A polyimide aerogel and a method for preparing the same

Abstract

The present application provides a kind of polyimide aerogel and its preparation method, wherein polyimide aerogel has biomimetic eggshell structure, can resist shrinkage caused by solvent surface tension in drying process by thin shell effect, the density of the polyimide aerogel is 20-60mg / cm 3 , shrinkage is 0.5%-1%, porosity is 90%-98%, specific surface area is 100-300m 2 / g, thermal conductivity is 0.025-0.038W·m ‑1 ·K ‑1 . The polyimide aerogel provided by the present application has biomimetic eggshell structure, has lower shrinkage, thermal conductivity and density, has higher porosity and specific surface area, and can be applied in catalysis, adsorption, sound insulation and thermal insulation fields.

Description

TECHNICAL FIELD

[0001] The present application relates to aerogels and a method for preparing the same, and more particularly to a polyimide aerogel and a freeze-drying method for preparing the same. BACKGROUND

[0002] Polyimide as an organic polymer has the characteristics of low thermal conductivity, good mechanical properties, low dielectric constant, radiation resistance, etc. In addition, due to the large number of benzene rings in the molecular chain, the thermal decomposition temperature of polyimide can reach 530℃ or even higher, which is much higher than that of general organic matter. Therefore, polyimide aerogel has a wide application prospect for high-temperature thermal insulation.

[0003] However, the biggest problem restricting the development of polyimide aerogel and even all aerogels is the drying cost. When the wet gel is dried, the surface tension of the solvent can easily cause the collapse and shrinkage of the aerogel structure. In order to obtain a complete aerogel structure, reducing the influence of the surface tension of the solvent is the most commonly used method. For example, using supercritical drying can almost completely eliminate the surface tension of the solvent, but the high equipment requirements and expensive cost seriously limit its industrialization. Although vacuum freeze-drying can greatly reduce the surface tension of the solvent, it often needs the wet gel to have superhydrophobicity to obtain a complete aerogel block with less shrinkage. Due to the superhydrophilicity of polyimide itself and the soft nanofiber microstructure, the polyimide aerogel prepared by freeze-drying will have obvious shrinkage and cracking, which will greatly affect the strength of the aerogel block. Even if the polyimide is hydrophobically modified, it is still difficult to obtain a perfect polyimide aerogel block by vacuum freeze-drying. Therefore, how to prepare a polyimide wet gel with superhydrophobicity, so that the solvent can be removed by freeze-drying to maintain the space skeleton and not easily crack and shrink, to obtain a high-porosity polyimide aerogel product is very important, and has great significance for the industrialization of polyimide aerogel. SUMMARY

[0004] The present application provides a kind of polyimide aerogel, compared with existing polyimide aerogel, the polyimide aerogel fiber is introduced by polyurethane, thereby has biomimetic eggshell structure, by "thin shell effect" can the shrinkage stress caused by solvent surface tension in wet gel drying process be evenly dispersed, play the role of reducing local pressure, can effectively avoid the shrinkage and collapse of gel skeleton. Thus can use freeze-drying to prepare microstructure complete and uniform polyimide aerogel, reduce drying cost.

[0005] The present application also provides a freeze-drying method for preparing polyimide aerogel. Compared with the existing process, the raw materials of the present process are widely available, the preparation process is simple, the reaction conditions are mild, the reaction process is controllable, the process flow is continuous, the front and rear treatment steps are less, the drying cost is lower, and large-scale production can be realized.

[0006] The present application comprises two aspects as follows:

[0007] In the first aspect, the polyimide aerogel has a biomimetic eggshell structure, and can resist shrinkage caused by solvent surface tension in a drying process through a thin shell effect, the polyimide aerogel fiber has a density of 20-60 mg / cm 3 , a shrinkage rate of 0.5%-1%, a porosity of 90%-98%, a specific surface area of 100-300 m 2 / g, and a thermal conductivity of 0.025-0.038 W·m -1 ·K -1 .

[0008] In the second aspect, the present application provides a freeze-drying preparation method of a polyimide aerogel, and the method comprises the following steps:

[0009] In step S1, diamine and polyurethane are dissolved in an organic solvent to obtain a mixed organic solution;

[0010] In step S2, dianhydride is added to the mixed organic solution to obtain a polyurethane-modified polyamide acid solution;

[0011] In step S3, a crosslinking agent, acetic anhydride and pyridine are added to the polyurethane-modified polyamide acid solution to obtain a polyimide organic gel; and

[0012] In step S4, the polyimide organic gel is subjected to solvent replacement and then freeze-drying to obtain a polyimide aerogel.

[0013] The polyimide aerogel provided by the present application has a biomimetic eggshell structure, has a low shrinkage rate, thermal conductivity and density, has a high porosity and specific surface area, and can be applied in the fields of catalysis, adsorption, sound insulation and heat preservation, etc.

[0014] In the freeze-drying preparation method of the polyimide aerogel provided by the present application, the raw materials are widely available, the preparation process is simple, the reaction conditions are mild, the reaction process is controllable, the process flow is continuous, the front and rear processing steps are few, the drying cost is low, and large-scale production can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is a sectional morphology SEM image (left) and a local magnified SEM image (right) of the polyimide aerogel prepared in Example 1 of the present application.

[0016] Figure 2 is a sectional morphology SEM image of the polyimide aerogel prepared in the comparative example of the present application. DETAILED DESCRIPTION

[0017] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions of the present application will be described clearly and completely below. Obviously, the described embodiments are only part of, rather than all of, the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present application shall fall within the scope of the present application.

[0018] Unless otherwise defined, the technical terms or scientific terms used in the present application shall have the meanings commonly understood by those of ordinary skill in the art to which the present application belongs.

[0019] In order to achieve the purposes of the present application, the present application adopts the following technical solutions:

[0020] According to a first aspect of the present application, a high-strength polyimide aerogel fiber is provided, characterized by having a biomimetic eggshell structure, capable of resisting shrinkage caused by solvent surface tension during the drying process through the thin-shell effect, the density of the polyimide aerogel being 20-60 mg / cm 3 , the shrinkage being 0.5%-1%, the porosity being 90%-98%, and the specific surface area being 100-300 m 2 / g, and the thermal conductivity being 0.025-0.037 W·m -1 ·K -1 .

[0021] According to an embodiment of the present application, the density of the polyimide aerogel is 20-60 mg / cm 3 , preferably 30-55 mg / cm 3 , and more preferably 40-50 mg / cm 3 , the density being obtained by measuring the mass and volume.

[0022] According to an embodiment of the present application, the shrinkage of the polyimide aerogel is 0.5%-1%, preferably 0.6%-1%, the shrinkage being obtained by measuring the volume of the sample before and after drying.

[0023] According to an embodiment of the present application, the porosity of the polyimide aerogel is 90%-98%, preferably 95%-98%, the porosity being obtained by measuring the volume of solvent volatilization during drying.

[0024] According to an embodiment of the present application, the specific surface area of the polyimide aerogel is 100-300 m 2 / g, preferably 150-250 m 2 / g, the specific surface area being obtained by testing the nitrogen adsorption-desorption curve of the aerogel using ELSORP-MAX G (Japan) and applying the BET method.

[0025] According to an embodiment of the present application, the polyimide aerogel has a thermal conductivity of 0.025-0.038 W·m -1 ·K -1 , preferably 0.03-0.038 W·m -1 ·K -1 , more preferably 0.035-0.037 W·m -1 ·K -1 , and the thermal conductivity is measured by TC3000E hot-wire thermal constant analyzer.

[0026] According to a second aspect of the present application, a method for preparing a polyimide aerogel fiber is provided, characterized in that it comprises the following steps:

[0027] Step S1, dissolving diamine and polyurethane in an organic solvent to obtain a mixed organic solution;

[0028] Step S2, adding dianhydride into the mixed organic solution to obtain a polyurethane modified polyamide acid solution;

[0029] Step S3, adding crosslinking agent, acetic anhydride and pyridine into the polyurethane modified polyamide acid solution to obtain a polyimide organic gel; and

[0030] Step S4, solvent exchanging the polyimide organic gel, and then freeze-drying to obtain a polyimide aerogel.

[0031] According to an embodiment of the present application, in the step S1, the mixed organic solution is formed by mixing diamine and polyurethane monomers in an organic solvent, wherein the molar ratio of the diamine and polyurethane monomers is 0.01-0.1:1;

[0032] wherein the diamine monomer is one or more selected from 4,4'-diamino diphenyl ether, p-phenylenediamine, methylene diphenyl diisocyanate, toluene diisocyanate, 4,4'-diaminobenzanilide, 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, and bis(4-aminophenoxy)benzene;

[0033] The polyurethane is one selected from aqueous polyurethane, polyurethane powder, and polyurethane particles, and the mass fraction of polyurethane in the mixed organic solution is 0.1%-10%, preferably 0.5%-3%;

[0034] The organic solvent is one selected from N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide, and dimethylacetamide.

[0035] According to the embodiment of the present application, in the step S2, the polyurethane modified polyamic acid solution is formed by polymerization of the dianhydride monomer and the diamine monomer in the organic solvent.

[0036] The dianhydride monomer is one or more selected from the group consisting of pyromellitic dianhydride, biphenyl tetracarboxylic dianhydride, diphenyl ether tetracarboxylic dianhydride, benzophenone tetracarboxylic anhydride, bisphenol A dianhydride, oxydiphthalic anhydride, biphenyl tetracarboxylic dianhydride, and pyromellitic anhydride; and the molar ratio of the dianhydride monomer and the diamine monomer is (n+1) / n, wherein n is 10-60, preferably 15-30.

[0037] For example, in one embodiment, in the step S2, the polyamic acid solution is obtained by reaction of pyromellitic anhydride and 4,4-diamino diphenyl ether. The reaction mechanism thereof is shown in the following reaction scheme (I):

[0038] According to the embodiment of the present application, in the step S3, the polyimide organic gel is obtained by gelation of the polyurethane modified polyamic acid solution under the action of the crosslinking agent and chemical imidization under the action of acetic anhydride and pyridine;

[0039] The crosslinking agent is one or more selected from the group consisting of 1,3,5-tris(4-aminophenoxy)benzene, bis[3-(trimethoxysilyl)propyl]amine, N1,N3,N5-tris(4-aminophenyl)benzene-1,3,5-tricarboxamide, tris(4-aminophenyl)amine, 2,4,6-tris(4-aminophenoxy)-1,3,5-triazine, 1,3,5-tris(4-aminophenyl)benzene, melamine and derivatives thereof;

[0040] The molar ratio of the crosslinking agent and the diamine monomer is 0.5-1; the molar ratio of the acetic anhydride and the pyridine is 1:1, and the molar ratio of the acetic anhydride and the dianhydride monomer is 2-20:1.

[0041] For example, in one embodiment, the reaction mechanism of the chemical imidization of the amic acid to form the imide is shown in the following reaction scheme (II):

[0042]

[0043] Further, in the step S3, the mass fraction of the polyimide in the polyimide organic gel is 2-10 wt%.

[0044] According to the embodiment of the present application, in the step S4, the solvent replacement is performed by multiple replacements with a solution of tert-butyl alcohol and water, the volume ratio of tert-butyl alcohol to water in the solution being 2:3, the volume ratio of the mixed solution to the polyimide organic gel being 5-15:1, and then freeze-drying is performed, the temperature during the freeze-drying being -50 to -30℃, the pressure being 1-50 Pa, and the drying time being preferably 24-48 h.

[0045] Compared with the existing polyimide aerogel, the polyimide aerogel of the present application is crosslinked between the polyamic acid molecular chains by introducing polyurethane under the action of the polar groups such as carboxyl and hydroxyl in the aqueous polyurethane. In the polar organic solvent, the polar groups in the polyamic acid are crosslinked inside the polyamic acid under the action of the polyurethane, the non-polar groups outside form a spherical structure under the action of the solvent surface tension, and the different spherical structures are crosslinked with each other, thereby having a biomimetic eggshell structure. From the thermodynamic point of view, this eggshell structure contains the lowest free energy and is thus the most stable. And in the drying process of the wet gel, this eggshell structure can evenly disperse the shrinkage stress caused by the solvent surface tension through the "thin shell effect" to reduce the local pressure, which can effectively avoid the shrinkage and collapse of the gel skeleton. Thus, the polyimide aerogel with complete and uniform microstructure can be prepared by freeze-drying, and the drying cost is reduced.

[0046] In order to make the purposes, features and advantages of the present application more obvious and easy to understand, the following describes the polyimide aerogel and the preparation method thereof according to the embodiment of the present application through multiple non-limiting examples.

[0047] Example

[0048] Example 1

[0049] The diamine monomer p-phenylenediamine 0.4866 g (4.50 mmol) was dissolved in 40 mL of the organic solvent N-methylpyrrolidone, and then 1.0 mL (about 0.1 mmol) of a 40% by mass aqueous polyurethane solution was added, and the mixture was stirred magnetically until it became a transparent liquid. Then the dianhydride monomer 3,3,4,4-biphenyltetracarboxylic dianhydride 1.4123 g (4.80 mmol) was added to the above solution while stirring, and the mixture was stirred magnetically at room temperature for 30 minutes. After the solution became completely transparent, a polyurethane-modified polyamide acid solution was obtained. Then the crosslinking agent 1,3,5-tris(4-aminophenoxy)benzene 0.0799 g (0.20 mmol) was dissolved in 10 mL of N-methylpyrrolidone, and the solution was slowly added to the polyurethane-modified polyamide acid solution, and the mixture was stirred for 10 minutes. Then acetic anhydride 2.286 mL (24.0 mmol) and pyridine 1.941 mL (24.0 mmol) were added to the polyurethane-modified polyamide acid solution containing the crosslinking agent, and the mixture was stirred rapidly until it became uniform. The mixture was then poured into a mold. The mixture was then gelled and aged at room temperature for 24 h to obtain a polyimide organic gel.

[0050] The mass fraction of the polyimide in the polyimide organic gel was 4.19 wt%, and the mass fraction of the polyurethane was 0.73%. The molar ratio of the dianhydride monomer, the diamine monomer, the crosslinking agent, and the polyurethane was 16:15:0.67:0.36. The molar ratio of the acetic anhydride and the pyridine was 1:1, and the molar ratio of the acetic anhydride and the dianhydride monomer was 5:1.

[0051] The polyimide organic gel was then solvent-exchanged four times in a solution of tert-butyl alcohol / water in which the volume ratio of tert-butyl alcohol to water was 2:3, and the volume of the solution was 10 times the volume of the polyimide organic gel, and the solvent-exchange time was 6 h each time. The polyimide aerogel with the eggshell structure was then obtained by vacuum freeze-drying. The temperature during the vacuum freeze-drying was controlled to be in the range of -50 to -30 °C, the pressure was controlled to be in the range of 1 to 50 Pa, and the drying time was 24 h. The density of the obtained polyimide aerogel was 47.5 mg / cm 3 , the shrinkage was 0.7%, the porosity was 96%, the specific surface area was 213.5 m 2 / g, and the thermal conductivity was 0.036 W·m -1 ·K -1 .

[0052] Figure 1 Figure 1 is a SEM image of the cross-section morphology of the polyimide aerogel prepared in Example 1 (left) and a local enlarged SEM image (right). It can be seen that, on the micrometer scale, an eggshell structure with a diameter of about 5 μm was formed, and from the enlarged view of a single eggshell structure, it can be seen that the eggshell wall was composed of a three-dimensional fiber network, i.e., on the nanometer scale, a typical three-dimensional fiber network structure of a PI aerogel was formed.

[0053] Example 2

[0054] Take diamine monomer 4,4'-diaminodiphenyl ether 0.6007 g (3.00 mmol), dissolved in 40 ml of organic solvent N-methylpyrrolidone, then add 40% mass fraction of aqueous polyurethane solution 1.25 mL (about 0.125 mmol), magnetic stirring to dissolve to form a transparent liquid. Then take dianhydride monomer 3,3,4,4-biphenyl tetracarboxylic dianhydride 0.9121 g (3.10 mmol), stirring while adding to the above solution, magnetic stirring at room temperature for thirty minutes, after the solution is completely transparent, polyurethane modified polyamide acid solution is obtained. Then take the crosslinking agent bis[3-(trimethoxysilyl)propyl]amine 0.0229 g (0.067 mmol) dissolved in 10 mL of N-methylpyrrolidone, slowly add to the above polyurethane modified polyamide acid solution, continue to stir for ten minutes. Then take acetic anhydride 0.886 mL (9.3 mmol) and pyridine 0.752 mL (9.3 mmol), add to the polyurethane modified polyamide acid solution with the crosslinking agent at the same time, and quickly stir evenly, and load into a mold. Then gel and age at room temperature for 24 h to obtain a polyimide organic gel.

[0055] The mass fraction of polyimide in the polyimide organic gel is 3.73 wt%, and the mass fraction of polyurethane is 0.94%. The molar ratio of dianhydride monomer, diamine monomer, crosslinking agent and polyurethane is 31:30:0.67:1.25. The molar ratio of acetic anhydride and pyridine is 1:1, and the molar ratio of acetic anhydride and dianhydride monomer is 3:1.

[0056] Then the polyimide organic gel is solvent exchanged 4 times in 10 times the volume of tert-butyl alcohol / water solution, each time for 6 hours, and the volume ratio of tert-butyl alcohol / water in the tert-butyl alcohol / water solution is 2:3. Then vacuum freeze-drying is carried out to obtain the polyimide aerogel with eggshell structure. The temperature is controlled at-50 to-30℃, the pressure is controlled at 1 to 50 Pa, and the drying time is 24 h. The density of the obtained polyimide aerogel is 35.5 mg / cm 3 , the shrinkage is 0.9%, the porosity is 98%, the specific surface area is 233.5 m 2 / g, and the thermal conductivity is 0.037 W·m -1 ·K -1 .

[0057] Example 3

[0058] 0.9553 g (4.50 mmol) of the diamine monomer 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl was dissolved in 40 mL of the organic solvent N-methylpyrrolidone. Then, 1.0 g (approximately 0.3 mmol) of polyurethane powder was added, and the solution was magnetically stirred until fully dissolved to form a transparent liquid. Next, 1.0470 g (4.80 mmol) of the dianhydride monomer pyromellitic anhydride was added to the above solution while stirring. The solution was magnetically stirred at room temperature for 30 minutes until completely transparent, yielding a polyurethane-modified polyamic acid solution. Then, 0.0799 g (0.20 mmol) of the crosslinking agent 1,3,5-tris(4-aminophenoxy)benzene was dissolved in 10 mL of N-methylpyrrolidone and slowly added to the above polyurethane-modified polyamic acid solution, while stirring for another 10 minutes. Then, 2.286 mL (24.0 mmol) of acetic anhydride and 1.941 mL (24.0 mmol) of pyridine were added simultaneously to a polyurethane-modified polyamic acid solution containing a crosslinking agent, and the mixture was quickly stirred until homogeneous. The solution was then poured into a mold. The mixture was then gelled at room temperature and aged for 24 h to obtain a polyimide organic gel.

[0059] In the polyimide organic gel, the mass fraction of polyimide is 5.23 wt%, and the mass fraction of polyurethane is 1.7%. The molar ratio of dianhydride monomer, diamine monomer, crosslinking agent, and polyurethane is 16:15:0.67:1. The molar ratio of acetic anhydride to pyridine is 1:1, and the molar ratio of acetic anhydride to dianhydride monomer is 5:1.

[0060] The polyimide organic gel was then solvent-displaced four times in a tert-butanol / water solution (10 times its own volume), each time for 6 hours. The volume ratio of tert-butanol to water in the tert-butanol / water solution was 2:3. The gel was then freeze-dried under vacuum to obtain the polyimide aerogel with an eggshell structure. During the freeze-drying process, the temperature was controlled at -50 to -30°C, the pressure at 1 to 50 Pa, and the drying time was 24 hours. The density of the obtained polyimide aerogel was 42.5 mg / cm³. 3 The shrinkage rate is 0.6%, the porosity is 98%, and the specific surface area is 237.5 m². 2 / g, thermal conductivity is 0.035W·m -1 ·K -1 .

[0061] Comparative example

[0062] 0.4866 g (4.50 mmol) of the diamine monomer p-phenylenediamine was dissolved in 40 mL of the organic solvent N-methylpyrrolidone and magnetically stirred until fully dissolved to form a transparent liquid. Then, 1.4123 g (4.80 mmol) of the dianhydride monomer 3,3,4,4-biphenyltetracarboxylic acid dianhydride was added to the above solution while stirring. The solution was magnetically stirred at room temperature for 30 minutes until completely transparent, yielding a polyurethane-modified polyamic acid solution. Next, 0.0799 g (0.20 mmol) of the crosslinking agent 1,3,5-tris(4-aminophenoxy)benzene was dissolved in 9 mL of N-methylpyrrolidone and slowly added to the above polyurethane-modified polyamic acid solution, with stirring continued for 10 minutes. Then, 2.286 mL (24.0 mmol) of acetic anhydride and 1.941 mL (24.0 mmol) of pyridine were simultaneously added to the polyurethane-modified polyamic acid solution containing the crosslinking agent, and the mixture was rapidly stirred until homogeneous. The solution was then poured into a mold. Then, the gel was gelled at room temperature and aged for 24 hours to obtain a polyimide organic gel.

[0063] In the polyimide organic gel, the mass fraction of polyimide is 3.46 wt%, the molar ratio of dianhydride monomer, diamine monomer and crosslinking agent is 16:15:0.67, the molar ratio of acetic anhydride and pyridine is 1:1, and the molar ratio of acetic anhydride and dianhydride monomer is 5:1.

[0064] The polyimide organic gel was then solvent-displaced four times in a tert-butanol / water solution (10 times its own volume), each time for 6 hours. The volume ratio of tert-butanol to water in the tert-butanol / water solution was 2:3. The gel was then freeze-dried under vacuum to obtain the polyimide aerogel with an eggshell structure. During the freeze-drying process, the temperature was controlled at -50 to -30°C, the pressure at 1 to 50 Pa, and the drying time was 24 hours. The density of the obtained polyimide aerogel was 51.4 mg / cm³. 3 The shrinkage rate is 3.7%, the porosity is 91%, and the specific surface area is 244.2 m². 2 / g, thermal conductivity is 0.033W·m -1 ·K -1 .

[0065] Figure 2 This is a SEM image of the cross-sectional morphology of the polyimide aerogel prepared as a comparative example. The image shows the typical three-dimensional fiber network structure of the polyimide aerogel, rather than a biomimetic eggshell structure.

[0066] While at least one embodiment has been given in the foregoing detailed description, it should be understood that numerous variations exist. It should also be understood that these embodiments are merely exemplary and are not intended to limit the scope of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient roadmap for implementing embodiments of the invention. It should be understood that various changes can be made to the function and arrangement of the elements described in the exemplary embodiments without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.

Claims

1. A method for preparing a polyurethane-modified polyimide aerogel, characterized by, The method comprises the following steps: Step S1, dissolving diamine and polyurethane in an organic solvent to obtain a mixed organic solution; Step S2, adding dianhydride into the mixed organic solution to obtain a polyurethane modified polyamide acid solution; Step S3, adding a crosslinking agent, acetic anhydride and pyridine into the polyurethane modified polyamide acid solution to obtain a polyimide organic gel; and Step S4, performing solvent replacement on the polyimide organic gel, and then performing freeze drying to obtain a polyimide aerogel, In the step S1, the mixed organic solution is formed by mixing diamine and polyurethane in an organic solvent; The diamine is one or more selected from 4,4'-diamino diphenyl ether, p-phenylenediamine, 4,4'-diamino benzalaniline, 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, bis(4-aminophenoxy)benzene; The polyurethane is one selected from polyurethane solution, polyurethane powder and polyurethane particles; The organic solvent is one selected from N-methyl pyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide and dimethylacetamide; The dianhydride is one or more selected from biphenyl tetracarboxylic dianhydride, diphenyl ether tetracarboxylic dianhydride, benzophenone tetracarboxylic anhydride, bisphenol A dianhydride, oxybisphthalic anhydride, biphenyl tetracarboxylic dianhydride and pyromellitic anhydride.

2. The production method according to claim 1, characterized by, In the step S2, the polyurethane modified polyamide acid solution is formed by polymerization of dianhydride and diamine in an organic solvent; The molar ratio of the dianhydride to the diamine is (n+1) / n, wherein n is 10-60.

3. The preparation method according to claim 1, characterized in that, In the step S3, the polyimide organic gel is formed by gelation of the polyurethane modified polyamide acid solution under the action of the crosslinking agent, and chemical imidization under the action of acetic anhydride and pyridine; The crosslinking agent is one or more selected from 1,3,5-tris(4-aminophenoxy)benzene, bis[3-(trimethoxysilyl)propyl]amine, N1,N3,N5-tris(4-aminophenyl)benzene-1,3,5-tricarboxamide, tris(4-aminophenyl)amine, 2,4,6-tris(4-aminophenoxy)-1,3,5-triazine, 1,3,5-tris(4-aminophenyl)benzene and melamine; The molar ratio of the acetic anhydride to the pyridine is 1:1, and the molar ratio of the acetic anhydride to the dianhydride is 2-20:

1.

4. The production method according to claim 1, characterized by, In the step S3, the mass fraction of the polyimide in the polyimide organic gel is 2-10 wt%.

5. The production method according to claim 1, characterized by, In the step S4, the solvent replacement is performed by using a solution of tert-butyl alcohol and water for multiple times, the volume ratio of the tert-butyl alcohol to the water in the solution of the tert-butyl alcohol and the water is 2:3, the volume ratio of the mixed organic solution to the polyimide organic gel is 5-15:1, and then freeze drying is performed, the temperature in the freeze drying process is -50 to -30 ℃, the pressure is 1-50 Pa, and the drying time is 24-48 h.

6. A polyurethane-modified polyimide aerogel prepared according to the method of any one of claims 1-5, characterized by The polyurethane modified polyimide aerogel has a biomimetic eggshell structure, can resist shrinkage caused by solvent surface tension in the drying process through the thin shell effect, the density of the polyimide aerogel is 20-60 mg / cm 3 , the shrinkage rate is 0.5%-1%, the porosity is 90%-98%, the specific surface area is 100-300 m 2 / g, and the thermal conductivity is 0.025-0.038 W·m -1 ·K -1 , wherein the polar groups in the polyamide acid are crosslinked under the action of polyurethane, the external non-polar groups form a spherical structure, and different spherical structures are crosslinked with each other, thereby having a biomimetic eggshell structure.

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