Preparation method of low-dielectric bidirectionally drawn polyimide film

Abstract

The invention relates to a preparation method of a low-dielectric bidirectionally drawn polyimide film, which comprises the following steps: synthesizing a polyamic acid solution with a side group containing hydroxyl, and introducing acetic anhydride and an organic basic catalyst into the polyamic acid solution for chemical imidization to obtain a gel film; and then bidirectionally drafting and volatilizing a solvent to prepare the bidirectionally-drafted polyimide film. The film provided by the invention not only has excellent dielectric properties, but also realizes collaborative optimization of mechanical properties and dielectric properties through the fixing effect and thermal rearrangement reaction of molecular chains, so that the film has wide application prospects in the fields of high-frequency electronic devices, microelectronic packaging and the like.

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 popularization of 5G communication technology around the world, electronic products require components to be miniaturized and intensive. However, the crosstalk and loss between circuits will increase greatly, thus affecting the propagation speed of signals. Therefore, finding materials with high mechanical strength, high heat resistance and low dielectric constant is the key to the development of 5G and even future 6G technology.

[0003] Polyimide is a polymer with an imide ring structure in the main chain. It has the advantages of excellent mechanical properties, high and low temperature resistance, solvent corrosion resistance, low dielectric constant and low thermal expansion coefficient. It is widely used in electronic appliances, aerospace and other fields, and is the first choice for many low dielectric insulation materials. However, the dielectric constant of unmodified aromatic polyimide is in the range of 3.0 to 3.5, which is not enough to meet the needs of current high-frequency and high-speed 5G technology. Therefore, the development of new low dielectric constant polyimide materials is one of the research focuses in the current field.

[0004] The methods for reducing the dielectric constant of polyimide materials are mainly divided into two aspects: one is to change the bulk structure of polyimide, such as introducing fluorine atoms, alicyclic structures or larger side groups to reduce the polarization ability of dipoles in the molecule. Patent CN110655472A relates to a diamine monomer with a large side group and a preparation method thereof. The molecular chain free volume of the prepared polyimide film is large, which can reduce the accumulation between molecular chains and reduce the molar polarization rate, thereby significantly reducing the dielectric constant of the polyimide film. The second is to introduce a porous structure into the material to form pores, thereby reducing the number of polarized molecules per unit volume, thereby reducing the dielectric constant of the material as a whole. Patent CN113336998A provides a method for preparing a low dielectric, low thermal conductivity benzocyclobutene side group cross-linked self-microporous polyimide film. The prepared film has the performance characteristics of low dielectric constant and low thermal conductivity, and its mechanical properties and heat resistance stability are excellent, and the dimensional stability is good. It has broad application prospects in the field of high-frequency circuit boards and thermal insulation materials. However, the above methods all have disadvantages: the steps to change the structure of polyimide are cumbersome and costly; the method of preparing porous structures in polyimide is difficult to control the structure and pore size of the pores, and often loses the mechanical properties of the material. Therefore, it is of great significance to develop a method that is easy to operate and can reduce the dielectric constant while maintaining the mechanical strength and dimensional stability of the material.

[0005] Bidirectional stretching by chemical imidization is a method that can greatly improve the mechanical strength and dimensional stability of polyimide film. U.S. Patent US5460890A describes a process for preparing gel film by chemical imidization and bidirectional stretching, which greatly improves the mechanical properties and dimensional stability of the film. However, after bidirectional stretching, the orientation degree of the molecular chain increases, resulting in a larger dielectric constant of the film. Patent CN116462630A provides a quinoline low-temperature catalyst, which is introduced into the polyamic acid molecular chain in the form of grafting, while ensuring a high imidization rate, improving the mechanical properties, thermal properties, dielectric properties and hydrophobicity of polyimide. The patent provides a method that takes into account the mechanical properties and dielectric properties of the film, but the material's use stability at high temperatures deteriorates. If the molecular structure is designed so that the alkaline catalyst still interacts with the polyamic acid molecular chain after catalyzing the imidization reaction, it is stably present before high-temperature heat setting, and the molecular chain spacing is increased while the molecular chain regularity can be improved by bidirectional stretching. After high-temperature heat setting, the molecular chains are fixed by tension, and the positions originally occupied by the organic base 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 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, so that it has 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: (1) adding an aromatic diamine monomer and an ortho-hydroxy diamine monomer into a non-protonic polar organic solvent, adding a dianhydride monomer after the monomers are completely dissolved, and synthesizing a polyamic acid solution containing a hydroxyl group on a side group under ice bath conditions; (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; (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.

[0008] 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.

[0009] Preferably, the o-hydroxydiamine monomer in step (1) is at least one of the following structures: .

[0010] 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.

[0011] 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.

[0012] Preferably, the non-protonic 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%.

[0013] 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.

[0014] 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.

[0015] Preferably, the stirring reaction temperature in step (2) is -30~0 °C, preferably -15 °C.

[0016] Preferably, the heating temperature in step (2) is 20-100 °C, preferably 50 °C.

[0017] 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.

[0018] Preferably, the heat setting process parameters in step (3) are: heating from 300 °C to 450 °C, and the heating rate is 1-5 °C / min.

[0019] The principle of the present invention is: First, a polyamic acid molecule containing a hydroxyl side group is designed and synthesized to provide a hydrogen bonding site, which is conducive to the uniform dispersion of the organic alkaline catalyst in the polymer matrix; a large-volume, high-boiling-point organic alkaline catalyst is selected to catalyze the chemical imidization reaction of acetic anhydride and polyamic acid, and then produce a hydrogen bond with the hydroxyl group on the molecular chain; the reaction temperature is regulated to prepare a gel film and bidirectional stretching is performed, and the molecular chain is oriented along the external force while the molecular chain spacing is expanded due to the presence of the large-volume catalyst of the side group; then high-temperature heat setting is performed, and the position originally occupied by the organic base is released due to the tension of the molecular chain, thereby forming a submicron-scale pore structure. Finally, the hydroxyl group on the polyimide and the adjacent imide ring are overheated and rearranged to convert into a benzoxazole structure, which further reduces the dielectric constant and loss of the film, and obtains a low-dielectric polyimide bidirectional stretch film.

[0020] Beneficial Effects (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 stacking of the molecular chains is effectively inhibited by the hydrogen bonding between the organic base and the polymer molecular chain. At the same time, submicron pores are formed after the organic base is removed at high temperature, which significantly reduces the dielectric constant and dielectric loss of the film. In addition, the thermal rearrangement reaction between the hydroxyl group on the polyimide and the adjacent imide ring further transforms the film structure into a benzoxazole structure with low dielectric properties, further optimizing the dielectric properties of the film.

[0021] (2) Ensure 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 due to pore defects, thereby reducing the dielectric properties while ensuring the mechanical strength and stability of the film.

[0022] (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.

[0023] (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

[0024] Figure 1This is an optical photograph of the gel film during bidirectional stretching.

[0025] Figure 2 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.

[0026] Figure 3 The dielectric constants of the biaxially stretched polyimide films prepared in Example 1 and Comparative Example 2. DETAILED DESCRIPTION

[0027] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it 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 within the scope limited by the appended claims of the application equally.

[0028] Example 1

[0029] (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 anhydride 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.

[0030] (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. The mixture was degassed by centrifugation and a 800 µm liquid film was scraped onto a clean glass plate. The mixture was then reacted at 50 °C for 10 min to obtain a partially imidized polyamic acid-imide gel film.

[0031] (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 temperature was then raised to 300 °C and kept 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 biaxially stretched film.

[0032] Example 2

[0033] (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 anhydride 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.

[0034] (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. The mixture was degassed by centrifugation and a 800 µm liquid film was scraped onto a clean glass plate. The mixture was then reacted at 50 °C for 10 min to obtain a partially imidized polyamic acid-imide gel film.

[0035] (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 temperature was then raised to 300 °C and kept 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 biaxially stretched film.

[0036] Example 3

[0037] (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 anhydride 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.

[0038] (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. The mixture was degassed by centrifugation and a 800 µm liquid film was scraped onto a clean glass plate. The mixture was then reacted at 50 °C for 10 min to obtain a partially imidized polyamic acid-imide gel film.

[0039] (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 temperature was then raised to 300 °C and kept 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 biaxially stretched film.

[0040] Example 4

[0041] 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).

[0042] Example 5

[0043] 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).

[0044] Example 6

[0045] 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 longitudinal and transverse stretching of 1.6 times, followed by heating to 300°C and keeping the temperature for 10 min to remove the solvent, and then heat treatment was performed 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.

[0046] Example 7

[0047] 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 longitudinal and transverse stretching of 1.6 times, followed by heating to 300 °C and keeping the temperature for 10 min to remove the solvent, and then heat treatment was performed 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.

[0048] Example 8

[0049] 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 1.4 times, followed by heating to 300°C and keeping the temperature for 10 min to remove the solvent, and then heat treatment was performed 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.

[0050] Example 9

[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 30 mm / min, a stretching temperature of 50°C, and a program of synchronous longitudinal and transverse stretching of 1.8 times, followed by heating to 300°C and keeping the temperature for 10 min to remove the solvent, and then heat treatment was performed 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 10

[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 30 mm / min, a stretching temperature of 80°C, and a program of synchronous longitudinal and transverse stretching of 1.6 times, followed by heating to 300°C and keeping the temperature for 10 min to remove the solvent, and then heat treatment was performed 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] Embodiment 11

[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 30°C, and a program of synchronous longitudinal and transverse stretching of 1.6 times, followed by heating to 300°C and keeping the temperature for 10 min to remove the solvent, and then heat treatment was performed 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] Comparative Example 1 (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 anhydride 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.

[0057] (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. The mixture was degassed by centrifugation and a 800 µm liquid film was scraped onto a clean glass plate. The mixture was then reacted at 50 °C for 10 min to obtain a partially imidized polyamic acid-imide gel film.

[0058] (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 rate of 1 °C / min from 300 °C to 450 °C to obtain a low dielectric polyimide film prepared by the chemical imidization method.

[0059] Comparative Example 2 (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 anhydride 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%.

[0060] (2) After the polyamic acid solution was centrifuged for degassing, a 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 temperature was then raised to 300 °C and kept for 10 min to remove the solvent. The polyamic acid gel 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.

[0061] Table 1 is a comparison of mechanical property data of each embodiment and the comparative example, wherein tensile strength, elongation at break and elastic modulus are tested according to the national standard GB / T 1040.3-2006, and the specific test steps are: 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 with an interval of 50 mm, and the spline edge is smooth without a gap; fixing the distance between the clamps of the tensile testing machine to 50 mm, vertically clamping the spline in the clamp, inputting the width, thickness, and length, and then testing at a tensile speed of 5 mm / min.

[0062] Table 1

[0063] 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 about 400°C, indicating that thermal rearrangement of the ortho-hydroxyl group and the imide ring occurs; while Example 1 produces thermal weight loss at about 300°C and about 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 after chemical imidization, biaxial stretching and high-temperature heat treatment, the dielectric constant of the film is greatly reduced; combined with the performance data in Table 1, after chemical imidization and biaxial stretching, the tensile strength, elongation at break and elastic modulus are significantly improved, 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 ortho-hydroxy diamine monomer into a non-protonic polar organic solvent, adding a dianhydride monomer after the monomers are completely dissolved, and synthesizing a polyamic acid solution containing hydroxyl groups on the side groups under ice bath conditions; (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; (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.

2. The preparation method according to claim 1, characterized in that: 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, characterized in that: The molar ratio of the aromatic diamine monomer to the o-hydroxy diamine monomer in step (1) is 9:1 to 1:

9.

4. The preparation method according to claim 1, characterized in that: 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; the molar ratio of the dianhydride monomer to the total amount of the aromatic diamine monomer and the o-hydroxy diamine monomer is 0.98:1 to 1.02:

1.

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

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

9. The preparation method according to claim 1, characterized in that: The stretching process parameters in step (3) are as follows: stretching temperature is 20-80°C, longitudinal stretching ratio is 1.1-1.9, transverse stretching ratio is 1.1-1.9, and stretching speed is 10-50 mm / s.

10. The preparation method according to claim 1, characterized in that: The heat setting process parameters in step (3) are: heating from 300 °C to 450 °C, and the heating rate is 1-5 °C / min.

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