High-voltage fabrication of ultrafine polyimide nanofiber membranes and their high-temperature filtration applications

Abstract

This invention discloses a high-voltage fabrication method for ultrafine polyimide nanofiber membranes and their high-temperature filtration applications. Using a needle-free electrospinning technique with metal wires, nanofiber membranes with an average fiber diameter of less than 130 nm were prepared under an 80 kV high voltage. This fine fiber structure not only enhances the interception efficiency of fine particulate matter (PM0.3), achieving a filtration efficiency of over 99.97%, but also significantly reduces the filtration resistance to 189.18 Pa through a slip effect, outperforming traditional commercial glass fiber filter materials. The fine-diameter PI nanofiber membrane exhibits good thermal stability below 550℃ and maintains high-efficiency PM0.3 filtration even after high-temperature treatment at 350℃~390℃. The ultrafine polyimide nanofiber membrane prepared by this invention possesses advantages such as high filtration efficiency, low resistance, good thermal stability, and hydrophobic properties, making it a promising candidate for high-temperature PM filtration.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of separation membrane technology, in particular to the preparation of ultrafine polyimide nanofiber membranes at high voltage and their high-temperature filtration applications. BACKGROUND

[0002] The emission of fine particulate matter (PM2.5-0.3) caused by industrialization and urbanization has become an environmental problem that needs to be addressed urgently. PM2.5-0.3 is mainly derived from the combustion of fossil fuels, biomass and waste, which is prevalent in industry, transportation, agriculture and home heating. PM is complex in composition, including inorganics such as silicates, sulfates, nitrates, and organics such as organic carbon and elemental carbon, and can carry up to 791 bacterial genera, increasing the risk of allergies and infections. The negative charge characteristic of PM affects its behavior in the atmosphere.

[0003] PM poses a serious threat to human health, and PM2.5 is more likely to enter the human body due to its small size, causing more serious diseases such as asthma, cardiovascular disease and lung inflammation. Studies have shown that when the concentration of PM2.5 exceeds 200 μg / m 3 , the number of respiratory clinic visits increases significantly. PM0.3 can cause cardiovascular disease, respiratory disease, diabetes, and lung cancer due to its smaller particle size.

[0004] Industrial dust collectors are effective for large particles, but ineffective for particles smaller than 2.5 μm. Electrostatic precipitators are costly and their removal efficiency is affected by the characteristics of PM. Nanofiber filters reduce resistance to airflow due to the "slip effect", so they have higher filtration efficiency and lower pressure drop than general ultrafine fiber filters, showing potential in the removal of particles smaller than 2.5 μm. Polyimide (PI) exhibits excellent mechanical properties due to its rigid segments, strong intermolecular interactions and high binding energy. Its high conjugation and high aromatic structure endows PI with good thermal stability, and its high dipole moment (6.2D) enhances its adsorption capacity for particulate matter, making PI an optimal choice for high-performance materials in the field of air filtration.

[0005] In the article "High Temperature Resistant Polyimide / Polyethersulfone Nano-Fibers Membrane for Removal of PM", PI / PES nanofiber membranes developed by Wei-Ting Chen and Yi-Hung Lai have a filtration efficiency of 99.54% for PM2.5 in 20 minutes of spinning time, a pressure drop of 92 Pa, and a significant thermal stability of about 500℃. In the article "Electrospun Polyimide / Metal-Organic Framework Nanofibrous Membrane with Superior Thermal Stability for Efficient PM2.5 Capture", the PI-ZIF nanofiber membrane prepared by Zhimin Hao's team combining PI and metal organic framework (ZIF-8) has a filtration efficiency of 96.6% ± 2.9% for PM2.5 at 300℃, showing good thermal stability and low pressure drop.

[0006] Although polyimide has made significant progress in PM2.5 filtration, there is still room for improvement in the filtration efficiency of PM0.3. In addition, the electrospinning time of PI nanofiber membranes in traditional electrospinning technology is usually in the range of 10kV to 40kV, and the electrospinning time is often more than 60min. The complexity of the preparation process also affects the economy and scalability of the material in industrial applications. SUMMARY

[0007] The purpose of the present application is to provide a high-voltage preparation of ultra-fine polyimide nanofiber membranes and their high-temperature filtration applications to solve the problems of poor filtration efficiency of PM0.3 and complex preparation process of the above-mentioned polyimide materials. The present application uses a metal wire needleless electrospinning process to electrospin PI nanofiber membranes, which avoids the problems of needle blockage and wear compared to the needle electrospinning process. The present application uses a voltage of 60kV to 80kV to enhance the droplet splitting ability, generating nanofibers with a finer diameter to improve filtration performance for high-temperature filtration. Compared to composite materials, the present application adjusts the electrospinning voltage to prepare fine-diameter nanofiber membranes to improve the filtration performance of PI nanofiber membranes, which is more convenient. With the help of the metal wire needleless electrospinning process, the fine-diameter PI nanofiber membrane 8~12min can be electrospun to obtain a high-efficiency filtration membrane.

[0008] To achieve the above-mentioned purpose, the first aspect of the present application provides a preparation method of an ultra-fine polyimide nanofiber membrane, comprising the following steps:

[0009] (1) Preparation of polyamide acid solution

[0010] 4,4'-diaminodiphenyl ether is dissolved in N,N-dimethylacetamide, and after 4,4'-diaminodiphenyl ether (ODA) is completely dissolved, a transparent 4,4'-diaminodiphenyl ether / N,N-dimethylacetamide solution is obtained; 4,4'-diaminodiphenyl ether / N,N-dimethylacetamide solution is added with phthalic anhydride monomers in multiple times, 4,4'-diaminodiphenyl ether and phthalic anhydride monomers are fully reacted by stirring, and a polyamide acid solution is obtained by standing;

[0011] (2) The polyamide acid solution is electrospun by a metal wire type needleless process.

[0012] (3) After electrospinning is completed, drying is performed to obtain a PAA nanofiber membrane, and then thermal imidization is performed to obtain a polyimide nanofiber membrane.

[0013] Preferably, the solution preparation process in step (1) is carried out at 0-5°C under nitrogen protection.

[0014] Preferably, 4,4'-diaminodiphenyl ether (ODA), N,N-dimethylacetamide (DMAc), and phthalic anhydride monomers (PMDA) in step (1) are vacuum dried at 120°C for 1-5h to remove residual solvents and moisture.

[0015] In the present application, PAA is polyamide acid, SE is an electrospinning electrode, CE is a collection electrode, PMDA is phthalic anhydride, ODA is 4,4'-diaminodiphenyl ether, and DMAC is N,N-dimethylacetamide.

[0016] Preferably, the molar ratio of ODA to PMDA in step (1) is 1:1-1.02.

[0017] Preferably, the concentration of the PAA solution in step (1) is 15-20wt%.

[0018] More preferably, the concentration of the PAA solution in step (1) is 16-18wt%.

[0019] Preferably, in the metal wire type needleless electrospinning process in step (2), the voltage is 60-80kV, the distance from the electrospinning electrode to the carrier is 240mm, the distance from the collection electrode to the carrier is 20mm, the running distance of the pipette car is 350mm, the cycle time is 6s, the electrospinning time is 8-12min, the spinning temperature is 15-25°C, and the humidity is 30-50%.

[0020] Preferably, the drying temperature in step (3) is 100-120°C, and the time is 8-15h.

[0021] Preferably, the specific step of the thermal imidization in step (3) is as follows:

[0022] The dried PAA nanofiber membrane is placed in a muffle furnace under air condition, and heated at a rate of 1-2℃ / min from room temperature to 350-400℃, and kept at 100℃, 200℃, 300℃ and 350℃ for 1h respectively, and then naturally cooled to room temperature to obtain a polyimide nanofiber membrane.

[0023] The second aspect of the present application provides a superfine polyimide nanofiber membrane prepared by the above preparation method.

[0024] Preferably, the average fiber diameter of the polyimide nanofiber membrane is 100-400nm.

[0025] The third aspect of the present application provides an application of the superfine polyimide nanofiber membrane, specifically, the application of the polyimide nanofiber membrane in high-temperature fine particulate matter filtration.

[0026] Preferably, the particle size of the fine particulate matter is 0.3μm.

[0027] Therefore, the present application adopts the above high-voltage to prepare the superfine polyimide nanofiber membrane and its high-temperature filtration application, which has the following beneficial effects:

[0028] (1) The present application adopts the metal wire type needleless electrospinning technology to electrospin the polyimide (PI) material, and 122nm average fiber diameter nanofiber membrane is prepared under 80kv high electrospinning voltage for 10 minutes.

[0029] (2) The fine diameter PI nanofiber membrane prepared by the present application has a filtration efficiency of 99.98% on PM0.3, and runs at a low resistance of 189.18Pa. After being treated at a high temperature of 390℃ for one hour, the filtration efficiency of the fine diameter PI nanofiber membrane on PM0.3 still remains above 99.95%, and can achieve complete filtration of PM2.5. This proves that the fine diameter PI nanofiber membrane has excellent stability and reliability under extreme temperature conditions.

[0030] (3) The membrane prepared by the present application also exhibits excellent hydrophobic performance, and the static hydrophobic angle measurement value is 130±3°, which helps to maintain its filtration performance in a humid environment.

[0031] (4) The fine diameter PI nanofiber membrane researched by the present application provides a new preparation strategy for high-temperature environment PM filtration, and provides new insights into its ultimate performance.

[0032] The technical solutions of the present application will be further described in detail below through the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1(a) is a process flow diagram of the preparation of the polyimide nanofiber membrane of the present application;

[0034] Figure 1(b) is a schematic diagram of the spinning process of the polyimide nanofiber membrane of the present application;

[0035] Figure 2 Figure 2 is the characterization results of Examples 1-5, wherein (a) is the Fourier transform infrared spectrometer (FTIR) diagram of PAA and PI, (b) is the tensile stress-strain curve diagram, (c) is the appearance comparison diagram before and after bending, and (d) is the principle diagram of the slippage flow effect;

[0036] Figure 3 Figure 3 is the scanning electron microscope (SEM) diagram and diameter distribution statistical diagram of the polyimide nanofiber membrane prepared in Examples 1-3 and Example 6, wherein (a) and (b) are the SEM diagrams of PI15-80-10, (c) is the SEM diagram of PI16-80-10, (d) is the diameter distribution statistical diagram of PI16-80-10, (e) is the SEM diagram of PI17-80-10, (f) is the diameter distribution statistical diagram of PI17-80-10, (g) is the SEM diagram of PI18-80-10, and (h) is the diameter distribution statistical diagram of PI18-80-10;

[0037] Figure 4 Figure 4 is the filtration performance of Examples 1-3 and Examples 7-10, wherein (a) is the filtration efficiency diagram of the PI nanofiber membrane prepared in Examples 1-3 and Examples 7-10 against PM0.3, (b) is the filtration efficiency diagram of the PI nanofiber membrane prepared in Examples 1-3 and Examples 7-10 against PM0.5-2.5, (c) is the filtration efficiency diagram of Example 1 and commercial glass fiber against PM0.3, and (d) is the filtration efficiency diagram of Example 1 and commercial glass fiber against PM0.5-2.5;

[0038] Figure 5 Figure 5 is the high temperature resistance performance, filtration performance and hydrophobic performance of Example 1, Examples 3-5 and Examples 11-14, wherein (a) is the thermogravimetric analysis (TG) diagram, differential thermogravimetric analysis (DTG) curve and differential scanning calorimetry (DSC) curve of the fine diameter PI nanofiber membrane of Example 3, (b) is the filtration efficiency diagram of the fine diameter PI nanofiber membrane of Example 3 and commercial glass fiber against PM0.3 after different temperature treatments, (c) is the filtration efficiency diagram of the fine diameter PI nanofiber membrane of Examples 1, 4-5 and 11-14 against PM0.5-2.5, and (d) is the comparison diagram of the surface water contact angle of the fine diameter PI nanofiber membrane after different temperature treatments and commercial glass fiber;

[0039] Figure 6SEM images and fiber diameter statistical distribution graphs of PI nanofiber membranes prepared for Example 1 and Examples 4-5, wherein (a) is an SEM image of PI350, (b) is a diameter distribution statistical graph of PI350, (c) is an SEM image of P380, (d) is a diameter distribution statistical graph of P380, (e) is an SEM image of P400, and (f) is a diameter distribution statistical graph of P400. DETAILED DESCRIPTION

[0040] The present application will be further described below. It should be noted that the present embodiment is based on the technical solution, and gives a detailed implementation and specific operation process, but the present application is not limited to the present embodiment.

[0041] Example 1

[0042] As shown in FIG. 1(a) and FIG. 1(b), a preparation method of a superfine polyimide nanofiber membrane, comprising the following steps:

[0043] (1) Preparation of PAA solution

[0044] ODA, DMAc, and PMDA monomers were vacuum dried at 120℃ for 1-5h to remove residual solvents and moisture;

[0045] Under the condition of nitrogen protection, ODA was dissolved in DMAc, and the molar ratio of ODA to PMDA was 1:1. After ODA was completely dissolved, a transparent ODA / DMAc solution was obtained;

[0046] PMDA monomers were added to the ODA / DMAc solution at 2℃ in three times with an interval of 30min, and ODA and PMDA monomers were fully reacted under stirring. The PAA solution was obtained by standing. The concentration of the PAA solution was 18wt%;

[0047] (2) Electrospinning of PAA solution

[0048] Electrospinning parameters: voltage was 80kV, SE to carrier distance was 240mm, CE to carrier distance was 20mm; spinning carriage running distance was 350mm, cycle time was 6s, electrospinning time was 10min; spinning temperature was 20℃, humidity was 40%;

[0049] (3) After electrospinning is completed, then dry at 120℃ for 12h to obtain PAA precursor, and then place the dried PAA precursor in a muffle furnace under air condition to heat at a rate of 1.66℃ / min, from room temperature to 350℃, and keep at 100℃, 200℃, 300℃ and 350℃ for 1h respectively, and then naturally cool to room temperature, and then heat imidize the PAA precursor into a PI nanofiber membrane, and the obtained polyimide nanofiber membrane is recorded as PI18-80-10 or PI350.

[0050] Example 2

[0051] The difference between this example and Example 1 is that the concentration of the PAA solution is different, and the concentration of the PAA solution in this example is 17wt%, and the obtained polyimide nanofiber membrane is recorded as PI17-80-10.

[0052] Example 3

[0053] The difference between this example and Example 1 is that the concentration of the PAA solution is different, and the concentration of the PAA solution in this example is 16wt%, and the obtained polyimide nanofiber membrane is recorded as PI16-80-10.

[0054] Example 4

[0055] The difference between this example and Example 1 is that the temperature of heat imidization is different, and the muffle furnace is heated from room temperature to 380℃ in this example, and the obtained polyimide nanofiber membrane is recorded as PI380.

[0056] Example 5

[0057] The difference between this example and Example 1 is that the temperature of heat imidization is different, and the muffle furnace is heated from room temperature to 400℃ in this example, and the obtained polyimide nanofiber membrane is recorded as PI400.

[0058] Example 6

[0059] The difference between this example and Example 1 is that the concentration of the PAA solution is different, and the concentration of the PAA solution in this example is 15wt%, and the obtained polyimide nanofiber membrane is recorded as PI15-80-10.

[0060] Example 7

[0061] The difference between this example and Example 3 is that the electrospinning time is different, and the electrospinning time in this example is 12 minutes, and the obtained polyimide nanofiber membrane is recorded as PI16-80-12.

[0062] Example 8

[0063] The difference between this embodiment and embodiment 3 is that the electrospinning time is different, the electrospinning time of this embodiment is 8 minutes, and the obtained polyimide nanofiber membrane is recorded as PI16-80-8.

[0064] Example 9

[0065] The difference between this embodiment and embodiment 3 is that the electrospinning voltage is different, the electrospinning voltage of this embodiment is 70kV, and the obtained polyimide nanofiber membrane is recorded as PI16-70-10.

[0066] Example 10

[0067] The difference between this embodiment and embodiment 3 is that the electrospinning voltage is different, the electrospinning voltage of this embodiment is 60kV, and the obtained polyimide nanofiber membrane is recorded as PI16-60-10.

[0068] Example 11

[0069] The difference between this embodiment and embodiment 1 is that the temperature of thermal imidization is different, in this embodiment, the muffle furnace is heated from room temperature to 360℃, and the obtained polyimide nanofiber membrane is recorded as PI360.

[0070] Example 12

[0071] The difference between this embodiment and embodiment 1 is that the temperature of thermal imidization is different, in this embodiment, the muffle furnace is heated from room temperature to 370℃, and the obtained polyimide nanofiber membrane is recorded as PI370.

[0072] Example 13

[0073] The difference between this embodiment and embodiment 1 is that the temperature of thermal imidization is different, in this embodiment, the muffle furnace is heated from room temperature to 390℃, and the obtained polyimide nanofiber membrane is recorded as PI390.

[0074] Example 14

[0075] The difference between this embodiment and embodiment 1 is that the temperature of thermal imidization is different, in this embodiment, the muffle furnace is heated from room temperature to 410℃, and the obtained polyimide nanofiber membrane is recorded as PI410.

[0076] Test example

[0077] (1) The polyimide nanofiber membranes prepared in examples 1-5 were characterized by FTIR and SEM.

[0078] The molecular structure of the synthesized material was characterized by using a Fourier infrared spectrometer of American Thermo Fisher Scientific Nicolet iS20. Fourier infrared spectroscopy (FTIR) is used to identify the functional groups and chemical bonds present in the material, and to determine whether the target nanofiber membrane has been obtained.

[0079] Detailed chemical composition analysis of PAA nanofibers and their thermally imidized PI nanofiber membranes was performed using FTIR. Figure 2 As shown in (a), the characteristic absorption peak of PAA is located at 1718.95 cm⁻¹. -1 (Asymmetric stretching vibration of C=O in COOH), 1649.49cm -1 (C=O symmetric stretching vibration in CONH), 1497.62cm -1 (Stretching vibration of the benzene ring skeleton) and 1233.38cm -1 (CO-NH symmetric stretching vibrations), which are consistent with the PAA structural formula. After thermal imidization, the characteristic absorption peak of PI appears at 1717.18 cm⁻¹. -1 (Asymmetric stretching vibration of C=O on the imide ring), 1775.13 cm -1 (Symmetrical stretching vibration of C=O) and 1374.76 cm -1 (Stretching vibration of CNC on the imide ring), and 1242.36 cm -1 (ether bond vibration) and 722.08 cm -1 (Bending vibration of -C=O in the imide ring). Furthermore, 2600 cm⁻¹ -1 Up to 3500cm -1 The absorption peak at that point is attributed to the -COOH and -NH- groups of PAA, which disappeared in the FTIR spectrum of PI, indicating that PAA was completely converted to PI after thermal imidization.

[0080] The morphology of the samples was examined using a Czech TESCAN MIRA LMS scanning electron microscope (SEM). Prior to SEM imaging, the samples underwent platinum sputtering for 45 seconds (10 mA) using an Oxford Quorum SC7620 sputtering system to enhance image resolution and contrast. SEM analysis provided detailed views of the sample surface morphology and microstructure.

[0081] The surface morphology of nanofiber membranes prepared with PAA solutions of different concentrations was analyzed using scanning electron microscopy (SEM). For example... Figure 3 As shown in (c), (e), and (g), when the PAA solution concentration was between 16 wt.% and 18 wt.%, the surface of the resulting fine-diameter PI nanofiber membrane was very smooth, and no beaded fibers were observed. This indicates that optimization of the electrospinning process parameters helps to obtain a uniform fiber morphology. However, when the PAA solution concentration was 15 wt.%, the surface of the resulting fine-diameter PI nanofiber membrane was very smooth, and no beaded fibers were observed. This suggests that optimization of the electrospinning process parameters helps to obtain a uniform fiber morphology. Figure 3(a) and (b) show that the fiber surface appears spindle structure and bonding phenomenon, which is due to the instability of fiber formation caused by low solution concentration. Further statistical analysis quantifies the SEM images, and the results show that as the PAA solution concentration increases, the average diameter of the nanofiber also increases accordingly. Specifically, as shown in Figure 3 (d), (f) and (h) show that the diameter distribution of the nanofiber is in the form of inverse Gaussian distribution, and the average diameter of PI16-80-10, PI17-80-10 and PI18-80-10 nanofiber is 139.28 nm, 150.90 nm and 174.27 nm, respectively. The membrane obtained by the present application exhibits a finer fiber diameter, which has a positive effect on improving the filtration performance.

[0082] The surface morphology of the fine-diameter PI nanofiber membrane treated at different temperatures was analyzed by SEM. As shown in Figure 6 (a), (c) and (e) show that even after further treatment at high temperature, the fiber surface remains smooth and does not appear to be damaged by bending or curling. Further statistical analysis shows that as the treatment temperature increases, the average diameter of the nanofiber also increases. Specifically, as shown in Figure 6 (b), (d) and (f) show that the average diameters of PI350, PI380 and PI400 nanofiber are 139.28 nm, 151.41 nm and 165.65 nm, respectively, and their diameter distributions are in the form of inverse Gaussian distribution.

[0083] (2) The fine-diameter PI nanofiber membrane sample was cut into a uniform size, i.e. a rectangular sample with a length of 30 mm and a width of 10 mm. This standardized sample size helps to ensure the consistency and comparability of the test results. Subsequently, the tensile test was carried out on the INSTRON 8872 tester. During the test, the sample was uniformly stretched at a strain rate of 2 mm / min until it broke.

[0084] As shown in Figure 2 (b), as the PAA concentration increases from 16 wt.% to 18 wt.%, the tensile strength of the fine-diameter PI nanofiber membrane significantly improves. Specifically, the PI18-80-10 sample exhibits the highest tensile strength, reaching 8.893 MPa, which is 28.53% higher than that of the PI16-80-10 sample, which is 6.355 MPa. This result shows that increasing the PAA concentration is an effective way to improve the mechanical strength of the PI nanofiber membrane. Further analysis shows that, as shown in Figure 3As shown in (c), (e), and (g), with increasing PAA concentration, the amount of fibers produced during electrospinning increases, leading to an increase in the thickness of the nanofiber membrane produced within the same electrospinning time. Furthermore, the increased PAA concentration also results in a decrease in the diameter of the nanofibers, which affects the tensile strength of the fine-diameter PI nanofiber membrane. The tensile strength of the fine-diameter PI nanofiber membrane obtained in this invention is 5–8 MPa.

[0085] In addition, Knudsen number (K n It can be calculated using the following formula:

[0086] Kn=2λ / d f

[0087] Where λ and d f These are the mean free path of air molecules (λ = 65.3 nm) and the fiber diameter, respectively.

[0088] The corresponding K values ​​for PI16-80-10, PI17-80-10, and PI18-80-10 nanofiber membranes n The values ​​are 0.937, 0.865, and 0.749, respectively. The airflow around the fine-diameter PI nanofibers belongs to the transition flow region. For example... Figure 2 (d) According to the principle of slip flow, when the fiber diameter is close to 65.3 nm, the airflow velocity on the surface of a single fiber is not zero. Therefore, the airflow resistance can be significantly reduced, and it is reasonable to assume that the fine-diameter PI nanofiber membrane exhibits the characteristics of a lip effect, especially for PI16-80-10.

[0089] K obtained through calculation n The airflow properties of PI16-80-10, PI17-80-10, and PI18-80-10 nanofiber membranes were evaluated. The K0 of these membranes was... n The values ​​are 0.937, 0.865, and 0.749, respectively, indicating that the airflow around the thin-diameter PI nanofibers is in the transition flow region. Based on the principle of slip flow, when the fiber diameter is close to the mean free path of air molecules, the airflow velocity on the surface of a single fiber is not expected to approach zero.

[0090] from Figure 2 (b) shows the mechanical properties; the increase in ambient temperature is accompanied by an increase in nanofiber diameter. The tensile strength of PI400 reaches 7.721 MPa, an increase of 21.49% compared to 6.355 MPa of PI16-80-10. However, the elongation at break of PI400 decreases from 25.60% for PI16-80-10 to 19.02%, a decrease of 25%. A comparison of the appearance before and after bending is shown in [reference needed]. Figure 2 (c)

[0091] (3) The thermal properties of the material, including stability and phase transitions, were determined using a Netzsch STA449F3 Jupiter simultaneous thermal analyzer (Germany). Samples were heated from 30°C to 800°C in a nitrogen atmosphere at a rate of 20°C / min without holding. This method allows for the assessment of the material's thermal degradation behavior and the identification of any phase transitions occurring during heating.

[0092] When discussing fundamental issues in particulate matter filtration under high-temperature environments, thermal stability is one of the key factors. For example... Figure 5 As shown in (a), the fine-diameter PI nanofiber membrane was characterized by thermogravimetric analysis (TGA). The results revealed minimal mass change in the membrane within a temperature range of 30 to 180 °C, attributed to the removal of residual organic solvents. Significant mass loss occurred at 553.40 °C, due to self-crosslinking, cyclization, and dehydrogenation reactions. Differential thermogravimetric analysis (DTG) curves showed that the main weight loss temperature of the fine-diameter PI nanofiber membrane was 610 °C, primarily caused by the oxidation of continuously released carbon and carbon monoxide. Therefore, the fine-diameter PI nanofiber membrane exhibits good thermal stability below 550 °C. Differential scanning calorimetry (DSC) curves showed no significant peak between 300 °C and 500 °C, further confirming the excellent thermal stability of the fine-diameter PI nanofiber membrane. However, as a filtration material, its ability to maintain filtration efficiency and avoid failure at high temperatures is of greater practical significance.

[0093] (4) The temperature resistance filtration test was conducted using a KJ-T1200-S6010-wQ vacuum tube furnace from Zhengzhou Kejia Electric Furnace Co., Ltd. To simulate real high-temperature conditions, the sample was heated to 410℃ in air at a rate of 10℃ / min, and isothermally held for 1 hour at 10℃ intervals within the range of 350℃ to 400℃. Then, the filtration efficiency and resistance were measured at different temperatures from 350℃ to 410℃.

[0094] The filtration performance of the membrane was evaluated on the LZC-K1 type filter media comprehensive performance testing bench of Jiangsu Suzhou Huada Instrument Equipment Co., Ltd., China. The membrane was placed in a circular clamp with a diameter of 11 cm, and its filtration efficiency and resistance were measured by adjusting the flow rate at different air velocities. The filtration efficiency of the filter was calculated using the following formula:

[0095]

[0096] In the formula, η is the filtration efficiency; C down C up These represent the particle number concentrations downstream and upstream of the filter, respectively.

[0097] The filtration performance analysis of fine-diameter PI nanofiber membranes mainly focuses on the filtration efficiency of PM0.3. For example...Figure 4 (a) and Figure 4 As shown in (b), the PI16-80-10 sample exhibited significant superiority. The PI16-80-10 filtration efficiency reached 99.98753%, exceeding 99.95% for all three samples (PI18-80-10: 99.989%; PI17-80-10: 99.97466%), achieving highly efficient filtration of PM0.3 and complete filtration of PM2.5. However, the PI16-80-10 showed an advantage in filtration resistance. The PI16-80-10 sample had a resistance of 189.18 Pa, the lowest compared to 348.86667 Pa for PI18-80-10 and 238.74 Pa for PI17-80-10. The PI15-80-10 sample was also tested. Due to the spindle-shaped structure of PI15-80-10 and the large amount of fiber bonding and sheet-like coverage on the surface, its filtration efficiency reached 99.99%, and the filtration resistance was extremely high, reaching 500Pa+. Due to the limitations of the testing instrument, the upper limit of the test was 500Pa.

[0098] Further comparison of samples under different electrospinning times (e.g., PI16-80-12) and different electrospinning voltages (e.g., PI16-70-10) revealed that extending the electrospinning time did not improve efficiency but instead increased resistance. While samples under different voltages, such as PI16-70-10, achieved a filtration efficiency of 99.92495%, their resistance was 178.74 Pa.

[0099] In exploring the performance of fine-diameter PI nanofiber membranes in practical applications, a series of filtration efficiency tests were conducted under various wind speed conditions, and a comparative analysis was performed with commercial glass fiber filter media. Figure 4 As shown in (c) and (d), at a wind speed of 1 cm / s, the fine-diameter PI nanofiber membrane (PI16-80-10) achieved a PM0.3 filtration efficiency of 99.9993%, slightly higher than the 99.9924% of the glass fiber filter media. While the filtration efficiency of the fine-diameter PI nanofiber membrane decreased slightly with increasing wind speed, it still maintained a high efficiency of 99.9736% for PM0.3 even at a wind speed of 10 cm / s. In contrast, the efficiency of the glass fiber filter media decreased from 99.9924% at 1 cm / s to 99.9317% at 10 cm / s.

[0100] Regarding filtration resistance, PI16-80-10 exhibits a resistance of 40.7 Pa at an air velocity of 1 cm / s, while the glass fiber filter media has a resistance of 72.5 Pa. This indicates that the fine-diameter PI nanofiber membrane possesses lower airflow resistance under low air velocity conditions. As the air velocity increases, the resistance of PI16-80-10 increases to 351.0 Pa, but it remains lower than the 500.5+ Pa resistance value of the glass fiber filter media at the same air velocity.

[0101] The temperature limit for maintaining high-efficiency filtration of fine-diameter PI nanofiber membranes was investigated. The fine-diameter PI nanofiber membrane was compared with commercial glass fiber under the same conditions, clarifying the gap between the two. In the experiment, both the fine-diameter PI nanofiber membrane (PI16-80-10) and commercial glass fiber were heated to 410℃ at a rate of 10℃ / min, and then isothermally held at 350℃ to 400℃ for 1 hour every 10℃ interval. Figure 5 As shown in (b), the filtration resistance of glass fiber increases slightly between 350°C and 400°C, from 302.63 Pa to 310.6 Pa. The filtration resistance of fine-diameter PI nanofiber membrane reaches a peak of 205.3 Pa at 360°C, but drops to 195.7 Pa at 400°C, and the filtration resistance decreases with increasing temperature.

[0102] Regarding filtration efficiency, both glass fiber and fine-diameter PI nanofiber membranes showed a decrease in filtration efficiency with increasing temperature. The filtration efficiency of glass fiber decreased from 99.98% at 28℃ to 99.92% at 400℃, a change of 0.06%. The filtration efficiency of fine-diameter PI nanofiber membranes decreased from 99.99% at 28℃ to 99.77% at 400℃, a change of 0.21%. Figure 5 (c) Although the fine-diameter PI nanofiber membrane had been operating at temperatures above 350°C for 6 hours at 400°C, its filtration efficiency decreased to the point where it could not achieve high-efficiency filtration of PM0.3 particles, but it could still guarantee complete filtration of PM2.5. When the temperature reached 410°C, PI410 still had a filtration efficiency of 99.96% for PM2.5.

[0103] (5) The surface contact angle was assessed using a JCY-2 contact angle measuring instrument from Shanghai Fangrui Instruments Co., Ltd. The membrane was cut into 1×1cm pieces. 2 The membrane was shaped into rectangles and dried in a drying oven at 60°C for 2 hours. A 2 μL droplet of water was placed on the membrane surface to measure the contact angle (CA), an indicator of the material's hydrophobicity or hydrophilicity. Each test was performed in triplicate to ensure the accuracy and repeatability of the results.

[0104] like Figure 5As shown in (d), glass fiber (GF) exhibits excellent hydrophobicity, with an initial contact angle of 128.53°. Over time, the contact angle slightly decreases to 126.29° at 300 seconds, indicating a slight degradation in hydrophobic properties, but overall stability. For fine-diameter PI nanofiber membranes, samples at different heat treatment temperatures all showed initial contact angles close to 130°, demonstrating extremely strong hydrophobicity. The decrease in contact angle over the observation time was small, indicating relatively stable hydrophobic properties of PI. In particular, the contact angle of the PI400 sample remained stable at 126.00°, slightly lower than the 127.23° of the PI16-80-10 sample, but the decrease was not significant. Despite slight differences in initial contact angles, PI samples at different heat treatment temperatures showed similar hydrophobic properties, suggesting that heat treatment temperature has little effect on the hydrophobic properties of PI. The enhanced hydrophobicity of the fine-diameter PI nanofiber membrane is attributed to a high-temperature imidization treatment at 350°C, which removes the hydrophilic -NH- groups, thereby increasing the hydrophobic angle. On the other hand, although the nanofiber diameter increases with increasing heat treatment temperature, it remains within the range of 120 nm to 165 nm. The finer nanofibers in this invention result in smaller pore sizes. These smaller pores cause water droplets to experience greater surface tension on the nanofibers, leading to a significantly improved surface water contact angle for the fine-diameter PI nanofiber membrane compared to the 100° to 110° observed in other studies.

[0105] Therefore, the present invention provides a method for preparing and applying a polyimide nanofiber membrane with the above-mentioned structure. The prepared polyimide nanofiber membrane has the advantages of high filtration efficiency, low resistance, good thermal stability and hydrophobic properties, making it promising for application in the field of high-temperature PM filtration.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing an ultrafine polyimide nanofiber membrane, characterized by: The steps are as follows: (1) Preparation of polyamide acid solution 4,4'-diamino diphenyl ether is dissolved in N,N-dimethylacetamide, and after 4,4'-diamino diphenyl ether is completely dissolved, a transparent 4,4'-diamino diphenyl ether / N,N-dimethylacetamide solution is obtained; 4,4'-diamino diphenyl ether / N,N-dimethylacetamide solution is added with multiple portions of pyromellitic dianhydride monomer, and 4,4'-diamino diphenyl ether and pyromellitic dianhydride monomer are fully reacted under stirring, and polyamide acid solution is obtained by standing; the molar ratio of 4,4'-diamino diphenyl ether and pyromellitic dianhydride monomer is 1:1~1.02, the solution preparation process is carried out under the conditions of 0~5℃ and nitrogen protection, and the concentration of the polyamide acid solution is 16~18wt%; (2) The polyamide acid solution is electrospun by using a metal wire type needleless process, the voltage is 60~80kV, the distance between the electrospinning electrode and the carrier is 240mm, the distance between the collection electrode and the carrier is 20mm, the electrospinning time is 8~12min, the running distance of the pipette car is 350mm, and the cycle time is 6s; the spinning temperature is 15℃~25℃, and the humidity is 30%~50%; (3) After electrospinning is completed, drying is performed to obtain a PAA nanofiber membrane, and then thermal imidization is performed to obtain a polyimide nanofiber membrane, and the average fiber diameter of the polyimide nanofiber membrane is 100~174.27nm; the specific steps of thermal imidization are as follows: the dried PAA nanofiber membrane is placed in a muffle furnace under air conditions and heated at a rate of 1~2℃ / min, from room temperature to 350~400℃, and the temperature is kept at 100℃, 200℃, 300℃ and 350℃ for 1h respectively, and then naturally cooled to room temperature to obtain a polyimide nanofiber membrane.

2. An ultra-fine polyimide nanofiber membrane, characterized by: The ultra-fine polyimide nanofiber membrane prepared by the preparation method of claim 1.

3. Use of ultra-fine polyimide nanofiber membranes, characterized in that: The application of the ultra-fine polyimide nanofiber membrane of claim 2 in high-temperature fine particulate matter filtration.

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