A black polyimide film and an in-situ controllable coloring preparation method and application thereof

Abstract

The application discloses a black polyimide film and an in-situ controllable coloring preparation method and application thereof. The black polyimide film is prepared by coating a polyamide acid solution prepared from 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride into a film, and then placing the film in a normal-pressure atmosphere environment with a molar ratio of nitrogen and oxygen satisfying 5.3<=n(N2):n(O2)<=20 and subjecting the film to a thermal imidization reaction. The method solves the problems of uneven color dispersion, poor mechanical properties and interface bonding effect caused by traditional external coloring fillers, is highly compatible with a continuous industrial production process of an existing polyimide film, simultaneously completes coloring and thermal imidization procedures, realizes preparation of the polyimide film with adjustable intrinsic black color while keeping inherent properties of the polyimide, and enables the polyimide to be applied to preparation of a precision optical film and a protective film capable of resisting high temperature or acid and alkali environments for a long time.

Description

Technical Field

[0001] This invention relates to the field of polyimide technology, and in particular to a black polyimide film and its in-situ controllable coloring preparation method and application. Background Technology

[0002] Polyimide is a class of high-performance polymers containing imide ring structures in its molecular chain, renowned for its excellent high-temperature resistance, superior mechanical strength, good chemical stability, and outstanding dielectric properties. Polyimide films play an irreplaceable role in aerospace, microelectronics packaging, flexible displays, and precision instruments. In the field of flexible electronics, such as flexible display substrates, touch screens, and flexible printed circuit boards, polyimide films are a core foundational material.

[0003] In addition to possessing comprehensive properties similar to traditional polyimide films, black polyimide films also exhibit excellent light-shielding characteristics, making them primarily used in cutting-edge technology fields with specific requirements for optical, thermal, and electrical properties, such as stealth coatings, optical shielding materials, flexible display cover plates / substrates, and chip packaging materials. Traditional black polyimide films are typically prepared using a composite method, which involves adding inorganic black fillers such as carbon black, graphite, and carbon nanotubes to polyamic acid (PAA) resin or coating the polyimide film with organic dyes. However, black polyimides prepared by physical blending methods have significant drawbacks: uneven dispersion of the colorant in the matrix easily leads to color differences; weak interfacial bonding between the filler and the matrix degrades the mechanical properties and reliability of the film; and the additional colorant and associated dispersion processes increase process complexity and cost. Therefore, designing intrinsically black polyimides is of great significance.

[0004] The published patent CN 120590794 A proposes a method to obtain a black insulating polyimide material by further imidization of a polyamic acid solution containing a mixture of elemental phosphorus. This mixture of elemental phosphorus and boron nitride or porous silicon, processed by ball milling, imparts a black characteristic to the polyimide while simultaneously improving its insulation and thermal conductivity. However, this approach is essentially a filler doping modification route, which suffers from problems such as uneven filler distribution, film color difference, decreased mechanical properties, and poor heat resistance, limiting the large-scale application of the material in demanding applications such as high-end microelectronics and aerospace. Furthermore, current synthetic routes for intrinsically black polyimides mainly focus on designing donor-acceptor structures with stronger CTC effects and introducing intrinsically chromogenic conjugated units, such as anthraquinone structures. However, these methods often involve high synthesis difficulty, high cost, or uncontrollable material color. Therefore, developing a novel polyimide with controllable blackness and a cost-effective preparation method has significant theoretical and practical application value. Summary of the Invention

[0005] The purpose of this invention is to provide an in-situ controllable coloring preparation method for an intrinsic black polyimide film that does not rely on external pigments and can achieve coloring solely based on changes in the chemical structure of the polyimide itself.

[0006] Another objective of this invention is to provide a method for preparing a black polyimide film using the above-described in-situ controllable coloring method and its application.

[0007] Therefore, the technical solution of the present invention is as follows:

[0008] A method for preparing a black polyimide film involves coating a polyamic acid solution formed by reacting 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine with 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride onto a substrate to form a wet polyamic acid film. After drying, the film is placed in an atmospheric pressure environment with a nitrogen-oxygen molar ratio of 5.3 ≤ n(N2):n(O2) ≤ 20, and then subjected to a thermal imidization reaction to obtain the film.

[0009] The chemical structural formula of 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine is as follows:

[0010] ,

[0011] The chemical structural formula of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride is:

[0012] ,

[0013] The repeating unit structure of polyamic acid formed by the reaction of 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride is as follows:

[0014] ,

[0015] The repeating unit structure of black polyimide formed by thermal imidization of polyamic acid is as follows:

[0016] ,

[0017] In the formula, the structures within the two square brackets are two repeating structural units that coexist in the molecular chain of black polyimide. These two repeating structural units repeat irregularly and disorderly, and have nitro groups at the ends of the molecular chain.

[0018] According to some embodiments of the present invention, during the preparation of black polyimide film, when the molar ratio of nitrogen to oxygen n(N2):n(O2) exceeds 20, although the blackness of the prepared black polyimide film will be further improved, the mechanical properties will be greatly reduced and cannot meet the application requirements.

[0019] In the molecular structure of the black polyimide film prepared by this invention, the tertiary nitrogen atom on the piperazine ring in the binary copolymer main chain has an easily oxidizable active site. During the thermal imidization stage, under a mild oxidizing environment with a controllable mixed atmosphere of nitrogen and oxygen, the piperazine nitrogen atom gradually undergoes an in-situ oxidation reaction to generate a nitrogen-oxygen single bond (NO) and a terminal nitro group chromogenic structure. These two types of nitrogen-containing oxidative conjugated groups are embedded in the polyimide molecular main chain conjugated system, which greatly broadens the light absorption range of the molecular charge transfer complex (CTC), enabling the material to absorb the visible light band of 450 nm-750 nm in the entire range, thus endowing the polyimide film with intrinsic black properties. In conventional imidization processes, thermal imidization reactions based on an inert pure nitrogen atmosphere cannot form the aforementioned NO bonds and nitro chromophores during imidization due to the lack of oxidative driving force. On the other hand, chemical imidization processes lead to premature completion of imide ring closure, a decrease in the overall energy of the molecular skeleton, and acetylation protection of the terminal amino groups, which blocks the reaction pathway for high-temperature oxidation to construct the conjugated chromophore structure, resulting in only the production of light yellow transparent films.

[0020] The broad-spectrum light-shielding mechanism of the polyimide film of this invention is further explained from the perspective of molecular frontier orbital (HOMO, LUMO) energy level theory. In this invention, 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine is an electron-rich donor and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride is an electron-deficient acceptor. Only a weak charge-transfer complexation effect exists within the molecule, resulting in a wide band gap between the highest occupied orbital (HOMO) and the lowest vacant orbital (LUMO). Electron transitions only absorb ultraviolet light, causing the polyimide film formed by these two elements to exhibit a light yellow transparent state. Furthermore, during the thermal imidization reaction of this invention, a mixture of nitrogen and oxygen... Under atmosphere-driven controlled oxidation, NO nitrogen-oxygen single bonds and terminal nitro conjugated chromophores are gradually generated at the piperazine tertiary nitrogen sites in the molecular chain. These two types of strong electron-withdrawing oxidizing groups are simultaneously embedded into the π-conjugated backbone of the main chain. On the one hand, this raises the HOMO energy level of the molecule and simultaneously lowers the LUMO energy level, significantly reducing the HOMO-LUMO orbital energy level difference and lowering the energy required for intramolecular electronic transitions. On the other hand, it expands the conjugated delocalization range of the molecular chain, strengthens intramolecular and intermolecular CTC charge transfer interactions, and extends the electronic transition absorption range to cover the entire visible light band of 450 nm-750 nm, thereby achieving near-complete absorption of visible light and ultimately preparing a black polyimide film.

[0021] Based on the above analysis, the in-situ controllable coloring preparation method of the present invention achieves controllable adjustment of the degree of piperazine nitrogen oxidation by precisely controlling the molar ratio of nitrogen and oxygen under normal pressure during the thermal imidization process. That is, it controls the generation content of NO and nitro chromophores in the polyimide molecular chain, so that the blackness gradient of the formed polyimide film can be adjusted. Since this preparation method does not require the addition of physical light-shielding fillers such as carbon black and organic dyes, it solves the inherent defects of traditional black polyimide fillers such as uneven dispersion, poor interfacial compatibility, and deterioration of mechanical properties at the molecular structure level.

[0022] A method for in-situ controllable coloring of the above-mentioned black polyimide film, the specific steps of which are as follows:

[0023] S1. Dissolve 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine in a polar aprotic organic solvent, and add 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride in batches at 0~25℃, and then continue stirring for 10h-48h to obtain a polyamic acid solution.

[0024] S2. Coat the substrate with a polyamic acid solution and obtain a polyamic acid wet film by scraping with a doctor blade;

[0025] S3. Vacuum dry the polyamic acid wet film to completely remove the solvent and obtain a polyamic acid dry film.

[0026] S4. The polyamic acid dry film is subjected to thermal imidization treatment to prepare a black polyimide film.

[0027] Preferably, in step S1, the molar ratio of 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine to 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride is 1:(1-1.1), more preferably 1:(1.02-1.08).

[0028] Preferably, in step S1, the polar aprotic organic solvent is a mixture of at least one or more of N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), and dimethyl sulfoxide (DMSO).

[0029] Preferably, in step S1, the solid content of the polyamic acid solution is 10 wt%-20 wt%, more preferably 14 wt%-20 wt%.

[0030] Preferably, in step S2, the polyamic acid solution is first degassed under vacuum, and then coated onto the substrate at a casting speed of 5 mm / s-30 mm / s to ensure uniform coating of the polyamic acid solution, thereby making the optical properties of the film uniform and stable. This avoids the problems of bubble wrinkles caused by low-speed thick films and the decrease in light-shielding performance caused by uneven film thickness at high speeds, ensuring that the overall optical properties of the final black polyimide film are uniform and stable. Subsequently, a wet film with a thickness of 25 μm-35 μm is obtained by casting with a casting blade. The substrate can be, but is not limited to, a glass substrate.

[0031] Preferably, in step S3, the vacuum drying conditions for the polyamic acid wet film are: drying in a vacuum drying oven at 70℃~80℃ for 2 h~4 h.

[0032] Preferably, in step S3, the heating program for the thermal imidization of the polyamic acid dry film is set as follows: the heating rate is controlled at 0.8℃ / min-3.0℃ / min, and the heating program is set to last for 0.5 h-3 h at 100℃ and 200℃ respectively, and for 20 min-50 min at 250℃ and 300℃ respectively, to complete the thermal imidization of the polyamic acid film.

[0033] A black polyimide film prepared using the above-mentioned in-situ controllable coloring method has an ultraviolet cutoff wavelength greater than 354 nm. It can be widely used in fields with stringent requirements for light shielding, stability, and lightweighting. Specifically, it can be used as a black matrix light shielding layer for flexible organic light-emitting diodes (flexible OLEDs) and micro-LED displays, as well as for light shielding shells for optical sensors and anti-glare apertures for camera modules. It can also be used to prepare high-temperature resistant markings, special insulating tapes, and flexible circuit covering films that can withstand high temperatures and acid and alkali environments for a long time, and as high-temperature protection products such as thermal blankets for aircraft. It can be applied to cutting-edge technology fields with extreme requirements for pollution-free and highly reliable optical shielding, such as light shielding components inside spacecraft and light shielding dielectric layers for microelectronic packaging.

[0034] Compared with existing technologies, this invention utilizes an in-situ controlled coloring method based on polyamic acid prepared from 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride, through a controlled thermal imidization reaction under a nitrogen and oxygen mixed atmosphere. This method achieves the preparation of an intrinsically black polyimide film. This preparation process avoids the problems of uneven dispersion, uneven color, decreased mechanical properties, and poor interfacial bonding caused by traditional physical colorants such as added carbon black and dyes. The resulting film exhibits excellent color consistency, flexibility, and long-term reliability. Furthermore, this preparation method… The process is highly compatible with existing continuous industrial production processes for polyimide films. The coloring process is completed simultaneously with the conventional thermal imidization process, eliminating the need for additional complex processes and equipment. This significantly simplifies the production process, reduces production costs, and has good prospects for large-scale application. Furthermore, the polyimide film products obtained using this preparation method not only have an adjustable intrinsic black color but also retain the excellent inherent properties of polyimide, such as thermal stability, chemical corrosion resistance, mechanical strength, and thermal expansion properties. This makes them promising for market application and promotion in the preparation of precision optical films and protective films that can withstand long-term high temperatures and / or acid and alkali environments. Attached Figure Description

[0035] Figure 1 A photograph of the polyimide film prepared in Example 1 of this invention;

[0036] Figure 2 The ultraviolet-visible spectral transmittance curves of the polyimide films prepared in Example 1 and Comparative Examples 1-5 of this invention in the range of 300 nm to 800 nm are shown.

[0037] Figure 3 The image shows the N1s XPS fine-fit spectrum of the polyimide film prepared in Example 8 of this invention.

[0038] Figure 4 The diagram shows the HOMO and LUMO front molecular orbital distributions and energy level band gaps of the polyimide films prepared in Example 1 and Comparative Examples 1-5 of this invention, respectively, in the polyimide molecular model. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention.

[0040] Example 1

[0041] In a flask-shaped reaction vessel equipped with mechanical stirring and a nitrogen atmosphere, 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine (2 mmol, 0.808 g) was dissolved in N,N-dimethylformamide, followed by the addition of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (2.16 mmol, 1.124 g) in multiple batches. The mixture was stirred continuously at 25°C for 48 h to prepare a transparent, viscous polyamic acid solution with a solid content of 20 wt%. The polyamic acid solution was subjected to vacuum degassing, and then coated onto a transparent glass substrate using a casting method at a casting speed of 30 mm / s. A wet film with a thickness of 30 μm was obtained by scraping with a casting doctor blade, and then dried in a vacuum drying oven at 80°C for 2 hours. The solvent was removed to obtain a polyamic acid film. Next, the polyamic acid film was placed in a muffle furnace, and a thermal imidization reaction atmosphere was established at atmospheric pressure with a nitrogen to oxygen molar ratio of 17.2:1 by controlling the vacuum level and introducing pure nitrogen gas. The heating rate was then controlled at 3.0℃ / min, with the heating program performing thermal imidization at 100℃ and 200℃ for 3 hours each, and at 250℃ and 300℃ for 50 minutes each. After cooling to room temperature, a black polyimide film 1 was obtained. Figure 1 As shown.

[0042] Example 2

[0043] In a flask-shaped reaction vessel equipped with mechanical stirring and a nitrogen atmosphere, 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine (2 mmol, 0.808 g) was dissolved in N-methylpyrrolidone, followed by the addition of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (2.06 mol, 1.040 g) in multiple batches. The mixture was stirred continuously at 3°C ​​for 10 h to prepare a transparent, viscous polyamic acid solution with a solid content of 13 wt%. The polyamic acid solution was subjected to vacuum degassing, and then coated onto a transparent glass substrate using a casting method at a casting speed of 5 mm / s. A wet film with a thickness of 30 μm was obtained by scraping with a casting doctor blade, and then dried in a vacuum drying oven at 70°C for 4 hours. h removes the solvent to obtain a polyamic acid film; then, the polyamic acid film is placed in a muffle furnace, and a thermal imidization reaction atmosphere of atmospheric pressure with a nitrogen to oxygen molar ratio of 8.4:1 is established by controlling the vacuum level and filling with pure nitrogen. Then, the heating rate is controlled at 1.0℃ / min, and the heating program is carried out by heating at 100℃ and 200℃ for 1h, and at 250℃ and 300℃ for 20min, respectively, to complete the thermal imidization of the polyamic acid film. After cooling to room temperature, a black polyimide film 2 is obtained.

[0044] Example 3

[0045] In a flask-shaped reaction vessel equipped with mechanical stirring and a nitrogen atmosphere, 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine (2 mmol, 0.808 g) was dissolved in dimethyl sulfoxide, followed by the addition of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (2.06 mol, 1.040 g) in multiple batches. The mixture was stirred continuously at 15 °C for 17 h to prepare a transparent, viscous polyamic acid solution with a solid content of 16 wt%. The polyamic acid solution was subjected to vacuum degassing, and then coated onto a transparent glass substrate using a casting method at a casting speed of 8 mm / s. A wet film with a thickness of 30 μm was obtained by scraping with a casting doctor blade, and then dried in a vacuum drying oven at 80 °C for 2 hours. The solvent was removed to obtain a polyamic acid film. Then, the polyamic acid film was placed in a muffle furnace, and a thermal imidization reaction atmosphere of atmospheric pressure with a nitrogen to oxygen molar ratio of 8.4:1 was established by controlling the vacuum level and filling with pure nitrogen. Then, the heating rate was controlled at 1.2℃ / min, and the heating program was carried out by heating at 100℃ and 200℃ for 1.5 h, and at 250℃ and 300℃ for 30 min, respectively, to complete the thermal imidization of the polyamic acid film. After cooling to room temperature, a black polyimide film 3 was prepared.

[0046] Example 4

[0047] In a flask-shaped reaction vessel equipped with mechanical stirring and a nitrogen atmosphere, 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine (2 mmol, 0.808 g) was dissolved in a mixture of N,N-dimethylacetamide and dimethyl sulfoxide in a volume ratio of 2:1. Then, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (2.04 mmol, 1.061 g) was added in multiple batches, and the mixture was stirred continuously at 20°C for 23 h to prepare a transparent, viscous polyamic acid solution with a solid content of 18 wt%. The polyamic acid solution was subjected to vacuum degassing, and then coated onto a transparent glass substrate using a casting method at a casting speed of 14 mm / s. A wet film with a thickness of 30 μm was obtained by scraping with a casting doctor blade, and then dried in a vacuum drying oven at 80°C for 2 hours. h removes the solvent to obtain a polyamic acid film; then, the polyamic acid film is placed in a muffle furnace, and a thermal imidization reaction atmosphere of atmospheric pressure with a nitrogen to oxygen molar ratio of 5.3:1 is established by controlling the vacuum level and filling with pure nitrogen. Then, the heating rate is controlled at 1.3 ℃ / min, and the heating program is carried out by heating at 100℃ and 200℃ for 2 h, and at 250℃ and 300℃ for 40 min, respectively, to complete the thermal imidization of the polyamic acid film. After cooling to room temperature, a black polyimide film 4 is prepared.

[0048] Example 5

[0049] In a flask-shaped reaction vessel equipped with mechanical stirring and a nitrogen atmosphere, 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine (2 mmol, 0.808 g) was dissolved in a mixture of N,N-dimethylacetamide and N,N-dimethylformamide in a volume ratio of 1:2. Then, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (2.08 mmol, 1.082 g) was added in multiple batches, and the mixture was stirred continuously at 25°C for 26 h to prepare a transparent, viscous polyamic acid solution with a solid content of 20 wt%. The polyamic acid solution was subjected to vacuum degassing, and then coated onto a transparent glass substrate using a casting method at a casting speed of 18 mm / s. A wet film with a thickness of 30 μm was obtained by scraping with a casting doctor blade, and then dried in a vacuum drying oven at 80°C for 2 hours. The solvent was removed to obtain a polyamic acid film. Then, the polyamic acid film was placed in a muffle furnace, and the reaction atmosphere for thermal imidization was established by controlling the vacuum level and filling with pure nitrogen gas. The reaction atmosphere was a normal pressure environment with a nitrogen to oxygen molar ratio of 20:1. Then, the heating rate was controlled at 1.5 °C / min, and the heating program was carried out by heating at 100 °C and 200 °C for 2 h, and at 250 °C and 300 °C for 50 min, respectively, to complete the thermal imidization of the polyamic acid film. After cooling to room temperature, a black polyimide film 5 was prepared.

[0050] Example 6

[0051] In a flask-shaped reaction vessel equipped with mechanical stirring and a nitrogen atmosphere, 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine (2 mmol, 0.808 g) was dissolved in dimethyl sulfoxide, followed by the addition of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (2.12 mmol, 1.103 g) in multiple batches. The mixture was stirred continuously at 7°C for 30 h to prepare a transparent, viscous polyamic acid solution with a solid content of 14 wt%. The polyamic acid solution was subjected to vacuum degassing, and then coated onto a transparent glass substrate using a casting method at a casting speed of 25 mm / s. A wet film with a thickness of 30 μm was obtained by scraping with a casting doctor blade, and then dried in a vacuum drying oven at 80°C for 2 hours. The solvent was removed to obtain a polyamic acid film. Then, the polyamic acid film was placed in a muffle furnace, and a thermal imidization reaction atmosphere of atmospheric pressure with a nitrogen to oxygen molar ratio of 17.2:1 was established by controlling the vacuum level and filling with pure nitrogen. Then, the heating rate was controlled at 2.0℃ / min, and the heating program was carried out by heating at 100℃ and 200℃ for 2.5h, and at 250℃ and 300℃ for 21min. After cooling to room temperature, a black polyimide film 6 was obtained.

[0052] Example 7

[0053] In a flask-shaped reaction vessel equipped with mechanical stirring and a nitrogen atmosphere, 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine (2 mmol, 0.808 g) was dissolved in N,N-dimethylacetamide, followed by the addition of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (2.14 mmol, 1.113 g) in multiple batches, and the mixture was stirred continuously at 16 °C for 48 h to prepare a transparent viscous polyamic acid solution with a solid content of 18 wt%. The polyamic acid solution was subjected to vacuum degassing, and then coated onto a transparent glass substrate using a casting method at a casting speed of 27 mm / s. A wet film with a thickness of 30 μm was obtained by scraping with a casting doctor blade, and then placed in a vacuum drying oven at 80 °C for 2 hours. The solvent was removed to obtain a polyamic acid film. Then, the polyamic acid film was placed in a muffle furnace, and a thermal imidization reaction atmosphere of atmospheric pressure with a nitrogen to oxygen molar ratio of 8.4:1 was established by controlling the vacuum level and filling with pure nitrogen. Then, the heating rate was controlled at 2.3℃ / min, and the heating program was carried out by heating at 100℃ and 200℃ for 3h, and at 250℃ and 300℃ for 38min. After cooling to room temperature, a black polyimide film 7 was obtained.

[0054] Example 8

[0055] In a flask-shaped reaction vessel equipped with mechanical stirring and a nitrogen atmosphere, 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine (2 mmol, 0.808 g) was dissolved in a mixture of N,N-dimethylacetamide and N-methylpyrrolidone in a volume ratio of 4:3. Then, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (2.16 mmol, 1.124 g) was added in multiple batches, and the mixture was stirred continuously at 5 °C for 29 h to prepare a transparent viscous polyamic acid solution with a solid content of 20 wt%. The polyamic acid solution was subjected to vacuum degassing, and then coated onto a transparent glass substrate using a casting method at a casting speed of 30 mm / s. A wet film with a thickness of 30 μm was obtained by scraping with a casting doctor blade, and then placed in a vacuum drying oven at 80 °C for 2 hours. The solvent was removed to obtain a polyamic acid film. Then, the polyamic acid film was placed in a muffle furnace, and a thermal imidization reaction atmosphere of atmospheric pressure with a nitrogen to oxygen molar ratio of 5.3:1 was established by controlling the vacuum level and filling with pure nitrogen. Then, the heating rate was controlled at 2.7℃ / min, and the heating program was carried out by heating at 100℃ and 200℃ for 0.5h, and at 250℃ and 300℃ for 40min. After cooling to room temperature, a black polyimide film 8 was obtained.

[0056] Comparative Example 1

[0057] Following the preparation process of Example 1, only the dianhydride monomer was replaced by an equimolar amount of 4,4'-bis(3,4-dicarboxyphenoxy)phenyl]propane dianhydride instead of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride. All other preparation processes were consistent with those in Example 1, and a yellow polyimide film 9 was finally prepared.

[0058] Comparative Example 2

[0059] Following the preparation process of Example 1, only the dianhydride monomer was replaced by an equimolar amount of 9,9-bis(trifluoromethyl)-2,3,6,7-oxanthracene tetracarboxylic dianhydride instead of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride. All other preparation processes were consistent with those of Example 1, and a yellow polyimide film 10 was finally prepared.

[0060] Comparative Example 3

[0061] Following the preparation process of Example 1, only the dianhydride monomer was replaced by an equimolar amount of 4,4'-oxobisphthalic anhydride instead of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride. All other preparation processes were consistent with those of Example 1, and a yellow polyimide film 11 was finally prepared.

[0062] Comparative Example 4

[0063] Following the preparation process of Example 1, only the dianhydride monomer was replaced by an equimolar amount of 3,3',4,4'-benzophenone tetracarboxylic dianhydride instead of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride. All other preparation processes were consistent with those of Example 1, and a yellow polyimide film 12 was finally prepared.

[0064] Comparative Example 5

[0065] Following the preparation process of Example 1, only the dianhydride monomer was replaced by an equimolar amount of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) with 4,4'-(hexafluoroisopropyl)bisphthalic anhydride. All other preparation processes were consistent with Example 1, and a yellow polyimide film 13 was finally prepared.

[0066] Comparative Example 6

[0067] A polyamic acid solution was prepared in the same manner as in Example 1. Then, acetic anhydride (8 mmol, 0.76 mL) and pyridine (0.25 mL) were added to the polyamic acid solution as a dehydrating agent. The mixture was heated to 80°C, stirred, and reacted for 5 h to prepare a polyimide solution using a chemical imidization process. Subsequently, the polyimide solution was subjected to vacuum degassing treatment, and a film casting method was used to coat the polyimide solution onto a transparent glass substrate at a casting speed of 30 mm / s. The substrate was then placed in a vacuum drying oven at 80°C for 2 h to remove the solvent, resulting in a yellow polyimide film 14.

[0068] Comparative Example 7

[0069] Following the preparation process of Example 1, only the mixed atmosphere of nitrogen and oxygen with a molar ratio of 17.2:1 was replaced with a pure nitrogen atmosphere. All other preparation processes remained the same as in Example 1, and a light brown polyimide film 15 was finally prepared.

[0070] Comparative Example 8

[0071] Following the preparation process of Example 2, only the mixed atmosphere of nitrogen and oxygen with a molar ratio of 8.4:1 was replaced with a pure nitrogen atmosphere. All other preparation processes remained the same as in Example 2, and a light brown polyimide film 16 was finally prepared.

[0072] Comparative Example 9

[0073] Following the preparation process of Example 1, only the mixed atmosphere of nitrogen and oxygen with a molar ratio of 17.2:1 was replaced with an air atmosphere. All other preparation processes were consistent with Example 1, and a black polyimide film 17 was finally prepared.

[0074] Comparative Example 10

[0075] Following the preparation process of Example 1, only the diamine monomer was replaced by an equimolar amount of 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine, while the rest of the preparation process remained the same as in Example 1, and a light yellow polyimide film 18 was finally obtained.

[0076] Performance testing

[0077] (a) Optical performance testing:

[0078] The transmittance of the polyimide films prepared in Examples 1-8 and Comparative Examples 1-10 was tested using a Beijing Purkinje TU-1901 dual-beam UV-Vis spectrophotometer. The test range was set to 200 nm-800 nm, and the resolution was set to 0.5 nm. The specific test method was as follows: with air as the background, the prepared polyimide film was placed in a sample holder, and the UV-Vis spectrum was measured; the UV transmittance of the polyimide film at 450 nm was denoted as T. 450 (%), the ultraviolet transmittance at 500 nm is denoted as T. 500 (%), the ultraviolet transmittance at 550 nm is denoted as T. 550 (%); the ultraviolet transmittance at 650 nm is denoted as T. 650 (%), the ultraviolet transmittance at 750 nm is denoted as T. 750 (%); cutoff wavelength is denoted as λ. 10 , where represents the wavelength (nm) at which transmittance reaches 10%. Specific test results for transmittance performance are shown in Table 1 below.

[0079] Table 1:

[0080]

[0081] As can be seen from the test results in Table 1, in Examples 1-8, the intrinsic black polyimide film prepared by copolymerizing polyamic acid from 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride under a controlled atmosphere of nitrogen and oxygen under normal pressure can be prepared by thermal imidization reaction, with a visible light standard range of 400 nm-760 nm. Specifically, in the characteristic wavelength tests at 450 nm and 550 nm, the transmittance of the black polyimide films prepared in Examples 1-8 was all below 0.3%, while the transmittance at 650 nm and 750 nm was significantly lower. In the characteristic wavelength test of nm, the transmittance of the black polyimide film prepared in Examples 1-8 remained as low as 16.5 and below. The above test results prove that the polyimide film prepared by the method of the present invention can achieve excellent ultraviolet-visible broad-spectrum absorption and high light-shielding properties without the addition of carbon black, organic dyes and other external colorants, and can be applied to the field of high-precision optical light-shielding devices.

[0082] Meanwhile, combined with Table 1 based on λ 10The cutoff wavelengths obtained under the test conditions (i.e., wavelengths with a transmittance of 10%) show a clear gradient distribution pattern among the polyimide films prepared in Examples 1-8 and Comparative Examples 7-9. Specifically, the cutoff wavelengths of the polyimide films obtained by thermal imidization reaction under a pure nitrogen atmosphere in Comparative Examples 7 and 8 are 600 nm and 601 nm, respectively, with the smallest cutoff wavelengths. The cutoff wavelengths of the polyimide films obtained by thermal imidization reaction under a controlled atmosphere of nitrogen and oxygen in Examples 1-8 of this invention are distributed in the range of 683 nm-690 nm, which is the second highest. The cutoff wavelengths of the polyimide films obtained by thermal imidization reaction under a pure nitrogen atmosphere in Comparative Examples 7 and 8 are 600 nm and 601 nm, respectively, with the smallest cutoff wavelengths. The polyimide film obtained by thermal imidization reaction under a pure air atmosphere in Comparative Example 9 has the longest cutoff wavelength, reaching 720 nm. nm; The above results fully demonstrate that for polyamic acid prepared by copolymerization of 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride, in the further thermal imidization reaction, the higher the oxygen partial pressure in the reaction atmosphere, the higher the degree of oxidation of nitrogen atoms on the piperazine ring, the more conjugated chromophores in the molecular chain, and the narrower the HOMO-LUMO energy level band gap, ultimately achieving the effect of redshifting the cutoff wavelength and improving the intrinsic blackness of the film. In Comparative Examples 7 and 8, the lack of an oxidizing medium during the thermal imidization reaction prevented piperazine from forming a conjugated structure with NO bonds and nitro groups, resulting in light brown polyimide films with significantly reduced light-shielding properties. While the polyimide film prepared in Comparative Example 9 had the best light-shielding effect, the excessively high oxygen partial pressure during the thermal imidization reaction caused molecular chain breakage and local carbonization, resulting in a significant deterioration in the mechanical and heat resistance properties of the polyimide film, rendering it unusable for practical applications. See Table 3 below for details.

[0083] In Comparative Examples 1 to 5, when 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride was replaced with other dianhydrides, the corresponding cutoff wavelengths were only distributed in the range of 467 nm to 515 nm, indicating that the prepared polyimide films had high visible light transmittance. Although these polyimide films used the same preparation process as the polyimide films prepared in Examples 1-8, the change in dianhydride structure resulted in insufficient oxidative coloring during the thermal imidization reaction, leading to higher transmittance and lower blackness. The polyimide film prepared in Comparative Example 6, due to the use of a chemical imidization preparation process, resulted in premature closure of its molecular chain, preventing the construction of a conjugated color-emitting structure based on piperazine oxidation, and only a light yellow film could be obtained. In Comparative Example 10, the cutoff wavelengths of other diamines in Comparative Example 10 were only distributed in the range of 467 nm to 515 nm. The visible light transmittance of the film is significantly higher at nm; however, in Comparative Example 10, when 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine is replaced with the diamine monomer 4,4'-diaminodiphenyl ether without a piperazine ring, the resulting polyimide film is a transparent film with no light-shielding ability due to the lack of oxidizable active sites. The above comparative results fully demonstrate that only by using the matching diamine monomer and dianhydride monomer of this invention, and by carrying out controlled thermal imidization under the nitrogen and oxygen mixed atmosphere specified in this invention, can an intrinsically black polyimide film with excellent light-shielding properties be prepared.

[0084] like Figure 2 The figures show the UV-Vis transmittance curves of the polyimide films prepared in Example 1 and Comparative Examples 1-5 in the range of 300 nm to 800 nm. As can be seen from the figures, all polyimide films exhibit almost no transmittance in the UV band, demonstrating excellent UV shielding performance. However, in the visible light region, compared to Comparative Examples 1-5, the polyimide film prepared in Example 1 shows significantly lower transmittance, achieving efficient light blocking in the 450 nm to 750 nm visible light band. This indicates that it has a richer conjugated chromophore structure, a narrower HOMO-LUMO band gap, and excellent intrinsic high blackness and visible light shielding capabilities.

[0085] like Figure 3 The image shows the N1s XPS fine-fit spectrum of the polyimide film prepared in Example 8. The fitted spectrum shows nitrogen characteristic peaks from three chemical environments, corresponding to pristine tertiary amine nitrogen, N–O bonded oxidized nitrogen, and nitro-type high-oxidation-state nitrogen, respectively. Among these, nitro-type high-oxidation-state nitrogen has the highest proportion, indicating that the polyimide film has the highest degree of oxidation, possesses a large number of conjugated chromogenic structures, effectively narrows the HOMO-LUMO band gap, and achieves high-black intrinsic coloration. This conclusion is consistent with the results shown in Table 1.

[0086] In summary, this invention enables the preparation of a black polyimide film without the addition of black filler by precisely controlling the oxygen content of the atmosphere. This fundamentally avoids the dispersion and interface problems caused by adding filler, and achieves the controllable preparation of a uniform, stable, and high-performance non-additive black polyimide.

[0087] (II) Color and haze performance testing:

[0088] The color parameters, haze, and transmittance of the polyimide films prepared in Examples 1-8 and Comparative Examples 1-10 were tested using a color haze meter (CHN Spec). During the tests, the standard light source was set to D65, the observation angle to 2°, and the test mode to transmittance mode. The haze, yellowness index (YI), lightness index (L*), red-green index (a*), and blue-yellow index (b*) of each polyimide film were measured. Parallel tests were conducted at three different locations for each sample group, and the average value was taken as the final test result to characterize the optical properties and color differences of each polyimide film. The specific test results are shown in Table 2 below.

[0089] Table 2:

[0090]

[0091] As shown in Table 2, the lightness index (L*) of the polyimide films prepared in Examples 1-8 and Comparative Example 9 is below 10, and the films exhibit high blackness. The lightness index of the polyimide films prepared in Comparative Examples 7 and 8 is above 15, and the films are light brown. The lightness index of the polyimide films prepared by Comparative Examples 1-5 and Comparative Example 10 by monomer substitution, and the polyimide film prepared by Comparative Example 6 using chemical imidization, are all above 60, and the films are light yellow and transparent. Therefore, based on the above data, it can be proven that only polyamic acid prepared from 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride, under controlled thermal imidization conditions with a defined nitrogen and oxygen molar ratio, can form an intrinsically black polyimide film with a high conjugated color structure, low lightness, and high blackness.

[0092] (III) Mechanical properties and thermal stability tests:

[0093] The mechanical properties of the polyimide films prepared in Examples 1-8 and Comparative Examples 1-10 were tested using a CMT 44503 universal tensile testing machine (China). All polyimide films were prepared with dimensions of 75 mm (L) × 4 mm (W) and a thickness of 30 μm, and the tensile rate was set to 5.0 mm / min according to GB / 1040.3-2006. The mechanical properties of each sample were determined by measuring three times and taking the average value.

[0094] Thermogravimetric analysis (TGA) was performed on the polyimide films prepared in Examples 1-8 and Comparative Examples 1-10 using a Netzsch STA 449F5 analyzer (Germany) to test the thermal stability of the polyimide films. The test conditions were set as follows: a nitrogen atmosphere was used. The test method was as follows: 5 mg of polyimide was weighed and placed in an alumina crucible, and the temperature was increased from room temperature to 800°C at a heating rate of 10°C / min for TGA testing; where T5% (°C) represents the 5% thermogravimetric temperature, and T... 10 % (°C) represents the temperature at which the body loses weight by 10%; RW 750 (%) indicates the carbon residue at 750℃; T s (MPa) represents tensile strength, T M (Gpa) represents the elastic modulus, E b (%) represents the elongation at break.

[0095] The specific test results are shown in Table 3 below.

[0096] Table 3:

[0097]

[0098] As can be seen from the test results in Table 3, the 5% thermogravimetric temperature of the polyimide films prepared in Examples 1-8, Comparative Examples 1-5, Comparative Examples 7-8, and Comparative Example 10 is concentrated between 394℃ and 424℃, and the tensile strength is distributed between 36.35 MPa and 48.62 MPa, exhibiting moderate thermal stability and mechanical properties typical of aromatic polyimides. However, the polyimide prepared by chemical imidization in Comparative Example 6 and the polyimide prepared by excessive oxidation in air in the comparative example show a significant decrease in thermal stability and mechanical properties, with 5% thermogravimetric temperatures of only 245℃ and 256℃, respectively, and tensile strengths less than 11 MPa. The thermal stability and mechanical properties are significantly deteriorated due to the MPa and elongation at break being less than 0.4%. The reason for this is that the chemical imidization process easily leads to incomplete imidization and numerous molecular defects inside the polyimide film. In a pure air environment, excessive oxidation occurs, causing polyimide molecular chain breakage and local carbonization. As a result, the heat resistance and mechanical properties of the polyimide film are greatly reduced. This also confirms that only by using the controlled thermal imidization method of this invention, which limits the molar ratio of nitrogen to oxygen, can the inherent excellent thermal stability and good mechanical properties of polyimide materials be preserved.

[0099] (iv) Wide-angle X-ray diffraction test:

[0100] Wide-angle X-ray diffraction tests were performed on the polyimide films prepared in Examples 1-8 using a Shimadzu 6100 X-ray diffractometer (Japan). The test method was set as follows: operating voltage 40 kV, operating current 200 mA, and test range 5... o -50 o A polyimide film with dimensions of 2 cm (L) × 1 cm (W) was fabricated and placed in a sample cell for XRD characterization; where 2θ (°) represents the X-ray diffraction angle and d (Å) represents the interlayer spacing of the molecular chains. The specific test results are shown in Table 4 below.

[0101] Table 4:

[0102]

[0103] In wide-angle X-ray diffraction (XRD) tests, intermolecular forces alter the aggregated structure of polyimide, thereby affecting the optical absorption properties and apparent color of the film. XRD can characterize the micro-aggregate structure of polyimide films, and by combining it with Bragg's law, the average interlayer spacing of molecular chains can be calculated, thereby analyzing the packing density of polyimide molecular chains.

[0104] As can be seen from the test results in Table 4, the black polyimide film prepared by this invention has a typical amorphous structure. The trifluoromethyl group in the molecular structure has strong electronegativity and large steric hindrance, which increases the repulsion between molecular chains, destroys the regularity of molecular chains, and inhibits the formation of ordered crystals. It also makes the molecular chains relatively loosely packed and the interlayer spacing larger. This structural feature is beneficial to improving the solubility and gas permeability of polyimide.

[0105] (v) Water contact angle and moisture absorption rate test:

[0106] The water contact angle of the polyimide films prepared in Examples 1-8 was tested using a Shanghai Zhongchen JC2000D1 dynamic contact angle measuring instrument. The test conditions were set to room temperature. A 1 cm × 1 cm polyimide film was attached to a quartz plate, and the water contact angle of the polyimide film was measured using the five-point fitting method.

[0107] The moisture absorption rate test was conducted in a constant temperature water bath at 25℃. Specifically, the dried polyimide film (100 mg) was immersed in deionized water for 48 h, and the mass change before and after was measured to determine its moisture absorption rate.

[0108] The specific test results are shown in Table 5 below.

[0109] Table 5: Test results of moisture absorption rate and water contact angle of polyimide film in each embodiment

[0110]

[0111] The water contact angle can characterize the hydrophobicity of a material surface, and together with the hygroscopic properties, it is regulated by molecular polarity, free volume, and monomer structure. As can be seen from the test results in Table 5, the polyimide film prepared by this invention exhibits both low moisture absorption and a moderate water contact angle. The reason for this is that, in the polyimide molecular chain structure, the isopropyl group provided by 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride B is a hydrophobic group, which can reduce the polarity of the molecular chain and reduce the adsorption sites of water molecules. Meanwhile, the hydrophobic effect of the trifluoromethyl group provided by 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine balances the hydrophilic effects of polar groups such as ether bonds and carbonyl groups, so that the polyimide achieves a good match between hydrophobicity and moisture resistance. This can effectively suppress surface water absorption problems in microelectronic and flexible device applications and reduce the risk of interface failure. At the same time, the high-density aromatic rings make the molecular chain compact and reduce the free volume to hinder the diffusion of water molecules, thus making the polyimide film exhibit low moisture absorption and a moderate water contact angle.

[0112] (vi) Solubility test:

[0113] The polyimide films prepared in Examples 1-8 were subjected to solubility tests. The solubility characterization method was as follows: 1 mg of polyimide film was added to 1 mL of solvent, and the solubility was qualitatively measured at 25°C. + indicates dissolution; +- indicates partial dissolution; and - indicates no dissolution. The specific test results are shown in Table 6 below.

[0114] Table 6:

[0115]

[0116] Table 6 shows the solubility of the polyimide films of each embodiment in six commonly used organic solvents. "+" indicates complete dissolution, "+-" indicates partial swelling or dissolution, and "-" indicates no dissolution. Based on the preparation process, it can be seen that the solvent system used in the polymerization process directly affects the degree of molecular chain ordering, thus controlling the solvent resistance of the film. Examples 1, 2, 6, and 7 were polymerized using DMF, NMP, DMAC, and DMSO as single solvents, respectively. These films exhibited a high degree of molecular chain ordering and stronger intermolecular forces, resulting in only slight swelling in DMF and no dissolution in NMP, DMAC, THF, CHCl3, and DMSO, demonstrating outstanding solvent resistance. Examples 3-5 and Example 8 were polymerized using DMSO as a single solvent. Alternatively, polymerization can be carried out using a DMAC-based mixed solvent system. The mixed solvent can disrupt the regular arrangement of molecular chains and increase molecular randomness. Therefore, these samples can all partially dissolve in NMP, DMF, DMAC, THF, and CHCl3. Among them, Example 3 can be completely dissolved in DMF and DMAC, and Example 5 can be completely dissolved in THF. The polyimide films prepared in all examples are insoluble in the highly polar solvent DMSO and have excellent resistance to organic solvent corrosion, which can meet the processing requirements of flexible optical devices and microelectronic packaging components and the requirements of long-term service environment.

[0117] (vii) Simulation of molecular orbital energies:

[0118] Molecular orbital energy simulations were performed on the polyimide films prepared in Examples 1-8 and Comparative Examples 1-10 using the Gaussian 09 software package. Specifically, a model of the polyimide was constructed within Gaussian 09, with the Method selected being DFT (Density Functional Theory) and the Job Type being Opt+Freq. All conformations were calculated under B3LYP / 6-31G(d). HOMO represents the highest occupied molecular orbital energy level, and LUMO represents the lowest unoccupied molecular orbital energy level; the energy difference between the two is the HOMO-LUMO band gap. The specific test results are shown in Table 7 below.

[0119] Table 7:

[0120]

[0121] In Table 7, the HOMO-LUMO band gap (Gap) can be used to reflect the color difference of the polyimide film. The smaller the Gap value, the darker the polyimide film; the larger the Gap value, the lighter the polyimide film. According to the test results in Table 7, the apparent color of the polyimide film is black when the Gap value is approximately 0.7 eV; brown when the Gap value is approximately 1.7 eV; and yellow when the Gap value is approximately 3 eV.

[0122] As shown in Table 7, the polyimide prepared in Examples 1-8 of this invention, after being oxidized in a controlled mixed atmosphere of nitrogen and oxygen, has a HOMO-LUMO band gap (Gap) of only 0.600 eV to 0.810 eV. This narrow band gap structure allows electrons to transition in the visible light band, making the polyimide film exhibit an intrinsic black color with high blackness. In Comparative Examples 1-5 and 10, the newly replaced dianhydride or diamine monomers could not form a NO conjugated chromophore structure, resulting in a band gap of 2.005 eV to 2.940 eV. These films could only absorb ultraviolet light, and the polyimide films appeared yellow and transparent, with no visible light blocking ability. The polyimide prepared by the chemical imidization process in Comparative Example 6, and the polyimide films prepared in pure nitrogen atmospheres in Comparative Examples 7 and 8, had a band gap between 1.713 eV and 2.940 eV due to weak oxidation. These films appeared light brown with poor blackness and light-blocking properties. The polyimide film prepared in air atmosphere in Comparative Example 9 had the highest oxidation level, resulting in a band gap as low as 0.580 eV. Although it had the best blackness, excessive oxidation caused molecular chain breakage and carbonization, leading to a significant decrease in mechanical properties. In summary, molecular orbital simulation results confirm that only by precisely controlling the molar ratio of nitrogen to oxygen in the mixed atmosphere during the thermal imidization reaction can the degree of oxidation of the polyimide molecular chain be accurately controlled, thereby controlling the band gap within the range of 0.6 eV to 0.8 eV. This approach imparts high light-shielding properties to the film while retaining the excellent comprehensive thermal and mechanical properties of polyimide.

[0123] like Figure 4The diagram shows the HOMO and LUMO front molecular orbital distributions and band gaps of the polyimide films prepared in Example 1 and Comparative Examples 1-5, respectively, in the polyimide molecular model. This reveals the mechanism of the high light-blocking performance of intrinsic black polyimide at the molecular level. Specifically, the HOMO orbitals are mainly delocalized within the conjugated molecular backbone, while the LUMO orbitals are concentrated in the electron-withdrawing dianhydride structural region; as shown... Figure 4 As shown, the molecular orbitals of Example 1 have a higher degree of overlap and a smaller band gap, enabling electronic transitions in the visible light band.

[0124] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for in-situ controllable coloring of a black polyimide film, characterized in that, It is prepared by reacting a polyamic acid solution formed by the reaction of 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine with 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride, coating it onto a substrate to form a polyamic acid wet film, drying it, and then placing it in an atmospheric pressure environment with a nitrogen and oxygen molar ratio of 5.3≤n(N2):n(O2)≤20 for thermal imidization reaction.

2. The in-situ controllable coloring preparation method according to claim 1, characterized in that, The preparation steps of the polyamic acid solution are as follows: 4,4'-bis(4-amino-2-trifluoromethylphenyl)piperazine is dissolved in a polar aprotic organic solvent, and then 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride is added in batches at 0~25℃ and stirred for 10h-48h.

3. The in-situ controllable coloring preparation method according to claim 2, characterized in that, The solid content of the polyamic acid solution is 10 wt%-20 wt%.

4. The in-situ controllable coloring preparation method according to claim 2, characterized in that, The polar aprotic organic solvent is a mixture of at least one or more of N,N-dimethylformamide, N-methylpyrrolidone, N,N-dimethylacetamide, and dimethyl sulfoxide.

5. The in-situ controllable coloring preparation method according to claim 1, characterized in that, The preparation steps of the polyamic acid wet film are as follows: the polyamic acid solution is subjected to vacuum degassing treatment, then cast onto the substrate at a casting speed of 5 mm / s-30 mm / s, and then coated into a polyamic acid wet film with uniform thickness.

6. The in-situ controllable coloring preparation method according to claim 1, characterized in that, The drying conditions for the polyamic acid wet film are: drying in a vacuum drying oven at 70℃~80℃ for 2 h~4 h.

7. The in-situ controllable coloring preparation method according to claim 1, characterized in that, The thermal imidization reaction steps are as follows: the heating rate is controlled at 0.8℃ / min-3.0℃ / min, and the heating program is set at 100℃ and 200℃ for 0.5h-3h respectively, and at 250℃ and 300℃ for 20min-50min respectively, to complete the thermal imidization of the polyamic acid film.

8. A black polyimide film prepared by the in-situ controllable coloring preparation method as described in any one of claims 1-7.

9. An application of the black polyimide film as described in claim 8, characterized in that, Used in precision optical films, including: black matrix light-shielding layers for flexible OLED and Micro-LED displays, light-shielding housings for optical sensors, anti-glare apertures for camera modules, and light-shielding dielectric layers for microelectronic packaging; Alternatively, it can be used to prepare protective films that can withstand high temperatures and / or acid and alkali environments for a long time, including: marking films, tape substrates, flexible circuit covering films, and aircraft thermal control blankets.

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