Ultralight polyimide aerogel and preparation method thereof

Abstract

The invention discloses ultralight polyimide aerogel and a preparation method thereof, and relates to polyimide aerogel and a preparation method thereof. The problem that the density of the polyimide aerogel is relatively high due to the fact that the contradiction between low solid content and high molecular weight is difficult to coordinate in the existing PI aerogel is solved. The ultralight polyimide aerogel is prepared from a polyimide precursor solution through freeze drying and thermal imidization, the solid content of the polyamic acid salt in the polyimide precursor solution is 0.1 wt%-0.2 wt%, and the molecular weight of the polyamic acid salt is gt; 200 thousand; the preparation method comprises the following steps: 1, preparing a polyimide precursor solution based on torque control; 2, freeze drying; and 3, carrying out thermal imidization. The invention is used for the ultralight polyimide aerogel and the preparation thereof.

Description

TECHNICAL FIELD

[0001] The present application relates to a polyimide aerogel and a preparation method thereof. BACKGROUND

[0002] In the field of deep space exploration, the probe such as lunar rover, Mars rover and the like faces the severe challenge of extreme thermal environment. Taking the lunar surface as an example, the diurnal temperature difference is extremely large, the temperature in the sunlit area can reach 127℃, and the temperature in the shadow area can be as low as -173℃, and the components need to withstand a severe thermal shock of more than 200℃ / min during the regional switching process. Therefore, a high-efficiency lightweight thermal insulation material must be used to protect the cabin to ensure the stable operation of the internal instruments and equipment. At the same time, the weight reduction design of the spacecraft is crucial. Studies have shown that every 1 kg of weight reduction can save 500,000 to 500,000 yuan in launch cost from low earth orbit to lunar orbit, and effectively improve the payload capacity of scientific instruments. Under this background, aerogel has become one of the ideal materials for the thermal protection system of high-end equipment such as spacecraft and extraterrestrial landing vehicles due to its extremely low density and excellent thermal insulation performance.

[0003] Silica (SiO2) aerogel is the earliest aerogel material successfully prepared. However, the inherent high brittleness of this type of aerogel makes it extremely easy to pulverize or break under the severe vibration during the launch stage, greatly limiting its wide application in the field of aerospace.

[0004] As a latecomer, polyimide (PI) aerogel combines the excellent mechanical properties, high and low temperature resistance of polyimide material and the lightweight porous structure of aerogel, and its molecular structure has high designability, showing broad application prospects. However, the currently reported PI aerogel density is mostly above 80 mg / cm³, and how to realize its ultra-lightweight is the key to expand its deep space application.

[0005] Currently, there are two technical routes for the preparation of PI aerogel: one is to synthesize the precursor in an organic solvent, and then to prepare it by chemical imidization and supercritical drying; the other is to synthesize the precursor in an aqueous solvent, and then to prepare it by freeze-drying and thermal imidization. The common bottleneck of these two routes is the difficulty in coordinating the contradiction between "low solid content" and "high molecular weight". When the solid content of the precursor solution is too low, the distance between monomers increases, and the collision probability decreases sharply, resulting in the inability to form long-chain structures with high molecular weight. The low molecular weight precursor lacks sufficient interchain entanglement, and cannot build a stable three-dimensional network at very low concentration, causing the collapse of the gel structure during the drying process, and the inability to form a complete aerogel. Therefore, the density of the PI aerogel prepared by the existing method is usually difficult to be less than 30 mg / cm³. 3 . SUMMARY

[0006] This invention aims to solve the problem that existing PI aerogels are difficult to coordinate between low solid content and high molecular weight, resulting in high density of polyimide aerogels, and thus provides an ultralight polyimide aerogel and its preparation method.

[0007] An ultralight polyimide aerogel is prepared by freeze-drying and thermal imidization of a polyimide precursor solution; the solid content of the polyamic acid salt in the polyimide precursor solution is 0.1wt%~0.2wt%, and the molecular weight of the polyamic acid salt is >200,000;

[0008] The polyimide precursor solution is prepared by torque control from aromatic diamine, aromatic dianhydride, tertiary amine and deionized water; the molar ratio of aromatic diamine to aromatic dianhydride is 1:1; the molar ratio of aromatic diamine to tertiary amine is 1:(2~2.5); the mass ratio of the total mass of aromatic diamine and aromatic dianhydride to the mass of deionized water is 1:(500~1000);

[0009] The repeating unit of the polyamic acid salt in the aqueous solution of the polyimide precursor is:

[0010] ;

[0011] R1 is and One or two combinations thereof;

[0012] R2 is or .

[0013] A method for preparing an ultralight polyimide aerogel, comprising the following steps:

[0014] I. Preparation of polyimide precursor solution based on torque control:

[0015] ① Weigh out the aromatic diamine, aromatic dianhydride, tertiary amine, and deionized water;

[0016] The molar ratio of the aromatic diamine to the aromatic dianhydride is 1:1; the molar ratio of the aromatic diamine to the tertiary amine is 1:(2~2.5); the mass ratio of the total mass of the aromatic diamine and the aromatic dianhydride to the mass of deionized water is 1:(500~1000).

[0017] ② Divide the weighed deionized water into two portions, with the total mass of the aromatic diamine and aromatic dianhydride being 1:(8~10) of the first portion of deionized water, and the remaining deionized water being the second portion of deionized water; preheat the second portion of deionized water to 60℃~80℃.

[0018] ③ Under a nitrogen atmosphere, add the aromatic diamine to the first portion of deionized water and stir to dissolve. Then raise the reaction temperature to 60℃~80℃, and then add the tertiary amine and aromatic dianhydride in sequence to obtain the reaction system.

[0019] ④ Under a nitrogen atmosphere, at a temperature of 60℃~80℃ and with stirring, stir the reaction system. When the stirring paddle torque is greater than 25 N·cm, add the second part of preheated deionized water dropwise. When the stirring paddle torque is 20 N·cm, stop adding the second part of preheated deionized water and continue stirring the reaction under a nitrogen atmosphere, at a temperature of 60℃~80℃ and with stirring.

[0020] ⑤ Repeat step ④ until the stirring paddle torque is maintained at 20 N·cm~25 N·cm for 1 hour of continuous stirring reaction, and obtain the reaction system after torque control;

[0021] ⑥ Add the remaining preheated second portion of deionized water to the torque-controlled reaction system all at once. Under nitrogen atmosphere, temperature of 60℃~80℃ and stirring conditions, stir and react for 2h~4h to obtain polyimide precursor solution.

[0022] II. Freeze-drying:

[0023] The polyimide precursor solution was poured into a mold and cooled to room temperature. Then, it was frozen and freeze-dried in sequence to obtain the polyimide precursor dry gel.

[0024] III. Thermal imidization:

[0025] Ultralight polyimide aerogels were prepared by thermal imidization of the dry gel of the polyimide precursor.

[0026] The beneficial effects of this invention are:

[0027] This invention achieves breakthroughs in controlling the molecular weight of polyimide precursors and the density of aerogels through molecular design and process innovation. Its beneficial effects and mechanisms of action are detailed below:

[0028] (1) This invention solves the contradiction between low solid content and high molecular weight in the polymerization process, achieving a breakthrough in product performance. This invention controls viscosity through torque feedback, enabling the polymerization reaction to proceed rapidly at a high monomer concentration, while avoiding problems such as runaway polymerization and rod climbing caused by viscosity runaway, and successfully synthesizing ultra-high molecular weight polyimide precursors with a number average molecular weight exceeding 200,000. The ultra-long molecular chains can still form a stable three-dimensional network structure through sufficient inter-chain entanglement and strong intermolecular forces even at extremely low solid content (0.1wt%). This breaks through the technical bottleneck of traditional methods where the gel structure inevitably collapses at low solid content due to insufficient molecular weight, and for the first time achieves the preparation of an ultra-light polyimide aerogel with self-support, bendability, and more than 90% rebound after 500 compressions at a solid content of 0.1wt%.

[0029] (2) An ultralight aerogel product with excellent comprehensive performance was obtained. a. Ultra-low density: Thanks to the extremely low solid content of the precursor solution, the density of the prepared polyimide aerogel is 2.08 mg / cm³. 3 This is significantly lower than most products currently reported (typically >30 mg / cm³). 3 (a) Provides key material support for achieving extreme weight reduction in aircraft. (b) Excellent mechanical properties and flexibility: The stable network constructed by ultra-high molecular weight makes the aerogel both ultra-lightweight and possesses good mechanical strength and flexibility. It can be bent multiple times without breaking and can rebound after multiple compressions, effectively overcoming the fatal shortcomings of traditional SiO2 aerogels, such as high brittleness and easy pulverization. (c) Excellent thermal insulation performance: The extremely low density and nanoporous structure ensure that the material has extremely low thermal conductivity. Combined with the excellent high and low temperature resistance of PI itself, it can serve as an effective thermal protection material in extreme environments ranging from -196 to 350℃. Attached Figure Description

[0030] Figure 1 The images show the physical photos of the ultralight polyimide aerogel prepared in Example 1: (a) a weighing photo; (b) a photo showing the measured dimensions.

[0031] Figure 2 Photographs of the bending, compression and rebound of the ultralight polyimide aerogel prepared in Example 1: (a) bending, (b) compression, (c) compression and rebound.

[0032] Figure 3 Compression cycle of the ultralight polyimide aerogel prepared in Example 1;

[0033] Figure 4 The infrared spectrum of the ultralight polyimide aerogel prepared in Example 1;

[0034] Figure 5The product temperature-viscosity curve is shown in step 1, ⑤ of Example 1, where the stirring paddle torque is maintained at 20 N·cm to 25 N·cm throughout the 1 hour of continuous stirring reaction.

[0035] Figure 6 The DSC test result is shown for the ultralight polyimide aerogel prepared in Example 1.

[0036] Figure 7 The results of gel permeation chromatography (GPC) of the polyimide precursor solution prepared in step one of Example 1;

[0037] Figure 8 The image shows a stirring impeller whose torque remained between 20 N·cm and 25 N·cm during the continuous stirring reaction for 1 hour in step 1, ⑤ of Example 1. Detailed Implementation

[0038] Specific Implementation Method 1: An ultralight polyimide aerogel of this embodiment is prepared by freeze-drying and thermal imidization of a polyimide precursor solution; the solid content of the polyamic acid salt in the polyimide precursor solution is 0.1wt%~0.2wt%, and the molecular weight of the polyamic acid salt is >200,000;

[0039] The polyimide precursor solution is prepared by torque control from aromatic diamine, aromatic dianhydride, tertiary amine and deionized water; the molar ratio of aromatic diamine to aromatic dianhydride is 1:1; the molar ratio of aromatic diamine to tertiary amine is 1:(2~2.5); the mass ratio of the total mass of aromatic diamine and aromatic dianhydride to the mass of deionized water is 1:(500~1000);

[0040] The repeating unit of the polyamic acid salt in the aqueous solution of the polyimide precursor is:

[0041] ;

[0042] R1 is and One or two combinations thereof;

[0043] R2 is or .

[0044] In this specific embodiment, an ultra-high molecular weight, ultra-low solid content polyimide precursor solution is synthesized from aromatic diamine, aromatic dianhydride and tertiary amine.

[0045] This specific embodiment achieves the construction of a three-dimensional network framework for ultra-low density polyimide aerogel through the synthesis of ultra-long molecular chains and the abundance of intermolecular hydrogen bonds.

[0046] This specific implementation method achieves a fundamental breakthrough in the synthesis of ultra-low solids content polyimide precursor solutions and the construction of ultralight aerogels through molecular structure design and an innovative "torque-controlled polymerization" process. Its core lies in the synergistic effect of the strong interchain entanglement generated by ultra-high molecular weight and the strong hydrogen bond network provided by the specific molecular structure, systematically resolving the contradiction between "low solids content" and "stable gel network." Its beneficial effects and mechanism of action are detailed below:

[0047] The key to this implementation lies in precisely maintaining the precursor solution viscosity within the optimal viscosity window of 2 Pa·s to 10 Pa·s by real-time monitoring and control of the torque (20 N·cm ~ 25 N·cm) of the polymerization system. This process solves the viscosity runaway problem that inevitably leads to rapid polymerization at high concentrations. This viscosity window ensures that the monomer has a sufficient collision frequency to maintain the high-speed polymerization reaction, while timely dilution with water at the same temperature avoids mass transfer obstruction, local overheating, and "bursting polymerization" caused by viscosity spikes. This creates an ideal kinetic environment for the continuous and stable growth of polyamic acid molecular chains, thereby successfully driving the continuous growth of molecular chain length and synthesizing ultra-high molecular weight precursors with molecular weights exceeding 200,000.

[0048] The formation and enhancement mechanism of a stable three-dimensional network under ultra-low solid content: First, the physical entanglement of ultra-long molecular chains. Ultra-long molecular chains with molecular weights exceeding 200,000 have enormous random coil sizes, which interpenetrate and entangle in solution, forming a transiently physically cross-linked network with extremely stable topological structure. Even under extreme conditions with a solid content as low as 0.1 wt%, this network formed by the entanglement of ultra-long chains is sufficient to macroscopically lock the solvent, resist structural shrinkage, and provide a skeletal basis for the gel. Second, the hydrogen bond enhancement effect of the specifically designed monomer. The aromatic diamine monomer selected in this embodiment has a molecular structure specifically designed to introduce strong hydrogen bonds, serving as key "physical cross-linking points" to further enhance the network. When 2-(3-aminophenyl)-5-aminobenzimidazole is used, the benzimidazole ring in its molecule is an extremely strong hydrogen bond donor and acceptor. It can form numerous strong hydrogen bonds with the amide carbonyl groups on adjacent molecular chains. These hydrogen bonds effectively fix the relative positions between molecular chains, greatly enhancing the stability of the entangled network. When 4',4-diaminodiphenyl sulfone is selected, its strong electron-withdrawing effect of the sulfone group can activate the hydrogen atoms on the adjacent amide group, enabling them to form stronger hydrogen bonds with the carbonyl group on another chain. At the same time, the sulfone itself can also act as a hydrogen bond acceptor, jointly constructing a robust inter-chain linkage.

[0049] Long chains and strong hydrogen bonds form a synergistic mechanism. "Ultra-long chain entanglement" constitutes the framework of the macroscopic network, while "strong hydrogen bonding" strengthens the entanglement points at the molecular scale. Together, they construct a relatively stable three-dimensional network even under extremely dilute conditions (0.1 wt% solid content). This network can effectively resist the huge capillary forces during subsequent freeze-drying, ensuring structural integrity and preventing collapse.

[0050] Based on the aforementioned synergistic design at the molecular and process levels, the ultralight polyimide aerogel prepared in this embodiment exhibits the following disruptive properties: 1. Achieving extreme lightweight: Directly benefiting from the ultra-low precursor solid content of 0.1 wt%, an ultralight aerogel with a density far lower than that of existing technologies was successfully prepared, with a minimum density of 2.08 mg / cm³. 3 This provides crucial material support for achieving stringent weight reduction goals in spacecraft. 2. Combining flexibility and compression resilience: The three-dimensional network constructed by ultra-high molecular weight and strong hydrogen bonds endows aerogel nanoporous materials with rare mechanical properties. The product not only possesses a certain level of mechanical strength, but more importantly, exhibits excellent flexibility and resilience, capable of bending without pulverizing, completely overcoming the brittleness bottleneck of traditional silica aerogels, making it perfectly suitable for the severe vibration environment during spacecraft launches. The extremely low density and porous structure ensure extremely low thermal conductivity. Combined with the excellent thermal stability of polyimide itself, this aerogel can serve as a highly efficient and reliable long-term thermal barrier in extreme temperature environments ranging from -196℃ to 350℃.

[0051] The beneficial effects of this specific implementation method are:

[0052] This specific implementation method achieves a breakthrough in controlling the molecular weight of polyimide precursors and the density of aerogels through molecular design and process innovation. Its beneficial effects and mechanisms of action are detailed below:

[0053] (1) This specific embodiment solves the contradiction between low solid content and high molecular weight in the polymerization process, achieving a breakthrough in product performance. This specific embodiment controls viscosity through torque feedback, enabling the polymerization reaction to proceed rapidly at a high monomer concentration, while avoiding problems such as runaway polymerization and rod climbing caused by viscosity runaway, and successfully synthesizing an ultra-high molecular weight polyimide precursor with a number average molecular weight exceeding 200,000. The ultra-long molecular chains can still form a stable three-dimensional network structure through sufficient inter-chain entanglement and strong intermolecular forces even at extremely low solid content (0.1wt%). This breaks through the technical bottleneck of traditional methods where the gel structure inevitably collapses at low solid content due to insufficient molecular weight, and for the first time achieves the preparation of an ultra-light polyimide aerogel with self-support, bendability, and more than 90% rebound after 500 compressions at a solid content of 0.1wt%.

[0054] (2) An ultralight aerogel product with excellent comprehensive performance was obtained. a. Ultra-low density: Thanks to the extremely low solid content of the precursor solution, the density of the prepared polyimide aerogel is 2.08 mg / cm³. 3 This is significantly lower than most products currently reported (typically >30 mg / cm³). 3 (a) Provides key material support for achieving extreme weight reduction in aircraft. (b) Excellent mechanical properties and flexibility: The stable network constructed by ultra-high molecular weight makes the aerogel both ultra-lightweight and possesses good mechanical strength and flexibility. It can be bent multiple times without breaking and can rebound after multiple compressions, effectively overcoming the fatal shortcomings of traditional SiO2 aerogels, such as high brittleness and easy pulverization. (c) Excellent thermal insulation performance: The extremely low density and nanoporous structure ensure that the material has extremely low thermal conductivity. Combined with the excellent high and low temperature resistance of PI itself, it can serve as an effective thermal protection material in extreme environments ranging from -196 to 350℃.

[0055] Specific Embodiment Two: This embodiment differs from Specific Embodiment One in that the aromatic diamine is one or a combination of two of 2-(3-aminophenyl)-5-aminobenzimidazole and 4,4'-diaminodiphenyl sulfone. Everything else is the same as in Specific Embodiment One.

[0056] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the aromatic dianhydride is 3,3',4,4'-biphenyltetracarboxylic dianhydride. Everything else is the same as in Specific Implementation Method One or Two.

[0057] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the tertiary amine is triethylamine or triethanolamine. Everything else is the same as in Specific Implementation Methods One to Three.

[0058] Specific Implementation Method 5: This implementation method provides a method for preparing ultralight polyimide aerogel, which is carried out according to the following steps:

[0059] I. Preparation of polyimide precursor solution based on torque control:

[0060] ① Weigh out the aromatic diamine, aromatic dianhydride, tertiary amine, and deionized water;

[0061] The molar ratio of the aromatic diamine to the aromatic dianhydride is 1:1; the molar ratio of the aromatic diamine to the tertiary amine is 1:(2~2.5); the mass ratio of the total mass of the aromatic diamine and the aromatic dianhydride to the mass of deionized water is 1:(500~1000).

[0062] ② Divide the weighed deionized water into two portions, with the total mass of the aromatic diamine and aromatic dianhydride being 1:(8~10) of the first portion of deionized water, and the remaining deionized water being the second portion of deionized water; preheat the second portion of deionized water to 60℃~80℃.

[0063] ③ Under a nitrogen atmosphere, add the aromatic diamine to the first portion of deionized water and stir to dissolve. Then raise the reaction temperature to 60℃~80℃, and then add the tertiary amine and aromatic dianhydride in sequence to obtain the reaction system.

[0064] ④ Under a nitrogen atmosphere, at a temperature of 60℃~80℃ and with stirring, stir the reaction system. When the stirring paddle torque is greater than 25 N·cm, add the second part of preheated deionized water dropwise. When the stirring paddle torque is 20 N·cm, stop adding the second part of preheated deionized water and continue stirring the reaction under a nitrogen atmosphere, at a temperature of 60℃~80℃ and with stirring.

[0065] ⑤ Repeat step ④ until the stirring paddle torque is maintained at 20 N·cm~25 N·cm for 1 hour of continuous stirring reaction, and obtain the reaction system after torque control;

[0066] ⑥ Add the remaining preheated second portion of deionized water to the torque-controlled reaction system all at once. Under nitrogen atmosphere, temperature of 60℃~80℃ and stirring conditions, stir and react for 2h~4h to obtain polyimide precursor solution.

[0067] II. Freeze-drying:

[0068] The polyimide precursor solution was poured into a mold and cooled to room temperature. Then, it was frozen and freeze-dried in sequence to obtain the polyimide precursor dry gel.

[0069] III. Thermal imidization:

[0070] Ultralight polyimide aerogels were prepared by thermal imidization of the dry gel of the polyimide precursor.

[0071] Specific Implementation Method Six: This implementation method differs from Specific Implementation Method Five in that the stirring speed described in steps ③ to ⑥ of step one is 200 rpm to 300 rpm. Everything else is the same as in Specific Implementation Method Five.

[0072] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Method Five or Six in that: in step one ③, under a nitrogen atmosphere, a temperature of 40℃~48℃, and with stirring, the aromatic diamine is added to the first portion of deionized water and stirred to dissolve for 1h~2h. Everything else is the same as in Specific Implementation Method Five or Six.

[0073] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods Five to Seven in that the freezing described in step two is specifically performed on a cold plate at a temperature of -30℃ to -40℃ for 1 to 2 hours. Everything else is the same as in Specific Implementation Methods Five to Seven.

[0074] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods Five to Eight in that the freeze-drying described in step two is specifically carried out at a temperature of -2℃ to -5℃ and a pressure of 0Pa to 20Pa for 2 to 3 days. Everything else is the same as in Specific Implementation Methods Five to Eight.

[0075] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods Five to Nine in that the thermal imidization described in step three is carried out according to the following steps: first, maintaining a temperature of 60℃~80℃ for 5h~6h; then maintaining a temperature of 150℃~160℃ for 1h~2h; then maintaining a temperature of 240℃~250℃ for 1h~2h; and finally maintaining a temperature of 290℃~300℃ for 1h~2h. Everything else is the same as in Specific Implementation Methods Five to Nine.

[0076] The beneficial effects of the present invention are verified using the following embodiments:

[0077] Example 1:

[0078] An ultralight polyimide aerogel is prepared by freeze-drying and thermal imidization of a polyimide precursor solution; the solid content of the polyamic acid salt in the polyimide precursor solution is 0.1 wt%.

[0079] The polyimide precursor solution is prepared by torque control from aromatic diamine, aromatic dianhydride, tertiary amine and deionized water; the molar ratio of aromatic diamine to aromatic dianhydride is 1:1; the molar ratio of aromatic diamine to tertiary amine is 1:2; the mass ratio of the total mass of aromatic diamine and aromatic dianhydride to the mass of deionized water is 1:1000.

[0080] The repeating unit of the polyamic acid salt in the aqueous solution of the polyimide precursor is:

[0081] ;

[0082] R1 is ;

[0083] R2 is .

[0084] The aromatic diamine is 2-((3-aminophenyl)-5-aminobenzimidazole.

[0085] The aromatic dianhydride is 3,3',4,4'-biphenyltetracarboxylic dianhydride.

[0086] The tertiary amine mentioned is triethylamine.

[0087] The above-mentioned method for preparing ultralight polyimide aerogel is carried out according to the following steps:

[0088] I. Preparation of polyimide precursor solution based on torque control:

[0089] ① Weigh out the aromatic diamine, aromatic dianhydride, tertiary amine, and deionized water;

[0090] The molar ratio of the aromatic diamine to the aromatic dianhydride is 1:1; the molar ratio of the aromatic diamine to the tertiary amine is 1:2; and the mass ratio of the total mass of the aromatic diamine and the aromatic dianhydride to the mass of deionized water is 1:1000.

[0091] ② Divide the weighed deionized water into two portions, with the total mass of the aromatic diamine and aromatic dianhydride being 1:9 of the mass of the first portion of deionized water, and the remaining deionized water being the second portion of deionized water; preheat the second portion of deionized water to 60°C.

[0092] ③ Under a nitrogen atmosphere, at a temperature of 40℃ and with stirring, the aromatic diamine was added to the first portion of deionized water and stirred to dissolve for 1 hour. Then the reaction temperature was raised to 60℃, and then the tertiary amine and aromatic dianhydride were added in sequence to obtain the reaction system.

[0093] ④ Under nitrogen atmosphere, temperature of 60℃ and stirring conditions, stir the reaction system. When the stirring paddle torque is greater than 25 N·cm, add the second part of preheated deionized water dropwise. When the stirring paddle torque is 20 N·cm, stop adding the second part of preheated deionized water and continue stirring the reaction under nitrogen atmosphere, temperature of 60℃ and stirring conditions.

[0094] ⑤ Repeat step ④ until the stirring paddle torque is maintained at 20 N·cm~25 N·cm for 1 hour of continuous stirring reaction, and obtain the reaction system after torque control;

[0095] ⑥ Add the remaining preheated second portion of deionized water to the torque-controlled reaction system all at once. Under nitrogen atmosphere, temperature of 60℃ and stirring conditions, stir and react for 3 hours to obtain a polyimide precursor solution.

[0096] II. Freeze-drying:

[0097] The polyimide precursor solution was poured into a mold and cooled to room temperature. Then, it was frozen and freeze-dried in sequence to obtain the polyimide precursor dry gel.

[0098] III. Thermal imidization:

[0099] Ultralight polyimide aerogels were prepared by thermal imidization of the dry gel of the polyimide precursor.

[0100] The stirring speed described in steps 1, ③ to 6, is 240 rpm.

[0101] The freezing process described in step two involves freezing the food on a cold plate at a temperature of -40°C for 2 hours.

[0102] The freeze-drying described in step two specifically involves freeze-drying for 3 days at a temperature of -2℃ and a pressure of 10Pa.

[0103] The thermal imidization described in step three is carried out in the following steps: first, it is kept at 80°C for 5 hours, then at 150°C for 1 hour, then at 240°C for 1 hour, and finally at 300°C for 1 hour.

[0104] Example 2: This example differs from Example 1 in that the aromatic diamine is 4',4-diaminodiphenyl sulfone; and R1 is... Everything else is the same as in Example 1.

[0105] Example 3: This example differs from Example 1 in that the tertiary amine is triethanolamine; and R2 is... Everything else is the same as in Example 1.

[0106] Example 4: This example differs from Example 1 in that the molar ratio of the aromatic diamine to the tertiary amine is 1:2.5. Everything else is the same as in Example 1.

[0107] Example 5: This example differs from Example 1 in that the solid content of polyamic acid salt in the polyimide precursor solution is 0.2 wt%; and the mass ratio of the total mass of the aromatic diamine and aromatic dianhydride to the mass of deionized water is 1:500. Everything else is the same as in Example 1.

[0108] Example 6: This example differs from Example 1 in that the aromatic diamine is a combination of 2-(3-aminophenyl)-5-aminobenzimidazole and 4,4'-diaminodiphenyl sulfone in a molar ratio of 1:1. Everything else is the same as in Example 1.

[0109] Example 7: This example differs from Example 1 in the following ways: Step 1 ②: Preheat the second portion of deionized water to 80°C; Step 3: Raise the reaction temperature to 80°C, then add the tertiary amine and aromatic dianhydride sequentially to obtain the reaction system; Step 4: Under a nitrogen atmosphere, at a temperature of 80°C, and with stirring, stir the reaction system. When the stirring paddle torque is greater than 25 N·cm, add the preheated second portion of deionized water dropwise. When the stirring paddle torque is 20 N·cm, stop adding the preheated second portion of deionized water and continue stirring under a nitrogen atmosphere, at a temperature of 80°C; Step 6: Under a nitrogen atmosphere, at a temperature of 80°C, and with stirring, stir the reaction for 3 hours to obtain the polyimide precursor solution. Everything else is the same as in Example 1.

[0110] Comparative Experiment 1: This comparative experiment differs from Example 1 in that the molar ratio of the aromatic diamine to the aromatic dianhydride is 1:1.05. Everything else is the same as in Example 1.

[0111] Comparative Experiment 2: This comparative experiment differs from Example 1 in that the aromatic diamine used is p-phenylenediamine. Everything else is the same as in Example 1.

[0112] Comparative Experiment 3: This comparative experiment differs from Example 1 in that the temperature of the second portion of deionized water in step 1, ② is 25°C. Everything else is the same as in Example 1.

[0113] Comparative Experiment 4: This comparative experiment differs from Example 1 in the following ways: In step 1, ④, the reaction system was stirred under a nitrogen atmosphere, at a temperature of 60°C, and with stirring. When the stirring paddle torque exceeded 15 N·cm, a second portion of preheated deionized water was added dropwise. When the stirring paddle torque reached 10 N·cm, the addition of the second portion of preheated deionized water was stopped, and the reaction was continued under a nitrogen atmosphere, at a temperature of 60°C, and with stirring. ⑤ Step 4 was repeated until the stirring paddle torque remained consistently between 10 N·cm and 15 N·cm for 1 hour of continuous stirring, thus obtaining a torque-controlled reaction system. Everything else was the same as in Example 1.

[0114] (1) The polyimide aerogels prepared in Examples 1 to 7 and Comparative Experiments 1 to 4 were subjected to various performance tests, and the test conditions were in accordance with the following standards (methods):

[0115] 1. Density determination: Weigh the sample (m) using an electronic balance (in milligrams). Measure the dimensions of the sample (in centimeters) using vernier calipers. Calculate the aerogel volume (V). The apparent density is obtained using the formula ρ = m / V (in mg / cm³). 3 .

[0116] 2. Glass transition temperature test: The glass transition temperature of the ultralight polyimide aerogel was tested according to GB / T 19466.2-2004.

[0117] 3. Compression strength test: The compression strength of the ultralight polyimide aerogel was tested using an electronic universal testing machine in accordance with GB / T 3159-2008 standard.

[0118] 4. Rebound test after 500 compression cycles: First, measure the thickness of the ultralight polyimide aerogel before the experiment in millimeters. Then, use a fatigue testing machine (INSTRON 8872, UK) to compress the aerogel to 50% and perform 500 compression cycles. After the experiment, measure the thickness of the aerogel again.

[0119] 5. Infrared test: The infrared spectrum of the ultralight polyimide aerogel prepared in Example 1 was tested.

[0120] 6. Thermal conductivity test: The thermal conductivity of the ultralight polyimide aerogel was tested using a Netzsch LFA 467 laser thermal conductivity meter (Germany); Thermal conductivity of the aerogel after immersion in liquid nitrogen (-196℃) for 1 min: The aerogel was completely immersed in liquid nitrogen for 1 min, removed and left to stand for 5 min before the thermal conductivity of the aerogel was tested; Thermal conductivity of the aerogel after 500 compressions: The aerogel was compressed 500 times using the method in test condition 4, and the thermal conductivity of the aerogel was measured; Thermal conductivity of the aerogel after aging in an oven at 350℃ for 30 min: The aerogel was placed in an oven at 350℃ for 30 min before the thermal conductivity of the aerogel was measured.

[0121] (2) Polyimide precursor solution test:

[0122] 1. Molecular weight test: The number-average molecular weight (Mn) of the solute in the polyimide precursor solution prepared in step one of Example 1 was determined using a FEI Sirion 200 chromatographic analyzer. The mobile phase was chromatographic grade tetrahydrofuran, and polystyrene was used as an internal standard. The sample was placed in an 80°C oven for 6 hours to remove the solvent, and then dissolved in chromatographic grade tetrahydrofuran.

[0123] 2. Viscosity-temperature curve test: The product sampled in step 1, ⑤ of Example 1, where the stirring paddle torque was maintained at 20 N·cm to 25 N·cm during continuous stirring reaction for 1 hour was taken using a TA HR30 rheometer for viscosity-temperature rheology test. The temperature range was 25℃ to 70℃, and the heating rate was 5℃ / min.

[0124] Table 1

[0125]

[0126] Figure 1 The images show the physical composition of the ultralight polyimide aerogel prepared in Example 1: (a) a weighing photograph; (b) a photograph showing the measured dimensions. As can be seen from the figures, the volume of the aerogel prepared in Example 1 is 4 × 6 × 1 cm. 3The ultralight polyimide aerogel weighs 0.0509 g, and its density is 2.08 mg / cm³. 3 .

[0127] Figure 2 The images show the bending, compression, and rebound of the ultralight polyimide aerogel prepared in Example 1: (a) bending, (b) compression, and (c) compression rebound. As can be seen from the images, the ultralight aerogel has good mechanical properties.

[0128] Figure 3 Compression cycling of the ultralight polyimide aerogel prepared in Example 1; as shown in the figure, after 500 compression cycles, the height of the aerogel recovered by more than 90%.

[0129] Figure 4 The infrared spectrum of the ultralight polyimide aerogel prepared in Example 1 is shown in the figure; as can be seen from the figure, from 1780 cm⁻¹... -1 and 1720cm -1 The presence of a carbonyl symmetric absorption peak indicates that iminolation is complete.

[0130] Figure 5 The figure shows the product temperature-viscosity curves during step 1, ⑤ of Example 1, where the stirring paddle torque remained between 20 N·cm and 25 N·cm for 1 hour of continuous stirring. As can be seen from the figure, the viscosity of the polyimide precursor solution during the polymerization process is between 2 Pa·s and 8 Pa·s.

[0131] Figure 6 The image shows the DSC test result of the ultralight polyimide aerogel prepared in Example 1. As can be seen from the image, the glass transition temperature of the ultralight polyimide aerogel is 386℃.

[0132] Figure 7 The figure shows the gel permeation chromatography (GPC) test results of the polyimide precursor solution prepared in step one of Example 1. As can be seen from the figure, the number average molecular weight Mn of the ultra-high molecular weight polyimide precursor solution is 200,822, the weight average molecular weight Mw is 225,300, the molecular weight is greater than 200,000 and the distribution is relatively narrow.

[0133] Table 2 shows the gel permeation chromatography results of the ultra-high molecular weight polyimide precursor solution prepared in Example 1.

[0134]

[0135] Figure 8 The image shows a photograph of the stirring paddle whose torque remained between 20 N·cm and 25 N·cm during the continuous stirring reaction for 1 hour in step 1 of Example 1. As can be seen from the figure, the torque during the polymerization process was 22.8 N·cm, which is within the range of 20 N·cm to 25 N·cm.

Claims

1. An ultralight polyimide aerogel, characterized in that... It is prepared by freeze-drying and thermal imidization of a polyimide precursor solution; the solid content of the polyamic acid salt in the polyimide precursor solution is 0.1wt%~0.2wt%, and the molecular weight of the polyamic acid salt is >200,000; The polyimide precursor solution is prepared by torque control from aromatic diamine, aromatic dianhydride, tertiary amine and deionized water; the molar ratio of aromatic diamine to aromatic dianhydride is 1:1; the molar ratio of aromatic diamine to tertiary amine is 1:(2~2.5); the mass ratio of the total mass of aromatic diamine and aromatic dianhydride to the mass of deionized water is 1:(500~1000); The repeating unit of the polyamic acid salt in the aqueous solution of the polyimide precursor is: ； R1 is and One or two combinations thereof; R2 is or .

2. The ultralight polyimide aerogel according to claim 1, characterized in that... The aromatic diamine is one or a combination of two of 2-(3-aminophenyl)-5-aminobenzimidazole and 4,4'-diaminodiphenyl sulfone.

3. The ultralight polyimide aerogel according to claim 1, characterized in that... The aromatic dianhydride is 3,3',4,4'-biphenyltetracarboxylic dianhydride.

4. The ultralight polyimide aerogel according to claim 1, characterized in that... The tertiary amine is triethylamine or triethanolamine.

5. The method for preparing an ultralight polyimide aerogel as described in claim 1, characterized in that... It is done in the following steps: I. Preparation of polyimide precursor solution based on torque control: ① Weigh out the aromatic diamine, aromatic dianhydride, tertiary amine, and deionized water; The molar ratio of the aromatic diamine to the aromatic dianhydride is 1:1; the molar ratio of the aromatic diamine to the tertiary amine is 1:(2~2.5); the mass ratio of the total mass of the aromatic diamine and the aromatic dianhydride to the mass of deionized water is 1:(500~1000). ② Divide the weighed deionized water into two portions, with the total mass of the aromatic diamine and aromatic dianhydride being 1:(8~10) of the first portion of deionized water, and the remaining deionized water being the second portion of deionized water; preheat the second portion of deionized water to 60℃~80℃. ③ Under a nitrogen atmosphere, add the aromatic diamine to the first portion of deionized water and stir to dissolve. Then raise the reaction temperature to 60℃~80℃, and then add the tertiary amine and aromatic dianhydride in sequence to obtain the reaction system. ④ Under a nitrogen atmosphere, at a temperature of 60℃~80℃ and with stirring, stir the reaction system. When the stirring paddle torque is greater than 25 N·cm, add the second part of preheated deionized water dropwise. When the stirring paddle torque is 20 N·cm, stop adding the second part of preheated deionized water and continue stirring the reaction under a nitrogen atmosphere, at a temperature of 60℃~80℃ and with stirring. ⑤ Repeat step ④ until the stirring paddle torque is maintained at 20 N·cm~25 N·cm for 1 hour of continuous stirring reaction, and obtain the reaction system after torque control; ⑥ Add the remaining preheated second portion of deionized water to the torque-controlled reaction system all at once. Under nitrogen atmosphere, temperature of 60℃~80℃ and stirring conditions, stir and react for 2h~4h to obtain polyimide precursor solution. II. Freeze-drying: The polyimide precursor solution was poured into a mold and cooled to room temperature. Then, it was frozen and freeze-dried in sequence to obtain the polyimide precursor dry gel. III. Thermal imidization: Ultralight polyimide aerogels were prepared by thermal imidization of the dry gel of the polyimide precursor.

6. The method for preparing an ultralight polyimide aerogel according to claim 5, characterized in that... The stirring speed described in steps 1, ③ to 6, is 200 rpm to 300 rpm.

7. The method for preparing an ultralight polyimide aerogel according to claim 5, characterized in that... In step 1, under a nitrogen atmosphere, at a temperature of 40℃~48℃ and with stirring, the aromatic diamine is added to the first portion of deionized water and stirred to dissolve for 1h~2h.

8. The method for preparing an ultralight polyimide aerogel according to claim 5, characterized in that... The freezing process described in step two involves freezing the food on a cold plate at a temperature of -30℃ to -40℃ for 1 to 2 hours.

9. The method for preparing an ultralight polyimide aerogel according to claim 5, characterized in that... The freeze-drying described in step two is specifically carried out at a temperature of -2℃ to -5℃ and a pressure of 0Pa to 20Pa for 2 to 3 days.

10. The method for preparing an ultralight polyimide aerogel according to claim 5, characterized in that... The thermal imidization described in step three is carried out in the following steps: first, keep warm at 60℃~80℃ for 5h~6h, then keep warm at 150℃~160℃ for 1h~2h, then keep warm at 240℃~250℃ for 1h~2h, and finally keep warm at 290℃~300℃ for 1h~2h.

Images