An air-cured high performance colorless transparent polyimide film

Abstract

The application discloses an air-cured high-performance colorless transparent polyimide film, which is prepared by the following method: aromatic tetracarboxylic dianhydride and fluorine-containing aromatic dianhydride are subjected to polymerization reaction with fluorene-containing aromatic diamine in a solvent to obtain polyimide, and the molar ratio of the aromatic tetracarboxylic dianhydride to the fluorine-containing aromatic dianhydride is (35-65):(65-35). The air-cured colorless transparent polyimide of the application meets the heat resistance performance under the premise of balancing high optical performance, and can be applied to the fields of semiconductor packaging process, OLED display screen, flexible solar cell and the like, and has wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of organic polymer materials technology, specifically relating to an air-cured high-performance colorless and transparent polyimide film and its preparation method. Background Technology

[0002] Polyimide is a polymer material synthesized from diamines and dianhydrides. Renowned for its excellent heat resistance, low-temperature resistance, flame retardancy, electrical properties, and mechanical properties, it is widely used in electronics, microelectronics, printed circuit boards, aerospace, optoelectronics, and many other fields. However, traditional polyimide films have a pyromellitic dianhydride-type molecular structure, with pyromellitic dianhydride and 4,4'-diaminodiphenyl ether as their main monomer raw materials. Due to the significant charge-transfer complex (CTC) effect in the molecular structure of polyimides made from these monomers, the resulting polyimide films are dark in color and have low visible light transmittance. This undoubtedly limits its application in fields requiring high transmittance, such as optical and optoelectronic devices, flexible / printed electronics, and semiconductor manufacturing. Furthermore, the preparation of colorless and transparent polyimide film materials typically involves curing under a protective atmosphere, which undoubtedly increases equipment costs and raises technical requirements.

[0003] Currently, in the field of colorless and transparent polyimides, modification strategies mainly focus on monomer selection to suppress the CTC effect during synthesis. Generally, selecting bulky side groups or fluorinated monomers can effectively suppress intramolecular or intermolecular coordination. For example, patent application number 201910758499.5 discloses a method using a Cardo structure (spirocyclic or bridged ring), introducing a compound of norbornene spirocyclic dianhydride (CPODA) and fluorene-containing diamine (FFDA), combined with aromatic diacid chlorides (such as terephthaloyl chloride). By increasing the interchain spacing and free volume, interchain stacking is hindered, reducing CTC formation. Furthermore, patent publication number CN105295046A discloses a polyimide obtained by reacting an aromatic diamine with a strong electron-withdrawing group containing fluorine atoms with an alicyclic dianhydride. Because it simultaneously uses an electron-withdrawing diamine and an alicyclic dianhydride as polymerizing monomers, it further suppresses the CTC effect and significantly improves the transparency of the polyimide.

[0004] However, simultaneously achieving high optical performance and heat resistance is challenging. Fluorine-containing groups or large-volume side groups introduced to suppress CTC-induced yellowing weaken intermolecular chain forces, leading to a decrease in glass transition temperature (Tg) or an increase in coefficient of thermal expansion (CTE), which may not meet the requirements of semiconductor packaging or LTPS processes. Furthermore, polyimide is prone to oxidation during high-temperature curing, resulting in a darker color and requiring a protective atmosphere, further increasing production costs. Therefore, developing high-performance air-cured polyimide film materials is essential. Summary of the Invention

[0005] This invention addresses the shortcomings of existing technologies by providing a high-performance, colorless, and transparent polyimide film that is air-cured. This invention achieves a balance between high optical performance and considerable heat resistance in air-cured colorless and transparent polyimide, making it applicable to semiconductor packaging processes, OLED displays, flexible solar cells, and other fields, with broad application prospects.

[0006] To achieve the above technical objectives, the present invention adopts the following technical solution: an air-cured high-performance colorless and transparent polyimide film is prepared by the following method: in a solvent, aromatic tetracarboxylic dianhydride and fluorinated aromatic dianhydride are polymerized with fluorene-containing aromatic diamine to obtain polyimide, wherein the molar ratio of aromatic tetracarboxylic dianhydride to fluorinated aromatic dianhydride is (35~65):(65~35).

[0007] Preferably, the aromatic tetracarboxylic dianhydride is selected from: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, 1,4,5,8-naphthalene tetracarboxylic dianhydride, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride, p-terphenyl-3,3'',4,4''-tetracarboxylic dianhydride, and 4,4'-bisphenol A diether tetracarboxylic dianhydride.

[0008] Preferably, the fluorinated aromatic dianhydride is selected from: 4,4'-(hexafluoroisopropyl)bisphthalic anhydride, 9-trifluoromethyl-2,3,6,7-oxanthracene tetracarboxylic anhydride, 9-pentafluoroethyl-2,3,6,7-oxanthracene tetracarboxylic anhydride, 2',3',5',6'-tetrafluoro[1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylic anhydride, spirocyclic dinorbornene tetracarboxylic anhydride, bis(trifluoromethyl)oxanthracene dianhydride, 4-trifluoromethoxyphenyl-3,3',4,4'-biphenyltetracarboxylic anhydride, 3,5-bis(trifluoromethyl)pyromellitic anhydride, perfluoroalkyl ether dianhydride, 9-(4-trifluoromethyl)pyromellitic anhydride, etc. 1,1-bis(3,4-dicarboxyphenyl)-oxanthracene dianhydride, 1,1-bis(3,4-dicarboxyphenyl)-1-(3-trifluoromethylphenyl)-2,2,2-trifluoroethane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, 4,4'-bis(3,4-dicarboxytrifluorophenoxy)biphenyl dianhydride, 2,3,6,7-tetrafluoronaphthalenetetracarboxylic dianhydride, bis(3,4-dicarboxy-2,5,6-trifluorophenyl)methane dianhydride, 4,4'-bis(3,4-dicarboxy-2-fluorophenoxy)diphenylsulfone dianhydride, 2-fluoropyromellitic dianhydride, 3,6-difluoronaphthalene-1,4,5,8-tetracarboxylic dianhydride, bis(3-fluoro-4,5-dicarboxyphenyl)ether dianhydride.

[0009] Preferably, the fluorenyl-containing aromatic diamine is selected from 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-amino-3-fluorophenyl)fluorene, 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene, 9,9-bis(3,5-dimethyl-4-aminophenoxyphenyl)fluorene, 9,9-bis(3,5-diethyl-4-aminophenyl)fluorene, 9,9-bis(3-tert-butyl-4-(4-aminophenoxy)phenyl)fluorene, 2,7-bis(4-aminophenyl)fluorene, 2,7-bis(4-aminophenoxy)fluorene, 9,9-bis(4-amino-3-trifluoromethylphenyl)fluorene, 9,9-bis(4-amino-2-methoxyphenyl)fluorene, 9,9-bis(3,5-diethyl-4-aminophenyl)fluorene, and 9,9-bis(3,5-diethyl-4-aminophenyl)fluorene. -di-tert-butyl-4-aminophenyl)fluorene, 9,9-bis(3-fluoro-4-aminophenyl)fluorene, 9,9-bis(4-amino-3-chlorophenyl)fluorene, 9,9-bis(4-amino-3-nitrophenyl)fluorene, 9,9-bis(3,5-difluoro-4-aminophenyl)fluorene, 9,9-bis(4-amino-2-methylphenyl)fluorene, 4,4'-(9,9-dimethylfluorene-2,7-diyl)diphenylamine, 4-(9,9-dimethyl-9H-fluorene-2-yl)aniline, 9,9-bis(3-phenyl-4-(4-amino-2-methylphenoxy)phenyl)fluorene, 9,9-bis(3,5-dimethoxy-4-(3-methyl-4-aminophenoxy)phenyl)fluorene.

[0010] Preferably, the solvent is selected from: N,N-Dimethylformamide, N,N-Dimethylacetamide, N-methyl-2-pyrrolidone, N-methylcaprolactam, 1,3-dimethylimidazolium ketone, tetramethylurea; Or lactone solvents such as γ-butyrolactone and γ-valerolactone, hexamethylphosphoramide, hexamethylphosphonic triamide; Or sulfur-containing solvents such as dimethyl sulfone, dimethyl sulfoxide, sulfolane, acetone, cyclohexanone, and methylcyclohexanone; Or methylpyridine, pyridine; Or (2-methoxy-1-methylethyl)acetate; Or phenol, o-cresol, m-cresol, p-cresol, 2,3-xylenol, 2,4-xylenol, 2,5-xylenol, 2,6-xylenol, 3,4-xylenol, 3,5-xylenol; Or 1,2-dimethoxyethane, bis(2-methoxyethyl) ether, 1,2-bis(2-methoxyethoxy)ethane, bis[2-(2-methoxyethoxy)ethyl] ether, tetrahydrofuran, 1,4-dioxane; Or diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate.

[0011] Preferably, the total molar ratio of the aromatic tetracarboxylic dianhydride and the fluorinated aromatic dianhydride to the diamine is 0.98-1.02:1.

[0012] As a preferred option, the following specific preparation steps are included: S1, under nitrogen atmosphere and ice-water bath cooling conditions, add solvent and fluorenyl aromatic diamine to the reaction vessel and stir until completely dissolved, while maintaining the system temperature at 0-5℃; S2, after mixing aromatic tetracarboxylic dianhydride and fluorinated aromatic dianhydride in proportion, and slowly adding the mixture into the reaction vessel in multiple portions while controlling the system temperature to not exceed 2℃, stir and react for 10-60 hours to obtain polyamic acid solution. S3, after removing impurities by filtering the obtained polyamic acid solution through a filter membrane, a diluent is added to control the solid content at 10-20 wt%; S4. A polyamic acid solution is coated onto a substrate, and after heating and imidization, the solution is cooled to room temperature and then peeled off from the substrate to obtain a colorless and transparent polyimide film.

[0013] Furthermore, before the polyamic acid solution undergoes the heated imidization treatment in step S4, it is allowed to stand under ventilation conditions to evaporate and remove excess solvent.

[0014] Furthermore, the imidization treatment in step S4 adopts a staged heating method: after heating to 80℃ and holding for 1 hour, the temperature is raised to 150℃ and held for 1 hour, then further heated to 200℃ and held for 1 hour, and finally heated to 250℃ and held for 1 hour, with a heating rate of 2℃ / min.

[0015] Beneficial effects: Compared with existing technologies, the colorless and transparent polyimide of the present invention comprises a copolymer backbone of fluorinated aromatic dianhydrides and rigid aromatic dianhydrides with fluorene-containing aromatic diamines. The strongly electron-withdrawing fluorinated groups and the bulky fluorene groups jointly suppress the CTC effect, reduce the generation of chromophores, and improve transmittance. Furthermore, the included rigid aromatic dianhydrides maintain the material's heat resistance to a certain extent. Moreover, it can be thermosetting in an air atmosphere, reducing equipment requirements and saving production costs. Detailed Implementation Plan

[0016] The present invention will be further illustrated below with specific synthetic examples. It should be understood that these synthetic examples are for illustrative purposes only and are not intended to limit the scope of the invention. After reading the present invention, any modifications of the present invention by those skilled in the art in various equivalent forms fall within the scope defined by the appended claims.

[0017] The present invention provides a high-performance colorless and transparent polyimide film that is air-cured. By using specific monomer selection and ratio, it can balance heat resistance and optical properties while ensuring that it maintains high transmittance during air curing.

[0018] To achieve the aforementioned objectives, this invention systematically studied the selection of monomers. The results showed that polyimides containing specific proportions of aromatic tetracarboxylic dianhydrides and fluorinated aromatic tetracarboxylic dianhydrides, along with a fluorene-containing aromatic diamine backbone, possess the properties required to satisfy the above-mentioned performance requirements and solve the problem. The dianhydrides used in this invention preferably include rigid and fluorinated aromatic tetracarboxylic dianhydrides, including 3,3',4,4'-biphenyltetracarboxylic dianhydride (35-65% molar weight of total dianhydrides) and 4,4'-(hexafluoroisopropylidene)phthalic anhydride (35-65% molar weight of total dianhydrides). The diamine includes fluorene-containing aromatic diamines, including 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene. The reactants involved in the preparation of the polyimide film of this invention include: (A) Dihydride The dianhydrides involved in this invention include aromatic tetracarboxylic acid dianhydrides such as: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, 1,4,5,8-naphthalene tetracarboxylic dianhydride, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride, p-terphenyl-3,3'',4,4''-tetracarboxylic dianhydride, and 4,4'-bisphenol A diether tetracarboxylic dianhydride.

[0019] Fluorinated aromatic tetracarboxylic dianhydrides include: 4,4'-(hexafluoroisopropyl)bisphthalic dianhydride, 9-trifluoromethyl-2,3,6,7-oxanthracene tetracarboxylic dianhydride, 9-pentafluoroethyl-2,3,6,7-oxanthracene tetracarboxylic dianhydride, 2',3',5',6'-tetrafluoro[1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylic dianhydride, spirocyclic dinorbornene tetracarboxylic dianhydride, bis(trifluoromethyl)oxanthracene dianhydride, 4-trifluoromethoxyphenyl-3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,5-bis(trifluoromethyl)pyromellitic dianhydride, perfluoroalkyl ether dianhydride, and 9-(4-trifluoromethylphenyl) 1,1-bis(3,4-dicarboxyphenyl)-1-(3-trifluoromethylphenyl)-2,2,2-trifluoroethane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, 4,4'-bis(3,4-dicarboxytrifluorophenoxy)biphenyl dianhydride, 2,3,6,7-tetrafluoronaphthalenetetracarboxylic dianhydride, bis(3,4-dicarboxy-2,5,6-trifluorophenyl)methane dianhydride, 4,4'-bis(3,4-dicarboxy-2-fluorophenoxy)diphenylsulfone dianhydride, 2-fluoropyromellitic dianhydride, 3,6-difluoronaphthalene-1,4,5,8-tetracarboxylic dianhydride, and bis(3-fluoro-4,5-dicarboxyphenyl)ether dianhydride.

[0020] (B) Diamine The diamines involved in this invention include at least one selected from the following: aromatic diamines containing a fluorene group and a benzene ring, such as 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-amino-3-fluorophenyl)fluorene, 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene, 9,9-bis(3,5-dimethyl-4-aminophenoxyphenyl)fluorene, 9,9-bis(3,5-diethyl-4-aminophenyl)fluorene, 9,9-bis(3-tert-butyl-4-(4-aminophenoxy)phenyl)fluorene, 2,7-bis(4-aminophenyl)fluorene, 2,7-bis(4-aminophenoxy)fluorene, 9,9-bis(4-amino-3-trifluoromethylphenyl)fluorene, and 9,9-bis(4-amino-2-methoxyphenyl)fluorene. 9,9-bis(3,5-di-tert-butyl-4-aminophenyl)fluorene, 9,9-bis(3-fluoro-4-aminophenyl)fluorene, 9,9-bis(4-amino-3-chlorophenyl)fluorene, 9,9-bis(4-amino-3-nitrophenyl)fluorene, 9,9-bis(3,5-difluoro-4-aminophenyl)fluorene, 9,9-bis(4-amino-2-methylphenyl)fluorene, 4,4'-(9,9-dimethylfluorene-2,7-diyl)diphenylamine, 4-(9,9-dimethyl-9H-fluorene-2-yl)aniline, 9,9-bis(3-phenyl-4-(4-amino-2-methylphenoxy)phenyl)fluorene, 9,9-bis(3,5-dimethoxy-4-(3-methyl-4-aminophenoxy)phenyl)fluorene.

[0021] (C) Solvent The organic solvents involved in this invention only need to be able to dissolve the polyamic acid produced without affecting the amidation reaction. Examples of such solvents include aprotic solvents, phenolic solvents, ether solvents, and carbonate solvents. Specific examples of aprotic solvents include amide solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, N-methylcaprolactam, 1,3-dimethylimidazolium ketone, and tetramethylurea; lactone solvents such as γ-butyrolactone and γ-valerolactone; phosphorus-containing amide solvents such as hexamethylphosphoramide and hexamethylphosphonic triamide; sulfur-containing solvents such as dimethyl sulfone, dimethyl sulfoxide, and sulfolane; ketone solvents such as acetone, cyclohexanone, and methylcyclohexanone; amine solvents such as methylpyridine and pyridine; and ester solvents such as (2-methoxy-1-methylethyl)acetate. Specific examples of phenolic solvents include phenol, o-cresol, m-cresol, p-cresol, 2,3-xylenol, 2,4-xylenol, 2,5-xylenol, 2,6-xylenol, 3,4-xylenol, and 3,5-xylenol. Specific examples of ether solvents include 1,2-dimethoxyethane, bis(2-methoxyethyl) ether, 1,2-bis(2-methoxyethoxy)ethane, bis[2-(2-methoxyethoxy)ethyl] ether, tetrahydrofuran, and 1,4-dioxane. Specific examples of carbonate solvents include diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, and propylene carbonate. Among the above-mentioned reaction solvents, amide solvents or lactone solvents are preferred. Furthermore, the above-mentioned reaction solvents can be used alone or in mixtures of two or more.

[0022] The colorless and transparent polyimide of this invention is polymerized from the aforementioned dianhydride and diamine in a certain proportion in a certain amount of the aforementioned solvent. The colorless and transparent polyimide film is obtained from the polymerized polyamic acid. Specifically, under a nitrogen atmosphere and ice-water bath cooling (temperature controlled at 0-5℃), the diamine and a polar aprotic organic solvent are first placed in a reaction vessel and stirred until completely dissolved. The dianhydride is then slowly added in batches, maintaining the system temperature below 2℃ during the addition process. After the addition is complete, the reaction is continued at room temperature for 10-60 hours, ultimately yielding a viscous polyamic acid solution. The polyamic acid solution is uniformly coated onto a clean glass substrate, followed by a stepped-heat imidization treatment. After staged solvent removal, imidization is completed. After cooling, the film is peeled off from the substrate to obtain a polyimide film of a certain thickness.

[0023] Example 1

[0024] In a 1000 mL open glass reactor under nitrogen protection, 0.06 mol (22.47 g) of 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene and 450 mL of anhydrous N-methylpyrrolidone were added. The mixture was stirred in an ice-water bath until completely dissolved, and the system temperature was lowered to 0-5 °C. Subsequently, a dianhydride mixture (a mixture of 0.036 mol of 4,4'-(hexafluoroisopropylidene) phthalic anhydride and 0.024 mol of 3,3',4,4'-biphenyltetracarboxylic dianhydride, totaling 25.65 g) was added in four portions, with a 1-hour interval between each addition. After stirring at low temperature for 5 hours, the reaction continued at room temperature for 24 hours. The system viscosity stabilizes and obvious stringing occurs, yielding a polyamic acid solution. Impurities are removed by filtration through a filter membrane, and N-methylpyrrolidone is added to dilute the solution to a solid content of 10-20 wt%. The solution is then uniformly coated onto a clean glass substrate using a spin coater and allowed to stand at room temperature in a fume hood for 30 minutes to allow initial solvent evaporation. The coated substrate is then transferred to an oven and subjected to step-heating thermal imidization (80℃ / 1h → 150℃ / 1h → 200℃ / 1h → 250℃ / 1h, heating rate 2℃ / min). After natural cooling to room temperature, a colorless, transparent polyimide film S1 with a thickness of approximately 25-30 μm is peeled off from the glass plate.

[0025] Example 2

[0026] In a 1000 mL open glass reactor under nitrogen protection, add 0.06 mol (31.96 g) of 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene and 450 mL of anhydrous N-methylpyrrolidone. Stir until completely dissolved under ice-water bath conditions, then lower the system temperature to 0-5 °C. Subsequently, add a dianhydride mixture (a mixture of 0.03 mol of 4,4'-(hexafluoroisopropylidene) phthalic anhydride and 0.03 mol of 3,3',4,4'-biphenyltetracarboxylic dianhydride, totaling 24.32 g) in four portions, with each addition spaced 1 hour apart. After stirring at low temperature for 5 hours, continue the reaction at room temperature for 24 hours until... The system viscosity stabilizes and obvious stringing occurs, yielding a polyamic acid solution. Impurities are removed by filtration through a filter membrane, and N-methylpyrrolidone is added to dilute the solution to a solid content of 10-20 wt%. The solution is then uniformly coated onto a clean glass substrate using a spin coater and allowed to stand at room temperature in a fume hood for 30 minutes to allow initial solvent evaporation. The coated substrate is then transferred to an oven and subjected to step-heating thermal imidization (80℃ / 1h → 150℃ / 1h → 200℃ / 1h → 250℃ / 1h, heating rate 2℃ / min). After natural cooling to room temperature, a colorless, transparent polyimide film S2 with a thickness of approximately 25-30 μm is peeled off from the glass plate.

[0027] Example 3

[0028] In a 1000 mL open glass reactor under nitrogen protection, 0.06 mol (31.96 g) of 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene and 450 mL of anhydrous N-methylpyrrolidone were added. The mixture was stirred in an ice-water bath until completely dissolved, and the system temperature was lowered to 0-5 °C. Subsequently, a dianhydride mixture (a mixture of 0.024 mol of 4,4'-(hexafluoroisopropylidene) phthalic anhydride and 0.036 mol of 3,3',4,4'-biphenyltetracarboxylic dianhydride, totaling 22.98 g) was added in four portions, with each addition spaced 1 hour apart. After stirring at low temperature for 5 hours, the reaction continued at room temperature for 24 hours. The system viscosity stabilizes and obvious stringing occurs, yielding a polyamic acid solution. Impurities are removed by filtration through a filter membrane, and N-methylpyrrolidone is added to dilute the solution to a solid content of 10-20 wt%. The solution is then uniformly coated onto a clean glass substrate using a spin coater and allowed to stand at room temperature in a fume hood for 30 minutes to allow initial solvent evaporation. The coated substrate is then transferred to an oven and subjected to step-heating thermal imidization (80℃ / 1h → 150℃ / 1h → 200℃ / 1h → 250℃ / 1h, heating rate 2℃ / min). After natural cooling to room temperature, a colorless, transparent polyimide film S3 with a thickness of approximately 25-30 μm is peeled off from the glass plate.

[0029] Comparative Example 1: In a 1000 mL open glass reactor under nitrogen protection, 0.06 mol (31.96 g) of 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene and 450 mL of anhydrous N-methylpyrrolidone were added. The mixture was stirred in an ice-water bath until completely dissolved, and the system temperature was lowered to 0-5 °C. Then, 0.06 mol (30.98 g) of 4,4'-(hexafluoroisopropylidene) phthalic anhydride was added in four portions, with a 1-hour interval between each addition. After stirring at low temperature for 5 hours, the reaction continued at room temperature for 24 hours until the system viscosity stabilized and obvious stringing occurred, yielding the desired product. Polyamic acid solution; the solution is filtered through a filter membrane to remove impurities, and N-methylpyrrolidone is added to dilute it to a solid content of 10-20 wt%. The solution is then uniformly coated onto a clean glass substrate using a spin coater and left to stand at room temperature in a fume hood for 30 minutes to allow the solvent to evaporate initially. The coated substrate is then transferred to an oven and subjected to step-heating thermal imidization (80℃ / 1h→150℃ / 1h→200℃ / 1h→250℃ / 1h, heating rate 2℃ / min). After naturally cooling to room temperature, a colorless and transparent polyimide film A1 with a thickness of approximately 25-30 μm is obtained by peeling it off from the glass plate.

[0030] Comparative Example 2: In a 1000 mL open glass reactor under nitrogen protection, 0.06 mol (31.96 g) of 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene and 450 mL of anhydrous N-methylpyrrolidone were added. The mixture was stirred in an ice-water bath until completely dissolved, and the system temperature was lowered to 0-5 °C. Then, 0.06 mol (17.65 g) of 3,3',4,4'-biphenyltetracarboxylic dianhydride was added in four portions, with a 1-hour interval between each addition. After stirring at low temperature for 5 hours, the reaction continued at room temperature for 24 hours until the system viscosity stabilized and obvious stringing occurred, yielding polyamide. Amino acid solution; filter the solution through a filter membrane to remove impurities, add N-methylpyrrolidone to dilute to a solid content between 10-20 wt%, and coat it evenly on a clean glass substrate using a spin coater. Let it stand at room temperature in a fume hood for 30 minutes to allow the solvent to evaporate initially. Then transfer the coated substrate to an oven and perform step-heating thermal imidization (80℃ / 1h→150℃ / 1h→200℃ / 1h→250℃ / 1h, heating rate 2℃ / min). After naturally cooling to room temperature, peel it off from the glass plate to obtain a colorless transparent polyimide film A2 with a thickness of about 25-30 μm.

[0031] The colorless and transparent polyimide films prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to performance tests, and the test methods are as follows: Optical performance evaluation: a) Transmittance Assessment: Cut the polyimide films prepared in the examples and comparative examples into flat samples ≥50mm × 50mm, with the thickness controlled at the target application value of 25-30µm, avoiding scratches or contamination. Use a UV-Vis spectrophotometer, with air as a reference (100% transmission baseline), to calibrate the wavelength range, ensuring the beam is perpendicularly incident on the sample. Fix the film to the sample holder, avoiding tension or wrinkles. Scan the wavelength range of 380-780nm and record the transmittance (T), taking the average of three measurements. Standard reference: ASTM D1003-13.

[0032] b) Haze Assessment: The polyimide films prepared in the examples and comparative examples are sampled according to the transmittance requirements. Using a haze meter, the haze (unit: %) is calculated by measuring the "scattered light flux / total transmitted light flux". Typically, three tests are performed and the average value is taken. Standard reference: ASTM D1003-13.

[0033] c) Yellowness Index Assessment: The polyimide films prepared in the examples and comparative examples were sampled according to the transmittance requirements. A colorimeter was used for testing, following the ASTM E313-15 standard procedure: First, the instrument was calibrated with a white board and a black board (or air) to ensure stability. Transmission mode was used. Samples were cleaned and equilibrated before testing. During measurement, the sample was fixed on the sample holder, and measurements were repeated at 3-5 points at different locations. The average value was taken as the result. The test conditions were a D65 light source, a 10° observation angle, room temperature (23±2℃), and relative humidity (50±5)%. The yellowness index is calculated based on the material's reflection or transmission characteristics in the red, green, and blue bands of visible light. The intensity of the yellow hue is quantified using tristimulus values ​​(X, Y, Z). The calculation formula (based on the ASTM E313-15 standard) is: YI=100(1.28X−1.06Z) / Y X, Y, and Z are the tristimulus values ​​specified by the CIE (International Commission on Illumination), and Y also represents the lightness of the material.

[0034] Thermal performance evaluation: d) Glass transition temperature (Tg): Cut the polyimide films prepared in the examples and comparative examples into uniform fragments, accurately weigh them (5-10 mg) using an analytical balance, place them in an aluminum crucible, and seal it with a pressure cap. Use a differential scanning calorimeter, first calibrate the instrument, control the flow rate under a nitrogen atmosphere to prevent oxidation, then perform the heating program, record the heating curve, and take the midpoint temperature according to ASTM D3418-15 standard. Perform the test 3 times and take the average value.

[0035] e) Coefficient of thermal expansion: The transparent polyimide films of the examples and comparative examples were cut into strips 20 mm long and 5 mm wide, and the initial gauge length (L0) of the samples was measured and recorded using precision calipers. The samples were then dried in a vacuum oven at 80°C for 2 hours to remove surface moisture and cooled to room temperature. During instrument calibration, the thermomechanical analyzer was calibrated using quartz standard samples according to ASTM E831-19. The pre-treated strip samples were fixed at both ends to the sample holders, keeping the samples naturally straight and avoiding stretching or relaxation. Nitrogen gas was introduced to purge air before testing. The test parameters were set as follows: a small constant load of 0.05-0.1 N was applied, the heating rate was 5°C / min, and the test temperature range was 50°C to 50°C below Tg. After starting the instrument, the temperature was held at 50°C for 10 min, and then increased at the set rate. The length change (ΔL) at different temperatures was recorded. The coefficient of thermal expansion (CTE) was calculated using the formula CTE = (ΔL / (L0×ΔT))×10. 6 The coefficient of thermal expansion was calculated in ppm / ℃. The average value of three tests was taken.

[0036] Mechanical performance evaluation: f) Tensile Strength: Dumbbell-shaped samples were cut from the prepared polyimide film, conforming to the ASTM D882-18 standard. The gauge length (test section) was 25 mm long and 6 mm wide, while the clamping section was 15 mm wide and 15 mm long. Before testing, the electronic universal testing machine was calibrated to ensure the accuracy of the force and displacement sensors. The sample was clamped vertically in the fixture, ensuring the narrow neck section was centered and free from twisting. The tensile rate was set to 50 mm / min, and an initial gauge length of 20 mm was marked. The testing machine was started to apply axial tension, and the maximum load at break was recorded (breakage at the narrow neck section is valid). After the test, the tensile strength was calculated using the formula: Tensile Strength = F / S. Where F is the maximum tensile force at break, and S is the initial cross-sectional area of ​​the gauge length (S = gauge length width × gauge length thickness).

[0037] The viscosity, solid content, and performance parameters of the colorless and transparent polyimide films prepared in Examples 1-3 and Comparative Examples 1-2 are shown in Tables 1 and 2, respectively. Table 1

[0038] Table 2

[0039] As shown in Tables 1 and 2, all polyimide samples were prepared by thermal imidization of polyamic acid in air. The polyimide films in the examples exhibited excellent optical properties, as well as high heat resistance and mechanical properties, making them suitable for use as flexible substrates in optoelectronic devices. In contrast, the polyimide films synthesized using a single dianhydride in the comparative examples had at least one defect.

[0040] This invention provides an air-curable, high-performance, colorless polyimide formulation. By employing a rigid aromatic dianhydride monomer to ensure heat resistance, it introduces a copolymer of fluorinated aromatic dianhydride and a fluorene-containing aromatic diamine to further suppress the CTC effect, improve optical properties, and synergistically achieve excellent overall performance. Crucially, the copolymer can undergo thermal imidization in air, which is more conducive to practical production.

[0041] Based on the above-described ideal synthesis example according to the present invention, and through the above description, those skilled in the art can make various changes and modifications without departing from the technical concept of the present invention. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A high-performance, colorless, transparent polyimide film that is air-cured, characterized in that... The polyimide is prepared by the following method: in a solvent, aromatic tetracarboxylic dianhydride and fluorinated aromatic dianhydride are polymerized with fluorenyl-containing aromatic diamine to obtain polyimide, wherein the molar ratio of aromatic tetracarboxylic dianhydride to fluorinated aromatic dianhydride is (35~65):(65~35).

2. The high-performance colorless and transparent polyimide film with air curing according to claim 1, characterized in that: The aromatic tetracarboxylic dianhydride is selected from: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, 1,4,5,8-naphthalene tetracarboxylic dianhydride, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride, p-terphenyl-3,3'',4,4''-tetracarboxylic dianhydride, and 4,4'-bisphenol A diether tetracarboxylic dianhydride.

3. The high-performance colorless and transparent polyimide film with air curing according to claim 1, characterized in that: The fluorinated aromatic dianhydride is selected from: 4,4'-(hexafluoroisopropyl)bisphthalic anhydride, 9-trifluoromethyl-2,3,6,7-oxanthracene tetracarboxylic anhydride, 9-pentafluoroethyl-2,3,6,7-oxanthracene tetracarboxylic anhydride, 2',3',5',6'-tetrafluoro[1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylic anhydride, spirocyclic dinorbornene tetracarboxylic anhydride, bis(trifluoromethyl)oxanthracene dianhydride, 4-trifluoromethoxyphenyl-3,3',4,4'-biphenyltetracarboxylic anhydride, 3,5-bis(trifluoromethyl)pyromellitic anhydride, perfluoroalkyl ether dianhydride, 9-(4-trifluoromethylphenyl) 1,1-bis(3,4-dicarboxyphenyl)-1-(3-trifluoromethylphenyl)-2,2,2-trifluoroethane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, 4,4'-bis(3,4-dicarboxytrifluorophenoxy)biphenyl dianhydride, 2,3,6,7-tetrafluoronaphthalenetetracarboxylic dianhydride, bis(3,4-dicarboxy-2,5,6-trifluorophenyl)methane dianhydride, 4,4'-bis(3,4-dicarboxy-2-fluorophenoxy)diphenylsulfone dianhydride, 2-fluoropyromellitic dianhydride, 3,6-difluoronaphthalene-1,4,5,8-tetracarboxylic dianhydride, and bis(3-fluoro-4,5-dicarboxyphenyl)ether dianhydride.

4. The high-performance colorless and transparent polyimide film cured by air according to claim 1, characterized in that: The fluorene-containing aromatic diamine is selected from 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-amino-3-fluorophenyl)fluorene, 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene, 9,9-bis(3,5-dimethyl-4-aminophenoxyphenyl)fluorene, 9,9-bis(3,5-diethyl-4-aminophenyl)fluorene, 9,9-bis(3-tert-butyl-4-(4-aminophenoxy)phenyl)fluorene, 2,7-bis(4-aminophenyl)fluorene, 2,7-bis(4-aminophenoxy)fluorene, 9,9-bis(4-amino-3-trifluoromethylphenyl)fluorene, 9,9-bis(4-amino-2-methoxyphenyl)fluorene, 9,9-bis(3,5-diethyl-4-amino ... tert-butyl-4-aminophenyl)fluorene, 9,9-bis(3-fluoro-4-aminophenyl)fluorene, 9,9-bis(4-amino-3-chlorophenyl)fluorene, 9,9-bis(4-amino-3-nitrophenyl)fluorene, 9,9-bis(3,5-difluoro-4-aminophenyl)fluorene, 9,9-bis(4-amino-2-methylphenyl)fluorene, 4,4'-(9,9-dimethylfluorene-2,7-diyl)diphenylamine, 4-(9,9-dimethyl-9H-fluorene-2-yl)aniline, 9,9-bis(3-phenyl-4-(4-amino-2-methylphenoxy)phenyl)fluorene, 9,9-bis(3,5-dimethoxy-4-(3-methyl-4-aminophenoxy)phenyl)fluorene.

5. The high-performance colorless and transparent polyimide film cured by air according to claim 1, characterized in that: The solvent is selected from: N,N-Dimethylformamide, N,N-Dimethylacetamide, N-methyl-2-pyrrolidone, N-methylcaprolactam, 1,3-dimethylimidazolium ketone, tetramethylurea; Or lactone solvents such as γ-butyrolactone and γ-valerolactone, hexamethylphosphoramide, hexamethylphosphonic triamide; Or sulfur-containing solvents such as dimethyl sulfone, dimethyl sulfoxide, sulfolane, acetone, cyclohexanone, and methylcyclohexanone; Or methylpyridine, pyridine; Or (2-methoxy-1-methylethyl)acetate; Or phenol, o-cresol, m-cresol, p-cresol, 2,3-xylenol, 2,4-xylenol, 2,5-xylenol, 2,6-xylenol, 3,4-xylenol, 3,5-xylenol; Or 1,2-dimethoxyethane, bis(2-methoxyethyl) ether, 1,2-bis(2-methoxyethoxy)ethane, bis[2-(2-methoxyethoxy)ethyl] ether, tetrahydrofuran, 1,4-dioxane; Or diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate.

6. The high-performance colorless and transparent polyimide film cured by air according to claim 1, characterized in that: The total molar ratio of the aromatic tetracarboxylic dianhydride and the fluorinated aromatic dianhydride to the diamine is 0.98-1.02:

1.

7. The high-performance colorless and transparent polyimide film with air curing according to claim 1, characterized in that: The specific preparation steps include the following: S1, under nitrogen atmosphere and ice-water bath cooling conditions, add solvent and fluorenyl aromatic diamine to the reaction vessel and stir until completely dissolved, while maintaining the system temperature at 0-5℃; S2, after mixing aromatic tetracarboxylic dianhydride and fluorinated aromatic dianhydride in proportion, and slowly adding the mixture into the reaction vessel in multiple portions while controlling the system temperature to not exceed 2℃, stir and react for 10-60 hours to obtain polyamic acid solution. S3, after removing impurities by filtering the obtained polyamic acid solution through a filter membrane, a diluent is added to control the solid content at 10-20 wt%; S4. A polyamic acid solution is coated onto a substrate, and after heating and imidization, the solution is cooled to room temperature and then peeled off from the substrate to obtain a colorless and transparent polyimide film.

8. The high-performance colorless and transparent polyimide film cured by air according to claim 7, characterized in that: Before the polyamic acid solution undergoes the heating imidization treatment in step S4, it is allowed to stand under ventilation conditions to evaporate and remove excess solvent.

9. The high-performance colorless and transparent polyimide film with air curing according to claim 8, characterized in that: In step S4, the imidization treatment is carried out in stages: the temperature is raised to 80℃ and held for 1 hour, then raised to 150℃ and held for 1 hour, then raised to 200℃ and held for 1 hour, and finally raised to 250℃ and held for 1 hour. The heating rate is 2℃ / min.

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