A polyimide aerogel fiber with a gradient pore structure having a porous core layer and a dense skin layer and a preparation method thereof

Abstract

The present invention relates to the technical field of aerogel preparation, and in particular to a polyimide aerogel fiber with a gradient pore structure having a porous core layer and a dense skin layer and a preparation method thereof. The method comprises: using a polyamic acid solution as a skin layer spinning solution, copolymerizing a diamine containing a carboxyl group or a hydroxyl group, other diamines and a dibasic acid anhydride, and grafting a photosensitive group to obtain a photocrosslinkable polyimide solution as a core layer spinning solution. Based on ultraviolet-assisted coaxial spinning technology, under the action of ultraviolet light, the core layer is induced to form a highly crosslinked gel network, while the skin layer polyamic acid remains in a non-crosslinked state, and solvent replacement is carried out in a coagulation bath. After drying at normal pressure, a polyimide aerogel fiber with a porous core layer and a dense skin layer structure is formed. The polyimide aerogel fiber obtained by the present invention has excellent thermal insulation properties given to the fiber by its porous core layer, and excellent mechanical properties given to the fiber by its dense skin layer, which solves the problem that the mechanical and thermal insulation properties of the aerogel fiber cannot be achieved at the same time.

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 pore structure having a porous core layer and a dense skin layer and a preparation method thereof. Background Art

[0002] Aerogel fibers are a type of aerogel material with a one-dimensional macroscopic fiber structure. They inherit the three-dimensional porous structure of aerogel and the flexibility of fiber. They possess advantages such as high porosity, low density, low thermal conductivity, and the ability to be weaved and worn. They are expected to become a new fiber material for efficient thermal management clothing. Furthermore, the high porosity and high specific surface area of ​​aerogel fibers allow for efficient compounding with phase change materials, photothermal conversion, and energy storage materials, endowing them with multiple functions such as thermal / electrical / photoresponsiveness, heat storage and release, and fire resistance. Therefore, they play an irreplaceable role in personal thermal management.

[0003] The porous structure and high porosity of aerogel fibers contribute to their improved thermal insulation properties, but this inevitably leads to a decrease in the fiber's mechanical properties. The tensile strength of aerogel fibers reported so far is far below that of solid fiber materials.

[0004] Currently, non-aerogel encapsulation layers are often used as the fiber sheath. Although mechanical properties are improved, the thermal insulation performance of aerogel fibers is significantly reduced. Therefore, how to effectively control the molecular / microstructure of polyimide fibers to achieve improved thermal insulation performance of aerogel fibers while maximally maintaining the mechanical properties of the fibers remains a huge challenge. Summary of the Invention

[0005] [Technical Issues]

[0006] Existing polyimide aerogels are difficult to combine excellent fiber mechanics and excellent thermal insulation properties.

[0007] [Technical solution]

[0008] On the one hand, a method for preparing a polyimide aerogel fiber having a gradient pore structure of a porous core layer and a dense skin layer is provided, comprising the following steps:

[0009] (1) Preparation of a cortical polyamic acid spinning solution: Under inert gas protection, a diamine is dissolved in a polar aprotic solvent, and a dibasic acid anhydride is slowly added to carry out a polycondensation reaction. When there is no significant change in viscosity, a cortical polyamic acid spinning solution is obtained; the cortical polyamic acid spinning solution has a polyamic acid solid content of 12 to 15 wt%;

[0010] (2) Preparation of core layer photocrosslinkable polyimide spinning solution:

[0011] Step 1, under the protection of inert gas, adding a diamine containing a carboxyl or hydroxyl group and other diamines to a polar aprotic solvent, stirring and dissolving, adding a fluorinated dianhydride to carry out a condensation reaction to obtain a polyamic acid solution; heating to a high temperature for imidization to obtain a soluble polyimide solution; wherein: the molar ratio of the diamine containing a carboxyl or hydroxyl group: the other diamines is 4:6 to 6:4, the ratio of the sum of the moles of the diamine containing a carboxyl or hydroxyl group and the other diamines to the mole of the fluorinated dianhydride is 1:(0.94 to 1.04), and the total mass of the diamine containing a carboxyl or hydroxyl group, the other diamines and the fluorinated dianhydride accounts for 10% to 20% of the total mass of the diamine containing a carboxyl or hydroxyl group, the other diamines, the fluorinated dianhydride and the polar aprotic solvent;

[0012] Step 2: adding a photosensitive group-containing acrylate monomer, a dehydrating agent, and a catalyst to a polar aprotic solvent, stirring until the solid is completely dissolved; adding the monomer to the soluble polyimide solution prepared in step 1, stirring at room temperature for reaction; separating the resulting product to obtain a solution; then precipitating the solution in deionized water, washing, and drying to obtain a photosensitive fluorinated polyimide; the molar ratio of the photosensitive group-containing acrylate monomer to the diamine containing a carboxyl group or a hydroxyl group in step 1 is (0.1-100):100;

[0013] Step 3: dissolving the photosensitive fluorinated polyimide and photoinitiator prepared in step 2 in a polar aprotic solvent in the dark to obtain a core layer photocrosslinkable polyimide spinning solution; the core layer photocrosslinkable polyimide spinning solution has a solid content of 5 to 30 wt%.

[0014] (3) Preparation of polyimide aerogel fiber: The sheath polyamide acid spinning solution prepared in step (1) and the core layer photocrosslinkable polyimide spinning solution prepared in step (2) are extruded through a coaxial needle, cured by ultraviolet irradiation, and the fibers are placed in a coagulation bath for solvent replacement, dried at normal pressure and subjected to thermal imidization treatment to obtain a polyimide aerogel fiber with a gradient pore structure having a porous core layer and a dense sheath; wherein the coaxial needle size is: the core layer diameter is 0.2 to 0.6 mm, and the sheath layer diameter is 1 to 2 mm.

[0015] In some embodiments, in step (1), the diamine includes 4,4'-diaminodiphenyl ether (ODA), p-phenylenediamine, m-phenylenediamine, 3,5-diaminobenzoic acid, 3,3'-diamino-5,5'-bis(trifluoromethyl)biphenyl, 4,4'-diamino-2,2'-dimethyl-1,1'biphenyl, 2(4-aminophenyl)5-aminobenzimidazole, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane. At least one; the dibasic acid anhydride includes at least one of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, bis(3,4-dicarboxylic acid phthalic anhydride) ether, 4,4'-(hexafluoroisopropylene) diphthalic anhydride or 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride; the molar ratio of the dibasic acid anhydride to the diamine is 1:(1.01-1.03).

[0016] In some embodiments, in step 1: 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); the other diamine includes at least one of 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl (TFMB), 4,4-diaminodiphenyl ether (ODA), or 2-(4-aminophenyl)-5-aminobenzimidazole (BIA); and the fluorinated dianhydride includes at least one of 4,4'-(hexafluoroisopropylene)diphthalic anhydride (6FDA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), or 2,3,3',4'-diphenylethertetracarboxylic dianhydride (A-ODPA).

[0017] In some embodiments, in step 2: the acrylate monomer containing a photosensitive group includes at least one of hydroxyethyl methacrylate (HEMA), glycidyl methacrylate (GMA), or 2-hydroxyethyl acrylate (HEA); the dehydrating agent includes N,N'-dicyclohexylcarbodiimide or 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride; the catalyst is 4-dimethylaminopyridine; and the molar ratio of the acrylate monomer containing a photosensitive group: the dehydrating agent: the catalyst is 10:10:1.

[0018] In some embodiments, in step 3: the initiator includes at least one of Irgacure-819, Irgacure-184, Irgacure-2959, Irgacure-651, Irgacure-369, Irgacure-907, Irgacure-1300, Irgacure-784, Irgacure-250, Irgacure819DW, Darocur-4265, Darocur BP, Darocur MBF, Darocur TPO or Darocur-1173; the amount of the initiator is 0.5-2% of the mass of the core layer photo-crosslinkable polyimide spinning solution.

[0019] In some embodiments, in step (3): coaxial needle extrusion parameters: the extrusion speed of the core layer spinning solution injection pump is 10-100 μL min -1 The extrusion pressure of the skin spinning solution is 100-700 kPa; the diameter ratio of the skin layer to the core layer is controlled to be (5-7):1; the power of ultraviolet radiation curing is 0.3-3 W cm -2 The curing time is 0.01 to 3 minutes; the air layer distance between the coaxial spinning nozzle and the coagulation bath liquid surface is controlled to be 1 to 5 cm; the drafting multiple is 1 to 6 times, and the collection speed is 0.3 to 15 min. -1 .

[0020] In some embodiments, the polar aprotic solvent in step (1), step 1 and / or step 2 is 1-methyl-2-pyrrolidone.

[0021] In some embodiments, in step 1: the reaction temperature of the polycondensation reaction is 0 to 25° C., and the reaction time is 5 to 24 hours.

[0022] In some embodiments, in step 3: the polar aprotic solvent includes at least one of 1-methyl-2-pyrrolidone, N,N-dimethylacetamide or N,N-dimethylformamide; the solvent used for solvent replacement includes at least one of water, acetone, ethanol, methanol, isopropanol or n-hexane; the replacement temperature is 25-100°C, the replacement time is 24-96 hours, and the number of replacements is 3-5 times; the temperature of atmospheric drying is 25-100°C, and the time is 2-48 hours.

[0023] In some embodiments, the thermal imidization treatment conditions in step (3) are 100-150°C for 10-30 min, 200-250°C for 20-40 min, 280-300°C for 30-40 min, and a heating rate of 1-1.5°C min -1 .

[0024] On the other hand, a polyimide aerogel fiber with a gradient pore structure of porous core layer and dense skin layer prepared by the above method is provided.

[0025] In another aspect, the present invention provides a use of the aforementioned polyimide aerogel fiber with a gradient pore structure having a porous core layer and a dense skin layer in thermal management clothing.

[0026] [Beneficial Effects]

[0027] (1) The present invention uses polyamic acid solution as the sheath spinning solution, copolymerizes diamines containing carboxyl or hydroxyl groups and other diamines with dibasic acid anhydrides, and grafts photosensitive groups to obtain a photocrosslinkable polyimide solution as the core spinning solution. Through UV-assisted coaxial wet spinning, solvent replacement, normal pressure drying, and thermal imidization, with the help of different molecular structures and different forming mechanisms of the inner and outer layer spinning solutions, under the action of ultraviolet light, the core layer is induced to form a highly crosslinked gel network, while the sheath polyamic acid remains in a non-crosslinked state; solvent replacement is carried out in a coagulation bath, and the non-crosslinked sheath polymer molecular chains have strong mobility and migrate to the surface under the action of the coagulation bath, while the movement of the highly crosslinked core layer polymer molecular chains is restricted, thereby maintaining a three-dimensional gel network structure; after drying at normal pressure, the formed polyimide aerogel fiber has a gradient pore structure of porous core layer-dense sheath. The method of the present invention successfully constructs a polyimide aerogel fiber with a gradient pore structure having a porous core layer-dense sheath;

[0028] (2) The present invention uses polyamic acid solution as the sheath spinning solution and organically soluble fluorinated polyimide as the core spinning solution. During normal pressure drying, the core layer polyimide undergoes cross-linking under ultraviolet irradiation, which limits the movement of the molecular chains and enhances the strength of the gel skeleton. In addition, the core layer spinning solution contains trifluoromethyl groups, which effectively reduces the capillary force and the shrinkage during normal pressure drying, thereby helping to form a fluffy porous structure with good thermal insulation performance. The sheath polyamic acid molecular chains have strong mobility and the gel skeleton has relatively weak strength. The pore structure will collapse to varying degrees during drying, and shrink significantly during normal pressure drying, forming a relatively dense pore structure that helps to enhance mechanical properties. At the same time, due to the large shrinkage of the sheath, the sheath can tightly wrap the core layer, and no large air layer will be generated between the sheath and core layers. This effectively improves the mechanical properties of the polyimide aerogel fiber while ensuring thermal insulation performance.

[0029] (3) The polyimide aerogel fiber with a gradient pore structure prepared by the present invention has a porous core layer that improves the thermal insulation performance of the fiber, and a dense skin layer that improves the mechanical properties of the fiber. By regulating the fiber microstructure, the problem that the mechanical and thermal insulation properties of the aerogel fiber cannot be achieved at the same time is solved;

[0030] (4) The present invention designs the structure of polyimide, uses polyamic acid as the spinning solution for the sheath layer, and uses organic soluble photosensitive polyimide as the spinning solution for the core layer. Based on the UV-assisted coaxial wet spinning technology, the core layer is induced to form a highly cross-linked gel network under the action of ultraviolet light. In the process of solvent replacement and normal pressure drying, the molecular chains of the sheath layer form a dense pore structure due to their better mobility and greater shrinkage, thereby improving the mechanical properties of the aerogel fiber. The fluffy porous structure formed in the core layer is conducive to improving the thermal insulation performance. The prepared polyimide aerogel fiber with a gradient pore structure of porous core layer and dense sheath has excellent thermal insulation performance, maintains a temperature difference of 80°C on a 200°C hot plate, and has good mechanical properties, achieving a balance between the mechanical and thermal insulation properties of the polyimide aerogel fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a scanning electron microscope image of the polyimide aerogel fiber with a gradient pore structure having a porous core layer and a dense skin layer prepared in Example 1;

[0032] Figure 2 The cross-sectional scanning electron micrograph, the cortex scanning electron micrograph, and the core scanning electron micrograph of the polyimide aerogel fiber obtained in Comparative Example 1 are shown;

[0033] Figure 3 The cross-sectional scanning electron micrograph, the cortex scanning electron micrograph, and the core scanning electron micrograph of the polyimide aerogel fiber obtained in Comparative Example 2 are shown;

[0034] Figure 4 is the hydrogen nuclear magnetic resonance spectrum of the photosensitive polyimide obtained in Example 1;

[0035] Figure 5 is the stress-strain curve of polyimide aerogel fibers with different fiber diameters;

[0036] Figure 6 This is an infrared thermal imaging photograph of the gradient pore structure polyimide aerogel fiber with a porous core layer and a dense skin layer prepared in Example 1. DETAILED DESCRIPTION

[0037] Performance testing method:

[0038] 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.

[0039] The microstructure of the aerogel fibers was observed using a field emission scanning electron microscope (SU8600, Japan).

[0040] The thermal insulation properties of aerogel fibers were observed using an infrared thermal imager (Ti480U, Fluke, China).

[0041] Example 1

[0042] A method for preparing a polyimide aerogel fiber with a gradient pore structure having a porous core layer and a dense skin layer comprises the following steps:

[0043] (1) Preparation of cortical polyamic acid spinning solution: In a three-necked round-bottom flask, under the protection of inert gas, 2.0023 g of 4,4′-diaminodiphenyl ether (ODA, as diamine) was stirred and dissolved in 28 mL of 1-methyl-2-pyrrolidone (NMP, as polar aprotic solvent), and 3.0305 g of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA, as dibasic acid anhydride, the molar ratio of diamine to dibasic acid anhydride was 1:1.03) was slowly added and polycondensed for 12 h (until the solution viscosity did not change significantly) to obtain the cortical polyamic acid spinning solution; the solid content of polyamic acid in the cortical polyamic acid spinning solution was 15 wt%;

[0044] (2) Preparation of core layer photocrosslinkable polyimide spinning solution:

[0045] Step 1, preparation of copolymerized polyimide:

[0046] In a three-necked round-bottom flask, under nitrogen protection, 0.7608 g of 3,5-diaminobenzoic acid (DABA, as a diamine containing a carboxyl group), 1.001 g of ODA (as a diamine) and 25 mL of NMP were added in sequence and stirred until DABA and ODA were completely dissolved; then, 4.4423 g of 4,4'-(hexafluoroisopropylene) diphthalic anhydride (6FDA, as a fluorinated dianhydride) was added and polycondensed at 25°C for 24 h; the temperature was raised to 120°C and kept for 1 h; the temperature was further raised to 160°C and kept for 1 h; the temperature was further raised to 200°C and kept for 10 h to obtain a soluble polyimide solution;

[0047] Step 2, preparation of hydroxyethyl methacrylate grafted polyimide:

[0048] 0.65 g of hydroxyethyl methacrylate (HEMA, 5 mmol, as an acrylate monomer containing a photosensitive group), 1.03 g of N,N'-dicyclohexylcarbodiimide (5 mmol), and 0.0608 g of 4-dimethylaminopyridine (0.50 mmol) were added to 6 mL of NMP in sequence, and stirred until the solid was completely dissolved. The mixture was then added to the soluble polyimide solution prepared in step 1, and stirred at room temperature for 24 h to allow Steglich esterification reaction to occur until a precipitate was formed. The solution was centrifuged to obtain a supernatant solution. The solution was then precipitated in deionized water, repeatedly washed, and dried to obtain a photosensitive polyimide, i.e., a polyimide grafted with hydroxyethyl methacrylate.

[0049] Step 3: Preparation of photosensitive polyimide precursor solution:

[0050] The photosensitive polyimide prepared in step 2 was dissolved in NMP according to a mass ratio of photosensitive polyimide: NMP: photoinitiator of 15:83:2, and then the photoinitiator Irgacure-819 was added. The mixture was stirred at room temperature in the dark for 20 minutes to obtain a photosensitive polyimide precursor solution, i.e., a core layer photocrosslinkable polyimide spinning solution. The solid content of the core layer photocrosslinkable polyimide spinning solution was 15 wt %.

[0051] (3) Preparation of polyimide aerogel fibers:

[0052] The skin layer polyamic acid spinning solution prepared in step (1) and the core layer photocrosslinkable polyimide spinning solution prepared in step (2) are extruded through a coaxial needle, wherein the core layer diameter of the coaxial spinning nozzle is 210 μm, the skin layer diameter of the coaxial spinning nozzle is 1.07 mm (the diameter ratio of the skin layer to the core layer is 5.1:1), and the core layer is extruded by an injection pump at an extrusion speed of 50 μL min -1 The cortex is extruded by an air pump at a pressure of 400 kPa. The power is 0.6 W cm -2 The ultraviolet light source was irradiated for 15 seconds, and the molecular chains were cross-linked and solidified to form wet gel fibers. After drawing, solvent replacement in a coagulation bath, normal pressure drying and thermal imidization, a gradient pore structure polyimide aerogel fiber with a porous core layer and a dense skin layer was obtained. The drawing and collection speed was 0.6 m / min. -1 , the drafting ratio is 2 times; the solvent used for solvent replacement is ethanol, the air layer distance between the coaxial spinning nozzle and the coagulation bath liquid surface is controlled to be 1.5 cm, the replacement temperature is 25°C, the replacement time is 48h, and the number of replacements is 5; the temperature of atmospheric drying is 25°C, and the time is 24h; the conditions for thermal imidization are: 150°C for 20min, 250°C for 30min, 300°C for 40min, and the heating rate is 1°C min -1 .

[0053] Figure 1This is a scanning electron microscope image of the gradient pore structure polyimide aerogel fiber with a porous core layer and a dense skin layer obtained in Example 1. Figure 1 It can be seen that both the skin and the core form a porous aerogel structure. However, the polyamide acid in the skin shrinks significantly during atmospheric drying due to the strong molecular chain mobility and weak gel skeleton strength. After thermal imidization, the pores formed are relatively dense and have a smaller pore size. The core layer strengthens the gel skeleton due to UV crosslinking, and the organic-soluble polyimide in the core layer effectively reduces the capillary force due to the presence of trifluoromethyl groups, reducing shrinkage during atmospheric drying and forming a more fluffy porous structure. This confirms the successful preparation of polyimide aerogel fibers with a gradient pore structure and a porous core and dense skin.

[0054] Figure 4 This is the H NMR spectrum of the photosensitive polyimide obtained in Example 1. The peaks at 5.8 ppm, 4.6 ppm, and 4.3 ppm correspond to hydrogen atoms on unsaturated double bonds and methylene groups in the aliphatic chains of the photosensitive polyimide, respectively, corresponding to H1, H2, and H3 in the structural formula. Hydroxyethyl methacrylate (HEMA) was successfully grafted onto the side chains of the polyimide via a Stiglitz esterification reaction, confirming the successful synthesis of the photosensitive polyimide.

[0055] Figure 6 This is an infrared thermal imaging photograph of the gradient pore structure polyimide aerogel fiber with a porous core layer and a dense skin layer obtained in Example 1. Figure 6 It can be seen that when the gradient pore structure polyimide aerogel fiber bundle with a porous core layer and a dense skin layer prepared in Example 1 is placed on a hot stage heated to 200°C, the temperature difference between the fiber bundle surface and the hot stage can reach 80°C, which confirms that the gradient pore structure polyimide aerogel fiber with a porous core layer and a dense skin layer has good thermal insulation properties.

[0056] Comparative Example 1

[0057] A method for preparing polyimide aerogel fibers, referring to Example 1, differs only in that: in step (3), the diameter ratio of the fiber's sheath to the core is adjusted to 4.5:1, the core diameter of the coaxial spinning nozzle is 300 μm, and the sheath diameter of the coaxial spinning nozzle is 1.35 mm.

[0058] Figure 2 These are the cross-sectional scanning electron microscope images, cortex scanning electron microscope images, and core layer scanning electron microscope images of the polyimide aerogel fiber obtained in Comparative Example 1; the cross-sectional scanning electron microscope image of the fiber shows the presence of finger-like holes in the cortex, which is not conducive to the mechanical and thermal insulation properties of the aerogel fiber, and due to the presence of the finger-like holes, the pore size at the edge of the finger-like holes is very small, and the scanning electron microscope image shows a very dense pore structure.

[0059] Comparative Example 2

[0060] A method for preparing polyimide aerogel fibers, referring to Example 1, differs only in that: in step (3), the diameter ratio of the fiber's sheath to core layer is adjusted to 2.7:1, the core layer diameter of the coaxial spinning nozzle is 260 μm, and the sheath diameter of the coaxial spinning nozzle is 0.7 mm.

[0061] Figure 3 The following SEM images show the cross-section, cortex, and core of the polyimide aerogel fiber obtained in Comparative Example 2. While the finger-like pores in the cortex have been largely eliminated, the cortex has experienced significant shrinkage, resulting in a more compact pore structure and poor thermal insulation. The SEM image of the core layer shows that due to the excessively thin cortex, the core layer's solvent replacement rate is accelerated, resulting in larger pores and a poor thermal insulation performance.

[0062] Figure 5 Figure 2 is the stress-strain curve of polyimide aerogel fibers with different fiber diameters. As can be seen from the figure, the fiber corresponding to Example 1 has the best mechanical properties, with a tensile strength of 55.3 MPa, which is the best mechanical strength compared to Comparative Example 1 (43 MPa) and Comparative Example 2 (53 MPa). At the same time, due to the presence of finger-like holes in the cortex in Comparative Example 1, the fracture strain is only 6.9%. Although the finger-like holes are eliminated in Comparative Example 2, the fiber toughness decreases, the rigidity is large, and it is very easy to break. This confirms that the skin-core diameter setting in Example 1 is reasonable, and the obtained polyimide aerogel fiber with a gradient pore structure of porous core layer-dense cortex has good mechanical properties.

[0063] Comparative Example 3

[0064] Zhou [1] et al. prepared polyimide aerogel fibers with excellent mechanical properties using wet spinning technology. However, due to the dense pore structure, the thermal conductivity was only 53 mW m -1 K -1 , its thermal insulation performance is poor, and when it is placed on a hot plate heated to 200℃, the temperature difference between the fiber bundle surface and the hot plate is far less than 80℃. [2] et al. prepared coaxial aerogel fibers through structural design. The thermal insulation performance of aerogel fibers can be improved through structural design, and the thermal conductivity is only 35 mW m -1 K -1 However, its mechanical properties are poor, and the mechanical strength is only 3-4 MPa. It can be seen that achieving a balance between the mechanical and thermal insulation properties of aerogel fibers is a bottleneck problem currently faced.

[0065] [1]Jian Zhou,You-Lo Hsieh.Nanocellulose aerogel-based porous coaxialfibers for thermal insulation.[J].Nano Energy,2020,104305,2211-2855.

[0066] [2]Mengmeng Li, Feng Gan, Jie Dong, Yuting Fang, Xin Zhao, and Qinghua Zhang. Facile Preparation of Continuous and Porous Polyimide Aerogel Fibers for Multifunctional Applications. [J]. ACS Applied Materials&Interfaces 2021 13(8),10416-10427.

[0067] Example 2

[0068] A method for preparing polyimide aerogel fibers is described with reference to Example 1, except that the solid content of the spinning solution is adjusted to 10 wt % in steps (1) and (2).

[0069] Example 3

[0070] A method for preparing polyimide aerogel fibers is described with reference to Example 1, except that the solid content of the spinning solution is adjusted to 20% in steps (1) and (2).

[0071] In Example 2, due to the lower solid content, the skin layer will have larger pores and lower mechanical properties. In Example 3, due to the high solid content, the viscosity of the spinning solution does not match the spinning process, resulting in a discontinuous spinning process and difficulty in efficiently and continuously preparing aerogel fibers. Therefore, the 15% solid content of the skin and core layer in Example 1 is more optimal.

[0072] 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 pore structure of a porous core layer and a dense skin layer, characterized in that: The steps include: (1) Preparation of cortical polyamic acid spinning solution: Under the protection of inert gas, diamine is dissolved in a polar aprotic solvent, and dibasic acid anhydride is slowly added to carry out polycondensation reaction. When there is no obvious change in viscosity, the cortical polyamic acid spinning solution is obtained; the solid content of polyamic acid in the cortical polyamic acid spinning solution is 12~15 wt%; (2) Preparation of core layer photocrosslinkable polyimide spinning solution: Step 1: Under the protection of an inert gas, a diamine containing a carboxyl group or a hydroxyl group and other diamines are added to a polar aprotic solvent, stirred and dissolved, and a fluorinated dianhydride is added to carry out a polycondensation reaction to obtain a polyamic acid solution; Heating to a high temperature for imidization to obtain a soluble polyimide solution; wherein: the molar ratio of the diamine containing a carboxyl or hydroxyl group: the other diamine is 4:6 to 6:4, the molar ratio of the sum of the moles of the diamine containing a carboxyl or hydroxyl group and the other diamines: the molar ratio of the fluorinated dianhydride is 1:(0.94 to 1.04), and the total mass of the diamine containing a carboxyl or hydroxyl group, the other diamines, and the fluorinated dianhydride accounts for 10% to 20% of the total mass of the diamine containing a carboxyl or hydroxyl group, the other diamines, the fluorinated dianhydride, and the polar aprotic solvent; Step 2: Adding a photosensitive group-containing acrylate monomer, a dehydrating agent, and a catalyst to a polar aprotic solvent, stirring until the solid is completely dissolved; adding the monomer to the soluble polyimide solution prepared in step 1, stirring at room temperature for reaction; and separating the resulting product to obtain a solution; The solution is then precipitated in deionized water, washed, and dried to obtain a photosensitive fluorinated polyimide; the molar ratio of the acrylate monomer containing a photosensitive group to the diamine containing a carboxyl group or a hydroxyl group in step 1 is (0.1-100):100; Step 3: dissolving the photosensitive fluorinated polyimide and the photoinitiator prepared in step 2 in a polar aprotic solvent in the dark to obtain a core layer photocrosslinkable polyimide spinning solution; the core layer photocrosslinkable polyimide spinning solution has a solid content of 5 to 30 wt%; (3) Preparation of polyimide aerogel fiber: The skin layer polyamide acid spinning solution prepared in step (1) and the core layer photocrosslinkable polyimide spinning solution prepared in step (2) are extruded through a coaxial needle, cured by ultraviolet irradiation, and the fiber enters a coagulation bath for solvent replacement, dried at normal pressure and subjected to thermal imidization treatment to obtain a polyimide aerogel fiber with a gradient pore structure having a porous core layer and a dense skin layer; wherein the coaxial needle size is: the core layer diameter is 0.2~0.6 mm, and the skin layer diameter is 1~2 mm; In step (3), the diameter ratio of the skin layer to the core layer is controlled to be (5-7):

1.

2. The method according to claim 1, characterized in that In step (1): The diamine includes at least one of 4,4'-diaminodiphenyl ether, p-phenylenediamine, m-phenylenediamine, 3,5-diaminobenzoic acid, 3,3'-diamino-5,5'-bis(trifluoromethyl)biphenyl, 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl, 2-(4-aminophenyl)-5-aminobenzimidazole, and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane; The dibasic acid anhydride includes at least one of 3,3',4,4'-biphenyltetracarboxylic dianhydride, pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, bis(3,4-dicarboxylic acid phthalic anhydride) ether, 4,4'-(hexafluoroisopropylene) diphthalic anhydride or 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride; The molar ratio of dibasic acid anhydride to diamine is 1: (1.01~1.03).

3. 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; Other diamines include at least one of 4,4'-diamino-2,2'-bistrifluoromethylbiphenyl, 4,4-diaminodiphenyl ether, or 2-(4-aminophenyl)-5-aminobenzimidazole; The fluorinated dibasic anhydride is 4,4'-(hexafluoroisopropylene) diphthalic anhydride.

4. The method according to claim 1, wherein In step 2: 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: the dehydrating agent: the catalyst is 10:10:

1.

5. The method according to claim 1, wherein In step 3: The photoinitiator includes at least one of Irgacure-819, Irgacure-184, Irgacure-2959, Irgacure-651, Irgacure-369, Irgacure-907, Irgacure-1300, Irgacure-784, Irgacure-250, Irgacure819DW, Darocur-4265, Darocur BP, Darocur MBF, Darocur TPO, or Darocur-1173; The amount of the photoinitiator used is 0.5-2% of the mass of the core layer photocrosslinkable polyimide spinning solution.

6. The method according to claim 1, characterized in that In step (3): Coaxial needle extrusion parameters: the extrusion speed of the core layer spinning solution injection pump is 10~100 μL·min -1 ;The extrusion pressure of the cortical spinning solution is 100~700 kPa; The power of UV radiation curing is 0.3~3 W·cm -2 , curing time is 0.01~3 min; Control the air layer distance between the coaxial spinning nozzle and the coagulation bath liquid surface to 1~5 cm; The drafting ratio is 1~6 times, and the collection speed is 0.3~15 m·min -1 .

7. The method according to claim 1, characterized in that In step 1: The reaction temperature of the polycondensation reaction is 0~25℃ and the time is 5~24 h.

8. The method according to claim 1, characterized in that In step 3: The polar aprotic solvent includes at least one of 1-methyl-2-pyrrolidone, N,N-dimethylacetamide, or N,N-dimethylformamide.

9. The method according to claim 1, characterized in that In step (3): The solvent used for solvent replacement includes at least one of water, acetone, ethanol, methanol, isopropanol, or n-hexane; the replacement temperature is 25-100°C, the replacement time is 24-96 hours, and the number of replacements is 3-5 times; The temperature of atmospheric pressure drying is 25~100 ℃ and the time is 2~48 h.

10. A polyimide aerogel fiber having a gradient pore structure of porous core layer and dense skin layer, obtained by the method according to any one of claims 1 to 9.

11. Use of the polyimide aerogel fiber with a gradient pore structure having a porous core layer and a dense skin layer as claimed in claim 10 in thermal management clothing.

Images