A crosslinking type polyimide and a method for preparing the same

Abstract

The application relates to a crosslinking type polyimide and a preparation method thereof, and relates to the technical field of polyimide synthesis. The application provides a crosslinking type polyimide preparation method with relatively wide applicability, and solves the problem of complex preparation process of the existing crosslinking type polyimide. The method specifically comprises the following steps: after a polyamide acid slurry is generated by condensation reaction of a diamine monomer and a tetracarboxylic dianhydride monomer, a compound containing multiple epoxy groups or oxazolyl groups is added as a crosslinking agent; then, during subsequent imidization, the carboxyl groups and the epoxy groups or the oxazolyl groups contained in the polyamide acid are utilized to react so that crosslinking occurs between polyimide molecular chains, and a crosslinking type polyimide is obtained. The solvent resistance, heat resistance and mechanical properties of the crosslinking type polyimide obtained by the preparation method are obviously improved, and the crosslinking type polyimide can be applied to the fields of polyimide films and fibers and the like by utilizing the performance advantages.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of polyimide synthesis, in particular to a cross-linked polyimide and a preparation method thereof. BACKGROUND

[0002] Polyimide (PI) is one of the best heat-resistant materials among existing polymers, and aromatic polyimide has excellent heat resistance, thermal stability, dimensional stability, mechanical properties, chemical stability and radiation resistance due to its rigid molecular chain structure and strong intermolecular force, and thus is widely used in the field of electrical / electronic components. However, due to the rigidity of the molecular chain structure of polyimide, its processing performance is poor, and it is generally modified by adding flexible chains or branched chains to improve the processing performance; in addition, for the needs of some special application fields such as the photoelectric field, polyimide also needs to be modified to meet the application requirements. However, these modifications have sacrificed the heat resistance, mechanical properties, solvent resistance and other properties of polyimide to some extent.

[0003] To further improve the high temperature resistance, mechanical properties and solvent resistance of polyimide resin, an effective method is to crosslink polyimide by crosslinking agent to form a three-dimensional network structure between the molecular chains. For example, Chinese patent document CN108586744A "A colorless transparent polyimide film and its preparation method" (published on September 28, 2018) discloses that in the polyamide acid containing terminal amino group, the end treatment is carried out by the monocarboxylic anhydride of unsaturated bond, and finally the crosslinking structure is formed by further polymerization of unsaturated bond through high temperature heating. Chinese patent document CN111683992A "Polyimide resin composition and polyimide film" (published on September 18, 2020) discloses a polyimide film preparation method, which synthesizes a polyimide with carboxyl side chains in part of the chain segments, then adds a crosslinking agent containing two oxazolyl groups, and finally forms a crosslinking structure by reacting oxazolyl groups with carboxyl groups to improve the solvent resistance of polyimide. Chinese patent document CN111073282A "A solvent-resistant and colorless transparent cross-linked polyimide film and its preparation method" (published on April 28, 2020) discloses a cross-linked polyimide film, which introduces a monomer containing a benzimidazole group structure into the diamine monomer of the synthesized polyimide, so that it can react with the epoxy group crosslinking agent to form a crosslinking structure and improve the solvent resistance. Chinese patent document CN111187436A "A high-performance colorless transparent polyimide film and its preparation method" (published on June 22, 2020) discloses a polyimide film, which introduces a monomer containing a POSS structure into the diamine monomer of the synthesized polyimide, and adds an end-capping agent containing an alkyne group, finally forms a crosslinking structure to improve the solvent resistance of the polyimide resin. Chinese patent document CN111732748A "A cross-linked colorless transparent high-temperature-resistant polyimide film material and its preparation method" (published on October 2, 2020) discloses a polyimide material, which synthesizes a branched diamine monomer containing unsaturated olefins, and finally forms a crosslinking structure by further polymerization of the olefins.

[0004] However, the methods described in the above documents have strict requirements for the types of polyimide monomers, or must introduce special structure monomers to achieve, and the crosslinking process requires additional heating process steps, which is complex and has high preparation cost, making it difficult to achieve mass production. SUMMARY

[0005] To solve the problem of complex preparation process of existing cross-linked polyimide, the present application provides a cross-linked polyimide and a preparation method thereof.

[0006] The technical scheme of the present application is as follows:

[0007] A preparation method of a cross-linked polyimide, comprising the following steps:

[0008] S1, under an inert gas atmosphere, adding a diamine monomer and a tetracarboxylic dianhydride monomer into an aprotic polar organic solvent, and performing a polycondensation reaction to obtain a polyamic acid slurry;

[0009] S2, adding a compound containing an epoxy group or an oxazole group as a crosslinking agent into the polyamic acid slurry, and mixing to form a homogeneous solution;

[0010] S3, performing imidization on the homogeneous solution to obtain a crosslinked polyimide.

[0011] Preferably, the tetracarboxylic dianhydride monomer is one or more of pyromellitic dianhydride, 3,3',4,4'-biphenyl tetracarboxylic dianhydride, 2,3,3',4'-biphenyl tetracarboxylic dianhydride, 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, 1,2,3,4-cyclobutane tetracarboxylic dianhydride, 1,2,4,5-cyclohexane tetracarboxylic dianhydride, and 4,4'(4,4'isopropyl diphenyl oxy) bis (phthalic anhydride).

[0012] Preferably, the diamine monomer is one or more of 4,4'oxydianiline, m-phenylenediamine, 4,4'oxydianiline, 4,4'oxydianiline, p-phenylenediamine, 3,4oxydianiline, 4,4'oxydianiline, 2,2'dimethyl 4,4'oxydianiline, 3,3'oxydianiline, 3,3'dimethyl 4,4oxydianiline, 3,3'oxydianiline, 4,4'oxydianiline, 2,2'bis(trifluoromethyl) 4,4'oxydianiline, 4,4'oxydianiline, 2,2'bis(4aminophenyl) hexafluoropropane, 1,3bis(4aminophenoxy) benzene, 2,2bis[4(4aminophenoxy) phenyl] hexafluoropropane, 1,4bis(2 trifluoromethyl 4aminophenoxy) benzene, 1,3bis(3aminophenoxy) benzene, and 2,2bis[4(4aminophenoxy) phenyl] propane.

[0013] Preferably, the molar ratio of the tetracarboxylic dianhydride monomer to the diamine monomer is 1.00:(0.99-1.01).

[0014] Preferably, the molar ratio of the epoxy group or the oxazole group to the carboxyl group in the polyamic acid is (1-15):100.

[0015] Preferably, the crosslinking agent containing epoxy groups or oxazoles groups contains at least two epoxy groups or oxazoles groups, and is specifically selected from one or more of 2,2'-(1,3-phenylene)-dioxazine, bisoxazoline-based benzene, 2,2-bis(2-oxazoline), 2,2'-(1,2-phenylene)-dioxazine, trimethylolpropane triglycidyl ether, and 1,6-hexanediol diglycidyl ether.

[0016] Preferably, the imidization is thermal imidization or chemical imidization.

[0017] The reaction temperature of the thermal imidization is 300-500 DEG C.

[0018] The catalyst used in the chemical imidization is one or more of pyridine, p- pyrroline, dimethylpyridine, trimethylpyridine, and quinoline; and the dehydrating agent is one or more of trifluoroacetic anhydride, acetic anhydride, and propionic anhydride.

[0019] Preferably, the temperature of the polycondensation reaction in step S1 is -10-100 DEG C, and the reaction time is 0.5-12 h.

[0020] The solid content of the polyimide paste is 5-30 wt%.

[0021] Preferably, the aprotic polar organic solvent is one or more of N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0022] A crosslinked polyimide material is prepared by the preparation method described above.

[0023] Compared with the prior art, the present application has the following specific advantages:

[0024] 1. The present application adds a compound containing multiple epoxy groups or oxazoles groups as a crosslinking agent in the process of preparing polyimide by a two-step method, and a crosslinking reaction occurs between the carboxyl groups contained in the polyamic acid itself and the epoxy groups or oxazoles groups in the crosslinking agent, thereby providing a preparation process of a crosslinked polyimide with a wide application range, all diamine monomers and tetracarboxylic dianhydride monomers can be used as reactants to prepare the crosslinked polyimide material; and the crosslinking and imidization processes occur simultaneously, without the need to introduce additional special monomers or additional heating process steps, and the process is simple and has a wide application range.

[0025] 2. The solvent resistance, heat resistance, and mechanical properties of the crosslinked polyimide obtained by the preparation method provided by the present application are obviously improved, and the performance advantages thereof can be applied in the fields of polyimide films and fibers. DETAILED DESCRIPTION

[0026] In order to make the technical solutions of the present application clearer, the technical solutions in the embodiments of the present application are described clearly and completely below. It should be noted that the following embodiments are only used to better understand the technical solutions of the present application, and should not be understood as a limitation of the present application.

[0027] It should be noted that the properties of the polyimide film in the following examples and comparative examples are measured by the following methods.

[0028] The film thickness is measured by a digital display micrometer thickness gauge of Aip Measurement Instrument Co., Ltd.; the tensile strength and tensile elastic modulus are tested by a universal material testing machine according to GB / T1040-2008; and the thermal decomposition temperature is measured by a thermal gravimetric analyzer, under a nitrogen atmosphere, starting from room temperature, and increasing to 810℃ at a rate of 10℃ / min, to obtain the temperature at which the weight loss is 5%.

[0029] Example 1.

[0030] S1, under a nitrogen atmosphere, 2,2'bis(trifluoromethyl)4,4'diaminobiphenyl 16.4281g was dissolved in 190.0g of N,N-dimethylacetamide, after stirring to complete dissolution, 3,3'4,4'-biphenyl tetracarboxylic dianhydride 15.0968g was added, the molar ratio of dianhydride to diamine was 1.00:1.00, and the reaction was carried out at room temperature for 12h to obtain a viscous polyamic acid solution with a solid content of 15wt%;

[0031] S2, 30.0g of the polyamic acid solution was taken, 2,2'-(1,3-phenylene)-dioxazoline 0.1687g was added, the molar ratio of oxazolyl to carboxyl was 10:100, and after stirring for 4h to complete dissolution, a homogeneous polyamic acid solution was obtained;

[0032] S3, the polyamic acid solution was uniformly coated on a clean and smooth glass plate by casting method, and then the glass plate coated with the polyamic acid solution was placed in an imidization furnace, and the temperature was increased in stages under a nitrogen atmosphere, the heating rate was 5℃ / min, and the temperature was kept constant at 80℃ for 0.5h, at 160℃ for 0.5h, and at 350℃ for 0.5h, and then naturally cooled; finally, the crosslinked polyimide product with a thickness of 30μm was peeled off from the glass plate.

[0033] The mechanical properties and thermal decomposition properties of the crosslinked polyimide film obtained in this example were tested, and the results are shown in Table 1; the solvent resistance test was carried out on the crosslinked polyimide film obtained in this example, the film was immersed in a polar solvent for 10min, then taken out and dried, and it was observed that there was no obvious change in the film.

[0034] Example 2.

[0035] In this example, 30.0 g of the polyamic acid slurry prepared in S1 of Example 1 was taken, 2,2'-(1,3-phenylene)-dioxazoline 0.1213 g was added, the molar ratio of oxazolyl to carboxyl was 7:100, and the remaining steps were the same as in Example 1. The test results of the mechanical properties and thermal decomposition properties of the crosslinked polyimide film thus obtained are shown in Table 1; the crosslinked polyimide film obtained in this example was subjected to solvent resistance testing, and after the film was immersed in a polar solvent for 10 min, it was taken out, dried, and observed, and no obvious change in the film was observed.

[0036] Example 3.

[0037] In this example, 30.0 g of the polyamic acid slurry prepared in S1 of Example 1 was taken, 2,2'-(1,3-phenylene)-dioxazoline 0.0908 g was added, the molar ratio of oxazolyl to carboxyl was 5:100, and the remaining steps were the same as in Example 1. The test results of the mechanical properties and thermal decomposition properties of the crosslinked polyimide film thus obtained are shown in Table 1; the crosslinked polyimide film obtained in this example was subjected to solvent resistance testing, and after the film was immersed in a polar solvent for 10 min, it was taken out, dried, and observed, and no obvious change in the film was observed.

[0038] Example 4

[0039] In this example, 30.0 g of the polyamic acid slurry prepared in S1 of Example 1 was taken, 2,2'-(1,3-phenylene)-dioxazoline 0.0453 g was added, the molar ratio of oxazolyl to carboxyl was 3:100, and the remaining steps were the same as in Example 1. The test results of the mechanical properties and thermal decomposition properties of the crosslinked polyimide film thus obtained are shown in Table 1; the crosslinked polyimide film obtained in this example was subjected to solvent resistance testing, and after the film was immersed in a polar solvent for 10 min, it was taken out, dried, and observed, and no obvious change in the film was observed.

[0040] Example 5.

[0041] In this example, 30.0 g of the polyamic acid slurry prepared in S1 of Example 1 was taken, 2,2'-(1,3-phenylene)-dioxazoline 0.0195 g was added, the molar ratio of oxazolyl to carboxyl was 1.00:100, and the remaining steps were the same as in Example 1. The test results of the mechanical properties and thermal decomposition properties of the crosslinked polyimide film thus obtained are shown in Table 1; the crosslinked polyimide film obtained in this example was subjected to solvent resistance testing, and after the film was immersed in a polar solvent for 10 min, it was taken out, dried, and observed, and no obvious change in the film was observed.

[0042] Comparative Example 1.

[0043] In this embodiment, 20.0 g of the polyamide acid slurry in Example 1 was taken, and 2,2'-(1,3-phenylene)-dioxazoline was not added, and the remaining steps were the same as in Example 1. The test results of the mechanical properties and thermal decomposition properties of the crosslinked polyimide film thus obtained are shown in Table 1; the solvent resistance test was performed on the crosslinked polyimide film obtained in this embodiment, and after the film was immersed in a polar solvent for 10 min, it was taken out and dried for observation, and it was observed that the film surface became hazy.

[0044] Table 1

[0045]

[0046]

[0047] Example 6.

[0048] S1, p-phenylenediamine 1.9476 g and m-phenylenediamine 0.6457 g were dissolved in 180.0 g of N-methylpyrrolidone under a nitrogen atmosphere at -10°C, and after stirring until complete dissolution, 3,3'4,4'-diphenyltetracarboxylic dianhydride 7.0645 g was slowly added, the molar ratio of dianhydride to diamine was 1.00:0.99, and the reaction was carried out at -10°C for 4 h, and then gradually warmed to 40°C, and the total reaction time was 12 h, to obtain a viscous polyamide acid slurry with a solid content of 5 wt%;

[0049] S2, 30.0 g of the polyamide acid slurry was taken, 1,6-hexanediol diglycidyl ether 0.0876 g was added as a crosslinking agent, the molar ratio of epoxy group to carboxyl group was 10:100, and after stirring for 4 h until complete dissolution, a polyamide acid solution was obtained;

[0050] S3, the polyamide acid solution was uniformly coated on a clean and smooth glass plate by casting, and then the glass plate coated with the polyamide acid solution was placed in an imidization furnace under a nitrogen atmosphere, and the temperature was gradually increased, the heating rate was 5°C / min, the temperature was kept at 80°C for 0.5 h, the temperature was kept at 160°C for 0.5 h, and the temperature was kept at 350°C for 0.5 h, and then naturally cooled; finally, the crosslinked polyimide product with a thickness of 30 μm was obtained by peeling off from the glass plate.

[0051] The test results of the mechanical properties and thermal decomposition properties of the crosslinked polyimide film obtained in this embodiment are shown in Table 2; the solvent resistance test was performed on the crosslinked polyimide film obtained in this embodiment, and after the film was immersed in a polar solvent for 10 min, it was taken out and dried for observation, and it was observed that the film had no obvious change.

[0052] Example 7.

[0053] In this example, 30.0 g of S1 polyamic acid slurry in Example 6 was taken, 1,6-hexanediol diglycidyl ether 0.0553 g was added, the molar ratio of epoxy group to carboxyl group was 6:100, and the remaining steps were the same as in Example 6. The test results of the mechanical properties and thermal decomposition properties of the cross-linked polyimide film thus obtained are shown in Table 2; the cross-linked polyimide film obtained in this example was subjected to solvent resistance test, and after the film was immersed in a polar solvent for 10 min, it was taken out and dried for observation, and no obvious change in the film was observed.

[0054] Example 8.

[0055] In this example, 30.0 g of S1 polyamic acid slurry in Example 6 was taken, 1,6-hexanediol diglycidyl ether 0.0370 g was added, the molar ratio of epoxy group to carboxyl group was 4:100, and the remaining steps were the same as in Example 6. The test results of the mechanical properties and thermal decomposition properties of the cross-linked polyimide film thus obtained are shown in Table 2; the cross-linked polyimide film obtained in this example was subjected to solvent resistance test, and after the film was immersed in a polar solvent for 10 min, it was taken out and dried for observation, and no obvious change in the film was observed.

[0056] Example 9.

[0057] In this example, 30.0 g of S1 polyamic acid slurry in Example 6 was taken, 1,6-hexanediol diglycidyl ether 0.0256 g was added, the molar ratio of epoxy group to carboxyl group was 3:100, and the remaining steps were the same as in Example 6. The test results of the mechanical properties and thermal decomposition properties of the cross-linked polyimide film thus obtained are shown in Table 2; the cross-linked polyimide film obtained in this example was subjected to solvent resistance test, and after the film was immersed in a polar solvent for 10 min, it was taken out and dried for observation, and no obvious change in the film was observed.

[0058] Example 10.

[0059] In this example, 30.0 g of S1 polyamic acid slurry in Example 6 was taken, 1,6-hexanediol diglycidyl ether 0.0114 g was added, the molar ratio of epoxy group to carboxyl group was 1:100, and the remaining steps were the same as in Example 6. The test results of the mechanical properties and thermal decomposition properties of the cross-linked polyimide film thus obtained are shown in Table 2; the cross-linked polyimide film obtained in this example was subjected to solvent resistance test, and after the film was immersed in a polar solvent for 10 min, it was taken out and dried for observation, and no obvious change in the film was observed.

[0060] Comparative Example 2.

[0061] In this embodiment, 10.0 g of the S1 polyamic acid slurry in Example 6 was taken, and 1,6-hexanediol diglycidyl ether was not added, and the remaining steps were the same as in Example 6. The test results of the mechanical properties and thermal decomposition properties of the cross-linked polyimide film thus obtained are shown in Table 2; the cross-linked polyimide film obtained in this embodiment was subjected to solvent resistance test. After the film was immersed in a polar solvent for 10 min, it was taken out and dried for observation. It was observed that the film surface became hazy.

[0062] Table 2

[0063]

[0064]

[0065] Example 11.

[0066] S1, 32.0391 g of 4,4'-oxydianiline was dissolved in 160.0 g of N-methylpyrrolidone under a nitrogen atmosphere at room temperature, and after stirring until complete dissolution, 31.3792 g of 1,2,3,4-cyclobutanetetracarboxylic dianhydride was slowly added, the molar ratio of dianhydride to diamine was 1.00:1.01, and the reaction was carried out at room temperature for 12 h to obtain a viscous polyamic acid slurry with a solid content of 30 wt%;

[0067] S2, 25.0 g of the polyamic acid slurry was taken, 0.0408 g of trimethylolpropane triglycidyl ether was added as a cross-linking agent, the molar ratio of epoxy groups to carboxyl groups was 1:100, and after stirring at 90°C for 4 h until complete dissolution, a polyamic acid solution was obtained;

[0068] S3, after the solution was cooled to 70°C, 2 ml of pyridine and 6 ml of acetic anhydride were added, the reaction was carried out for 24 h, and then the reaction liquid was discharged into 500 ml of anhydrous ethanol, the solid crude product was collected by suction filtration, the crude product was purified by Soxhlet extractor using anhydrous ethanol for 48 h, and finally dried to obtain a cross-linked polyimide resin;

[0069] S4, the polyimide resin obtained was dissolved in dimethylacetamide to obtain a polyimide resin solution with a solid content of 10 wt%, which was uniformly coated on a clean and smooth glass plate by casting, and then the glass plate coated with the polyimide resin solution was placed in a vacuum oven, and pre-dried at 80°C for 0.5 h, 160°C for 0.5 h, and 200°C for 0.5 h in turn, and then naturally cooled; finally, the cross-linked polyimide product with a thickness of 28 μm was peeled off from the glass plate.

[0070] The related performance test results of the polyimide film obtained in the example are shown in Table 3; the solvent resistance test was performed on the crosslinked polyimide film obtained in the example, the film was immersed in a polar solvent for 10 min, then taken out and dried for observation, and it was observed that the film had no obvious change.

[0071] Example 12.

[0072] In the example, 25.0 g of S1 polyamide acid slurry in Example 11 was taken, 0.2043 g of trimethylolpropane triglycidyl ether was added, the molar ratio of epoxy group to carboxyl group was 5:100, and the remaining steps were the same as in Example 11. The mechanical properties and thermal decomposition properties test results of the crosslinked polyimide film thus obtained are shown in Table 3; the solvent resistance test was performed on the crosslinked polyimide film obtained in the example, the film was immersed in a polar solvent for 10 min, then taken out and dried for observation, and it was observed that the film had no obvious change.

[0073] Example 13.

[0074] In the example, 25.0 g of S1 polyamide acid slurry in Example 11 was taken, 0.2043 g of trimethylolpropane triglycidyl ether was added, the molar ratio of epoxy group to carboxyl group was 5:100, and the remaining steps were the same as in Example 11. The mechanical properties and thermal decomposition properties test results of the crosslinked polyimide film thus obtained are shown in Table 3; the solvent resistance test was performed on the crosslinked polyimide film obtained in the example, the film was immersed in a polar solvent for 10 min, then taken out and dried for observation, and it was observed that the film had no obvious change.

[0075] Example 14.

[0076] In the example, 25.0 g of S1 polyamide acid slurry in Example 11 was taken, 0.2043 g of trimethylolpropane triglycidyl ether was added, the molar ratio of epoxy group to carboxyl group was 5:100, and the remaining steps were the same as in Example 11. The mechanical properties and thermal decomposition properties test results of the crosslinked polyimide film thus obtained are shown in Table 3; the solvent resistance test was performed on the crosslinked polyimide film obtained in the example, the film was immersed in a polar solvent for 10 min, then taken out and dried for observation, and it was observed that the film had no obvious change.

[0077] Example 15.

[0078] In this embodiment, 25.0 g of S1 polyamide acid slurry in Example 11 is taken, 0.3854 g of trimethylolpropane triglycidyl ether is added, the molar ratio of epoxy group to carboxyl group is 10:100, the remaining steps are the same as in Example 11, and the test results of the mechanical properties and thermal decomposition properties of the cross-linked polyimide film thus obtained are shown in Table 3; the cross-linked polyimide film obtained in this embodiment is subjected to solvent resistance test, after the film is immersed in a polar solvent for 10 min, it is taken out and dried for observation and judgment, and it can be observed that the film has no obvious change.

[0079] Comparative Example 3.

[0080] In this embodiment, 20.0 g of S1 polyamide acid slurry in Example 11 is taken, no trimethylolpropane triglycidyl ether is added, and the remaining steps are the same as in Example 11, and the test results of the mechanical properties and thermal decomposition properties of the cross-linked polyimide film thus obtained are shown in Table 3; the cross-linked polyimide film obtained in this embodiment is subjected to solvent resistance test, after the film is immersed in a polar solvent for 10 min, it is taken out and dried for observation and judgment, and it can be observed that the film surface has a misting phenomenon.

[0081] Table 3

[0082]

[0083]

[0084] Through comparison, it can be proved that the solvent resistance, heat resistance and mechanical properties of the cross-linked polyimide obtained by the preparation method provided in the application are obviously improved, and its performance advantages can be applied in the field of polyimide film and fiber.

[0085] In the process of preparing polyimide by two-step method, the compound containing multiple epoxy groups or oxazoline groups is added as a cross-linking agent in the application, the cross-linking reaction between the carboxyl group in the polyamide acid and the epoxy group or oxazoline group in the cross-linking agent is utilized, a cross-linked polyimide preparation process with a wide application range is provided, all diamine monomers and tetracarboxylic dianhydride monomers can be used as reactants to prepare the cross-linked polyimide material; and the cross-linking and imidization processes occur simultaneously, without the need to introduce additional special monomers and additional heating process steps, the process is simple and has a wide application range.

Claims

1. A method for preparing cross-linked polyimide, characterized in that, Includes the following steps: S1. Under an inert gas atmosphere, diamine monomers and tetracarboxylic acid dianhydride monomers are added to an aprotic polar organic solvent, and polycondensation reaction is carried out to generate polyamic acid slurry. S2. A compound containing multiple epoxy groups or oxazole groups is added to the polyamic acid slurry as a crosslinking agent, and after thorough mixing, a homogeneous solution is formed. S3. The homogeneous solution is imidized to obtain a cross-linked polyimide; The molar ratio of the epoxy group or oxazole group to the carboxyl group in the polyamic acid is (1~15):100; The crosslinking agent containing epoxy groups or oxazole groups contains at least two epoxy groups or oxazole groups, specifically selected from one or more of 2,2'-(1,3-phenylene)-dioxazoline, bisoxazolinebenzene, 2,2-bis(2-zoline), trimethylolpropane triglycidyl ether, and 1,6-hexanediol diglycidyl ether.

2. The method for preparing cross-linked polyimide according to claim 1, characterized in that, The tetracarboxylic acid dianhydride monomers are one or more of the following: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, and 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride).

3. The method for preparing cross-linked polyimide according to claim 1, characterized in that, The diamine monomers are 4,4'-diaminodiphenyl ether, m-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminobiphenyl, p-phenylenediamine, 3,4-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 2,2'-dimethyl-4,4'-diaminobiphenyl, 3,3'-diaminodiphenyl sulfone, 3,3'-dimethyl-4,4'-diaminobiphenyl, 3,3'-diamino-benzophenone, 4,4'-diamino-benzophenone, 2,2'- One or more of the following: bis(trifluoromethyl)-4,4'-diaminobiphenyl, 4,4'-diaminobenzoylaniline, 2,2'-bis(4-aminophenyl)hexafluoropropane, 1,3-bis(4-aminophenoxy)benzene, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 1,4-bis(2-trifluoromethyl-4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, and 2,2-bis[4-(4-aminophenoxy)phenyl]propane.

4. The method for preparing cross-linked polyimide according to claim 1, characterized in that, The molar ratio of the tetracarboxylic acid dianhydride monomer to the diamine monomer is 1.00:(0.99~1.01).

5. The method for preparing cross-linked polyimide according to claim 1, characterized in that, The imidization is either thermal imidization or chemical imidization; The reaction temperature for thermal imidization is 300℃~500℃; The catalyst used in the chemical imidization is one or a mixture of at least two of pyridine, p-pyrroline, dimethylpyridine, trimethylpyridine, and quinoline; the dehydrating agent is one or more of trifluoroacetic anhydride, acetic anhydride, and propionic anhydride.

6. The method for preparing cross-linked polyimide according to claim 1, characterized in that, The temperature of the polycondensation reaction in step S1 is -10℃ to 100℃, and the reaction time is 0.5h to 12h. The solid content of the polyimide slurry is 5wt%~30wt%.

7. The method for preparing cross-linked polyimide according to claim 1, characterized in that, The aprotic polar organic solvent is one or more of N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide.

8. A cross-linked polyimide material, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 7.

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