Polyimide aerogel fiber with gradient cross-linking structure and preparation method thereof

Abstract

The present invention relates to the field of aerogel preparation, and relates to polyimide aerogel fibers with a gradient cross-linking structure and a preparation method thereof. The method comprises: copolymerizing a diamine containing carboxyl or hydroxyl groups, other diamines, and a dibasic acid anhydride; controlling the grafting density of the photosensitive groups to obtain a photosensitive polyimide with different photosensitive group contents as a spinning solution; and subjecting the spinning solution to three-layer coaxial wet spinning, ultraviolet light irradiation, solvent replacement, and normal pressure drying to obtain a polyimide aerogel fiber with a gradient cross-linking structure. The polyimide aerogel fiber comprises a cortex, an intermediate layer, and a core layer. The cortex structure is the densest, while the core and intermediate layers are porous structures of a three-dimensional interconnected nanofiber network. The core layer structure has the highest fluffiness, and the intermediate layer has the highest fluffiness. The photosensitive group grafting density of the photosensitive polyimide precursor solution in the spinning solution increases from the inner layer to the outer layer. By regulating the fiber microstructure, the problem of aerogel fibers being unable to achieve both high strength and high toughness is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of aerogel preparation, and in particular to a polyimide aerogel fiber with a gradient cross-linking structure and a preparation method thereof. Background Art

[0002] Aerogel is a new type of material with a three-dimensional porous structure using air as a dispersion medium. Due to its extremely low density, high porosity and low thermal conductivity, it has broad application prospects in the fields of thermal insulation such as aerospace, energy-saving buildings, and clothing. Among them, aerogel fiber is an aerogel material with a one-dimensional macroscopic fiber structure. It combines the three-dimensional porous structure of aerogel with the flexibility of fiber, and has high porosity, low density, and low thermal conductivity (23-50mW m -1 K -1 ) and advantages such as weavability and wearability, it has the potential to become a new fiber material for efficient thermal management clothing.

[0003] Polyimide aerogel fiber prepared from polyimide, known as the "top material of the polymer material pyramid", has played an irreplaceable role in thermal protective textiles used in ultra-low and high temperature environments, such as space suits and fire suits, because of its excellent temperature resistance and excellent mechanical properties as well as the ultra-low thermal conductivity given by the three-dimensional porous structure of the aerogel.

[0004] The porous structure and high porosity of aerogel fibers can give them excellent thermal insulation properties, but will inevitably lead to a decrease in the mechanical properties of aerogel fibers.

[0005] The mechanical strength of polyimide aerogel fibers reported so far is no higher than 15 MPa, and the breaking strain is no higher than 30%. [1] et al. prepared polyimide aerogel fibers through confined sol-gel strategy, with a mechanical strength of 11 MPa and a breaking strain of 25%. [2] et al. prepared polyimide aerogel fibers using a freeze-spinning strategy. Their mechanical strength was 10.6 MPa, which is far lower than that of solid fiber materials, and the fracture strain was 28.9%. This shows that although porous polyimide aerogel fibers have great application potential, due to their high porosity and fragile mechanical properties, it is a challenge to prepare polyimide aerogel fibers with both high strength and high toughness. Therefore, the preparation of high-strength and high-toughness polyimide aerogel fibers is of great significance to the development of the aerogel fiber field.

[0006] [1]Xin Li, Guoqing Dong, Zengwei Liu, and Xuetong Zhang.PolyimideAerogel Fibers with Superior Flame Resistance,Strength,Hydrophobicity,andFlexibility Made via a Universal Sol-Gel Confined Transition Strategy.[J].ACSNano 2021 15(3),4759-4768.

[0007] [2] Yujie Wang, Ying Cui, Ziyu Shao, Weiwei Gao, Wei Fan, Tianxi Liu, HaoBai. Multifunctional polyimide aerogel textile inspired by polar bear hair forthermoregulation in extreme environments. [J]. Chemical Engineering Journal, 2020, 124623, 1385-8947. Summary of the Invention

[0008] [Technical Issues]

[0009] Existing polyimide aerogel fibers have difficulty in achieving both high strength and high toughness.

[0010] [Technical solution]

[0011] On the one hand, a method for preparing a polyimide aerogel fiber having a gradient cross-linking structure is provided, which comprises the following steps:

[0012] (1) subjecting a diamine containing a carboxyl group or a hydroxyl group, other diamines and a dibasic acid anhydride to a condensation reaction to form a polyamic acid; and chemically imidizing the polyamic acid to obtain a polyimide;

[0013] (2) using the polyimide obtained in step (1) as a raw material, preparing photosensitive polyimides with different grafting densities of photosensitive groups, and adding a photoinitiator and a polar aprotic solvent to respectively prepare photosensitive polyimide precursor solutions with different grafting densities of photosensitive groups;

[0014] (3) The photosensitive polyimide precursor solutions with different grafting densities of photosensitive groups prepared in step (2) are respectively used as spinning solutions and are defoamed, and then extruded from a three-layer coaxial needle, wherein the grafting density of the photosensitive groups of the photosensitive polyimide precursor solutions corresponding to the spinning solutions from the inner layer to the outer layer increases successively; irradiated with an ultraviolet light source to form wet gel fibers, and then stretched, solvent replaced, dried at normal pressure, and collected to obtain polyimide aerogel fibers with a gradient cross-linking structure; the polyimide aerogel fibers with a gradient cross-linking structure include a cortex, an intermediate layer, and a core layer, wherein the cortex structure is the densest, the core layer and the intermediate layer are porous structures of a three-dimensional interconnected nanofiber network, the core layer structure has the highest fluffiness, and the intermediate layer has the second highest fluffiness.

[0015] In some embodiments, in step (1):

[0016] The diamine containing a carboxyl group or a hydroxyl group includes at least one of 3,5-diaminobenzoic acid (DABA) or 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP);

[0017] The other diamines include at least one of 4,4-diaminodiphenyl ether (ODA), 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl (TFMB), or 2-(4-aminophenyl)-5-aminobenzimidazole (BIA);

[0018] The dibasic acid anhydride includes at least one of 4,4'-(hexafluoroisopropylene) diphthalic anhydride (6FDA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), or 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride (A-ODPA).

[0019] In some embodiments, step (1) specifically includes the following steps:

[0020] Under the protection of inert gas, a diamine containing carboxyl or hydroxyl groups and other diamines are completely dissolved in a polar aprotic solvent, and a dibasic acid anhydride is added to carry out a condensation reaction to obtain polyamic acid; and a high-temperature imidization is carried out to obtain a polyimide having a molecular chain containing carboxyl or hydroxyl groups; wherein:

[0021] The molar ratio of the diamine containing carboxyl or hydroxyl groups to other diamines is 4:6 to 6:4; the ratio of the total molar number of the diamine containing carboxyl or hydroxyl groups to the other diamines to the molar number of the dibasic acid anhydride is 1:(0.94 to 1.04); and the ratio of the total mass of the diamine containing carboxyl or hydroxyl groups, other diamines and dibasic acid anhydride to the total mass of the diamine containing carboxyl or hydroxyl groups, other diamines, dibasic acid anhydride and polar aprotic solvent is (10 to 20):100.

[0022] In some embodiments, the temperature of the polycondensation reaction in step (1) is 0 to 25° C., and the time of the polycondensation reaction is 5 to 24 hours.

[0023] In some embodiments, the high-temperature imidization reaction conditions in step (1) are: first heating to 120° C. for reaction for 0.5 to 1 h, then heating to 160° C. for reaction for 0.5 to 1 h, and finally heating to 200° C. for reaction for 6 to 12 h.

[0024] In some embodiments, in step (2):

[0025] The photosensitive group is acrylate.

[0026] In some embodiments, in step (2):

[0027] The photosensitive group includes at least one of hydroxyethyl methacrylate (HEMA), glycidyl methacrylate (GMA), or 2-hydroxyethyl acrylate (HEA).

[0028] In some embodiments, the method for preparing polyimides with different grafting densities of photosensitive groups in step (2) comprises the following steps:

[0029] Adding a photosensitive group-containing acrylate monomer, a dehydrating agent and a catalyst to a polar aprotic solvent and stirring until the solid is completely dissolved; then adding the monomer to the result of step (1), stirring at room temperature for 24 hours, separating the solution from the product, precipitating in water, and drying to obtain a photosensitive polyimide; wherein:

[0030] The acrylate monomer containing a photosensitive group includes at least one of hydroxyethyl methacrylate, glycidyl methacrylate or 2-hydroxyethyl acrylate;

[0031] Dehydrating agents include N,N'-dicyclohexylcarbodiimide or 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride;

[0032] The catalyst is 4-dimethylaminopyridine;

[0033] The molar ratio of the acrylate monomer containing a photosensitive group to the diamine containing a carboxyl group or a hydroxyl group is (0.1-100):100;

[0034] The molar ratio of the acrylate monomer containing a photosensitive group: the dehydrating agent: the catalyst is 10:10:1.

[0035] In some embodiments, the photoinitiator in step (2) includes at least one of Irgacure-184, Irgacure-2959, Irgacure-651, Irgacure-369, Irgacure-907, Irgacure-1300, Irgacure-784, Irgacure-250, Irgacure-819, Irgacure819DW, Darocur-4265, Darocur BP, Darocur MBF, Darocur TPO or Darocur-1173, and the amount of the photoinitiator is 0.5-2 wt % of the total mass of the photosensitive polyimide precursor solution, and the solid content of the photosensitive polyimide precursor solution is 5-30 wt %.

[0036] In some embodiments, the polar aprotic solvent in the method for preparing polyimides with different grafting densities of photosensitive groups in step (1) or step (2) is N-methylpyrrolidone.

[0037] In some embodiments, the polar aprotic solvent used in preparing the photosensitive polyimide precursor solution in step (2) includes N-methylpyrrolidone, N,N-dimethylacetamide or N,N-dimethylformamide.

[0038] In some embodiments, in step (3):

[0039] The photosensitive group grafting densities of the photosensitive polyimide precursor solution corresponding to the spinning solutions from the inner layer to the outer layer are 20%, 50% and 100% respectively.

[0040] In some embodiments, in step (3), the extrusion speed of the syringe pump used for the three-layer coaxial needle extrusion is 10 to 100 μL min -1 The dimensions of the three-layer coaxial needle are: the core diameter is 100-300 μm; the middle layer diameter is 400-700 μm; the skin diameter is 1.07-1.35 mm; the power of the ultraviolet light source is 0.3-3 W cm -2 The UV light irradiation time is 0.01 to 3 minutes; the drafting multiple is 1 to 6 times, and the collection speed is 0.3 to 15 minutes. -1 .

[0041] In some embodiments, the solvent used for solvent replacement in step (3) includes at least one of water, acetone, ethanol, methanol, isopropanol or n-hexane, the temperature of solvent replacement is 25-100° C., the time of solvent replacement is 24-96 h, and the number of replacements is 3-5 times.

[0042] In some embodiments, the temperature of the atmospheric pressure drying in step (3) is 25 to 100° C., and the time is 2 to 48 hours.

[0043] On the other hand, a polyimide aerogel fiber having a gradient cross-linking structure prepared by the aforementioned method is provided.

[0044] In another aspect, the present invention provides an application of the aforementioned polyimide aerogel fiber with a gradient cross-linking structure in the field of intelligent thermal management.

[0045] In some embodiments, the polyimide aerogel fiber having a gradient cross-linking structure is used in the fields of architectural decoration, aerospace, or firefighting clothing.

[0046] [Beneficial Effects]

[0047] (1) Different from the existing coaxial polyimide aerogel fiber, which has no interaction between the skin and core layers and poor mechanical properties, the present invention provides a polyimide aerogel fiber with a gradient cross-linking structure and a preparation method thereof, wherein polyimide with different photosensitive group grafting densities is used as a spinning solution, wherein the photosensitive group grafting density of the photosensitive polyimide corresponding to the spinning solution from the inner layer to the outer layer increases in sequence, and the gel fiber is obtained by UV-assisted coaxial wet spinning based on a three-layer coaxial needle, which is subjected to solvent replacement in a coagulation bath and dried at normal pressure to obtain a polyimide aerogel fiber with a gradient cross-linking structure, and successfully realizes the integrated molding of a three-layer gradient cross-linking polyimide aerogel fiber. The three-layer gradient cross-linking structure includes a skin layer, an intermediate layer and a core layer, wherein the skin layer structure is the densest, the core layer and the intermediate layer are a porous structure of a three-dimensional interconnected nanofiber network, the core layer structure has the highest fluffiness, and the intermediate layer has the highest fluffiness. Based on the above-mentioned specific structure, the polyimide aerogel fiber is endowed with high mechanical strength, high toughness and excellent thermal insulation performance;

[0048] (2) The polyimide aerogel fiber of the present invention has a three-layer gradient cross-linked structure. Under ultraviolet irradiation, the photosensitive groups in different layers of spinning solution quickly undergo free radical polymerization reaction to achieve the integrated molding of the gradient cross-linked structure aerogel fiber; wherein, as the cross-linking density increases, the mechanical strength increases, so the high cross-linking degree of the skin layer provides the mechanical strength of the fiber, and the low cross-linking degree of the core layer provides the flexibility and toughness of the fiber; that is, the skin layer adopts a high photosensitive group grafting density to provide high mechanical strength for the fiber, and the core layer adopts a low photosensitive group grafting density to provide toughness and flexibility for the fiber. At the same time, due to the compatibility of the spinning solution, the different layers are chemically cross-linked by the photosensitive groups during the preparation process, which further improves the mechanical properties of the fiber. Therefore, the triaxial gradient cross-linked polyimide aerogel fiber of the present invention achieves a balance between strength and toughness, and gives the fiber better weaving properties; further, the selection of fluorine-containing dibasic acid anhydride or diamine can effectively reduce surface tension and reduce the shrinkage rate of the aerogel fiber during normal pressure drying. The method of the present invention solves the problem that aerogel fibers cannot have both high strength and high toughness by regulating the fiber microstructure;

[0049] (3) The method of the present invention first polymerizes a diamine containing a carboxyl structure or a hydroxyl structure with a dibasic acid anhydride compound to form polyamic acid, and obtains polyimide through chemical imidization; the grafting density is controlled by controlling the content of the photosensitive compound to obtain a fluorine-containing polyimide spinning solution with different photosensitive group contents. The defoamed spinning solution is then extruded through a three-axis needle. Under ultraviolet light irradiation, the photosensitive groups rapidly undergo free radical polymerization to form gel networks of different strengths, thereby obtaining stable gel fibers. At the same time, due to the compatibility of different layers of spinning solution, chemical crosslinking exists between different layers. After obtaining the gel fiber, it is placed in a coagulation bath for solvent replacement, and dried at normal pressure to obtain a polyimide aerogel fiber with a gradient crosslinking structure.

[0050] (4) The polyimide aerogel fiber with a gradient cross-linking structure based on coaxial spinning and atmospheric pressure drying of the present invention has a gradient cross-linking structure due to the different grafting densities of the photosensitive groups in the inner and outer spinning solutions. In addition, the photosensitive group acrylate is introduced into the polyimide through polycondensation and Steglich esterification reaction. The coaxial wet spinning process directly applies an ultraviolet light source to form a polyimide wet gel fiber, and then the polyimide aerogel fiber with a gradient cross-linking structure is prepared through solvent replacement and atmospheric pressure drying.

[0051] (5) The present invention synthesizes an organically soluble photosensitive polyimide, extrudes it through a triaxial needle, irradiates it with ultraviolet light, and then crosslinks the spinning solution, replaces the solvent, and then dries it under normal pressure to form a polyimide aerogel fiber with a gradient crosslinking structure. SEM images demonstrate the formation of the gradient crosslinking structure, and stress-strain curves also demonstrate that the resulting fiber has strength and elongation at break that exceed the average level of current aerogel fibers. This simultaneously achieves a balance between strength and toughness in the polyimide aerogel fiber, significantly improving the performance compared to existing polyimide aerogel fibers. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is a scanning electron microscope image of the polyimide aerogel fiber with a gradient cross-linking structure prepared in Example 1;

[0053] Figure 2 The test results of hydrogen nuclear magnetic resonance spectroscopy of the photosensitive polyimide and polyimide with different grafting densities in Example 1 are as follows;

[0054] Figure 3 Optical photograph showing the flexibility and continuity of the polyimide aerogel fiber with a gradient cross-linking structure prepared in Example 1;

[0055] Figure 4 This is an optical demonstration photograph showing that the polyimide aerogel fiber with a gradient cross-linking structure prepared in Example 1 can bear a 200g weight;

[0056] Figure 5 These are the stress-strain curves of the polyimide aerogel fiber with a gradient cross-linking structure (GCPAF-1) prepared in Example 1 and the polyimide aerogel fibers with single components of different grafting densities (CPI-20, CPI-50, and CPI-100). DETAILED DESCRIPTION

[0057] Test method:

[0058] An electronic universal testing machine (Model E42, MTS, USA) was used with a sensor of 50 N and a force of 0.8 mm s -1 , the mechanical properties of aerogel fibers were tested in tensile mode.

[0059] The microstructure of the aerogel fibers was observed using a field emission scanning electron microscope (SU8600, Japan).

[0060] Example 1

[0061] A method for preparing a polyimide aerogel fiber with a gradient cross-linking structure comprises the following steps:

[0062] (1) Preparation of copolymerized polyimide:

[0063] In a three-necked round-bottom flask, under nitrogen protection, 3,5-diaminobenzoic acid (DABA, 0.7608 g), 4,4-diaminodiphenyl ether (ODA, 1.001 g) and N-methylpyrrolidone (NMP, 25 mL) were added in sequence and stirred until DABA and ODA were completely dissolved; then 4,4'-(hexafluoroisopropylene) diphthalic anhydride (6FDA, 4.4423 g) was added and reacted at 25°C for 24 h; the temperature was raised to 120°C and kept for reaction for 1 h, then continued to rise to 160°C and kept for reaction for 1 h, and then continued to rise to 200°C and kept for reaction for 10 h to obtain a soluble polyimide (co-polyimide) solution;

[0064] (2) Preparation of photosensitive polyimides with different grafting densities of hydroxyethyl methacrylate:

[0065] Preparation of a photosensitive polyimide with 100% hydroxyethyl methacrylate grafting amount: Hydroxyethyl methacrylate (HEMA, 5 mmol), N,N'-dicyclohexylcarboximide (5 mmol) and 4-dimethylaminopyridine (0.50 mmol) were added to N-methylpyrrolidone (NMP, 6 mL) in sequence and stirred until the solid disappeared. The mixture was added to the soluble polyimide solution of step (1) and stirred at room temperature for 24 h to cause Steglich esterification reaction until a precipitate was formed. The upper layer solution was obtained by centrifugation from the product, and the solution was then precipitated into deionized water, repeatedly washed, and dried to obtain a photosensitive polyimide with 100% grafting amount.

[0066] Preparation of a photosensitive polyimide with a 50% hydroxyethyl methacrylate grafting amount: The preparation steps of the photosensitive polyimide with a 100% hydroxyethyl methacrylate grafting amount were referred to, except that the amounts of the reactants, hydroxyethyl methacrylate (HEMA, 2.5 mmol), N,N'-dicyclohexylcarbamide (2.5 mmol), and 4-dimethylaminopyridine (0.25 mmol), were changed to obtain a photosensitive polyimide with a 50% grafting amount.

[0067] Preparation of a photosensitive polyimide with a 20% hydroxyethyl methacrylate grafting amount: The preparation steps of the photosensitive polyimide with a 100% hydroxyethyl methacrylate grafting amount were referred to, except that the amounts of the reactants, hydroxyethyl methacrylate (HEMA, 1 mmol), N,N'-dicyclohexylcarbodiimide (1 mmol), and 4-dimethylaminopyridine (0.1 mmol), were changed to obtain a photosensitive polyimide with a 20% grafting amount.

[0068] Note: The above-mentioned hydroxyethyl methacrylate grafting amount is based on 100% hydroxyethyl methacrylate grafting, indicating a molar ratio of 1:1 between the grafted hydroxyethyl methacrylate and the carboxyl group content in the copolymerized polyimide prepared in the previous step. This is the theoretical grafting amount. The gradient design here is based on the theoretical grafting amount.

[0069] (3) Preparation of photosensitive polyimide precursor solution:

[0070] The photosensitive polyimides with different grafting amounts obtained above were dissolved in N-methylpyrrolidone at a mass ratio of 15:83:2, and then photoinitiator Irgacure-819 was added and stirred for 20 minutes to obtain photosensitive polyimide precursor solutions with different grafting amounts; the solid content of the photosensitive polyimide precursor solution was 15 wt %.

[0071] (4) Preparation of polyimide aerogel fibers:

[0072] The photosensitive polyimide precursor solutions with different grafting amounts prepared in step (3) were used as spinning solutions and, after degassing, were extruded from a three-layer coaxial needle via an injection pump, wherein the hydroxyethyl methacrylate grafting amounts of the photosensitive polyimide corresponding to the spinning solutions from the inner layer to the outer layer were 20%, 50% and 100% respectively; the coaxial needle dimensions were: core layer diameter was 190 μm, middle layer diameter was 400 μm, and skin layer diameter was 1.07 mm; the extrusion speed of the injection pump was 50 μL min -1 ; 0.6W cm -2 After 15 seconds of UV irradiation, wet gel fibers were formed. After drawing, solvent replacement, drying at normal pressure, and collection, gradient cross-linked polyimide aerogel fibers were obtained, which were recorded as GCPAF-1. The drawing ratio was 2 times, and the collection speed was 0.6 m / min. -1 The solvent used for solvent replacement was ethanol, the temperature was 25°C, the time was 48h, and the number of replacements was 4 times; the temperature for normal pressure drying was 25°C, and the time was 24h.

[0073] Figure 1 is a scanning electron microscope image of the polyimide aerogel fiber with a gradient cross-linking structure prepared in Example 1; Figure 1 It can be seen that the obtained polyimide aerogel fiber successfully formed a gradient cross-linked structure, including the cortex, middle layer and core layer. Among them, the cortex structure is the densest, the core layer and the middle layer are porous structures of three-dimensional interconnected nanofiber networks, the core layer structure has the highest fluffiness, and the middle layer is second to third.

[0074] Figure 2The results of proton nuclear magnetic resonance (HNMR) spectroscopy of the photosensitive polyimide (PPI) and polyimide with different grafting densities in Example 1 are shown. As shown in the figure, the peaks at 5.8 ppm, 4.6 ppm, and 4.3 ppm correspond to hydrogen atoms on unsaturated double bonds and methylene atoms in the aliphatic chains of the photosensitive polyimide (PPI), respectively, corresponding to H1, H2, and H3 in the structural formula. Furthermore, the intensity of the characteristic peaks increases with increasing grafting density. The above analysis demonstrates that HEMA was successfully grafted onto the side chains of the polyimide (PI) via the Stiglitz esterification reaction, confirming the successful synthesis of the photosensitive polyimide (PPI).

[0075] Figure 3 The optical photographs show the flexibility and continuity of the polyimide aerogel fiber with gradient cross-linking structure prepared in Example 1; Figure 3 It can be seen that polyimide aerogel fibers with a gradient cross-linking structure of a certain length can be entangled without breaking, which confirms that GCPAF-1 has good flexibility and continuity.

[0076] Figure 4 This is an optical display photograph showing that the polyimide aerogel fiber with a gradient cross-linking structure prepared in Example 1 can withstand a 200g weight; Figure 4 It can be seen that the polyimide aerogel fiber with a gradient cross-linking structure in Example 1 can withstand a 200 g weight without breaking, thereby confirming that the fiber has good mechanical properties.

[0077] Comparative Example 1

[0078] A method for preparing a polyimide aerogel fiber with a gradient cross-linked structure is described with reference to Example 1, except that in step (4), the amounts of hydroxyethyl methacrylate grafted onto the photosensitive polyimide in the spinning solutions from the inner layer to the outer layer are 100%, 50%, and 20%, respectively. The resulting gradient cross-linked polyimide aerogel fiber is designated GCPAF-2.

[0079] The experimental results show that the grafting amount of hydroxyethyl methacrylate from the inside to the outside is a gradient of 100%, 50% and 20% respectively. Since the grafting density of the photosensitive group in the outermost layer is 20%, the sol-gel transition of the fiber takes longer time, which is difficult to match the overall spinning speed during the spinning process, and a continuous three-layer coaxial structure cannot be obtained. Therefore, the aerogel fiber with a gradient cross-linking structure cannot be formed.

[0080] Comparative Example 2

[0081] A method for preparing a polyimide aerogel fiber is described with reference to Example 1, except that only a photosensitive polyimide grafted with 20% hydroxyethyl methacrylate is used as the spinning solution, and UV-assisted wet spinning is performed using a needle with an inner diameter of 1.07 mm. The resulting cross-linked polyimide aerogel fiber is designated CPI-20.

[0082] Comparative Example 3

[0083] A method for preparing a polyimide aerogel fiber is described with reference to Example 1, except that only a photosensitive polyimide grafted with 50% hydroxyethyl methacrylate is used as the spinning solution, and UV-assisted wet spinning is performed using a needle with an inner diameter of 1.07 mm. The resulting cross-linked polyimide aerogel fiber is designated CPI-50.

[0084] Comparative Example 4

[0085] A method for preparing a polyimide aerogel fiber is described with reference to Example 1, except that only a photosensitive polyimide grafted with 100% hydroxyethyl methacrylate is used as the spinning solution, and UV-assisted wet spinning is performed using a needle with an inner diameter of 1.07 mm. The resulting cross-linked polyimide aerogel fiber is designated CPI-100.

[0086] Figure 5 The stress-strain curves of the polyimide aerogel fiber with gradient cross-linking structure (GCPAF-1) and the polyimide aerogel fibers with different grafting densities (CPI-20, CPI-50, CPI-100) prepared in Example 1 are shown. Figure 5 It can be seen that the polyimide aerogel fiber with a gradient cross-linking structure has a higher breaking strength of 24.5 MPa and a higher breaking strain of 49.1% than the cross-linked polyimide aerogel fiber with a single structure. This confirms that the aerogel fiber obtained by designing a three-layer coaxial structure of the aerogel fiber and ultraviolet-assisted coaxial wet spinning has a strength that is better than the current aerogel fiber level of 8 to 15 MPa, and the toughness is significantly improved, thereby realizing the preparation of high-strength and high-toughness aerogel fibers.

[0087] Comparative Example 5

[0088] The mechanical properties of polyimide aerogel fibers prepared in the current literature were investigated. [1] et al. prepared polyimide aerogel fibers through confined sol-gel strategy, with a mechanical strength of 11 MPa and a breaking strain of 25%. [2]et al. used a freeze-spinning strategy to prepare polyimide aerogel fibers with a mechanical strength of 10.6 MPa and a fracture strain of 28.9%. Therefore, it is a challenge to prepare polyimide aerogel fibers with both high strength and high toughness.

[0089] The embodiments provided above are not intended to limit the scope of the present invention, nor are the steps described to limit their execution order. Any obvious improvements to the present invention made by those skilled in the art in combination with existing common knowledge shall fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a polyimide aerogel fiber with a gradient cross-linking structure, characterized in that: The steps include: (1) A diamine containing a carboxyl group or a hydroxyl group, another diamine and a dibasic acid anhydride are subjected to a condensation reaction to form a polyamic acid; the polyamic acid is subjected to high-temperature imidization to obtain a polyimide; wherein the diamine or dibasic acid anhydride contains fluorine; (2) using the polyimide obtained in step (1) as a raw material, adding an acrylate monomer containing a photosensitive group to prepare photosensitive polyimides with different grafting densities of photosensitive groups, and adding a photoinitiator and a polar aprotic solvent to respectively prepare photosensitive polyimide precursor solutions with different grafting densities of photosensitive groups; (3) The photosensitive polyimide precursor solutions with different grafting densities of photosensitive groups prepared in step (2) are used as spinning solutions and are defoamed before being extruded from a three-layer coaxial needle, wherein the grafting density of the photosensitive groups of the photosensitive polyimide precursor solutions corresponding to the spinning solutions from the inner layer to the outer layer increases in sequence; wet gel fibers are formed by irradiation with an ultraviolet light source, and polyimide aerogel fibers with a gradient cross-linking structure are obtained by stretching, solvent replacement, drying at normal pressure, and collection; the polyimide aerogel fibers with a gradient cross-linking structure include a cortex, an intermediate layer, and a core layer, wherein the cortex structure is the densest, the core layer and the intermediate layer are porous structures of a three-dimensional interconnected nanofiber network, the core layer structure has the highest fluffiness, and the intermediate layer is the second.

2. The method according to claim 1, characterized in that In step (1): The diamine containing a carboxyl group or a hydroxyl group includes at least one of 3,5-diaminobenzoic acid or 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane; The other diamines include at least one of 4,4-diaminodiphenyl ether, 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl or 2-(4-aminophenyl)-5-aminobenzimidazole; The dibasic acid anhydride includes at least one of 4,4'-(hexafluoroisopropylene) diphthalic anhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, or 2,3,3',4'-diphenylether tetracarboxylic dianhydride.

3. The method according to claim 1, characterized in that In step (3): The photosensitive group grafting densities of the photosensitive polyimide precursor solution corresponding to the spinning solutions from the inner layer to the outer layer are 20%, 50% and 100% respectively.

4. The method according to claim 1, wherein Step (1) specifically includes the following steps: Under the protection of inert gas, a diamine containing carboxyl or hydroxyl groups and other diamines are completely dissolved in a polar aprotic solvent, and a dibasic acid anhydride is added to carry out a condensation reaction to obtain polyamic acid; and a high-temperature imidization is carried out to obtain a polyimide having a molecular chain containing carboxyl or hydroxyl groups; wherein: The molar ratio of the diamine containing a carboxyl or hydroxyl group to other diamines is 4:6~6:4; the ratio of the total molar number of the diamine containing a carboxyl or hydroxyl group and other diamines to the molar number of the dibasic acid anhydride is 1:(0.94~1.04); and the ratio of the total mass of the diamine containing a carboxyl or hydroxyl group, other diamines and dibasic acid anhydride to the total mass of the diamine containing a carboxyl or hydroxyl group, other diamines, dibasic acid anhydride and polar aprotic solvent is (10~20):

100.

5. The method according to claim 1, wherein The method for preparing polyimide with different grafting densities of photosensitive groups in step (2) comprises the following steps: Adding a photosensitive group-containing acrylate monomer, a dehydrating agent and a catalyst to a polar aprotic solvent and stirring until the solid is completely dissolved; then adding the monomer to the result of step (1), stirring at room temperature for reaction, separating the solution from the product, precipitating in water, and drying to obtain a photosensitive polyimide; wherein: The acrylate monomer containing a photosensitive group includes at least one of hydroxyethyl methacrylate, glycidyl methacrylate or 2-hydroxyethyl acrylate; Dehydrating agents include N,N'-dicyclohexylcarbodiimide or 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride; The catalyst is 4-dimethylaminopyridine; The molar ratio of the acrylate monomer containing a photosensitive group to the diamine containing a carboxyl group or a hydroxyl group is (0.1-100):100; The molar ratio of the acrylate monomer containing a photosensitive group: the dehydrating agent: the catalyst is 10:10:

1.

6. The method according to claim 1, wherein The photoinitiator in step (2) includes at least one of Irgacure-184, Irgacure-2959, Irgacure-651, Irgacure-369, Irgacure-907, Irgacure-1300, Irgacure-784, Irgacure-250, Irgacure-819, Irgacure819DW, Darocur-4265, Darocur BP, Darocur MBF, Darocur TPO or Darocur-1173, and the amount of the photoinitiator is 0.5-2 wt% of the total mass of the photosensitive polyimide precursor solution, and the solid content of the photosensitive polyimide precursor solution is 5-30 wt%.

7. The method according to claim 1, wherein In step (3), the extrusion speed of the syringe pump used for the three-layer coaxial needle extrusion is 10~100 μL min -1 The dimensions of the three-layer coaxial needle are: core diameter 100-300 μm; middle layer diameter 400-700 μm; skin diameter 1.07-1.35 mm; and UV light source power 0.3-3 W cm -2 The UV light irradiation time is 0.01~3 min; the drafting multiple is 1~6 times, and the collection speed is 0.3~15 m / min -1 .

8. A polyimide aerogel fiber having a gradient cross-linked structure obtained by the method according to any one of claims 1 to 7.

9. Application of the polyimide aerogel fiber with a gradient cross-linking structure according to claim 8 in the field of intelligent thermal management.

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