Polyamic acid-polyimide varnish, preparation method and application in preparation of polyimide film

Abstract

The invention belongs to the technical field of high-insulating-strength polyimide varnish materials, and particularly relates to polyamide acid-polyimide varnish, a preparation method and application in preparation of a polyimide film. The preparation method comprises the following steps: S1, adding first diamine and dianhydride into a polar solvent to carry out condensation polymerization, and then adding an anhydride end-capping reagent to slow down a decomposition reaction so as to prepare first polyamide acid; s2, adding second diamine and an auxiliary agent into the stable polyamic acid for reaction to obtain second polyamic acid; and S3, adding a dehydrating agent and a catalyst into the low-viscosity polyimide, and carrying out chemical reaction to obtain the polyamide acid-polyimide varnish. According to the invention, resorcinol bis (diphenyl phosphate) is used as an auxiliary agent in polyamide acid, and the molecular structure of polyamide acid and the interaction force between chains are influenced through the interaction between the resorcinol bis (diphenyl phosphate) and the molecular chains of polyamide acid, so that the viscosity of the polyamide acid is influenced.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-insulation-strength polyimide varnish materials, and particularly relates to a polyamic acid-polyimide varnish, a preparation method thereof, and an application thereof in the preparation of polyimide films. Background Art

[0002] Polyimide insulating varnish, a kind of insulating material, is used for making enameled wires. It is mainly used in aspects of high-temperature resistance performance, such as coils of motors and high-voltage components, in the aerospace industry, the outer coatings of spacecraft and military aircraft, and in the precision electronics manufacturing industry, electronic circuit board making, etc. In the above-mentioned high-tech fields represented, there is a quite urgent need for the mechanical properties, high tensile modulus, high tensile strength, high and low temperature resistance, chemical corrosion resistance, radiation resistance, etc. shown by polyimide.

[0003] The polyimide varnish in enameled wires usually requires a lower viscosity, mainly to ensure: 1. Better impregnation: The low-viscosity polyimide varnish can more easily penetrate into the tiny gaps and surfaces of copper wires, ensuring uniform coating of the paint film and avoiding insulation failure or decreased conductivity caused by uneven paint film. 2. Improve coating efficiency: In the production of enameled wires, methods such as dipping or spraying are usually used to coat the paint on the surface of copper wires. The low-viscosity paint has better fluidity and can be evenly coated in a shorter time, reducing production time and improving production efficiency. 3. Avoid over-thick coating: If the viscosity of the paint is too high, it will cause the coating to be too thick, and the paint film is prone to cracking or peeling during the curing process. The low viscosity can control the thickness of the coating and ensure the formation of a uniform and firm insulating layer. 4. Improve curing quality: The polyimide varnish needs good fluidity during curing to form a dense paint film. Too high viscosity may lead to incomplete curing of the paint film or problems such as bubbles and cracks during the curing process. 5. Adapt to high production speeds: In the production of enameled wires, a relatively high production speed is usually required. The low-viscosity paint can reduce the burden on equipment during the coating process and ensure the quality of the coating, avoiding coating difficulties or unevenness caused by too high viscosity.

[0004] Meanwhile, the polyimide varnish in enameled wire usually requires a high solid content. The specific reasons are as follows: 1. Increase the coating thickness: The solid content refers to the proportion of non-volatile solid components in the varnish. A high solid content means that more polyimide components can be left during the curing process, thus forming a thicker insulating coating on the copper wire surface, directly affecting the insulation performance, withstand voltage, and service life. 2. Reduce the solvent evaporation amount. This helps to reduce environmental pollution and improve the stability and safety during the production process. 3. Improve production efficiency: The polyimide varnish with a high solid content can be quickly cured after coating to form a thicker insulating layer, reducing the number of repeated coating times during the coating process and improving production efficiency. 4. Save solvents and reduce evaporation losses: The varnish with a high solid content contains less solvent compared to the varnish with a low solid content, which helps to save solvent costs and reduce the waste caused by solvent evaporation. In addition, reducing the evaporated solvent also helps to meet environmental protection requirements and reduce harmful gas emissions during the production process. 5. Enhance the physical properties of the paint film: The polyimide varnish with a higher solid content can form a denser and more stable paint film, improving the physical properties such as insulation, abrasion resistance, and high-temperature resistance of the paint film, thereby improving the overall quality and service life of the enameled wire. 6. Improve electrical properties: As the insulating layer of the enameled wire, the polyimide varnish needs to have good electrical insulation properties. A high solid content helps to form a uniform and high-quality paint film, reducing defects such as bubbles and voids, thereby improving the insulation of the enameled wire and ensuring its safety and stability in electrical equipment.

[0005] Generally speaking, the low viscosity and high solid content of the polyimide varnish can improve production efficiency, reduce costs, improve the coating quality, enhance the physical and electrical properties of the paint film while ensuring good coating performance, meeting the requirements of the enameled wire industrial production. Summary of the Invention

[0006] Aiming at the deficiencies of the prior art, the object of the present invention can be achieved through the following technical solutions:

[0007] A preparation method of a polyamic acid-polyimide varnish, the preparation method comprising the following steps:

[0008] S1. Put the first diamine and dianhydride in a polar solvent for polycondensation reaction, and then add an acid anhydride capping agent to slow down the decomposition reaction to prepare the first polyamic acid;

[0009] S2. Add the second diamine and an auxiliary agent to the stable polyamic acid for reaction to obtain the second polyamic acid;

[0010] S3. Add a dehydrating agent and a catalyst to the low-viscosity polyimide, and react by a chemical method to obtain the polyamic acid-polyimide varnish.

[0011] Further, in step S1, the molar ratio of the first diamine, dianhydride, and acid anhydride end-capping agent is 1:0.98:0.02, and the dianhydride, first diamine, and acid anhydride end-capping agent account for 35% of the total mass of the reaction system; the first diamine accounts for 0.01 - 0.1 of the total mass of the diamine, dianhydride, and acid anhydride end-capping agent in S1, and the auxiliary agent accounts for 0.01 - 0.1 of the total mass of the first diamine, dianhydride, and acid anhydride end-capping agent in S1; the dehydrating agent and catalyst respectively account for 0.01 - 0.1 of the total mass of the first diamine, dianhydride, and acid anhydride end-capping agent in S1.

[0012] Further, the first diamine in step S1 is selected from one or more of 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 1,4'-phenylenediamine, 4,4-diaminodiphenyl ether, 4,4'-diaminobenzanilide, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, 2-(4-aminophenyl)-5-aminobenzoxazole, 4,4'-bis-4-(nitrophenoxy)biphenyl, 4,4'-diamino-2,2'-dimethylbiphenyl, 4,4'-diaminodiphenylmethane, 4,4'-diaminobenzophenone, and diaminodiphenyl sulfone.

[0013] Further, the dianhydride in step S1 is selected from one or more of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenylethertetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 4,4-oxybisphthalic anhydride, and bisphenol A type diether dianhydride.

[0014] Further, the polar solvent in step S1 is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, γ-caprolactone, toluene, and xylene; the temperature of the polar solvent is 25 - 45°C; the acid anhydride end-capping agent is selected from one or more of phthalic anhydride and 1,2,4-benzenetricarboxylic anhydride.

[0015] Further, the second diamine is selected from one or more of 1,4'-phenylenediamine, 4,4-diaminodiphenyl ether, 4,4'-diaminobenzanilide, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, 2-(4-aminophenyl)-5-aminobenzoxazole, 4,4'-bis-4-(nitrophenoxy)biphenyl, 4,4'-diamino-2,2'-dimethylbiphenyl, 4,4'-diaminodiphenylmethane, 4,4'-diaminobenzophenone, and diaminodiphenyl sulfone; the auxiliary agent is selected from resorcinol bis(diphenyl phosphate), the dehydrating agent is selected from one of acetic anhydride and N,N'-dicyclohexylcarbodiimide, and the catalyst is selected from one of isoquinoline, triethylamine, and pyridine.

[0016] A polyamic acid-polyimide varnish is prepared by the preparation method of a polyamic acid-polyimide varnish described above.

[0017] A polyimide film, wherein the polyimide film is prepared from the polyamic acid-polyimide varnish described above.

[0018] A preparation method of a polyimide film, specifically: defoaming, coating, desolventizing and imidizing the polyamic acid-polyimide varnish described in claim 7, and then a polyimide film can be obtained.

[0019] Furthermore, the temperature of the desolventizing treatment is 60 - 150 °C, and the time is 20 - 40 min; the temperature of the imidizing treatment is 400 - 450 °C, and the time is 20 - 30 min; the thickness of the polyimide film is 12.5 μm; the modulus of the polyimide film is 2.9 - 3.4 GPa.

[0020] Advantages of the present invention:

[0021] 1. In the present invention, resorcinol bis(diphenyl phosphate) is used as an additive in polyamic acid. Through its interaction with the molecular chain of polyamic acid, it affects the molecular structure and the intermolecular force of polyamic acid, thereby reducing its viscosity.

[0022] 2. The present invention uses an acid anhydride end-capping agent and an additive to promote the imidization reaction of polyamic acid and accelerate the efficiency of the imidization reaction, thereby extending the molecular chain of polyimide, increasing its molecular weight, and improving the mechanical properties and thermal stability of the film. At the same time, it can avoid the breakage or uneven formation of polymer chains, thereby improving the uniformity and stability of the polyimide film. The partially imidized enameled wire can be cured at a lower temperature, and the coating can react further through subsequent heating steps more quickly, reducing the dependence on high-temperature equipment and long heating time. Therefore, this technology can improve the working efficiency of the production line, shorten the production cycle, and thus improve the overall production efficiency. While reducing production costs, the high performance of the enameled wire coating is maintained.

[0023] 3. The present invention uses 1,3-bis(4-aminophenoxy)benzene with a rigid structure as a diamine monomer. At the same time, due to the presence of two (4-aminophenoxy) groups, there will be a certain steric hindrance effect in space for the molecule, which will affect the interaction between the diamine and the dianhydride. When the reaction occurs, it will limit the contact of the active sites of the reaction, thereby affecting the reaction rate and selectivity; when forming a polymer or supramolecular structure, it will also affect the arrangement and packing mode of the molecules, and thus improve its high solid content. Specific embodiments

[0024] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0025] A preparation method of a polyamic acid-polyimide varnish, the preparation method comprising the following steps:

[0026] S1. Put a first diamine and a dianhydride in a polar solvent for polycondensation reaction, and then add an acid anhydride end-capping agent to slow down the decomposition reaction to prepare a first polyamic acid;

[0027] S2. Add a second diamine and an auxiliary agent to the stable polyamic acid for reaction to obtain a second polyamic acid;

[0028] S3. Add a dehydrating agent and a catalyst to the low-viscosity polyimide, and react by a chemical method to obtain a polyamic acid-polyimide varnish.

[0029] To better understand the above technical solutions, the following will combine specific embodiments to prepare a polyimide film from the polyimide varnish prepared by the above method, and then evaluate the performance of the polyimide varnish by testing the performance of the polyimide film.

[0030] Example 1:

[0031] This example provides a preparation method of a polyimide film, comprising the following steps:

[0032] (1) Add 110 ml of N,N-dimethylacetamide to a 250 ml glass flask, add 14.6 g of 1,3-bis(4-aminophenoxy)benzene and 10 g of 4,4-diaminodiphenyl ether. After it is dissolved, gradually add 15.5 g of 4,4-oxybisphthalic anhydride, 16.1 g of 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and 0.5 g of phthalic anhydride to obtain a polyamic acid resin with a viscosity of 12000 mPa·s;

[0033] (2) Add 0.5 g of 4,4-diaminodiphenyl ether and 0.1 g of resorcinol bis(diphenyl phosphate) to the solution, and stir evenly to obtain a polyamic acid resin with a viscosity of 9000 mPa·s;

[0034] (3) Gradually add 0.1 g of acetic anhydride and 0.1 g of isoquinoline to the solution, and stir evenly to obtain a polyimide varnish with a viscosity of 7800 mPa·s;

[0035] (4) The polyimide varnish is subjected to vacuum defoaming treatment, and then coated on a glass plate and dried in a forced-air drying oven at 90 °C for 40 min to obtain a gel film. The obtained gel film is peeled off and fixed on a metal frame, and then put into an imidization furnace for imidization at 400 °C for 30 min to obtain a polyimide film with a thickness of 12.5 μm.

[0036] Example 2

[0037] This example provides a method for preparing a polyimide film, which includes the following steps:

[0038] (1) Add 110 ml of N,N-dimethylacetamide to a 250-ml glass flask, add 14.6 g of 1,3-bis(4'-aminophenoxy)benzene and 10 g of 4,4'-diaminodiphenyl ether. After dissolution, gradually add 31 g of 4,4'-oxydiphthalic anhydride and 0.5 g of phthalic anhydride to obtain a low-molecular-weight polyamic acid resin with a viscosity of 11000 mPa·s;

[0039] (2) Add 0.5 g of 4,4'-diaminodiphenyl ether and 0.1 g of resorcinol bis(diphenyl phosphate) to the solution, and stir evenly to obtain a polyamic acid resin with a viscosity of 9200 mPa·s;

[0040] (3) Gradually add 0.1 g of acetic anhydride and 0.1 g of isoquinoline to the solution, and stir evenly to obtain a polyimide varnish with a viscosity of 8200 mPa·s;

[0041] (4) The polyimide varnish is subjected to vacuum defoaming treatment, and then coated on a glass plate and dried in a forced-air drying oven at 90 °C for 40 min to obtain a gel film. The obtained gel film is peeled off and fixed on a metal frame, and then put into an imidization furnace for imidization at 400 °C for 30 min to obtain a polyimide film with a thickness of 12.5 μm.

[0042] Example 3

[0043] This example provides a method for preparing a polyimide film, which includes the following steps:

[0044] (1) Add 110 ml of N,N-dimethylacetamide to a 250-ml glass flask, add 14.6 g of 1,3-bis(4'-aminophenoxy)benzene and 10 g of 4,4'-diaminodiphenyl ether. After dissolution, gradually add 32.2 g of 3,3',4,4'-benzophenone tetracarboxylic dianhydride and 0.5 g of phthalic anhydride to obtain a low-molecular-weight polyamic acid resin with a viscosity of 11000 mPa·s;

[0045] (2) Add 0.5 g of 4,4'-diaminodiphenyl ether and 0.1 g of resorcinol bis(diphenyl phosphate) to the dissolution solution. After stirring evenly, a polyamic acid resin with a viscosity of 9100 mPa·s is obtained;

[0046] (3) Gradually add 0.1 g of acetic anhydride and 0.1 g of isoquinoline to the dissolution solution. After stirring evenly, a polyimide varnish with a viscosity of 8100 mPa·s is obtained;

[0047] (4) Perform vacuum degassing treatment on the polyimide varnish, then coat it on a glass plate, and place it in a forced-air drying oven at 90 °C for 40 min to obtain a gel film. Peel off the obtained gel film and fix it on a metal frame, and put it into an imidization furnace for imidization at 400 °C for 30 min to obtain a polyimide film with a thickness of 12.5 μm.

[0048] Example 4

[0049] This example provides a method for preparing a polyimide film, which includes the following steps:

[0050] (1) Add 110 ml of N,N-dimethylacetamide to a 250 ml glass flask, add 14.6 g of 1,3-bis(4'-aminophenoxy)benzene and 10 g of 4,4'-diaminodiphenyl ether. After it is dissolved, gradually add 21.8 g of pyromellitic dianhydride and 0.5 g of PA to obtain a low-molecular-weight polyamic acid resin with a viscosity of 11000 mPa·s;

[0051] (2) Add 0.5 g of 4,4'-diaminodiphenyl ether and 0.1 g of resorcinol bis(diphenyl phosphate) to the dissolution solution. After stirring evenly, a polyamic acid resin with a viscosity of 9200 mPa·s is obtained;

[0052] (3) Gradually add 0.1 g of acetic anhydride and 0.1 g of isoquinoline to the dissolution solution. After stirring evenly, a polyimide varnish with a viscosity of 7950 mPa·s is obtained;

[0053] (4) Perform vacuum degassing treatment on the polyimide varnish, then coat it on a glass plate, and place it in a forced-air drying oven at 90 °C for 40 min to obtain a gel film. Peel off the obtained gel film and fix it on a metal frame, and put it into an imidization furnace for imidization at 400 °C for 30 min to obtain a polyimide film with a thickness of 12.5 μm.

[0054] Example 5

[0055] This example provides a method for preparing a polyimide film, which includes the following steps:

[0056] (1) Add 110 ml of N,N-dimethylacetamide to a 250-ml glass flask, add 29.2 g of 1,3-bis(4'-aminophenoxy)benzene, and after it dissolves, gradually add 15.5 g of 4,4'-oxydiphthalic anhydride, 16.1 g of 3,3',4,4'-benzophenonetetracarboxylic dianhydride, and 0.5 g of phthalic anhydride to obtain a low-molecular-weight polyamic acid resin with a viscosity of 10,000 mPa·s;

[0057] (2) Add 0.5 g of 4,4'-diaminodiphenyl ether and 0.1 g of resorcinol bis(diphenyl phosphate) to the solution, and after stirring evenly, obtain a polyimide-polyamic acid resin with a viscosity of 9,000 mPa·s;

[0058] (3) Gradually add 0.1 g of acetic anhydride and 0.1 g of isoquinoline to the solution, and after stirring evenly, obtain a polyimide varnish with a viscosity of 8,300 mPa·s;

[0059] (4) Perform vacuum degassing on the polyimide varnish, then coat it on a glass plate, place it in a forced-air drying oven and dry it at 90 °C for 40 min to obtain a gel film. Peel off the obtained gel film and fix it on a metal frame, and put it into an imidization furnace for imidization at 400 °C for 30 min to obtain a polyimide film with a thickness of 12.5 μm.

[0060] Comparative Example 1

[0061] This comparative example provides a traditional process for preparing a polyimide film, including the following steps:

[0062] (1) Add 83 ml of N,N-dimethylacetamide to a 250-ml glass flask, then add 20 g of 4,4'-diaminodiphenyl ether, and after it dissolves, gradually add 21.8 g of pyromellitic dianhydride to obtain a polyimide varnish with a viscosity of 195,000 mPa·s;

[0063] (2) Perform vacuum degassing on the polyimide-polyamic acid resin, then coat it on a glass plate, place it in a forced-air drying oven and dry it at 90 °C for 40 min to obtain a gel film. Peel off the obtained gel film and fix it on a metal frame, and put it into an imidization furnace for imidization at 400 °C for 30 min to obtain a polyimide film with a thickness of 12.5 μm.

[0064] Comparative Example 2

[0065] This comparative example provides a traditional process for preparing a polyimide film, including the following steps:

[0066] (1) Add 80 ml of N,N-dimethylacetamide to a three-necked flask, and then add 10.8 g of p-phenylenediamine. After dissolution, gradually add 29.4 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride to obtain a polyimide varnish with a viscosity of 120,000 mPa·s;

[0067] (2) Perform vacuum degassing on the above polyamic acid solution, then coat it on a glass plate, and place it in a forced-air drying oven at 80 °C for 40 min to obtain a gel film. Peel off the obtained gel film and fix it on a metal frame, and put it into an imidization furnace for imidization at 360 °C for 25 min to obtain a polyimide film with a thickness of 25 μm.

[0068] Comparative Example 3

[0069] This comparative example provides a traditional process for preparing polyimide films, including the following steps:

[0070] Add 83 ml of N,N-dimethylacetamide to a 250 ml glass flask, then add 20 g of 4,4'-diaminodiphenyl ether. After dissolution, gradually add 21.8 g of pyromellitic dianhydride to obtain a polyamic acid resin with a viscosity of 190,000 mPa·s;

[0071] Gradually add 0.1 g of acetic anhydride and 0.1 g of isoquinoline to the dissolution solution, and stir evenly to obtain a polyimide varnish with a viscosity of 180,000 mPa·s;

[0072] Perform vacuum degassing on the polyimide-polyamic acid resin, then coat it on a glass plate, and place it in a forced-air drying oven at 90 °C for 40 min to obtain a gel film. Peel off the obtained gel film and fix it on a metal frame, and put it into an imidization furnace for imidization at 400 °C for 30 min to obtain a polyimide film with a thickness of 12.5 μm.

[0073] Detect the properties of the polyimide films prepared in the above Examples 1-5 and Comparative Examples 1-3, and the results are shown in Table 1.

[0074] Table 1 Performance test results of the obtained polyimide films

[0075]

[0076] As can be seen from Table 2, the viscosity of the polyimide film prepared by the method of the present invention is significantly lower than that of the polyimide film prepared by the traditional process. The reason is that 1,3-bis(4-aminophenoxy)benzene weakens the intermolecular force between the polyamic acid segments through hydrogen bonding, electrostatic interaction and π-π interaction with the polyamic acid, playing a plasticizing role. Moreover, the phosphate groups of resorcinol bis(diphenyl phosphate) interact with the amino or amide groups in the polyamic acid molecules, which can reduce the intermolecular force. The low-viscosity polyimide-polyamic acid resin can meet the use requirements of enameled wires. At the same time, the properties such as the elongation at break and elastic modulus of the examples are improved compared with those of the comparative examples. The operation process of the preparation method of the present invention is simple and feasible, and has a significant improvement effect on the comprehensive properties of the resin and the prepared film including viscosity, and has potential application value in the industrial production field.

[0077] In the description of this specification, the descriptions referring to the terms "one embodiment", "example", "specific example", etc. mean that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0078] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art of this industry should understand that the present invention is not limited by the above embodiments. The above embodiments and the descriptions in the specification only illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed.

Claims

1. A preparation method of polyamic acid-polyimide varnish, characterized in that, The preparation method includes the following steps: S1. Put the first diamine and dianhydride into a polar solvent for polycondensation reaction, and then add an acid anhydride end-capping agent to slow down the decomposition reaction to prepare the first polyamic acid; S2. Add the second diamine and an auxiliary agent to the stable polyamic acid for reaction to obtain the second polyamic acid; S3. Add a dehydrating agent and a catalyst to the low-viscosity polyimide, and react through a chemical method to obtain a polyamic acid-polyimide varnish.

2. The preparation method of a polyamic acid-polyimide varnish according to claim 1, characterized in that, In step S1, the molar ratio of the first diamine, dianhydride, and acid anhydride end-capping agent is 1:0.98:0.02, and the dianhydride, the first diamine, and the acid anhydride end-capping agent account for 35% of the total mass of the reaction system; the first diamine accounts for 0.01-0.1 of the total mass of the diamine, dianhydride, and acid anhydride end-capping agent in S1, and the auxiliary agent accounts for 0.01-0.1 of the total mass of the first diamine, dianhydride, and acid anhydride end-capping agent in S1; the dehydrating agent and the catalyst respectively account for 0.01-0.1 of the total mass of the first diamine, dianhydride, and acid anhydride end-capping agent in S1.

3. The preparation method of a polyamic acid-polyimide varnish according to claim 1, characterized in that, The first diamine in step S1 is selected from one or more of 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 1,4'-phenylenediamine, 4,4-diaminodiphenyl ether, 4,4'-diaminobenzanilide, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, 2-(4-aminophenyl)-5-aminobenzoxazole, 4,4'-bis-4-(nitrophenoxy)biphenyl, 4,4'-diamino-2,2'-dimethylbiphenyl, 4,4'-diaminodiphenylmethane, 4,4'-diaminobenzophenone, and diaminodiphenyl sulfone.

4. The preparation method of a polyamic acid-polyimide varnish according to claim 1, characterized in that, The dianhydride in step S1 is selected from one or more of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenylethertetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 4,4-oxybisphthalic anhydride, and bisphenol A type diether dianhydride.

5. The preparation method of a polyamic acid-polyimide varnish according to claim 1, characterized in that, The polar solvent in step S1 is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, γ-caprolactone, toluene, and xylene; the temperature of the polar solvent is 25-45°C; the acid anhydride end-capping agent is selected from one or more of phthalic anhydride and 1,2,4-benzenetricarboxylic anhydride.

6. The preparation method of a polyamic acid-polyimide varnish according to claim 1, characterized in that, The second diamine is selected from one or more of 1,4'-phenylenediamine, 4,4-diaminodiphenyl ether, 4,4'-diaminobenzanilide, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, 2-(4-aminophenyl)-5-aminobenzoxazole, 4,4'-bis-4-(nitrophenoxy)biphenyl, 4,4'-diamino-2,2'-dimethylbiphenyl, 4,4'-diaminodiphenylmethane, 4,4'-diaminobenzophenone, and diaminodiphenyl sulfone; the auxiliary agent is selected from resorcinol bis(diphenyl phosphate), the dehydrating agent is selected from one of acetic anhydride and N,N'-dicyclohexylcarbodiimide, and the catalyst is selected from one of isoquinoline, triethylamine, and pyridine.

7. A polyamic acid - polyimide varnish is prepared by the preparation method of a polyamic acid - polyimide varnish according to any one of claims 1 - 6.

8. A polyimide film, characterized in that, The polyimide film is prepared from the polyamic acid - polyimide varnish according to claim 7.

9. A method for preparing a polyimide film, characterized in that, The specific preparation method is as follows: subject the polyamic acid - polyimide varnish according to claim 7 to defoaming, coating, desolventization treatment and imidization treatment, and a polyimide film can be obtained.

10. A method for preparing a polyimide film according to claim 9, characterized in that, The temperature of the desolventization treatment is 60 - 150 °C, and the time is 20 - 40 min; the temperature of the imidization treatment is 400 - 450 °C, and the time is 20 - 30 min; the thickness of the polyimide film is 12.5 μm; the modulus of the polyimide film is 2.9 - 3.4 GPa.

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