Microwave-assisted rapid synthesis process for reducing polydispersity of aromatic polyimide

Abstract

The invention belongs to the field of chemistry and chemical engineering, and provides a microwave-assisted rapid synthesis process for reducing polydispersity of aromatic polyimide. According to the process, the molecular weight is increased and the polydispersity coefficient is reduced by utilizing the characteristics of quick temperature rise, uniform heating and molecular responsiveness of microwave heating and the physicochemical characteristics of a high-melting-point and high-boiling-point acidic organic solvent through quick response adjustment of reaction temperature and staged programmed precise control; the aromatic polyimide has the two significant advantages that the total time consumption of the polymerization process is less than 5 hours, and is shortened by 50%-80% compared with the traditional process; the polydispersity coefficient is effectively reduced while the molecular weight is increased, for polyimide 6FDA-Dure, the weight-average molecular weight of a product synthesized through a traditional process is 130000 Daltons, the polydispersity coefficient is 1.70, and the weight-average molecular weight of a product synthesized through the process exceeds 180000 Daltons, and the polydispersity coefficient is smaller than 1.30.

Description

Technical Field

[0001] The present invention belongs to the fields of chemistry and chemical engineering, and relates to a microwave-assisted rapid synthesis process for fully utilizing the physical and chemical properties of high-melting-point and high-boiling-point acidic organic solvents to reduce the polydispersity of polyimide. Background Art

[0002] Polyimide refers to a polymer material containing an imide ring structure in the main chain of the molecule. It has excellent comprehensive properties and is widely used in the fields of aviation, aerospace, microelectronics, electrical, solar energy, nanometer, liquid crystal, separation membrane, laser, etc. According to the chemical structure of the repeating unit, polyimide can be divided into aliphatic, semi-aromatic and aromatic polyimides. Aromatic polyimide is usually obtained by polycondensation of aromatic diamine and aromatic dianhydride. It has extremely high heat resistance, low temperature resistance, mechanical properties and radiation resistance, as well as very low thermal expansion coefficient and dielectric constant, and is at the top of the engineering plastic pyramid.

[0003] In the synthesis process of aromatic polyimide, commonly used solvents include halogenated hydrocarbon solvents and aprotic polar solvents, which have good solubility for both monomers and polymerized products. Commonly used halogenated hydrocarbon solvents include reagents such as dichloromethane, chloroform, dichloroethane, etc., with relatively low boiling points. Its advantage is that deep solvent removal can be achieved by evaporation, which is beneficial to the subsequent processing and application of polyimide; its disadvantage is that the condensation reaction temperature is relatively low, and highly active catalysts must be added, and the monomers themselves must also have high activity. Commonly used aprotic polar solvents include reagents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, and sulfolane, with relatively high boiling points. Its advantage is that high-temperature reactions can be carried out, so the requirements for catalyst activity and monomer activity are relatively low, adapting to more synthesis systems. High temperature can also significantly enhance the migration ability of molecular segments in the solution environment, which is beneficial to the mutual condensation of oligomers, thereby obtaining higher polymer molecular weights and meeting the requirements of polymer materials for special scenarios; its disadvantage is that it is difficult to achieve deep removal of high-boiling-point solvents by evaporation, and non-solvent precipitation is usually required for the synthesized product to obtain purified polyimide products.

[0004] In addition to increasing the molecular weight and length of the chain segments, reducing the polydispersity coefficient of the chain segments and narrowing the molecular weight distribution range are also key factors in improving the comprehensive properties of aromatic polyimide materials [Broseta D, Fredrickson G H, Helfand E, Leibler L, Molecular weight and polydispersity effects at polymer-polymer interfaces, Macromolecules, 1990, 23, 132; Wu Haiyan, Cao Liulin, Wang Jing, Control of polymer molecular weight distribution by output feedback method, Journal of Chemical Industry and Engineering (China), 2012, 9, 2836]. For the polycondensation process of synthesizing polyimide, the key to reducing the polydispersity of the chain segments is to control the reaction uniformity and consistency at each stage of chain growth. In the initial stage of the reaction, adjacent dianhydride monomers and diamine monomers undergo condensation reactions to form a large number of oligomers with reactive end groups; in the middle stage of the reaction, the oligomers react with adjacent monomers or oligomers with different reactive groups (anhydride and amine are different reactive groups) to achieve chain extension; in the final stage of the reaction, adjacent polymer chain segments with different reactive end groups undergo condensation reactions to achieve multiplicative chain extension [Ren Guangyuan, Wang Chunyan, Huang Bin, Qian Yong, Synthesis and property characterization of several soluble polyimides, Polymer Materials Science & Engineering, 2011, 27, 26; Liaw D J, Wang K L, Huang Y C, Lee K R, Lai J Y, Ha C S, Advanced polyimide materials: Syntheses, physical properties and applications, Progress in Polymer Science, 2012, 37, 907]. Since the molecular weight of the oligomers is much higher than that of the dianhydride or diamine monomers, significant growth differences in the chain segment molecular weight are shown in the middle stage of the polymerization reaction, resulting in a broad molecular weight distribution range for the finally obtained molecular chain segments D M, de La Campa J G, de Abajo J, Lozano A E. Experimental and theoretical study of an improved activated polycondensation method for aromatic polyimides. Macromolecules, 2007, 40, 8225]. To reduce the polydispersity coefficient of the chain segment molecular weight distribution and obtain a more concentrated molecular weight distribution range, in the synthesis process of aromatic polyimides, it is necessary to improve the initial reaction uniformity and consistency, so that the monomers can reach a higher conversion rate as much as possible, thereby reducing the remaining monomers and their influence on the chain growth process in the middle stage of the polymerization reaction, and making the condensation reaction between oligomers mainly occur in the middle stage of the reaction.

[0005] Microwave-assisted synthesis is a new process intensification approach. Compared with traditional heating methods, microwaves directly provide energy to polar molecules in the reaction system through the electromagnetic field, without relying on the thermal conductivity and convection intensity of the heated medium, and has the advantages of faster heating speed and more uniform heating. In addition to increasing the temperature of the reaction system, the electromagnetic field can also directly change the energy state of polar functional groups, significantly improving the reaction activity. In summary, introducing microwave-assisted synthesis technology in the preparation process of aromatic polyimides can achieve rapid response regulation and programmed precise control of the reaction temperature, meeting the specific requirements of the system temperature at different polymerization stages. Through microwave-assisted synthesis technology, it is expected to balance the improvement of molecular weight and the reduction of the polydispersity coefficient, and then obtain aromatic polyimide materials with more excellent comprehensive properties. Summary of the Invention

[0006] The purpose of the present invention is to provide a microwave-assisted rapid synthesis process for aromatic polyimide materials, making full use of the characteristics of fast heating, uniform heating, and molecular responsiveness during the microwave heating process, as well as the physical and chemical properties of high-melting-point and high-boiling-point acidic organic solvents. Through rapid response regulation of the reaction temperature and staged programmed precise control, while increasing the molecular weight, the polydispersity coefficient is reduced, and aromatic polyimide materials with excellent comprehensive properties are obtained.

[0007] The technical solution of the present invention:

[0008] A microwave-assisted rapid synthesis process for reducing the polydispersity of aromatic polyimides, the steps are as follows:

[0009] (1) Preparation of polyimide synthesis reaction system: Add aromatic diamine into acidic organic solvent, and fully grind and mix in a dry inert gas atmosphere by a ball mill to ensure that the particle diameter is less than 10 μm; control the molar ratio of acidic organic solvent to aromatic diamine between 4.00 and 6.00; perform heat treatment on the fully ground aromatic diamine - acidic organic solvent mixture, control the heat treatment temperature at 5 - 10 °C above the melting temperature of the acidic organic solvent, maintain for 20 - 30 min after reaching the set temperature, and then slowly cool to ambient temperature; add aromatic dianhydride into the aromatic diamine - acidic organic solvent mixture after heat treatment, and fully grind and mix in a dry inert gas atmosphere by a ball mill to ensure that the particle diameter is less than 5 μm; control the molar ratio of aromatic dianhydride to aromatic diamine between 0.99 and 1.01; transfer the fully ground ternary mixture to a microwave reactor, and then inject dry inert gas to form a protective atmosphere;

[0010] (2) The first stage of polyimide synthesis reaction: Raise the temperature of the ternary mixture to 5 - 10 °C below the melting temperature of the acidic organic solvent by microwave - assisted heating, maintain for 10 - 15 min after reaching the set temperature; rapidly heat the ternary mixture to 140 - 180 °C within 2.0 min, and maintain for 3 - 5 min after reaching the set temperature; turn off the microwave - assisted heating, and rapidly cool the ternary mixture to 5 - 10 °C below the melting temperature of the acidic organic solvent by convective heat transfer, and maintain for 10 - 15 min after reaching the set temperature; repeat the above rapid heating and rapid cooling operation steps more than 5 times to ensure that the two monomers fully react to form oligomers;

[0011] (3) The second stage of polyimide synthesis reaction: On the basis of the first stage of polyimide synthesis reaction, raise the temperature to 180 - 210 °C, maintain the reaction time for more than 120 min after reaching the set temperature, and make the liquefied ternary mixture in a state of high - frequency vibration and disturbance through a pulse element; turn off the microwave - assisted heating and the pulse element, and cool the ternary mixture to ambient temperature;

[0012] (4) Separation and purification stage of polyimide material: The mixture obtained in the second stage of the polyimide synthesis reaction is soaked and washed batch by batch with anhydrous ethanol to recover the acidic organic solvent through selective dissolution. The mass of anhydrous ethanol used for each washing is not less than 2.5 times the total amount of acidic organic solvent used in the synthesis process, the soaking and washing time for each time is not less than 30 min, and the number of soaking and washing times is not less than 5 times; Aprotic polar solvent is used to prepare a polymer solution from the polymer remaining after ethanol washing. The concentration of the polymer solution is controlled within the range of 8.0 - 12.0 wt%, and then a stainless steel sieve with a pore size less than 50 μm is used to filter out the undissolved solid residues; The polymer solution is dispersed and added to methanol or ethanol to form particles or fibers with a diameter less than 500 μm through solvent exchange. Then, the obtained particles or fibers are soaked and washed batch by batch with methanol or ethanol. The mass of methanol or ethanol required for each washing is not less than 5.0 times the mass of the particles or fibers, the soaking time for each time is not less than 300 min, and the number of washing times is not less than 2 times; The washed particles or fibers are transferred to a vacuum oven to deeply remove methanol or ethanol, and a refined high-performance aromatic polyimide is obtained.

[0013] The acidic organic solvent mentioned specifically refers to aromatic organic acids with both high melting points and high boiling points. Its melting point is required to be higher than 105 °C, the boiling point is required to be higher than 245 °C, and at the same time, it has an appropriate acid strength, and the negative logarithm index value of the acid dissociation constant is between 3.00 and 4.50. The acidic organic solvent can be but is not limited to trifluoromethylbenzoic acid, dimethylbenzoic acid, ethylbenzoic acid, methylbenzoic acid, and benzoic acid.

[0014] The aprotic polar solvent is but is not limited to N-methylpyrrolidone, N,N-dimethylformamide, dimethylacetamide, dioxane, dimethyl sulfoxide, sulfolane, pyridine.

[0015] Advantages of the present invention: Through the characteristic matching of microwave-assisted heating and high melting point organic solvents, the present invention realizes the rapid response regulation of the polymerization reaction temperature and the precise control of the staged programming; Greatly shortens the synthesis time of polyimide, and the total time-consuming of the polymerization reaction process does not exceed 5 hours, saving 50% to 80% of the time compared with the traditional polymerization process; While increasing the molecular weight of aromatic polyimide, the polydispersity coefficient is reduced, breaking through the inherent balance effect in the traditional polycondensation reaction process. Taking the synthesis of polyimide material 6FDA-Durene as an example, the weight average molecular weight of the product prepared by the traditional heating method and the polymerization solution system is about 130,000 Daltons, and the corresponding polydispersity coefficient is about 1.70. The product prepared by the technical solution of the present invention has a weight average molecular weight exceeding 180,000 Daltons, and the corresponding polydispersity coefficient is less than 1.30. Description of the Drawings

[0016] Figure 1It is the microwave-assisted rapid synthesis process flow for reducing the polydispersity of aromatic polyimides described in the present invention. Specific Embodiments

[0017] The following further illustrates the specific embodiments of the present invention in conjunction with the accompanying drawings and technical solutions.

[0018] Example 1

[0019] In Example 1, trimethylbenzene diamine DAM and hexafluorodiacid dianhydride 6FDA were used as monomers, and benzoic acid BZA was used as a high-melting-point acidic organic solvent. An aromatic polyimide material 6FDA-DAM was prepared by a microwave-assisted rapid synthesis process. The specific synthesis steps are as follows:

[0020] (1) Accurately weigh the monomers and acidic organic solvents: 6FDA, DAM, and BZA were purified by sublimation crystallization with a purity exceeding 99.5 wt%. Weigh 100.00 grams of purified 6FDA, 33.82 grams of purified DAM, and 123.71 grams of purified BZA for standby;

[0021] (2) Prepare the polyimide synthesis reaction system: Add DAM to BZA and thoroughly grind and mix them in a dry nitrogen atmosphere using a ball mill to ensure that the particle diameter is less than 10 μm; Heat the ground DAM-BZA mixture. After the temperature reaches 130 °C, maintain it for 25 min, and then slowly cool it to room temperature; Add 6FDA to the heat-treated DAM-BZA mixture and thoroughly grind and mix them in a dry nitrogen atmosphere using a ball mill to ensure that the particle diameter is less than 5 μm; Transfer the thoroughly ground ternary mixture of 6FDA-DAM-BZA to a microwave-assisted synthesis reaction device and then inject dry nitrogen;

[0022] (3) The first stage of the polyimide synthesis reaction: Heat the ternary mixture to 115 °C by microwave. After reaching the set temperature, maintain it for 10 min, then increase the heating power and raise the temperature of the ternary mixture to 150 °C within 2.0 min. After reaching the set temperature, maintain it for 4 min; Turn off the microwave heating and quickly cool the ternary mixture to 115 °C by convective heat transfer. After reaching the set temperature, maintain it for 10 min; Repeat the above rapid heating and rapid cooling operation steps 5 times to ensure that the two monomers fully react to form oligomers;

[0023] (4) The second stage of the polyimide synthesis reaction: On the basis of the first stage of the synthesis reaction, further increase the microwave-assisted heating power and raise the temperature of the ternary mixture to 185 °C. After reaching the set temperature, maintain it for 120 min, and make the liquefied ternary mixture in a state of high-frequency vibration and perturbation through a pulse element; Turn off the microwave-assisted heating and the pulse element, and cool the ternary mixture to the ambient temperature;

[0024] (5) Separation and purification stage of polyimide material: The mixture obtained in the second stage of the synthesis reaction was soaked and washed batch by batch with absolute ethanol. Each time, 350 g of absolute ethanol was used for washing, and the soaking and washing time was 40 min each time, and the soaking and washing was carried out 5 times; N-methylpyrrolidone was used to prepare a solution of the polymer solid remaining after ethanol washing, and the polymer concentration was controlled at 8.0 wt%. Then, a 400-mesh stainless steel screen was used to filter out the solid residues; the polymer solution was dispersed and added to ethanol, and particles with a diameter less than 500 μm were formed through solvent exchange. Then, the particles obtained by solvent exchange were soaked and washed batch by batch with ethanol. Each time, 650 g of ethanol was used for washing, and the soaking time was 300 min each time, and the washing was carried out 2 times; the washed particles were transferred to a vacuum oven to deeply remove ethanol, and the refined high-performance aromatic polyimide material 6FDA-DAM was obtained.

[0025] The yield of the aromatic polyimide 6FDA-DAM synthesized in Example 1 was calculated and characterized by GPC test. The results showed that the polymer product was 110.70 g, the yield was 82.72%, the corresponding weight-average molecular weight was greater than 138,000 Daltons, and the polydispersity coefficient was less than 1.33.

[0026] Example 2

[0027] In Example 2, diamino diphenyl ether ODA and pyromellitic dianhydride PMDA were used as polyimide monomers, and trifluoromethyl benzoic acid TFMBZA was used as a high-melting-point acidic organic solvent. An aromatic polyimide material PMDA-ODA was prepared by a microwave-assisted rapid synthesis process. The specific synthesis steps are as follows:

[0028] (1) Accurately weigh the monomers and acidic organic solvents: PMDA, ODA, and TFMBZA were refined by sublimation crystallization, and the purity exceeded 99.5 wt%; 100.00 g of purified PMDA, 84.92 g of purified ODA, and 366.08 g of purified TFMBZA were weighed and reserved.

[0029] (2) Preparation of the polyimide synthesis reaction system: ODA was added to TFMBZA, and the mixture was thoroughly ground and mixed by a ball mill in a dry nitrogen atmosphere to ensure that the particle diameter was less than 10 μm; the ground ODA-TFMBZA mixture after heat treatment was heated, and after the temperature reached 120 °C, it was maintained for 30 min, and then slowly cooled to the ambient temperature; PMDA was added to the heat-treated ODA-TFMBZA mixture, and the mixture was thoroughly ground and mixed by a ball mill in a dry nitrogen atmosphere to ensure that the particle diameter was less than 5 μm; the thoroughly ground PMDA-ODA-TFMBZA ternary mixture was transferred to a microwave-assisted synthesis reaction device, and then dry nitrogen was injected.

[0030] (3) The first stage of the polyimide synthesis reaction: Heat the ternary mixture to 105 °C by microwave. After reaching the set temperature, maintain it for 12 min, then increase the heating power and raise the temperature of the ternary mixture to 160 °C within 2.0 min. After reaching the set temperature, maintain it for 3.5 min; turn off the microwave heating and quickly cool the ternary mixture to 105 °C by convective heat transfer. After reaching the set temperature, maintain it for 12 min; repeat the above steps of rapid heating and rapid cooling 6 times to ensure that the two monomers fully react to form oligomers;

[0031] (4) The second stage of the polyimide synthesis reaction: On the basis of the first stage of the synthesis reaction, further increase the microwave-assisted heating power, raise the temperature of the ternary mixture to 190 °C, maintain it for 150 min after reaching the set temperature, and make the liquefied ternary mixture in a state of high-frequency vibration and perturbation through a pulse element; turn off the microwave-assisted heating and the pulse element, and cool the ternary mixture to the ambient temperature;

[0032] (5) The separation and purification stage of the polyimide material: Soak and wash the mixture obtained in the second stage of the synthesis reaction with anhydrous ethanol in batches. Each time, 950 g of anhydrous ethanol is used for washing, and the soaking and washing time each time is 35 min. Soak and wash 5 times; Use N-methylpyrrolidone to prepare a solution of the polymer solid remaining after ethanol washing, control the polymer concentration to 10.0 wt%, and then use a 400-mesh stainless steel sieve to filter out the solid residues; Disperse the polymer solution into ethanol, form fibers with a diameter less than 500 μm through solvent exchange, and then soak and wash the fibers obtained by solvent exchange with ethanol in batches. Each time, 1500 g of ethanol is used for washing, and the soaking time each time is 300 min. Wash 2 times; Transfer the washed particles to a vacuum oven to deeply remove ethanol to obtain the refined high-performance aromatic polyimide material PMDA-ODA.

[0033] The yield of the aromatic polyimide PMDA-ODA synthesized in Example 2 was calculated and characterized by GPC test. The results showed that the polymer product was 161.37 g, the yield was 87.26%, the corresponding weight-average molecular weight was greater than 172,000 Daltons, and the polydispersity coefficient was less than 1.35.

[0034] Example 3

[0035] Example 3 uses tetramethyl-p-phenylenediamine (Durene) and hexafluorodiacid anhydride (6FDA) as polyimide monomers, and dimethylbenzoic acid (DMBZA) as a high-melting-point acidic organic solvent to prepare the aromatic polyimide material 6FDA-Durene through a microwave-assisted rapid synthesis process. The specific synthesis steps are as follows:

[0036] (1) Accurately weigh the monomers and acidic organic solvents: 6FDA, Durene, and DMBZA are refined by sublimation crystallization with a purity exceeding 99.5 wt%; weigh 100.00 g of purified 6FDA, 36.97 g of purified Durene, and 169.03 g of purified DMBZA for standby;

[0037] (2) Preparation of the polyimide synthesis reaction system: Add Durene to DMBZA and thoroughly grind and mix them in a dry nitrogen atmosphere using a ball mill to ensure that the particle diameter is less than 10 μm; heat the thoroughly ground Durene-DMBZA mixture. After the temperature reaches 150 °C, maintain it for 20 min, and then slowly cool it to the ambient temperature; add 6FDA to the heat-treated Durene-DMBZA mixture and thoroughly grind and mix them in a dry nitrogen atmosphere using a ball mill to ensure that the particle diameter is less than 5 μm; transfer the thoroughly ground ternary mixture of 6FDA-Durene-DMBZA to a microwave-assisted synthesis reaction device and then inject dry nitrogen;

[0038] (3) The first stage of the polyimide synthesis reaction: Heat the ternary mixture to 135 °C by microwave. After reaching the set temperature, maintain it for 15 min, and then increase the heating power. Raise the temperature of the ternary mixture to 165 °C within 2.0 min. After reaching the set temperature, maintain it for 3.5 min; turn off the microwave heating and quickly cool the ternary mixture to 135 °C by convective heat transfer. After reaching the set temperature, maintain it for 15 min; repeat the above steps of rapid heating and rapid cooling 6 times to ensure that the two monomers fully react to form oligomers;

[0039] (4) The second stage of the polyimide synthesis reaction: Based on the first stage of the synthesis reaction, further increase the microwave-assisted heating power to raise the temperature of the ternary mixture to 180 °C. After reaching the set temperature, maintain it for 210 min, and make the liquefied ternary mixture in a state of high-frequency vibration and disturbance through a pulse element; turn off the microwave-assisted heating and the pulse element, and cool the ternary mixture to the ambient temperature;

[0040] (5) Separation and purification stage of polyimide material: The mixture obtained in the second stage of the synthesis reaction was soaked and washed batch by batch with anhydrous ethanol. Each washing used 850 grams of anhydrous ethanol, and the soaking and washing time for each time was 35 minutes. The soaking and washing were carried out 5 times; N-methylpyrrolidone was used to prepare a solution of the polymer solid remaining after ethanol washing, and the polymer concentration was controlled at 8.0 wt%. Then, a 400-mesh stainless steel sieve was used to filter and remove the solid residue; the polymer solution was dispersed and added to ethanol, and particles with a diameter less than 500 μm were formed through solvent exchange. Then, the particles obtained by solvent exchange were soaked and washed batch by batch with ethanol. Each washing used 700 grams of ethanol, and the soaking time for each time was 300 minutes. The washing was carried out 2 times; the washed particles were transferred to a vacuum oven to deeply remove ethanol, and a refined high-performance aromatic polyimide material 6FDA-Durene was obtained.

[0041] The yield of the aromatic polyimide 6FDA-Durene synthesized in Example 3 was calculated and characterized by GPC test. The results showed that the polymer product was 114.68 grams, the yield was 83.72%, the corresponding weight-average molecular weight exceeded 180,000 Daltons, and the polydispersity coefficient was less than 1.30.

[0042] Example 4

[0043] In Example 4, tetramethyl-p-phenylenediamine Durene and hexafluorodiacid anhydride 6FDA were used as polyimide monomers, and N-methylpyrrolidone (NMP), a commonly used solvent in the classical synthesis of polyimide, was used as the solvent. An aromatic polyimide material 6FDA-Durene was prepared by a traditional thermal catalytic synthesis process. The specific synthesis steps are as follows:

[0044] (1) Accurately weigh the monomers and acidic organic solvents: 6FDA and Durene were refined by sublimation crystallization, and the purity exceeded 99.5 wt%; NMP was deeply dehydrated by molecular sieves; 100.00 grams of purified 6FDA, 36.97 grams of purified Durene, and 750 milliliters of purified NMP were weighed and reserved.

[0045] (2) Preparation of the polyimide synthesis reaction system: The diamine monomer Durene and the high-boiling solvent NMP were added to the polymerization reaction kettle and stirred and dissolved in a dry nitrogen atmosphere at room temperature. Subsequently, the dianhydride monomer 6FDA was slowly added and stirred and dissolved at room temperature; the prepared polymerization solution system was placed in an oil bath and heated to 80 °C, and at the same time, 90 milliliters of isoquinoline was added dropwise. After the temperature reached 80 °C, it was maintained for 1 hour.

[0046] (3) The first stage of the polyimide synthesis reaction: The polymerization solution system was heated to 120 °C through a high-temperature oil bath. After reaching the set temperature, it was maintained for 2 hours to allow the dianhydride monomer and the diamine monomer to fully react to form polyimide acid.

[0047] (4) The second stage of the polyimide synthesis reaction: On the basis of the first stage of the synthesis reaction, further increase the oil bath heating temperature, heat the polymerization solution system to 180 °C, maintain it for 6 hours after reaching the set temperature, and cause the polyamic acid formed in the first stage of the synthesis reaction to undergo a dehydration cyclization reaction to be converted into polyimide;

[0048] (5) The separation and purification stage of the polyimide material: The mixed solution obtained in the second stage of the synthesis reaction is cooled to room temperature and then slowly dropped into 5000 g of anhydrous ethanol while mechanically stirring the anhydrous ethanol, and the polyimide in the mixed solution precipitates to form polymer particles with a diameter of less than 500 μm; The polymer solid particles are formulated into a solution using NMP, the polymer concentration is controlled at 8.0 wt%, and then a 400-mesh stainless steel sieve is used to filter out the solid residues; The polymer solution after filtration and impurity removal is dispersed and added to ethanol, and particles with a diameter of less than 500 μm are formed through solvent exchange, and then the polymer particles are soaked and washed with ethanol, 5000 g of ethanol is used each time, soaked for 300 min, and washed 2 times; The washed particles are transferred to a vacuum oven to deeply remove ethanol, and the refined high-performance aromatic polyimide material 6FDA-Durene is obtained.

[0049] The yield of the aromatic polyimide 6FDA-Durene synthesized in Example 4 was calculated and characterized by GPC testing. The results showed that the polymer product was 107.53 g, the yield was 78.50%, the corresponding weight-average molecular weight was approximately 130,000 Daltons, and the polydispersity coefficient was approximately 1.70.

Claims

1. A microwave-assisted rapid synthesis process for reducing the polydispersity of aromatic polyimides, characterized in that, The steps are as follows: (1) Preparation of the polyimide synthesis reaction system: Add aromatic diamine into an acidic organic solvent, grind it in a dry inert gas atmosphere until thoroughly mixed, ensuring that the particle diameter is less than 10 μm; conduct heat treatment on the thoroughly ground aromatic diamine - acidic organic solvent mixture, control the heat treatment temperature at 5 - 10 °C above the melting temperature of the acidic organic solvent, maintain for 20 - 30 min after reaching the set temperature, and then slowly cool to the ambient temperature; add aromatic dianhydride into the aromatic diamine - acidic organic solvent mixture after heat treatment, grind it in a dry inert gas atmosphere until thoroughly mixed, ensuring that the particle diameter is less than 5 μm; transfer the thoroughly ground ternary mixture to a microwave reactor, and then inject dry inert gas to form a protective atmosphere; (2) The first stage of the polyimide synthesis reaction: Raise the temperature of the ternary mixture to 5 - 10 °C below the melting temperature of the acidic organic solvent by microwave - assisted heating, maintain for 10 - 15 min after reaching the set temperature; rapidly heat the ternary mixture to 140 - 180 °C within 2.0 min, maintain for 3 - 5 min after reaching the set temperature; turn off the microwave - assisted heating, and rapidly cool the ternary mixture to 5 - 10 °C below the melting temperature of the acidic organic solvent by convective heat transfer, maintain for 10 - 15 min after reaching the set temperature; (3) The second stage of the polyimide synthesis reaction: Based on the first stage of the polyimide synthesis reaction, raise the temperature to 180 - 210 °C, maintain the reaction time for more than 120 min after reaching the set temperature, and make the liquefied ternary mixture in a state of high - frequency vibration and disturbance through a pulse element; turn off the microwave - assisted heating and the pulse element, and cool the ternary mixture to the ambient temperature; (4) Separation and purification stage of the polyimide material: Soak and wash the mixture obtained in the second stage of the polyimide synthesis reaction with anhydrous ethanol multiple times, and recover the acidic organic solvent through selective dissolution; use an aprotic polar solvent to prepare a polymer solution from the residue of the ethanol - washed polymer, control the concentration of the polymer solution within the range of 8.0 - 12.0 wt%, and then use a stainless - steel sieve with a pore diameter less than 50 μm to filter out the undissolved solid residues; disperse the polymer solution into methanol or ethanol, form particles or fibers with a diameter less than 500 μm through solvent exchange, and then soak and wash the obtained particles or fibers with methanol or ethanol multiple times; transfer the washed particles or fibers to a vacuum oven to deeply remove methanol or ethanol to obtain refined high - performance aromatic polyimide.

2. The microwave - assisted rapid synthesis process for reducing the polydispersity of aromatic polyimide according to claim 1, wherein, In step (1), the molar ratio of the acidic organic solvent to the aromatic diamine is controlled between 4.00 and 6.00; the molar ratio of the aromatic dianhydride to the aromatic diamine is controlled between 0.99 and 1.

01.

3. The microwave - assisted rapid synthesis process for reducing the polydispersity of aromatic polyimide according to claim 1, wherein, In step (2), Repeat the above rapid heating and rapid cooling operation steps more than 5 times to ensure that the two monomers fully react to form oligomers.

4. The microwave-assisted rapid synthesis process for reducing the polydispersity of aromatic polyimide according to claim 1, wherein in step (4), clean the mixture obtained in the second stage of the polyimide synthesis reaction. Each time of cleaning uses anhydrous ethanol with a mass not less than 2.5 times the total mass of the acidic organic solvent used in the synthesis process. Each soaking and cleaning time is not less than 30 min, and the number of soaking and cleaning times is not less than 5 times.

5. The microwave-assisted rapid synthesis process for reducing the polydispersity of aromatic polyimide according to claim 1, wherein in step (4), clean the obtained particles or fibers. Each time of cleaning requires methanol or ethanol with a mass not less than 5.0 times the mass of the particles or fibers. Each soaking time is not less than 300 min, and the number of cleaning times is not less than 2 times.

6. The microwave-assisted rapid synthesis process for reducing the polydispersity of aromatic polyimide according to claim 1, wherein the acidic organic solvent is an aromatic organic acid with both high melting point and high boiling point. Its melting point is required to be higher than 105 °C, and its boiling point is required to be higher than 245 °C. At the same time, it has an appropriate acidic strength, and the negative logarithm index value of the acid dissociation constant is between 3.00 and 4.

50.

7. The microwave-assisted rapid synthesis process for reducing the polydispersity of aromatic polyimide according to claim 1, wherein the acidic organic solvent is trifluoromethylbenzoic acid, dimethylbenzoic acid, ethylbenzoic acid, methylbenzoic acid, benzoic acid.

8. The microwave-assisted rapid synthesis process for reducing the polydispersity of aromatic polyimide according to claim 1, wherein the aprotic polar solvent is N-methylpyrrolidone, N,N-dimethylformamide, dimethylacetamide, dioxane, dimethyl sulfoxide, sulfolane, pyridine.

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