Biimidazole polyimide film and preparation method thereof

Abstract

The preparation method comprises the following steps: taking para aminobenzoic acid, 3, 3-diaminobiphenyl and phosphorus pentoxide as raw materials, and carrying out high-temperature dehydration cyclization reaction in a polyphosphoric acid solvent to obtain a diamine monomer containing biimidazole; the preparation method comprises the following steps: adding a bis-imidazole-containing diamine monomer, 2-(4-aminophenyl)-5-aminobenzoxazole and an organic solvent into a reactor, stirring and dissolving, after the solution is clarified, adding pyromellitic dianhydride, stirring and reacting, and after the reaction is completed, obtaining a polyamide acid membrane casting solution; and carrying out film casting on the polyamide acid film casting solution, drying at low temperature, and transferring to a muffle furnace to carry out imidization reaction after solvent volatilization is finished, so as to obtain the bis-imidazole polyimide film. The biimidazole polyimide film constructed by the invention is applied to a flexible OLED substrate, and has relatively good thermal performance and mechanical performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible display films. More specifically, the present invention relates to a bis-imidazole polyimide film and a preparation method thereof. Background Art

[0002] With the continuous progress of technology and the growing demand for intelligent and convenient electronic devices, traditional display technologies are gradually showing their limitations in meeting the needs of new applications. Traditional display technologies such as liquid crystal displays and organic light-emitting diodes, although providing excellent display effects for various electronic products in the past few decades, are gradually unable to meet the diverse needs of modern intelligent terminal devices in terms of flexibility, thinness, integration, and innovative functions, especially in emerging fields such as smartphones, wearable devices, and in-vehicle electronics. With the increasing demand of consumers for a more efficient, convenient, and personalized display experience, flexible display technology has gradually become a new direction for the development of the industry, driving a revolution in display technology. Through flexible display technology, electronic products can not only achieve a more convenient operation experience, but also break through the design limitations of traditional display technologies and realize more innovative product forms. However, flexible display films face a series of technical challenges in practical applications, especially in terms of materials, performance, processes, and costs, and still need to be continuously broken through.

[0003] Since the basic working principle of organic light-emitting diodes was first proposed and successfully demonstrated by DuPont in the United States in 1987, many research teams have begun to explore how to combine organic light-emitting diode displays with flexible substrate materials (such as thin films, plastic substrates) to enable the displays to have the ability to bend and fold. As one of the key components of organic light-emitting diode displays, the functions of flexible display films mainly include the following two aspects: (1) As the substrate of the display, maintaining the shape and structure stability of the screen, enhancing the durability and damage resistance of the display screen, and extending the service life of the display screen; (2) Transmitting charges in the display materials to ensure stable current transmission of the display screen in different forms, ensuring that the color performance and brightness of the display screen are not affected by the film material, and achieving a high-definition and high-color-reduction display effect. Currently, polyimide films such as Kapton H of DuPont, Upilex R of Ube Industries, and Novax of Mitsubishi are high-performance polymer materials with the highest heat resistance grades and excellent comprehensive performance available on the market. However, the thermal properties of these commercial polyimide film materials are still insufficient, especially thermal expansion, which limits the maximum process temperature as a substrate material. Therefore, it is urgently needed in science and engineering to develop polyimide films with higher thermal stability and lower thermal expansion. Summary of the Invention

[0004] An object of the present invention is to solve at least the above problems and / or defects and provide at least the advantages described hereinafter.

[0005] To achieve these objects and other advantages in accordance with the present invention, a bis-imidazole polyimide film is provided, and its chemical structural formula is:

[0006]

[0007] A method for preparing a bis-imidazole polyimide film includes the following steps:

[0008] Step 1: Using p-aminobenzoic acid, 3,3-diaminobiphenyl, and phosphorus pentoxide as raw materials, and performing a high-temperature dehydration cyclization reaction in a polyphosphoric acid solvent to obtain a bis-imidazole diamine monomer;

[0009] Step 2: Adding the bis-imidazole diamine monomer, 2-(4-aminophenyl)-5-aminobenzoxazole, and an organic solvent into a reactor and stirring to dissolve. After the solution becomes clear, adding pyromellitic dianhydride and stirring for reaction. After the reaction is completed, a polyamic acid casting solution is obtained;

[0010] Step 3: Casting the polyamic acid casting solution into a film, drying at a low temperature. After the solvent evaporation is completed, transferring it to a muffle furnace for imidization reaction to obtain the bis-imidazole polyimide film.

[0011] Preferably, in the said Step 1, the synthesis method of the bis-imidazole diamine monomer includes the following steps:

[0012] Step I: Weigh 10 - 60 parts by weight of phosphorus pentoxide and 160 - 250 parts by weight of polyphosphoric acid and add them to a reaction vessel in sequence; stir at 100 - 150 °C for 1 - 2 h. After the solution becomes clear, add 2 - 14 parts by weight of 3,3-diaminobiphenyl; stir at 140 - 160 °C for 1 - 2 h, add 2 - 19 parts by weight of p-aminobenzoic acid; after stirring for 1 - 2 h, raise the temperature to 160 - 200 °C and react for 12 - 24 h; after the reaction is completed, pour it into a deionized water solvent, and brown precipitates will precipitate; filter and collect the filter cake, pour the filter cake into a saturated sodium bicarbonate solution until no bubbles are generated and the pH value is slightly alkaline, and after filtering, washing, and drying, a crude bis-imidazole diamine monomer is obtained;

[0013] Step II: Add the crude bis-imidazole diamine monomer and absolute ethanol to a reaction vessel in sequence, and stir at 50 - 70 °C for 0.5 - 5 h; after the reaction is completed, filter and collect the filtrate, pour the filtrate into deionized water, and yellow precipitates will form. After washing and drying, the bis-imidazole diamine monomer is obtained.

[0014] Preferably, in the said Step II, the dosage ratio of the crude bis-imidazole diamine monomer to absolute ethanol is 4 - 40 g:50 - 200 mL.

[0015] Preferably, in the second step, the reaction includes: under the protection of nitrogen, dissolving 2-(4-aminophenyl)-5-aminobenzoxazole and bisimidazole diamine monomer in an organic solution, heating to 25-50°C and stirring until the solid substances are dissolved, then adding pyromellitic dianhydride and continuing to stir, and the reaction time is 24-48h.

[0016] Preferably, in the second step, the molar ratio of 2-(4-aminophenyl)-5-aminobenzoxazole to bisimidazole diamine monomer is 5-10.0:1.0-5.0; the molar ratio of the total molar amount of 2-(4-aminophenyl)-5-aminobenzoxazole and bisimidazole diamine monomer to the molar amount of pyromellitic dianhydride is 1.0:1.0±0.02; the mass ratio of the organic solution to the total mass of 2-(4-aminophenyl)-5-aminobenzoxazole and bisimidazole diamine monomer is 5-10.0:1.0-2.0.

[0017] Preferably, in the third step, the drying temperature of the polyamic acid casting solution is 60-80°C, and the drying time is 12-24h; the temperature change rate of imidization in the muffle furnace is 50-100°C / h, the imidization temperature is 100-400°C, and the temperature of deionized water is 50-80°C.

[0018] Preferably, the organic solvent is any one or a mixture of two or more of N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone.

[0019] Preferably, the thickness of the bisimidazole polyimide film is 20-50μm.

[0020] An application of a bisimidazole polyimide film, wherein the bisimidazole polyimide film is applied to a flexible OLED substrate as a substrate material for flexible display.

[0021] The present invention at least includes the following beneficial effects:

[0022] (1) For the bisimidazole polyimide film of the present invention, due to the introduction of a rigid backbone structure in the main chain to increase the stiffness and linearity of the molecular chain, it is beneficial to reduce the thermal expansion coefficient and improve the thermal stability.

[0023] (2) For the bisimidazole polyimide film of the present invention, due to the introduction of a hydrogen bond structure to increase the interaction between molecular chains and restrict the movement of molecular chains, it is beneficial to improve the thermal performance and to a certain extent improve the mechanical properties.

[0024] (3) For the bisimidazole polyimide film of the present invention, due to the presence of a benzoxazole heterocyclic structure introduced, the regularity between chains is improved, which is beneficial to the improvement of mechanical properties.

[0025] Other advantages, objects and features of the present invention will be partly reflected by the following description, and partly will be understood by those skilled in the art through the research and practice of the present invention. Description of the Drawings

[0026] Figure 1 It is the synthesis flow chart of the bis-imidazole diamine monomer in Example 1 of the present invention;

[0027] Figure 2 It is the 1H NMR spectrum of the bis-imidazole diamine monomer in Example 1 of the present invention;

[0028] Figure 3 It is the infrared spectrum of the bis-imidazole diamine monomer in Example 1 of the present invention;

[0029] Figure 4 It is the synthesis flow chart of the bis-imidazole polyimide film in Example 1 of the present invention;

[0030] Figure 5 It is the infrared spectrum of the bis-imidazole polyimide film in Example 2 of the present invention;

[0031] Figure 6 It is the coefficient of thermal expansion diagram of the bis-imidazole polyimide film in Example 3 of the present invention;

[0032] Figure 7 It is the thermogravimetric diagram of the bis-imidazole polyimide film in Example 3 of the present invention;

[0033] Figure 8 It is the glass transition temperature diagram of the bis-imidazole polyimide in Example 3 of the present invention;

[0034] Figure 9 It is the mechanical property diagram of the bis-imidazole polyimide film in Example 3 of the present invention. Detailed Description of the Invention

[0035] The present invention will be further described in detail below with reference to the drawings, so that those skilled in the art can implement it according to the description in the specification.

[0036] It should be understood that the terms such as "having", "comprising" and "including" used herein do not exclude the presence or addition of one or more other elements or their combinations.

[0037] Example 1:

[0038] As Figure 4 shown, a preparation method of a bis-imidazole imide film, the steps of which are:

[0039] Step 1: Under the protection of nitrogen, add 7 mmol of 2-(4-aminophenyl)-5-aminobenzoxazole (BOA) and 3 mmol of bisimidazole diamine monomer (DBIA) to a 100 mL single-necked flask, which are dissolved in a mixed solution of 45.0 mL of dimethyl sulfoxide and N,N-dimethylformamide. Heat the solution to 35 °C and stir until the solids dissolve. Then add 9.98 mmol of pyromellitic dianhydride (PDMA) and continue stirring for 32 h to obtain a polyamic acid polymer.

[0040] Step 2: Pour the polyamic acid casting solution prepared in Step 1 onto a dry and clean glass plate to form a film by casting. Then place the glass plate in an oven and dry it at 70 °C for 16 h. After the solvent has completely evaporated, transfer it to a muffle furnace and perform imidization reactions at 100 °C, 200 °C, 300 °C, and 400 °C in sequence at a heating rate of 100 °C / h. After the reaction is completed, soak the film in deionized water at 55 °C and peel it off to obtain a bisimidazole polyimide film. The thickness of the prepared bisimidazole polyimide film is 28 μm. Store it in a dry place.

[0041] In Step 1, the synthesis method of the bisimidazole diamine monomer includes the following steps:

[0042] Step I: Weigh 25.00 g of phosphorus pentoxide and 190 g of polyphosphoric acid and add them to the reaction vessel in sequence. Stir at 110 °C for 2 h. After the solution becomes clear, add 8.56 g of 3,3-diaminobiphenyl. Stir at 150 °C for 1 h, then add 10.96 g of p-aminobenzoic acid. After stirring for 2 h, raise the temperature to 190 °C and react for 24 h. After the reaction is completed, pour it into a deionized water solvent, and brown precipitates will form. Filter and collect the filter cake, and pour the filter cake into a saturated sodium bicarbonate solution until no bubbles are generated and the pH value is slightly alkaline. After filtration, washing, and drying, a crude bisimidazole diamine monomer is obtained.

[0043] Step II: Add 25.00 g of the crude bisimidazole diamine monomer and 125.0 mL of absolute ethanol to a three-necked flask in sequence and stir at 50 °C for 2 h. After the reaction is completed, filter and collect the filtrate, and pour the filtrate into deionized water, then yellow precipitates will form. After washing and drying, the bisimidazole diamine monomer is obtained.

[0044] The nuclear magnetic resonance hydrogen spectrum of the bisimidazole diamine monomer prepared in this example is as Figure 2 shown. The peaks with chemical shifts in the range of 6.0 - 12.0 ppm originate from the hydrogen atoms of the benzene ring of the bisbenzimidazole diamine monomer; the peak appearing at 5.59 ppm is from the hydrogen atom of the amino group of the bisbenzimidazole diamine monomer, and the peak value at 12.47 ppm corresponds to the hydrogen atom on the imidazole ring, preliminarily indicating that the bisimidazole diamine monomer has been successfully synthesized.

[0045] The infrared spectrum of the bisimidazole diamine monomer prepared in this example is asFigure 3 As shown. At 3200~3500cm -1 Appears within range - NH 2 Stretching vibration peak. At 1600~1800cm -1 There is no peak at 1610 and 1484 cm -1 The peak at can be attributed to the C=N symmetric vibration peak of the imidazole ring. This further confirmed that the bisimidazole diamine monomer had been successfully synthesized.

[0046] Embodiment 2:

[0047] A method for preparing a bisimidazole polyimide film, comprising the following steps:

[0048] Step 1: Under the protection of nitrogen, 8 mmol of 2-(4-aminophenyl)-5-aminobenzoxazole and 2 mmol of bisimidazole diamine monomer were added to a 100 mL single-necked flask and dissolved in 50.0 mL of N, N-dimethylformamide solution, and the temperature was raised to 30 ° C and stirred until the solid was dissolved, and then 10.02 mmol of pyromellitic dianhydride was added and continued to stir. The reaction time was 48 hours to obtain a polyamic acid polymer;

[0049] Step 2: Pour the polyamic acid liquid casting liquid obtained in step 1 onto a dry and clean glass plate to cast it into a film, then place the glass plate in an oven at 80°C for 18 hours, transfer it to a muffle furnace after the solvent is completely evaporated, and carry out imidization reaction at 100°C, 200°C, 300°C, and 400°C in sequence at a heating rate of 100°C / h. After the reaction is completed, immerse the film in deionized water at 60°C and peel it off to obtain a bisimidazole polyimide film; the thickness of the obtained bisimidazole polyimide film is 30μm; dry and store it.

[0050] The infrared spectrum of the bisimidazolyl polyimide film prepared in this example is shown in FIG. Figure 5 As shown. At 1778 and 1712cm -1 The peaks at 1355cm-1 belong to the asymmetric and symmetric C=O stretching vibration peaks. -1 The CN asymmetric stretching characteristic peak is found at 1660cm -1 The carbonyl peak of PAA did not appear at 1238 cm -1 With 920cm -1 The peak at can be attributed to the stretching vibration peaks of COC and OCN of the oxazole ring, which proves that the synthesis of bisimidazole polyimide is successful.

[0051] In the step 1, the synthesis method of the bisimidazole diamine monomer comprises the following steps:

[0052] Step Ⅰ: Weigh 30.00 g of phosphorus pentoxide and 180 g of polyphosphoric acid in portions and add them to the reaction vessel successively. Stir at 120 °C for 1.5 h. After the solution becomes clear, add 4.28 g of 3,3-diaminobiphenyl. Stir at 160 °C for 2 h, then add 5.48 g of p-aminobenzoic acid. After stirring for 2 h, raise the temperature to 180 °C and react for 18 h. After the reaction is completed, pour it into deionized water as a solvent, and brown precipitates will precipitate. Filter and collect the filter cake. Pour the filter cake into saturated sodium bicarbonate solution until no bubbles are generated and the pH value is slightly alkaline. After filtration, washing and drying, the crude product of bisimidazole diamine monomer is obtained.

[0053] Step II: Add 15.00 g of the crude bisimidazole diamine monomer and 75.0 mL of absolute ethanol to a three-necked flask successively, and stir at 60 °C for 1 h. After the reaction is completed, filter and collect the filtrate. Pour the filtrate into deionized water, and yellow precipitates will form. After washing and drying, the bisimidazole diamine monomer is obtained.

[0054] Example 3:

[0055] A preparation method of a bisimidazole polyimide film, the steps are as follows:

[0056] Step 1: Under the protection of nitrogen, add 9 mmol of 2-(4-aminophenyl)-5-aminobenzoxazole (BOA) and 1 mmol of bisimidazole diamine monomer (DBIA) dissolved in 41.0 mL of dimethyl sulfoxide solution to a 100 mL single-necked flask, and raise the temperature to 50 °C and stir until the solid substances dissolve. Then add 10 mmol of pyromellitic dianhydride (PDMA) and continue stirring. The reaction time is 48 h, and a polyamic acid polymer is obtained.

[0057] Step 2: Pour the polyamic acid casting solution prepared in Step 1 onto a dry and clean glass plate to form a film by casting. Then place the glass plate in an oven and dry it at 60 °C for 12 h. After the solvent has completely volatilized, transfer it to a muffle furnace and carry out imidization reactions at 100 °C, 200 °C, 300 °C, and 400 °C in sequence at a heating rate of 100 °C / h. After the reaction is completed, soak the film in deionized water at 50 °C and peel it off to obtain the bisimidazole polyimide film. The thickness of the obtained bisimidazole polyimide film is 25 μm, and it is stored after drying.

[0058] The thermal expansion coefficient diagram of the bisimidazole polyimide film prepared in this example is as Figure 6 . It can be seen that it shows good dimensional stability at 50 - 350 °C, and the thermal expansion coefficient is 5.0 ppm K -1 , indicating that the bisimidazole polyimide film prepared in this example has a low thermal expansion coefficient, making the application of this polyimide film in flexible displays feasible.

[0059] The thermogravimetric graph of the bis-imidazole polyimide film prepared in this example is as follows Figure 7 . At a heating rate of 10 °C / min -1 , in a nitrogen atmosphere, from room temperature to 800 °C, its 5% weight loss temperature (T d5％ ) is 550 °C, and its 10% weight loss temperature (T d10％ ) is 580 °C, with a char yield of 63.1%, indicating that the bis-imidazole polyimide film prepared in this example has excellent thermal stability.

[0060] The glass transition temperature graph of the bis-imidazole polyimide film prepared in this example is as follows Figure 8 . At a heating rate of 5 °C / min -1 , in a nitrogen atmosphere, from room temperature to 500 °C, its glass transition temperature is 452 °C, meaning that the material has strong rigidity, is less likely to deform, and has the potential to work at higher temperatures.

[0061] The mechanical property graph of the bis-imidazole polyimide film prepared in this example is as follows Figure 9 . As an important index for flexible display applications, the mechanical properties determine the performance and lifespan of the display. The tensile strength, elastic modulus, and elongation at break of the bis-imidazole polyimide film prepared in this example are 168 MPa, 5.2 GPa, and 5.0% respectively, and its mechanical properties can basically meet the requirements of flexible displays.

[0062] As shown in Figure 1 , in step one, the synthesis method of the bis-imidazole diamine monomer includes the following steps:

[0063] Step I: Weigh 20.00 g of phosphorus pentoxide and 160 g of polyphosphoric acid and add them to the reaction vessel in sequence; stir at 100 °C for 1 h, add 2.14 g of 3,3-diaminobiphenyl after the solution becomes clear; stir at 150 °C for 1 h, add 2.74 g of p-aminobenzoic acid; after stirring for 1 h, raise the temperature to 200 °C and react for 24 h; after the reaction is completed, pour it into deionized water as a solvent, and brown precipitates will precipitate; filter and collect the filter cake, pour the filter cake into saturated sodium bicarbonate solution until no bubbles are generated and the pH value is slightly alkaline, and after filtration, washing, and drying, the crude bis-imidazole diamine monomer is obtained;

[0064] Step II: Add 10.00 g of the crude bis-imidazole diamine monomer and 50.0 mL of absolute ethanol to a three-necked flask in sequence, and stir at 50 °C for 2 h; after the reaction is completed, filter and collect the filtrate, pour the filtrate into deionized water, and yellow precipitates will form, and after washing and drying, the bis-imidazole diamine monomer is obtained.

[0065] Example 4:

[0066] A preparation method of a bis-imidazole polyimide film, and its steps are as follows:

[0067] Step 1: Under the protection of nitrogen, add 6 mmol of 2-(4-aminophenyl)-5-aminobenzoxazole and 4 mmol of bisimidazole diamine monomer to a 100 mL single-necked flask, dissolve them in 60.0 mL of N,N-dimethylacetamide solution, heat to 40 °C and stir until the solid dissolves. Then add 9.99 mmol of pyromellitic dianhydride and continue stirring for 48 h to obtain a polyamic acid polymer.

[0068] Step 2: Pour the polyamic acid casting solution prepared in Step 1 onto a dry and clean glass plate to form a film by casting. Then place the glass plate in an oven and dry it at 80 °C for 12 h. After the solvent has completely evaporated, transfer it to a muffle furnace and carry out imidization reactions at 100 °C, 200 °C, 300 °C, and 400 °C in sequence at a heating rate of 100 °C / h. After the reaction is completed, soak the film in deionized water at 60 °C and peel it off to obtain a bisimidazole polyimide film; the thickness of the prepared bisimidazole polyimide film is 30 μm; store it after drying.

[0069] In the above Step 1, the synthesis method of the bisimidazole diamine monomer includes the following steps:

[0070] Step I: Weigh 20.00 g of phosphorus pentoxide and 200 g of polyphosphoric acid and add them to the reaction vessel in sequence; stir at 100 °C for 1 h. After the solution becomes clear, add 8.56 g of 3,3-diaminobiphenyl; stir at 160 °C for 1.5 h, add 10.96 g of p-aminobenzoic acid; after stirring for 1 h, raise the temperature to 180 °C and react for 18 h; after the reaction is completed, pour it into a deionized water solvent, and brown precipitates will precipitate; filter and collect the filter cake, pour the filter cake into a saturated sodium bicarbonate solution until no bubbles are generated and the pH value is slightly alkaline, and after filtering, washing, and drying, the crude bisimidazole diamine monomer is obtained.

[0071] Step II: Add 30.00 g of the crude bisimidazole diamine monomer and 150.0 mL of absolute ethanol to a three-necked flask in sequence, and stir at 60 °C for 1.5 h; after the reaction is completed, filter and collect the filtrate, pour the filtrate into deionized water, and yellow precipitates will form. After washing and drying, the bisimidazole diamine monomer is obtained.

[0072] Example 5:

[0073] A preparation method of a bisimidazole polyimide film, the steps of which are as follows:

[0074] Step 1: Under the protection of nitrogen, add 5 mmol of 2-(4-aminophenyl)-5-aminobenzoxazole and 5 mmol of bisimidazole diamine monomer to a 100 mL single-necked flask, which are dissolved in a mixed solution of 40.0 mL of dimethyl sulfoxide and N,N-dimethylacetamide. Then heat to 30 °C and stir until the solid dissolves. Subsequently, add 10.0 mmol of pyromellitic dianhydride and continue stirring for 24 h to obtain a polyamic acid polymer.

[0075] Step 2: Pour the polyamic acid casting solution prepared in Step 1 onto a dry and clean glass plate to form a film by casting. Then place the glass plate in an oven and dry it at 80 °C for 24 h. After the solvent has completely evaporated, transfer it to a muffle furnace and perform imidization reactions at 100 °C, 200 °C, 300 °C, and 400 °C in sequence with a heating rate of 100 °C / h. After the reaction is completed, soak the film in deionized water at 50 °C for peeling to obtain a bisimidazole polyimide film; the thickness of the prepared bisimidazole polyimide film is 35 μm; store it after drying.

[0076] In the above Step 1, the synthesis method of the bisimidazole diamine monomer includes the following steps:

[0077] Step I: Weigh 18.00 g of phosphorus pentoxide and 160 g of polyphosphoric acid and add them to the reaction vessel in sequence; stir at 130 °C for 1.5 h. After the solution becomes clear, add 3.21 g of 3,3-diaminobiphenyl; stir at 160 °C for 1 h, add 4.11 g of p-aminobenzoic acid; after stirring for 1 h, raise the temperature to 200 °C and react for 18 h; after the reaction is completed, pour it into a deionized water solvent, and brown precipitates will precipitate; filter and collect the filter cake, pour the filter cake into a saturated sodium bicarbonate solution until no bubbles are generated and the pH value is slightly alkaline, and after filtration, washing, and drying, the crude product of the bisimidazole diamine monomer is obtained.

[0078] Step II: Add 35.0 g of the crude product of the bisimidazole diamine monomer and 165.0 mL of absolute ethanol to a three-necked flask in sequence, and stir at 60 °C for 1.5 h; after the reaction is completed, filter and collect the filtrate, pour the filtrate into deionized water, and yellow precipitates will form. After washing and drying, the bisimidazole diamine monomer is obtained.

[0079] Example 6:

[0080] A preparation method of a bisimidazole polyimide film, the steps of which are as follows:

[0081] Step 1: Under the protection of nitrogen, add 5.5 mmol of 2-(4-aminophenyl)-5-aminobenzoxazole and 4.5 mmol of bisimidazole diamine monomer to 30.0 mL of a mixed solution of N,N-dimethylacetamide and N-methylpyrrolidone in a 100 mL single-necked flask. Heat to 50 °C and stir until the solid dissolves. Then add 10.01 mmol of pyromellitic dianhydride and continue stirring for 28 h to obtain a polyamic acid polymer.

[0082] Step 2: Pour the polyamic acid casting solution prepared in Step 1 onto a dry and clean glass plate to form a film by casting. Then place the glass plate in an oven at 65 °C and dry for 12 h. After the solvent has completely evaporated, transfer it to a muffle furnace and perform imidization reactions at 100 °C, 200 °C, 300 °C, and 400 °C in sequence at a heating rate of 100 °C / h. After the reaction is completed, soak the film in deionized water at 60 °C and peel it off to obtain a bisimidazole polyimide film; the thickness of the prepared bisimidazole polyimide film is 40 μm; store it after drying.

[0083] In the above Step 1, the synthesis method of the bisimidazole diamine monomer includes the following steps:

[0084] Step I: Weigh 20.00 g of phosphorus pentoxide and 200 g of polyphosphoric acid and add them to the reaction vessel in turn; stir at 100 °C for 2 h. After the solution becomes clear, add 5.35 g of 3,3-diaminobiphenyl; stir at 155 °C for 1.5 h, add 6.85 g of p-aminobenzoic acid; after stirring for 1 h, raise the temperature to 185 °C and react for 18 h; after the reaction is completed, pour it into a deionized water solvent, and brown precipitate will precipitate; filter and collect the filter cake, pour the filter cake into a saturated sodium bicarbonate solution until no bubbles are generated and the pH value is slightly alkaline, and after filtering, washing, and drying, the crude bisimidazole diamine monomer is obtained.

[0085] Step II: Add 15.00 g of the crude bisimidazole diamine monomer and 100.0 mL of absolute ethanol to a three-necked flask in turn, and stir at 50 °C for 5 h; after the reaction is completed, filter and collect the filtrate, pour the filtrate into deionized water, and yellow precipitate will form. After washing and drying, the bisimidazole diamine monomer is obtained.

[0086] Example 7:

[0087] A preparation method of a bisimidazole polyimide film, the steps of which are as follows:

[0088] Step 1: Under the protection of nitrogen, add 6.5 mmol of 2-(4-aminophenyl)-5-aminobenzoxazole and 3.5 mmol of bisimidazole diamine monomer to 40.0 mL of N-methylpyrrolidone solution in a 100 mL single-necked flask, heat to 30 °C and stir until the solid dissolves. Then add 10.0 mmol of pyromellitic dianhydride and continue stirring for 48 h to obtain a polyamic acid polymer;

[0089] Step 2: Pour the polyamic acid casting solution prepared in Step 1 onto a dry and clean glass plate to form a film by casting. Then place the glass plate in an oven and dry it at 65 °C for 18 h. After the solvent has completely evaporated, transfer it to a muffle furnace and carry out imidization reactions at 100 °C, 200 °C, 300 °C, and 400 °C in sequence at a heating rate of 100 °C / h. After the reaction is completed, soak the film in deionized water at 70 °C and peel it off to obtain a bisimidazole polyimide film; the thickness of the prepared bisimidazole polyimide film is 25 μm; store it after drying.

[0090] In the above Step 1, the synthesis method of the bisimidazole diamine monomer includes the following steps:

[0091] Step I: Weigh 19.00 g of phosphorus pentoxide and 185 g of polyphosphoric acid and add them to the reaction vessel in sequence; stir at 130 °C for 1 h. After the solution becomes clear, add 5.35 g of 3,3-diaminobiphenyl; stir at 160 °C for 1 h, add 6.00 g of p-aminobenzoic acid; after stirring for 1.5 h, heat to 190 °C and react for 24 h; after the reaction is completed, pour it into deionized water solvent, and brown precipitate will precipitate; filter and collect the filter cake, pour the filter cake into saturated sodium bicarbonate solution until no bubbles are generated and the pH value is slightly alkaline, and then filter, wash, and dry to obtain the crude bisimidazole diamine monomer;

[0092] Step II: Add 25.00 g of the crude bisimidazole diamine monomer and 150.0 mL of absolute ethanol to a three-necked flask in sequence, stir at 60 °C for 2 h; after the reaction is completed, filter and collect the filtrate, pour the filtrate into deionized water, and yellow precipitate will form. After washing and drying, the bisimidazole diamine monomer is obtained.

[0093] Example 8:

[0094] A preparation method of a bisimidazole polyimide film, the steps are as follows:

[0095] Step 1: Under the protection of nitrogen, add 8.5 mmol of 2-(4-aminophenyl)-5-aminobenzoxazole and 1.5 mmol of bisimidazole diamine monomer to 35.0 mL of dimethyl sulfoxide solution in a 100 mL single-necked flask, heat to 35 °C and stir until the solid dissolves. Then add 10.00 mmol of pyromellitic dianhydride and continue stirring for 24 h to obtain a polyamic acid polymer;

[0096] Step 2: Pour the polyamic acid solution casting solution prepared in Step 1 onto a dry and clean glass plate to form a film by casting. Then, place the glass plate in an oven and dry it at 60 °C for 16 h. After the solvent has completely evaporated, transfer it to a muffle furnace and perform imidization reactions at 100 °C, 200 °C, 300 °C, and 400 °C in sequence at a heating rate of 100 °C / h. After the reaction is completed, soak the film in deionized water at 70 °C for peeling, and then a bis-imidazole polyimide film can be obtained; the thickness of the prepared bis-imidazole polyimide film is 35 μm; store it dry.

[0097] In the said Step 1, the synthesis method of the bis-imidazole diamine monomer includes the following steps:

[0098] Step I: Weigh 25.00 g of phosphorus pentoxide and 150 g of polyphosphoric acid and add them to the reaction vessel in sequence; stir at 120 °C for 1 h. After the solution becomes clear, add 2.34 g of 3,3-diaminobiphenyl; stir at 150 °C for 2 h, and then add 2.99 g of p-aminobenzoic acid; after stirring for 1 h, raise the temperature to 185 °C and react for 16 h; after the reaction is completed, pour it into a deionized water solvent, and brown precipitates will precipitate; filter and collect the filter cake, pour the filter cake into a saturated sodium bicarbonate solution until no bubbles are generated and the pH value is slightly alkaline, and after filtration, washing, and drying, a crude bis-imidazole diamine monomer can be obtained;

[0099] Step II: Add 20.00 g of the crude bis-imidazole diamine monomer and 120.0 mL of absolute ethanol to a three-necked flask in sequence, and stir at 60 °C for 1 h; after the reaction is completed, filter and collect the filtrate, pour the filtrate into deionized water, and yellow precipitates will form. After washing and drying, the bis-imidazole diamine monomer can be obtained.

[0100] The equipment quantities and processing scales described here are used to simplify the description of the present invention. Applications, modifications, and variations of the present invention will be obvious to those skilled in the art.

[0101] Although the embodiments of the present invention have been disclosed as above, they are not limited to only the applications listed in the specification and embodiments. It can be fully applied to various fields suitable for the present invention. For those familiar with the field, additional modifications can be easily achieved. Therefore, without departing from the general concept defined by the claims and the equivalent scope, the present invention is not limited to specific details and the illustrated examples described herein.

Claims

1. A bisimidazolyl polyimide film, characterized in that: The chemical structural formula of the bisimidazole polyimide film is:

2. A method for preparing a bisimidazolyl polyimide film as claimed in claim 1, characterized in that: The following steps are involved: Step 1: using p-aminobenzoic acid, 3,3-diaminobiphenyl and phosphorus pentoxide as raw materials, and subjecting them to a high-temperature dehydration cyclization reaction in a polyphosphoric acid solvent to obtain a diimidazole-containing diamine monomer; Step 2: adding a diimidazole monomer, 2-(4-aminophenyl)-5-aminobenzoxazole and an organic solvent into a reactor and stirring to dissolve; after the solution is clarified, adding pyromellitic dianhydride and stirring to react; after the reaction is completed, a polyamic acid casting solution is obtained; Step 3: Cast the polyamic acid casting solution into a film, dry it at low temperature, and after the solvent evaporates, transfer it to a muffle furnace for imidization reaction to obtain a bisimidazole polyimide film.

3. The method for preparing a bisimidazolyl polyimide film according to claim 2, wherein: In the step 1, the synthesis method of the bisimidazole diamine monomer comprises the following steps: Step I, weigh 10 to 60 parts of phosphorus pentoxide and 160 to 250 parts of polyphosphoric acid by weight, and add them to a reaction container in sequence; stir at 100 to 150° C. for 1 to 2 hours, and add 2 to 14 parts by weight of 3,3-diaminobiphenyl after the solution is clarified; stir at 140 to 160° C. for 1 to 2 hours, and add 2 to 19 parts by weight of p-aminobenzoic acid; stir for 1 to 2 hours, and then heat to 160 to 200° C. and react for 12 to 24 hours; after the reaction is completed, pour into a deionized water solvent, and a brown precipitate will precipitate; filter and collect the filter cake, and pour the filter cake into a saturated sodium bicarbonate solution until no bubbles are generated and the pH value is slightly alkaline, and filter, wash, and dry to obtain a crude product of a diimidazole diamine monomer; Step II, add the crude product of diimidazole diamine monomer and anhydrous ethanol to the reaction container in sequence, and stir at 50-70° C. for 0.5-5 h; after the reaction is completed, filter and collect the filtrate, pour the filtrate into deionized water, and a yellow precipitate is generated. After washing and drying, the diimidazole diamine monomer is obtained.

4. The method for preparing a bisimidazolyl polyimide film according to claim 3, wherein: In the step II, the usage ratio of the crude product of the bisimidazolyl diamine monomer and anhydrous ethanol is 4-40 g:50-200 mL.

5. The method for preparing a bisimidazolyl polyimide film according to claim 2, wherein: In the step 2, the reaction includes: under the protection of nitrogen, dissolving 2-(4-aminophenyl)-5-aminobenzoxazole and diimidazole monomers in an organic solution, heating to 25-50° C. and stirring until the solid is dissolved, then adding pyromellitic anhydride and continuing to stir, and the reaction time is 24-48 hours.

6. The method for preparing a bisimidazolyl polyimide film according to claim 2, wherein: In the step 2, the molar ratio of 2-(4-aminophenyl)-5-aminobenzoxazole to the diimidazole monomer is 5 to 10.0:1.0 to 5.0; the ratio of the total molar amount of 2-(4-aminophenyl)-5-aminobenzoxazole and diimidazole monomers to the molar amount of pyromellitic dianhydride is 1.0:1.0±0.02; and the total mass ratio of the organic solution to the 2-(4-aminophenyl)-5-aminobenzoxazole and diimidazole monomers is 5 to 10.0:1.0 to 2.

0.

7. The method for preparing a bisimidazolyl polyimide film according to claim 2, wherein: In the step 3, the drying temperature of the polyamic acid casting solution is 60-80°C, and the drying time is 12-24h; the temperature change rate of the muffle furnace imidization is 50-100°C / h, the imidization temperature is 100-400°C, and the deionized water temperature is 50-80°C.

8. The method for preparing a bisimidazole polyimide film according to claim 2 or 5, characterized in that: The organic solvent is any one of N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone, or a mixture of two or more thereof.

9. The method for preparing a bisimidazolyl polyimide film according to claim 2, characterized in that: The thickness of the bisimidazolyl polyimide film is 20-50 μm.

10. A use of the bisimidazolyl polyimide film as claimed in claim 1, characterized in that: The bisimidazolyl polyimide film is applied to a flexible OLED substrate as a substrate material for a flexible display.

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