High-strength and high-temperature-resistant ultrathin PBO nanofiber membrane as well as preparation method and application of high-strength and high-temperature-resistant ultrathin PBO nanofiber membrane

Abstract

The invention belongs to the technical field of nanofiber thin film materials, and relates to a high-strength and high-temperature-resistant ultra-thin PBO nanofiber membrane as well as a preparation method and application of the high-strength and high-temperature-resistant ultra-thin PBO nanofiber membrane. The prepared ultra-light and ultra-thin PBO nanofiber membrane comprises a three-dimensional interconnected network structure formed by PBO nanofibers, and the nanofibers are crosslinked through pi-pi interaction; the three-dimensional interconnected network structure is compressed and densified at room temperature to form a layer-by-layer stacked layered structure, the thickness of the ultra-light and ultra-thin PBO nanofiber membrane is 10-50 microns, the tensile strength is greater than or equal to 120 MPa, and the thermal decomposition temperature is greater than or equal to 650 DEG C; the preparation process provided by the invention is simple, does not involve high-temperature treatment and is low in energy consumption, specifically, a uniform aerogel intermediate is formed through freeze drying, and then room-temperature compression densification is combined, so that the problems of fiber segregation and interface stress concentration caused by high-temperature treatment are avoided, and the prepared PBO nanofiber film is light in weight, high in strength, flame-retardant and high-temperature-resistant, and has good application prospects. The application prospect is wide.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanofiber film materials, and particularly relates to an ultra-thin, high-strength and high-temperature-resistant PBO nanofiber membrane, and a preparation method and application thereof. Background Art

[0002] Disclosing the information of this background art section is only intended to enhance the understanding of the overall background of the present invention, and is not necessarily regarded as an admission or an implication in any form that this information constitutes the prior art already known to those of ordinary skill in the art.

[0003] Due to the excellent mechanical properties, high thermal stability and chemical corrosion resistance endued by the rigid molecular chain structure, poly(p-phenylene benzobisoxazole) (PBO) fiber has become a key candidate material in the fields of aerospace thermal protection systems, high-grade bulletproof composite materials and special flame retardant materials.

[0004] Currently, the preparation systems of PBO nanofiber membranes are mainly based on electrospinning method and proton dissociation-thermocompression method.

[0005] (1) The electrospinning method can prepare PBO solution into nanoscale fibers. However, in the spinning process of this technology, there are problems such as too high molecular weight resulting in too high viscosity of the spinning solution, difficult volatilization of strong acid solvent during film formation, uneven fiber morphology and extremely low strength of the fiber membrane, which greatly increases the difficulty of obtaining PBO nanofiber membrane by electrospinning.

[0006] (2) The films prepared by the proton dissociation-thermocompression method usually show high strength and increase the possibility of large-scale preparation. However, during the deprotonation process, the non-equilibrium phase transition caused by the reagent concentration gradient leads to anisotropic shrinkage of the gel film during the solvent replacement stage, resulting in the collapse of the three-dimensional network structure and inducing local agglomeration of nanofibers, and a uniform three-dimensional network structure cannot be formed. At the same time, the rapid water escape during the thermocompression stage causes the instability of the gas-liquid-solid three-phase interface, resulting in fiber orientation segregation and stress concentration effects, and finally leading to the generation of microcracks and early fracture of the film. The above problems essentially stem from the failure of the coordinated regulation of the multi-scale structure (molecular orientation-nanoassembly-macroscopic interface) of the material.

[0007] Therefore, developing a new preparation strategy to realize the topological structure directional transformation from a gel-state uniform three-dimensional network structure to a high-strength and high-density film is still an urgent problem to be solved. Summary of the Invention

[0008] In view of the deficiencies of the prior art, the present invention provides a super-thin PBO nanofiber membrane with high strength and high temperature resistance, and its preparation method and application. The present invention proposes a room-temperature densification strategy based on the regulation of a three-dimensional network structure, constructs an aerogel intermediate with a three-dimensional network topological structure with uniform pores, and utilizes the elastic deformation characteristics of nanofibers to achieve high-density recombination under room-temperature compression, and densifies into a PBO nanofiber thin film with a smooth surface and high strength. That is, through the directional transformation of the aerogel intermediate structure, the uniform densification stacking of nanofibers is realized, thus avoiding the problem of structural deterioration caused by high-temperature treatment; at the same time, the screening and proportion optimization of the deprotonating reagent solve the problem of uneven gel network in the traditional process, and are applicable to high-value-added fields such as aerospace thermal protection and flexible electronic devices, expanding the application scenarios of PBO materials.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] In the first aspect of the present invention, there is provided a super-thin PBO nanofiber membrane with high strength and high temperature resistance. The super-thin PBO nanofiber membrane includes a three-dimensional interconnected network structure formed by PBO nanofibers, and the nanofibers are crosslinked through π-π interactions; the three-dimensional interconnected network structure is densified by room-temperature compression to form a layered structure stacked layer by layer; the thickness of the super-thin PBO nanofiber membrane is 10-50 μm, the tensile strength ≥ 120 MPa, and the thermal decomposition temperature ≥ 650 °C.

[0011] Preferably, the average diameter of the PBO nanofibers is 24 ± 3 nm, and the aspect ratio is (1.1-1.4)×10 5 .

[0012] Preferably, the toughness of the super-thin PBO nanofiber membrane ≥ 12 MJ / m3, and the areal density is 10-30 g / m 2 .

[0013] In the second aspect of the present invention, there is provided a preparation method of the above-mentioned super-thin PBO nanofiber membrane with high strength and high temperature resistance, including the following steps:

[0014] S1. Mix and react the pretreated PBO nanofibers with trifluoroacetic acid and methanesulfonic acid to obtain a PBO nanofiber acid solution;

[0015] S2. Add a deprotonating reagent to the PBO nanofiber acid solution for reaction, replace the obtained acidic gel with deionized water and an organic solvent, and freeze-dry the obtained wet gel to obtain a nanofiber aerogel intermediate;

[0016] S3. Compress the nanofiber aerogel intermediate to obtain a PBO nanofiber thin film.

[0017] Preferably, in step S1, the specific method of the pretreatment is as follows: soak the PBO nanofibers in absolute ethanol, and obtain them after ultrasonic washing and vacuum drying.

[0018] Preferably, in step S1, the mass ratio of the PBO nanofibers, trifluoroacetic acid, and methanesulfonic acid is 0.1:49.95:49.95 to 1:49.5:49.5.

[0019] Preferably, in step S1, the method of the mixed reaction is mechanical stirring in a sealed environment, with a rotation speed of 500 - 1000 rpm; the dissociation time is 48 - 72 h; the mass concentration of the PBO nanofiber acid solution is 0.1 - 1 wt.%.

[0020] Preferably, in step S2, the mass ratio of the PBO nanofibers to the deprotonating agent is 1:1 to 5.

[0021] Preferably, in step S2, the reaction time is 2 - 5 h. Pour the PBO nanofiber acid sol after the reaction into a petri dish with a diameter of 40 - 60 mm, and gel it for 12 - 24 h in a low-temperature sealed environment to obtain an acidic gel.

[0022] Preferably, in step S2, the acid gel is subjected to solvent exchange with deionized water, ethanol, or isopropanol in turn, and the replacement is carried out every 2 - 4 h until the pH is close to 7; freeze-dry the gel after solvent exchange at -30 to -50 °C for 24 - 48 h to obtain an aerogel intermediate.

[0023] Preferably, in step S2, the deprotonating agent is selected from one or more of sodium sulfate, ferric sulfate, magnesium sulfate, sodium carbonate, and aluminum chloride.

[0024] Preferably, in step S3, press the freeze-dried aerogel at 20 - 30 °C and a pressure of 1 - 10 MPa for 30 - 120 min to obtain the PBO nanofiber film.

[0025] In the third aspect of the present invention, there is provided an application of the high-strength, high-temperature-resistant, ultra-thin PBO nanofiber film described in the first aspect in aerospace, flexible electronic devices, or flame-retardant materials.

[0026] The beneficial effects obtained by one or more of the above technical solutions of the present invention are as follows:

[0027] (1) The preparation of traditional PBO nanofiber membranes mostly relies on high-temperature hot pressing or complex solvent replacement processes (such as problems like gel shrinkage caused by traditional direct water / alcohol solvent replacement and complex process flows of gradient solvent replacement). In contrast, the present invention forms a uniform aerogel intermediate through freeze-drying and then combines room-temperature compression densification, avoiding the problems of fiber segregation and interfacial stress concentration caused by high-temperature treatment. This process combination not only simplifies the process but also preserves the three-dimensional network integrity of the nanofibers through structure setting at low temperature, thereby significantly improving the mechanical properties of the film.

[0028] (2) In the prior art, the deprotonation process often causes non-equilibrium phase transitions due to concentration gradients, leading to structural collapse. The present invention screens inorganic salts such as sodium sulfate as deprotonating reagents by regulating the mass ratio of PBO nanofibers to deprotonating reagents, effectively shielding the surface charges of the fibers, enhancing π-π interactions, and forming a uniform three-dimensional interconnected network gel. This regulation strategy solves the problem of anisotropic shrinkage caused by traditional solvent replacement, avoids fiber agglomeration, and provides a structural basis for subsequent densification.

[0029] (3) The present invention dissociates micron-scale PBO fibers into nanofibers successfully under a mixed acid system of methanesulfonic acid and trifluoroacetic acid, and the concentration of the acid sol is reasonably controlled, avoiding the viscosity problem of the traditional electrospinning method; by combining solvent replacement, it effectively reduces the damage of capillary stress to the aerogel structure and preserves the intermediate with a uniform three-dimensional topological structure. Finally, high-density recombination is achieved through the elastic deformation of the nanofibers. The final film has the characteristics of light weight (thickness 10 - 50 μm), high strength (>120 MPa), and high-temperature resistance (decomposition temperature >650°C).

[0030] (4) Compared with traditional methods (such as the tensile strength in Comparative Example 1 being only 79.2 MPa), the tensile strength of the PBO nanofiber membrane prepared by the present invention can be increased to 161.8 MPa, the toughness reaches 22.4 MJ / m3, and the thermal decomposition temperature reaches 652.7°C. Moreover, the whole process does not require high-temperature treatment (the traditional hot pressing method requires 200 - 300°C), with low energy consumption and avoiding structural deterioration caused by high temperature, meeting the trend of green manufacturing; the thin, flexible, and flame-retardant characteristics of the film (no melting droplets during combustion) make it suitable for high-value-added fields such as aerospace thermal protection and flexible electronic devices, expanding the application scenarios of PBO materials.

[0031] (5) In the prior art, deacidification treatment is carried out by acid volatilization, and finally hot pressing is directly performed to prepare the film. This process is prone to secondary pollution due to acid volatilization. Moreover, after acid volatilization, the fiber surface still carries a large amount of positive charges (protonated state), which is not conducive to the generation of a three-dimensional cross-linked network. The migration rates of charged fibers in polar solvents vary greatly, forming a density gradient, and stress cracks are likely to occur after drying. However, the deprotonating reagent of the present invention effectively neutralizes the charges on the fiber surface. The neutral fibers have a consistent diffusion rate in the solvent, and the gel network is more uniform after replacement. At the same time, the present invention can moderately compact the film by performing room-temperature compression after freeze-drying on the premise of avoiding thermal damage, enhancing the mechanical interlocking between fibers, rather than relying on strong chemical bonds, and can improve the dimensional stability at high temperatures. In addition, the concentration of the mixed acid solution added in the present invention is relatively high. If the concentration is low, potential problems such as low fiber decomposition efficiency and incomplete decomposition and peeling may occur. The incompletely peeled fiber fragments may become stress concentration points, reducing the overall strength of the film. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention.

[0033] Figure 1 It is the internal SEM morphology diagram of the PBO nanofiber aerogel intermediate prepared in Comparative Example 1 and Examples 1 to 3 of the present invention; wherein, (a) is the internal SEM morphology diagram of the intermediate in Comparative Example 1; (b) is the internal SEM morphology diagram of the intermediate in Example 1; (c) is the internal SEM morphology diagram of the intermediate in Example 2; (d) is the internal SEM morphology diagram of the intermediate in Example 3;

[0034] Figure 2 It is the cross-sectional SEM morphology diagram of the PBO nanofiber film prepared in Comparative Example 1 and Examples 1 to 3 of the present invention; wherein, (a) is the cross-sectional SEM image of the film in Comparative Example 1; (b) is the cross-sectional SEM image of the film in Example 1; (c) is the cross-sectional SEM image of the film in Example 2; (d) is the cross-sectional SEM image of the film in Example 3;

[0035] Figure 3 It is the strain curve (a) and toughness (b) of the PBO nanofiber film prepared in Examples 1 to 3 and Comparative Examples 1 to 4 of the present invention;

[0036] Figure 4 It is the thermogravimetric curve of the PBO nanofiber film prepared in Examples 1 to 3 and Comparative Examples 1 to 4 of the present invention;

[0037] Figure 5Pictures of the combustion tests of the PBO nanofiber films prepared in Comparative Example 1 and Examples 1 to 3 of the present invention. Among them, (a) is the combustion test diagram of the film in Comparative Example 1; (b) is the combustion test diagram of the film in Example 1; (c) is the combustion test diagram of the film in Example 2; (d) is the combustion test diagram of the film in Example 3. Detailed implementation manners

[0038] It should be noted that the following detailed descriptions are all exemplary and are intended to provide further explanations of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0039] It should be noted that the raw material PBO fiber (short-cut PBO fiber) involved in the present invention was purchased from Zhongke Jinqi Co., Ltd., with the model JQDQ-3, the specification of 3 mm, and the linear density of 1.6 to 3.3 dtex;

[0040] TFA (trifluoroacetic acid) was purchased from Macklin Biochemical Technology Co., Ltd., Cas No.: 76-05-1, and the specification: 99%;

[0041] MSA (methanesulfonic acid) was purchased from Macklin Biochemical Technology Co., Ltd., Cas No.: 76-75-2, and the specification: 99%.

[0042] The following is a further detailed description of the present invention in combination with specific examples. It should be noted that the specific examples are explanations rather than limitations of the present invention.

[0043] Example 1 : This example provides a super-thin PBO nanofiber membrane with high strength and high temperature resistance and its preparation method

[0044] (1) Immerse the PBO fiber in clean anhydrous ethanol and ultrasonically clean it to remove organic and inorganic impurities on the fiber surface. Subsequently, put the cleaned fiber into a vacuum drying oven for drying for later use.

[0045] (2) Add 0.3 g of short-cut PBO fiber into a mixed acid solution of 49.85 g of TFA (trifluoroacetic acid) and 49.85 g of MSA (methanesulfonic acid), and mechanically stir for 24 h in a sealed environment at room temperature to obtain a uniform dark yellow 0.3 wt.% PBO nanofiber acid solution.

[0046] (3) Add 0.3 g of Na2SO4 to the above acid solution, seal and stir for 2 h until Na2SO4 is completely dissolved. Then, pour 13.3 g of the PBO nanofiber acid sol into a petri dish with a diameter of 50 mm and incubate at 5 °C for 30 h to obtain an acidic gel.

[0047] (4) First, replace the acid with deionized water solvent once every 2 h until the pH approaches 7, and then replace it with isopropanol solvent twice.

[0048] (5) Freeze-dry the wet gel after the above solvent replacement at -50 °C for 24 h to obtain a nanofiber aerogel intermediate.

[0049] (6) At room temperature, compress the above intermediate at a pressure of 5 MPa for 60 min to obtain a brownish-yellow PBO nanofiber film. The thickness of the film is about 24 μm, and the areal density is 20.4 g / cm 2 .

[0050] Example 2 : This example provides a super-thin PBO nanofiber film with high strength and high temperature resistance and its preparation method.

[0051] (1) Immerse the PBO fibers in clean anhydrous ethanol and ultrasonically clean to remove organic and inorganic impurities on the fiber surface. Subsequently, put the cleaned fibers into a vacuum drying oven and dry for later use.

[0052] (2) Add 0.5 g of chopped PBO fibers to a mixed acid solution of 49.75 g of TFA (trifluoroacetic acid) and 49.75 g of MSA (methanesulfonic acid), and mechanically stir for 54 h in a sealed environment at room temperature to obtain a uniform dark yellow 0.5 wt.% PBO nanofiber acid solution.

[0053] (3) Add 1.5 g of Na2SO4 to the above acid solution, seal and stir for 3 h until Na2SO4 is completely dissolved. Then, pour 8 g of the PBO nanofiber acid sol into a petri dish with a diameter of 50 mm and culture at 5 °C for 24 h to obtain an acidic gel.

[0054] (4) First, replace the acid with deionized water solvent once every 2 h until the pH approaches 7, and then replace it with isopropanol solvent twice.

[0055] (5) Freeze-dry the wet gel after the above solvent replacement at -50 °C for 24 h to obtain a nanofiber aerogel intermediate.

[0056] (6) At room temperature, compress the above intermediate at a pressure of 5 MPa for 60 min to obtain a brownish-yellow PBO nanofiber film. The thickness of the film is about 23 μm, and the areal density is 20.4 g / cm 2 .

[0057] Example 3 : This example provides a super-thin PBO nanofiber film with high strength and high temperature resistance and its preparation method

[0058] (1) Immerse the PBO fibers in clean anhydrous ethanol and ultrasonically clean them to remove organic and inorganic impurities on the fiber surface. Subsequently, put the cleaned fibers into a vacuum drying oven for drying and reserve.

[0059] (2) Add 0.7 g of chopped PBO fibers to a mixed acid solution of 49.65 g of TFA (trifluoroacetic acid) and 49.65 g of MSA (methanesulfonic acid), and mechanically stir for 72 h in a sealed environment at room temperature to obtain a uniform dark yellow 0.7 wt.% PBO nanofiber acid solution.

[0060] (3) Add 3.5 g of Na2SO4 to the above acid solution, seal and stir for 4 h until Na2SO4 is completely dissolved. Then, pour 5.7 g of the PBO nanofiber acid sol into a petri dish with a diameter of 50 mm and incubate at 5 °C for 20 h to obtain an acidic gel.

[0061] (4) First, use deionized water solvent to displace the acid, replacing it every 2 h until the pH is close to 7, and then use isopropanol solvent to displace it 2 times.

[0062] (5) Freeze-dry the wet gel after the above solvent displacement at -50 °C for 24 h to obtain a nanofiber aerogel intermediate.

[0063] (6) At room temperature, compress the above intermediate at a pressure of 5 MPa for 60 min to obtain a brownish-yellow PBO nanofiber film. The thickness of this film is about 22 μm and the areal density is 20.4 g / cm 2 。

[0064] Comparative Example 1 :This comparative example provides a PBO nanofiber membrane and its preparation method

[0065] (1) Immerse the PBO fibers in clean anhydrous ethanol and ultrasonically clean them to remove organic and inorganic impurities on the fiber surface. Subsequently, put the cleaned fibers into a vacuum drying oven for drying and reserve.

[0066] (2) Add 1 g of chopped PBO fibers to a mixed acid solution of 99.5 g of TFA (trifluoroacetic acid) and 99.5 g of MSA (methanesulfonic acid), and mechanically stir for 72 h in a sealed environment at room temperature to obtain a uniform dark yellow 0.5 wt.% PBO nanofiber acid solution.

[0067] (3) Pour 8 g of the PBO nanofiber acid sol into a petri dish with a diameter of 50 mm and incubate at 5 °C for 30 h to obtain an acidic gel.

[0068] (4) First, use deionized water solvent to displace the acid, replacing it every 2 h until the pH is close to 7, and then use isopropanol solvent to displace it 2 times.

[0069] (5) Freeze-dry the wet gel after the above solvent replacement at -50 °C for 24 h to obtain a nanofiber aerogel intermediate.

[0070] (6) At room temperature, compress the above intermediate under a pressure of 5 MPa for 60 min to obtain a brownish-yellow PBO nanofiber film. The thickness of the film is about 23 μm, and the areal density is 20.4 g / m 2 .

[0071] Comparative Example 2 : This comparative example provides a PBO nanofiber membrane and its preparation method

[0072] (1) Immerse the PBO fibers in clean anhydrous ethanol and ultrasonically clean to remove organic and inorganic impurities on the fiber surface. Subsequently, place the cleaned fibers in a vacuum drying oven and dry for later use.

[0073] (2) Add 0.7 g of chopped PBO fibers to a mixed acid solution of 49.65 g of TFA (trifluoroacetic acid) and 49.65 g of MSA (methanesulfonic acid), and mechanically stir for 72 h in a sealed environment at room temperature to obtain a uniform dark yellow 0.7 wt.% PBO nanofiber acid solution.

[0074] (3) Add 4.9 g of Na2SO4 to the above acid solution, seal and stir for 5 h until Na2SO4 is completely dissolved. Then, pour 5.7 g of the PBO nanofiber acid sol into a petri dish with a diameter of 50 mm and incubate at 5 °C for 20 h to obtain an acidic gel.

[0075] (4) First, replace the acid with deionized water solvent, replacing it every 2 h until the pH is close to 7, and then replace it with isopropanol solvent twice.

[0076] (5) Freeze-dry the wet gel after the above solvent replacement at -50 °C for 24 h to obtain a nanofiber aerogel intermediate.

[0077] (6) At room temperature, compress the above intermediate under a pressure of 5 MPa for 60 min to obtain a brownish-yellow PBO nanofiber film. The thickness of the film is about 22 μm, and the areal density is 20.4 g / cm 2 .

[0078] Comparative Example 3 : This comparative example provides a PBO nanofiber membrane and its preparation method

[0079] (1) Immerse the PBO fibers in clean anhydrous ethanol and ultrasonically clean to remove organic and inorganic impurities on the fiber surface. Subsequently, place the cleaned fibers in a vacuum drying oven and dry for later use.

[0080] (2) Add 0.4 g of chopped PBO fibers to a mixed acid solution of 49.8 g of TFA (trifluoroacetic acid) and 49.8 g of MSA (methanesulfonic acid), and mechanically stir for 24 h in a sealed environment at room temperature to obtain a uniform dark yellow 0.4 wt.% PBO nanofiber acid solution.

[0081] (3) Add 0.2 g of Na2SO4 to the above acid solution, seal and stir for 1 h until Na2SO4 is completely dissolved. Then, pour 10 g of the PBO nanofiber acid sol into a petri dish with a diameter of 50 mm and incubate at 5 °C for 28 h to obtain an acidic gel.

[0082] (4) First, replace the acid with deionized water solvent every 2 h until the pH is close to 7, and then replace it with isopropanol solvent twice.

[0083] (5) Freeze-dry the wet gel after the above solvent replacement at -50 °C for 24 h to obtain a nanofiber aerogel intermediate.

[0084] (6) At room temperature, compress the above intermediate at a pressure of 5 MPa for 60 min to obtain a brownish-yellow PBO nanofiber film. The thickness of the film is about 23 μm, and the areal density is 20.4 g / cm 2 .

[0085] Comparative Example 4 : This comparative example provides a PBO nanofiber membrane and a preparation method thereof

[0086] (1) Immerse the PBO fibers in clean anhydrous ethanol and ultrasonically clean to remove organic and inorganic impurities on the fiber surface. Subsequently, put the cleaned fibers into a vacuum drying oven and dry for later use.

[0087] (2) Add 0.5 g of chopped PBO fibers to a mixed acid solution of 49.75 g of TFA (trifluoroacetic acid) and 49.75 g of MSA (methanesulfonic acid), and mechanically stir for 54 h in a sealed environment at room temperature to obtain a uniform dark yellow 0.5 wt.% PBO nanofiber acid solution.

[0088] (3) Add 1.5 g of Na2SO4 to the above acid solution, seal and stir for 3 h until Na2SO4 is completely dissolved. Then, pour 8 g of the PBO nanofiber acid sol into a petri dish with a diameter of 50 mm and incubate at 5 °C for 24 h to obtain an acidic gel.

[0089] (4) First, replace the acid with deionized water solvent every 2 h until the pH is close to 7, and then replace it with isopropanol solvent twice.

[0090] (5) The wet gel after the above solvent replacement was freeze-dried at -50°C for 24 h to obtain a nanofiber aerogel intermediate.

[0091] Experimental Example 1 : In this example, the PBO nanofiber membranes prepared in Examples 1 to 3 and Comparative Example 1 were subjected to structural characterization tests.

[0092] Figure 1 (a)-(d) are internal SEM micrographs of the PBO nanofiber intermediates prepared in Comparative Example 1 and Examples 1 to 3 of the present invention; Figure 2 (a)-(d) are cross-sectional SEM micrographs of the PBO nanofiber thin films prepared in Comparative Example 1 and Examples 1 to 3 of the present invention;

[0093] It can be seen from the figure that compared with the fibers in Comparative Example 1, which show significant aggregation and adhesion due to strong electrostatic repulsion, the nanofiber network of the gel prepared in Example 2 is the most uniform, without obvious fiber aggregation, effectively preventing excessive fiber aggregation. At the same time, the cross-section of the thin film shows a structure of dense stacking layer by layer.

[0094] Test Example 1 : In this test example, the PBO nanofiber membranes prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were subjected to mechanical property and heat resistance tests.

[0095] (1) Determination of mechanical properties

[0096] Experimental process: The mechanical properties of the samples were tested by a universal tensile testing machine. The samples were cut into long test strips of 20 mm × 5 mm, and the tensile speed was 0.5 mm / min.

[0097] Figure 3 are the stress-strain curves (a) and toughness (b) of the PBO nanofiber thin films prepared in Comparative Examples 1 to 5 and Examples 1 to 4 of the present invention; it can be seen that the thin films prepared in Examples 1 to 3 of the present invention all have a tensile strength greater than 120 MPa, and the toughness is 15.3 MJ / m 3 , especially the thin film of Example 2 has the highest tensile strength of 161.8 MPa and the toughness is 22.4 MJ / m 3 , while the tensile strength of Comparative Example 4 is only 6.1 MPa, and the toughness is extremely low, only 0.03 MJ / m 3 , and the specific test data are shown in Table 1.

[0098] (2) Determination of heat resistance and flame retardancy

[0099] Experimental procedure: Thermogravimetric analysis was carried out at a heating rate of 10 °C / min in an N2 atmosphere from 30 to 800 °C to evaluate the thermal stability and content of the samples. The flame retardancy was evaluated by clamping the film with tweezers and observing the morphological changes of the film before and after combustion after continuously burning it with a lit alcohol lamp under the film for 30 s.

[0100] Figure 4 Figures 1(a)-(d) are the thermogravimetric curves of the PBO nanofiber films prepared in Comparative Examples 1-3 and Examples 1-4 of the present invention. The films all showed good temperature tolerance. The thermal decomposition temperatures of the films prepared in Examples 1-3 were all higher than 650 °C, and the residual weights were all as high as 72%. The specific test data are shown in Table 1.

[0101] Figure 5 (a)-(d) are the optical images of the PBO nanofiber films prepared in Comparative Example 1 and Examples 1-3 of the present invention after the combustion test. The films did not have open flames during combustion and did not produce molten droplets, showing good flame retardancy.

[0102] Table 1 Performance test results of the films obtained in Examples 1-3 and Comparative Examples 1-4

[0103] Number Tensile Strength (MPa) <![CDATA[Toughness (MJ / m 3 )]]> Thermal Decomposition Temperature (°C) Example 1 124.1 15.3 650.1 Example 2 161.8 22.4 652.7 Example 3 145.4 12.0 653.2 Comparative Example 1 79.2 6.3 639.8 Comparative Example 2 98.35 10.8 647.3 Comparative Example 3 113.1 9.8 638.3 Comparative Example 4 6.1 0.03 646.2

[0104] As shown in Table 1, compared with Examples 1-3, in Comparative Example 1, due to the absence of a deprotonating agent, the unneutralized positive charges on the surface of PNF generated a strong electrostatic barrier, hindering the effective cross-linking of the three-dimensional network. During the solvent replacement process, the directional rearrangement of molecular chains was blocked, inducing the collapse of the macroscopic structure. The amount of chopped PBO fibers in Comparative Examples 2-3 changed with the addition amount of the deprotonating agent: when sodium sulfate was in excess, the super-threshold charge screening effect almost completely eliminated the Coulomb repulsion, and the overload of the π-π stacking interaction between molecular chains caused fiber adhesion. While a small amount of deprotonating agent only neutralized part of the charges, and the residual repulsion made the molecular chains adopt an extended conformation. When the osmotic pressure was insufficient, the hydration layer was destroyed, leading to dehydration shrinkage. The network underwent volume collapse under the capillary force due to the lack of critical cross-linking density, and local stress cracking was likely to occur after densification. In Comparative Example 4, the room-temperature compression process was not carried out, and its three-dimensional porous network structure was connected by van der Waals forces between loosely packed PBO nanofibers, contributing limitedly to the mechanical properties.

[0105] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A super-thin PBO nanofiber membrane with high strength and high temperature resistance, characterized in that, The ultra-thin PBO nanofiber membrane includes a three-dimensional interconnected network structure formed by PBO nanofibers, and the nanofibers are crosslinked by π-π interactions; The three-dimensional interconnected network structure is compressed and densified at room temperature to form a layered structure stacked layer by layer; the thickness of the ultra-thin PBO nanofiber membrane is 10-50 μm, the tensile strength ≥ 120 MPa, and the thermal decomposition temperature ≥ 650 °C.

2. The high-strength, high-temperature resistant ultra-thin PBO nanofiber membrane according to claim 1, characterized in that, The average diameter of the PBO nanofibers is 24 ± 3 nm, and the aspect ratio is (1.1 - 1.4) × 10 5 .

3. The high-strength, high-temperature resistant ultra-thin PBO nanofiber membrane according to claim 1, characterized in that, The toughness of the ultra-thin PBO nanofiber membrane ≥ 12 MJ / m3, and the areal density is 10-30 g / m2.

4. A method for preparing a high-strength, high-temperature resistant ultra-thin PBO nanofiber membrane according to any one of claims 1 to 3, characterized in that, It includes the following steps: S1. Mix and react the pretreated PBO nanofibers with trifluoroacetic acid and methanesulfonic acid to obtain a PBO nanofiber acid solution; S2. Add a deprotonating reagent to the PBO nanofiber acid solution for reaction. The obtained acidic gel is replaced with deionized water and an organic solvent, and the obtained wet gel is freeze-dried to obtain a nanofiber aerogel intermediate; S3. Compress the nanofiber aerogel intermediate to obtain a PBO nanofiber film.

5. The preparation method according to claim 4, characterized in that, In step S1, the specific method of the pretreatment is: soak the PBO nanofibers in absolute ethanol, wash them by ultrasonic and dry them under vacuum.

6. The preparation method according to claim 4, characterized in that, In step S1, the mass ratio of the PBO nanofibers to trifluoroacetic acid and methanesulfonic acid is 0.1:49.95:49.95-1:49.5:49.

5. The mixing reaction method is mechanical stirring in a sealed environment, the rotation speed is 500-1000 rpm; the dissociation time is 48-72 h; the concentration of the PBO nanofiber acid solution is 0.1-1 wt%.

7. The preparation method according to claim 4, characterized in that, In step S2, the mass ratio of the deprotonating reagent to the PBO nanofibers is 1:1-5; Preferably, the reaction time is 2-5 h, and the reacted PBO nanofiber acid sol is poured into a petri dish with a diameter of 40-60 mm and gelled for 12-24 h in a low-temperature sealed environment to obtain an acidic gel; Preferably, the acid gel is successively subjected to solvent exchange with deionized water, ethanol or isopropanol, and the replacement is carried out every 2-4 h until the pH is close to 7; the gel after solvent exchange is freeze-dried at -30 to -50 °C for 24-48 h to obtain an aerogel intermediate.

8. The preparation method according to claim 4, characterized in that, In step S2, the deprotonating reagent is selected from one or more of sodium sulfate, ferric sulfate, magnesium sulfate, sodium carbonate and aluminum chloride.

9. The preparation method according to claim 4, characterized in that, In step S3, the freeze-dried aerogel is pressed at 20-30 °C and a pressure of 1-10 MPa for 30-120 min to obtain a PBO nanofiber film.

10. Application of the high-strength and high-temperature resistant ultra-thin PBO nanofiber membrane according to claims 1-3 in aerospace, flexible electronic devices or flame retardant materials.

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