A method for preparing a low-thermal expansion coefficient colorless transparent polyimide composite film

Abstract

This invention discloses a method for preparing a colorless and transparent polyimide composite film with a low coefficient of thermal expansion. The film is prepared in a solvent by combining polyamic acid and inorganic nanoparticles. The polyamic acid is synthesized by polycondensation of a rigid alicyclic dianhydride and a fluorinated aromatic dianhydride with a sulfone-containing aromatic diamine. The inorganic nanoparticles are zirconium tungstate nanoparticles. This invention uses a specific type of monomer to polymerize polyamic acid, and then disperses the added nanoparticles in an organic solvent for further composite preparation with the polyamic acid to form a polyimide film. This film has broad application prospects in semiconductor packaging processes, OLED displays, flexible solar cells, and other fields.

Description

Technical Field

[0001] This invention belongs to the field of organic polymer materials technology, specifically relating to a colorless and transparent polyimide composite film with a low coefficient of thermal expansion and its preparation method. Background Technology

[0002] Polyimide possesses excellent heat resistance, mechanical properties, dielectric properties, and chemical stability, making it an irreplaceable high-performance polymer material in many fields. However, traditional polyimide, due to the conjugated aromatic ring structure in its main chain, easily forms intramolecular / intermolecular charge-transfer complexes (CTCs), resulting in strong absorption in the visible light region and a brownish-yellow color (low transmittance), which cannot meet the transparency requirements of optical devices or transparent packaging. Furthermore, heat resistance is also crucial for colorless and transparent polyimide to meet the demands of various high-temperature processes and maintain long-term reliability and dimensional stability. The balance between heat resistance and colorability has always been a key challenge for polyimide, and finding a solution to this problem is currently urgent.

[0003] Currently, in polyimide synthesis, achieving high transmittance mainly involves introducing large-volume substituents, fluorinated groups, flexible groups, asymmetric structures, alicyclic structures, and non-coplanar structures into the molecular chain. Among strategies for achieving a low coefficient of thermal expansion, adding nanoparticles is one of the most effective methods. Patent publication number CN117304690A proposes a polyimide composite film doped with inorganic nanoparticles, obtained by combining polyimide resin and AlOOH inorganic nanoparticles. The hydrogen bonds and physical crosslinking between the polyimide resin and AlOOH inorganic nanoparticles effectively restrict the thermal motion of polymer molecules, reducing the linear coefficient of thermal expansion of the film. Patent publication number CN110372895A proposes a method of uniformly dispersing nano-SiO2 in a polyamic acid solution via a sol-gel method, followed by thermal imidization to form a composite film. SiO2 forms hydrogen bonds and chemical bonds with the polyimide molecular chain, significantly reducing the coefficient of thermal expansion, and maintaining a transmittance of over 85% after selecting specific monomers.

[0004] However, existing high-performance polyimide synthesis methods have many shortcomings. Simply improving heat resistance or optical properties is insufficient to meet the needs of high-end applications, and the complex synthesis processes of some materials are not conducive to industrialization. Therefore, there is a need to develop a polyimide film that is relatively easy to synthesize and possesses both high heat resistance and optical properties, thereby meeting market demands from multiple perspectives. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing a colorless and transparent polyimide composite film with a low coefficient of thermal expansion. The prepared polyimide composite film has a high glass transition temperature, a low coefficient of thermal expansion, and excellent light transmittance, and can be applied in semiconductor packaging processes, OLED displays, flexible solar cells, and other fields, showing broad application prospects.

[0006] To achieve the above technical objectives, the present invention adopts the following technical solution: a method for preparing a colorless and transparent polyimide composite film with a low coefficient of thermal expansion, wherein the film is prepared in a solvent by compositing polyamic acid and inorganic nanoparticles, wherein the polyamic acid is obtained by polycondensation of a rigid alicyclic dianhydride and a fluorinated aromatic dianhydride with a sulfone-containing aromatic diamine, and the inorganic nanoparticles are zirconium tungstate nanoparticles.

[0007] Preferably, the rigid alicyclic dianhydride is selected from: cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, bicyclo(2,2,2)-7-octene-2,3,5,6-tetracarboxylic dianhydride, bicyclo(2.2,2)octane-2,3,5,6-tetracarboxylic acid 2,3:5,6-dianhydride, decahydronaphthalenetetracarboxylic dianhydride, isomerized dicyclohexyltetracarboxylic dianhydride, norbornene-2-spirocyclic-α-cyclopentanone-α'-spirocyclic-2''-norbornane- One or more of the following: 5,5'',6,6''-tetracarboxylic dianhydride, 1,4-cyclohexanetetracarboxylic dianhydride, hydrogenated pyromellitic dianhydride, hydrogenated biphenyltetracarboxylic dianhydride, hexahydro-4,7-methylene-1H-inden-1,3,5,7-tetracarboxylic dianhydride, 5-norbornene-2,3-dicarboxylic dianhydride, bicyclo[3.3.0]octane-2,4,6,8-tetracarboxylic dianhydride, 2R,5R,7S,10S-naphthalenetetracarboxylic dianhydride, and adamantanetetracarboxylic dianhydride. Due to the rigid alicyclic structure, the rigid alicyclic dianhydride in this invention imparts excellent heat resistance and dimensional stability to the materials during the synthesis of high-performance polymers such as polyimides.

[0008] Preferably, the fluorinated aromatic dianhydride is selected from: 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, 3,6-bis(trifluoromethyl)-pyromellitic anhydride, 3-trifluoromethyl-6-methylpyromellitic anhydride, 2,2-bis(3,4-dicarboxyphenyl dianhydride)hexafluoropropane, 1-phenyl-1,1-bis(3,4-dicarboxyphenyl dianhydride)trifluoroethane, 1-(3,5-bistrifluoromethylphenyl)-1,1-bis(3,4-dicarboxyphenyl dianhydride)trifluoroethane, 2,2-bis(4-(3,4-dicarboxyphenoxy)phenyl)hexafluoropropane dianhydride, 2,6-bis(3'-... One or more of the following: (4'-dicarboxyphenyl)-4-(3'',5''-bis(trifluoromethylphenyl)pyridine dianhydride, 1,4-bis(trifluoromethyl)-2,3,5,6-benzenetetracarboxylic acid dianhydride, 4,4'-oxybis(3-(trifluoromethyl)phthalic anhydride), 4,4'-oxybis(3-(oxytrifluoromethyl)phthalic anhydride), 4,4'-oxybis(3-fluorophthalic anhydride), 3,3',4,4'-tetrafluorobiphenyltetracarboxylic dianhydride, 3,3',4,4'-tetrafluorobenzophenone tetracarboxylic dianhydride, and 3,3',4,4'-tetrafluorodiphenyl ether tetracarboxylic dianhydride. In this invention, fluorinated aromatic dianhydrides, due to the fluorinated groups they contain, can impart better optical properties to polyimide materials.

[0009] Preferably, the sulfone-containing aromatic diamine is selected from: 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, bis(aminophenoxyphenyl) sulfone, 2,4'-trifluoromethylaminophenoxy)diphenyl sulfone, 2,2'-bis(3,5-bis(4-trifluoromethylphenyl)-4-(4-aminophenoxy)phenyl) sulfone, 2,2'-bis(3-(4-trifluoromethylphenyl)-4-(3-aminophenoxy)phenyl) sulfone, 2,2'-bis{3-(3,5-bis(trifluoromethyl)phenyl)-4-(3-aminophenoxy)phenyl} sulfone, 4,4'-diamino-3,3'-dimethyldiphenyl sulfone, 4,4'-diamino-2,2'-dimethyldiphenyl sulfone, 4,4'-diamino-3,3'-diethoxydiphenyl sulfone, 4,4'-diamino-2, 2'-Diethoxydiphenyl sulfone, 4,4'-Diamino-3-fluorodiphenyl sulfone, 4,4'-Diamino-2-fluorodiphenyl sulfone, 4,4'-Diamino-3-chlorodiphenyl sulfone, 4,4'-Diamino-2-chlorodiphenyl sulfone, 4,4'-Diamino-3,3'-dinitrodiphenyl sulfone, 4,4'-Diamino-2,2'-dinitrodiphenyl sulfone, 4,4'-Diamino-3,3'-dibromodiphenyl sulfone, 4 4,4'-Diamino-2,2'-dibromodiphenyl sulfone, 4,4'-bis(4-aminophenoxy)diphenyl sulfone, 4,4'-bis(3-aminophenoxy)diphenyl sulfone, 3,3'-bis(4-aminophenoxy)diphenyl sulfone, 3,4'-bis(4-aminophenoxy)diphenyl sulfone, 2,2'-dimethyl-4,4'-diaminodiphenyl sulfone, 3,3'-dimethoxy-4,4'-diaminodiphenyl sulfone. Sulfone-containing aromatic diamines contain strongly polar sulfone groups and aromatic rings, which can impart excellent thermal stability to polyimides when polymerized with dianhydrides.

[0010] Preferably, the solvent is selected from: N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), 1,3-dimethyl-2-imidazolinone (DMI), γ-butyrolactone (GBL), sulfolane, or dimethyl sulfoxide (DMSO). The solvent is preferably N-methylpyrrolidone (NMP) or N,N-dimethylacetamide (DMAc).

[0011] Preferably, the added zirconium tungstate nanoparticles account for 1-15% of the polyamic acid.

[0012] Furthermore, the method for preparing a colorless and transparent polyimide composite film with a low coefficient of thermal expansion includes the following steps: S1, under nitrogen atmosphere and ice-water bath cooling conditions, add solvent and sulfone-containing aromatic diamine into the reaction vessel, stir until completely dissolved, and control the system temperature at 0-5℃; S2, add rigid alicyclic dianhydride and fluorinated aromatic dianhydride in batches slowly, keeping the system temperature below 2°C during the addition process; after the addition is complete, stir and react at room temperature for 20-30 hours to obtain a viscous polyamic acid solution; S3, add zirconium tungstate nanoparticles to the polyamic acid solution obtained in step S2 according to the mass ratio, stir and disperse evenly to obtain a polyamic acid composite solution with a solid content of 10-25%; S4. The polyamic acid composite solution obtained in step S4 is coated on the substrate and subjected to imidization treatment at a higher temperature. After imidization, the solvent is removed, and after cooling, the solution is peeled off from the substrate to obtain a colorless and transparent polyimide composite film.

[0013] Furthermore, the total amount of the rigid alicyclic dianhydride and the fluorinated aromatic dianhydride added is in a molar ratio of 0.98 to 1.02:1 to the sulfone-containing aromatic diamine; the molar ratio between the rigid alicyclic dianhydride and the fluorinated aromatic dianhydride is 0.98 to 1.02:1.

[0014] Furthermore, the zirconium tungstate nanoparticles are prepared by the following method: (1) Add cyclohexane, n-hexanol and polyethylene glycol nonylphenyl ether to the reaction vessel and stir to mix evenly; (2) Hydrated ammonium metatungstate is dissolved in distilled water to obtain a transparent and homogeneous aqueous solution of ammonium metatungstate, and hydrated zirconium dichloride is dissolved in distilled water to obtain a transparent and homogeneous aqueous solution of zirconium dichloride. The two are mixed by dropwise addition to form a transparent and colorless premixed solution. (3) Slowly drop the premixed solution obtained in step (2) into the reaction vessel of step (1) and stir rapidly for at least 30 minutes to obtain a transparent and colorless reaction product; (4) After the reaction continues for at least 3 hours, anhydrous ethanol is added to the reaction system to obtain a white turbid suspension; after stirring and centrifuging for more than 1 hour, a white gel-like precipitate is obtained. (5) After repeatedly washing the obtained white gel-like precipitate with anhydrous ethanol, it was transferred to a container, distilled water was added to make a slurry, and then hydrochloric acid solution was added. The reaction solution was refluxed for 24-36 hours, and the white solution was centrifuged to obtain the white precipitate. (6) After washing the white precipitate with water several times, it was aged at room temperature for at least three days. The aged precipitate was then calcined in air at 600±20℃ for 6-12 hours to obtain a yellowish-white ZrW2O8 precipitate. (7) Yellow-white ZrW2O8 precipitate was ultrasonically treated in N-methylpyrrolidone (NMP) solvent for more than 2 hours to depolymerize the clusters. After removing the NMP solvent, it was washed multiple times with tetrahydrofuran. The obtained solid product was dried at 100±40℃ under vacuum. Under a nitrogen atmosphere, the dried ZrW2O8 was dispersed in NMP solvent and 3-aminopropyltriethoxysilane was added. The mixture was stirred at 50±5℃ for more than 12 hours. The precipitate obtained by cooling and centrifugation was washed and then dried under vacuum to complete the process.

[0015] Beneficial Effects: Compared with existing technologies, this invention introduces inorganic nanoparticles as additives into polyamic acid to prepare polyimide nanocomposite films with excellent comprehensive performance, specifically exhibiting high transparency and low coefficient of thermal expansion. The interaction between polyimide and inorganic nanoparticles breaks down the organic-inorganic interface, achieving a combination of inorganic and organic materials. Zirconium tungstate, as a negative expansion material, has a certain limiting and offsetting effect on the thermal expansion of polyimide. Rigid alicyclic rings and sulfone groups restrict the movement of polyimide molecular chains, improving the material's heat resistance. Simultaneously, the introduced non-planar structure breaks conjugation and generates steric hindrance, and the strong electron-withdrawing fluorine-containing groups further suppress the CTC effect, ensuring optical performance. Under this composite strategy, the prepared polyimide film exhibits excellent performance in all aspects, especially with a significantly reduced coefficient of thermal expansion, providing promising applications in flexible display substrates and optoelectronic devices. 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 core technical route of the method for preparing a colorless and transparent polyimide composite film with a low coefficient of thermal expansion provided by this invention lies in the following: it is prepared in a solvent by combining polyamic acid and inorganic nanoparticles. The polyamic acid is formed by the condensation polymerization of a rigid alicyclic dianhydride and a fluorinated aromatic dianhydride with a sulfone-containing aromatic diamine. The inorganic nanoparticles are zirconium tungstate nanoparticles. The structural formulas of the fluorinated aromatic dianhydride, the rigid alicyclic dianhydride, and the sulfone-containing aromatic diamine in this embodiment are shown below: , , .

[0018] The preferred amount of nano-sized zirconium tungstate added in this invention is between 1% and 15%, and the preparation method is as follows: I. Synthesis of Nanoscale Zirconium Tungstate (ZrW₂O₈) (1) Preparation of nano-sized zirconium tungstate ZrW2O8: Nanoscale ZrW₂O₈ was prepared using a combination of sol-gel and reverse micelle synthesis. The following reagents were added to a 500 mL round-bottom flask and stirred rapidly: 160 mL cyclohexane, 7 mL n-hexanol, and 20.0 mL polyethylene glycol nonylphenyl ether. 4.09 g of ammonium metatungstate hydrate was dissolved in 7 mL of distilled water to form a clear, colorless solution. 2.8 g of zirconium dichloride hydrate was dissolved in 3.5 mL of distilled water to prepare a clear, colorless solution. These two aqueous solutions were simultaneously and slowly added dropwise to form a clear, colorless premixed solution. This premixed solution was added dropwise to the aforementioned 500 mL reaction flask and stirred rapidly. The reaction solution immediately became turbid. After 10 minutes of reaction, the system separated into a clear, colorless liquid phase and a white precipitate. After approximately 1 hour of reaction, the precipitate dissolved, and the entire reaction solution became clear and colorless. After 4 hours of reaction, 100 mL of anhydrous ethanol was added to the flask, and the reaction solution immediately formed a white, turbid suspension. After stirring for another hour, centrifugation yielded a white, gel-like precipitate. The gel-like precipitate was suspended and washed five times with anhydrous ethanol, then transferred to a 500 mL round-bottom flask. A slurry was prepared using 100 mL of distilled water, and 100 mL of 6M hydrochloric acid was added to the flask. The reaction solution was refluxed for 48 hours. The flask was then cooled to room temperature, and the white solution was centrifuged to obtain a white precipitate. The precipitate was washed three times with 15 mL of water and aged at room temperature for one week. The aged precipitate was calcined at 600°C in air for 10 hours to obtain a yellowish-white ZrW₂O₈ precipitate. The prepared ZrW₂O₈ had a particle size between 150-350 nm in length and 50-100 nm in width, smaller than the visible light wavelength, and its addition to polyimide did not affect its total transmittance.

[0019] (2) Surface treatment of ZrW2O8: The synthesized ZrW₂O₈ exhibits extremely poor inherent affinity with the aromatic polyimide matrix, necessitating interface modification through functionalization to improve their interaction. Considering the presence of free hydroxyl groups on the surface of the synthesized inorganic oxide, which readily react with silanes, a short-chain aliphatic silane—3-aminopropyltriethoxysilane—was chosen for functionalization of the ZrW₂O₈ surface. Its ethoxy groups react with the free hydroxyl groups on the ZrW₂O₈ surface, introducing free NH₂ groups. These NH₂ groups can serve as reaction sites, binding with anhydride-terminated polyimide oligomer chains, thereby enhancing the affinity and interaction with the polyimide matrix.

[0020] The specific method is as follows: ZrW₂O₈ was ultrasonically treated in NMP for 4 hours to depolymerize the clusters. After centrifugation to remove NMP, it was washed repeatedly with tetrahydrofuran. The solid product was dried at 100°C under vacuum for 48 hours. Approximately 2g of dried ZrW₂O₈ was dispersed in 25mL of NMP (nitrogen atmosphere), and 1.5mL of 3-aminopropyltriethoxysilane was added to the flask. The mixture was kept at a constant temperature of 50°C and stirred overnight under nitrogen protection. After cooling to room temperature, it was centrifuged. The precipitate was washed repeatedly with tetrahydrofuran and acetone and centrifuged. The final product was dried at 100°C under vacuum for 48 hours.

[0021] II. Preparation of Colored Transparent Polyimide Composite Films The invention will be described in more detail below with reference to the following examples. However, these examples are provided to illustrate the invention, and the scope of the invention is not limited thereto. In the following synthesis examples, the system viscosity was monitored by rotational viscometry, by immersing a specific rotor in the reaction system, setting the rotational speed, and directly reading the viscosity value.

[0022] Example 1

[0023] In a 1000 mL open glass reactor under nitrogen protection, add 0.08 mol (19.84 g) of 4,4'-diaminodiphenyl sulfone and 500 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 0.04 mol 4,4'-(hexafluoroisopropylidene) phthalic anhydride and 0.04 mol norbornene-2-spirocyclic-α-cyclopentanone-α'-spirocyclic-2''-norbornane-5,5'',6,6''-tetracarboxylic acid dianhydride, with masses of 20.48 and 16.88 g respectively) in four separate additions. After a 1-hour interval, the mixture was stirred at low temperature for 5 hours and then reacted at room temperature for another 24 hours. Then, 1.6 mmol (0.82 g) of 4,4'-(hexafluoroisopropylidene) phthalic anhydride was added, and the mixture was stirred for another 4 hours until the viscosity of the system stabilized. After standing for 30 minutes, the solution was filtered through a filter membrane to remove impurities, and a polyamic acid solution was obtained for later use. 5% by mass of surface-modified nano-ZrW2O8 was uniformly dispersed in the polyamic acid solution and stirred for 6 hours. Subsequently, anhydrous N-methylpyrrolidone was added for dilution to ensure that the solid content of the final solution was within 10-20%. After sonication for 30 minutes, the final mixed polyamic acid solution was obtained.

[0024] The mixed polyamic acid solution was uniformly coated onto a clean glass substrate using a spin coater. In a nitrogen-protected oven, the substrate was heated to 50-80℃ for 1 hour, then to 120-150℃ for 1 hour, 200-220℃ for 1 hour, 250℃ for 2 hours, and finally to 280℃ for 1 hour. The heating rate at each stage was controlled at 2℃ / min. After natural cooling to room temperature under nitrogen protection, a colorless, transparent polyimide film with a thickness of approximately 25-30 μm was peeled off from the glass substrate. This film is labeled S1.

[0025] Example 2

[0026] The synthesis method for polyamic acid was the same as in Example 1, except for the change in the ratio of the mixed dianhydrides. The dianhydride mixture (a mixture of 0.024 mol 4,4'-(hexafluoroisopropylidene)phthalic anhydride and 0.056 mol norbornene-2-spirocyclic-α-cyclopentanone-α'-spirocyclic-2''-norbornane-5,5'',6,6''-tetracarboxylic acid dianhydride, with masses of 12.29 g and 23.63 g respectively) was added in four separate additions. After coating and stepwise thermal imidization, a colorless and transparent polyimide film with a thickness of approximately 25-30 μm was obtained. This film is labeled S2.

[0027] Example 3

[0028] The synthesis method for polyamic acid was the same as in Example 1, except for the change in the ratio of the mixed dianhydrides. The dianhydride mixture (a mixture of 0.056 mol 4,4'-(hexafluoroisopropylidene)phthalic anhydride and 0.024 mol norbornene-2-spirocyclic-α-cyclopentanone-α'-spirocyclic-2''-norbornane-5,5'',6,6''-tetracarboxylic acid dianhydride, with masses of 28.67 g and 10.12 g respectively) was added in four separate additions. After coating and stepwise thermal imidization, a colorless and transparent polyimide film with a thickness of approximately 25-30 μm was obtained. This film is labeled S3.

[0029] Example 4

[0030] The method for synthesizing polyamic acid is the same as in Example 1, except that 10% by mass of nano-ZrW2O8 is added. After coating and stepwise thermal imidization, a colorless and transparent polyimide film with a thickness of approximately 25-30 μm is obtained. This film is labeled S4.

[0031] Comparative Example 1: The method for synthesizing polyamic acid is the same as in Example 1, except that only 0.08 mol of 4,4'-(hexafluoroisopropylidene) phthalic anhydride, weighing 40.96 g, is added. After coating and stepwise thermal imidization, a colorless and transparent polyimide film with a thickness of approximately 25-30 μm is obtained. This film is labeled A1.

[0032] Comparative Example 2: The synthesis method for polyamic acid was the same as in Example 1, except that only 0.08 mol of norbornene-2-spirocyclic-α-cyclopentanone-α'-spirocyclic-2''-norbornane-5,5'',6,6''-tetracarboxylic acid dianhydride was added, with a mass of 33.76 g. After coating and stepwise thermal imidization, a colorless and transparent polyimide film with a thickness of approximately 25-30 μm was obtained. This film is labeled A2.

[0033] Comparative Example 3: The method for synthesizing polyamic acid is the same as in Example 1, except that nano-ZrW2O8 is not added. After coating and stepwise thermal imidization, a colorless and transparent polyimide film with a thickness of approximately 25-30 μm is obtained. This film is labeled A3.

[0034] The following table shows the reaction components and their contents in each embodiment and comparative example.

[0035] Table 1

[0036] III. Performance Testing The colorless and transparent polyimide films prepared in Examples 1-4 and Comparative Examples 1-3 were tested for optical, thermal, and mechanical properties, respectively. The test methods are as follows: (1) Optical performance evaluation Transmittance Evaluation: Cut the polyimide films prepared in the examples and comparative examples into flat samples ≥50 mm × 50 mm, with the thickness controlled at the target application value of 25-30 μm, avoiding scratches or contamination. Using a UV-Vis spectrophotometer, with air as a reference (100% transmission baseline), 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 380-780 nm and record the transmittance (T), taking the average of three measurements. Standard Reference: ASTM D1003-13.

[0037] 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.

[0038] 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, quantifying the intensity of the yellow hue through tristimulus values ​​(X, Y, Z). The calculation formula (based on the ASTM E313-15 standard) is: YI=100(1.28X−1.06Z) / Y Where 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.

[0039] (2) Thermal performance evaluation Glass transition temperature (Tg): The polyimide films prepared in the examples and comparative examples were cut into uniform fragments, accurately weighed (5-10 mg) using an analytical balance, and placed in an aluminum crucible, which was then sealed with a pressure cap. A differential scanning calorimeter was used. The instrument was first calibrated, and under a nitrogen atmosphere, the flow rate was controlled to prevent oxidation. The heating program was then executed, and the heating curve was recorded. The midpoint temperature was taken according to ASTM D3418-15 standard. The test was performed three times, and the average value was taken.

[0040] Thermal expansion coefficient: 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 an 80°C vacuum oven 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 them 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.

[0041] (3) Mechanical performance evaluation Tensile Strength: Dumbbell-shaped samples were cut from the prepared polyimide film, conforming to ASTM D882-18 standards. 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).

[0042] The optical, thermal, and mechanical properties of the examples and comparative examples were tested according to the above test methods, and the test results are shown in Table 2.

[0043] Table 2

[0044] As shown in Table 2, the polyimide obtained by this invention exhibits excellent heat resistance, especially in terms of thermal expansion resistance, and also demonstrates excellent colorless transparency and mechanical strength. Therefore, it has broad development prospects in the fields of flexible display substrates, flexible circuit boards, and other electrical / electronic materials and optical materials.

[0045] This invention provides a high-performance, colorless polyimide formulation with a low coefficient of thermal expansion. By employing fluorinated aromatic dianhydride monomers and dianhydride monomers containing rigid alicyclic rings, combined with a rigid, strongly electron-withdrawing sulfone diamine monomer, it ensures both high heat resistance and excellent optical properties. Furthermore, this invention provides a composite film of inorganic nanomaterial zirconium tungstate and polyimide. As a material with a negative coefficient of thermal expansion, zirconium tungstate partially offsets the thermal expansion of polyimide, further reducing the coefficient of thermal expansion of the polyimide film and significantly improving the dimensional stability of the material.

[0046] 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 method for preparing a colorless and transparent polyimide composite film with a low coefficient of thermal expansion, characterized in that: The polyamic acid is prepared by compositing polyamic acid and inorganic nanoparticles in a solvent. The polyamic acid is formed by polycondensation of a rigid alicyclic dianhydride and a fluorinated aromatic dianhydride with a sulfone-containing aromatic diamine. The inorganic nanoparticles are zirconium tungstate nanoparticles.

2. The method for preparing the colorless and transparent polyimide composite film with low thermal expansion coefficient according to claim 1, characterized in that: The rigid alicyclic dianhydride is selected from: cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, bicyclo(2,2,2)-7-octene-2,3,5,6-tetracarboxylic dianhydride, bicyclo(2.2.2)octane-2,3,5,6-tetracarboxylic acid 2,3:5,6-dianhydride, decahydronaphthalenetetracarboxylic dianhydride, isomerized dicyclohexyltetracarboxylic dianhydride, norbornene-2-spirocyclic-α-cyclopentanone-α'-spirocyclic-2''-norbornane-5,5 One or more of the following: '',6,6''-tetracarboxylic dianhydride, 1,4-cyclohexanetetracarboxylic dianhydride, hydrogenated pyromellitic dianhydride, hydrogenated biphenyltetracarboxylic dianhydride, hexahydro-4,7-methylene-1H-indene-1,3,5,7-tetracarboxylic dianhydride, 5-norbornene-2,3-dicarboxylic dianhydride, bicyclo[3.3.0]octane-2,4,6,8-tetracarboxylic dianhydride, 2R,5R,7S,10S-naphthalenetetracarboxylic dianhydride, and adamantanetetracarboxylic dianhydride.

3. The method for preparing the colorless and transparent polyimide composite film with low thermal expansion coefficient according to claim 1, characterized in that: The fluorinated aromatic dianhydride is selected from: 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, 3,6-di(trifluoromethyl)-pyromellitic anhydride, 3-trifluoromethyl-6-methylpyromellitic anhydride, 2,2-bis(3,4-dicarboxyphenyl dianhydride)hexafluoropropane, 1-phenyl-1,1-bis(3,4-dicarboxyphenyl dianhydride)trifluoroethane, 1-(3,5-bistrifluoromethylphenyl)-1,1-bis(3,4-dicarboxyphenyl dianhydride)trifluoroethane, 2,2-bis(4-(3,4-dicarboxyphenoxy)phenyl)hexafluoropropane dianhydride, 2,6-bis(3,4'-dicarboxyphenyl)hexafluoropropane dianhydride, and 2,6-bis(3,4'-dicarboxyphenyl)hexafluoropropane dianhydride. One or more of the following: '-dicarboxyphenyl)-4-(3'',5''-bis(trifluoromethylphenyl)pyridine dianhydride, 1,4-bis(trifluoromethyl)-2,3,5,6-benzenetetracarboxylic acid dianhydride, 4,4'-oxybis(3-(trifluoromethyl)phthalic anhydride), 4,4'-oxybis(3-(oxytrifluoromethyl)phthalic anhydride), 4,4'-oxybis(3-fluorophthalic anhydride), 3,3',4,4'-tetrafluorobiphenyltetracarboxylic dianhydride, 3,3',4,4'-tetrafluorobenzophenone tetracarboxylic dianhydride, and 3,3',4,4'-tetrafluorodiphenyl ether tetracarboxylic dianhydride.

4. The method for preparing the colorless and transparent polyimide composite film with low thermal expansion coefficient according to claim 1, characterized in that: The sulfone-containing aromatic diamine is selected from: 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, bis(aminophenoxyphenyl) sulfone, 2,4'-trifluoromethylaminophenoxy)diphenyl sulfone, 2,2'-bis(3,5-bis(4-trifluoromethylphenyl)-4-(4-aminophenoxy)phenyl) sulfone, 2,2'-bis(3-(4-trifluoromethylphenyl)-4-(3-aminophenoxy)phenyl) sulfone, 2,2'-bis{3-(3,5-bis(trifluoromethyl)phenyl)-4-(3-aminophenoxy)phenyl} sulfone, 4,4'-diamino-3,3'-dimethyldiphenyl sulfone, 4,4'-diamino-2,2'-dimethyldiphenyl sulfone, 4,4'-diamino-3,3'-diethoxydiphenyl sulfone, 4,4'-diamino-2,2'-diphenyl sulfone Diethoxydiphenyl sulfone, 4,4'-diamino-3-fluorodiphenyl sulfone, 4,4'-diamino-2-fluorodiphenyl sulfone, 4,4'-diamino-3-chlorodiphenyl sulfone, 4,4'-diamino-2-chlorodiphenyl sulfone, 4,4'-diamino-3,3'-dinitrodiphenyl sulfone, 4,4'-diamino-2,2'-dinitrodiphenyl sulfone, 4,4'-diamino-3,3'-dibromodiphenyl sulfone, 4, 4'-Diamino-2,2'-Dibromodiphenyl sulfone, 4,4'-Di(4-aminophenoxy)diphenyl sulfone, 4,4'-Di(3-aminophenoxy)diphenyl sulfone, 3,3'-Di(4-aminophenoxy)diphenyl sulfone, 3,4'-Di(4-aminophenoxy)diphenyl sulfone, 2,2'-Dimethyl-4,4'-Diaminodiphenyl sulfone, 3,3'-Dimethoxy-4,4'-Diaminodiphenyl sulfone.

5. The method for preparing the colorless and transparent polyimide composite film with low thermal expansion coefficient according to claim 1, characterized in that: The solvent is selected from: N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), 1,3-dimethyl-2-imidazolinone (DMI), γ-butyrolactone (GBL), sulfolane, or dimethyl sulfoxide (DMSO).

6. The method for preparing the colorless and transparent polyimide composite film with low thermal expansion coefficient according to claim 1, characterized in that: The solvent is selected from N-methylpyrrolidone (NMP) and N,N-dimethylacetamide (DMAc).

7. The method for preparing the colorless and transparent polyimide composite film with low thermal expansion coefficient according to claim 1, characterized in that: The added zirconium tungstate nanoparticles account for 1-15% of the polyamic acid.

8. The method for preparing a colorless and transparent polyimide composite film with a low coefficient of thermal expansion according to any one of claims 1-7, characterized in that... Includes the following steps: S1, under nitrogen atmosphere and ice-water bath cooling conditions, add solvent and sulfone-containing aromatic diamine into the reaction vessel, stir until completely dissolved, and control the system temperature at 0-5℃; S2, add rigid alicyclic dianhydride and fluorinated aromatic dianhydride in batches slowly, keeping the system temperature below 2°C during the addition process; after the addition is complete, stir and react at room temperature for 20-30 hours to obtain a viscous polyamic acid solution; S3, add zirconium tungstate nanoparticles to the polyamic acid solution obtained in step S2 according to the mass ratio, stir and disperse evenly to obtain a polyamic acid composite solution with a solid content of 10-25%; S4. The polyamic acid composite solution obtained in step S4 is coated on the substrate and subjected to imidization treatment at a higher temperature. After imidization, the solvent is removed, and after cooling, the solution is peeled off from the substrate to obtain a colorless and transparent polyimide composite film.

9. The method for preparing the colorless and transparent polyimide composite film with low thermal expansion coefficient according to claim 8, characterized in that: The total amount of rigid alicyclic dianhydride and fluorinated aromatic dianhydride added is in a molar ratio of 0.98 to 1.02:1 to that of sulfone-containing aromatic diamine; the molar ratio between the rigid alicyclic dianhydride and the fluorinated aromatic dianhydride is 0.98 to 1.02:

1.

10. The method for preparing the colorless and transparent polyimide composite film with low thermal expansion coefficient according to claim 1, characterized in that: The zirconium tungstate nanoparticles were prepared by the following method: (1) Add cyclohexane, n-hexanol and polyethylene glycol nonylphenyl ether to the reaction vessel and stir to mix evenly; (2) Hydrated ammonium metatungstate is dissolved in distilled water to obtain a transparent and homogeneous aqueous solution of ammonium metatungstate, and hydrated zirconium dichloride is dissolved in distilled water to obtain a transparent and homogeneous aqueous solution of zirconium dichloride. The two are mixed by dropwise addition to form a transparent and colorless premixed solution. (3) Slowly drop the premixed solution obtained in step (2) into the reaction vessel of step (1) and stir rapidly for at least 30 minutes to obtain a transparent and colorless reaction product; (4) After continuing the reaction for at least 3 hours, anhydrous ethanol was added to the reaction system to obtain a white turbid suspension; Continue stirring and centrifuging for more than 1 hour to obtain a white gel-like precipitate; (5) After repeatedly washing the obtained white gel-like precipitate with anhydrous ethanol, it was transferred to a container, distilled water was added to make a slurry, and then hydrochloric acid solution was added. The reaction solution was refluxed for 24-36 hours, and the white solution was centrifuged to obtain the white precipitate. (6) After washing the white precipitate with water several times, it was aged at room temperature for at least three days. The aged precipitate was then calcined in air at 600±20℃ for 6-12 hours to obtain a yellowish-white ZrW2O8 precipitate. (7) Yellow-white ZrW2O8 precipitate was ultrasonically treated in N-methylpyrrolidone (NMP) solvent for more than 2 hours to depolymerize the clusters. After removing the NMP solvent, it was washed multiple times with tetrahydrofuran. The obtained solid product was dried at 100±40℃ under vacuum. Under a nitrogen atmosphere, the dried ZrW2O8 was dispersed in NMP solvent and 3-aminopropyltriethoxysilane was added. The mixture was stirred at 50±5℃ for more than 12 hours. The precipitate obtained by cooling and centrifugation was washed and then dried under vacuum to complete the process.

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