Micro-nano integrated polyimide fiber cotton and water-soluble preparation method and application thereof

Abstract

The present application relates to a kind of micro-nano integrated polyimide fiber cotton and its water-soluble preparation method and application, the micro-nano integrated polyimide fiber cotton includes 0.5-5 μm diameter micron fiber and 30-499 nm diameter nanometer fiber, the specific surface area of the micro-nano integrated polyimide fiber cotton is not less than 2 m 2 / g.The polyimide fiber cotton provided by the present application can take into account the advantages of nanometer polyimide fiber and micron polyimide fiber.In addition, the present application uses water-soluble polyamide acid solution as spinning solution, reduces the difficulty and energy consumption of solvent recovery treatment in spinning process, meets the requirements of engineering scale-up process;High production efficiency is achieved by using high-speed airflow spinning technology.

Description

Technical Field

[0001] This invention relates to the field of polyimide fiber technology, specifically to a micro / nano integrated polyimide fiber cotton, its water-soluble preparation method, and its application. Background Technology

[0002] Polyimide materials, with their unique rigid aromatic heterocyclic structure, possess excellent high and low temperature resistance, flame retardant and self-extinguishing properties, and low thermal conductivity. They can be prepared into micron- or nano-scale porous aerogels and other products through methods such as freeze-drying and electrospinning, showing broad application prospects in thermal insulation and other fields. However, freeze-dried samples are limited by the size of the freeze dryer equipment, and electrospinning methods have low preparation efficiency, restricting the large-scale continuous production of related products. Using polyimide fibers as raw materials, through processes such as opening, carding, and web laying, products with micron-scale basic structures, such as fiber cotton or wadding (e.g., CN112359483A, CN115652652A, CN102965846A), can also be obtained, meeting the requirements of large-scale industrial production. However, since the fiber diameter is mostly on the order of 10 μm, it affects the specific surface area and thermal insulation performance of the products.

[0003] Furthermore, the spinning solution for polyimide fibers is mainly a polyamic acid solution or a soluble polyimide solution, and the solvent is mainly aprotic polar solvent, such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO). These solvents have high boiling points, requiring solvent removal and recovery during the spinning process, which increases energy consumption and the complexity of equipment and processes.

[0004] Polyimide fibers with diameters in the micrometer scale are mainly prepared through wet spinning, wet-dry spinning, or dry spinning processes, as described in CN102345177A and CN104294391A. These processes are characterized by high mechanical strength and high production efficiency, but the specific surface area is lower than that of fibers in the nanometer scale. Existing technologies also utilize electrospinning to prepare polyimide fibers, such as those described in CN110106635A and CN119524652A. Electrospinned fibers typically have diameters in the hundreds of nanometers and are relatively uniformly distributed. However, because the fiber diameters are at the nanometer level, their mechanical properties are lower than those of fibers with diameters in the micrometer scale, and the production efficiency is low, which is not conducive to large-scale production. Summary of the Invention

[0005] To overcome the shortcomings of existing polyimide fiber technologies in terms of overall performance, particularly the difficulty in combining the advantages of nanoscale and microscale polyimide fibers, this invention proposes a micro-nano integrated polyimide fiber that simultaneously maintains the advantages of large specific surface area of ​​nanoscale fibers and good mechanical properties of microscale fibers. Furthermore, addressing the shortcomings of existing polyimide fiber preparation technologies, such as the difficulty in handling organic solvents and the low efficiency of electrospinning, this invention also proposes a water-soluble preparation method for the aforementioned micro-nano integrated polyimide fiber. This invention uses a water-soluble polyamic acid solution as the spinning solution, enabling the recovery of polar solvents such as DMAc or DMF before spinning, reducing the difficulty and energy consumption of solvent recovery during the spinning process, and better meeting the requirements of engineering scale-up processes. Simultaneously, it innovatively employs high-speed airflow spinning technology, which can improve production efficiency by two orders of magnitude compared to electrospinning technology. By adjusting parameters such as shear rate and viscosity, micro-nano integrated polyimide fibers can be obtained. In a preferred embodiment of the present invention, the micro-nano integrated fiber exhibits a cross-linked dendritic structure, with micron fibers as the trunk and nanofibers as branches. This combines the mechanical properties of micron fibers with the high specific surface area of ​​nanofibers, resulting in better thermal insulation, sound absorption, and heat preservation effects. Its integrated cross-linked network structure, unlike fused or bonded networks formed through post-processing, possesses excellent structural stability, macroscopically manifested as superior compression resilience, allowing it to better retain still air and further improve thermal insulation, sound absorption, and other properties. Although existing technologies also include air-jet spinning processes for fiber preparation, there are no reports of preparing micro-nano integrated polyimide fibers.

[0006] Specifically, the present invention provides the following technical solutions to achieve the above objectives:

[0007] The first objective of this invention is to provide a micro / nano integrated polyimide fiber cotton, comprising micron-sized fibers with a diameter of 0.5-5 μm and nano-sized fibers with a diameter of 30-499 nm, wherein the diameter of the micron-sized fibers can be more than twice that of the nano-sized fibers; the specific surface area (BET method test) of the micro / nano integrated polyimide fiber cotton is not less than 2 m². 2 / g.

[0008] Furthermore, the micro-nano integrated polyimide fiber cotton has a dendritic cross-linked structure, comprising a trunk, branches, and nodes. Fibers with a diameter of 0.5-5 μm serve as the trunk, fibers with a diameter of 30-499 nm serve as branches, and the nodes of the trunk and branches serve as cross-linking points. The cross-linking point density is 1-15 points / 100 μm. 2 Preferably 3-10 per 100μm 2 More preferably, 3.5-8 per 100μm 2The optimal value is 5.24-7.86 particles / 100μm. 2 .

[0009] Furthermore, the trunk diameter of the dendritic micro-nano integrated polyimide fiber cotton is 0.5-3.5 μm, the branch diameter is 35-400 nm, and the specific surface area is 2.5-10 m². 2 / g, with a preferred specific surface area of ​​3-5 m² / g. 2 / g, more preferably 3.5-4.3m 2 / g, the optimal value is 3.89-4.53 m 2 / g.

[0010] The second objective of this invention is to provide a water-soluble preparation method for the above-mentioned micro / nano integrated polyimide fiber cotton, comprising the following steps:

[0011] (S1) Add dianhydride monomer and diamine monomer to an aprotic polar organic solvent and prepare polyamic acid solution by solution polycondensation.

[0012] (S2) Mix the polyamic acid solution with water, dry the precipitate, and obtain polyamic acid filaments;

[0013] (S3) Add polyamic acid filament to water, add organic amine weak base to obtain polyamic acid aqueous solution with viscosity of 10-80 Pa·s at 25℃;

[0014] (S4) Polyamic acid aqueous solution was blown out of the spinneret using high-speed airflow spinning technology to obtain micro-nano integrated polyamic acid fibers; the shear rate of high-speed airflow spinning was 5.0 × 10⁻⁶. 4 ~5.0×10 7 s -1 ;

[0015] (S5) Micro-nano integrated polyamic acid fiber is heat-treated to obtain micro-nano integrated polyimide fiber cotton.

[0016] Preferably, the viscosity of the polyamic acid aqueous solution at 25°C is 10-80 Pa·s, more preferably 20-60 Pa·s; the shear rate of the high-speed airflow spinning is 1.2 × 10⁻⁶ Pa·s. 5 -9.1×10 6 s -1 More preferably, the shear rate of high-speed airflow spinning is 6.6 × 10⁻⁶. 5 -5.4×10 6 s -1Within the aforementioned shear rate range, and with an appropriate viscosity of the polyamic acid aqueous solution, a micro-nano integrated cross-linked network structure with micron-scale trunks and nano-scale branches can be obtained. This structure, with its suitable cross-linked network, can further improve the mechanical properties of fiber cotton while maintaining thermal insulation, sound absorption, and heat insulation properties. However, it is necessary to control the shear rate appropriately. An excessively high shear rate results in fiber cotton with a high proportion of nano-branches, which is detrimental to resilience. A moderate ratio of micron-scale trunks to nano-scale branches is required to obtain fiber cotton that possesses a large specific surface area, good thermal insulation, sound absorption, and excellent resilience.

[0017] Further, in step (S1), the dianhydride monomer is selected from at least one of pyromellitic dianhydride (PMDA), diphenyl ether tetracarboxylic dianhydride (ODPA), biphenyl tetracarboxylic dianhydride (BPDA), and 3,3',4,4'-triphenyl diether tetracarboxylic dianhydride (HQDPA); the diamine monomer is selected from 4,4'-diaminodiphenyl ether (ODA), 3,4'-diaminodiphenyl ether (3,4'- At least one of the following: ODA, p-phenylenediamine (p-PDA), m-phenylenediamine (m-PDA), 2-(4-aminophenyl)-5-aminobenzimidazole (BIA), 3,5'-diaminobenzoic acid (3,5'-DABA), and 4,4-diaminobiphenyl-2,2'-dicarboxylic acid (2,2'-DCB); further, the molar ratio of the dianhydride monomer to the diamine monomer is 0.8-1.2:0.8-1.2, preferably 1:0.9-1.1, more preferably 1:0.95-1.05, such as 1:0.98, 1:0.99, 1:1, 1:1.01, and 1:1.02.

[0018] In a preferred embodiment of the present invention, the diamine monomer contains 15-30 mol% of a carboxyl-containing diamine monomer, wherein the carboxyl-containing diamine monomer is selected from at least one of 3,5'-diaminobenzoic acid (3,5'-DABA) and 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid (2,2'-DCB). The presence of carboxyl groups on the diamine monomer is more conducive to the subsequent reaction of polyamic acid with a weak organic base, increases the water solubility of the polyamic acid and the rheological properties of the solution, and is more beneficial for high-speed airflow spinning.

[0019] Further, in step (S1), the intrinsic viscosity of the obtained polyamic acid is 1.1-2.8 dL / g, preferably 1.5-2.1 dL / g.

[0020] Further, in step (S1), the aprotic polar organic solvent is selected from at least one of N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), and N-methylpyrrolidone (NMP); preferably, the amount of aprotic polar organic solvent used is 3-8 times the total mass of the dianhydride monomer and the diamine monomer. The solution polycondensation conditions in step (S1) are 0-40°C, preferably 10-30°C, and the reaction time is 3-15 h, preferably 5-10 h.

[0021] Furthermore, in step (S2), the polyamic acid solution and water are mixed by slowly extruding the polyamic acid solution in the form of fine filaments or pouring it into water, where it solidifies to form polyamic acid filaments; or by immersing the spinneret in water, and then extruding the polyamic acid solution through the spinneret in the form of filaments, which directly solidify upon contact with water, and then drawing the polyamic acid filaments; even further, the diameter of the obtained polyamic acid filaments is 0.1-2 mm.

[0022] Further, in step (S3), the water is at least one of distilled water, deionized water, and ultrapure water; the organic amine weak base is selected from triethylamine, triethanolamine, diethanolamine, N,N-dimethylethanolamine, pyridine, morpholine, and N-methylmorpholine; the molar ratio of the nitrogen-containing group in the organic weak base to the carboxyl group in the polyamic acid is 1-3:1, preferably 1.5-2.5:1, such as 1.8:1, 1.9:1, 2:1, 2.1:1, or 2.2:1. The amount of water added is such that the viscosity of the polyamic acid aqueous solution is 20-60 Pa·s at 25°C.

[0023] Furthermore, in step (S4), the shear rate of the high-speed airflow spinning is 1.0 × 10⁻⁶. 5 -5.0×10 7 s -1 Furthermore, the high-speed airflow spinning process parameters are as follows: airflow slit width 0.05-0.30 mm, gas flow rate 1-80 L / min, air pressure 0.01-0.4 MPa, airflow temperature 10-50℃, spinneret diameter 0.4-1.0 mm, single-hole extrusion speed of spinning solution 2-80 mL / h, and fiber receiving distance 25-50 cm; preferably, the shear rate of high-speed airflow spinning is 1.2 × 10⁻⁶. 5 -9.1×10 6 s -1 More preferably, the shear rate of high-speed airflow spinning is 6.6 × 10⁻⁶. 5 -5.4×10 6 s -1 .

[0024] In this invention, although the exact reason is not fully understood, micro-nano integrated polyamic acid fibers are prepared by spinning a polyamic acid aqueous solution of appropriate viscosity using high-speed airflow spinning technology. The core process parameter of this invention is the shear rate of high-speed airflow spinning; shear rates that are too fast or too slow will not successfully produce micro-nano integrated polyamic acid fibers. At a preferred shear rate, dendritic polyamic acid fiber cotton with a micron-scale trunk and nano-scale branches, exhibiting a cross-linked network structure, can also be obtained. A possible reason is that when the shear rate is too low, micron-scale fibers are obtained, which are independent of each other and do not form a dendritic structure. When the shear rate reaches a certain range, some micron-scale fibers split into branches of nano-scale fibers (thus, the nano-scale fibers and micron-scale fibers appear to be cross-linked). When the shear rate continues to increase, exceeding a certain range, the nano-scale fibers completely split or break from the micron-scale fibers, completely separating from them, thus forming a mixture of micron-scale fibers and nano-scale fibers. Further increasing the shear rate yields single nano-scale fibers.

[0025] Preferably, a suitable polyamic acid viscosity, combined with a suitable shear rate, can yield micro-nano integrated polyimide fiber cotton. In a more preferred shear rate range, dendritic micro-nano integrated polyimide fiber cotton with a cross-linked structure can be obtained.

[0026] Further, in step (S5), the heat treatment is performed at 300-450℃ for 10-100 minutes. The heat treatment is to prepare polyimide by thermal imidization of polyamic acid at high temperature. When polyamic acid fibers are thermally imidized to obtain polyimide fibers, the diameter of the fibers will be slightly thinner, but the microstructure of the dendritic micro-nano cross-linked structure is basically unaffected.

[0027] This invention also provides the use of the micro-nano integrated polyimide fiber cotton prepared by the above method in the preparation of thermal insulation materials, heat insulation materials, and sound insulation materials. For example, its use in the preparation of cold-proof clothing, thermal blankets, sleeping bags, and in sound insulation and noise reduction for automobiles, high-speed trains, and airplanes.

[0028] Compared with the prior art, the present invention achieves the following beneficial effects:

[0029] I. This invention provides a micro / nano integrated polyimide fiber cotton, which differs from uniform nanofiber cotton obtained by electrospinning and uniform micron-sized fiber cotton obtained by conventional methods. It not only possesses the large specific surface area imparted by the nanofiber structure but also the mechanical support provided by the micron-sized fibers, which is beneficial for heat insulation, sound absorption, and thermal insulation. Preferably, by adjusting the shear rate, a micro / nano integrated polyimide fiber cotton with a dendritic structure can be obtained. The dendritic cross-linked network structure can better provide spatial support, helping to maintain its fluffy properties and better retain still air. In this dendritic cross-linked structure, the micron-sized trunk and nano-sized branches are an integrated growth structure, rather than a fused or adhered network formed by post-processing, exhibiting better continuity and network stability, giving the fiber cotton better resilience. The micro / nano integrated polyimide fiber cotton of this invention has a wide range of applications. Due to its good sound absorption, it can be used to prepare sound insulation cotton for noise reduction in automobiles, high-speed trains, and airplanes; due to its good heat insulation and thermal insulation properties, it can be applied in the field of heat insulation, such as in cold-weather clothing, thermal blankets, and sleeping bags.

[0030] Second, the preparation process of this invention adopts high-speed airflow spinning technology, which has high production efficiency and can improve by two orders of magnitude compared with electrospinning technology.

[0031] Third, the high-speed airflow spinning technology of the present invention processes polyamic acid aqueous solution, avoiding the defects of difficult processing and high energy consumption of high-boiling-point aprotic polar organic solvents. Attached Figure Description

[0032] Figure 1 The image shows an SEM image of the polyimide fiber cotton prepared in Example 1, a schematic diagram of the density of micro-nano cross-linking points calculated according to the five-point sampling method, and a diameter distribution diagram of micron and nanofibers in the SEM image.

[0033] Figure 2 The image shows an SEM image of the polyimide fiber cotton prepared in Example 2, a schematic diagram of the density of micro-nano cross-linking points calculated according to the five-point sampling method, and a diameter distribution diagram of micron and nanofibers in the SEM image.

[0034] Figure 3 SEM image and fiber diameter distribution diagram of the polyimide fiber cotton prepared in Example 7;

[0035] Figure 4 SEM images and fiber diameter distribution diagrams of the polyimide fiber cotton prepared in Comparative Example 1 are shown.

[0036] Figure 5 SEM images and fiber diameter distribution diagrams of the polyimide fiber cotton prepared in Comparative Example 2 are shown. Detailed Implementation

[0037] The present invention will now be described in detail with reference to specific embodiments.

[0038] The present invention employs the following testing method:

[0039] Fiber diameter: observed using a scanning electron microscope.

[0040] Micro-nano cross-linking point density: The intersection of micron-scale trunk fibers and nano-scale branch fibers is taken as the micro-nano cross-linking point, and the ratio of the diameter of the micron-scale trunk fiber connected to the cross-linking point to the diameter of the nano-scale branch fiber is ≥2. The number of cross-linking points per unit area in the SEM image is calculated and counted using a five-point sampling method (unit: points / 100μm). 2 The five-point sampling method involves selecting five rectangular regions (6.5 μm × 9.4 μm) at the center and one-quarter and three-quarters of the diagonal of the SEM image. The number of micro-nano cross-linking points in each region is observed, and the average number of points across the five regions is calculated. This average number of points per unit area is the micro-nano cross-linking point density. Following the conventional five-point sampling method in this field, when sampling points at the pressure line, the left side is counted, but the right side is not.

[0041] Specific surface area: Refer to GB / T 19587 Determination of specific surface area of ​​solid substances by gas adsorption BET method.

[0042] Thermal conductivity: Refer to GB / T 10297 Determination of thermal conductivity of non-metallic solid materials by hot wire method.

[0043] Thermal resistance: Thermal resistance is calculated based on thermal conductivity and sample thickness, as shown in the following formula. Where R is the thermal resistance, l is the sample thickness, and k is the thermal conductivity. The higher the thermal resistance, the better the insulation properties of the fiber cotton.

[0044] (1)

[0045] Sound absorption coefficient: Refer to GB / T 18696.2-2002 Measurement of sound absorption coefficient and acoustic impedance in acoustic impedance tubes - Part 2: Transfer function method. The sound absorption coefficient at 6300Hz is used to characterize the sound insulation and noise reduction performance. The larger the sound absorption coefficient, the better the sound insulation and noise reduction performance.

[0046] Viscosity: The viscosity of the solution was measured using a rheometer.

[0047] Intrinsic viscosity: The intrinsic viscosity of the solution is measured using an Ubbelohde viscometer and is determined by extrapolating the specific viscosity at infinite dilution or the logarithmic viscosity at infinite dilution.

[0048] Compression resilience: The recovery rate is tested according to "FZ / T64003-2021 Spray-bonded cotton sheet". The recovery rate is used to characterize the compression resilience. The higher the recovery rate, the better the compression resilience.

[0049] Example 1

[0050] (S1) A polyamic acid solution was prepared by solution polycondensation. 0.08 mol of 4,4'-diaminodiphenyl ether (ODA) and 0.02 mol of p-phenylenediamine (p-PDA) were added to N,N-dimethylacetamide (DMAc) solvent, followed by 0.05 mol of pyromellitic dianhydride (PMDA) and 0.05 mol of biphenyl dianhydride (BPDA). The reaction was continued for 10 h to obtain a polyamic acid solution with a solid content of 14.9% and an intrinsic viscosity of 1.9 dL / g.

[0051] (S2) The polyamic acid solution was precipitated in water by wet spinning, and the precipitate was dried to obtain polyamic acid filament with a diameter of 0.5 mm;

[0052] (S3) The polyamic acid filament is added back into deionized water, and then triethylamine is added. The molar ratio of triethylamine to the carboxyl groups in the polyamic acid filament is 2:1, to obtain a polyamic acid aqueous solution with a viscosity of 20 Pa·s at 25°C.

[0053] (S4) Polyamic acid fiber cotton was obtained by shearing the above aqueous solution with a high-speed airflow. The airflow temperature was 25±2℃. By controlling parameters such as the spinneret diameter, airflow slit width, gas flow rate, and single-hole extrusion speed of the spinning solution (spinneret diameter 0.55mm, airflow slit width 0.15mm, gas flow rate 16L / min, air pressure 0.07MPa, single-hole extrusion speed of the spinning solution 25mL / h), the shear rate was adjusted to 2.1×10⁻⁶. 6 s -1 The receiving distance is 40cm;

[0054] (S5) The polyamic acid fiber cotton obtained in step (S4) is heated to obtain polyimide fiber cotton. The heat treatment temperature is 350℃ and the heat treatment time is 25min.

[0055] Example 2

[0056] The other conditions are the same as in Example 1, except that in step (S1), the diamine monomer is replaced with 0.085 mol of 4,4'-diaminodiphenyl ether (ODA) and 0.015 mol of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid (2,2'-DCB); in step (S3), triethylamine is replaced with triethanolamine, and the molar ratio of triethanolamine to carboxyl groups in the polyamic acid filament is 2:1.

[0057] Example 3

[0058] The other conditions are the same as in Example 1, except that in step (S1), the diamine monomer is replaced with 0.07 mol of 4,4'-diaminodiphenyl ether (ODA) and 0.03 mol of 3,5'-diaminobenzoic acid (3,5'-DABA).

[0059] Example 4

[0060] Other conditions are the same as in Example 3, except that in step (S3), the amount of deionized water added is adjusted so that the viscosity of the polyamic acid aqueous solution at 25°C is 60 Pa·s; and in step (S4), the shear rate is adjusted to 5.4 × 10⁻⁶ by adjusting the air pressure and gas flow rate parameters. 6 s -1 .

[0061] Example 5

[0062] Other conditions are the same as in Example 3, except that in step (S3), the amount of deionized water added is adjusted so that the viscosity of the polyamic acid aqueous solution at 25°C is 20 Pa·s; and in step (S4), the shear rate is adjusted to 6.6 × 10⁻⁶ by adjusting the wind pressure and gas flow rate parameters. 5 s -1 .

[0063] Example 6

[0064] Other conditions are the same as in Example 3, except that in step (S3), the amount of deionized water added is adjusted so that the viscosity of the polyamic acid aqueous solution at 25°C is 80 Pa·s; and in step (S4), the shear rate is adjusted to 9.1 × 10⁻⁶ by adjusting the air pressure and gas flow rate. 6 s -1 .

[0065] Example 7

[0066] Other conditions are the same as in Example 3, except that in step (S4), the shear rate is adjusted to 1.2 × 10⁻⁶ by adjusting the spinneret diameter, airflow slit width, gas flow rate, and single-hole extrusion speed of the spinning solution. 7 s -1 .

[0067] Example 8

[0068] Other conditions are the same as in Example 3, except that in step (S4), the shear rate is adjusted to 3.2 × 10⁻⁶ by adjusting the spinneret diameter, airflow slit width, gas flow rate, and single-hole extrusion speed parameters of the spinning solution. 7 s -1 .

[0069] Comparative Example 1

[0070] Other conditions are the same as in Example 1, except that in step (S4), the shear rate is adjusted to 1.9 × 10⁻⁶ by adjusting the spinneret diameter, airflow slit width, gas flow rate, and single-hole extrusion speed of the spinning solution. 4 s -1 .

[0071] Comparative Example 2

[0072] Other conditions are the same as in Example 1, except that in step (S4), the shear rate is adjusted to 8.5 × 10⁻⁶ by adjusting the spinneret diameter, airflow slit width, gas flow rate, and single-hole extrusion speed of the spinning solution. 7 s -1 .

[0073] Comparative Example 3

[0074] The other conditions are the same as in Example 1, except that in step (S3), the amount of deionized water added is such that the viscosity of the resulting polyamic acid aqueous solution at 25°C is 2 Pa·s. Because the viscosity of the spinning solution is too low, the spinning solution is discontinuous when blown out of the spinneret and cannot form fibers.

[0075] Comparative Example 4

[0076] Other conditions are the same as in Example 1, except that in step (S3), the amount of deionized water added is such that the viscosity of the resulting polyamic acid aqueous solution at 25°C is 100 Pa·s. The spinning solution is extruded at the spinneret in the form of liquid spheres, gradually gathers, and is blown off after reaching a certain size, making it impossible to obtain continuous fiber products.

[0077] Figure 1 and Figure 2 The images show SEM images of the polyimide fiber cotton prepared in Examples 1 and 2, a schematic diagram of the density of micro-nano cross-linking points calculated using the five-point sampling method, and diameter distribution diagrams of micron-fibers and nanofibers. Figure 4 SEM images and fiber diameter distribution diagrams of the polyimide fiber cotton prepared in Comparative Example 1 are shown. Figure 5 SEM images and fiber diameter distribution diagrams of the polyimide fiber cotton prepared in Comparative Example 2 are shown. Following the conditions of Examples 1-6 of this invention, polyimide fiber cotton with a dendritic micro-nano integrated cross-linked network structure can be prepared. The main structure consists of 0.5-5 μm micron-sized fibers as the backbone and 30-499 nm nano-sized fibers as branches, together forming a three-dimensional network structure. To obtain this micro-nano integrated cross-linked network structure fiber cotton, the shear rate in the high-speed airflow spinning process parameters needs to be limited. In Comparative Example 1, the shear rate was too low, resulting in single micron-sized fibers; in Comparative Example 2, the shear rate was too high, resulting in single nano-sized fibers.

[0078] Examples 7 and 8 yielded non-crosslinked micro / nano integrated polyimide fiber cotton, wherein the microstructure of the polyimide fiber cotton obtained in Example 7 is as follows: Figure 3 As shown. The inventors believe that this may be due to the excessively high shear rate, which causes the nanoscale branches splitting from the micron-scale fibers to further split or break, completely separating them from the main trunk.

[0079] The BET specific surface area of ​​the polyimide fiber cotton obtained in Example 1 was 3.89 m². 2 / g, far exceeding the single micron-sized fiber of Comparative Example 1. The inventors unexpectedly discovered that the single nanoscale fiber of Comparative Example 2 had a BET specific surface area of ​​3.30 m². 2 / g, but the BET specific surface area is smaller than that of the micro-nano integrated fiber of Example 1 of the present invention. The inventors believe that the possible reason is that the single nano-fiber is easy to bond and spin due to its small size, so the specific surface area is smaller.

[0080] The properties of the polyimide fiber cotton obtained in the above embodiments and comparative examples were tested as follows, and the results are shown in Table 1. The thermal resistance and sound absorption coefficient were calculated when the areal density of the polyimide fiber cotton was 100 g / m³. 2 Tested under the following conditions.

[0081] Table 1 Performance Tests of Polyimide Fiber Cotton

[0082] .

Claims

1. A micro / nano integrated polyimide fiber cotton, characterized in that, The micro-nano integrated polyimide fiber cotton has a dendritic cross-linked structure, comprising a trunk, branches, and nodes. The trunk consists of micron-sized fibers with a diameter of 0.5-5 μm, and the branches consist of nano-sized fibers with a diameter of 30-499 nm. The diameter of the micron-sized fibers is more than twice that of the nano-sized fibers. The nodes of the trunk and branches serve as cross-linking points. The density of micro-nano integrated polyimide fiber cotton across micro-nano cross-linking points is 1-15 per 100 μm. 2 The specific surface area of ​​the micro-nano integrated polyimide fiber cotton is not less than 2 m². 2 / g.

2. The micro-nano integrated polyimide fiber cotton according to claim 1, characterized in that, The density of micro-nano integrated polyimide fiber cotton is 3-10 cross-linking points per 100μm. 2 .

3. The micro-nano integrated polyimide fiber cotton according to claim 1, characterized in that, The main stem diameter of the micro-nano integrated polyimide fiber cotton is 0.5-3.5μm, the branch diameter is 35-400nm, and the specific surface area is 2.5-10m². 2 / g.

4. The micro-nano integrated polyimide fiber cotton according to claim 3, characterized in that, The specific surface area of ​​the micro-nano integrated polyimide fiber cotton is 3-5 m². 2 / g.

5. The method for preparing the micro-nano integrated polyimide fiber cotton according to any one of claims 1-4, characterized in that, Includes the following steps: (S1) Add dianhydride monomer and diamine monomer to an aprotic polar organic solvent and prepare polyamic acid solution by solution polycondensation. (S2) Mix the polyamic acid solution with water, dry the precipitate, and obtain polyamic acid filaments; (S3) Add polyamic acid filament to water, add organic amine weak base to obtain polyamic acid aqueous solution with viscosity of 10-80 Pa·s at 25℃; (S4) Polyamic acid aqueous solution was blown out of the spinneret using high-speed airflow spinning technology to obtain micro-nano integrated polyamic acid fibers; the shear rate of high-speed airflow spinning was 5.0 × 10⁻⁶. 4 ~5.0×10 7 s -1 ; (S5) Micro-nano integrated polyamic acid fiber is heat-treated to obtain micro-nano integrated polyimide fiber cotton.

6. The preparation method according to claim 5, characterized in that, In step (S1), the dianhydride monomer is selected from at least one of pyromellitic dianhydride (PMDA), diphenyl ether tetracarboxylic dianhydride (ODPA), biphenyl tetracarboxylic dianhydride (BPDA), and 3,3',4,4'-triphenyl diether tetracarboxylic dianhydride (HQDPA); the diamine monomer is selected from at least one of 4,4'-diaminodiphenyl ether (ODA), 3,4'-diaminodiphenyl ether (3,4'-ODA), p-phenylenediamine (p-PDA), m-phenylenediamine (m-PDA), 2-(4-aminophenyl)-5-aminobenzimidazole (BIA), 3,5'-diaminobenzoic acid (3,5'-DABA), and 4,4-diaminobiphenyl-2,2'-dicarboxylic acid (2,2'-DCB).

7. The preparation method according to claim 5, characterized in that, The molar ratio of dianhydride monomer to diamine monomer is 0.8-1.2:0.8-1.

2.

8. The preparation method according to claim 5, characterized in that, The molar ratio of dianhydride monomer to diamine monomer is 1:0.9-1.

1.

9. The preparation method according to claim 5, characterized in that, The molar ratio of dianhydride monomer to diamine monomer is 1:0.95-1.

05.

10. The preparation method according to claim 5, characterized in that, The diamine monomer contains 15-30 mol% of a carboxyl-containing diamine monomer, wherein the carboxyl-containing diamine monomer is selected from at least one of 3,5'-diaminobenzoic acid (3,5'-DABA) and 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid (2,2'-DCB).

11. The preparation method according to claim 5, characterized in that, In step (S1), the intrinsic viscosity of the obtained polyamic acid is 1.1-2.8 dL / g.

12. The preparation method according to claim 5, characterized in that, In step (S1), the intrinsic viscosity of the obtained polyamic acid is 1.5-2.1 dL / g.

13. The preparation method according to claim 5, characterized in that, In step (S1), the aprotic polar organic solvent is selected from at least one of N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), and N-methylpyrrolidone (NMP); the solution polycondensation conditions in step (S1) are 0-40°C and the reaction time is 3-15 h; and / or In step (S2), mixing the polyamic acid solution and water involves slowly extruding the polyamic acid solution in the form of fine filaments or pouring it into water, where it solidifies to form polyamic acid filaments; or immersing the spinneret in water, then extruding the polyamic acid solution through the spinneret in the form of filaments, which directly solidify upon contact with water, and obtaining polyamic acid filaments through drawing; and / or In step (S3), the water is at least one of distilled water, deionized water, and ultrapure water; the organic amine weak base is selected from triethylamine, triethanolamine, diethanolamine, N,N-dimethylethanolamine, pyridine, morpholine, and N-methylmorpholine; the molar ratio of the nitrogen-containing group in the organic weak base to the carboxyl group in the polyamic acid is 1-3:1; the amount of water added is such that the viscosity of the polyamic acid aqueous solution is 20-60 Pa·s at 25°C; and / or In step (S4), the shear rate of high-speed airflow spinning is 1.0 × 10⁻⁶. 5 -5.0×10 7 s -1 ; and / or In step (S5), the heat treatment is performed at 300-450℃ for 10-100 minutes.

14. The preparation method according to claim 13, characterized in that, In step (S1), the amount of aprotic polar organic solvent used is 3-8 times the total mass of the dianhydride monomer and the diamine monomer.

15. The preparation method according to claim 13, characterized in that, The solution polycondensation conditions for step (S1) are 10-30℃ and the reaction time is 5-10h.

16. The preparation method according to claim 13, characterized in that, In step (S2), the diameter of the obtained polyamic acid filament is 0.1-2 mm.

17. The preparation method according to claim 13, characterized in that, In step (S3), the molar ratio of nitrogen-containing groups in the organic weak base to carboxyl groups in the polyamic acid is 1.5-2.5:

1.

18. The preparation method according to claim 13, characterized in that, In step (S4), the high-speed airflow spinning spinneret process parameters are as follows: airflow slit width 0.05-0.30 mm, gas flow rate 1-80 L / min, air pressure 0.01-0.4 MPa, airflow temperature 10-50℃, spinneret diameter 0.4-1.0 mm, single-hole extrusion speed of spinning solution 2-80 mL / h, and fiber receiving distance 25-50 cm.

19. The preparation method according to claim 13, characterized in that, In step (S4), the shear rate of high-speed airflow spinning is 1.2 × 10⁻⁶. 5 -9.1×10 6 s -1 .

20. The preparation method according to claim 13, characterized in that, In step (S4), the shear rate of high-speed airflow spinning is 6.6 × 10⁻⁶. 5 -5.4×10 6 s -1 .

21. The use of the micro-nano integrated polyimide fiber cotton according to any one of claims 1-4, or the micro-nano integrated polyimide fiber cotton prepared by the preparation method according to any one of claims 5-20, in the preparation of thermal insulation materials, heat preservation materials, and sound insulation materials.

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