A polyamide-imide film with low coefficient of thermal expansion and a method for producing the same

Abstract

The application discloses a polyamide-imide film with low thermal expansion coefficient and a preparation method thereof, and belongs to the polyamide-imide film field. In view of the poor high-temperature dimensional stability of the existing polyamide-imide film, the method is as follows: aromatic diamine and aromatic dianhydride monomers are added in a polar aprotic solution under an inert atmosphere for polymerization; then at least one kind of monomer containing an amide bond is added, and a polyamide-imide acid solution is obtained after reaction; a compound containing copper ions is added, and a film forming solution is obtained after reaction; the film forming solution is coated on a substrate and is subjected to programmed temperature rising to form a film, so that the polyamide-imide film is obtained. The heterocyclic ring and the amide bond are introduced into the polyimide structure, the strong intermolecular interaction and the ordered molecular chain arrangement generated thereby improve the thermal dimensional stability of the film, and the film has excellent mechanical properties: the tensile strength is greater than 200 MPa, and the tensile elastic modulus is greater than 5 GPa. The polyamide-imide film prepared by the application has a wide application prospect in the flexible device field.

Description

Technical Field

[0001] This invention belongs to the field of polyamide-imide films, and more specifically, relates to a polyamide-imide film with a low coefficient of thermal expansion and a method for preparing the same. Background Technology

[0002] Polyimide (PI) films possess excellent high and low temperature resistance, chemical stability, low dielectric constant, and radiation resistance, making them a typical high-performance polymer material widely used in microelectronics and aerospace fields. To date, to meet more diverse application requirements and cope with increasingly complex application scenarios, higher performance demands have been placed on polyimide. As one of the commonly used substrate materials for flexible display devices, polyimide films need to exhibit both excellent thermal stability and a low linear coefficient of thermal expansion during the multiple heating-cooling cycles in device manufacturing. Furthermore, they must possess a high glass transition temperature (PI). T g Excellent thermal stability and mechanical properties are also requirements for polyimide applications in organic solar cells, flexible printed circuit boards, aerospace, and other fields. When the thermal dimensional stability of the plastic substrate is insufficient, even at lower temperatures than the substrate material... T g At certain temperatures, significant expansion and contraction occur with increasing heating-cooling cycles. This expansion and contraction can lead to serious problems such as misalignment and bonding failure of various electronic components, lamination warping, and transparent electrode breakdown. The most direct way to improve the thermal dimensional stability of plastic substrates is to reduce the coefficient of thermal expansion (CTE) in the planar direction of the film in the glassy state. Regarding thermal dimensional stability, the substrate... T g It must be as high as possible or almost impossible to detect by high-sensitivity methods such as dynamic mechanical analysis.

[0003] Among commercially available polyimide materials, those such as DuPont's Kapton H, Ube Industries' Upilex R, and Mitsubishi's Novax have the highest heat resistance and excellent overall performance. Kapton H... T g Upilex R can reach 385℃. T g It can reach 285℃, Novax's T g It can reach 350℃. However, the thermal properties of these PI materials, especially... T g Thermal expansion limits the maximum processing temperature for substrates. Therefore, developing substrates with higher... T gThe need for lower CTE and PI is urgent. The thermal stability of polyimide materials is typically improved by introducing inorganic fillers or structural modification.

[0004] Chinese patent CN 114015090B discloses a method for preparing a polyimide film with a low coefficient of thermal expansion. The method involves incorporating carbon nanotubes into a polyamic acid solution of a certain viscosity to prepare a polyimide composite film with a three-layer structure. The preparation process is relatively complex and can easily lead to uneven mixing. Furthermore, the incorporation of carbon nanotubes can significantly reduce other properties of the film.

[0005] Chinese patent CN 118879071 A discloses a method for preparing a polyamide-imide film with a high glass transition temperature and low thermal expansion. This method reduces the coefficient of thermal expansion by introducing an amide structure. However, the reaction uses aliphatic anhydrides, which have a limited effect on reducing the coefficient of thermal expansion of the film.

[0006] Chinese patent CN 115558103 B discloses a method for preparing a high-toughness polyimide antistatic film based on metal coordination crosslinking. This method first involves the combination of a ligand diamine molecule and a metal ion to form a crosslinked structure, then reacting it with a dianhydride monomer, and finally obtaining a metal-coordinated polyimide film through programmed temperature rise. It is primarily intended for use as an antistatic film, and the coefficient of thermal expansion is not measured.

[0007] In the manufacturing process of electronic devices, polyimide films, used as substrate materials, often need to be bonded or in contact with other support materials with low coefficients of thermal expansion. Therefore, matching the coefficients of thermal expansion between the polyimide film and the metal or inorganic layer is crucial. Since the CTE of metal or inorganic materials is typically below 20 ppm / K, while the CTE of commercial PI films generally exceeds 30 ppm / K, this significant difference can generate destructive thermal stress at the interface, directly affecting the yield and performance of device fabrication. This places stringent requirements on the high-temperature thermal dimensional stability of PI films, and developing novel PI materials that combine high heat resistance and wide-temperature-range dimensional stability has become an urgent problem to be solved in the field of materials science.

[0008] Introducing rigid molecular chains can improve the coefficient of thermal expansion of polyimide materials, but it also reduces the flexibility of the molecular chains, leading to increased film brittleness. In addition, polyimide films with low coefficients of thermal expansion usually require high modulus, but high modulus is accompanied by low elongation at break, thus affecting the flexibility of flexible devices. Therefore, the balance between dimensional stability and mechanical properties is also particularly important. Summary of the Invention

[0009] This invention provides a polyamide-imide film with high glass transition temperature, high strength, and low coefficient of thermal expansion, as well as its preparation method. Addressing the problem of poor high-temperature dimensional stability in existing polyimide films, this invention introduces heterocyclic rings and amide bonds into the polyimide structure. The strong intermolecular interactions and ordered molecular chain arrangement enhance the thermal dimensional stability of the film, improving its heat resistance and dimensional stability while maintaining a certain level of mechanical strength. Furthermore, this film exhibits excellent mechanical properties, with a tensile strength greater than 200 MPa and a tensile modulus greater than 5 GPa. The polyamide-imide film prepared by this invention has broad application prospects in the field of flexible devices.

[0010] To address the aforementioned technical problems, the present invention adopts the following technical solution:

[0011] The film of the present invention is prepared by copolymerization of a nitrogen-containing aromatic heterocyclic diamine with acyl chloride and dianhydride monomers. By introducing aromatic heterocycles and amide bonds into the polyimide molecular chain, the rigidity of the molecular chain is increased; at the same time, metal ions are added for coordination, resulting in a polyamide-imide film with both low thermal expansion coefficient and high mechanical strength.

[0012] The purpose of this invention is to provide a method for preparing a polyamide-imide film with a low coefficient of thermal expansion, comprising the following steps:

[0013] Step 1: Under an inert atmosphere, add aromatic diamine and aromatic dianhydride monomers to a polar aprotic solution to carry out a polymerization reaction to generate a precursor polyamic acid solution.

[0014] Step 2: Then add at least one monomer containing an amide bond, react at room temperature for a period of time to obtain a polyamide-amic acid solution with a solid content of 8% to 35%.

[0015] Step 3: Then add a compound containing copper ions, stir and react for a certain period of time to obtain a film-forming solution;

[0016] Step 4: Then, the coating is applied to the substrate, and the film is formed by programmed temperature rise to finally obtain a polyimide film.

[0017] Further specifying, in step 1, the inert atmosphere is one or more of nitrogen, argon, and helium.

[0018] Further specifying, in step 1, the polar aprotic solution is one or more of dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc).

[0019] Further specifying, the diamine monomer must contain one or more selected from structure I and structure II, and may optionally contain other aromatic diamines that are not structurally restricted, such as p-phenylenediamine, m-phenylenediamine, 9,9-bis(4-aminophenyl)fluorene, 2,4,6-trimethyl-1,3-phenylenediamine, 2,2'-bis(trifluoromethyl)diaminobiphenyl, etc.

[0020] Structure I,

[0021] Structure II, Ar represents an aromatic group, such as benzene ring, naphthalene, anthracene, etc.

[0022] Further specifying, in step 2, the reaction is carried out at room temperature for 5 h to 12 h.

[0023] Further specifying, in step 3, the stirring reaction is carried out for 2 h to 10 h.

[0024] Further specifying, the aromatic dianhydride is selected from one or more of pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), bisphenol A type diether dianhydride (BPADA), and hexafluorodianhydride (6FDA).

[0025] Further specifying, the molar ratio of aromatic dianhydride to aromatic diamine is (1~9):10.

[0026] Further specifying, the monomer containing the amide bond is trimellitic anhydride chloride and / or terephthaloyl chloride.

[0027] Further specifying, the molar ratio of the monomer containing the amide bond to the aromatic diamine is (1~9):10.

[0028] Further specifying, the copper-containing compounds are copper tetraacetonitrile hexafluorophosphate, copper chloride, copper trifluoroacetate hydrate, or copper trifluoromethanesulfonate.

[0029] Further specifying the temperature program, the drying process is as follows: drying at 80℃ for 4 hours, drying at 120℃ for 1 hour, drying at 150℃ for 1 hour, drying at 180℃ for 1 hour, drying at 250℃ for 1 hour, and drying at 300℃ for 1 hour.

[0030] Another object of the present invention is to provide a polyamide-imide film prepared by any of the above methods, which has a low coefficient of thermal expansion, a tensile strength greater than 200 MPa, and a tensile modulus greater than 5 GPa.

[0031] Nanofillers can also be incorporated into polyamic acid to obtain films with low coefficient of thermal expansion and excellent mechanical properties.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] This invention synthesizes a series of polyamide-imide films with metal ion complexes by introducing aromatic heterocycles and amide bonds into the molecular chain and then complexing them with copper ions. The prepared polyamide-imide films have the characteristics of low coefficient of thermal expansion and excellent thermal and mechanical properties.

[0034] This invention introduces heterocyclic rings and amide bonds into the polyimide structure. The strong intermolecular interactions and ordered molecular chain arrangement enhance the thermal dimensional stability of the film. Simultaneously, the film exhibits excellent mechanical properties, with a tensile strength greater than 200 MPa and a tensile elastic modulus greater than 5 GPa. The polyamide-imide film prepared by this invention has broad application prospects in the field of flexible devices.

[0035] For a deeper understanding of the features and technical content of this invention, please refer to the accompanying detailed description and drawings. It should be noted that the drawings are provided for illustrative purposes only and are not intended to limit the scope of the invention. Attached Figure Description

[0036] Figure 1 This is the DMA test result of Implementation Example 1;

[0037] Figure 2 This is the DMA test result of Implementation Example 2;

[0038] Figure 3 This is the DMA test result of Implementation Example 3;

[0039] Figure 4 This is the DMA test result for Comparative Example 1. Detailed Implementation

[0040] The present invention will be described in detail below with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but should not be considered as limiting the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0041] Example 1: The preparation method of the polyamide-imide film with low thermal expansion coefficient in this example is carried out according to the following steps:

[0042] Step 1: Under nitrogen protection with a volume purity of 99.999%, add 45.855 g of N-methylpyrrolidone, 2.613 g (0.01 mol) of 2,5-bis(4-aminophenyl)pyridine and 1.471 g (0.005 mol) of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride to a three-necked flask equipped with a mechanical stirrer. The polymerization reaction is carried out at room temperature for 14 h to generate a precursor polyamic acid solution.

[0043] Step 2: Then add 1.011 g (0.005 mol) of trimellitic anhydride chloride and react at room temperature for 10 h to obtain a polyamide ammonium acid solution;

[0044] Step 3: Add 0.027 g (0.0002 mol) of copper chloride and stir at 200 rpm / min for 6 h to obtain the film-forming solution;

[0045] Step 4: Then, the coating is applied to a glass plate, and the temperature is increased by program to perform thermal imidization to form a film, thereby obtaining a polyamide-imide film with a low coefficient of thermal expansion.

[0046] The temperature program in step 4 is as follows: drying at 80℃ for 4 hours, drying at 120℃ for 1 hour, drying at 150℃ for 1 hour, drying at 180℃ for 1 hour, drying at 250℃ for 1 hour, and drying at 300℃ for 1 hour.

[0047] The CTE of the thin film prepared by the method in this embodiment was measured to be 4.573 ppm / K. T g The temperature is 324℃.

[0048] Example 2: The preparation method of the polyamide-imide film with low thermal expansion coefficient in this example is carried out according to the following steps:

[0049] Step 1: Under nitrogen protection with a volume purity of 99.999%, add 94.182 g of N-methylpyrrolidone, 6.533 g (0.025 mol) of 2,5-bis(4-aminophenyl)pyridine and 3.678 g (0.0125 mol) of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride to a three-necked flask equipped with a mechanical stirrer. The polymerization reaction is carried out at room temperature for 14 h to generate a precursor polyamic acid solution.

[0050] Step 2: Then add 2.632 g (0.0125 mol) of trimellitic anhydride chloride and react at room temperature for 10 h to obtain a polyamide amic acid solution;

[0051] Step 3: Add 0.269 g (0.002 mol) of copper chloride and stir at 200 rpm / min for 10 h to obtain the film-forming solution;

[0052] Step 4: Then, the coating is applied to a glass plate, and the temperature is increased by program to perform thermal imidization to form a film, thereby obtaining a polyamide-imide film with a low coefficient of thermal expansion.

[0053] The temperature program in step 4 is as follows: drying at 80℃ for 4 hours, drying at 120℃ for 1 hour, drying at 150℃ for 1 hour, drying at 180℃ for 1 hour, drying at 250℃ for 1 hour, and drying at 300℃ for 1 hour.

[0054] The CTE of the thin film prepared by the method in this embodiment was measured to be 4.438 ppm / K. T g It is 329℃.

[0055] Example 3: The preparation method of the polyamide-imide film with low thermal expansion coefficient in this example is carried out according to the following steps:

[0056] Step 1: Under nitrogen protection with a volume purity of 99.999%, add 45.855 g of N-methylpyrrolidone, 2.613 g (0.01 mol) of 2,5-bis(4-aminophenyl)pyridine and 1.471 g (0.005 mol) of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride to a three-necked flask equipped with a mechanical stirrer. The polymerization reaction is carried out at room temperature for 14 h to generate a precursor polyamic acid solution.

[0057] Step 2: Then add 1.011 g (0.005 mol) of trimellitic anhydride chloride and react at room temperature for 10 h to obtain a polyamide ammonium acid solution;

[0058] Step 3: Add 0.040 g (0.0003 mol) of copper chloride and stir at 200 rpm / min for 6 h to obtain the film-forming solution;

[0059] Step 4: Then, the coating is applied to a glass plate, and the temperature is increased by program to perform thermal imidization to form a film, thereby obtaining a polyamide-imide film with a low coefficient of thermal expansion.

[0060] The temperature program in step 4 is as follows: drying at 80℃ for 4 hours, drying at 120℃ for 1 hour, drying at 150℃ for 1 hour, drying at 180℃ for 1 hour, drying at 250℃ for 1 hour, and drying at 300℃ for 1 hour.

[0061] The CTE of the thin film prepared by the method in this embodiment was measured to be 4.340 ppm / K. Tg The temperature is 326℃.

[0062] Example 4: The preparation method of the polyamide-imide film with low thermal expansion coefficient in this example is carried out according to the following steps:

[0063] Step 1: Under nitrogen protection with a volume purity of 99.999%, add 82.447 g of N-methylpyrrolidone, 5.226 g (0.02 mol) of 2,5-bis(4-aminophenyl)pyridine and 2.942 g (0.01 mol) of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride to a three-necked flask equipped with a mechanical stirrer. The polymerization reaction is carried out at room temperature for 14 h to generate a precursor polyamic acid solution.

[0064] Step 2: Then add 2.106 g (0.01 mol) of trimellitic anhydride chloride and react at room temperature for 10 h to obtain a polyamide ammonium acid solution;

[0065] Step 3: Add 0.373 g (0.0008 mol) of copper hexafluorophosphate tetraacetonitrile and stir at 300 rpm / min for 5 h to obtain the film-forming solution;

[0066] Step 4: Then, the coating is applied to a glass plate, and the temperature is increased by program to perform thermal imidization to form a film, thereby obtaining a polyamide-imide film with a low coefficient of thermal expansion.

[0067] The temperature program in step 4 is as follows: drying at 80℃ for 4 hours, drying at 120℃ for 1 hour, drying at 150℃ for 1 hour, drying at 180℃ for 1 hour, drying at 250℃ for 1 hour, and drying at 300℃ for 1 hour.

[0068] The CTE of the thin film prepared by the method in this embodiment was measured to be 2.618 ppm / K.

[0069] Example 5: The preparation method of the polyamide-imide film with low thermal expansion coefficient in this example is carried out according to the following steps:

[0070] Step 1: Under nitrogen protection with a volume purity of 99.999%, add 82.438 g of N-methylpyrrolidone, 5.226 g (0.02 mol) of 2,5-bis(4-aminophenyl)pyridine and 2.942 g (0.01 mol) of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride to a three-necked flask equipped with a mechanical stirrer. The polymerization reaction is carried out at room temperature for 14 h to generate a precursor polyamic acid solution.

[0071] Step 2: Then add 2.021 g (0.01 mol) of trimellitic anhydride chloride and react at room temperature for 10 h to obtain a polyamide ammonium acid solution;

[0072] Step 3: Add 0.250 g (0.001 mol) of copper sulfate pentahydrate and stir at 200 rpm / min for 8 hours to obtain the film-forming solution;

[0073] Then it is coated onto a glass plate, and thermal imidization is performed by programmed temperature rise to obtain a polyamide-imide film with a low coefficient of thermal expansion;

[0074] The temperature program in step 4 is as follows: drying at 80℃ for 4 hours, drying at 120℃ for 1 hour, drying at 150℃ for 1 hour, drying at 180℃ for 1 hour, drying at 250℃ for 1 hour, and drying at 300℃ for 1 hour.

[0075] The CTE of the thin film prepared by the method in this embodiment was measured to be 1.962 ppm / K.

[0076] Example 6: The preparation method of the polyamide-imide film with low thermal expansion coefficient in this example is carried out according to the following steps:

[0077] Step 1: Under nitrogen protection with a volume purity of 99.999%, add 82.438 g of N-methylpyrrolidone, 5.226 g (0.02 mol) of 2,5-bis(4-aminophenyl)pyridine and 2.942 g (0.01 mol) of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride to a three-necked flask equipped with a mechanical stirrer. The polymerization reaction is carried out at room temperature for 14 h to generate a precursor polyamic acid solution.

[0078] Step 2: Then add 2.021 g (0.01 mol) of trimellitic anhydride chloride and react at room temperature for 10 h to obtain a polyamide ammonium acid solution;

[0079] Step 3: Add 0.289 g (0.0008 mol) of copper trifluoromethanesulfonate and stir at 300 rpm / min for 6 h to obtain the film-forming solution;

[0080] Step 4: Then, the coating is applied to a glass plate, and the temperature is increased by program to perform thermal imidization to form a film, thereby obtaining a polyamide-imide film with a low coefficient of thermal expansion.

[0081] The temperature program in step 4 is as follows: drying at 80℃ for 4 hours, drying at 120℃ for 1 hour, drying at 150℃ for 1 hour, drying at 180℃ for 1 hour, drying at 250℃ for 1 hour, and drying at 300℃ for 1 hour.

[0082] The CTE of the thin film prepared by the method in this embodiment was measured to be 5.233 ppm / K.

[0083] Example 7: The preparation method of the polyamide-imide film with low thermal expansion coefficient in this example is carried out according to the following steps:

[0084] Step 1: Under nitrogen protection with a volume purity of 99.999%, 73.773 g of N-methylpyrrolidone, 3.920 g (0.015 mol) of 2,5-bis(4-aminophenyl)pyridine and 3.972 g (0.0135 mol) of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride were added to a three-necked flask equipped with a mechanical stirrer. The polymerization reaction was carried out at room temperature for 14 h to generate a precursor polyamic acid solution.

[0085] Step 2: Then add 0.305 g (0.0015 mol) of terephthaloyl chloride and react at room temperature for 10 h to obtain a polyamide-amic acid solution;

[0086] Step 3: Add 0.054 g (0.0004 mol) of copper chloride and stir at 200 rpm / min for 8 h to obtain the film-forming solution;

[0087] Step 4: Then, the coating is applied to a glass plate, and the temperature is increased by program to perform thermal imidization to form a film, thereby obtaining a polyamide-imide film with a low coefficient of thermal expansion.

[0088] The temperature program in step 4 is as follows: drying at 80℃ for 4 hours, drying at 120℃ for 1 hour, drying at 150℃ for 1 hour, drying at 180℃ for 1 hour, drying at 250℃ for 1 hour, and drying at 300℃ for 1 hour.

[0089] The CTE of the thin film prepared by the method in this embodiment was measured to be 3.600 ppm / K. T g The temperature is 362℃.

[0090] Comparative Example 1:

[0091] Under nitrogen protection with a volume purity of 99.999%, 2.613 g (0.01 mol) of 2,5-bis(4-aminophenyl)pyridine, 1.471 g (0.005 mol) of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 1.011 g (0.005 mol) of trimellitic anhydride chloride, and 45.855 g of N-methylpyrrolidone were added to a three-necked flask equipped with a mechanical stirrer. The reaction was carried out at room temperature for 24 h to obtain a polyamide-amic acid solution. The film-forming solution was then coated onto a glass plate, and thermal imidization was performed by programmed temperature rise to obtain a polyamide-imide film.

[0092] The temperature program in step 4 is as follows: drying at 80℃ for 4 hours, drying at 120℃ for 1 hour, drying at 150℃ for 1 hour, drying at 180℃ for 1 hour, drying at 250℃ for 1 hour, and drying at 300℃ for 1 hour.

[0093] The CTE of the film was measured to be 9.734 ppm / K. T g The temperature is 320℃.

[0094] Comparative Example 2:

[0095] Under nitrogen protection with a volume purity of 99.999%, 3.920 g (0.015 mol) of 2,5-bis(4-aminophenyl)pyridine, 3.972 g (0.0135 mol) of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 0.305 g (0.0015 mol) of terephthaloyl chloride, and 73.773 g of N-methylpyrrolidone were added to a three-necked flask equipped with a mechanical stirrer. The reaction was carried out at room temperature for 24 h to obtain a polyamide-amic acid solution. The film-forming solution was then coated onto a glass plate, and thermal imidization was performed by programmed temperature rise to obtain a polyimide film.

[0096] The temperature program in step 4 is as follows: drying at 80℃ for 4 hours, drying at 120℃ for 1 hour, drying at 150℃ for 1 hour, drying at 180℃ for 1 hour, drying at 250℃ for 1 hour, and drying at 300℃ for 1 hour.

[0097] The CTE of the film was measured to be 3.518 ppm / K. T g The temperature is 350℃.

[0098] Figures 1-4 The DMA image of the sample is shown. Figure 1 It can be seen that the storage modulus begins to decrease most rapidly around 320℃, and tan δ shows that it reaches its peak at 324℃. From the peak value of tan δ, we can obtain the following information about the sample. T g It is 324℃. From Figures 2-4 It can be concluded that tan δ reaches its peak at 329℃, 326℃, and 320℃, respectively, and is read as the peak value of these samples. T g .

[0099] Mechanical properties were tested on the listed examples, and the results are shown in Table 1.

[0100] Table 1. Test results of mechanical properties of polyimide films prepared in the examples of this invention.

[0101]

[0102] The specific embodiments of the present invention have been described in detail above. It should be noted that the present invention is not limited to the specific embodiments described above. Various modifications or alterations can be made by those skilled in the art without departing from the scope of protection defined by the claims, and all such modifications or alterations fall within the scope of the present invention.

Claims

1. A method for producing a polyamide-imide film having a low coefficient of thermal expansion, characterized by, The method comprises the following steps: Step 1, under the protection of inert atmosphere, aromatic diamine and aromatic dianhydride monomers are added in a polar aprotic solution to generate a precursor polyamide acid solution through polymerization, Step 2, then at least one kind of amide bond-containing monomer is added, and the reaction is carried out at room temperature for a period of time to obtain a polyamide amic acid solution, Step 3, then a copper ion-containing compound is added, and the reaction is carried out for a period of time to obtain a film-forming solution; Step 4, then the film-forming solution is coated on a substrate, and film formation is carried out through programmed temperature rising, and finally a polyamide-imide film is obtained; The diamine monomer comprises one or more selected from structure I and structure II, and other non-structure-limited aromatic diamines can be optionally added, Structure I, Structure II, Ar represents an aromatic group; The copper ion-containing compound is copper tetrafluorophosphate hexafluoro-phosphate, copper chloride, copper trifluoroacetate hydrate or copper trifluoromethanesulfonate, and the molar ratio of the copper ion-containing compound to the aromatic diamine is 1: (100-2).

2. The method of claim 1, wherein, The inert atmosphere is one or more of nitrogen, argon and helium.

3. The method of claim 1, wherein, The polar aprotic solution is one or more of dimethyl sulfoxide (DMSO), N-methyl pyrrolidone (NMP), N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAc).

4. The method of claim 1, wherein, The aromatic dianhydride is one or more selected from pyromellitic dianhydride (PMDA), 3,3',4,4'-diphenyl tetracarboxylic dianhydride (BPDA), bisphenol A type diether dianhydride (BPADA) and hexafluoro dianhydride (6FDA); and the molar ratio of the aromatic dianhydride to the aromatic diamine is (1-9):

10.

5. The method of claim 1, wherein, The polymerization is carried out at -10-50℃.

6. The method of claim 1, wherein, The amide bond-containing monomer is trimellitic anhydride chloride and / or terephthaloyl chloride, and the molar ratio of the amide bond-containing monomer to the aromatic diamine is (1-9):

10.

7. The method of claim 1, wherein, The programmed temperature rising process is: drying at 80℃ for 4 h, and then drying at 120℃, 150℃, 180℃, 250℃ and 300℃ for 1 h respectively.

8. A polyamide-imide film prepared by the process of any one of claims 1 to 7, characterized in that The polyamide-imide film has a low coefficient of thermal expansion, a tensile strength greater than 200 MPa and a tensile elastic modulus greater than 5 GPa.

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