Polyimide film as well as preparation method and application thereof

Abstract

The invention relates to the technical field of polymer dielectrics, and discloses a polyimide film preparation method, which comprises: (1) carrying out a condensation polymerization reaction on an alicyclic dianhydride monomer and an ortho-side group aromatic diamine monomer in an organic solvent to obtain a precursor solution; (2) forming a film from the precursor solution, and then carrying out drying and imidization reaction to obtain a polyimide film; the imidization reaction is carried out under a vacuum condition, and the imidization reaction is a temperature programming reaction and specifically comprises the following steps: preserving heat at 180-210 DEG C for 1-2 hours, raising the temperature to 240-260 DEG C, preserving heat for 1-2 hours, continuously raising the temperature to T, and preserving heat for 0.5-1 hour; wherein T = Tg-t, Tg is the glass transition temperature of the polymer, and t is larger than or equal to 10 DEG C and smaller than or equal to 20 DEG C. The polyimide film prepared by the preparation method disclosed by the invention has relatively high insulating property and heat resistance, so that the energy storage application temperature can reach 300 DEG C.

Description

Technical Field

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

[0002] Capacitors are one of the three basic passive components and are key components of modern electronic power systems. Film capacitors are an important category and are widely used in rail transportation, solar cells, new energy electric vehicles and other fields. Polymer dielectrics are the main functional components of film capacitors. The glass transition temperature (T g ) and melting point (T m ) is often used to evaluate the heat resistance of polymers, and electrical conductivity (S) and energy storage efficiency (η) are often used to evaluate the insulation of polymers in the field of high-temperature electrostatic energy storage. 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 polyamideimide (PAI) have received extensive attention, but these polymers have a large number of conjugated structures and a large number of delocalized electrons in the molecules. Therefore, under high temperature and high electric field, the leakage current and energy storage loss of the polymers increase sharply, making them unsuitable for use in the field of high-temperature energy storage.

[0003] Patent CN116535644A discloses a film preparation and application of a semi-alicyclic polyimide and / or a fully alicyclic polyimide, which achieves low conductivity and loss at high temperature by directly destroying the conjugated structure in the polyimide. The fully alicyclic polyimide in the patent has the best insulation, but is limited by its low T g The maximum energy storage application temperature is limited to 250°C; semi-alicyclic polyimide has the best heat resistance, T g The maximum temperature can reach 350°C, but due to its low insulation, the energy storage application temperature is also limited to 250°C. Although directly destroying the conjugated structure in polyimide can inhibit leakage current under high electric fields, it will also seriously reduce the heat resistance of the polymer. Therefore, how to balance the heat resistance and insulation of polymer dielectrics to obtain a better energy storage application temperature is an urgent problem to be solved. 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 energy storage application temperature of the existing polyimide film.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

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

[0007] (1) alicyclic dianhydride monomers and ortho-side aromatic diamine monomers are subjected to polycondensation reaction in an organic solvent to obtain a precursor solution;

[0008] (2) forming a film from the precursor solution, and then drying and performing imidization reaction to obtain a polyimide film;

[0009] The imidization reaction is carried out under vacuum conditions. The imidization reaction is a programmed temperature reaction, specifically: keeping the temperature at 180-210° C. for 1-2 hours, then heating to 240-260° C. for 1-2 hours, and continuing to heat to T for 0.5-1 hour;

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

[0011] Preferably, the molar ratio of the alicyclic dianhydride monomer to the ortho-side 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 aromatic diamine monomer includes one or more of 4,4'-methylene di-o-toluidine (ME-MEA), 4,4'-methylene bis(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 m-cresol.

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

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

[0017] Preferably, the method further comprises the following steps: cooling the polyimide film and then drying it to obtain a dry polyimide film.

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

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

[0020] The present invention provides a method for preparing a polyimide film. Compared with the prior art, the method has the following beneficial effects:

[0021] The invention adopts ortho-side aromatic diamine monomers and alicyclic dianhydride monomers as raw materials, introduces a side group at the ortho position of the amino group of the diamine monomer, increases the dihedral angle between the imide ring (acceptor) and the adjacent benzene ring (donor) in the polymer structure, reduces the conjugation effect of the dianhydride part and the diamine part in the polymer, and has higher insulation performance; at the same time, because the ortho position of the amino group in the diamine monomer introduces a relatively small flexible group, the interaction between the molecular chains will not be excessively destroyed, and the torsion barrier at the connection between the donor and the acceptor is increased, thereby improving the rigidity of the polymer, so that the polymer still maintains a high glass transition temperature and has higher heat resistance.

[0022] The polyimide film prepared by the present invention has high insulation performance and heat resistance. At 300°C, it still has a high Weibull static breakdown of 404MV / m and an energy storage efficiency of nearly 90% under an electric field of 200MV / m, and can be recycled nearly 40,000 times under the conditions of 300°C & 200MV / m. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0024] Figure 1 Schematic diagram of molecular structural unit of polyimide film of Example 1-2 and Comparative Example 1-2 of the present invention;

[0025] Figure 2 is a flow chart of preparing polyimide films of Examples 1-2 and Comparative Example 2 of the present invention;

[0026] Figure 3 is a Fourier transform infrared spectrum (FT-IR) graph of the polyimide film of Example 1-2 of the present invention and Comparative Example 1-2;

[0027] Figure 4is a differential scanning calorimetry (DSC) graph of the polyimide film of Example 1-2 of the present invention and Comparative Example 1-2;

[0028] Figure 5 is an Arrhenius fitting diagram of the conductivity and temperature of the polyimide films of Examples 1-2 of the present invention and Comparative Examples 1-2;

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

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

[0031] Figure 8 It is a summary diagram of the maximum energy storage density (when the efficiency is greater than 90%) of the polyimide film of Examples 1-2 of the present invention and Comparative Examples 1-2 as a function of temperature;

[0032] Fig. 9 1 is a graph showing the change in energy storage density and energy storage efficiency of the polyimide film of Example 1 of the present invention at 200° C., 250° C. and 300° C. under the condition of 200 MV / m with increasing cycle times. DETAILED DESCRIPTION

[0033] The present invention will be described below by specific examples, and it will be appreciated by those skilled in the art that the following specific examples are only for illustrative purposes, and do not limit the scope of the present invention in any way. In addition, in the following examples, unless otherwise specified, the reagents and equipment used are all commercially available. If in the following examples, specific treatment conditions and treatment methods are not clearly described, then conditions and methods known in the art can be used to process.

[0034] As described in the background technology, the prior art directly destroys the conjugated structure in the polyimide to achieve low conductivity and loss at high temperature, while the present application introduces alicyclic dianhydride monomers and ortho-side aromatic diamine monomers to effectively reduce the conjugation effect and increase the rigidity of the material, thereby adjusting the heat resistance and insulation of the polyimide film, breaking the contradiction between the insulation performance and heat resistance in the polyimide dielectric that cannot be effectively balanced, and greatly improving its insulation performance without reducing the heat resistance of the polyimide as much as possible, thereby achieving a breakthrough in high electric field electrostatic energy storage at a temperature of 300°C.

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

[0036] (1) alicyclic dianhydride monomers and ortho-side aromatic diamine monomers are subjected to polycondensation reaction in an organic solvent to obtain a precursor solution;

[0037] (2) Forming the precursor solution into a film, and then drying and performing imidization reaction to obtain a polyimide film, i.e., a polyimide film.

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

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

[0040] In some embodiments of the present invention, the molar ratio of the alicyclic dianhydride monomer to the ortho-side aromatic diamine monomer is 1-1.02:1, for example, 1:1, 1.01:1, 1.02:1, etc. It has been found through experiments that a slightly excessive amount of alicyclic dianhydride monomer can avoid the influence of water decomposition, thereby increasing the degree of polymerization, but an excessive amount of alicyclic dianhydride monomer will inhibit the progress of the polycondensation reaction; in addition, if the ortho-side aromatic diamine monomer is added in excess compared to the alicyclic dianhydride monomer, the degree of polymerization will decrease.

[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), 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA), etc., which have a certain rigid structure to maintain a high glass transition temperature of the polyimide.

[0042] In some embodiments of the present invention, the ortho-side 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'-methylene di-o-toluidine (ME-MEA), 4,4'-methylenebis(2-ethylaniline) (M-OEA), and 2,5-dimethyl-1,4-phenylenediamine.

[0043] The dianhydride monomer used in the present invention is an aliphatic cyclodianhydride with a low conjugated structure and high reactivity, which can reduce the conjugation degree of the polyimide and improve the insulation performance; the diamine monomer used is an aromatic diamine with an ortho-side group and high reactivity, which maintains the rigid structure of the polyimide and maintains partial thermal stability. Specifically, the ortho-side group introduced into the diamine monomer is a flexible group with a small volume such as a methyl group and an ethyl group, which will not excessively destroy the effect between molecular chains and improve the torsion barrier at the connection between the donor and the acceptor, thereby improving the rigidity of the polymer, so that the polymer still maintains a high glass transition temperature and has high heat resistance. In addition, by introducing the ortho-side group, the conjugation effect of the dianhydride monomer and the diamine monomer is reduced, and the polyimide film has high insulation performance and heat resistance at the same time.

[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 m-cresol. The organic solvent is a non-protonic polar solvent with a high boiling point, which can quickly dissolve the reactants and increase the reaction activity. In addition, the organic solvent can form a complex with the carboxyl group of the polyamic acid formed after polycondensation during the polymerization process, so that the reaction proceeds in the forward direction and the degree of polymerization is increased.

[0045] In some embodiments of the present invention, the amount of the organic solvent is based on the mass fraction of the precursor solution being 13-30%, for example, it can be 13%, 15%, 18%, 20%, 23%, 25%, 28%, 30%, etc. Specifically, the mass fraction here is the ratio of the sum of the mass of the alicyclic dianhydride monomer and the ortho-side aromatic diamine monomer to the sum of the mass of the alicyclic dianhydride monomer, the ortho-side aromatic diamine monomer and the organic solvent. Generally speaking, the higher the reactivity of the dianhydride monomer and the diamine monomer, the greater the viscosity of the precursor solution. At this time, a lower mass fraction is required to increase the degree of polymerization, and at the same time, the film-forming process can be smoother.

[0046] In some embodiments of the present invention, the polycondensation reaction is carried out at room temperature, and the reaction time is 24-72 hours, for example, 24 hours, 48 ​​hours, 60 hours, 72 hours, etc., to ensure that a polyimide film with a 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 film on a substrate to obtain a wet film, and then the wet film is dried and imidized to obtain a polyimide film, wherein the substrate can be a glass plate, and the film forming method can be cast film, coating film, etc., which are not particularly limited.

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

[0049] In the present invention, the polyamic acid solution can be dehydrated and ring-closed by programmed temperature reaction to generate cyclic polyimide. Specifically, in the process of synthesizing polyimide by thermal imidization, it is necessary to ensure that the corresponding T g The polyimide film has corresponding flexibility, so the final temperature is selected as T g -t,T g The lower the value, the better the flexibility of the molecular chain. The last program selects the value close to T g Heat treatment at a temperature of 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°C, for example, 40°C, 50°C, 60°C, 70°C, 80°C, 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, there is no special limitation on the drying method. The drying process is preferably carried out in a forced air drying oven that is better at removing the solvent. A vacuum drying oven may be not conducive to the removal of the solvent due to its closed environment.

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

[0052] In some embodiments of the present invention, the following steps are also included: cooling and drying the polyimide film to obtain a dry polyimide film. Specifically, the polyimide film is peeled off from the substrate after cooling, and then the peeled polyimide film is dried to obtain a dry polyimide film, wherein the peeling process can be carried out in deionized water, and the temperature of the deionized water used in the peeling process can be changed according to the bonding strength between the polyimide film and the substrate, wherein the greater the bonding strength, the higher the temperature of the deionized water, the more conducive to rapid peeling, and the drying can be dried at 80°C for 12 hours, and there is no special limitation on this, as long as the peeling and drying process can be achieved.

[0053] In another aspect of the present invention, the present invention also provides a polyimide film prepared by any of the preparation methods described above, 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. The specific thickness of the polyimide film can be adjusted according to actual conditions. Since the polyimide film is prepared by the aforementioned preparation method, the polyimide film has all the characteristics and advantages possessed by the aforementioned method, which will not be described in detail here.

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

[0055] The technical solutions in the present invention will be described clearly and completely below in conjunction 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 work are within the scope of protection of the present 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 a reaction container, and after stirring to dissolve, 0.224 mg (1 mmol) of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) was added. After reacting for 48 hours, a precursor solution with a mass fraction of 27% was obtained;

[0059] (2) The precursor solution is placed in a vacuum condition to remove low-boiling point small molecules, and then poured onto a clean glass plate for casting to obtain an unimidized wet film, which is then placed in a forced air drying oven and dried at 60°C for 2 h, and then moved to a vacuum drying oven and treated at 200°C for 2 h, 250°C for 1 h, and 320°C for 1 h. As the vacuum drying oven cools to room temperature, the film is soaked in warm water above 50°C until the film is separated from the glass plate, and then the film is placed in a vacuum drying oven and dried at 80°C for 12 h to obtain a semi-aromatic polyimide film (ME-PI) with a thickness of about 10 μm and having ortho-methyl groups. The schematic diagram of the molecular structural unit of ME-PI is shown in 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 a reaction container, and after stirring to dissolve, 0.224 mg (1 mmol) of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) was added. After reacting for 48 hours, a precursor solution with a mass fraction of 28% was obtained;

[0063] (2) The precursor solution is placed in a vacuum condition to remove low-boiling point small molecules, and then poured onto a clean glass plate for casting to obtain an unimidized wet film, which is then placed in a forced air drying oven and dried at 60°C for 2 h, and then moved to a vacuum drying oven and treated at 200°C for 2 h, 250°C for 1 h, and 270°C for 1 h. As the vacuum drying oven cools to room temperature, the film is soaked in warm water above 50°C until the film is separated from the glass plate, and then placed in a vacuum drying oven and dried at 80°C for 12 h to obtain a semi-aromatic polyimide film (MO-PI) with a thickness of about 10 μm and having ortho-methyl groups. The schematic diagram of the molecular structural unit of MO-PI is shown in Figure 1 .

[0064] Comparative Example 1

[0065] This comparative example provides a method for preparing a polyimide film, and the specific steps 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 a reaction container, and after stirring to dissolve, 0.218 mg (1 mmol) of pyromellitic dianhydride (PMDA) was added and reacted for 24 h to obtain a precursor solution with a mass fraction of 13%;

[0067] (2) The precursor solution is placed under vacuum conditions to remove low-boiling point small molecules, and then poured onto a clean glass plate for casting to obtain an unimidized wet film, which is then placed in a forced air drying oven and dried at 70°C for 2 h, and then moved to a vacuum drying oven and treated at 100°C, 150°C, 200°C, 250°C, 300°C and 350°C for 1 h, respectively. As the vacuum drying oven cools to room temperature, the film is soaked in hot water until it is separated from the glass plate, and then the film is placed in a vacuum drying oven and dried at 80°C for 12 h to obtain a fully aromatic polyimide film (Regular-PI) with a typical structure of about 10 μm in thickness. The molecular structure unit diagram of Regular-PI is shown in 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 a reaction vessel, stirred to dissolve, and then 0.224 mg (1 mmol) of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) was added. The mixture was reacted for 48 h to obtain a precursor solution with a mass fraction of 24%;

[0071] (2) The precursor solution is placed under vacuum conditions to remove low-boiling point small molecules, and then poured onto a clean glass plate for casting to obtain an unimidized wet film, which is then placed in a forced air drying oven and dried at 60°C for 2 h, and then moved to a vacuum drying oven and treated at 200°C for 2 h, 250°C for 1 h, and 330°C for 1 h. As the vacuum drying oven cools to room temperature, the film is soaked in warm water above 50°C until the film is separated from the glass plate, and then the film is placed in a vacuum drying oven at 80°C for 12 h to obtain a semi-aromatic polyimide film (MD-PI) with a thickness of about 10 μm and a typical structure. The schematic diagram of the molecular structure unit of MD-PI is shown in Figure 1 .

[0072] like Figure 3 As shown in the figure, it is the FT-IR graph of the polyimide film prepared in Example 1-2 and Comparative Example 1-2. It can be seen from the figure that each of them has obtained Figure 1 The molecular structures of the polyimide films shown, 1605 cm -1 The disappearance of the polyamic acid peak nearby indicates that the imidization process is basically completed. For the semi-aromatic polyimide (Example 1, Example 2, Comparative Example 2), the -CH bending vibration on the benzene ring structure corresponds to 724 cm -1 and 824cm -1 Due to the introduction of the adjacent side groups, the peaks of Example 1 and Example 2 are split here, while Comparative Example 2 has only a single peak here; only Example 2 has a peak at 2960cm -1 and 2870cm -1 There is an obvious peak at this point, which is because it corresponds to the symmetric and asymmetric stretching vibrations of -CH on the fatty chain. For Example 1, since its methyl group is directly connected to the benzene ring, the hyperconjugation effect between them suppresses the stretching vibration of the methyl group, while Example 2 has a freer -CH bond on the ortho-ethyl group, and an obvious stretching vibration peak appears on the surface.

[0073] like Figure 4As shown, it is the DSC graph of the polyimide film prepared in Example 1-2 and Comparative Example 1-2. It can be seen from the figure that the glass transition temperature of the polyimide film in Example 1 is 340°C, the glass transition temperature of the polyimide film in Example 2 is 298°C, the glass transition temperature of the polyimide film in Comparative Example 1 is greater than 350°C, and the glass transition temperature of the polyimide film in Comparative Example 2 is about 350°C. Compared with Comparative Example 1 or Comparative Example 2, the glass transition temperature of the polyimide film prepared by the present invention is only reduced by 10-50°C, indicating that the polyimide film prepared by the present invention can maintain a relatively high heat resistance.

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

[0075] like Figure 6 The figure shows the Weibull distribution diagram of the breakdown electric field of the polyimide films prepared in Examples 1-2 and Comparative Examples 1-2 at a series of temperatures, where no data indicates that the breakdown is too low to be tested. It can be seen from the figure that the dielectric application of the polyimide films of Comparative Examples 1 and 2 is limited to below 200°C, the polyimide film of Example 2 is limited to below 250°C, and the polyimide film of Example 1 can be effectively tested for breakdown 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 increases with the enhancement of the insulation properties. For example, at 200°C, the insulation properties are arranged as follows: Example 2 > Example 1 > Comparative Example 2 > Comparative Example 1, and the Weibull breakdown electric field is 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 in a high temperature environment.

[0076] like Figure 7As shown, it is a comparison diagram of the energy storage efficiency and energy storage density of the polyimide film prepared by Example 1-2 and Comparative Example 1-2 at a series of temperatures as the electric field changes, where no data represents that the breakdown is very low and cannot be tested. In this figure, the horizontal axis represents the electric field, the left vertical axis represents the energy storage density discharged in a test, and the right vertical axis represents the energy storage efficiency in a charge and discharge process. Generally speaking, the higher the energy storage density and efficiency, the better, and the energy storage efficiency is preferably above 90%. It can be seen from the figure that due to the dual requirements of heat resistance and insulation, the high electric field dielectric application of the polyimide film of Comparative Example 1 is limited to below 200°C, because it has an energy storage efficiency of almost 0 at high electric fields (for example, 300MV / m). It can be expected that it will naturally show worse performance at higher temperatures. Although Comparative Example 2 shows good energy storage performance at 200°C, it cannot be effectively tested due to the extremely low electrical breakdown caused by poor electrical insulation performance at temperatures of 250°C and above, and the energy storage temperature under high electric fields will not be higher than 300°C. The polyimide film of Example 2 has a low glass transition temperature T g =298°C is limited to below 300°C, while the polyimide film of Example 1 can be effectively tested for energy storage at 300°C. 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 the enhancement of insulation. For example, at 200°C, 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 / cm 3 )>Example 1(2.84J / cm 3 )>Comparative Example 2 (1.25 J / cm 3 )>Comparative Example 1(0.32J / cm 3 ), which is because leakage current is the main source of energy storage loss in high temperature environment.

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

[0078] like Fig. 9 As shown in the figure, the energy storage density and energy storage efficiency of the polyimide film prepared in Example 1 at 200 MV / m at 200°C, 250°C and 300°C respectively change with the increase of the number of cycles. It can be seen from the figure that the polyimide film of Example 1 can perform more than 40,000 energy storage cycles even at a condition as high as 300°C, indicating that it has excellent high-temperature energy storage performance.

[0079] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a polyimide film, characterized in that: The following steps are involved: (1) alicyclic dianhydride monomers and ortho-side aromatic diamine monomers are subjected to polycondensation reaction in an organic solvent to obtain a precursor solution; (2) forming a film from the precursor solution, and then drying and performing imidization reaction to obtain a polyimide film; The imidization reaction is carried out under vacuum conditions. The imidization reaction is a programmed temperature reaction, specifically: keeping the temperature at 180-210° C. for 1-2 hours, then heating to 240-260° C. for 1-2 hours, and continuing to heat to T for 0.5-1 hour; Where T = T g -t,T g is the glass transition temperature of the polymer, 10℃≤t≤20℃.

2. The method for preparing a polyimide film according to claim 1, characterized in that: The molar ratio of the alicyclic dianhydride monomer to the ortho-side aromatic diamine monomer is 1-1.02:

1.

3. The method for preparing a polyimide film according to claim 1, characterized in that: The alicyclic dianhydride monomer includes one or more of 1,2,4,5-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, and 1,2,3,4-cyclopentanetetracarboxylic dianhydride.

4. The method for preparing a polyimide film according to claim 1, characterized in that: The ortho-side aromatic diamine monomer includes one or more of 4,4'-methylene di-o-toluidine, 4,4'-methylene bis(2-ethylaniline), and 2,5-dimethyl-1,4-phenylenediamine.

5. The method for preparing a polyimide film according to claim 1, characterized in that: The organic solvent includes one or more of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide and meta-phenol.

6. The method for preparing a polyimide film according to claim 1, characterized in that: The polycondensation reaction in step (1) is carried out at room temperature, and the reaction time is 24-72 hours.

7. The method for preparing a polyimide film according to any one of claims 1 to 6, characterized in that: The following steps are also included: The polyimide film is cooled and then dried to obtain a dry polyimide film.

8. A polyimide film prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The thickness of the polyimide film is 6-25 μm.

9. Use of a polyimide film prepared by the preparation method according to any one of claims 1 to 7 or a polyimide film according to claim 8 in a capacitor.

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