A method for preparing a low-dielectric polyimide biaxially stretched film

Abstract

The present invention relates to a method for preparing a low-dielectric polyimide biaxially stretched film, comprising first synthesizing a polyamic acid solution containing hydroxyl groups on side groups, introducing acetic anhydride and an organic alkaline catalyst therein to chemically imidize the solution to obtain a gel film; and then preparing the polyimide biaxially stretched film by biaxial stretching and solvent volatilization. The film not only exhibits excellent dielectric properties but also achieves synergistic optimization of mechanical and dielectric properties through molecular chain fixation and thermal rearrangement reactions, resulting in broad application prospects in high-frequency electronic devices, microelectronic packaging, and other fields.

Description

Technical Field

[0001] The invention belongs to the technical field of polyimide films, and in particular relates to a method for preparing a low-dielectric polyimide biaxially stretched film. Background Art

[0002] With the global adoption of 5G communications technology, electronic products are demanding smaller and more compact components. However, this significantly increases crosstalk and losses between circuits, affecting signal propagation speed. Therefore, finding materials with high mechanical strength, high heat resistance, and low dielectric constant is crucial for the development of 5G and even future 6G technologies.

[0003] Polyimide, a polymer containing an imide ring structure in its backbone, boasts excellent mechanical properties, high and low temperature resistance, solvent corrosion resistance, low dielectric constant, and a low thermal expansion coefficient. It is widely used in electronics, aerospace, and other fields, and is the preferred low-dielectric insulating material for many applications. However, the dielectric constant of unmodified aromatic polyimide is in the range of 3.0 to 3.5, insufficient to meet the current high-frequency and high-speed requirements of 5G technology. Therefore, the development of new low-dielectric-constant polyimide materials is a current research priority in this field.

[0004] There are two main approaches to reducing the dielectric constant of polyimide materials. First, modifying the bulk structure of the polyimide, such as by introducing fluorine atoms, alicyclic structures, or bulky side groups, reduces the polarizability of the dipoles within the molecule. Patent CN110655472A relates to a diamine monomer with bulky side groups and its preparation method. The resulting polyimide film has a large free volume in the molecular chains, which reduces interchain stacking and molar polarizability, thereby significantly lowering the dielectric constant of the polyimide film. Second, introducing a porous structure into the material to form pores reduces the number of polarizable molecules per unit volume, thereby lowering the overall dielectric constant of the material. Patent CN113336998A provides a method for preparing a low-dielectric, low-thermal-conductivity, self-microporous polyimide film cross-linked with benzocyclobutene side groups. The resulting film exhibits low dielectric constant and low thermal conductivity, as well as excellent mechanical properties, thermal stability, and dimensional stability. It has broad application prospects in high-frequency circuit boards and thermal insulation materials. However, all of these methods have drawbacks: modifying the polyimide structure requires complex procedures and is costly. Creating porous structures in polyimide also makes it difficult to control the pore structure and diameter, often resulting in a loss of mechanical properties. Therefore, developing a simpler method that can reduce the dielectric constant while maintaining the material's mechanical strength and dimensional stability is of great significance.

[0005] Bidirectional stretching with the help of chemical imidization is a method that can significantly improve the mechanical strength and dimensional stability of polyimide films. U.S. Patent No. 5,460,890A describes a process for preparing a gel film using a chemical imidization method and performing bidirectional stretching, which significantly improves the mechanical properties and dimensional stability of the film. However, after bidirectional stretching, the orientation degree of the molecular chain increases, causing the dielectric constant of the film to increase. Patent CN116462630A provides a quinoline-based low-temperature catalyst that is introduced into the polyamic acid molecular chain in the form of a graft, ensuring a high imidization rate while improving the mechanical properties, thermal properties, dielectric properties and hydrophobicity of the polyimide. This patent provides a method that takes into account both the mechanical properties and dielectric properties of the film, but the material's stability in use at high temperatures deteriorates. If, through molecular structure design, the alkaline catalyst is made to interact with the polyamic acid molecular chain after catalyzing the imidization reaction, it will be stably present before high-temperature heat setting, and the molecular chain spacing will be increased while bidirectional stretching can improve the regularity of the molecular chain. After high-temperature heat setting, the molecular chains are fixed under tension, and the positions originally occupied by organic bases are released, thereby forming a submicron pore structure, which significantly reduces the dielectric constant and dielectric loss of the film. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for preparing a low-dielectric polyimide biaxially stretched film. The obtained film not only exhibits excellent dielectric properties, but also achieves synergistic optimization of mechanical properties and dielectric properties through the fixation of molecular chains and thermal rearrangement reactions, making it have broad application prospects in high-frequency electronic devices, microelectronic packaging and other fields.

[0007] The present invention provides a method for preparing a low-dielectric polyimide biaxially stretched film, comprising the following steps:

[0008] (1) adding aromatic diamine monomer and o-hydroxy diamine monomer into a non-protonic polar organic solvent, adding dianhydride monomer after complete dissolution, and synthesizing polyamic acid solution containing hydroxyl groups in an ice bath;

[0009] (2) Adding acetic anhydride and an organic alkaline catalyst to the polyamic acid solution, stirring until the reaction is complete, scraping the film and heating to obtain a gel film;

[0010] (3) The gel film obtained in step (2) is stretched in a biaxial stretching device and then heat-set to obtain a low dielectric polyimide biaxially stretched film.

[0011] Preferably, the aromatic diamine monomer in step (1) is at least one of p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenyl ether, 2-(4-aminophenyl)-5-aminobenzimidazole, 4,4'-diaminodiphenylmethane, 2,2'-dimethylbenzidine, and 2,2'-dimethylbenzidine.

[0012] Preferably, the o-hydroxydiamine monomer in step (1) is at least one of the following structures:

[0013] .

[0014] Preferably, the molar ratio of the aromatic diamine monomer to the o-hydroxydiamine monomer in step (1) is 9:1 to 1:9, preferably 5:5.

[0015] Preferably, the dianhydride monomer in step (1) is at least one of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl ketone tetracarboxylic dianhydride, 2,3',3,4'-biphenyltetracarboxylic dianhydride, and 4,4'-biphenyl ether dianhydride; and the molar ratio of the dianhydride monomer to the total amount of the aromatic diamine monomer and the o-hydroxydiamine monomer is 0.98:1 to 1.02:1.

[0016] Preferably, the aprotic polar organic solvent in step (1) is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; and the solid content of the polyamic acid solution containing hydroxyl groups on the side groups is 10-15 wt%.

[0017] Preferably, the organic alkaline catalyst in step (2) is at least one of 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,5-diazabicyclo[4.3.0]nonene, 8-hydroxyisoquinoline, 8-hydroxyquinoline, 1,1'-biisoquinoline, 2,2'-biquinoline, 2,2'-bi-4,4'-dimethylquinoline, and triphenylamine.

[0018] Preferably, the molar amount of acetic anhydride in step (2) is 1 to 10 times that of dianhydride, preferably 3 times; the molar amount of the organic alkaline catalyst is 0.1 to 10 times that of o-hydroxydiamine, preferably 3 times.

[0019] Preferably, the stirring reaction temperature in step (2) is -30~0 °C, preferably -15 °C.

[0020] Preferably, the heating temperature in step (2) is 20-100 °C, preferably 50 °C.

[0021] Preferably, the stretching process parameters in step (3) are as follows: the stretching temperature is 20-80 °C, preferably 50 °C; the longitudinal stretching ratio of the gel film is 1.1-1.9, preferably 1.6; the transverse stretching ratio is 1.1-1.9, preferably 1.6; and the stretching rate is 10-50 mm / min, preferably 30 mm / min.

[0022] Preferably, the heat setting process parameters in step (3) are: heating from 300 °C to 450 °C, with a heating rate of 1-5 °C / min.

[0023] The principle of the present invention is:

[0024] First, a polyamic acid molecule containing hydroxyl side groups was designed and synthesized to provide hydrogen bonding sites, facilitating the uniform dispersion of the organic alkaline catalyst within the polymer matrix. A large, high-boiling-point organic alkaline catalyst was selected to catalyze the chemical imidization reaction between acetic anhydride and polyamic acid, generating hydrogen bonds with the hydroxyl groups on the molecular chain. The reaction temperature was controlled to prepare a gel film and biaxially stretch it. This oriented the molecular chains along the external force, while the presence of the large catalyst side groups expanded the distance between the molecular chains. Subsequently, high-temperature heat setting was performed. As the molecular chains were fixed under tension, the sites previously occupied by the organic base were released, thereby forming a submicron-scale porous structure. Finally, the hydroxyl groups on the polyimide rearranged with adjacent imide rings through thermal transformation, transforming them into benzoxazole structures, further reducing the dielectric constant and loss of the film, resulting in a low-dielectric polyimide biaxially stretched film.

[0025] Beneficial effects

[0026] (1) Significantly reduce dielectric constant and loss: The present invention synergistically optimizes the molecular chain structure and the aggregate structure to modify the dielectric properties. The hydrogen bonding between the organic base and the polymer molecular chain effectively inhibits the stacking of the molecular chain. At the same time, submicron pores are formed after the organic base is removed at high temperature, significantly reducing the dielectric constant and dielectric loss of the film. In addition, the thermal rearrangement reaction between the hydroxyl groups on the polyimide and the adjacent imide rings further transforms the film structure into a benzoxazole structure with low dielectric properties, further optimizing the dielectric properties of the film.

[0027] (2) Guaranteeing the mechanical properties of the film: The uniform dispersion of the organic base and the small and evenly distributed microporous structure formed after high-temperature removal avoid the degradation of mechanical properties caused by pore defects, thereby ensuring the mechanical strength and stability of the film while reducing the dielectric properties.

[0028] (3) Improving the comprehensive performance of the film: The film of the present invention not only performs well in dielectric properties, but also achieves synergistic optimization of mechanical properties and dielectric properties through the fixation of molecular chains and thermal rearrangement reactions, making it have broad application prospects in high-frequency electronic devices, microelectronic packaging and other fields.

[0029] (4) The process is simple and controllable: The preparation process of the present invention does not rely on complex equipment or expensive materials. The removal of organic bases and structural transformation can be achieved through simple high-temperature treatment. The process conditions are easy to control and are suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is an optical photograph of the gel film during biaxial stretching.

[0031] Figure 2 These are the thermogravimetric curves of the biaxially stretched polyimide films prepared in Example 1 and Comparative Example 2 that were not subjected to high-temperature heat treatment above 300°C.

[0032] Figure 3 The dielectric constants of the biaxially stretched polyimide films prepared in Example 1 and Comparative Example 2 are shown in FIG. DETAILED DESCRIPTION

[0033] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0034] Example 1

[0035] (1) Under nitrogen protection, 0.05 mol of 3,3'-dihydroxybenzidine, 0.05 mol of p-phenylenediamine and 241.6 g of N,N-dimethylacetamide were added to a 500 mL three-necked flask. After the solid was dissolved, 0.1 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid solution with a solid content of 15 wt% and a side group containing hydroxyl groups.

[0036] (2) At room temperature, 0.15 mol of 1,8-diazabicyclo[5.4.0]undec-7-ene was added to the polyamic acid solution containing hydroxyl groups and reacted for 2 h. Subsequently, 0.6 mol of acetic anhydride was added at -15 °C and stirred for 20 min. After centrifugation and degassing, an 800 µm liquid film was scraped onto a clean glass plate and then reacted at 50 °C for 10 min to obtain a partially imidized polyamic acid-imide gel film.

[0037] (3) The gel film was fixed in a biaxial stretching machine and subjected to synchronous biaxial stretching at a stretching speed of 30 mm / min, a stretching temperature of 50 °C, and a program of synchronous longitudinal and transverse stretching of 1.6 times. The film was then heated to 300 °C and kept at this temperature for 10 min to remove the solvent. The film was then heat treated at a temperature control program of heating from 300 °C to 450 °C at a heating rate of 1 °C / min to obtain a low dielectric polyimide biaxial stretch film.

[0038] Example 2

[0039] (1) Under nitrogen protection, 0.05 mol of 2,4-diaminophenol, 0.05 mol of p-phenylenediamine and 215.51 g of N,N-dimethylacetamide were added to a 500 mL three-necked flask. After the solid was dissolved, 0.1 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid solution with a solid content of 15 wt% and a side group containing hydroxyl groups.

[0040] (2) At room temperature, 0.15 mol of 1,8-diazabicyclo[5.4.0]undec-7-ene was added to the polyamic acid solution containing hydroxyl groups and reacted for 2 h. Subsequently, 0.6 mol of acetic anhydride was added at -15 °C and stirred for 20 min. After centrifugation and degassing, an 800 µm liquid film was scraped onto a clean glass plate and then reacted at 50 °C for 10 min to obtain a partially imidized polyamic acid-imide gel film.

[0041] (3) The gel film was fixed in a biaxial stretching machine and subjected to synchronous biaxial stretching at a stretching speed of 30 mm / min, a stretching temperature of 50 °C, and a program of synchronous longitudinal and transverse stretching of 1.6 times. The film was then heated to 300 °C and kept at this temperature for 10 min to remove the solvent. The film was then heat treated at a temperature control program of heating from 300 °C to 450 °C at a heating rate of 1 °C / min to obtain a low dielectric polyimide biaxial stretch film.

[0042] Example 3

[0043] (1) Under nitrogen protection, 0.05 mol of 2,5-diaminophenol, 0.05 mol of p-phenylenediamine and 215.51 g of N,N-dimethylacetamide were added to a 500 mL three-necked flask. After the solid was dissolved, 0.1 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid solution with a solid content of 15 wt% and a side-group hydroxyl group.

[0044] (2) At room temperature, 0.15 mol of 1,8-diazabicyclo[5.4.0]undec-7-ene was added to the polyamic acid solution containing hydroxyl groups and reacted for 2 h. Subsequently, 0.6 mol of acetic anhydride was added at -15 °C and stirred for 20 min. After centrifugation and degassing, an 800 µm liquid film was scraped onto a clean glass plate and then reacted at 50 °C for 10 min to obtain a partially imidized polyamic acid-imide gel film.

[0045] (3) The gel film was fixed in a biaxial stretching machine and subjected to synchronous biaxial stretching at a stretching speed of 30 mm / min, a stretching temperature of 50 °C, and a program of synchronous longitudinal and transverse stretching of 1.6 times. The film was then heated to 300 °C and kept at this temperature for 10 min to remove the solvent. The film was then heat treated at a temperature control program of heating from 300 °C to 450 °C at a heating rate of 1 °C / min to obtain a low dielectric polyimide biaxial stretch film.

[0046] Example 4

[0047] A low dielectric polyimide biaxially stretched film was prepared according to the method of Example 1, except that 1,8-diazabicyclo[5.4.0]undec-7-ene was replaced by 1,5-diazabicyclo[4.3.0]nonene in step (2).

[0048] Example 5

[0049] A low dielectric polyimide biaxially stretched film was prepared according to the method of Example 1, except that 1,8-diazabicyclo[5.4.0]undec-7-ene was replaced by 8-hydroxyisoquinoline in step (2).

[0050] Example 6

[0051] A low dielectric polyimide biaxially stretched film was prepared according to the method of Example 1, except that in step (3), the gel film was fixed in a biaxial stretching machine, and synchronous biaxial stretching was performed at a stretching speed of 10 mm / min, a stretching temperature of 50 °C, and a program of synchronous stretching of 1.6 times in the longitudinal and transverse directions. The film was then heated to 300 °C and kept warm for 10 min to remove the solvent. The film was then heat treated according to a temperature control program of heating from 300 °C to 450 °C at a heating rate of 1 °C / min to obtain a low dielectric polyimide biaxially stretched film.

[0052] Example 7

[0053] A low dielectric polyimide biaxially stretched film was prepared according to the method of Example 1, except that in step (3), the gel film was fixed in a biaxial stretching machine, and synchronous biaxial stretching was performed at a stretching speed of 50 mm / min, a stretching temperature of 50 °C, and a program of synchronous stretching of 1.6 times in the longitudinal and transverse directions. The film was then heated to 300 °C and kept warm for 10 min to remove the solvent. The film was then heat treated according to a temperature control program of heating from 300 °C to 450 °C at a heating rate of 1 °C / min to obtain a low dielectric polyimide biaxially stretched film.

[0054] Example 8

[0055] A low dielectric polyimide biaxially stretched film was prepared according to the method of Example 1, except that in step (3), the gel film was fixed in a biaxial stretching machine, and synchronous biaxial stretching was performed at a stretching speed of 30 mm / min, a stretching temperature of 50 °C, and a program of synchronous stretching of 1.4 times in the longitudinal and transverse directions. The film was then heated to 300 °C and kept warm for 10 min to remove the solvent. The film was then heat treated according to a temperature control program of heating from 300 °C to 450 °C at a heating rate of 1 °C / min to obtain a low dielectric polyimide biaxially stretched film.

[0056] Example 9

[0057] A low dielectric polyimide biaxially stretched film was prepared according to the method of Example 1, except that in step (3), the gel film was fixed in a biaxial stretching machine, and synchronous biaxial stretching was performed at a stretching speed of 30 mm / min, a stretching temperature of 50 °C, and a program of synchronous longitudinal and transverse stretching of 1.8 times. Subsequently, the temperature was raised to 300 °C and kept at this temperature for 10 min to remove the solvent. Then, the film was heat treated according to a temperature control program of raising the temperature from 300 °C to 450 °C at a heating rate of 1 °C / min, thereby obtaining a low dielectric polyimide biaxially stretched film.

[0058] Example 10

[0059] A low dielectric polyimide biaxially stretched film was prepared according to the method of Example 1, except that in step (3), the gel film was fixed in a biaxial stretching machine, and synchronous biaxial stretching was performed at a stretching speed of 30 mm / min, a stretching temperature of 80 °C, and a program of synchronous longitudinal and transverse stretching of 1.6 times. Subsequently, the temperature was raised to 300 °C and kept at this temperature for 10 min to remove the solvent. The film was then heat-treated according to a temperature control program of heating from 300 °C to 450 °C at a heating rate of 1 °C / min to obtain a low dielectric polyimide biaxially stretched film.

[0060] Example 11

[0061] A low dielectric polyimide biaxially stretched film was prepared according to the method of Example 1, except that in step (3), the gel film was fixed in a biaxial stretching machine, and synchronous biaxial stretching was performed at a stretching speed of 30 mm / min, a stretching temperature of 30 °C, and a program of synchronous stretching of 1.6 times in the longitudinal and transverse directions. The film was then heated to 300 °C and kept warm for 10 min to remove the solvent. The film was then heat treated according to a temperature control program of heating from 300 °C to 450 °C at a heating rate of 1 °C / min to obtain a low dielectric polyimide biaxially stretched film.

[0062] Comparative Example 1

[0063] (1) Under nitrogen protection, 0.05 mol of 3,3'-dihydroxybenzidine, 0.05 mol of p-phenylenediamine and 241.6 g of N,N-dimethylacetamide were added to a 500 mL three-necked flask. After the solid was dissolved, 0.1 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid solution with a solid content of 15 wt% and a side group containing hydroxyl groups.

[0064] (2) At room temperature, 0.15 mol of 1,8-diazabicyclo[5.4.0]undec-7-ene was added to the polyamic acid solution containing hydroxyl groups and reacted for 2 h. Subsequently, 0.6 mol of acetic anhydride was added at -15 °C and stirred for 20 min. After centrifugation and degassing, an 800 µm liquid film was scraped onto a clean glass plate and then reacted at 50 °C for 10 min to obtain a partially imidized polyamic acid-imide gel film.

[0065] (3) The gel film was heated to 300 °C and kept at this temperature for 10 min to remove the solvent, and then heat treated at a temperature control program of 300 °C to 450 °C at a heating rate of 1 °C / min to obtain a low dielectric polyimide film prepared by the chemical imidization method.

[0066] Comparative Example 2

[0067] (1) Under nitrogen protection, 0.05 mol of 3,3'-dihydroxybenzidine, 0.05 mol of p-phenylenediamine and 241.6 g of N,N-dimethylacetamide were added to a 500 mL three-necked flask. After the solid was dissolved, 0.1 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid solution with a solid content of 15 wt%.

[0068] (2) After the polyamic acid solution was centrifuged for degassing, an 800 μm liquid film was scraped onto a clean glass plate and then reacted at 80°C for 60 min to obtain a polyamic acid gel film. The film was then heated to 300°C and kept for 10 min to remove the solvent. The film was then heat treated at a heating rate of 1°C / min from 300°C to 450°C to obtain a thermal imidization polyimide film.

[0069] Table 1 compares the mechanical properties of various examples and comparative examples, where tensile strength, elongation at break, and elastic modulus were tested according to the national standard GB / T 1040.3-2006. The specific testing steps are as follows: cutting a long spline with a length of 150 mm, a width of 10 mm, and a thickness of less than 100 μm, marking the middle of the spline with parallel markings spaced 50 mm apart, and ensuring that the spline edges are smooth and free of notches; fixing the distance between the clamps of a tensile testing machine to 50 mm, vertically clamping the spline in the clamps, inputting the width, thickness, and length, and then testing at a tensile speed of 5 mm / min.

[0070] Table 1

[0071]

[0072] from Figure 1 It can be seen that the gel film has good stretching processability. Figure 2 Comparative Example 2 produces thermal weight loss at around 400°C, indicating that thermal rearrangement of the ortho-hydroxyl group and the imide ring occurs; while Example 1 produces thermal weight loss at around 300°C and around 400°C, indicating that the catalyst is removed first, followed by thermal rearrangement of the ortho-hydroxyl group and the imide ring. Figure 3 The results show that the dielectric constant of the film is significantly reduced after chemical imidization, biaxial stretching and high-temperature heat treatment. Combined with the performance data in Table 1, the tensile strength, elongation at break and elastic modulus are significantly improved after chemical imidization and biaxial stretching, indicating that this process improves the mechanical properties of the film while reducing the dielectric properties.

Claims

1. A method for preparing a low-dielectric polyimide biaxially stretched film, comprising the following steps: (1) adding an aromatic diamine monomer and an o-hydroxy diamine monomer into an aprotic polar organic solvent, and adding a dianhydride monomer after the monomers are completely dissolved, and synthesizing a polyamic acid solution containing hydroxyl groups in an ice bath; (2) adding acetic anhydride and an organic alkaline catalyst to the polyamic acid solution, stirring and reacting until the reaction is complete, scraping the film and heating to obtain a gel film; (3) The gel film obtained in step (2) is stretched in a biaxial stretching device, and then heat-set to obtain a low-dielectric polyimide biaxial stretch film; the stretching process parameters are: stretching temperature of 20 to 80°C, longitudinal stretching ratio of 1.1 to 1.9, transverse stretching ratio of 1.1 to 1.9, and stretching speed of 10 to 50 mm / s; the heat-setting process parameters are: heating from 300°C to 450°C, and a heating rate of 1 to 5°C / min.

2. The preparation method according to claim 1, wherein: The aromatic diamine monomer in step (1) is at least one of p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenyl ether, 2-(4-aminophenyl)-5-aminobenzimidazole, 4,4'-diaminodiphenylmethane, 2,2'-dimethylbenzidine, and 2,2'-dimethylbenzidine; the o-hydroxydiamine monomer is at least one of the following structures:

3. The preparation method according to claim 1, wherein: The molar ratio of the aromatic diamine monomer to the o-hydroxydiamine monomer in step (1) is 9:1 to 1:

9.

4. The preparation method according to claim 1, wherein: The dianhydride monomer in step (1) is at least one of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl ketone tetracarboxylic dianhydride, 2,3',3,4'-biphenyltetracarboxylic dianhydride, and 4,4'-biphenyl ether dianhydride; and the molar ratio of the dianhydride monomer to the total amount of the aromatic diamine monomer and the o-hydroxydiamine monomer is 0.98:1 to 1.02:

1.

5. The preparation method according to claim 1, wherein: The aprotic polar organic solvent in step (1) is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; and the solid content of the polyamic acid solution containing hydroxyl groups on the side groups is 10 to 15 wt%.

6. The preparation method according to claim 1, wherein: The organic alkaline catalyst in step (2) is at least one of 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,5-diazabicyclo[4.3.0]nonene, 8-hydroxyisoquinoline, 8-hydroxyquinoline, 1,1'-biisoquinoline, 2,2'-biquinoline, 2,2'-bi-4,4'-dimethylquinoline, and triphenylamine.

7. The preparation method according to claim 1, wherein: The molar amount of acetic anhydride in step (2) is 1 to 10 times that of dianhydride; and the molar amount of the organic alkaline catalyst is 0.1 to 4.5 times that of o-hydroxydiamine.

8. The preparation method according to claim 1, wherein: The stirring reaction temperature in step (2) is -30 to 0°C; The heating temperature is 20-100°C.

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