A polyimide film, a method for preparing the same, and an application thereof

Abstract

This invention relates to the field of polymer dielectric technology and discloses a method for preparing a polyimide film, comprising the following steps: (1) alicyclic dianhydride monomer and an ortho-side-group aromatic diamine monomer undergo a polycondensation reaction in an organic solvent to obtain a precursor solution; (2) the precursor solution is formed into a film, and then dried and subjected to an imidization reaction to obtain a polyimide film; the imidization reaction is carried out under vacuum conditions, and the imidization reaction is a programmed temperature rise reaction, specifically: holding at 180-210℃ for 1-2 h, then raising the temperature to 240-260℃ and holding for 1-2 h, and continuing to raise the temperature to T and holding for 0.5-1 h; wherein, T = T g -t,T g The glass transition temperature of the polymer is 10℃≤t≤20℃. The polyimide film prepared by this invention exhibits high insulation and heat resistance properties, allowing for energy storage applications at temperatures up to 300℃.

Description

Technical Field

[0001] This invention relates to the field of polymer dielectric materials technology, and in particular to a polyimide film, its preparation method, and its application. Background Technology

[0002] Capacitors are one of the three basic passive components and a key component in modern electronic power systems. Film capacitors are an important category, widely used in rail transportation, solar cells, and new energy electric vehicles. The polymer dielectric is the main functional component of film capacitors, and its glass transition temperature (Tg) is a key factor in their performance. g ) and melting point (T m Polymer conductivity (S) is often used to evaluate the heat resistance, electrical conductivity (S), and energy storage efficiency (η) of polymers. It is also commonly used in high-temperature electrostatic energy storage to assess the insulation properties of polymers. Currently, high T... g (For amorphous polymers) and high T m Highly heat-resistant polymers (for crystalline polymers) such as polycarbonate (PC), polyetheretherketone (PEEK), polyetherimide (PEI), polyimide (PI), and polyamide-imide (PAI) have attracted widespread attention. However, these polymers possess numerous conjugated structures and a large number of delocalized electrons within their molecules. Consequently, under high temperatures and high electric fields, the leakage current and energy storage losses of these polymers increase dramatically, making them unsuitable for use in high-temperature energy storage applications.

[0003] Patent CN116535644A discloses the preparation and application of films of polyimides with half-alicyclic and / or full-alicyclic structures. It achieves low conductivity and loss at high temperatures by directly disrupting the conjugated structure in the polyimide. The full-alicyclic polyimide in this patent exhibits optimal insulation properties; however, it is limited by its low thermal conductivity (T). g The maximum energy storage application temperature is limited to 250°C; semi-alicyclic polyimide exhibits optimal heat resistance. g While its maximum temperature can reach 350℃, its relatively low insulation properties limit its energy storage application temperature to around 250℃. Although directly disrupting the conjugated structure of polyimide can suppress leakage current under high electric fields, it also severely reduces the polymer's heat resistance. Therefore, balancing the heat resistance and insulation properties of the polymer dielectric to achieve a better energy storage application temperature is a pressing issue. Summary of the Invention

[0004] In view of this, the present invention provides a method for preparing a polyimide film to solve the problem of low temperature for energy storage applications of existing polyimide films.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] On one hand, the present invention provides a method for preparing a polyimide film, comprising the following steps:

[0007] (1) Alicyclic dianhydride monomer and ortho-side group aromatic diamine monomer undergo polycondensation reaction in an organic solvent to obtain a precursor solution;

[0008] (2) The precursor solution is formed into a film, and then dried and imidized to obtain a polyimide film;

[0009] The imidization reaction is carried out under vacuum conditions and is a temperature-programmed reaction, specifically: holding at 180-210℃ for 1-2 hours, then raising the temperature to 240-260℃ and holding for 1-2 hours, and continuing to raise the temperature to T and holding for 0.5-1 hours.

[0010] Where T = T g -t,T g The glass transition temperature of the polymer is 10℃≤t≤20℃.

[0011] Preferably, the molar ratio of the alicyclic dianhydride monomer to the ortho-side-chain aromatic diamine monomer is 1-1.02:1.

[0012] Preferably, the alicyclic dianhydride monomer includes one or more of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA), 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), and 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA).

[0013] Preferably, the ortho-side-group aromatic diamine monomer includes one or more of 4,4'-methylenedi-o-toluidine (ME-MEA), 4,4'-methylenebis(2-ethylaniline) (M-OEA), and 2,5-dimethyl-1,4-phenylenediamine.

[0014] Preferably, the organic solvent includes one or more of N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), and resorcinol (m-cresol).

[0015] Preferably, the polycondensation reaction in step (1) is carried out at room temperature for 24-72 hours.

[0016] Preferably, the drying temperature in step (2) is 40-80℃ and the drying time is 2-6h.

[0017] Preferably, the method further includes the following step: cooling and drying the polyimide film to obtain a dried polyimide film.

[0018] On the other hand, the present invention also provides a polyimide film prepared by any of the above preparation methods, wherein the thickness of the polyimide film is 6-25 μm.

[0019] Furthermore, the present invention also provides a polyimide film prepared by any of the above-described preparation methods, or the application of the above-described polyimide film in a capacitor.

[0020] This invention provides a method for preparing a polyimide film, which has the following advantages compared with the prior art:

[0021] This invention uses ortho-side-group aromatic diamine monomers and alicyclic dianhydride monomers as raw materials. By introducing side groups at the ortho position of the amino group in the diamine monomer, the dihedral angle between the imide ring (acceptor) and the adjacent benzene ring (donor) in the polymer structure is increased, reducing the conjugation effect between the dianhydride and diamine parts in the polymer, resulting in higher insulation performance. At the same time, since the ortho position of the amino group in the diamine monomer is a small-volume flexible group, it will not excessively disrupt the interaction between molecular chains, and it increases the torsional barrier at the donor-acceptor junction, thereby improving the stiffness of the polymer and allowing the polymer to maintain a high glass transition temperature, resulting in high heat resistance.

[0022] The polyimide film prepared by this invention has high insulation and heat resistance properties. It still has a high Weibull static breakdown of 404MV / m at 300℃ and an energy storage efficiency of nearly 90% under an electric field of 200MV / m. Furthermore, it can be used for nearly 40,000 cycles under the conditions of 300℃ and 200MV / m. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0024] Figure 1 These are schematic diagrams of the polyimide film molecular structure units of Examples 1-2 and Comparative Examples 1-2 of the present invention;

[0025] Figure 2 This is a flowchart illustrating the preparation process of polyimide films in Examples 1-2 and Comparative Example 2 of the present invention;

[0026] Figure 3 These are Fourier transform infrared (FT-IR) spectra of the polyimide films of Examples 1-2 and Comparative Examples 1-2 of the present invention;

[0027] Figure 4These are differential scanning calorimetry (DSC) images of the polyimide films of Examples 1-2 and Comparative Examples 1-2 of the present invention;

[0028] Figure 5 This is an Arrhenius fitting graph of the conductivity versus temperature of the polyimide films of Examples 1-2 and Comparative Examples 1-2 of the present invention.

[0029] Figure 6 The diagram shows the Weibull distribution of the breakdown electric field of the polyimide films of Examples 1-2 and Comparative Examples 1-2 of this invention at room temperature RT (approximately 30°C), 150°C, 200°C, 250°C, and 300°C.

[0030] Figure 7 This is a comparison graph showing the energy storage efficiency and energy storage density of the polyimide films of Examples 1-2 and Comparative Examples 1-2 of the present invention as a function of electric field at room temperature RT (approximately 30°C), 150°C, 200°C, 250°C and 300°C.

[0031] Figure 8 This is a summary graph showing the maximum energy storage density (at an efficiency greater than 90%) of the polyimide films of Examples 1-2 and Comparative Examples 1-2 as a function of temperature.

[0032] Figure 9 This is a graph showing the changes in energy storage density and energy storage efficiency of the polyimide film of Example 1 of the present invention at 200℃, 250℃ and 300℃ under 200MV / m conditions as the number of cycles increases. Detailed Implementation

[0033] 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.

[0034] As described in the background section, existing technologies achieve low conductivity and loss at high temperatures by directly disrupting the conjugated structure in polyimide. However, this application, by introducing alicyclic dianhydride monomers and ortho-side-group aromatic diamine monomers, can effectively reduce the conjugation effect and increase the rigidity of the material. This allows for the adjustment of the heat resistance and insulation properties of the polyimide film, breaking the contradiction between the insulation and heat resistance properties in polyimide dielectrics. It significantly improves the insulation properties of polyimide without reducing its heat resistance as much as possible, thereby achieving a breakthrough in high-field electrostatic energy storage at 300°C.

[0035] In one aspect of the present invention, a method for preparing a polyimide film is provided, comprising the following steps:

[0036] (1) Alicyclic dianhydride monomer and ortho-side group aromatic diamine monomer undergo polycondensation reaction in an organic solvent to obtain a precursor solution;

[0037] (2) The precursor solution is formed into a film, and then dried and imidized to obtain a polyimide film, i.e., a polyimide film.

[0038] In some embodiments of the present invention, an alicyclic dianhydride monomer and an ortho-side-group aromatic diamine monomer undergo a polycondensation reaction in an organic solvent to obtain a precursor solution, wherein the precursor solution is a viscous polyamic acid solution, and the polycondensation reaction is carried out under a protective atmosphere.

[0039] In some embodiments of the present invention, the ortho-side-group aromatic diamine monomer is first dissolved in an organic solvent, and then an alicyclic dianhydride monomer is added to carry out a polycondensation reaction to obtain a precursor solution. By dissolving the ortho-side-group aromatic diamine monomer before adding the alicyclic dianhydride monomer, the decomposition of the alicyclic dianhydride monomer upon contact with water can be reduced, thereby making it more conducive to improving the degree of polymerization.

[0040] In some embodiments of the present invention, the molar ratio of the alicyclic dianhydride monomer to the ortho-side-chain aromatic diamine monomer is 1-1.02:1, for example, 1:1, 1.01:1, 1.02:1, etc. Experiments have shown that a slight excess of the alicyclic dianhydride monomer can avoid the effects of water decomposition, thereby increasing the degree of polymerization; however, an excess of the alicyclic dianhydride monomer will inhibit the condensation reaction. Furthermore, if the ortho-side-chain aromatic diamine monomer is added in excess compared to the alicyclic dianhydride monomer, it will lead to a decrease in the degree of polymerization.

[0041] In some embodiments of the present invention, the alicyclic dianhydride monomer includes one or more of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA), 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA), and 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA) that have a certain rigid structure in order to maintain a high glass transition temperature of polyimide.

[0042] In some embodiments of the present invention, the ortho-side-group aromatic diamine monomer is a diamine compound with a side group at the ortho position of the aromatic ring, including one or more of 4,4'-methylenedi-o-toluidine (ME-MEA), 4,4'-methylenebis(2-ethylaniline) (M-OEA), and 2,5-dimethyl-1,4-phenylenediamine.

[0043] The dianhydride monomer used in this invention is an alicyclic dianhydride with a low conjugation structure and high reactivity, which can reduce the conjugation degree of polyimide and improve insulation performance. The diamine monomer used is an aromatic diamine with ortho-side groups and high reactivity, which maintains the rigid structure of polyimide and preserves some thermal stability. Specifically, the ortho-side groups introduced into the diamine monomer are flexible groups with small volume, such as methyl and ethyl groups, which will not excessively disrupt the interaction between molecular chains, and will increase the torsional barrier at the donor-acceptor junction, thereby improving the stiffness of the polymer and allowing the polymer to maintain a high glass transition temperature and high heat resistance. Furthermore, by introducing ortho-side groups, the conjugation effect of the dianhydride monomer and the diamine monomer is reduced, resulting in high insulation performance. Thus, the polyimide film has both high insulation performance and high heat resistance.

[0044] In some embodiments of the present invention, the organic solvent includes one or more of N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), and resorcinol (m-cresol). The organic solvent is an aprotic polar solvent with a high boiling point, enabling rapid dissolution of the reactants, increasing reactivity, and forming complexes with the carboxyl groups of the polyamic acid formed after polymerization during the polymerization process, thus promoting the reaction and increasing the degree of polymerization.

[0045] In some embodiments of the present invention, the amount of organic solvent is such that the mass fraction of the precursor solution is 13-30%, for example, 13%, 15%, 18%, 20%, 23%, 25%, 28%, 30%, etc. Specifically, the mass fraction here is the ratio of the sum of the masses of the alicyclic dianhydride monomer and the ortho-side-group aromatic diamine monomer to the sum of the masses of the alicyclic dianhydride monomer, the ortho-side-group aromatic diamine monomer, and the organic solvent. Generally, the higher the reactivity of the dianhydride monomer and the diamine monomer, the higher the viscosity of the precursor solution. In this case, a lower mass fraction is needed to increase the degree of polymerization and also to make the film formation process smoother.

[0046] In some embodiments of the present invention, the polycondensation reaction is carried out at room temperature for a reaction time of 24-72 hours, such as 24 hours, 48 ​​hours, 60 hours, 72 hours, etc., to ensure that a polyimide film with sufficient degree of polymerization is obtained.

[0047] In some embodiments of the present invention, the precursor solution is formed into a film, and then dried and imidized to obtain a polyimide film. For example, the precursor solution can be formed into a wet film on a substrate, and then the wet film can be dried and imidized to obtain a polyimide film. The substrate can be a glass plate, and the film forming method can be casting, coating, etc., without particular limitation.

[0048] In some embodiments of the present invention, the imidization reaction is carried out under vacuum conditions and is a temperature-programmed reaction, specifically: holding at 180-210°C for 1-2 hours, then raising the temperature to 240-260°C and holding for 1-2 hours, and continuing to raise the temperature to T and holding for 0.5-1 hours; wherein, T = T g -t,T g The glass transition temperature of the polymer is 10℃≤t≤20℃.

[0049] In this invention, a programmed temperature-controlled reaction allows for intramolecular dehydration and ring closure of the polyamic acid solution, generating cyclic polyimide. Specifically, during the thermal imidization synthesis of polyimide, it is necessary to ensure the presence of a corresponding Tx. g The polyimide film has corresponding flexibility, therefore the final temperature selection is T. g -t,T g The lower the value, the better the flexibility of the molecular chain. The final procedure should be close to T. g Heat treatment at a certain temperature can ensure that the polymer chains can move fully, reduce internal stress, and improve film flexibility.

[0050] In some embodiments of the present invention, the drying temperature in step (2) is 40-80℃, for example, 40℃, 50℃, 60℃, 70℃, 80℃, etc., and the drying time is 2-6h, for example, 2h, 3h, 4h, 5h, 6h, etc. Since the purpose of drying is to remove the solvent on the surface and inside of the wet film and to remove some residual small molecules, the drying method is not specifically limited. The drying process is preferably carried out in a blower drying oven, which has a better solvent removal effect. A vacuum drying oven may not be conducive to solvent removal due to the closed environment.

[0051] In some embodiments of the present invention, before the precursor solution is formed into a film, the following step is further included: placing the precursor solution in a vacuum environment at room temperature to remove low-boiling-point small molecules from the precursor solution.

[0052] In some embodiments of the present invention, the following step is further included: cooling and drying the polyimide film to obtain a dried polyimide film. Specifically, this can be done by: cooling the polyimide film and peeling it off to the substrate, and then drying the peeled polyimide film to obtain a dried polyimide film. The peeling process can be carried out in deionized water, and the temperature of the deionized water used in the peeling process can be varied according to the adhesion strength between the polyimide film and the substrate. The greater the adhesion strength, the higher the temperature of the deionized water, which is more conducive to rapid peeling. Drying can be done at 80°C for 12 hours, and there are no special limitations on this, as long as the peeling and drying processes can be achieved.

[0053] In another aspect, the present invention also provides a polyimide film prepared by any of the above-described preparation methods, wherein the thickness of the polyimide film is 6-25 μm, for example, 6 μm, 8 μm, 10 μm, 15 μm, 20 μm, 25 μm, etc., and the specific thickness of the polyimide film can be adjusted according to actual conditions. Since this polyimide film is prepared by the aforementioned preparation method, it possesses all the features and advantages of the methods described above, and will not be repeated here.

[0054] Furthermore, the present invention also provides a polyimide film prepared by any of the above-described preparation methods, or the application of said polyimide film in a capacitor.

[0055] The technical solutions of this invention will be clearly and completely described below with reference to specific embodiments. The embodiments of this application are only examples, and all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0056] Example 1

[0057] This embodiment provides a method for preparing a polyimide film, such as... Figure 2 As shown, the specific steps are as follows:

[0058] (1) At room temperature, 0.226 mg (1 mmol) of 4,4'-methylenedi-o-toluidine (ME-MDA) and 1.2 mL of N-methylpyrrolidone were added to the reaction vessel. After stirring and dissolving, 0.224 mg (1 mmol) of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) was added. After reacting for 48 h, a precursor solution with a mass fraction of 27% was obtained.

[0059] (2) The precursor solution was placed under vacuum to remove low-boiling-point small molecules, and then poured onto a clean glass plate for casting to obtain an uniminated wet film. This film was then dried in a forced-air drying oven at 60°C for 2 hours, followed by treatment in a vacuum drying oven at 200°C for 2 hours, 250°C for 1 hour, and 320°C for 1 hour. After cooling to room temperature in the vacuum drying oven, the film was soaked in warm water above 50°C until it separated from the glass plate. Finally, the film was dried in a vacuum drying oven at 80°C for 12 hours to obtain a semi-aromatic polyimide film (ME-PI) with an ortho-methyl group and a thickness of approximately 10 μm. A schematic diagram of the molecular structure unit of ME-PI can be found in [reference needed]. Figure 1 .

[0060] Example 2

[0061] This embodiment provides a method for preparing a polyimide film, such as... Figure 2 As shown, the specific steps are as follows:

[0062] (1) At room temperature, 0.254 mg (1 mmol) of 4,4'-methylenebis(2-ethylaniline) (M-OEA) and 1.2 mL of N-methylpyrrolidone were added to the reaction vessel. After stirring and dissolving, 0.224 mg (1 mmol) of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) was added. After reacting for 48 h, a precursor solution with a mass fraction of 28% was obtained.

[0063] (2) The precursor solution was placed under vacuum to remove low-boiling-point small molecules, and then poured onto a clean glass plate for casting to obtain an uniminated wet film. This film was then dried in a forced-air drying oven at 60°C for 2 hours, followed by treatment in a vacuum drying oven at 200°C for 2 hours, 250°C for 1 hour, and 270°C for 1 hour. After cooling to room temperature in the vacuum drying oven, the film was soaked in warm water above 50°C until it separated from the glass plate. The film was then dried in a vacuum drying oven at 80°C for 12 hours to obtain a semi-aromatic polyimide film (MO-PI) with an ortho-methyl group and a thickness of approximately 10 μm. A schematic diagram of the molecular structure unit of MO-PI can be found in [reference needed]. Figure 1 .

[0064] Comparative Example 1

[0065] This comparative example provides a method for preparing a polyimide film, the specific steps of which are as follows:

[0066] (1) At room temperature, 0.200 mg (1 mmol) of 4,4'-diaminodiphenyl ether (ODA) and 2.8 mL of N-methylpyrrolidone were added to the reaction vessel. After stirring and dissolving, 0.218 mg (1 mmol) of pyromellitic dianhydride (PMDA) was added. The reaction was carried out for 24 h to obtain a precursor solution with a mass fraction of 13%.

[0067] (2) The precursor solution was placed under vacuum to remove low-boiling-point small molecules, and then poured onto a clean glass plate for casting to obtain an uniminated wet film. This film was then dried in a forced-air drying oven at 70°C for 2 hours, followed by treatment in a vacuum drying oven at 100°C, 150°C, 200°C, 250°C, 300°C, and 350°C for 1 hour each. After cooling to room temperature in the vacuum drying oven, the film was soaked in hot water until it separated from the glass plate. Finally, the film was dried in a vacuum drying oven at 80°C for 12 hours to obtain a typical all-aromatic polyimide film (Regular-PI) with a thickness of approximately 10 μm. A schematic diagram of the molecular structure unit of Regular-PI can be found here. Figure 1 .

[0068] Comparative Example 2

[0069] This comparative example provides a method for preparing a polyimide film, such as... Figure 2 As shown, the specific steps are as follows:

[0070] (1) At room temperature, 0.198 mg (1 mmol) of 4,4'-diaminodiphenylmethane (MDA) and 1.3 mL of N-methylpyrrolidone were added to the reaction vessel. After stirring and dissolving, 0.224 mg (1 mmol) of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) was added. The reaction was carried out for 48 h to obtain a precursor solution with a mass fraction of 24%.

[0071] (2) The precursor solution was placed under vacuum to remove low-boiling-point small molecules, and then poured onto a clean glass plate for casting to obtain an uniminated wet film. This film was then dried in a forced-air drying oven at 60°C for 2 hours, followed by treatment in a vacuum drying oven at 200°C for 2 hours, 250°C for 1 hour, and 330°C for 1 hour. After cooling to room temperature in the vacuum drying oven, the film was soaked in warm water above 50°C until it separated from the glass plate. The film was then dried in a vacuum drying oven at 80°C for 12 hours to obtain a semi-aromatic polyimide film (MD-PI) with a typical structure and a thickness of approximately 10 μm. A schematic diagram of the molecular structure unit of MD-PI can be found in [reference needed]. Figure 1 .

[0072] like Figure 3 The image shows the FT-IR spectra of the polyimide films prepared in Examples 1-2 and Comparative Examples 1-2. It can be seen from the images that each film obtained... Figure 1 The molecular structures corresponding to the various polyimide films shown are 1605 cm⁻¹. -1 The disappearance of the nearby polyamic acid peak indicates that the imidization process is basically complete. For semi-aromatic polyimides (Examples 1, 2, and Comparative Example 2), the -CH bending vibration on the benzene ring structure corresponds to 724 cm⁻¹. -1 and 824cm -1 The peaks in Examples 1 and 2 split due to the introduction of adjacent side groups, while Comparative Example 2 shows only a single peak; only Example 2 shows a peak at 2960 cm⁻¹. -1 and 2870cm -1 The presence of a distinct peak at this location corresponds to the symmetric and asymmetric stretching vibrations of -CH on the aliphatic chain. In Example 1, the hyperconjugation effect between the methyl group directly connected to the benzene ring suppresses the stretching vibration of the methyl group. In Example 2, the ortho-ethyl group has a freer -CH bond, resulting in a distinct stretching vibration peak.

[0073] like Figure 4The figure shows the DSC diagrams of the polyimide films prepared in Examples 1-2 and Comparative Examples 1-2. It can be seen from the figures that the glass transition temperature (GTH) of the polyimide film in Example 1 is 340°C, the GTH of the polyimide film in Example 2 is 298°C, the GTH of the polyimide film in Comparative Example 1 is greater than 350°C, and the GTH of the polyimide film in Comparative Example 2 is approximately 350°C. Compared to Comparative Example 1 or Comparative Example 2, the GTH of the polyimide film prepared by this invention is only reduced by 10-50°C, indicating that the polyimide film prepared by this invention can maintain high heat resistance.

[0074] like Figure 5 The figure shows the Arrhenius fitting plots of the conductivity versus temperature for the polyimide films prepared in Examples 1-2 and Comparative Examples 1-2. It can be seen from the figure that the apparent activation energy E of the polyimide film in Example 1 is... a The apparent activation energy E of the polyimide film in Example 2 is 0.44 eV. a The apparent activation energy E of the polyimide film in Comparative Example 1 is 0.45 eV. a The apparent activation energy E of the polyimide film in Comparative Example 2 is 0.27 eV. a The apparent activation energy is 0.40 eV. Compared with Comparative Example 1 or Comparative Example 2, the polyimide film prepared by the present invention has a larger apparent activation energy and a lower conductivity by an order of magnitude at the same temperature, indicating that the introduction of side groups in the present invention can significantly improve the insulation properties of the polyimide film.

[0075] like Figure 6 The figure shows the Weibull distribution of the breakdown electric field of the polyimide films prepared in Examples 1-2 and Comparative Examples 1-2 at a series of temperatures. No data indicates very low breakdown that cannot be tested. It can be seen from the figure that the dielectric applications of the polyimide films in Comparative Examples 1 and 2 are limited to below 200°C, the polyimide film in Example 2 is limited to below 250°C, while the polyimide film in Example 1 can undergo effective breakdown testing at 300°C. Under the same temperature conditions, the Weibull breakdown electric field of the polyimide films of Examples 1-2 and Comparative Examples 1-2 increased with the increase of insulation performance. For example, at 200°C, the insulation performance was arranged as follows: Example 2 > Example 1 > Comparative Example 2 > Comparative Example 1, and the Weibull breakdown electric field was arranged as follows: Example 2 (600MV / m) > Example 1 (576MV / m) > Comparative Example 2 (546MV / m) > Comparative Example 1 (323MV / m). This is because the electrothermal breakdown mechanism is dominant under high temperature conditions.

[0076] like Figure 7The graph shows a comparison of the energy storage efficiency and energy storage density of the polyimide films prepared in Examples 1-2 and Comparative Examples 1-2 at a series of temperatures as a function of electric field. No data indicates very low breakdown that cannot be tested. In this graph, the horizontal axis represents the electric field, the left vertical axis represents the energy storage density during a single test, and the right vertical axis represents the energy storage efficiency during a single charge-discharge process. Generally, higher energy storage density and efficiency are better, with an energy storage efficiency ideally above 90%. As can be seen from the graph, due to the dual requirements of heat resistance and insulation, the high-field dielectric application of the polyimide film in Comparative Example 1 is limited to below 200°C, because its energy storage efficiency is almost zero at high electric fields (e.g., 300 MV / m). It is expected that it will exhibit even worse performance at higher temperatures. Although Comparative Example 2 shows good energy storage performance at 200°C, its extremely low electrical breakdown due to poor electrical insulation at temperatures of 250°C and above prevents effective testing; the energy storage temperature under high electric fields will not exceed 300°C. The polyimide film of Example 2 has a low glass transition temperature T0. g =298℃ is limited to below 300℃, while the polyimide film of Example 1 can undergo effective energy storage testing at 300℃. Under the same temperature conditions, the maximum energy storage density (efficiency greater than 90%) of the polyimide films of Examples 1-2 and Comparative Examples 1-2 increases with increasing insulation. For example, at 200℃, the insulation performance is arranged as follows: Example 2 > Example 1 > Comparative Example 2 > Comparative Example 1, and the maximum energy storage density (efficiency greater than 90%) is arranged as follows: Example 2 (4.53 J / cm²) 3 Example 1 (2.84 J / cm) 3 ) > Comparative Example 2 (1.25 J / cm 3 ) > Comparative Example 1 (0.32 J / cm 3 This is because leakage current is the main source of energy storage loss in high-temperature environments.

[0077] like Figure 8 The figure shows a summary graph of the maximum energy storage density (at an efficiency greater than 90%) of the polyimide films prepared in Examples 1-2 and Comparative Examples 1-2 as a function of temperature. Figure 7 The maximum energy storage density shown is above 90%. As can be seen from the figure, the polyimide films of Comparative Example 1 and Comparative Example 2 can only be used for electrostatic energy storage applications below 200°C. Although the polyimide films of Comparative Example 1 and Comparative Example 2 have high glass transition temperatures and excellent heat resistance, they cannot be used above 200°C due to their poor insulation properties. For the polyimide films of Example 1 and Example 2, due to their more balanced insulation and heat resistance properties, they can be used for electrostatic energy storage applications at 250°C or even 300°C.

[0078] like Figure 9 The figure shows the changes in energy storage density and energy storage efficiency of the polyimide film prepared in Example 1 at 200℃, 250℃ and 300℃ under the condition of 200MV / m as the number of cycles increases. It can be seen from the figure that the polyimide film of Example 1 can undergo more than 40,000 energy storage cycles even under conditions as high as 300℃, indicating that it has excellent high-temperature energy storage performance.

[0079] 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 method for producing a polyimide film, characterized by, Comprise the following steps: (1) alicyclic dianhydride monomer and ortho side group aromatic diamine monomer in an organic solvent for condensation reaction, to obtain precursor solution; (2) the precursor solution is formed into a film, and then dried and imidized to obtain a polyimide film; The imidization reaction is carried out under vacuum, and the imidization reaction is a programmed temperature reaction, specifically: 180-210 DEG C for 1-2 h, then heated to 240-260 DEG C for 1-2 h, continue to heat to T for 0.5-1 h; Wherein, T=Tg-t, Tg is the glass transition temperature of polyimide, 10 DEG C≤t≤20 DEG C; The condensation reaction in step (1) is carried out at room temperature, and the reaction time is 24-72 h, The molar ratio of the alicyclic dianhydride monomer to the ortho side group aromatic diamine monomer is 1-1.02:1, The alicyclic dianhydride monomer includes one or more of 1,2,4,5-cyclohexane tetracarboxylic dianhydride, 1,2,3,4-cyclobutane tetracarboxylic dianhydride, 1,2,3,4-cyclopentane tetracarboxylic dianhydride, The ortho side group aromatic diamine monomer includes one or more of 4,4'-methylene di-o-toluidine, 4,4'-methylene bis(2-ethylaniline), 2,5-dimethyl-1,4-phenylenediamine.

2. The method for producing a polyimide film according to claim 1, characterized by, The organic solvent includes one or more of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, m-phenol.

3. The method for producing a polyimide film according to claim 1 or 2, characterized by, Further comprising the following steps: The polyimide film is cooled and dried to obtain a dried polyimide film.

4. A polyimide film produced by the production method according to any one of claims 1 to 3, characterized by The thickness of the polyimide film is 6-25 μm.

5. The polyimide film prepared by the preparation method of any one of claims 1-3 or the polyimide film of claim 4 is applied in a capacitor.

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