Process for the preparation of polyamide acid and its use in the filtration of particulate matter at high temperatures

Abstract

The application relates to the technical field of high polymer materials and filtering materials, in particular to a preparation method of polyamide acid and application of the polyamide acid in particulate matter filtration under a high-temperature environment. The method comprises the following steps: dissolving 4,4'-diaminodiphenyl methane (MDA) in an organic solvent with a water content lower than 100 ppm under an inert atmosphere, adding dried aromatic dianhydride in batches under low-temperature conditions to carry out a polymerization reaction, and obtaining a polyamide acid solution; wherein the molar ratio of the MDA to the aromatic dianhydride is 1:(1-1.02). Through control of the water content of a polymerization system, the molar ratio of the MDA to the aromatic dianhydride and the low-temperature batch feeding process of the dianhydride, high molecular weight polyamide acid suitable for electrostatic spinning is obtained, and a high-temperature resistant fiber filtering membrane is further prepared, which still maintains high filtering efficiency and low airflow resistance after high-temperature treatment.

Description

Technical Field

[0001] This invention relates to the field of polymer materials and filter materials, and in particular to a method for preparing polyamic acid and its application in particulate matter filtration under high temperature conditions. Background Technology

[0002] With the increasing demands for air filtration and personal protective equipment in high-temperature industrial production, fire rescue, and other special operational scenarios, particulate matter filtration materials suitable for high-temperature environments are receiving growing attention. For example, smoke from forest fires typically contains a large number of fine particulate matter, some of which, with smaller diameters, can enter the human respiratory system through respiration, posing a threat to human health. Therefore, achieving efficient filtration of fine particulate matter in high-temperature environments is of great significance.

[0003] Electrostatic electret filter materials can capture particulate matter using electrostatic forces in addition to mechanical interception, thus achieving high particulate filtration efficiency while maintaining low airflow resistance. However, the charges in electret materials are prone to migration and decay at high temperatures, reducing the material's electrostatic capture capacity and consequently decreasing filtration performance. Therefore, filter materials used in high-temperature environments not only need good thermal stability but also need to maintain good particulate filtration performance after exposure to high temperatures.

[0004] Polyimide is a class of polymeric materials with good heat resistance and chemical stability. When prepared into nanofiber membranes, it can form a porous network composed of fine fibers, thus possessing the potential for use as a high-temperature particulate filtration material. Currently, polyimide nanofiber membranes typically use polyamic acid as a precursor, forming polyamic acid nanofiber membranes through electrospinning, followed by thermal imidization treatment to obtain polyimide nanofiber membranes. Further electret treatment can enhance its particulate filtration performance.

[0005] In the above preparation process, the polymerization state of the polyamic acid precursor directly affects the subsequent electrospinning process. The polyamic acid solution used for electrospinning needs to have suitable molecular weight, viscosity, and flow state to ensure that the spinning process can proceed stably and form continuous nanofibers. If the degree of polymerization of the obtained polyamic acid is insufficient or the solution state is not suitable for spinning, it will be difficult to further obtain a structurally stable polyimide nanofiber membrane.

[0006] Therefore, how to stably obtain polyamic acid precursors suitable for electrospinning and further prepare polyimide nanofiber filter materials that can maintain good particulate matter filtration performance after being subjected to high temperature environments remains a technical problem that needs to be further solved in this field. Summary of the Invention

[0007] This invention covers the following technical solutions: One aspect of the present invention relates to a method for preparing polyamic acid, comprising: dissolving 4,4'-diaminodiphenylmethane (MDA) in an organic solvent with a water content of less than 100 ppm under an inert atmosphere, and adding dried aromatic dianhydride in batches at low temperature to carry out a polymerization reaction to obtain a polyamic acid solution; wherein the molar ratio of MDA to aromatic dianhydride is 1:(1 to 1.02).

[0008] In some embodiments, the aromatic dianhydride is selected from at least one of 4,4'-diphenyl ether dianhydride (ODPA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), and pyromellitic dianhydride (PMDA).

[0009] In some embodiments, the aromatic dianhydride is added to the organic solution of the MDA in batches, wherein, as the aromatic dianhydride is added and the polymerization reaction proceeds, the temperature of the polymerization reaction system is increased in the range of 3 ℃ to 8 ℃, and / or the feeding rate of the aromatic dianhydride is decreased.

[0010] In some embodiments, the aromatic dianhydride is added in five batches, wherein: the addition temperature of the first and second batches is 3 ℃~4 ℃, the addition temperature of the third batch is 4 ℃~5 ℃, the addition temperature of the fourth batch is 5 ℃~6 ℃, and the addition temperature of the fifth batch is 7 ℃~8 ℃; the addition times of the first to fourth batches are 0.4 h~0.6 h, 0.6 h~0.9 h, 0.8 h~1.2 h, and 1.2 h~1.8 h respectively, and the fifth batch of aromatic dianhydride is added in multiple small batches.

[0011] In some embodiments, the organic solvent is N,N-dimethylformamide (DMF).

[0012] In some embodiments, the aromatic dianhydride is dried under vacuum conditions above 160 °C for 20 h to 28 h.

[0013] In some embodiments, the preparation method further includes adding a desiccant to the obtained polyamic acid solution and storing it in an inert atmosphere at -10 °C to -5 °C.

[0014] Another aspect of the present invention relates to polyamic acid prepared by the above-described preparation method.

[0015] Another aspect of the present invention relates to a fiber filter membrane, the main component of which is the aforementioned polyamic acid.

[0016] In some embodiments, the fiber filter membrane is obtained by electrospinning and thermal imidization of the polyamic acid described above; in some embodiments, the polyimide nanofiber membrane is further subjected to negative corona discharge treatment.

[0017] Another aspect of the present invention relates to the application of the above-mentioned fiber filter membrane in particulate matter filtration under high temperature conditions.

[0018] In some embodiments, the temperature of the high-temperature environment is 100 ℃ to 275 ℃, preferably 150 ℃ to 270 ℃, and more preferably 200 ℃ to 270 ℃.

[0019] This invention utilizes 4,4'-diaminodiphenylmethane as a diamine monomer, polymerizing it with a dried aromatic dianhydride in a low-water-content organic solvent under an inert atmosphere. The diamine and dianhydride are controlled to be approximately equimolar, and a low-temperature batch feeding method is employed to carry out the polymerization reaction, resulting in polyamic acid with a high molecular weight and suitable solution state. Furthermore, by adjusting the batch feeding process of the aromatic dianhydride, the temperature of the reaction system and the dianhydride feeding rate are adapted to the polymerization process, which is beneficial for obtaining a polyamic acid solution suitable for electrospinning. The resulting polyamic acid, after electrospinning and thermal imidization, can form a polyimide nanofiber membrane, which can be further treated with negative corona discharge to obtain a fiber filter membrane with particulate matter filtration capabilities.

[0020] This invention has at least one of the following beneficial effects: (1) By controlling the water content of the organic solvent used in the polymerization reaction and drying the aromatic dianhydride, the present invention can reduce the influence of water in the polymerization system on the polymerization reaction. At the same time, controlling the molar ratio of MDA to aromatic dianhydride in a range close to equimolar can facilitate the full growth of polymer molecular chains, thereby obtaining polyamic acid with a higher molecular weight.

[0021] (2) The present invention adopts a low-temperature batch addition of aromatic dianhydride polymerization method, and can increase the reaction system temperature and / or decrease the addition rate of aromatic dianhydride as the aromatic dianhydride is added and the polymerization reaction proceeds, so that the addition process of aromatic dianhydride is adapted to the polymerization reaction process, which is beneficial to control the local reactant concentration and instantaneous reaction rate, and maintain the stable progress of the polymerization reaction under the condition that the viscosity of the system gradually increases, thereby obtaining a polyamic acid solution with suitable molecular weight, viscosity and flow state.

[0022] (3) The polyamic acid obtained by the present invention has good electrospinning processing performance, which can further form continuous and uniform polyamic acid nanofibers, and form polyimide nanofiber membranes through thermal imidization, thereby realizing the transformation from the polymerization control of polyamic acid precursor to polyimide nanofiber filter material.

[0023] (4) The fiber filter membrane prepared by the polyamic acid of the present invention has good high temperature filtration performance. After being treated in a high temperature environment, it can still maintain a high particulate matter filtration efficiency and a low breathing resistance, and is therefore suitable for particulate matter filtration in a high temperature environment. Attached Figure Description

[0024] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0025] Figure 1 Fourier transform infrared spectrum of the MDA-ODPA system.

[0026] Figure 2 Solid-state ¹³C NMR spectrum of MDA-ODPA polyimide.

[0027] Figure 3 SEM image of MDA-ODPA polyimide nanofiber membrane.

[0028] Figure 4 Fourier transform infrared spectrum of the MDA-BPDA system.

[0029] Figure 5 Solid-state ¹³C NMR spectrum of MDA-BPDA polyimide.

[0030] Figure 6 SEM image of MDA-BPDA polyimide nanofiber membrane.

[0031] Figure 7 Fourier transform infrared spectrum of the MDA-PMDA system.

[0032] Figure 8 Solid-state ¹³C NMR spectrum of MDA-PMDA polyimide.

[0033] Figure 9 SEM image of MDA-PMDA polyimide nanofiber membrane. Detailed Implementation

[0034] The embodiments of the present invention will now be described in detail. The following examples are only used to explain the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. After reading this specification, those skilled in the art can make various modifications, substitutions or variations to the technical solutions of the present invention without departing from the concept and technical principles of the present invention, and all such modifications, substitutions or variations should fall within the scope of protection of the present invention.

[0035] Unless otherwise stated, the technical and scientific terms used herein shall have the meanings commonly understood by one of ordinary skill in the art. Unless otherwise specified, the experimental methods described herein shall be performed under standard experimental conditions, in accordance with relevant laboratory manuals, or as recommended by the manufacturers of reagents or instruments, or using other equivalent methods known in the art.

[0036] The terms "comprising," "including," and "containing" as used herein are open-ended expressions, and unless explicitly specified, they do not exclude the presence of other components, steps, or structures not explicitly listed. The terms "preferred," "optional," and "optional" as used herein are only for illustrating some embodiments and do not constitute a limitation on the scope of protection of this invention.

[0037] Unless otherwise specified, the numerical ranges mentioned in this article include any integers, fractions, decimals, and their upper and lower limits within that range.

[0038] Unless otherwise specified, the singular expressions such as "a," "a kind," and "the" used in this article also include the plural forms; "multiple" usually refers to two or more.

[0039] As used in this document, the term "and / or" refers to any one or any combination of the relevant objects.

[0040] Where there is no contradiction, the technical features of the various embodiments and examples in this specification can be combined with each other. Any conventional adjustments, substitutions, or equivalent modifications made by those skilled in the art based on the content of this specification should be considered part of the disclosure herein.

[0041] The references cited in this invention are only used to illustrate the background technology or related technical content of this invention, and their disclosures may be used for reference only if they do not conflict with the technical solutions of this invention.

[0042] In this invention, "polyamic acid (PAA)" refers to a polymer whose molecular chain contains ammonium acid structural units, formed by the polymerization reaction of diamine monomers and dianhydride monomers. The polyamic acid can undergo dehydration cyclization reactions through heating or other methods to form the corresponding polyimide. In this invention, the polyamic acid is formed by the polymerization of 4,4'-diaminodiphenylmethane (MDA) and aromatic dianhydride, and serves as a precursor for subsequent electrospinning and the preparation of polyimide nanofiber membranes.

[0043] In this invention, "aromatic dianhydride" refers to a compound that contains two anhydride groups in its molecular structure and has an aromatic structure, which can react with the amino group of MDA to form polyamic acid.

[0044] In this invention, "dried aromatic dianhydride" refers to aromatic dianhydride that has undergone a process to reduce its moisture content before participating in the polymerization reaction. The drying process can be carried out using methods such as vacuum drying, which can reduce the moisture content of the aromatic dianhydride without causing significant adverse effects.

[0045] In this invention, "organic solvent" refers to an organic liquid medium that can dissolve MDA and is suitable for the polymerization reaction of MDA with aromatic dianhydrides.

[0046] In this invention, the "water content" of an organic solvent refers to the amount of water in the organic solvent. In some embodiments, the water content can be determined using the Karl Fischer method or other methods suitable for determining trace amounts of water in organic solvents.

[0047] In this invention, "inert atmosphere" refers to a gaseous environment that does not substantially participate in the polyamic acid polymerization reaction under the corresponding process conditions and can reduce direct contact between the reaction system and the external environment. The inert atmosphere can be formed from nitrogen, argon, or other gases that do not substantially react with the reaction system under the corresponding conditions.

[0048] In this invention, "low-temperature conditions" refers to maintaining the reaction system at a temperature below normal room temperature during the addition of aromatic dianhydride and the polymerization reaction. In some embodiments, the low-temperature conditions are 3°C to 8°C. During the batch addition of aromatic dianhydride, it is not required that each batch be added or reacted at the same temperature; the temperature of the reaction system can vary within the stated temperature range as the aromatic dianhydride is added and the polymerization reaction proceeds.

[0049] In this invention, "batch addition" refers to adding the aromatic dianhydride used in the polymerization reaction into the organic solution of MDA in two or more batches, rather than adding it all at once. In different batches, the amount of aromatic dianhydride added, the feeding time, and the feeding rate can be the same or different, and the batches can be carried out continuously or at intervals.

[0050] In this invention, "reducing the feeding rate of aromatic dianhydride" means that, as the aromatic dianhydride is added and the polymerization reaction proceeds, the amount of aromatic dianhydride added per unit time in at least the later stage is lower than the amount added per unit time in at least the earlier stage. The feeding rate can be adjusted by extending the feeding time of a certain amount of aromatic dianhydride, reducing the amount added per unit time, further dividing the corresponding batch into multiple small batches, or a combination thereof.

[0051] In this invention, "desiccant" refers to a material that is added to a polyamic acid solution and can adsorb or bind moisture in the system.

[0052] In this invention, "polyimide nanofiber membrane" refers to a fiber membrane composed of polyimide fibers, wherein at least a portion of the fibers have diameters at the nanoscale.

[0053] In this invention, "thermal imidization" refers to the process of dehydrating and cyclizing the ammonium acid structure in polyamic acid to form an imide ring structure by heating. The thermal imidization can be carried out at a single temperature or in a stepped heat treatment process using two or more temperature stages.

[0054] In this invention, "negative corona discharge treatment" refers to a process in which a polyimide nanofiber membrane acquires and / or traps charges using negative corona discharge. After negative corona discharge treatment, the polyimide nanofiber membrane can enhance the capture of aerosol particles through electrostatic interactions.

[0055] In this invention, "particulate filtration" refers to the use of the fiber filter membrane to intercept or capture solid and / or liquid particles suspended in a gas.

[0056] In this invention, "high temperature environment" refers to an environment with a temperature higher than normal temperature that imposes heat resistance requirements on the structural stability and / or filtration performance of the filter material.

[0057] Preparation of polyamic acid This invention provides a method for preparing polyamic acid, comprising: dissolving 4,4'-diaminodiphenylmethane (MDA) in an organic solvent with a water content of less than 100 ppm under an inert atmosphere, and adding dried aromatic dianhydride in batches at low temperature to carry out a polymerization reaction to obtain a polyamic acid solution; wherein the molar ratio of MDA to aromatic dianhydride is 1:(1 to 1.02).

[0058] This invention uses MDA as a diamine monomer to polymerize with aromatic dianhydride to form polyamic acid. During the polymerization process, the moisture content of the reactants and solvent is controlled to be below 100 ppm, and dried aromatic dianhydride is used. Simultaneously, the molar ratio of MDA to aromatic dianhydride is controlled to be close to equimolar, and the aromatic dianhydride is added in batches under low-temperature conditions. These combined conditions promote the full growth of the polymer molecular chains and yield a polyamic acid solution with suitable molecular weight, viscosity, and flow properties.

[0059] In some embodiments, the molar ratio of the MDA to the aromatic dianhydride is 1:(1 to 1.02), for example, it can be 1:1, 1:1.002, 1:1.005, 1:1.008, 1:1.01, 1:1.012, 1:1.015, 1:1.018 or 1:1.02, as well as a range formed by any of the above values ​​as endpoints.

[0060] Maintaining the molar ratio of MDA to aromatic dianhydride within a near equimolar range allows for a suitable stoichiometric relationship between the amino groups of the diamine monomer and the anhydride groups of the dianhydride monomer, which is beneficial for the continuous growth of the molecular chain during stepwise polymerization. The results of the examples and comparative examples show that when the molar ratio of MDA to dianhydride is changed from 1:(1–1.02) to 1:(0.8–0.9), the resulting PAA has a lower molecular weight and is difficult to further prepare into the desired fibrous membrane.

[0061] Aromatic dianhydrides The aromatic dianhydride described in this invention can undergo a polymerization reaction with MDA to form the corresponding polyamic acid.

[0062] In some embodiments, the aromatic dianhydride is selected from at least one of 4,4'-diphenyl ether dianhydride (ODPA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), and pyromellitic dianhydride (PMDA).

[0063] In some embodiments, the aromatic dianhydride is ODPA, thereby forming MDA-ODPA polyamic acid; in other embodiments, the aromatic dianhydride is BPDA, thereby forming MDA-BPDA polyamic acid; and in still other embodiments, the aromatic dianhydride is PMDA, thereby forming MDA-PMDA polyamic acid.

[0064] The aromatic dianhydride is dried before participating in the polymerization reaction. This drying process reduces the amount of moisture carried into the polymerization system by the aromatic dianhydride.

[0065] In some embodiments, the aromatic dianhydride is dried under vacuum conditions. In some embodiments, the drying temperature is 160 °C or higher, for example, 160 °C, 165 °C, 170 °C, 175 °C, or 180 °C or higher. In some embodiments, the drying time is 20 h to 28 h, for example, 20 h, 21 h, 22 h, 23 h, 24 h, 25 h, 26 h, 27 h, or 28 h, and a range formed by any of the above values ​​as endpoints.

[0066] In some specific embodiments, aromatic dianhydrides are placed in a brown container and dried under vacuum conditions of approximately 0.1 MPa and above 160°C for approximately 24 hours, and then stored in a vacuum-drying environment after drying. The above method was used to pretreat ODPA, BPDA, and PMDA in the examples.

[0067] Organic solvent and moisture control This invention uses an organic solvent that can dissolve MDA and is suitable for the polymerization reaction of MDA with aromatic dianhydrides.

[0068] In some embodiments, the organic solvent is N,N-dimethylformamide (DMF).

[0069] The organic solvent has a water content of less than 100 ppm. For example, the water content of the organic solvent may be less than 100 ppm, 90 ppm, 80 ppm, 70 ppm, 60 ppm or 50 ppm.

[0070] In some embodiments, a desiccant can be used to remove water from the organic solvent. The desiccant can be a molecular sieve, and in some specific embodiments, it is an activated 4A molecular sieve.

[0071] In some specific embodiments, the 4A molecular sieve is pre-dried and activated before being added to an organic solvent for dehydration. The 4A molecular sieve can be first dried under vacuum to remove water, and then activated at high temperature under an inert atmosphere. In the examples, the 4A molecular sieve was first vacuum dried at approximately 100 °C, then activated at above 400 °C for 5–6 hours under nitrogen protection, and subsequently added to DMF and sealed for deep dehydration.

[0072] By simultaneously controlling the moisture in the organic solvent and the moisture carried by the aromatic dianhydride, the adverse effects of moisture on the polymerization system can be reduced.

[0073] Low-temperature batch polymerization This invention employs a low-temperature, batch feeding method to polymerize MDA and aromatic dianhydrides.

[0074] In some embodiments, MDA is first added to an organic solvent under an inert atmosphere and fully dissolved to form a homogeneous MDA organic solution, and then aromatic dianhydride is added in batches to the MDA organic solution under stirring conditions.

[0075] The inert atmosphere can be a nitrogen atmosphere, an argon atmosphere, or an atmosphere formed by other gases that do not substantially participate in the polymerization reaction. In some specific embodiments, the inert atmosphere is a nitrogen atmosphere.

[0076] In some embodiments, the aromatic dianhydride is added in batches at a low temperature of 3°C to 8°C. The temperature of the polymerization reaction system is not required to remain constant during the addition of the aromatic dianhydride, but can vary as the aromatic dianhydride is added and the polymerization reaction proceeds.

[0077] In some embodiments, as the aromatic dianhydride is added and the polymerization reaction proceeds, the temperature of the polymerization reaction system is increased in the range of 3 °C to 8 °C, and / or the feeding rate of the aromatic dianhydride is decreased.

[0078] It should be noted that the "increasing the temperature of the polymerization reaction system" mentioned in this article does not require that the temperature of every two adjacent feeding stages be increased. For example, two consecutive initial feeding stages can use the same or substantially the same reaction temperature, while one or more subsequent feeding stages can use a reaction temperature higher than that of the initial feeding stages.

[0079] Similarly, the “reduction of the feeding rate of aromatic dianhydride” described herein does not require a fixed reduction between every two adjacent feeding stages. Instead, it can be achieved by extending the feeding time of the corresponding batch, reducing the amount added per unit time, further dividing the corresponding batch into multiple small batches for addition, or a combination thereof, so that the feeding rate of at least one subsequent feeding stage is lower than that of at least one earlier feeding stage, depending on the progress of the polymerization reaction.

[0080] Without being limited by specific theories, in a preferred embodiment, during the early stages of polymerization, the viscosity of the reaction system is relatively low, allowing for rapid contact and reaction between MDA and the newly added aromatic dianhydride. As the polymerization continues, the polymer molecular chains gradually lengthen, the system viscosity increases accordingly, and the mixing and diffusion states of the reactants within the system change. Therefore, adjusting the reaction temperature and the feeding rate of the aromatic dianhydride according to the polymerization process is beneficial for adapting the addition of the aromatic dianhydride to the mixing, diffusion, and polymerization processes within the system.

[0081] Meanwhile, using a batch addition method can avoid adding all the aromatic dianhydrides to the reaction system at once in a short period of time. By controlling the amount of aromatic dianhydrides entering the reaction system at each stage and the rate of addition, the possibility of high dianhydride concentrations in local areas and excessively rapid instantaneous reactions can be reduced, allowing the reaction system to gradually approach the final set molar ratio of diamine to dianhydride.

[0082] In some embodiments, the aromatic dianhydride is added to the organic solution of MDA in five batches.

[0083] The feeding temperature for the first and second batches of aromatic dianhydrides can be 3℃ to 4℃, for example, 3℃, 3.2℃, 3.5℃, 3.8℃ or 4℃; the feeding temperature for the third batch of aromatic dianhydrides can be 4℃ to 5℃, for example, 4℃, 4.2℃, 4.5℃, 4.8℃ or 5℃; the feeding temperature for the fourth batch of aromatic dianhydrides can be 5℃ to 6℃, for example, 5℃, 5.2℃, 5.5℃, 5.8℃ or 6℃; the feeding temperature for the fifth batch of aromatic dianhydrides can be 7℃ to 8℃, for example, 7℃, 7.2℃, 7.5℃, 7.8℃ or 8℃.

[0084] In some embodiments, the feeding time for the first batch of aromatic dianhydride is 0.4 h to 0.6 h, for example, 0.4 h, 0.45 h, 0.5 h, 0.55 h, or 0.6 h; the feeding time for the second batch of aromatic dianhydride is 0.6 h to 0.9 h, for example, 0.6 h, 0.7 h, 0.75 h, 0.8 h, or 0.9 h; the feeding time for the third batch of aromatic dianhydride is 0.8 h to 1.2 h, for example, 0.8 h, 0.9 h, 1.0 h, 1.1 h, or 1.2 h; and the feeding time for the fourth batch of aromatic dianhydride is 1.2 h to 1.8 h, for example, 1.2 h, 1.3 h, 1.4 h, 1.5 h, 1.6 h, 1.7 h, or 1.8 h.

[0085] The fifth batch of aromatic dianhydride can be added in multiple small batches. The amount added in a single batch of the fifth batch of aromatic dianhydride can be less than the amount added in at least one of the first four batches, and the next small batch of aromatic dianhydride can be added after the aromatic dianhydride added in the previous batch has been fully dispersed and / or reacted for a certain period of time.

[0086] In some specific embodiments, the first and second batches were added at approximately 3 °C, the third batch at approximately 4 °C, the fourth batch at approximately 5 °C, and the fifth batch at approximately 8 °C; the feeding times for the first to fourth batches were approximately 0.5 h, 0.75 h, 1 h, and 1.5 h, respectively, and the fifth batch was added in multiple small batches. Examples 1 to 3 all employed the above polymerization procedure.

[0087] In some embodiments, after each batch of aromatic dianhydride is added, stirring can continue for a certain period of time before adding the next batch. In some specific embodiments, stirring continues for about 0.5 hours after the first batch is added, about 0.75 hours after the second batch is added, about 0.75 hours after the third batch is added, and about 1 hour after the fourth batch is added. After all the aromatic dianhydrides have been added, stirring can continue to carry out the polymerization reaction.

[0088] In some specific embodiments, after all the aromatic dianhydrides have been added, stirring is continued for about 6 hours to allow the polymerization reaction to proceed further, resulting in a high-viscosity, homogeneous, and fluid PAA solution.

[0089] In some embodiments, the feeding and solvent usage are controlled to achieve a final solids content of 15 wt.% to 20 wt.% in the resulting polyamic acid solution. In some specific embodiments, a final solids content of approximately 20 wt.% is targeted. All existing embodiments aim for a final solids content of 20 wt.% during polymerization.

[0090] Storage of polyamic acid Once the polyamic acid is prepared, it can be used directly for subsequent electrospinning, or it can be stored at low temperature before electrospinning.

[0091] In some embodiments, a desiccant is added to the resulting polyamic acid solution, and the solution is sealed and stored at low temperature under an inert atmosphere.

[0092] The desiccant can be a material capable of adsorbing or binding moisture in the polyamic acid solution without significantly hindering subsequent use. In some embodiments, the desiccant is a molecular sieve. In some specific embodiments, the desiccant is an activated 4A molecular sieve.

[0093] In some embodiments, the polyamic acid solution is stored in a sealed container at -10°C to -5°C, for example, at -10°C, -9°C, -8°C, -7°C, -6°C, or -5°C, or within a temperature range defined by any of the above values ​​as endpoints.

[0094] In some specific embodiments, activated 4A molecular sieves are added to the obtained PAA solution, nitrogen gas is introduced and the solution is sealed, and then stored at -10 ℃ to -5 ℃.

[0095] Fiber filtration membrane The present invention also provides a fiber filter membrane prepared further from the above-mentioned polyamic acid.

[0096] In some embodiments, the polyamic acid solution is electrospun to first form a polyamic acid nanofiber membrane; the obtained polyamic acid nanofiber membrane is dried and further subjected to thermal imidization treatment to cause the polyamic acid to undergo dehydration and cyclization, forming a polyimide nanofiber membrane.

[0097] In some embodiments, a polyamic acid precursor solution is loaded into a syringe, polyamic acid nanofibers are formed by electrospinning, and a roller receiver is used to collect the nanofiber membrane.

[0098] The electrospinning process conditions can be adjusted according to the viscosity, solid content, and target fiber morphology of the polyamic acid solution.

[0099] In some embodiments, the electrospinning voltage is 20 kV to 25 kV, for example, 20 kV, 21 kV, 22 kV, 23 kV, 24 kV or 25 kV; the propulsion speed of the polyamic acid solution can be about 1 mL / h; and the distance from the nozzle to the receiver can be about 18 cm.

[0100] In some embodiments, a roller receiver is used to collect nanofibers, and the roller rotation speed can be 70 rpm to 90 rpm, for example, 70 rpm, 75 rpm, 80 rpm, 85 rpm or 90 rpm.

[0101] In some embodiments, the electrospinning environment temperature is 26 ℃ to 30 ℃, for example, 26 ℃, 27 ℃, 28 ℃, 29 ℃ or 30 ℃; the relative humidity of the environment is less than 40%.

[0102] In some specific embodiments, the PAA precursor solution is loaded into a 5 mL syringe, and electrospinning is performed with a spinning voltage of 20 kV to 25 kV, a feed rate of 1 mL / h, a distance of 18 cm from the nozzle to the roller receiver, a roller rotation speed of 70 rpm to 90 rpm, a reciprocating speed of 1000 rpm, a roller and reciprocating width of 30 cm, a spinning time of about 1 h, an ambient temperature of 26 ℃ to 30 ℃ and a relative humidity of less than 40%, to obtain a PAA nanofiber membrane.

[0103] The resulting polyamic acid nanofiber membrane can be further dried to reduce the content of residual organic solvents. In some specific embodiments, the PAA nanofiber membrane is first dried at about 60°C for 8 h under a vacuum of about 0.1 MPa, and then dried at about 80°C for more than 4 h to remove residual DMF.

[0104] In some embodiments, the dried polyamic acid nanofiber membrane is thermally imidized to convert the polyamic acid into polyimide.

[0105] Thermal imidization can be performed using a single-stage heat treatment or a stepped heat treatment with two or more temperature stages. In some embodiments, thermal imidization is performed using a stepped heating method, causing the polyamic acid to gradually undergo dehydration and cyclization.

[0106] In some specific embodiments, the PAA nanofiber membrane is subjected to stepwise thermal imidization under nitrogen protection: the first stage is held at about 100 °C for 60 min; the second stage is held at about 200 °C for 60 min; the third stage is held at about 280 °C for 30 min; the temperature is increased at a rate of about 5 °C / min between different stages, and finally the PI nanofiber membrane is obtained.

[0107] Through the above-mentioned thermal imidization treatment, while maintaining the basic fiber network structure of the nanofiber membrane, the amyl acid structure in PAA can undergo dehydration and cyclization to form an imide structure.

[0108] In some embodiments, the polyimide nanofiber membrane obtained by thermal imidization is subjected to negative corona discharge treatment.

[0109] Negative corona discharge treatment can enable polyimide nanofiber membranes to acquire and / or capture charges, thereby allowing the resulting fiber membranes to not only mechanically capture particulate matter through the fiber network, but also enhance the capture of aerosol particles through electrostatic interactions.

[0110] The voltage, electric field strength, treatment distance, and treatment time of negative corona discharge treatment can be adjusted according to the thickness, dielectric properties, and breakdown strength of the polyimide nanofiber membrane.

[0111] In some specific embodiments, the PI nanofiber membrane is subjected to negative corona discharge treatment for about 3 minutes, the distance between the needle tip and the fiber membrane is about 30 mm, and the electric field strength is lower than the breakdown strength of the fiber membrane.

[0112] Applications of high-temperature particulate matter filtration The present invention also provides the application of the above-mentioned fiber filter membrane in particulate matter filtration under high temperature environment.

[0113] The particulate matter can be solid particles and / or liquid particles suspended in a gas. In some embodiments, the particulate matter is aerosol particles. In some embodiments, the particulate matter is inorganic particulate matter. In some specific embodiments, the particulate matter is inorganic salt aerosol particles, such as NaCl aerosol particles.

[0114] In some embodiments, the temperature of the high-temperature environment is 100 ℃ to 275 ℃, for example, it can be 100 ℃, 120 ℃, 150 ℃, 180 ℃, 200 ℃, 220 ℃, 240 ℃, 250 ℃, 260 ℃, 270 ℃ or 275 ℃, as well as a range formed by any of the above values ​​as endpoints.

[0115] In some embodiments, the temperature of the high-temperature environment is 150 ℃ to 270 ℃; in other embodiments, it is 200 ℃ to 270 ℃.

[0116] The high-temperature environment can be the environment in which the fiber filter membrane is located during particulate matter filtration, or it can be the high-temperature environment that the fiber filter membrane experiences before particulate matter filtration. In the latter case, the filter membrane can still be used for particulate matter filtration after experiencing the high-temperature environment.

[0117] In some specific embodiments, the fiber filter membrane was incubated at 270 °C for 1 h, and then its particulate filtration performance and breathing resistance were tested. Existing experiments used NaCl aerosol with a particle size of approximately 300 nm as the test particles, with a test flow rate of 32 L / min and a corresponding surface velocity of 5.33 cm / s; the expiratory and inspiratory resistances of the fiber membrane before and after treatment at 270 °C were also tested.

[0118] Therefore, the polyamic acid described in this invention can be further processed to form a continuous polyimide nanofiber network, and a fiber filter membrane for particulate matter filtration can be obtained through electret treatment. The MDA-ODPA, MDA-BPDA, and MDA-PMDA systems obtained in the examples can all form a continuous, randomly distributed fiber structure, with the prepared PAA having a high molecular weight; the further formed PI nanofiber filter membrane can still maintain good filtration performance and low airflow resistance after being treated at 270 °C.

[0119] The embodiments of the present invention will be described in detail below with reference to the examples. It should be understood that these embodiments are only used to illustrate the technical content of the present invention and are not intended to limit the scope of protection of the present invention. Unless otherwise specified, the specific experimental conditions in the following embodiments are given priority reference to the guidelines provided in this specification, or may be carried out according to generally accepted experimental manuals or conventional experimental conditions, or other experimental methods known in the art, or according to the conditions recommended by the relevant reagent or instrument manufacturers. In specific embodiments, unless otherwise specified, minor deviations within the weighing accuracy range are allowed for the measurement parameters involving raw material components; reasonable deviations due to instrument detection accuracy or operational accuracy are also allowed for parameters such as temperature and time.

[0120] Raw materials and general preparation conditions used in the examples In the following examples, 4,4'-diaminodiphenylmethane is abbreviated as MDA, 4,4'-biphenyl ether dianhydride is abbreviated as ODPA, 3,3',4,4'-biphenyltetracarboxylic dianhydride is abbreviated as BPDA, pyromellitic dianhydride is abbreviated as PMDA, polyamic acid is abbreviated as PAA, and polyimide is abbreviated as PI.

[0121] Before use, the dianhydride monomer is placed in a brown bottle and dried in a vacuum oven at -0.1 MPa and above 160 °C for 24 h, then stored in a vacuum desiccator. The washed 4A molecular sieve is first dried in a vacuum oven at 100 °C to remove moisture, and then activated in a muffle furnace under nitrogen protection at above 400 °C for 5–6 h; when the molecular sieve temperature drops to about 150 °C, it is quickly added to ultra-dry DMF, sealed and stored for one week before use.

[0122] Unless otherwise specified, the polymerization conditions in all three examples were as follows: A dry four-necked flask was purged under a nitrogen atmosphere and placed in a cold bath. MDA and 100 mL of DMF were added, and the mixture was stirred for 0.5 h while maintaining the solution temperature at 3 °C to obtain a homogeneous diamine solution. Subsequently, the corresponding dianhydride was added in five batches. The first batch was added slowly over 0.5 h at 3 °C, followed by stirring for another 0.5 h; the second batch was added slowly over 0.75 h at 3 °C, followed by stirring for another 0.75 h; the third batch was added slowly over 1 h at 4 °C, followed by stirring for another 0.75 h; the fourth batch was added slowly over 1.5 h at 5 °C, followed by stirring for another 1 h; and the fifth batch was added in multiple small batches at 8 °C. After all the additions were completed, the reaction was stirred for another 6 h to obtain a high-viscosity, homogeneous PAA solution. The system aims for a final solid content of 20 wt.%, with a molar ratio of MDA to dianhydride of 1:(1–1.02) and a solvent water content of less than 100 ppm.

[0123] The obtained PAA solution was added with activated 4A molecular sieve, sealed under a nitrogen atmosphere, and stored frozen at -10 to -5 °C.

[0124] During electrospinning, the PAA precursor solution was loaded into a 5 mL syringe. The spinning voltage was 20 kV–25 kV, the feed rate was 1 mL / h, the distance from the nozzle to the roller receiver was 18 cm, the roller speed was 70–90 rpm, the reciprocating speed was 1000 mm / min, the roller and reciprocating width were both 30 cm, the spinning time was approximately 1 h, the ambient temperature was 26–30 ℃, and the relative humidity was below 40%, resulting in a PAA nanofiber membrane. The obtained fiber membrane was dried in a vacuum oven at -0.1 MPa at 60 ℃ for 8 h, and then dried at 80 ℃ for at least 4 h to remove residual DMF.

[0125] The dried PAA nanofiber membrane was subjected to stepwise thermal imidization under nitrogen protection: 100 °C for 60 min, then increased to 200 °C for 60 min, and finally increased to 280 °C for 30 min, with a heating rate of 5 °C / min, to obtain the PI nanofiber membrane. Subsequently, the PI nanofiber membrane was subjected to negative corona discharge treatment for 3 min, with the distance between the needle tip and the fiber membrane being 30 mm, and the electric field strength controlled below the fiber membrane breakdown strength.

[0126] The specific feed amounts for each embodiment are shown in Table 1.

[0127] Table 1 Monomer Feed Amounts for Each Embodiment

[0128] Example 1: Preparation of MDA-ODPA polyimide nanofiber membrane Polymerization was carried out using MDA as the diamine monomer and ODPA as the dianhydride monomer under the general preparation conditions described above. 9.13 g of MDA was dissolved in 100 mL of DMF, followed by the sequential addition of 2.18 g, 2.90 g, 3.64 g, 5.10 g, and 0.750 g of ODPA. After polymerization, the mixture was treated under the aforementioned electrospinning, drying, thermal imidization, and negative corona discharge conditions to obtain the MDA-ODPA polyimide nanofiber membrane.

[0129] The resulting PAA fiber membrane was pure white, while the PI fiber membrane after thermal imidization was white and insoluble in DMF and DMAc. FTIR results are as follows: Figure 1 As shown: MDA at 2885 cm - A -CH2 stretching vibration peak appears at position ¹, at 3437 cm⁻¹. - ¹ and 3335 cm - A -NH2 stretching vibration peak appears at ¹; ODPA peaks are observed at 1840 cm⁻¹.- ¹ and 1777 cm - The asymmetric and symmetric stretching vibration peaks of the acid anhydride C=O appear at ¹, respectively, and at 1271 cm⁻¹ - ¹ and 1244 cm - A weak shoulder peak for the diphenyl ether structure appears at position ¹. PAA is at 1715 cm⁻¹. - ¹ and 1649cm - A characteristic C=O peak appears at ¹, and at 3044 cm⁻¹ - ¹、3266 cm - ¹, 1600 cm - ¹ and 2920 cm - Characteristic peaks for OH stretching vibration, NH stretching vibration, NH plane bending vibration, and -CH2 stretching vibration appear at ¹, respectively; the characteristic peaks for acid anhydrides and diamines disappear, indicating that PAA has formed. PI at 1777 cm⁻¹ - ¹, 1715 cm - ¹, 1364 cm - ¹ and 725 cm - The presence of a characteristic peak for the imide structure at position ¹ indicates that PAA completes cyclization via a stepwise heating process.

[0130] GPC analysis results showed that the number-average molecular weight (Mn) of this PAA was 16870 g / mol, the weight-average molecular weight (Mw) was 31852 g / mol, and the molecular weight distribution index (Mw / Mn) was 1.89. The solid-state ¹³C NMR spectrum of MDA-ODPA polyimide is shown below. Figure 2 As shown, the chemical shifts of the imine carbonyl carbon are 164.4, the chemical shifts of the substituted aromatic carbons are 155.8 and 139.5, the chemical shifts of the protonated aromatic carbons are 131.7 and 126.5, and the chemical shift of the methylene carbon is 37.9. SEM images are shown below. Figure 3 As shown, the fibers are distributed in a continuous, random pattern.

[0131] Example 2 Preparation of MDA-BPDA polyimide nanofiber membrane Polymerization was carried out using MDA as the diamine monomer and BPDA as the dianhydride monomer under the general preparation conditions described above. 9.429 g of MDA was dissolved in 100 mL of DMF, followed by the sequential addition of 2.14 g, 2.85 g, 3.57 g, 4.99 g, and 0.721 g of BPDA. After polymerization, the mixture was treated under the aforementioned electrospinning, drying, thermal imidization, and negative corona discharge conditions to obtain the MDA-BPDA polyimide nanofiber membrane.

[0132] The resulting PAA fiber membrane was pure white, while the PI fiber membrane after thermal imidization was pale yellow and insoluble in DMF and DMAc. FTIR results are as follows: Figure 4 As shown: BPDA at 1840 cm - ¹ and 1777 cm - At position ¹, asymmetric and symmetric stretching vibration peaks of the acid anhydride C=O appear, respectively, at 1418 cm⁻¹. - ¹A C=C stretching vibration peak of the biphenyl group appears near 675 cm⁻¹. - An out-of-plane bending vibration peak of the biphenyl skeleton appears at position ¹. The remaining major characteristic peaks of PAA and PI are consistent with the polymerization and cyclization characteristics described in Example 1, indicating that PAA has been successfully formed and further thermal imidization has been completed.

[0133] GPC analysis results showed that the Mn of this PAA was 21026 g / mol, the Mw was 38383 g / mol, and the Mw / Mn ratio was 1.83. The solid-state ¹³C NMR spectrum of MDA-BPDA polyimide is shown below. Figure 5 As shown, the chemical shift of the imine carbonyl carbon is 164.4, the chemical shift of the substituted aromatic carbon is 141.1, the chemical shifts of the protonated aromatic carbon are 128.7 and 122.3, and the chemical shift of the methylene carbon is 38.6. SEM images are shown below. Figure 6 As shown, the fibers are distributed in a continuous, random pattern.

[0134] Example 3 Preparation of MDA-PMDA polyimide nanofiber membrane Polymerization was carried out using MDA as the diamine monomer and PMDA as the dianhydride monomer under the general preparation conditions described above. 11.168 g of MDA was dissolved in 100 mL of DMF, followed by the sequential addition of 1.87 g, 2.50 g, 3.13 g, 4.38 g, and 0.652 g of PMDA. After polymerization, the mixture was treated under the aforementioned electrospinning, drying, thermal imidization, and negative corona discharge conditions to obtain an MDA-PMDA polyimide nanofiber membrane.

[0135] The resulting PAA fiber membrane was pure white, while the PI fiber membrane after thermal imidization was pale yellow and insoluble in DMF and DMAc. FTIR results are as follows: Figure 7 As shown: PMDA at 1840 cm - ¹ and 1777 cm - At position ¹, peaks representing asymmetric and symmetric stretching vibrations of the anhydride C=O were observed, respectively. The remaining major characteristic peaks of PAA and PI were consistent with the polymerization and cyclization characteristics described in Example 1, indicating that PAA had been successfully formed and further thermal imidization had been completed.

[0136] GPC analysis results showed that the Mn of this PAA was 24125 g / mol, the Mw was 46149 g / mol, and the Mw / Mn ratio was 1.91. The solid-state ¹³C NMR spectrum of MDA-PMDA polyimide is shown below. Figure 8 As shown, the chemical shift of the imine carbonyl carbon is 163.1, the chemical shift of the substituted aromatic carbon is 134.5, the chemical shifts of the protonated aromatic carbon are 126.9 and 116.6, and the chemical shift of the methylene carbon is 37.4. SEM images are shown below. Figure 9 As shown, the fibers are distributed in a continuous, random pattern.

[0137] The GPC detection results of the three PAAs are summarized in Table 2, and the solid-state ¹³C NMR assignment results of the three PIs are summarized in Table 3.

[0138] Table 2. GPC test results of three PAA types

[0139] Table 3. Solid-state ¹³C NMR chemical shift assignments for three types of PI

[0140] Comparative Example 1: Substitution of Non-Dianhydride Acid Monomers Using the same operating conditions as in the examples, the dianhydride monomers in the examples were replaced with 1,2,3,4-benzenetetracarboxylic acid, 1,2,3,5-benzenetetracarboxylic acid, 4-methylbenzoic acid, 4-ethylbenzoic acid, 4-chlorobenzoic acid, or 2,3,4,5-tetrafluorobenzoic acid, respectively, while maintaining the molar ratio of MDA to the replaced acid monomer at 1:(1 to 1.02). As a result, high molecular weight PAA suitable for subsequent electrospinning was not obtained.

[0141] Comparative Example 2: Replacement of Diamine Monomer Using the same operating conditions as in the examples, with ODPA, BPDA, or PMDA as the dianhydride monomers, MDA was replaced with 4,4'-methylenebis(2-chloroaniline), 2,2-bis(4-aminophenyl)hexafluoropropane, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, 3,5-diaminotrifluorotoluene, or 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, respectively, while maintaining the molar ratio of diamine to dianhydride at 1:(1–1.02). As a result, high molecular weight PAA suitable for subsequent electrospinning was not obtained.

[0142] Comparative Example 3: No water removal and rapid one-time feeding Without performing the dehydration treatment of the raw materials and solvents described in the examples, conventional DMF with a water content exceeding 2% was used, and all dianhydrides were added at once over approximately 10 minutes. The reaction temperature during the addition process was not controlled, and the remaining operations were the same as in the examples. As a result, a high molecular weight PAA suitable for subsequent electrospinning was not obtained.

[0143] Comparative Example 4: Non-Isomolal Combinations The molar ratio of MDA to dianhydride was adjusted from 1:(1-1.02) in the example to 1:(0.8-0.9), and the remaining operations were the same as in the example. The resulting PAA had a low molecular weight, which could not meet the requirements for subsequent electrospinning and therefore could not be used to prepare subsequent fiber membranes.

[0144] Experimental Example 1: Test of expiratory and inspiratory resistance The breathing resistance was tested using a BRT-300 respirator resistance tester. The PI fiber membranes obtained in Examples 1-3 were cut into N95 mask models, with four samples prepared for each type of fiber membrane. The sample membranes were airtightly mounted on matching test head molds using sealing tape, ensuring that the fixing method did not affect the effective ventilation area of ​​the filter element or deform the mask. The ventilation rate was adjusted to (85±1) L / min, and the maximum expiratory and inspiratory resistances were measured and recorded. The samples were then incubated at 270 ℃ for 1 h and tested using the same method. The results are shown in Table 4.

[0145] Table 4. Expiratory and inspiratory resistance of three types of PI fiber membranes

[0146] As shown in Table 4, after being kept at 270 ℃ for 1 h, the exhalation resistance and inhalation resistance of the three types of PI fiber membranes were both lower than the 210 Pa limit of KN95 masks in GB2626—2019 "Respiratory Protective Self-priming Filtering Particulate Respirators".

[0147] Experimental Example 2: Filtration Performance Test Before and After High Temperature Treatment The filtration performance of the PI fiber membranes obtained in Examples 1-3 was tested using a CSI-506A automatic filter. NaCl aerosol particles with a diameter of approximately 300 nm were used as the test particles, with a flow rate of 32 L / min and a corresponding surface velocity of 5.33 cm / s. First, the negatively corona-treated fiber membranes were tested at 26 °C. Then, the corona-treated fiber membranes were placed on aluminum foil and heated in a muffle furnace at 270 °C for 1 h. The filtration efficiency and resistance were then measured under the same conditions, and the quality factor was calculated. The results are shown in Table 5.

[0148] Table 5. Filtration performance of three types of PI fiber membranes before and after high-temperature treatment.

[0149] As a control group, a commercially available Kapton precursor polymer was used to prepare a polyamic acid fiber membrane under conditions where the equipment parameters, including electrospinning, muffle furnace heat treatment, corona polarization, and automatic filtration, were completely consistent with those in the examples. This membrane was then converted to a polyimide fiber membrane via high-temperature thermal cyclization. Subsequently, its PM0.3 filtration efficiency and filtration resistance were evaluated through negative corona discharge treatment, and the corresponding quality factor was calculated. The results showed that the fiber membrane without high-temperature treatment had a quality factor of 0.061 Pa at room temperature. - ¹, After high-temperature treatment at 270 ℃, the quality factor is 0.045 Pa. - ¹. The above results show that MDA-ODPA and MDA-BPDA fiber membranes can still maintain high filtration efficiency and quality factor after high-temperature treatment, while the filtration efficiency of MDA-PMDA fiber membrane remains above 99% after high-temperature treatment.

[0150] Test Example 3: Flame Retardant Performance Test The TTech-GBT5455A textile vertical burning tester was used for testing. The samples were fixed with sample clamps, and after being clamped on both sides, they were placed in a vertical burning test chamber. Industrial propane gas was used as the ignition source, and ignition lasted for 12 seconds. After ignition, the afterflame time, smoldering time, and damage length were measured. The results are shown in Table 6.

[0151] Table 6 Flame retardant properties of three types of PI fiber films and commercial PP films

[0152] The afterflame time and smoldering time of the three types of PI fiber membranes were all 0 s, and the damaged length was less than 100 mm, which met the relevant requirements of GA869—2010; the smoldering time of commercial PP membranes was 3 to 4 s, and the damaged length was 15 to 17 cm.

[0153] The above embodiments are used to illustrate specific implementation methods of the present invention.

Claims

1. A method for preparing polyamic acid, characterized in that, include: Under an inert atmosphere, 4,4'-diaminodiphenylmethane (MDA) was dissolved in an organic solvent with a water content of less than 100 ppm, and a dried aromatic dianhydride was added in batches at low temperature to carry out a polymerization reaction, thereby obtaining a polyamic acid solution. The molar ratio of MDA to aromatic dianhydride is 1:(1 to 1.02).

2. The preparation method according to claim 1, characterized in that, The aromatic dianhydride is selected from at least one of 4,4'-diphenyl ether dianhydride (ODPA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), and pyromellitic dianhydride (PMDA).

3. The preparation method according to claim 1 or 2, characterized in that, The aromatic dianhydride is added in batches to the organic solution of the MDA. As the aromatic dianhydride is added and the polymerization reaction proceeds, the temperature of the polymerization reaction system is increased within the range of 3 ℃ to 8 ℃, and / or the feeding rate of the aromatic dianhydride is decreased.

4. The preparation method according to claim 3, characterized in that, The aromatic dianhydride was added in five batches, wherein: The feeding temperature for the first and second batches was 3 ℃~4 ℃, the feeding temperature for the third batch was 4 ℃~5 ℃, the feeding temperature for the fourth batch was 5 ℃~6 ℃, and the feeding temperature for the fifth batch was 7 ℃~8 ℃. The feeding times for the first to fourth batches were 0.4 h~0.6 h, 0.6 h~0.9 h, 0.8 h~1.2 h, and 1.2 h~1.8 h, respectively. The aromatic dianhydride for the fifth batch was added in multiple small batches.

5. The preparation method according to any one of claims 1, 2, and 4, characterized in that, The organic solvent is N,N-dimethylformamide (DMF).

6. The preparation method according to any one of claims 1, 2, and 4, characterized in that, The aromatic dianhydride was dried under vacuum conditions above 160°C for 20 to 28 hours.

7. The preparation method according to any one of claims 1, 2, and 4, characterized in that, The method further includes adding a desiccant to the obtained polyamic acid solution and storing it in an inert atmosphere at -10 °C to -5 °C.

8. The polyamic acid prepared by the preparation method according to any one of claims 1 to 7.

9. A fiber filter membrane, characterized in that, Its main component is the polyamic acid described in claim 8; Optionally, the fiber filter membrane is obtained from the polyamic acid of claim 8 by electrospinning and thermal imidization. Optionally, the process also includes subjecting the polyimide nanofiber membrane to negative corona discharge treatment.

10. The application of the fiber filter membrane according to claim 9 in particulate matter filtration under high temperature conditions; Optionally, the temperature of the high-temperature environment is 100℃~275℃; preferably 150℃~270℃, and more preferably 200℃~270℃.

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