A polyimide film and its preparation method

Abstract

This invention discloses a polyimide film and its preparation method. The polyimide film is a crystalline material with grains oriented along directions parallel to the film surface plane. Specifically, the crystal orientation factor in the MD direction is 0.08–0.65, and the crystal orientation factor in the TD direction is also 0.08–0.65. The polyimide film exhibits a dielectric constant less than or equal to 3.1 at 10 GHz, a dielectric loss less than or equal to 0.010 at 10 GHz, a dielectric constant less than or equal to 3.1 at 30 GHz, and a dielectric loss less than or equal to 0.012 at 30 GHz. The polyimide film possesses characteristics such as low dielectric constant, low dielectric loss, and low moisture absorption, and the crystal orientation in the MD / TD directions is individually controllable.

Description

Technical Field

[0001] This invention belongs to the field of polyimide, and particularly relates to polyimide films and their preparation methods. Background Technology

[0002] Polyimide (PI) is a type of polymer containing an imide ring structure in its molecular backbone. It is one of the best-performing functional polymer materials currently available and has been widely used in high-precision fields such as aerospace and microelectronics. PI materials have long been one of the most important basic materials in the microelectronics industry, with PI films and adhesives widely used in the manufacturing and packaging of microelectronic devices. However, with technological advancements, the performance requirements for basic materials across industries are constantly increasing. In particular, the miniaturization, integration, and flexibility of microelectronic devices, along with the continuous advancement of data transmission towards high frequency and high speed, have rendered traditional polyimide materials increasingly inadequate.

[0003] Especially with the advent of the 5G era, ultra-high-speed, ultra-high-data-density millimeter-wave communication technology will be fully deployed. To ensure high-fidelity transmission of millimeter-wave signals, the dielectric materials used in signal transmitting and receiving components must have very low dielectric constants and dielectric losses to minimize signal delay and loss. Traditional PI substrates suffer from high dielectric constants, dielectric losses, and hygroscopicity, making them unsuitable for application requirements. Therefore, there is an urgent need to develop new dielectric materials to drive the rapid development of the 5G industry. Summary of the Invention

[0004] To overcome the problems existing in the prior art, the present invention provides a polyimide film and its preparation method, wherein the polyimide film has advantages such as low dielectric constant, low dielectric loss, and low moisture absorption rate, and the polyimide film has the characteristic that the crystal orientation in the MD / TD direction can be individually controlled.

[0005] One of the objectives of this invention is to provide a polyimide film, which is a crystalline material with grains oriented along a direction parallel to the plane of the film surface, wherein the crystal orientation factors in the MD direction and the TD direction are each independently 0.08 to 0.65, and the difference between the crystal orientation factor in the MD direction and the orientation factor in the TD direction is -0.50 to 0.50.

[0006] For example, the crystal orientation factor in the MD direction is 0.08, 0.1, 0.12, 0.15, 0.18, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, or 0.65, and the crystal orientation factor in the TD direction is 0.08, 0.1, 0.12, 0.15, 0.18, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, or 0.65. The difference between the crystal orientation factor in the MD direction and the orientation factor in the TD direction is -0.50, -0.40, -0.30, -0.20, -0.10, 0, 0.10, 0.20, 0.30, 0.40, or 0.50.

[0007] Among them, when the absolute value of the difference between the orientation factors in the two directions of MD / TD is too large, (1) the film is prone to pores or tears, and (2) if the anisotropy difference of the film is too large, it is not suitable for application.

[0008] In this invention, the MD direction refers to the longitudinal direction, and the TD direction refers to the transverse direction. The crystal orientation factors in both directions can be adjusted through the preparation process.

[0009] In a preferred embodiment, the crystal orientation factors in the MD direction and the TD direction are each independently 0.10 to 0.55, and the difference between the crystal orientation factor in the MD direction and the orientation factor in the TD direction is -0.50 to 0.50.

[0010] In a preferred embodiment, the average value of the crystal orientation factor in the MD direction and the crystal orientation factor in the TD direction is ≤0.40, preferably ≤0.38.

[0011] Experiments revealed that the crystal orientation factor of the material, including both the unidirectional (MD / TD) and average crystal orientation factors in both directions, should not be too large. While theoretically a high orientation factor is beneficial for improving the dielectric properties and reducing moisture absorption, an excessively high crystal orientation factor can easily lead to an increase in defects in the thin film material (micropores, gaps, etc.), reducing processing yield. Specifically, when the average crystal orientation factor in both directions is greater than 0.4, it results in an increase in defects such as micropores and gaps on the thin film surface.

[0012] In a preferred embodiment, the moisture absorption rate of the polyimide film is less than or equal to 1.0%, preferably less than or equal to 0.8%, and more preferably ≤0.6%.

[0013] In a preferred embodiment, the dielectric constant of the polyimide film at a frequency of 10 GHz is less than or equal to 3.1, preferably less than or equal to 3.0.

[0014] In a preferred embodiment, the dielectric loss of the polyimide film at a frequency of 10 GHz is less than or equal to 0.010, preferably less than or equal to 0.008.

[0015] In a preferred embodiment, the dielectric constant of the polyimide film at a frequency of 30 GHz is less than or equal to 3.1, preferably less than or equal to 3.0.

[0016] In a preferred embodiment, the dielectric loss of the polyimide film at a frequency of 30 GHz is less than or equal to 0.012, preferably less than or equal to 0.009.

[0017] In a preferred embodiment, the polyimide film satisfies the condition of formula (1):

[0018]

[0019] In formula (1), ε1 represents the dielectric constant of the polyimide film at 10 GHz after being conditioned at 25°C and 65% humidity for 72 h; ε1 represents the dielectric constant of the polyimide film at 10 GHz after being dried in a vacuum oven at 150°C for 48 h.

[0020] This ratio represents the ratio of the dielectric constant of the film in its hygroscopic state to that in its dry state. The higher the moisture absorption rate, the higher the dielectric constant of the material in the hygroscopic state, and the larger the ratio of the dielectric constant in the dry state. Therefore, this ratio actually reflects the effect of moisture absorption on the dielectric constant value of the material.

[0021] The polyimide film described in this invention is a homogeneous heterogeneous polyimide film. Due to the crystal orientation effect, the molecular chain segments in the polyimide film material are tightly packed, and the molecular chain spacing is reduced, which effectively restricts the penetration and adsorption of water molecules, and the moisture absorption rate of the material is reduced. At the same time, the increased intermolecular interaction due to the grain orientation weakens the mobility of molecular segments under an external electric field, thus reducing the dielectric constant and dielectric loss.

[0022] In this invention, the crystallinity of the polyimide film is 8.0-60%, preferably 10-60%.

[0023] For example, the crystallinity of the polyimide film is 8.0%, 10%, 20%, 30%, 40%, 50%, or 60%.

[0024] A second objective of this invention is to provide a method for preparing the polyimide film described in one objective of this invention, comprising: subjecting an initial polyimide film to biaxial stretching and / or heat treatment annealing to obtain the polyimide film.

[0025] In a preferred embodiment, the initial polyimide film can be purchased directly or prepared by means of methods disclosed in the prior art, preferably but not limited to the following:

[0026] A polyamic acid solution is cast or coated, then dried at 40–150°C (e.g., 40°C, 50°C, 60°C, 80°C, 100°C, 120°C, 130°C, or 150°C) under a protective atmosphere to remove the solvent. Finally, imidization is performed under a protective atmosphere by gradually increasing the temperature to 300–600°C (e.g., 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, or 600°C) (imidization of the polyamic acid film can be carried out by high-temperature thermal imidization or chemical imidization) to obtain the initial polyimide film; or,

[0027] Prepare a polyimide solution, cast or coat the polyimide solution, and then dry it at 40–150°C (e.g., 40°C, 50°C, 60°C, 80°C, 100°C, 120°C, 130°C, or 150°C) under a protective atmosphere to remove the solvent and obtain the initial polyimide film.

[0028] In a further preferred embodiment, the polyamic acid solution or the polyimide solution is prepared using dianhydride monomers and diamine monomers.

[0029] The polyamic acid solution can be obtained using any method disclosed in the prior art, such as polymerization of diamine monomers and dianhydride monomers; the polyimide solution can be obtained using any method disclosed in the prior art, such as: (1) direct polymerization of dianhydride and diamine in a high-boiling-point solvent to generate polyimide; (2) reaction of dianhydride and diamine monomers in aprotic polar solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), and N-methylpyrrolidone (NMP) to form polyimide. (2) Polyamic acid solution is formed, and then polyimide solution is obtained by imidization of polyamic acid solution (imidization of polyamic acid film can be carried out by high temperature thermal imidization or chemical imidization); (3) Polyimide is obtained by polyisoimide, which means that polyamic acid solution is first obtained by reaction of dicarboxylic acid anhydride and diamine, and then polyamic acid is dehydrated and cyclized into polyisoimide solution under the action of dehydrating agent, and finally isomerized into polyimide solution under the action of catalysts such as acid or alkali or under heat treatment conditions (100~250℃).

[0030] In a further preferred embodiment, the diamine monomer is preferably an aromatic diamine, such as 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 4,4'-diamino-2,2'-dimethylbiphenyl, 4,4'-diaminobiphenyl, 3,3'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 3,3'-bis(methyl)-4,4'-diaminobiphenyl, p-phenylenediamine, m-phenylenediamine, 2,3,5,6-tetrafluoro-1,4-phenylenediamine, 4,5-difluorophenyl-1,2-diamine, 2-fluoro-1,4-phenylenediamine, 2,2',3,3',5,5',6,6'-octafluoro-[1,1'-bisphenyl]-4,4'-diamine, 4,4'-diamino-2,2 At least one of '-bis(trifluoromethoxy)p-diaminobiphenyl, 2,2'-bis(1,1,2-trifluoro-2-(perfluoropropoxy)ethoxy)-[1,1'-bisphenyl]-4,4'-diamine, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 1,3-bis(2-trifluoromethyl-4-aminophenoxy)benzene, 1,3-bis(3-trifluoromethyl-4-aminophenoxy)benzene, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 1,3-bis(3'-aminophenoxy)benzene, 1,3-bis(4'-aminophenoxy)benzene, and 1,4-bis(3'-aminophenoxy)benzene.

[0031] In a further preferred embodiment, the dianhydride monomer is selected from aromatic tetracarboxylic dianhydrides, preferably from pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,2'-dimethyl-3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,2'-di(trifluoromethyl)-3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,2'-diphenyl-3,3',4,4'-biphenyltetracarboxylic dianhydride, and 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride. At least one of the following: 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 5,5'-methylenebis(isobenzofuran-1,3-dione), 1,3-dioxo-1,3-bisisobenzofuran-5-yl1,3-dioxo-1,3-bisisobenzofuran-5-carboxylic acid ester, 4,4'-(4,4'-isopropyldiphenoxy)phthalic anhydride, 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride), and p-phenylene-bisphenyltriester dianhydride.

[0032] Preferably, the diamine monomer and dianhydride monomer are polymerized in a polar aprotic solvent to form a polyamic acid or polyimide solution. The polymerization process can consist of multiple steps at different temperature ranges; this invention is not limited to this and any method disclosed in the prior art can be used. After the reaction is complete, a small amount of end-capping groups can be added to block the reactive groups at the ends of the polymer chains, such as phthalic anhydride or maleic anhydride. This invention is not limited to this in detail and any method disclosed in the prior art can be used.

[0033] In a preferred embodiment, when biaxial stretching is performed, the stretching ratios in the MD direction and TD direction are each independently 1.05 to 1.50.

[0034] In a further preferred embodiment, when biaxial stretching is performed, the stretching ratios in the MD direction and the TD direction are each independently 1.10 to 1.45.

[0035] For example, when biaxial stretching is performed, the stretch ratios in the MD and TD directions are independently 1.10, 1.15, 1.2, 1.25, 1.3, 1.35, 1.35, 1.4, or 1.45.

[0036] In a further preferred embodiment, the biaxial stretching process is either a synchronous biaxial stretching process or an asynchronous biaxial stretching process.

[0037] The synchronous bidirectional stretching process refers to stretching in two directions simultaneously; the asynchronous bidirectional stretching process refers to stretching in two directions not simultaneously, that is, stretching in the MD direction can be performed first and then stretching in the TD direction, or stretching in the TD direction can be performed first and then stretching in the MD direction.

[0038] In a preferred embodiment, when biaxial stretching is performed, the sum of the stretch ratios in the MD direction and the TD direction shall not exceed 2.70; otherwise, the average orientation factor of the film will be too high, causing problems such as tearing, pores and increased surface defects of the film.

[0039] In a preferred embodiment, when performing the stretching process, the stretching temperature is (Tg-30℃) to (Tg+100℃), where Tg represents the initial glass transition temperature of the polyimide film.

[0040] In a further preferred embodiment, when performing the stretching treatment, the stretching temperature is (Tg-20℃) to (Tg+70℃), where Tg represents the initial glass transition temperature of the polyimide film.

[0041] For example, when performing a tensile treatment, the tensile temperature is (Tg-30℃), (Tg-20℃), (Tg-10℃), Tg, (Tg+10℃), (Tg+20℃), (Tg+30℃), (Tg+40℃), (Tg+50℃), (Tg+60℃), (Tg+70℃), (Tg+80℃), (Tg+90℃), or (Tg+100℃).

[0042] When the temperature is too low, the molecular chain segments are basically frozen and it is not easy for them to be oriented. If they are forcibly stretched, the film is prone to defects such as tearing and pores. When the temperature is too high, the molecular chain segments move too violently, and the orientation caused by stretching is not easy to be fixed. Therefore, the orientation factor of the obtained polyimide film is not high, and it is also easy to cause wrinkles on the film surface.

[0043] In a preferred embodiment, when heat treatment annealing is performed, the initial stress in the MD direction and the TD direction are each independently 10 to 3000 MPa, preferably 50 to 2500 MPa.

[0044] For example, the initial stresses in the MD and TD directions can be independently set to 10 MPa, 50 MPa, 100 MPa, 150 MPa, 200 MPa, 300 MPa, 400 MPa, 500 MPa, 600 MPa, 800 MPa, 1000 MPa, 1200 MPa, 1500 MPa, 1800 MPa, 2000 MPa, 2200 MPa, 2500 MPa, 2800 MPa, or 3000 MPa. The initial stresses in the MD and TD directions should not be too high, otherwise the film may develop pores or tears.

[0045] In a preferred embodiment, when heat treatment annealing is performed, the heat treatment temperature is (Tg-20℃) to (Tg+100℃), where Tg represents the initial glass transition temperature of the polyimide film.

[0046] In a further preferred embodiment, when heat treatment annealing is performed, the heat treatment temperature is (Tg-10℃) to (Tg+70℃), where Tg represents the initial glass transition temperature of the polyimide film.

[0047] When the temperature is too low, the molecular chain segments are basically frozen and it is not easy for them to be oriented. If excessive initial stress is forcibly applied, it is easy to cause defects such as tearing and pores in the film. When the temperature is too high, the molecular chain segments move too violently, and the orientation caused by thermal annealing is not easy to be fixed. Therefore, the orientation factor of the obtained polyimide film is not high, and it is also easy to cause wrinkles on the film surface.

[0048] In a preferred embodiment, when performing heat treatment annealing, the heat treatment time is 50 to 6000 s, preferably 100 s to 4500 s.

[0049] For example, when performing heat treatment annealing, the heat treatment time can be 50s, 100s, 200s, 300s, 500s, 800s, 1000s, 1500s, 2000s, 2500s, 3000s, 3500s, 4000s, 4500s, 5000s, 5500s, or 6000s. Among these, the heat treatment annealing time should not be too short, as this may result in an excessively small orientation factor, while being too long may lead to phenomena such as wrinkles in the film.

[0050] In a preferred embodiment, during heat treatment annealing, when the annealing temperature is (Tg+50℃)~(Tg+100℃), the heat treatment time is controlled to be 50~2000s, where Tg represents the initial glass transition temperature of the polyimide film.

[0051] In a further preferred embodiment, when performing heat treatment annealing, when the annealing temperature is (Tg+70℃)~(Tg+100℃), the heat treatment time is controlled to be 50~1000s, where Tg represents the glass transition temperature of the initial polyimide film.

[0052] In a preferred embodiment, when performing heat treatment annealing, when the annealing temperature is (Tg-20℃) to (Tg+50℃) and does not include (Tg+50℃), the heat treatment time is controlled to be 1000 to 6000 s and does not include 1000 s, where Tg represents the glass transition temperature of the initial polyimide film.

[0053] In a further preferred embodiment, when performing heat treatment annealing, when the annealing temperature is (Tg-20℃)~(Tg+70℃) and does not include (Tg+70℃), the heat treatment time is controlled to be 2000~6000s and does not include 2000s, where Tg represents the glass transition temperature of the initial polyimide film.

[0054] When the heat treatment temperature is higher than the glass transition temperature, the chain segments are more mobile, and the heat treatment time should not be too long, otherwise the film will wrinkle significantly; when the heat treatment temperature is lower, the heat treatment time can be longer.

[0055] A third objective of this invention is to provide a polyimide film obtained by the preparation method described in the second objective of this invention.

[0056] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values; these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. In the following, various technical solutions can, in principle, be combined with each other to obtain new technical solutions, which should also be considered as specifically disclosed herein.

[0057] Compared with the prior art, the present invention has the following beneficial effects: the polyimide film has the characteristics of low dielectric constant, low dielectric loss, and low moisture absorption rate, and the crystal orientation in the MD / TD direction is individually controllable. Detailed Implementation

[0058] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0059] It should also be noted that the various specific technical features described in the following embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the various possible combinations will not be described separately in this invention.

[0060] Furthermore, various embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention. The resulting technical solutions are part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0061] Unless otherwise specified, the raw materials used in the examples and comparative examples are all disclosed in the prior art, such as those that can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0062] The testing methods used are as follows:

[0063] Dielectric constant and dielectric loss: Polyimide films were tested using the resonant cavity method at 10 GHz and 30 GHz according to the IPC-TM-650-2.5.5.9 standard.

[0064] Moisture absorption rate: The moisture absorption rate of the prepared polyimide film was tested according to the IPC-TM-650-2.6.2 standard.

[0065] Glass transition temperature: The sample was tested using a thermomechanical analyzer under a load of 0.05 N and a heating rate of 10 °C / min.

[0066] Crystallinity was obtained as follows:

[0067] Calculate using the following equation.

[0068] Cr (%) = S c / (S c +S a )

[0069] Where Cr represents the percentage crystallinity; S c S is the integral area of ​​the crystallization peak; a S is the integral area of ​​the amorphous peak. c With S a The X-ray diffraction data were obtained by fitting and dividing the data using Origin software.

[0070] Orientation factor: The prepared polyimide film was tested by grazing incidence X-ray diffraction. By adjusting the sample placement so that the incident X-rays were along the MD / TD direction, the tests were performed separately. To quantitatively characterize the degree of grain orientation, the absorption peak with the highest diffraction intensity in the range of 2θ13° to 30° was selected. According to the Hermans orientation model, the orientation factor of the sample in the MD / TD direction was calculated using the following formula.

[0071]

[0072] Where f is the orientation factor. For orientation parameters,

[0073]

[0074] in It is the azimuth angle. The XRD diffraction intensity is at a specific azimuth angle.

[0075]

Example 1-1

[0076] 16.0115 g (0.0500 mol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB) and 70 g of N-methyl-2-pyrrolidone (NMP) were added to a three-necked flask and stirred until dissolved. Then, 10.0322 g (0.0341 mol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 3.2064 g (0.0147 mol) pyromellitic dianhydride (PMDA) were added and the mixture was stirred at 70 °C under nitrogen protection for 48 h to obtain a polyamic acid solution.

[0077] After coating, the polyamic acid solution was first subjected to nitrogen protection at 100°C to remove most of the solvent. Then, under nitrogen protection, the temperature was gradually increased to 450°C to complete imidization. The resulting polyimide film had a glass transition temperature of 375°C. The obtained film was then subjected to asynchronous biaxial stretching (first in the MD direction with a stretching ratio of 1.50, then in the TD direction with a stretching ratio of 1.15) at 450°C under nitrogen protection to obtain the polyimide film of [Example 1-1], with a crystallinity of 56.8%.

[0078]

Examples 1-2

[0079] The preparation process of the polyamic acid solution and the imidization process are exactly the same as in [Example 1-1]. The asynchronous biaxial stretching MD and TD stretching ratios are also the same, but the TD stretching is performed first and then the MD stretching is performed to obtain the polyimide film of [Example 1-2], which has a crystallinity of 55.3%.

[0080]

Comparative Example 1

[0081] The preparation process of the polyamic acid solution and the imidization process were exactly the same as those in [Example 1-1] and [Example 1-2], but without the asynchronous biaxial stretching step, resulting in the polyimide film of [Comparative Example 1-1].

[0082]

Example 2-1

[0083] 8.4916 g (0.0400 mol) of 4,4'-diamino-2,2'-dimethylbiphenyl (DMDB), 2.0712 g (0.0115 mol) of 2,3,5,6-tetrafluoro-1,4-phenylenediamine (6FPDA) and 418 g of N,N-dimethylacetamide (DMAc) were added to a three-necked flask. After stirring and dissolving, 16.1115 g (0.0500 mol) of 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) was added. The mixture was stirred and reacted at 60 °C under nitrogen protection for 24 h to obtain a polyamic acid solution.

[0084] After coating, the above-mentioned polyamic acid solution was first subjected to nitrogen protection at 100°C to remove most of the solvent. Then, under nitrogen protection, the temperature was gradually increased to 320°C to complete imidization. The glass transition temperature of the resulting polyimide film was 351°C. The obtained film was then annealed at 340°C under nitrogen protection for 5500 s (initial stress 30 MPa in the MD direction and 2500 MPa in the TD direction) to obtain the polyimide film of [Example 2-1], with a crystallinity of 31.31%.

[0085]

Example 2-2

[0086] The preparation process of the polyamic acid solution and the imidization process were exactly the same as in [Example 2-1]. After the obtained film was annealed at 340°C under nitrogen protection for 2700s (initial stress in the MD direction 2700MPa, initial stress in the TD direction 50MPa), the polyimide film of [Example 2-2] was obtained, with a crystallinity of 32.89%.

[0087]

Example 2-3

[0088] The preparation process of the polyamic acid solution and the imidization process were exactly the same as in [Example 2-1]. After the obtained film was annealed at 440°C under nitrogen protection for 60s (initial stress in the MD direction 1500MPa, initial stress in the TD direction 1000MPa), the polyimide film of [Example 2-4] was obtained, with a crystallinity of 29.48%.

[0089]

Examples 2-4

[0090] The preparation process of the polyamic acid solution and the imidization process are exactly the same as in [Example 2-1]. After the obtained film is simultaneously biaxially stretched (MD direction stretch ratio 1.50, TD direction stretch ratio 1.10) at 380°C under nitrogen protection, the polyimide film of [Example 2-4] is obtained, with a crystallinity of 43.67%.

[0091]

Examples 2-5

[0092] The preparation process of the polyamic acid solution and the imidization process are exactly the same as in [Example 2-1]. The resulting film is subjected to asynchronous biaxial stretching (first in the MD direction, stretching ratio 1.25, then in the TD direction, stretching ratio 1.25) at 320°C under nitrogen protection to obtain the polyimide film of [Example 2-5], which has a crystallinity of 36.55%.

[0093] Comparative Example 2-1

[0094] The preparation process of the polyamic acid solution was exactly the same as that of [Example 2-1], but without the annealing step. After coating and imidization, the polyimide film of [Comparative Example 2-1] was obtained.

[0095] Comparative Example 2-2

[0096] The preparation process of the polyamic acid solution and the imidization conditions were exactly the same as in [Example 2-1]. The resulting film was annealed at 315°C under nitrogen protection for 300s (constant stress of 3.0MPa in the MD direction and constant stress of 3.0MPa in the TD direction) to obtain the polyimide film of [Comparative Example 2-2].

[0097] [Comparative Examples 2-3]

[0098] The preparation process of the polyamic acid solution and the imidization conditions were exactly the same as in [Example 2-1]. The resulting film was annealed at 465°C under nitrogen protection for 300s (constant stress of 1.0MPa in the MD direction and constant stress of 1.0MPa in the TD direction) to obtain the polyimide film of [Comparative Example 2-3].

[0099] [Comparative Examples 2-4]

[0100] The preparation process of the polyamic acid solution and the imidization conditions were exactly the same as in [Example 2-1]. The resulting film was annealed at 340°C under nitrogen protection for 4500s (initial stress 30MPa in the MD direction and initial stress 4000MPa in the TD direction) to obtain the polyimide film of [Comparative Example 2-4].

[0101] [Comparative Examples 2-5]

[0102] The preparation process of the polyamic acid solution and the imidization conditions were exactly the same as in [Example 2-1]. The resulting film was annealed at 340°C under nitrogen protection for 35 seconds (initial stress in the MD direction 30 MPa, initial stress in the TD direction 2500 MPa) to obtain the polyimide film of [Comparative Example 2-5].

[0103] [Comparative Examples 2-6]

[0104] The preparation process of the polyamic acid solution and the imidization conditions were exactly the same as in [Example 2-1]. The resulting film was annealed at 340°C under nitrogen protection for 9000s (initial stress 30MPa in the MD direction and initial stress 2500MPa in the TD direction) to obtain the polyimide film of [Comparative Example 2-6].

[0105] [Comparative Examples 2-7]

[0106] The preparation process of the polyamic acid solution and the imidization process were exactly the same as in [Example 2-1]. The resulting film was then subjected to simultaneous biaxial stretching (MD direction stretching ratio 1.50, TD direction stretching ratio 1.10) at 315°C under nitrogen protection to obtain the polyimide film of [Comparative Example 2-7].

[0107] [Comparative Examples 2-8]

[0108] The preparation process of the polyamic acid solution and the imidization process were exactly the same as in [Example 2-1]. The resulting film was then subjected to simultaneous biaxial stretching (MD direction stretching ratio 1.50, TD direction stretching ratio 1.10) at 470°C under nitrogen protection to obtain the polyimide film of [Comparative Example 2-8].

[0109] Comparative Examples 2-9

[0110] The preparation process of the polyamic acid solution and the imidization process were exactly the same as in [Example 2-1]. The resulting film was then subjected to simultaneous biaxial stretching (MD direction stretching ratio 1.05, TD direction stretching ratio 1.05) at 340°C under nitrogen protection to obtain the polyimide film of [Comparative Example 2-9].

[0111] Comparative Example 2-10

[0112] The preparation process of the polyamic acid solution and the imidization process were exactly the same as in [Example 2-1]. The resulting film was subjected to asynchronous biaxial stretching (first in the MD direction, stretching ratio 1.65, then in the TD direction, stretching ratio 1.10) at 340°C under nitrogen protection to obtain the polyimide film of [Comparative Example 2-10].

[0113]

Example 3-1

[0114] 9.6069 g (0.030 mol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB), 6.5598 g (0.0309 mol) of 4,4'-diamino-2,2'-dimethylbiphenyl (DMDB) and 135 g of N,N-dimethylacetamide (DMAc) were added to a three-necked flask. After stirring and dissolving, 17.6520 g (0.0600 mol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) was added. The mixture was stirred and reacted at 20 °C under nitrogen protection for 36 h to obtain a polyamic acid solution.

[0115] After coating, the polyamic acid solution was first subjected to nitrogen protection at 100°C to remove most of the solvent. Then, under nitrogen protection, the temperature was gradually increased to 400°C to complete imidization. The resulting polyimide film had a glass transition temperature of 375°C. The obtained film was then subjected to simultaneous biaxial stretching at 450°C under nitrogen protection (MD direction stretching ratio 1.45, TD direction stretching ratio 1.10) to obtain the polyimide film of [Example 3-1], with a crystallinity of 48.17%.

[0116]

Example 3-2

[0117] The preparation process of the polyamic acid solution is exactly the same as that in [Example 3-1]. The resulting film is first subjected to simultaneous biaxial stretching (MD direction stretching ratio 1.15, TD direction stretching ratio 1.15) at 350°C under nitrogen protection to obtain the polyimide film of [Example 3-2], which has a crystallinity of 23.65%.

[0118]

Example 3-3

[0119] The preparation process of the polyamic acid solution is exactly the same as that in [Example 3-1]. The resulting film is then subjected to simultaneous biaxial stretching (MD direction stretching ratio 1.25, TD direction stretching ratio 1.50) at 405°C under nitrogen protection to obtain the polyimide film of [Example 3-3], which has a crystallinity of 43.55%.

[0120]

Examples 3-4

[0121] The preparation process of the polyamic acid solution is exactly the same as that in [Example 3-1]. The resulting film is first subjected to asynchronous biaxial stretching at 430°C under nitrogen protection (first in the MD direction, stretching ratio 1.45, and then in the TD direction, stretching ratio 1.30) to obtain the polyimide film of [Example 3-4], which has a crystallinity of 45.29%.

[0122]

Examples 3-5

[0123] The preparation process of the polyamic acid solution is exactly the same as that in [Example 3-1]. The resulting film is first annealed at 405°C under nitrogen protection for 3000s (initial stress in the MD direction is 1500MPa, and initial stress in the TD direction is 100MPa) to obtain the polyimide film of [Example 3-5], which has a crystallinity of 30.98%.

[0124]

Examples 3-6

[0125] The preparation process of the polyamic acid solution is exactly the same as that in [Example 3-1]. The resulting film is first annealed at 450°C under nitrogen protection for 200s (initial stress in the MD direction is 50MPa, and initial stress in the TD direction is 300MPa) to obtain the polyimide film of [Example 3-6], which has a crystallinity of 19.62%.

[0126] Comparative Example 3-1

[0127] The preparation process of the polyamic acid solution was exactly the same as that of [Example 3-1], but without the simultaneous biaxial stretching step. After coating and imidization, the polyimide film of [Comparative Example 3-1] was obtained.

[0128] Comparative Example 3-2

[0129] The preparation process of the polyamic acid solution was exactly the same as that in [Example 3-1]. After coating and imidization, the resulting film was simultaneously biaxially stretched at 485°C under nitrogen protection (MD stretch ratio 1.45, TD stretch ratio 1.10) to obtain the polyimide film of [Comparative Example 3-2].

[0130] Comparative Example 3-3

[0131] The preparation process of the polyamic acid solution was exactly the same as that in [Example 3-1]. After coating and imidization, the resulting film was simultaneously biaxially stretched at 335°C under nitrogen protection (MD stretch ratio 1.45, TD stretch ratio 1.10) to obtain the polyimide film of [Comparative Example 3-3].

[0132] Comparative Examples 3-4

[0133] The preparation process of the polyamic acid solution was exactly the same as that in [Example 3-1]. The resulting film was first annealed at 460°C under nitrogen protection for 3000s (initial stress 1500MPa in the MD direction and 100MPa in the TD direction) to obtain the polyimide film of [Comparative Example 3-5].

[0134] Comparative Examples 3-5

[0135] The preparation process of the polyamic acid solution was exactly the same as that in [Example 3-1]. The resulting film was first annealed at 345°C under nitrogen protection for 3000s (initial stress in the MD direction 1500MPa, initial stress in the TD direction 100MPa) to obtain the polyimide film of [Comparative Example 3-5].

[0136] [Comparative Examples 3-6]

[0137] The preparation process of the polyamic acid solution is exactly the same as that in [Example 3-1]. The resulting film is first subjected to asynchronous biaxial stretching at 430°C under nitrogen protection (first in the MD direction, stretching ratio 1.60, and then in the TD direction, stretching ratio 1.40) to obtain the polyimide film of [Example 3-6].

[0138] [Comparative Examples 3-7]

[0139] The preparation process of the polyamic acid solution is exactly the same as that in [Example 3-1]. The resulting film is first subjected to asynchronous biaxial stretching at 450°C under nitrogen protection (first in the MD direction, stretching ratio 1.50, and then in the TD direction, stretching ratio 1.25) to obtain the polyimide film of [Example 3-7].

[0140]

Example 4-1

[0141] 16.0115 g (0.050 mol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB) and 286 g of N,N-dimethylacetamide (DMAc) were added to a three-necked flask and stirred until dissolved. Then, 9.60 g (0.044 mol) of pyromellitic dianhydride (PMDA) and 2.71 g (0.0052 mol) of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) (BPADA) were added. The mixture was stirred and reacted at 40 °C under nitrogen protection for 48 h to obtain a polyamic acid solution.

[0142] After coating, the polyamic acid solution was first subjected to nitrogen protection at 100°C to remove most of the solvent. Then, under nitrogen protection, the temperature was gradually increased to 400°C to complete imidization. The resulting polyimide film had a glass transition temperature of 310°C. The obtained film was then subjected to simultaneous biaxial stretching at 350°C under nitrogen protection (MD stretching ratio 1.10, TD stretching ratio 1.10), resulting in the polyimide film of [Example 4] with a crystallinity of 13.23%.

[0143]

Example 4-2

[0144] The preparation of the polyamic acid solution and the imidization process were exactly the same as in [Example 4]. After coating and imidization, the resulting film was first annealed at 310°C under nitrogen protection for 100s (constant stress of 10MPa in the MD direction and constant stress of 10MPa in the TD direction), and then simultaneously biaxially stretched at 365°C under nitrogen protection (stretch ratio of 1.30 in the MD direction and stretch ratio of 1.30 in the TD direction) to obtain the polyimide film of [Example 4-2], which had a crystallinity of 15.74%.

[0145]

Example 5

[0146] 9.2857 g (0.0504 mol) of 4,4'-diaminobiphenyl (BzD), 2.0712 g (0.0115 mol) of 2,3,5,6-tetrafluoro-1,4-phenylenediamine (6FPDA) and 87 g of N,N-dimethylacetamide (DMAc) were added to a three-necked flask and stirred until dissolved. Then, 17.6520 g (0.0600 mol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) was added. The mixture was stirred at 40 °C under nitrogen protection for 48 h to obtain a polyamic acid solution. Then, 0.5628 g (0.0038 mol) of phthalic anhydride was added and the mixture was stirred for another 4 h to block the terminal amine groups.

[0147] After coating, the polyamic acid solution was first subjected to nitrogen protection at 100°C to remove most of the solvent. Then, under nitrogen protection, the temperature was gradually increased to 450°C to complete imidization. The resulting polyimide film had a glass transition temperature of 386°C. The obtained film was then subjected to simultaneous biaxial stretching at 425°C under nitrogen protection (1.20 stretch ratio in both the MD and TD directions), resulting in the polyimide film of Example 5, with a crystallinity of 33.43%.

[0148]

Example 6

[0149] 4.0048 g (0.0200 mol) of 4,4'-diaminodiphenyl ether (ODA), 12.6171 g (0.0394 mol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB) and 250 g of N,N-dimethylacetamide (DMAc) were added to a three-necked flask. After stirring and dissolving, 27.4998 g (0.0600 mol) of p-phenylene-bisphenyltripterygium dianhydride (TAHQ) was added. The mixture was stirred and reacted at 40 °C under nitrogen protection for 48 h to obtain a polyamic acid solution.

[0150] After coating, the polyamic acid solution was first subjected to nitrogen protection at 130°C to remove most of the solvent. Then, under nitrogen protection, the temperature was gradually increased to 420°C to complete imidization, resulting in a polyimide film with a glass transition temperature of 404°C. The obtained film was then subjected to simultaneous biaxial stretching at 430°C under nitrogen protection (1.20 stretch ratio in both the MD and TD directions), yielding the polyimide film of [Example 6], with a crystallinity of 41.55%.

[0151] Table 1: Test Results of Examples and Comparative Examples

[0152]

[0153] In this invention, synchronous / asynchronous biaxial stretching and / or thermal annealing operations can greatly enhance the in-plane orientation factor of polyimide films, while grain orientation can significantly improve the dielectric properties and moisture absorption of polyimide film materials.

[0154] As shown in Table 1, compared with the comparative examples whose orientation factor was less than 0.08, the examples with grain orientation characteristics all exhibited significantly reduced dielectric constant, dielectric loss, and hygroscopicity. This is mainly because grain orientation leads to denser molecular chain packing and reduced inter-chain spacing, making it less susceptible to water molecule penetration. Simultaneously, grain orientation increases intermolecular interactions, weakening the mobility of molecular segments under an external electric field, thus decreasing the dielectric constant and dielectric loss.

[0155] Furthermore, as shown in Table 1, the orientation factor of the polyimide film in the MD / TD direction can be adjusted by regulating the stretching ratio in the MD / TD direction during the biaxial stretching process and the initial stress in the MD / TD direction during the thermal annealing process. This is because by adjusting the stretching ratio or the initial stress, the molecules in the film tend to orient themselves along the stretching or stress direction, and the degree of grain orientation is closely related to the stretching ratio and the initial stress. Within a certain temperature range, the greater the stretching ratio and / or the initial stress, the more obvious the grain orientation.

[0156] The selection of stretching and heat annealing temperatures is crucial to the final performance of the polyimide film. At excessively low temperatures, the molecular chains are essentially frozen, making orientation difficult. Forced stretching or excessive initial stress can easily lead to defects such as tearing and pores. Conversely, at excessively high temperatures, the molecular chain activity is too vigorous, making it difficult for the orientation induced by stretching or heat annealing to solidify. Therefore, the resulting polyimide film has a low orientation factor and is prone to surface wrinkling. Because the microscopic molecular chain motion and crystallization states of polyimide materials with different chemical structures vary at different temperatures, the optimal stretching and heat annealing temperatures differ for different polyimide film systems and must be precisely adjusted according to their respective glass transition temperatures.

[0157] In addition, experiments have shown that the orientation factor of the material, including the unidirectional and average orientation factors of MD / TD in both directions, should not be too large. Although theoretically a high orientation factor is beneficial to improving the dielectric properties of the material and reducing the moisture absorption rate, an excessively high orientation factor can easily lead to an increase in defects in the thin film material (micropores, gaps, etc.) and reduce the processing yield.

[0158] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A polyimide film, which is a crystalline material with grains oriented along a direction parallel to the plane of the film surface, wherein, The crystal orientation factors in the MD and TD directions are each independently 0.08~0.55, the difference between the crystal orientation factor in the MD direction and the orientation factor in the TD direction is -0.50~0.50, and the average value of the crystal orientation factors in the MD direction and the crystal orientation factors in the TD direction is ≤0.

40. The diamine monomer used to prepare the polyimide film is an aromatic diamine, and the dianhydride monomer used to prepare the polyimide film is an aromatic tetracarboxylic dianhydride.

2. The polyimide film according to claim 1, characterized in that, The crystal orientation factors for the MD and TD directions are each independently 0.10~0.

55.

3. The polyimide film according to claim 1, characterized in that, The average crystal orientation factor in the MD direction and the average crystal orientation factor in the TD direction are ≤0.

38.

4. The polyimide film according to claim 1, characterized in that, The moisture absorption rate of the polyimide film is less than or equal to 1.0%; and / or, The dielectric constant of the polyimide film is less than or equal to 3.1 at a frequency of 10 GHz; and / or, The dielectric loss of the polyimide film at a frequency of 10 GHz is less than or equal to 0.010; and / or, The dielectric constant of the polyimide film is less than or equal to 3.1 at a frequency of 30 GHz; and / or, The dielectric loss of the polyimide film at a frequency of 30 GHz is less than or equal to 0.

012.

5. The polyimide film according to claim 1, characterized in that, The moisture absorption rate of the polyimide film is less than or equal to 0.8%; and / or, The polyimide film has a dielectric constant of less than or equal to 3.0 at a frequency of 10 GHz; and / or, The dielectric loss of the polyimide film at a frequency of 10 GHz is less than or equal to 0.008; and / or, The polyimide film has a dielectric constant of less than or equal to 3.0 at a frequency of 30 GHz; and / or, The dielectric loss of the polyimide film at a frequency of 30 GHz is less than or equal to 0.

009.

6. The polyimide film according to any one of claims 1 to 5, characterized in that, The polyimide film satisfies the condition of equation (1): In formula (1), ε1 represents the dielectric constant of the polyimide film at 10 GHz after being conditioned at 25 ℃ and 65% humidity for 72 h; ε1 represents the dielectric constant of the polyimide film at 10 GHz after being dried in a vacuum oven at 150 ℃ for 48 h.

7. A method for preparing a polyimide film according to any one of claims 1 to 6, comprising: The polyimide film is obtained by biaxial stretching and / or heat treatment annealing of the initial polyimide film; when biaxial stretching is performed, the stretch ratio in the MD direction is 1.05~1.45, the stretch ratio in the TD direction is 1.05~1.50, the sum of the stretch ratios in the MD and TD directions does not exceed 2.70, and the stretching temperature is (Tg-30℃)~(Tg+100℃); when heat treatment annealing is performed, the initial stress in the MD and TD directions is independently 10~3000MPa, and the heat treatment temperature is (Tg-20℃)~(Tg+100℃). The initial polyimide film is obtained as follows: a polyamic acid solution is cast or coated, then dried at 40-150°C under a protective atmosphere to remove the solvent, and finally imidized by gradually raising the temperature to 300-600°C under a protective atmosphere to obtain the initial polyimide film; the polyamic acid solution is prepared using a dianhydride monomer and a diamine monomer, wherein the diamine monomer is an aromatic diamine and the dianhydride monomer is an aromatic tetracarboxylic dianhydride.

8. The preparation method according to claim 7, characterized in that, The bidirectional stretching process can be either synchronous or asynchronous.

9. The preparation method according to claim 7, characterized in that, When performing the stretching process, the stretching temperature is (Tg-20℃)~(Tg+70℃), where Tg represents the initial glass transition temperature of the polyimide film.

10. The preparation method according to any one of claims 7 to 9, characterized in that, During heat treatment annealing, the initial stresses in the MD and TD directions are independently 50~2500MPa; and / or, When heat treatment annealing is performed, the heat treatment time is 50~6000s; and / or, When heat treatment annealing is performed, the heat treatment temperature is (Tg-10℃)~(Tg+70℃), where Tg represents the initial glass transition temperature of the polyimide film.

11. The preparation method according to any one of claims 7 to 9, characterized in that, When performing heat treatment annealing, the heat treatment time is 100s~4500s.

12. The preparation method according to claim 10, characterized in that, During heat treatment annealing, when the annealing temperature is (Tg+50℃)~(Tg+100℃), the heat treatment time is controlled to be 50~2000s, where Tg represents the initial glass transition temperature of the polyimide film; and / or, When performing heat treatment annealing, when the annealing temperature is (Tg-20℃)~(Tg+50℃) and does not include (Tg+50℃), the heat treatment time is controlled to be 1000~6000s and does not include 1000s, where Tg represents the glass transition temperature of the initial polyimide film.

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