A conjugated planar cross-linked polyimide film and a preparation method and application thereof

Abstract

This invention provides a conjugated planar cross-linked polyimide film, its preparation method, and its applications, belonging to the field of polymer materials technology. The cross-linked structure of this polyimide film effectively improves the thermal conduction path between polymer molecular chains, thereby significantly enhancing the material's thermal conductivity. Simultaneously, the coplanar structure between chains forms an ordered stack with the reluctantly polar benzene rings, which helps to reduce the dielectric constant and enhance the thermal and dimensional stability of the molecular chains. The polyimide film obtained by this invention possesses excellent thermal conductivity, low dielectric properties, and excellent thermal stability, making it suitable for high-performance electronic packaging, flexible printed circuit boards, and thermal management of microelectronic devices.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a conjugated planar cross-linked polyimide film, its preparation method, and its application. Background Technology

[0002] With the rapid development of 5G communication, IoT, and industrial internet technologies, electronic components are evolving towards miniaturization and high power. However, high-frequency, high-speed signal transmission generates a significant amount of Joule heat. If this heat cannot be efficiently dissipated, it will lead to performance degradation and shorten the lifespan of the equipment. Furthermore, 5G high-frequency signals are susceptible to dielectric loss; excessively high dielectric loss can cause signal attenuation, energy loss, and unstable data transmission. Therefore, developing materials with both high thermal conductivity and low dielectric loss is crucial for achieving efficient heat dissipation, high-fidelity signal transmission, and system stability in 5G communication equipment.

[0003] Polyimide (PI) films have become the preferred key functional material in optoelectronic devices due to their excellent thermal stability, superior mechanical strength, and outstanding chemical stability. However, ordinary polyimide films have low intrinsic thermal conductivity and high dielectric constant, making it difficult to meet the requirements of high-frequency electronic devices. In the prior art, although the thermal conductivity of polyimide can be improved or the dielectric constant can be reduced by introducing liquid crystal units or nanofillers, it is difficult to achieve synergistic optimization of the two properties. Chinese Patent 202110395408.3 discloses "an intrinsically high thermal conductivity liquid crystal polyimide film and its preparation method", which introduces phthalimide groups (rigid mesocrystalline units) and a large number of ether bonds (flexible chains) into the polyimide molecular chain, giving the polyimide thermotropic liquid crystal properties. The end-capping groups can fix the ordered structure of the molecular chain, so that it retains the high order of the molecular chain even after cooling to room temperature, thereby improving the intrinsic thermal conductivity of the film. However, the rigid mesocrystalline units introduced into this material increase the polarity of the material, resulting in high dielectric constant and high dielectric loss. Chinese patent 202411709465.4 discloses "A Low-Dielectric Fluorinated Silane-Modified Polyimide and Its Application," which introduces a bulky fluorene group structure, flexible ether bonds, a silane structure, and highly electronegative fluorine atoms into polyimide. The resulting fluorinated silane polyimide exhibits low dielectric properties and improves the polyimide's processability and solubility in various organic solvents. However, the flexible groups can lead to material distortion and entanglement, reducing the material's order and consequently shortening the phonon mean free path, thus reducing the material's thermal conductivity. Therefore, these methods often struggle to simultaneously achieve a synergistic optimization of high thermal conductivity and low dielectric loss. Summary of the Invention

[0004] The purpose of this invention is to provide a conjugated planar cross-linked polyimide film, its preparation method and application. By introducing a conjugated planar cross-linked structure, high thermal conductivity and low dielectric loss can be synergistically optimized, while also having high thermal stability, meeting the requirements of high-frequency electronic devices.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a conjugated planar crosslinked polyimide film, wherein the conjugated planar crosslinked polyimide in the film has the structure shown in Formula I: Formula I; In equation (Ⅰ), m>1, n>1, and m and n are both integers; Ar represents an aromatic dianhydride group, including one or more of the following: ; Ar'' represents an aromatic diamino group, including one or more of the following:

[0006] Ar' for .

[0007] This invention provides a method for preparing the conjugated planar crosslinked polyimide film described above, comprising the following steps: Diamine monomer, dianhydride monomer and organic solvent are mixed and polycondensation reaction is carried out in a protective gas to obtain polyamic acid solution; The polyamic acid solution was placed on a substrate, subjected to thermal imidization, and then separated and dried to obtain a conjugated planar cross-linked polyimide film. The diamine monomer includes a first diamine monomer and a second diamine monomer; The first diamine monomer is 3,3′-(but-1,3-diyne-1,4-diyl)diphenylamine; The second diamine monomer includes one or more of 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 4,4'-diaminobiphenyl, 2,2'-di(trifluoromethyl)diaminobiphenyl, 4,4'-diaminobenzoylaniline, 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether, and 4,4'-(hexafluoroisopropyl)bis(p-phenoxy)diphenylamine; The dianhydride monomer includes one or more of the following: 4,4-hexafluoroisopropylphthalic anhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride, bisphenol A type diether dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, and pyromellitic dianhydride.

[0008] Preferably, the molar amount of the first diamine monomer accounts for 5 to 20% of the total molar amount of the diamine monomer.

[0009] Preferably, the molar ratio of the dianhydride monomer to the diamine monomer is 1:1.

[0010] Preferably, the organic solvent includes one or more of N-methylpyrrolidone, N,N'-dimethylformamide and N,N'-dimethylacetamide; the mass ratio of the total mass of the diamine monomer and the dianhydride monomer to the mass of the organic solvent is 10~20:100.

[0011] Preferably, the temperature of the polycondensation reaction is -10 to 40°C, and the time is 1 to 24 hours; the protective gas is nitrogen.

[0012] Preferably, the temperature range for thermal imidization is 60~300℃, and the processing time is 6~16h.

[0013] Preferably, the gradient procedure of the thermal imidization is as follows: a first thermal imidization is performed at 120~140℃ for 0.5~1.5h, a second thermal imidization is performed at 180~200℃ for 0.5~1.5h, a third thermal imidization is performed at 220~240℃ for 0.5~1.5h, and a fourth thermal imidization is performed at 280~300℃ for 0.5~1.5h.

[0014] This invention provides the application of the conjugated planar cross-linked polyimide film described in the above technical solution or the conjugated planar cross-linked polyimide film prepared by the preparation method described in the above technical solution in the fields of 5G / 6G high-frequency communication packaging, flexible circuit boards and microelectronic thermal management.

[0015] This invention provides a conjugated planar crosslinked polyimide film. Through molecular structure design, this invention achieves a synergistic breakthrough in dielectric properties, thermal management, and structural stability via a triple synergistic mechanism: First, the conjugated units and benzene rings form an electron localization network, suppressing disordered charge migration and significantly reducing high-frequency signal transmission loss; second, the molecular chains are directionally crosslinked through the conjugated planar structure, constructing ordered phonon transport channels and overcoming the thermal conductivity bottleneck of traditional polymer materials; finally, the three-dimensional rigid network locks the molecular chain conformation through steric hindrance, achieving structural stability over a wide temperature range.

[0016] This invention uses 3,3′-(but-1,3-diyne-1,4-diyl)diphenylamine as a functional monomer, and reacts it with acid anhydride and diamine in a two-step process to prepare polyimide. First, a polyamic acid containing a diacetylene structure is generated through a condensation reaction; subsequently, during thermal imidization, the diacetylene groups on the molecular chain undergo thermal polymerization, forming a benzene ring structure and constructing large-volume coplanar crosslinking points between the molecular chains. This unique crosslinking structure effectively improves the thermal conduction path between polymer molecular chains, thereby significantly increasing the thermal conductivity of the material. Simultaneously, the coplanar structure between chains forms an ordered stack with the non-polar benzene rings, which helps to reduce the dielectric constant and enhance the thermal and dimensional stability of the molecular chains. The polyimide film obtained by this invention possesses excellent thermal conductivity, low dielectric properties, and excellent thermal stability, making it suitable for high-performance electronic packaging, flexible printed circuit boards, and thermal management of microelectronic devices, especially for thermal management and signal transmission in high-frequency electronic devices.

[0017] The conjugated planar crosslinked polyimide film provided by this invention combines low loss and high thermal conductivity, exhibiting low dielectric constant, high thermal conductivity, and high thermal stability. This material achieves this at 10... 6 Exhibits excellent dielectric properties at high frequency (Hz) (D k =2.4, tanδ=0.005). At the same time, it breaks through the thermal conductivity bottleneck of polymer materials, with an intrinsic thermal conductivity of 0.30 W / (m·K), which is more than 40% higher than that of traditional PI films, and has excellent insulation and heat dissipation capabilities.

[0018] The polyimide film of this invention possesses a conjugated planar crosslinked network formed by in-situ thermal induction of butyryl-1,3-diyne groups. When the amount of 3,3′-(butyryl-1,3-diyne-1,4-diyl)diphenylamine added is 15% (mol%) of the total diamine, this structure results in a dielectric loss of less than 0.005 at 1 MHz. Its constructed three-dimensional thermally stable system achieves wide temperature range adaptability: the glass transition temperature is increased to 296°C, and the 5% thermogravimetric temperature reaches 558°C. This advanced material, combining high-frequency, low-loss signal transmission, efficient thermal diffusion, and global thermal protection, provides a revolutionary packaging solution for cutting-edge fields such as 5G / 6G high-frequency communication, aerospace thermal control systems, and superconducting power equipment. Attached Figure Description

[0019] Figure 1 The infrared spectra of the polyimide films in Examples 3, 5-7 and Comparative Example 1 are shown in comparison. Figure 2 AFM images of the polyimide films in Example 3 and Comparative Example 1; Figure 3 The DSC curves of the polyimide films in Example 3 and Comparative Example 1 are compared. Figure 4The TGA curves of the polyimide films in Example 3 and Comparative Example 1 are compared. Figure 5 The DTG curves of the polyimide films in Example 3 and Comparative Example 1 are compared. Detailed Implementation

[0020] In this invention, unless otherwise specified, the raw materials or reagents required for preparation are all commercially available products well known to those skilled in the art.

[0021] This invention provides a conjugated planar crosslinked polyimide film, wherein the conjugated planar crosslinked polyimide in the film has the structure shown in Formula I: Formula I; In equation (Ⅰ), m>1, n>1, and m and n are both integers; Ar represents an aromatic dianhydride group, including one or more of the following: ; Ar'' represents an aromatic diamino group, including one or more of the following: ; Ar' for .

[0022] In this invention, Ar is preferably... Ar'' is preferred .

[0023] This invention provides a method for preparing the conjugated planar crosslinked polyimide film described above, comprising the following steps: Diamine monomer, dianhydride monomer and organic solvent are mixed and polycondensation reaction is carried out in a protective gas to obtain polyamic acid solution; The polyamic acid solution was placed on a substrate, subjected to thermal imidization, and then separated and dried to obtain a conjugated planar cross-linked polyimide film. The diamine monomer includes a first diamine monomer and a second diamine monomer; The first diamine monomer is 3,3′-(but-1,3-diyne-1,4-diyl)diphenylamine; The second diamine monomer includes one or more of 4,4'-diaminodiphenyl sulfone (4,4'-DDS), 4,4'-diaminodiphenyl ether (ODA), 4,4'-diaminodiphenylmethane (MDA), 4,4'-diaminobiphenyl, 2,2'-di(trifluoromethyl)diaminobiphenyl (TFMB), 4,4'-diaminobenzoylaniline (DABA), 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether (6FODA), and 4,4'-(hexafluoroisopropyl)bis(p-phenoxy)diphenylamine (HFBAPP); The dianhydride monomers include one or more of the following: 4,4-hexafluoroisopropylphthalic anhydride (6FDA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride (DSDA), bisphenol A type diether dianhydride (BPADA), 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride (ODPA), and pyromellitic dianhydride (PMDA).

[0024] In this invention, the first diamine monomer is 3,3′-(but-1,3-diyne-1,4-diyl)diphenylamine (Formula II); the second diamine monomer is preferably 4,4'-diaminodiphenyl ether (ODA) (Formula III); and the dianhydride monomer is preferably 4,4-hexafluoroisopropylphthalic anhydride (6FDA) (Formula IV).

[0025] Equation (II); Formula (Ⅲ); Formula (Ⅳ).

[0026] In this invention, the molar ratio of the dianhydride monomer to the diamine monomer is 1:1; the molar amount of the first diamine monomer accounts for 5-20% (mol%) of the total molar amount of the diamine monomer, more preferably 10-15% (mol%).

[0027] In this invention, the organic solvent preferably includes one or more of N-methylpyrrolidone, N,N'-dimethylformamide and N,N'-dimethylacetamide; when the organic solvent is two or more of the above, this invention does not have a special limitation on the ratio of different types of organic solvents, and any ratio is acceptable.

[0028] In this invention, the mass ratio of the total mass of the diamine monomer and the dianhydride monomer to the organic solvent is preferably 10~20:100, more preferably 12~15:100, and even more preferably 13.5:100.

[0029] In this invention, the process of mixing the diamine monomer, dianhydride monomer, and organic solvent includes: dissolving the diamine monomer in an organic solvent under a nitrogen atmosphere, adding the dianhydride monomer in batches to the resulting mixture, and carrying out a polycondensation reaction to obtain a polyamic acid solution.

[0030] In this invention, the temperature of the polycondensation reaction is -10 to 40°C, more preferably 0 to 5°C, and the time is preferably 1 to 24 hours, more preferably 4 to 12 hours; the protective gas is preferably nitrogen. This invention preferably carries out the polycondensation reaction under protective gas purging. This invention preferably uses a casting method to coat the polyamic acid solution onto a substrate, and after drying at 60 to 90°C for 1 to 3 hours to remove the solvent, thermal imidization is performed. This invention does not have a specific limitation on the substrate; any suitable substrate known in the art can be used; in the embodiments, a glass plate is specifically used.

[0031] In this invention, the preferred temperature range for thermal imidization is 60~300℃, and the preferred processing time is 6~16h.

[0032] In this invention, the preferred thermal imidization gradient procedure is as follows: a first thermal imidization is performed at 120-140°C for 0.5-1.5 h, a second thermal imidization is performed at 180-200°C for 0.5-1.5 h, a third thermal imidization is performed at 220-240°C for 0.5-1.5 h, and a fourth thermal imidization is performed at 280-300°C for 0.5-1.5 h.

[0033] The present invention does not have any special limitations on the separation and drying process, which can be carried out according to the process known in the art.

[0034] This invention provides the application of the conjugated planar cross-linked polyimide film described in the above technical solution or the conjugated planar cross-linked polyimide film prepared by the preparation method described in the above technical solution in the fields of 5G / 6G high-frequency communication packaging, flexible circuit boards and microelectronic thermal management.

[0035] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0036] Unless otherwise specified, the experimental methods described in the various embodiments of this invention are conventional methods; unless otherwise specified, the reagents and raw materials described below are all commercially available.

[0037] Example 1

[0038] Under a nitrogen atmosphere, 4.75 mmol of 4,4-diaminodiphenyl ether (ODA) (0.951 g), 0.25 mmol of 3,3′-(but-1,3-diyne-1,4-diyl)diphenylamine (mDA) (0.058 g), and 25.1 mL of N,N-dimethylformamide (23.8 g) were added to a dry 100 mL round-bottom flask and stirred until completely dissolved. 5.0 mmol of 4,4-hexafluoroisopropylphthalic anhydride (6FDA) (2.22 g) was added to the above mixed solution in three portions, and the reaction was carried out at 0 °C for 4 h to obtain a polyamic acid solution. The polyamic acid solution was uniformly coated onto a sterilized and cleaned glass plate using a casting method. The plate was then placed in a forced-air drying oven and dried at 80°C for 2 hours to remove N,N-dimethylformamide solvent. Subsequently, the resulting polyamic acid film underwent gradient thermal imidization in a forced-air drying oven. The gradient thermal imidization program was 120°C / 1.5h + 180°C / 1.5h + 240°C / 1.5h + 300°C / 1.5h. The glass plate with the attached polyimide composite film was then immersed in deionized water, allowing the polyimide composite film to naturally separate from the glass plate. The separated polyimide composite film was then removed and dried in an 80°C oven to obtain a conjugated planar crosslinked polyimide film, denoted as 6FDA-(ODA-co-mDA)-0.05.

[0039] Example 2

[0040] Under a nitrogen atmosphere, 4.5 mmol of 4,4-diaminodiphenyl ether (ODA) (0.901 g), 0.5 mmol of 3,3′-(but-1,3-diyne-1,4-diyl)diphenylamine (mDA) (0.116 g), and 25.1 mL of N,N-dimethylformamide (23.7 g) were added to a dry 100 mL round-bottom flask and stirred until completely dissolved. 5.0 mmol of 4,4-hexafluoroisopropylphthalic anhydride (6FDA) (2.22 g) was added to the above mixed solution in three portions, and the reaction was carried out at 0 °C for 4 h to obtain a polyamic acid solution. The polyamic acid solution was uniformly coated onto a sterilized and cleaned glass plate using a casting method. The plate was then placed in a forced-air drying oven and dried at 80°C for 2 hours to remove N,N-dimethylformamide solvent. Subsequently, the resulting polyamic acid film underwent gradient thermal imidization in a forced-air drying oven. The gradient thermal imidization program was 120°C / 1.5h + 180°C / 1.5h + 240°C / 1.5h + 300°C / 1.5h. The glass plate with the polyimide composite film attached was then immersed in deionized water to allow the polyimide composite film to separate naturally from the glass plate. The separated polyimide composite film was then removed and dried in an 80°C oven to obtain a conjugated planar crosslinked polyimide film, denoted as 6FDA-(ODA-co-mDA)-0.1.

[0041] Example 3

[0042] Under a nitrogen atmosphere, 4.25 mmol of 4,4-diaminodiphenyl ether (ODA) (0.851 g), 0.75 mmol of 3,3′-(but-1,3-diyne-1,4-diyl)diphenylamine (mDA) (0.174 g), and 25.2 mL of N,N-dimethylformamide (23.8 g) were added to a dry 100 mL round-bottom flask and stirred until completely dissolved. 5.0 mmol of 4,4-hexafluoroisopropylphthalic anhydride (6FDA) (2.22 g) was added to the above mixed solution in three portions, and the reaction was carried out at 0 °C for 4 h to obtain a polyamic acid solution. The polyamic acid solution was uniformly coated onto a sterilized and cleaned glass plate using a casting method. The plate was then placed in a forced-air drying oven and dried at 80°C for 2 hours to remove N,N-dimethylformamide solvent. Subsequently, the resulting polyamic acid film underwent gradient thermal imidization in a forced-air drying oven. The gradient thermal imidization program was 120°C / 1.5h + 180°C / 1.5h + 240°C / 1.5h + 300°C / 1.5h. The glass plate with the polyimide composite film attached was then immersed in deionized water to allow the polyimide composite film to separate naturally from the glass plate. The separated polyimide composite film was then removed and dried in an 80°C oven to obtain a conjugated planar crosslinked polyimide film, denoted as 6FDA-(ODA-co-mDA)-0.15.

[0043] Example 4

[0044] Under a nitrogen atmosphere, 4.0 mmol of 4,4-diaminodiphenyl ether (ODA) (0.801 g), 1.0 mmol of 3,3′-(but-1,3-diyne-1,4-diyl)diphenylamine (mDA) (0.232 g), and 25.3 mL of N,N-dimethylformamide (23.9 g) were added to a dry 100 mL round-bottom flask and stirred until completely dissolved. 5.0 mmol of 4,4-hexafluoroisopropylphthalic anhydride (6FDA) (2.22 g) was added to the above mixed solution in three portions, and the reaction was carried out at 0 °C for 4 h to obtain a polyamic acid solution. The polyamic acid solution was uniformly coated onto a sterilized and cleaned glass plate using a casting method. The plate was then placed in a forced-air drying oven and dried at 80°C for 2 hours to remove N,N-dimethylformamide solvent. Subsequently, the resulting polyamic acid film underwent gradient thermal imidization in a forced-air drying oven. The gradient thermal imidization program was 120°C / 1.5h + 180°C / 1.5h + 240°C / 1.5h + 300°C / 1.5h. The glass plate with the polyimide composite film attached was then immersed in deionized water to allow the polyimide composite film to separate naturally from the glass plate. The separated polyimide composite film was then removed and dried in an 80°C oven to obtain a conjugated planar crosslinked polyimide film, denoted as 6FDA-(ODA-co-mDA)-0.2.

[0045] Example 5

[0046] Under a nitrogen atmosphere, 4.25 mmol of 4,4-diaminodiphenyl ether (ODA) (0.851 g), 0.75 mmol of 3,3′-(but-1,3-diyne-1,4-diyl)diphenylamine (mDA) (0.174 g), and 16.5 mL of N,N-dimethylformamide (15.5 g) were added to a dry 100 mL round-bottom flask and stirred until completely dissolved. Then, 5 mmol of pyromellitic dianhydride (PMDA) (1.091 g) was added to the mixture in three portions and the mixture was reacted at 0 °C for 4 h to obtain a polyamic acid solution. The polyamic acid solution was uniformly coated onto a sterilized and cleaned glass plate using a casting method. The plate was then placed in a forced-air drying oven and dried at 80°C for 2 hours to remove N,N-dimethylformamide solvent. Subsequently, the resulting polyamic acid film underwent gradient thermal imidization in a forced-air drying oven. The gradient thermal imidization program was 120°C / 1.5h + 180°C / 1.5h + 240°C / 1.5h + 300°C / 1.5h. The glass plate with the polyimide composite film attached was then immersed in deionized water to allow the polyimide composite film to separate naturally from the glass plate. The separated polyimide composite film was then removed and dried in an 80°C oven to obtain a polyimide film, denoted as PMDA-(ODA-co-mDA)-0.15.

[0047] Example 6

[0048] Under a nitrogen atmosphere, 4.25 mmol of 4,4'-diaminobenzoyl aniline (DABA) (0.966 g), 0.75 mmol of 3,3'-(but-1,3-diyne-1,4-diyl)diphenylamine (mDA) (0.174 g), and 21.0 mL of N,N-dimethylformamide (19.7 g) were added to a dry 100 mL round-bottom flask and stirred until completely dissolved. 5 mmol of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride (ODPA) (1.55 g) was added to the above mixed solution in three portions, and the reaction was carried out at 0 °C for 4 h to obtain a polyamic acid solution. The polyamic acid solution was uniformly coated onto a sterilized and cleaned glass plate using a casting method. The plate was then placed in a forced-air drying oven at 80°C for 2 hours to remove N,N-dimethylformamide solvent. Subsequently, the resulting polyamic acid film underwent gradient thermal imidization in a forced-air drying oven. The gradient thermal imidization program was 120°C / 1.5h + 180°C / 1.5h + 240°C / 1.5h + 300°C / 1.5h. The glass plate with the polyimide composite film attached was then immersed in deionized water to allow the polyimide composite film to separate naturally from the glass plate. The separated polyimide composite film was then removed and dried in an 80°C oven to obtain a polyimide film, denoted as ODPA-(DABA-co-mDA)-0.15.

[0049] Example 7

[0050] Under a nitrogen atmosphere, 4.25 mmol of 4,4'-(hexafluoroisopropyl)bis(p-phenoxy)diphenylamine (HFBAPP) (2.20 g), 0.75 mmol of 3,3'-(but-1,3-diyne-1,4-diyl)diphenylamine (mDA) (0.174 g) and 30.5 mL of N,N-dimethylformamide (28.8 g) were added to a dry 100 mL round-bottom flask and stirred until completely dissolved. Then, 5 mmol of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride (ODPA) (1.55 g) was added to the above mixed solution in three portions and reacted at 0 °C for 4 h to obtain a polyamic acid solution. The polyamic acid solution was uniformly coated onto a sterilized and cleaned glass plate using a casting method. The plate was then placed in a forced-air drying oven and dried at 80°C for 2 hours to remove N,N-dimethylformamide solvent. Subsequently, the resulting polyamic acid film underwent gradient thermal imidization in a forced-air drying oven. The gradient thermal imidization program was 120°C / 1.5h + 180°C / 1.5h + 240°C / 1.5h + 300°C / 1.5h. The glass plate with the polyimide composite film attached was then immersed in deionized water to allow the polyimide composite film to separate naturally from the glass plate. The separated polyimide composite film was then removed and dried in an 80°C oven to obtain a polyimide film, denoted as ODPA-(HFBAPP-co-mDA)-0.15.

[0051] Comparative Example 1

[0052] Under a nitrogen atmosphere, 5.0 mmol of 4,4-diaminodiphenyl ether and 40 mL of N,N-dimethylformamide were added to a dry 100 mL round-bottom flask and mechanically stirred until completely dissolved. Then, 5.0 mmol of 4,4-hexafluoroisopropylphthalic anhydride was added to the solution in three portions, and the mixture was reacted in an ice bath at 0 °C for 4 h to obtain a polyamic acid solution. The polyamic acid solution was uniformly coated onto a sterilized and cleaned glass plate using a casting method. The plate was then placed in a forced-air drying oven and dried at 80°C for 2 hours to remove N,N-dimethylformamide solvent. Subsequently, the resulting polyamic acid film underwent gradient thermal imidization in a forced-air drying oven. The gradient thermal imidization program was 120°C / 1.5h + 180°C / 1.5h + 240°C / 1.5h + 300°C / 1.5h. The glass plate with the polyimide composite film attached was then immersed in deionized water to allow the polyimide composite film to separate naturally from the glass plate. The separated polyimide composite film was then removed and dried in an 80°C oven to obtain a polyimide film, denoted as 6FDA-ODA.

[0053] Performance testing

[0054] 1) Dielectric property testing

[0055] The dielectric properties of the thin film were tested using a broadband dielectric impedance spectrometer (Concept 40) at a temperature of 25°C. The test frequency range was 10^6 Hz. 2 Hz-10^ 6The test method involved cutting polyimide films into uniformly sized and thick circular sheets. During testing, a 10 mm diameter circular silver electrode was sprayed onto both sides of the circular film. The dielectric properties of the polyimide films in the examples and comparative examples were characterized using a broadband dielectric impedance spectrometer. The voltage amplitude was 1.0 V, the DC potential was 0.4 V, and the frequency scan range was 100–1,000,000 Hz. The test results are shown in Table 1.

[0056] Table 1. Dielectric properties of polyimide films in Examples 1-7 and Comparative Example 1 at a frequency of 1 MHz.

[0057] As shown in Table 1, the dielectric constant and dielectric loss of the polyimide film gradually decrease with a moderate increase in the amount of mDA added. This is mainly attributed to the unique rigid conjugated planar crosslinked network constructed by the introduction of mDA in the system: this structure significantly restricts the orientation freedom of the dipoles and reduces the local polarization response by enhancing the overall rigidity of the molecular chains, thereby effectively suppressing the orientation polarization effect at the source and endowing the film with excellent low dielectric properties. However, when the molar percentage of mDA in the total diamine reaches 20% (mol%) (as shown in Example 4), the dielectric constant and loss of the film both increase sharply. The mechanism is that excessive crosslinking density leads to the generation of microscopic physical defects, vacancies, or local stress concentration points in the film during film formation and imidization. Under the action of an electric field, these microscopic defect sites easily accumulate space charge (carriers), inducing a significant Maxwell-Wagner-Sillars (MWS) interfacial polarization effect, resulting in a sharp increase in the conductivity loss of the material under high-frequency conditions.

[0058] At the same crosslinking monomer ratio (15 mol%), the effects of different matrix monomer components on the electrical properties of the thin film varied significantly. Example 3 (6FDA-ODA-mDA system) exhibited the best dielectric stability, with a dielectric loss (D... f The dielectric constant (DmDA) is only 0.0042. In contrast, Example 5 (PMDA-ODA-mDA system) has a very high density of imide rings due to the regular structure of PMDA and the lack of low-polarity fluorine atoms, resulting in tight molecular chain packing and increased polarizability. Therefore, its dielectric constant (DmDA) is much higher. kBoth dielectric loss and dielectric strength were the highest in all groups. Compared to Example 6, Examples 3 and 7 significantly optimized dielectric properties by introducing a large-volume, low-polarization -CF3 group, which effectively increased intermolecular free volume and reduced electronic polarization. Example 3 showed the best performance: thanks to the highly electronegative fluorine element introduced at the dianhydride position and its strong steric hindrance effect, it had significant advantages in both suppressing electronic polarization and increasing free volume. In contrast, although Example 7 underwent fluorination modification at the diamine end, its dielectric reduction effect was still insufficient to completely offset the polarization increase brought about by the non-fluorinated dianhydride (ODPA). Therefore, Example 3 exhibited the best overall dielectric performance.

[0059] 2) Thermal conductivity test

[0060] The thermal diffusivity (α) of the thin film was tested using a laser flash thermal conductivity analyzer (LFA-467). The test method involved cutting the thin film into 25mm pieces. A circular film of uniform thickness and size (25 mm) was used for testing. A layer of graphite was sprayed onto both sides of the circular film. The thermal diffusivity of the film was measured at 30°C.

[0061] Thermal conductivity (λ) is calculated using the following formula.

[0062] Where Cp (J / g / K) is the specific heat capacity, and ρ (g / cm³) is the specific heat capacity. 3 ) represents density, α (mm) 2 / s) represents the thermal diffusivity.

[0063] The specific heat capacity at 30°C was measured using a DSC2500 (TA Instruments) and the sapphire three-step method. The film density (ρ) was determined at room temperature using a solid densitometer with anhydrous ethanol as the reference solvent. The results are shown in Table 2.

[0064] Table 2. Coefficients relating polyimide films to thermal conductivity in Example 3 and Comparative Example 1.

[0065] As shown in Table 2, compared with Comparative Example 1, the 6FDA-(ODA-co-mDA)-0.15 polyimide film prepared in Example 3 has a higher thermal conductivity. This is because the conjugated planar crosslinked structure, by introducing high-bond-energy covalent bonds and a π-conjugated system, makes the molecular chains planar and structurally ordered, which can significantly suppress the twisting and entanglement of molecular chains, increase the phonon mean free path, and at the same time, its ordered and regular conformation can reduce phonon scattering, thereby greatly improving the thermal conductivity efficiency and making its thermal conductivity significantly higher than that of the original film. This structural feature gives the polyimide film excellent thermal conductivity, providing a structural basis for its application in high-temperature electronic packaging and thermal management.

[0066] 3) Polyimide structure testing

[0067] Figure 1 The image shows a comparison of the infrared spectra of the polyimide films in Examples 3, 5-7, and Comparative Example 1. First, looking at the full spectrum, all samples are within the range of 1785 cm⁻¹. -1 and 1720 cm -1 A sharp peak of symmetric and asymmetric stretching vibration of the carbonyl group (C=O) in the imide ring appeared at 1375 cm⁻¹. -1 A distinct CN stretching vibration peak was observed at 1660 cm⁻¹, and not at 1660 cm⁻¹. -1 The presence of characteristic absorptions of polyamic acid nearby confirms that complete imidization was successfully achieved in both the above examples and the comparative examples. Furthermore, compared to Comparative Example 1, the C=O absorption peaks in Examples 3 and 5-7 showed a red shift towards lower wavenumbers. This indicates that the introduction of the mDA crosslinking component enhanced the intermolecular electronic delocalization effect and lowered the vibrational energy level of the carbonyl group.

[0068] 4) Morphological performance testing

[0069] The surface morphology of the polyimide films in Example 3 and Comparative Example 1 was tested using atomic force microscopy (AFM). The test results are as follows: Figure 2 As shown. By Figure 2 As can be seen, the polyimide film in Example 3 exhibits a smoother and flatter surface due to the introduction of a conjugated planar crosslinking structure, which promotes the formation of an ordered crosslinked phase. In contrast, the surface of the comparative polyimide film shows obvious irregular protrusions, resulting in a rougher overall morphology. Further analysis of the surface roughness (Ra) reveals that the surface roughness of the 6FDA-ODA polyimide film is 18.20 nm, while the surface roughness of the 6FDA-ODA-mDA polyimide film is significantly reduced to 4.72 nm, accompanied by the appearance of a continuous and uniform surface domain structure. This improvement in surface smoothness is attributed to the crosslinking reaction of the diacetylene groups in the mDA unit during thermal imidization, generating a conjugated planar structure. This promotes the ordered arrangement of polymer molecular chains and increases the crosslinking density, making the film surface more dense and smooth. The introduction of the conjugated plane facilitates the π–π stacking and orientation of molecular chains, forming a regular and dense crosslinked network within the polymer. On the one hand, the cross-linked structure effectively suppresses the orientation polarization and dipole migration of polar groups, reducing the degrees of freedom and free volume of the molecular chains, thereby significantly reducing the dielectric constant and dielectric loss. On the other hand, the planarized and ordered arrangement of molecular chains provides a continuous and efficient conduction path for phonon transport, reducing interface scattering and phonon scattering, and significantly improving the thermal conductivity of the material.

[0070] 5) Thermal performance test

[0071] Differential scanning calorimetry (DSC) tests were performed on the 6FDA-(ODA-co-mDA)-0.15 polyimide film in Example 3 and the 6FDA-ODA polyimide film in Comparative Example 1. The tests were conducted under a nitrogen atmosphere, and the heating and cooling program was as follows: first, the temperature was increased from 100℃ to 350℃ at a rate of 20℃ / min; then, it was decreased to 100℃ at a rate of 20℃ / min; finally, it was increased to 350℃ again at a rate of 10℃ / min. The third heating curve was selected as the analysis object, and the resulting DSC curve is shown below. Figure 3 As shown. By Figure 3 It can be seen that the glass transition temperature (Tg) of the 6FDA-(ODA-co-mDA)-0.15 film in Example 3 is 296℃, which is about 10℃ higher than the Tg (286℃) of the comparative example 6FDA-ODA polyimide film. The increase in glass transition temperature indicates that the movement of the molecular chains in this film is more restricted, and its thermal deformation ability and thermal stability are enhanced. This increase in Tg is mainly attributed to the planar conjugated crosslinking network introduced by the thermal polymerization of the mDA unit in the molecular structure. This crosslinking structure forms a certain degree of rigid spatial grid between molecules, which not only increases the interaction force and structural regularity between molecular chains, but also effectively inhibits the free rotation and thermal motion of molecular chain segments, thereby improving the overall thermal stability and structural integrity of the polyimide system.

[0072] Thermogravimetric analysis (TGA) was performed on the 6FDA-(ODA-co-mDA)-0.15 polyimide film in Example 3 and the 6FDA-ODA polyimide film in Comparative Example 1.

[0073] The thermogravimetric analysis (TGA) process and conditions were as follows: Under a nitrogen atmosphere, the protective gas flow rate was 30 mL / min, the purge gas flow rate was 30 mL / min, and the temperature was increased from 50℃ to 800℃ at a heating rate of 20℃ / min to obtain the thermogravimetric spectrum of the sample.

[0074] Figure 4 The TGA curves of the polyimide films in Example 3 and Comparative Example 1 are compared; by Figure 4 It can be seen that the decomposition temperature (T) of the 6FDA-(ODA-co-mDA)-0.15 film in Example 3 at a mass loss of 5% is... d5% The temperature was 558℃, compared to the T of the 6FDA-ODA polyimide film in Comparative Example 1. d5% The temperature increase of approximately 15°C (from approximately 543°C) indicates a significant increase in the thermal decomposition initiation temperature. Furthermore, the residual mass of the sample at 800°C still reaches 57%, significantly higher than the comparative film, demonstrating that this conjugated cross-linked structure effectively suppresses the high-temperature decomposition process and endows the material with excellent thermal stability.

[0075] Figure 5 The DTG curves of the polyimide films in Example 3 and Comparative Example 1 are compared; by Figure 5 The DTG curves show that the maximum thermogravimetric rate temperature of the 6FDA-(ODA-co-mDA)-0.15 film in Example 3 is approximately 590°C. This temperature is higher than the peak value of the main decomposition rate of the comparative sample, further indicating that the introduction of the conjugated planar crosslinked structure makes the molecular skeleton more stable, and the thermal decomposition reaction is delayed to a higher temperature. The mechanism is that the diacetylene structure in the mDA unit forms a stable conjugated planar crosslinked network during heat treatment. This structure enhances the π–π stacking effect and intermolecular interaction force between molecular chains, thereby restricting the thermal motion and breakage of chain segments. In addition, the presence of the crosslinked structure increases the rigidity of the molecular skeleton, enabling the polyimide system to maintain high structural integrity and pyrolysis resistance at high temperatures. In summary, the 6FDA-(ODA-co-mDA)-0.15 film in Example 3 exhibits excellent thermal stability and high-temperature resistance under high-temperature conditions, and has potential application value in high-temperature electronic packaging, flexible heat-resistant films, and aerospace thermal protection materials.

[0076] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A conjugated planar crosslinked polyimide film, characterized in that, The conjugated planar cross-linked polyimide film has the structure shown in Formula I: Formula I; In equation (Ⅰ), m>1, n>1, and m and n are both integers; Ar represents an aromatic dianhydride group, including one or more of the following: ； Ar'' represents an aromatic diamino group, including one or more of the following: ； Ar' for .

2. The method for preparing the conjugated planar crosslinked polyimide film according to claim 1, characterized in that, Includes the following steps: Diamine monomer, dianhydride monomer and organic solvent are mixed and polycondensation reaction is carried out in a protective gas to obtain polyamic acid solution; The polyamic acid solution was placed on a substrate, subjected to thermal imidization, and then separated and dried to obtain a conjugated planar cross-linked polyimide film. The diamine monomer includes a first diamine monomer and a second diamine monomer; The first diamine monomer is 3,3′-(but-1,3-diyne-1,4-diyl)diphenylamine; The second diamine monomer includes one or more of 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 4,4'-diaminobiphenyl, 2,2'-di(trifluoromethyl)diaminobiphenyl, 4,4'-diaminobenzoylaniline, 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether, and 4,4'-(hexafluoroisopropyl)bis(p-phenoxy)diphenylamine; The dianhydride monomer includes one or more of the following: 4,4-hexafluoroisopropylphthalic anhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride, bisphenol A type diether dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, and pyromellitic dianhydride.

3. The preparation method according to claim 2, characterized in that: The molar amount of the first diamine monomer accounts for 5 to 20% of the total molar amount of the diamine monomer.

4. The preparation method according to claim 3, characterized in that, The molar ratio of the dianhydride monomer to the diamine monomer is 1:

1.

5. The preparation method according to claim 2, characterized in that, The organic solvent includes one or more of N-methylpyrrolidone, N,N'-dimethylformamide and N,N'-dimethylacetamide; the mass ratio of the total mass of the diamine monomer and the dianhydride monomer to the mass of the organic solvent is 10~20:

100.

6. The preparation method according to claim 2, characterized in that, The polycondensation reaction is carried out at a temperature of -10 to 40°C for 1 to 24 hours; the protective gas is nitrogen.

7. The preparation method according to claim 2, characterized in that, The temperature range for thermal imidization is 60~300℃, and the processing time is 6~16h.

8. The preparation method according to claim 7, characterized in that, The gradient program for thermal imidization is as follows: a first thermal imidization is performed at 120~140℃ for 0.5~1.5h, a second thermal imidization is performed at 180~200℃ for 0.5~1.5h, a third thermal imidization is performed at 220~240℃ for 0.5~1.5h, and a fourth thermal imidization is performed at 280~300℃ for 0.5~1.5h.

9. The application of the conjugated planar cross-linked polyimide film of claim 1 or the conjugated planar cross-linked polyimide film prepared by the preparation method of any one of claims 2 to 8 in the fields of 5G / 6G high-frequency communication packaging, flexible circuit boards and microelectronic thermal management.

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