Preparation method of soluble micro-crosslinking type end-capped transparent heat-resistant polyimide film

Abstract

The invention discloses a preparation method of a soluble micro-crosslinking type end-capped transparent heat-resistant polyimide film, and relates to the technical field of polyimide materials. The cross-linking agent and the linear polyimide resin with an ammonia end group or an anhydride end group are subjected to micro-cross-linking reaction, the cross-linking degree is controlled by adjusting the type, the dosage and the reaction time of the cross-linking agent, and the prepared polyimide resin has a micro-cross-linking structure; by adding the multifunctional cross-linking agent containing three or more amino groups, the polyimide resin has high reaction activity, and a cross-linking reaction can be quickly carried out at room temperature; fluorine-containing aromatic diamine and a dianhydride monomer are selected to react with a specific cross-linking agent to prepare micro-cross-linked polyimide resin, the resin has good solubility in an organic solvent, and the soluble micro-cross-linked transparent polyimide film can be obtained by heat treatment under the vacuum condition of not more than 220 DEG C; the film has the characteristics of high transmittance and low yellowness.

Description

Technical Field

[0001] The invention relates to the technical field of polyimide materials, and in particular to a method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film. Background Art

[0002] In recent years, the rapid development of flexible display technology has brought about the innovation of flexible substrate materials, which are used in folding devices, wearable devices, transparent devices, and flexible solar cells. The development of these fields has also brought greater demand for polymer-based flexible substrate materials and higher requirements on performance. They are not only expected to have a low linear thermal expansion coefficient, but also to have better performance in terms of transparency and mechanical properties, so as to improve their stability and reliability during operation. Traditional polyimide is a type of polymer obtained by the polymerization of diamine and dianhydride. It has a strong rigid conjugated aromatic structure skeleton and strong intermolecular force, and its heat resistance and mechanical properties are excellent. However, due to its strong intermolecular and intramolecular electron complex effect (CTC), traditional polyimide appears light yellow or dark brown, which limits its application in the field of optical films. In the application of flexible materials, the demand for high transmittance, high mechanical properties and high heat resistance of optoelectronic devices is increasing, and research on new optical films with high transmittance and high heat resistance is also receiving more and more attention. Among them, colorless and transparent polyimide has the advantages of high transmittance and high heat resistance, and is currently one of the best materials for preparing flexible substrates.

[0003] Nowadays, it is known that there are many structural design methods to prepare transparent polyimide, such as introducing fluorine-containing structures, asymmetric structures, bulky side groups, etc. to suppress the CTC effect. However, the above methods usually cause the mechanical properties and heat resistance of polyimide films to decrease, such as becoming brittle, not resistant to bending, and reducing the thermal decomposition temperature. Therefore, it is very important to achieve a balance between optical properties, heat resistance, and mechanical properties. However, it is very difficult to improve the optical transparency and high temperature resistance of polyimide at the same time, because the improvement of the former performance often leads to a decrease in the latter performance. At the same time, improving the processability of polyimide is also important for optimizing the preparation method of transparent polyimide film. Therefore, it is of great significance to prepare polyimide with solubility, high heat resistance, high transparency and low thermal expansion performance, and it is a difficult point in technological development. Summary of the invention

[0004] The invention provides a method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film to solve the problem that the heat resistance, optical properties and low thermal expansion properties of the polyimide film cannot be taken into account at the same time.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film comprises the following steps:

[0007] Step 1, adding a fluorine-containing aromatic diamine monomer and a dianhydride monomer to an organic solvent at -5°C to 5°C and under the protection of an inert gas atmosphere, reacting for 5 to 20 minutes, adding a crosslinking agent, and continuing the reaction for 2 to 6 hours to obtain a polyamic acid resin solution with a solid content of 10wt.% to 30wt.%;

[0008] Step 2, after adjusting the reaction temperature to room temperature, adding a catalyst and a dehydrating agent to the polyamic acid resin solution, and performing a dehydration reaction for 6 to 12 hours; then adding an amino cage-type silsesquioxane, reacting for 8 to 12 hours, and then precipitating and drying to obtain a slightly cross-linked polyimide resin with an end-capped structure;

[0009] Step 3, dissolving the slightly cross-linked polyimide resin obtained in step 2 in an organic solvent, controlling the solid content to be 10wt.% to 30wt.%, obtaining a slightly cross-linked end-capped polyimide resin solution, and filtering with a needle filter membrane to obtain a homogeneous slightly cross-linked end-capped polyimide resin solution;

[0010] Step 4: coating the homogeneous micro-crosslinked end-capped polyimide resin solution obtained in step 3 on a glass substrate, and heat-treating the solution under vacuum to obtain a micro-crosslinked end-capped transparent heat-resistant polyimide film.

[0011] Furthermore, in step 1, the molar ratio of the fluorine-containing aromatic diamine monomer to the dianhydride monomer is m:n=(0.9-1.06):1, and m≠n (excluding the case of m:n=1:1).

[0012] Furthermore, in step 1, the cross-linking agent is a multifunctional cross-linking agent containing three or more amino groups.

[0013] Preferably, the multifunctional crosslinking agent is one or more of 4,4',4"-(1,3,5-triazine-2,4,6-triyl)triphenylamine, octa(aminopropyl)silsesquioxane, melamine, and tri(4-aminophenyl)amine.

[0014] The structural formula of the above-mentioned multifunctional cross-linking agent is as follows:

[0015]

[0016] Further, in step 1, the fluorine-containing aromatic diamine monomer is one or more of 2,2'-bis(trifluoromethyl)diaminobiphenyl, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 4,4'-diaminophenyl sulfone, 4,4'-diaminodiphenyl ether, and 4,4'-diaminophenyl sulfone, and the structural formula is as follows:

[0017]

[0018] Further, in step 1, the dianhydride monomer is one or more of pyromellitic anhydride, 1,2,3,4-cyclobutanetetracarboxylic anhydride, 1,2,3,4-cyclobutanetetracarboxylic anhydride, 1,4,5,8-naphthalenetetracarboxylic anhydride, 4,4′-(hexafluoroisopropylene) diphthalic anhydride, 3,3,4,4-diphenylsulfonetetracarboxylic anhydride, and 2,3,6,7-anthracenetetracarboxylic anhydride, and the structural formula is as follows:

[0019]

[0020] Furthermore, in step 1, the organic solvent is at least one of N-methylpyrrolidone (NMP), γ-butyrolactone (GBL), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO).

[0021] Furthermore, in step 2, the catalyst is one or more of quinoline, isoquinoline, triethylamine, and pyridine, and the structural formula is as follows:

[0022]

[0023] Furthermore, in step 2, the dehydrating agent is one or more of acetic anhydride, propionic anhydride, and butyric anhydride, and the structural formula is as follows:

[0024]

[0025] Furthermore, in step 2, the amino cage silsesquioxane is one or both of aminopropyl isobutyl silsesquioxane and octaaminopropyl silsesquioxane, and the structural formula is as follows:

[0026]

[0027] Furthermore, in step 3, the organic solvent is at least one of N-methylpyrrolidone (NMP), γ-butyrolactone (GBL), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO).

[0028] Furthermore, in step 1, the solid content of the cross-linking agent is 1 wt.% to 5 wt.%.

[0029] Furthermore, in step 2, the molar amount of the catalyst is 2 to 4 times the molar amount of the fluorinated aromatic diamine monomer; the molar amount of the dehydrating agent is 2 to 4 times the molar amount of the fluorinated aromatic diamine monomer; and the solid content of the amino cage silsesquioxane is 1 wt.% to 5 wt.%.

[0030] Furthermore, in step 2, the precipitation treatment process is: adding the reaction solution to a precipitant for precipitation treatment, and soaking the obtained precipitate in deionized water to remove by-products, wherein the precipitant is water, or a mixture of water and at least one of methanol, ethanol, propanol, butanol, and isopropanol; the drying process is: heating the precipitated micro-cross-linked polyimide resin in a forced air drying oven at 120° C. for 12 to 24 hours.

[0031] Furthermore, in step 4, the heat treatment process is: first vacuum drying at 60°C for 2 to 4 hours, then vacuum drying at 100°C for 1 to 4 hours, then vacuum drying at 200°C for 1 to 4 hours, and finally vacuum drying at 180 to 250°C for 1 to 3 hours.

[0032] The micro-crosslinked end-capped transparent heat-resistant polyimide film prepared by the present invention has excellent optical properties, mechanical properties and heat resistance. The film thickness is 50±2μm, the average transmittance in the range of 380-780nm is greater than 88%, the yellowness index is less than 3, the tensile strength is greater than 150MPa, the elastic modulus is greater than 4GPa, and the thermal expansion coefficient is as low as 17.36ppm / K.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] (1) The present invention uses a cross-linking agent to react with a linear polyimide resin having amino terminal groups or anhydride terminal groups to carry out a micro-cross-linking reaction. The type, amount and reaction time of the cross-linking agent are adjusted to control the degree of cross-linking. The prepared polyimide resin has a micro-cross-linking structure (i.e., local cross-linking). By adding a specific cross-linking agent (a multifunctional cross-linking agent containing three or more amino groups), the polyimide resin has a higher reactivity and can undergo a cross-linking reaction quickly at room temperature.

[0035] (2) The present invention prepares a micro-crosslinked polyimide resin by selecting a specific fluorine-containing aromatic diamine and dianhydride monomer to react with a specific crosslinking agent. The resin has good solubility in organic solvents. Therefore, in the process of preparing the film, heat treatment is performed under vacuum conditions not exceeding 220°C to obtain a soluble micro-crosslinked transparent polyimide film, avoiding the problem of film oxidation coloring caused by traditional thermal crosslinking reaction and high-temperature treatment of thermal imidization reaction (above 300°C). The film has the characteristics of high transmittance and low yellowness. In terms of mechanical properties, by controlling the degree of crosslinking, the problem of increased film brittleness and reduced tensile strength caused by excessive crosslinking is avoided.

[0036] (3) The present invention can increase the molecular distance, weaken the van der Waals force between molecular chains, and increase the free volume of molecules by performing a micro-crosslinking reaction to form a micro-crosslinking structure. Due to its steric hindrance, the micro-branched PI molecular chain needs to absorb more energy to move, which is manifested as an increase in the glass transition temperature on a macro scale. After the end-capping agent (amino cage silsesquioxane) is introduced, the interaction between molecules is weakened and the free volume is increased. After the cross-linking network is formed, the molecular chain can be effectively anchored under the synergistic effect of the cross-linking agent and the end-capping agent (amino cage silsesquioxane), thereby reducing the CTE value, improving the optical transmittance of the polyimide film, increasing its solubility in organic solvents, and limiting the excessive cross-linking of the polyimide molecular chain.

[0037] (4) The synthesis method of the present invention is simple and has strong applicability. The micro-crosslinked capped polyimide film has excellent thermal expansion performance, high temperature resistance, strong mechanical properties, high and stable transmittance, and the curing process does not require a high temperature process above 220°C, and is suitable for industrial large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 The following are hydrogen nuclear magnetic resonance spectra of the polyimide films prepared in the examples; wherein (a) corresponds to examples 1 to 3, and (b) corresponds to examples 1, 4 and 5;

[0039] Figure 2 is the Fourier transform infrared spectrum of the polyimide film prepared in Example;

[0040] Figure 3 Graph showing the optical properties of the polyimide film obtained in the embodiment;

[0041] Figure 4 : is the stress-strain diagram of the polyimide film prepared in Example. DETAILED DESCRIPTION

[0042] In order to make the technical scheme of the present invention more clearly understood by those skilled in the art, the present invention is further described in detail below through the accompanying drawings and embodiments. It should be noted that the given embodiments cannot be understood as limiting the present invention. Non-essential improvements and adjustments made by relevant technical personnel to the present invention based on the content of the present invention should still fall within the scope of protection of the present invention.

[0043] The percentage content in the present invention is by weight unless otherwise specified. The methods are conventional methods unless otherwise specified. The raw materials can be obtained from commercial sources unless otherwise specified. The thickness of the polyimide film in the embodiment can be controlled by the distance between the scraper and the substrate or the solid content of the solution.

[0044] Example 1

[0045] (1) Add 3.202 g (10 mmol) of 2,2'-bis(trifluoromethyl)diaminobiphenyl (TFMB) and 33.6 ml of N,N-dimethylacetamide (DMAc) into a three-necked flask, and stir under nitrogen protection until completely dissolved; lower the temperature to 0°C, add 4.575 g (10.3 mmol) of 4,4'-(hexafluoroisopropylene)diphthalic anhydride (6FDA), and continue to react at the same temperature for 4 hours after dissolution; add 4.083 g (40 mmol) of acetic anhydride and 2.695 g (34 mmol) of pyridine, dehydrate and react at room temperature for 12 hours, then pour the obtained 20 wt.% linear polyimide resin solution into a large amount of ethanol for precipitation, and wash the precipitate three times with deionized water, ethanol and methanol in sequence, and finally dry it at 120°C in vacuum;

[0046] (2) Add 0.2 g of polyimide resin and 0.8 g of DMAc solution into a three-necked flask to obtain a linear polyimide resin solution with a solid content of 20 wt.%. After the solution is filtered through a needle filter to remove impurities, it is coated on a glass substrate with a smooth surface, and the thickness of the solution on the glass substrate is controlled with a scraper. In a vacuum, it is kept at 60°C for 1 hour, 100°C for 1 hour, 200°C for 1 hour, and 220°C for 1 hour. The obtained glass substrate is placed in 30°C warm water for demolding. After demolding, it is washed with alcohol several times and placed in a 60°C drying oven for 24 hours to obtain a PI film. The polyimide film is named CPI-1.

[0047] Example 2

[0048] (1) Add 3.202 g (10 mmol) of 2,2'-bis(trifluoromethyl)diaminobiphenyl (TFMB) and 33.6 ml of N,N-dimethylacetamide (DMAc) to a three-necked flask and stir until completely dissolved under nitrogen protection; then lower the temperature to 0°C and add 4.575 g (10.3 mmol) of 4,4'-(hexafluoroisopropylene) diphthalic anhydride (6FDA). After dissolution, continue the reaction at the same temperature for 15 minutes, then add 0.10 6 g (0.3 mmol) of 4,4',4"-(1,3,5-triazine-2,4,6-triyl)triphenylamine, and continue to react for 5 hours; after adding 4.083 g (40 mmol) of acetic anhydride and 2.695 g (34 mmol) of pyridine, dehydration reaction was carried out at room temperature for 12 hours, and the obtained 20 wt.% polyimide resin solution was poured into a large amount of ethanol for precipitation, and the precipitate was washed three times with deionized water, ethanol and methanol, and finally vacuum dried at 120°C;

[0049] (2) Add 0.2 g of micro-cross-linked polyimide resin and 0.8 g of DMAc solution into a three-necked flask to obtain a micro-cross-linked polyimide resin solution with a solid content of 20 wt.%. After the solution is filtered through a needle filter to remove impurities, it is coated on a glass substrate with a smooth surface, and the thickness of the solution on the glass substrate is controlled with a scraper. In a vacuum, it is kept at 60°C for 2 h, 100°C for 1 h, 200°C for 1 h, and 220°C for 1 h. The obtained glass substrate is placed in 30°C warm water for demolding. After demolding, it is washed with alcohol several times and placed in a 60°C drying oven for 24 h to obtain a PI film. The polyimide film is named CPI-2.

[0050] Example 3

[0051] (1) Add 3.202 g (10 mmol) of 2,2'-bis(trifluoromethyl)diaminobiphenyl (TFMB) and 33.6 ml of N,N-dimethylacetamide (DMAc) into a three-necked flask and stir under nitrogen until completely dissolved; then lower the temperature to 0°C and add 4.575 g (10.3 mmol) of 4,4'-(hexafluoroisopropylene) diphthalic anhydride (6FDA). After dissolution, continue the reaction at the same temperature for 15 minutes, then add 0.106 g (0.3 mmol) of 4,4',4"-(1,3,5 -triazine-2,4,6-triyl) triphenylamine, and continue to react for 5 hours; after adding 4.083 grams (40 mmol) of acetic anhydride and 2.695 grams (34 mmol) of pyridine, after dehydration reaction at room temperature for 12 hours, 0.236 grams (0.27 mmol) of aminopropyl isobutyl silsesquioxide was added, and the reaction was continued for 10 hours at room temperature, and the obtained 20wt.% slightly cross-linked end-capped polyimide resin solution was poured into a large amount of ethanol for precipitation, and the precipitate was washed three times with deionized water, ethanol and methanol, and finally vacuum dried at 120°C;

[0052] (2) Add 0.2 g of micro-cross-linked end-capped polyimide resin and 0.8 g of DMAc solution into a three-necked flask to obtain a micro-cross-linked end-capped polyimide resin solution with a solid content of 20 wt.%. After the solution is filtered through a needle filter to remove impurities, it is coated on a glass substrate with a smooth surface, and the thickness of the solution on the glass substrate is controlled with a scraper. In a vacuum, it is kept at 60°C for 2 h, 100°C for 1 h, 200°C for 1 h, and 220°C for 1 h. The obtained glass substrate is placed in 30°C warm water for demolding. After demolding, it is washed with alcohol several times and placed in a 60°C drying oven for 24 h to obtain a PI film. The polyimide film is named CPI-3.

[0053] Example 4

[0054] (1) Add 3.202 g (10 mmol) of 2,2'-bis(trifluoromethyl)diaminobiphenyl (TFMB) and 33.6 ml of N,N-dimethylacetamide (DMAc) into a three-necked flask, and stir under nitrogen protection until completely dissolved; reduce the temperature to 0°C, add 4.575 g (10.3 mmol) of 4,4'-(hexafluoroisopropylene)diphthalic anhydride (6FDA), continue to react at the same temperature for 15 minutes after dissolution, and then add 0.2341 g (0.03 mmol) of octa(aminopropyl)silsesquioxane; add 4.083 g (40 mmol) of acetic anhydride and 2.695 g (34 mmol) of pyridine, dehydrate and react at room temperature for 12 hours, then pour the obtained 20 wt.% slightly cross-linked polyimide resin solution into a large amount of ethanol for precipitation, and wash the precipitate with deionized water, ethanol and methanol three times, and finally dry it at 120°C in vacuum;

[0055] (2) Add 0.2 g of micro-crosslinked polyimide resin and 0.8 g of DMAc solution into a three-necked flask to obtain a micro-crosslinked polyimide resin solution with a solid content of 20 wt.%. After the solution is filtered through a needle filter to remove impurities, it is coated on a glass substrate with a smooth surface, and the thickness of the solution on the glass substrate is controlled with a scraper. In a vacuum, it is kept at 60°C for 2 h, 100°C for 1 h, 200°C for 1 h, and 220°C for 1 h. The obtained glass substrate is placed in 30°C warm water for demolding. After demolding, it is washed with alcohol several times and placed in a 60°C drying oven for 24 h to obtain a PI film. The polyimide film is named CPI-4.

[0056] Example 5

[0057] (1) Add 3.204 g (10 mmol) of 2,2'-bis(trifluoromethyl)diaminobiphenyl (TFMB) and 33.6 ml of N,N-dimethylacetamide (DMAc) into a three-necked flask and stir under nitrogen until completely dissolved; then lower the temperature to 0°C and add 4.575 g (10.3 mmol) of 4,4'-(hexafluoroisopropylene) diphthalic anhydride (6FDA). After the mixture is dissolved, continue the reaction at the same temperature for 15 minutes, then add 0.234 g (0.03 mmol) of octa(amino)benzene. propyl) silsesquioxane, and continue to react for 6 hours; after adding 4.083 grams (40mmol) of acetic anhydride and 2.695 grams (34mmol) of pyridine, after dehydration reaction at room temperature for 12 hours, 0.236 grams (0.27mmol) of aminopropyl isobutyl silsesquioxane was added, and the reaction was continued for 10 hours at room temperature, and the obtained 20wt.% slightly cross-linked end-capped polyimide resin solution was poured into a large amount of ethanol for precipitation, and the precipitate was washed three times with deionized water, ethanol and methanol, and finally vacuum dried at 120°C;

[0058] (2) Add 0.2 g of micro-cross-linked end-capped polyimide resin and 0.8 g of DMAc solution into a three-necked flask to obtain a micro-cross-linked end-capped polyimide resin solution with a solid content of 20 wt.%. After the solution is filtered through a needle filter to remove impurities, it is coated on a glass substrate with a smooth surface, and the thickness of the solution on the glass substrate is controlled with a scraper. In a vacuum, it is kept at 60°C for 2 h, 100°C for 1 h, 200°C for 1 h, and 220°C for 1 h. The obtained glass substrate is placed in 30°C warm water for demolding. After demolding, it is washed with alcohol several times and placed in a 60°C drying oven for 24 h to obtain a PI film. The polyimide film is named CPI-5.

[0059] Table 1. Thermal and mechanical properties of micro-crosslinked transparent polyimide films *

[0060]

[0061] Table 2. Optical properties of slightly cross-linked transparent polyimide films

[0062]

[0063] The tensile properties of the film were measured using a universal material testing machine according to ASTMD882;

[0064] The optical properties of the films were measured using a UV-visible spectrophotometer and a colorimeter. av It is the average light transmittance in the wavelength range of 380-780nm. The yellowness index (YI) and turbidity (Haze) are measured according to ASTM E313 standard.

[0065] From the data in Tables 1 and 2, it can be seen that the thermal and mechanical properties of the polyimide film after adding a crosslinking agent and end-capping are improved while maintaining considerable optical properties.

[0066] According to the above examples and test results, it can be seen that the slightly cross-linked end-capped heat-resistant transparent polyimide film material of the present invention has significantly improved thermal and mechanical properties, and retains excellent optical properties.

[0067] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film, characterized in that: The following steps are involved: Step 1, adding a fluorine-containing aromatic diamine monomer and a dianhydride monomer to an organic solvent at -5°C to 5°C and under inert gas protection, reacting for 5 to 20 minutes, adding a crosslinking agent, and continuing the reaction for 2 to 6 hours to obtain a polyamic acid resin solution with a solid content of 10wt.% to 30wt.%; Step 2, after adjusting the reaction temperature to room temperature, adding a catalyst and a dehydrating agent to the polyamic acid resin solution, and performing a dehydration reaction for 6 to 12 hours; then adding an amino cage-type silsesquioxane, reacting for 8 to 12 hours, and then precipitating and drying to obtain a slightly cross-linked polyimide resin with an end-capped structure; Step 3, dissolving the slightly cross-linked polyimide resin obtained in step 2 in an organic solvent, controlling the solid content to be 10wt.% to 30wt.%, obtaining a slightly cross-linked end-capped polyimide resin solution, and filtering with a needle filter membrane to obtain a homogeneous slightly cross-linked end-capped polyimide resin solution; Step 4: coating the homogeneous micro-crosslinked end-capped polyimide resin solution obtained in step 3 on a substrate, and heat-treating the solution under vacuum to obtain a micro-crosslinked end-capped transparent heat-resistant polyimide film.

2. The method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film according to claim 1, characterized in that: In step 1, the molar ratio of the fluorine-containing aromatic diamine monomer to the dianhydride monomer is m:n=(0.9-1.06):1, and m≠n.

3. The method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film according to claim 1, characterized in that: In step 1, the cross-linking agent is a multifunctional cross-linking agent containing three or more amino groups.

4. The method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film according to claim 3, characterized in that: The multifunctional crosslinking agent is one or more of 4,4',4"-(1,3,5-triazine-2,4,6-triyl)triphenylamine, octa(aminopropyl)silsesquioxane, melamine, and tri(4-aminophenyl)amine.

5. The method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film according to claim 1, characterized in that: In step 1, the fluorine-containing aromatic diamine monomer is one or more of 2,2'-bis(trifluoromethyl)diaminobiphenyl, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 4,4'-diaminophenyl sulfone, 4,4'-diaminodiphenyl ether, and 4,4'-diaminophenyl sulfone.

6. The method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film according to claim 1, characterized in that: In step 1, the dianhydride monomer is one or more of pyromellitic anhydride, 1,2,3,4-cyclobutanetetracarboxylic anhydride, 1,2,3,4-cyclobutanetetracarboxylic anhydride, 1,4,5,8-naphthalenetetracarboxylic anhydride, 4,4′-(hexafluoroisopropylene) diphthalic anhydride, 3,3,4,4-diphenylsulfonetetracarboxylic anhydride, and 2,3,6,7-anthracenetetracarboxylic anhydride.

7. The method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film according to claim 1, characterized in that: In step 2, the catalyst is one or more of quinoline, isoquinoline, triethylamine, and pyridine; the dehydrating agent is one or more of acetic anhydride, propionic anhydride, and butyric anhydride; and the amino cage-type silsesquioxane is one or two of aminopropyl isobutyl silsesquioxane and octaaminopropyl silsesquioxane.

8. The method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film according to claim 1, characterized in that: In step 1, the solid content of the cross-linking agent is 1 wt.% to 5 wt.%.

9. The method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film according to claim 1, characterized in that: In step 2, the molar amount of the catalyst is 2 to 4 times the molar amount of the fluorinated aromatic diamine monomer; the molar amount of the dehydrating agent is 2 to 4 times the molar amount of the fluorinated aromatic diamine monomer; and the solid content of the amino cage silsesquioxane is 1 wt.% to 5 wt.%.

10. The method for preparing a soluble slightly cross-linked end-capped transparent heat-resistant polyimide film according to claim 1, characterized in that: In step 4, the heat treatment process is: first vacuum drying at 60°C for 2 to 4 hours, then vacuum drying at 100°C for 1 to 4 hours, then vacuum drying at 200°C for 1 to 4 hours, and finally vacuum drying at 180 to 250°C for 1 to 3 hours.

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