the figure that the individual molecular twisted conformation will reduce the dielectric constant, but the molecular twisted conformation lock helps to increase the dielectric constant. Because the lock in the molecular twisted 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 and increases its dielectric constant.

[0074] As Figure 8 shown, the energy storage performance diagrams of PEI, MPEI, and CMPEI at 200 °C and 250 °C are presented. It can be seen from the figure that the presence of the molecular twisted conformation lock is beneficial to increasing the polymer's maximum service temperature and suppressing losses, and improving the energy storage performance of the dielectric polymer film at the service temperature near the glass transition temperature.

[0075] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill 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, characterized in that, The polyimide is obtained by the polymerization reaction of a dianhydride monomer and a diamine monomer; The diamine monomer includes a methylene bridging group and a phenyl group with substituents, and the substituents include at least one methyl group.

2. The polyimide according to claim 1, characterized in that, 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'-diaminobibenzyl.

3. The polyimide according to claim 1, characterized in that, The dianhydride monomer includes one or more of pyromellitic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride, 4,4'-diphthalic anhydride, 4,4'-oxybisphthalic anhydride, 4,4'-(4,4'-isopropyl diphenoxy) diphthalic anhydride, 4,4'-(hexafluoroisopropyl) diphthalic anhydride, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride.

4. The polyimide according to any one of claims 1-3, characterized in that, The molar ratio of the diamine monomer to the dianhydride monomer is 1 - 1.02:

1.

5. A polyimide film, characterized in that, It is prepared from the polyimide according to any one of claims 1 - 4.

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

7. A method for preparing the polyimide film according to claim 5 or 6, characterized in that, It includes the following steps: (1) In an organic solvent, the dianhydride monomer and the diamine monomer are subjected to a polymerization reaction to obtain a prepolymer solution; (2) The prepolymer solution is formed on the surface of a substrate and subjected to thermal imidization in an atmosphere containing oxygen to obtain a polyimide film.

8. The method for preparing a polyimide film according to claim 7, characterized in that, 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.

9. The preparation method of the polyimide film according to claim 7, characterized in that, The thermal imidization is carried out under a stepped temperature rising program; The stepped temperature rising program is as follows: The first stage is to raise the temperature from room temperature to 60 - 80 °C and keep it constant for 0.5 - 2 h, the second stage is to raise the temperature to 75 - 95 °C and keep it constant for 0.5 - 2 h, the third stage is to raise the temperature to 100 - 120 °C and keep it constant for 0.5 - 2 h, the fourth stage is to raise the temperature to 150 - 170 °C and keep it constant for 0.5 - 2 h, the fifth stage is to raise the temperature to 210 - 230 °C and keep it constant for 0.5 - 2 h, the sixth stage is to raise the temperature to 260 - 320 °C and keep it constant for 0.5 - 2 h.

10. Application of the polyimide film according to claim 5 or 6 or the polyimide film prepared by the method according to any one of claims 7 - 9 in a capacitor.

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