CN114350010B

CN114350010B - Nanofiber-reinforced polyimide composite aerogel with semi-interpenetrating network structure and preparation method thereof

Info

Publication number: CN114350010B
Application number: CN202210059355.2A
Authority: CN
Inventors: 徐洋洋; 潘玉; 冯燕来; 李净珊; 张会
Original assignee: Zhongyuan University of Technology
Current assignee: Zhongyuan University of Technology
Priority date: 2022-01-19
Filing date: 2022-01-19
Publication date: 2023-01-24
Anticipated expiration: 2042-01-19
Also published as: CN114350010A

Abstract

The invention relates to a polyimide composite aerogel with a nanofiber-reinforced and semi-interpenetrating network structure and a preparation method thereof, wherein the polyimide composite aerogel adopts polyimide as a matrix material, short nanofibers as a reinforcing material and bismaleimide as a cross-linking agent, and the preparation method comprises the following steps: dispersing the nano-fibers in triethylamine aqueous solution of polyamic acid salt by using a homogenizer, carrying out freeze drying, carrying out heat treatment at 220 ℃ to initiate a crosslinking reaction, forming sufficient connection points between the fibers and the polyimide matrix, and then heating to 300 ℃ to carry out full thermal imidization to obtain the polyimide composite aerogel with the semi-interpenetrating network structure. The polyimide composite aerogel with the nanofiber reinforced and semi-interpenetrating network structure, which is prepared by the invention, has good mechanical and thermal properties, and the preparation process is simple, easy to operate, green and environment-friendly.

Description

Nanofiber-reinforced polyimide composite aerogel with semi-interpenetrating network structure and preparation method thereof

Technical Field

The invention belongs to the field of aerogel materials, and particularly relates to a nanofiber-reinforced polyimide composite aerogel with a semi-interpenetrating network structure, a preparation method and application thereof.

Background

The aerogel is a porous material with a continuous three-dimensional grid structure, has the characteristics of small density, light weight, high porosity, low thermal conductivity and the like, and has wide application prospects in the fields of military affairs, buildings, energy sources, medical treatment, environmental protection and the like. The earliest studies were directed to inorganic silica aerogels, which however are brittle and friable, making them susceptible to strength degradation and structural collapse under external forces. In recent years, the research of Polyimide (PI) aerogels has been receiving attention because the aerogels have superior mechanical properties and are less likely to break compared to inorganic aerogels. However, the existing PI aerogels often have the disadvantages of insufficient mechanical strength, poor elasticity and high shrinkage rate, so that the development of PI aerogels with excellent mechanical properties still has certain challenges.

The introduction of reinforcing fillers in the polymer matrix is an effective method to improve the mechanical properties of PI aerogels. In recent years, nanofillers such as silica particles, carbon nanotubes, graphene, boron nitride, clay, etc. have been introduced into PI aerogels, however, these fillers exhibit limited mechanical properties due to weak interfacial adhesion and low compatibility with the matrix. When the nano-fibers are used as reinforcing materials to introduce the PI aerogel, the load stress can be borne, the sliding of a PI aerogel framework is restrained, and the strength of the aerogel is enhanced. CN 110372907A discloses a nanofiber-reinforced polyimide aerogel material and a preparation method thereof, the mechanical property of the aerogel is improved by adding nanofibers, however, the PI aerogel in the patent has too high Young modulus, too strong rigidity and poor flexibility, and cannot meet the bending and deformation of a human body in the wearing process, and the application of the PI aerogel in the aspect of keeping warm is limited, so that the preparation of the polyimide thermal aerogel with excellent mechanical and flexible properties still has certain difficulty. The PAI nano fiber with a similar molecular structure with a matrix (PI) is used as a reinforcing filler, the compatibility between the filler and the matrix is improved, a novel cross-linking agent BMI is loaded on the nano fiber, and the interfacial adhesion between the filler and the matrix is improved through high-temperature initiation, so that the polyimide thermal aerogel with excellent mechanical, elastic and flexible properties is obtained.

Disclosure of Invention

The invention aims to overcome the defects of low strength, poor elasticity and poor flexibility of polyimide aerogel, and provides a nanofiber-reinforced polyimide aerogel with a semi-interpenetrating network structure, a preparation method and application thereof.

The polyimide aerogel with the nanofiber reinforced and semi-interpenetrating network structure is prepared by scattering a PAI/BMI nanofiber membrane in triethylamine aqueous solution of polyamic acid salt by using a homogenizer, carrying out heat treatment at 220 ℃ after freeze drying to initiate BMI crosslinking reaction, forming sufficient connecting points between fibers and a polyimide matrix, and then heating to 300 ℃ for full thermal imidization. On the one hand, nanofiber can restrain the shrink of aerogel, improves the intensity, elasticity and the pliability of aerogel, and on the other hand, cross-linking agent BMI can make the aerogel form half interpenetrating network structure through thermal initiation, between fibre and fibre, forms a large amount of tie points between fibre and the PI matrix to make the PI aerogel have better structural stability, keep original structure not destroyed when receiving external force. The polyimide thermal aerogel with excellent mechanical, elastic and flexible properties is obtained by changing the mass ratio of the polyamic acid salt to the PAI/BMI nano fibers and regulating and controlling the mechanical and thermal insulating properties of the polyimide aerogel with the nano fiber reinforced and semi-interpenetrating network structure.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows:

(1) Preparation of PAAs powder: dissolving diamine and a dicarboxylic anhydride monomer in a solvent, dropwise adding a triethylamine solution after uniformly stirring, pouring the mixed solution into deionized water to precipitate, washing for multiple times, and drying in an oven to obtain polyamic acid salt powder;

(2) Preparation of PAI/BMI nanofibers: preparing a polyamide imide (PAI) spinning solution loaded with a crosslinking agent Bismaleimide (BMI), and preparing a PAI/BMI nanofiber membrane through electrostatic spinning equipment;

(3) Preparation of PAI/BMI/PAAs aerogels: scattering the PAI/BMI nano-fiber membrane in a triethylamine aqueous solution of PAAs into a uniform dispersion liquid by using a homogenizer, quickly freezing by using liquid nitrogen, and drying by using a freeze dryer;

(4) Preparation of PAI/BMI/PI aerogels: and (4) putting the sample obtained in the step (3) into a carbonization furnace filled with nitrogen, and performing thermal imidization treatment by a gradient heating method to finally obtain the nanofiber reinforced polyimide composite aerogel with the semi-interpenetrating network structure.

Further, in the step (1), diamine monomer is selected from 4,4' -diaminodiphenyl ether or p-phenylenediamine, dicarboxylic anhydride monomer is selected from pyromellitic anhydride or biphenyl tetracarboxylic dianhydride, and solvent is selected from N, N-dimethylformamide or N, N-dimethylacetamide.

Further, in the step (1), the total mass fraction of diamine and dicarboxylic anhydride monomers is 8-12%, the molar ratio of diamine to dicarboxylic anhydride is 1: 1-1.

Further, the mass concentration of PAI in the spinning solution in the step (2) is 30-35%, and the mass concentration of BMI as a crosslinking agent is 5-8%.

Further, the preparation method of the spinning solution in the step (2) is as follows: firstly, PAI powder is dissolved in N, N-dimethylformamide to prepare PAI solution, BMI powder is added after stirring and dissolving, and then 8-16 h is stirred at room temperature to obtain spinning solution.

Further, in the step (2), the cross-linking agent Bismaleimide (BMI) is a bifunctional compound with maleimide as an active terminal group, wherein the five-membered ring of the maleimide contains a polymerizable double bond, and the type of bismaleimide includes, but is not limited to, N ' -vinylbismaleimide, N ' -tetramethylene bismaleimide, N ' -cyclohexanedimaleimide, N ' - (4,4 ' -methylenediphenyl) bismaleimide, and 2,2-bis [4- (4-maleimidophenoxy) phenyl ] propane.

Further, in the step (2), the electrostatic spinning condition is that the voltage is 20-24 kV, the distance from the spray head to the receiving roller is 15-18 cm, the receiving speed is 100-140 r/min, the translation speed is 300-500 mm/min, and the injection speed is 0.04-0.10 mm/min.

Further, in the step (3), the mass ratio of the water-soluble polyamic acid salt to the PAI/BMI composite nano-fiber is 2:1-1:2, the rotation speed of a homogenizer is 8000-12000 r/min, the homogenizing time is 5-20 min, and the freeze-drying time is 24-60 h.

Further, in the step (4), the temperature is raised to 220 ℃ at a heating rate of 2-10 ℃/min and is kept for 1-2 h, and the temperature is raised to 300 ℃ at a heating rate of 2-10 ℃/min and is kept for 1-3h.

Compared with the prior art, the technical scheme of the invention can obtain the following beneficial effects: (1) The preparation method is simple in preparation process, green, environment-friendly, easy to operate, convenient and efficient; (2) The PI aerogel prepared by the invention introduces PAI/BMI nano fibers, and due to the hydrogen bond effect among PI, PAI and BMI and the crosslinking effect of BMI, sufficient connection points are formed between fibers and a PI matrix, so that the PI aerogel is endowed with a unique network structure, namely the PAI/BMI nano fibers are wrapped on PI sheet layers, and meanwhile, the nano fibers are crosslinked and longitudinally distributed among the PI sheet layers; (3) The PAI/BMI nano-fiber is added to effectively inhibit the shrinkage of the aerogel and improve the strength, elasticity and flexibility of the aerogel; (4) The PI composite aerogel with the semi-interpenetrating network cross-linked structure is obtained through thermal initiation, has ultra-light weight, excellent elasticity, flexibility, mechanics and thermal insulation performance, and widens the application range of the PI aerogel.

Drawings

FIG. 1 is a schematic diagram of a process for preparing a nanofiber reinforced polyimide composite aerogel with a semi-interpenetrating network structure according to an embodiment of the present invention;

FIG. 2 is a scanning electron micrograph of a PAI/BMI/PI-1:1 aerogel prepared in example 1 of the present invention;

FIG. 3 is an SEM image of PAI/BMI/PI-2:1 (A, B) and PAI/BMI/PI-1:2 (C, D) aerogels prepared in examples 2 and 3 of the present invention;

FIG. 4 is a scanning electron micrograph of PAI/PI-1:1 (A, B) and PI (C, D) aerogels prepared in examples 4 and 5 of the present invention;

FIG. 5 is a graph of the porosity of the PAI/BMI/PI-2:1, PAI/BMI/PI-1:1, PAI/BMI/PI-1:2 aerogels prepared in examples 1, 2, 3 of the present invention;

FIG. 6 is a graph of compressive stress strain at various strains for the PAI/BMI/PI-1:1 aerogel prepared in example 1 of the present invention;

FIG. 7 is a graph of the compressive stress strain at 80% strain for PAI/BMI/PI-2:1, PAI/BMI/PI-1:1, PAI/BMI/PI-1:2 aerogels made in examples 1, 2, 3 of the present invention;

FIG. 8 is a histogram of Young's modulus of PAI/BMI/PI-2:1, PAI/BMI/PI-1:1, PAI/BMI/PI-1:2 aerogels made in examples 1, 2, 3 of the present invention;

FIG. 9 is a histogram of the density of the PAI/BMI/PI-2:1, PAI/BMI/PI-1:1 and PAI/BMI/PI-1:2 aerogels prepared in examples 1, 2, 3 of the present invention;

FIG. 10 is a graph of a compressed recovery and curved folding of the PAI/BMI/PI-1:1 aerogel prepared in example 1 of the present invention;

FIG. 11 is a graph of the compressive stress strain curves for the PAI/BMI/PI-1:1, PAI/PI-1:1, PI aerogels prepared in examples 1, 4, 5 of the present invention;

FIG. 12 is a graph of the tensile stress strain curves for PAI/BMI/PI-1:1, PAI/PI-1:1, PI aerogels made in examples 1, 4, 5 of the present invention;

FIG. 13 is a graph of thermal conductivity at room temperature for PAI/BMI/PI-2:1, PAI/BMI/PI-1:1, PAI/BMI/PI-1:2 aerogels made in examples 1, 2, 3 of the present invention;

FIG. 14 is a graph of the thermal conductivity of PAI/BMI/PI-1:1 aerogel prepared in example 1 of the present invention at various temperatures;

FIG. 15 is a plot of the storage modulus, loss modulus, and damping ratio of PAI/BMI/PI-1:1 aerogel prepared in example 1 of the present invention;

FIG. 16 is a graphic representation of the low temperature infrared thermal imaging of PAI/BMI/PI-1:1 aerogel made in example 1 of the present invention with cotton cloth of the same thickness.

Detailed Description

The present invention will be further clearly and completely described below with reference to specific examples, which are only a part of examples of the present invention and are intended to illustrate the present invention without limiting the scope of the invention.

Example 1

The preparation method of the nanofiber-reinforced polyimide composite aerogel with the semi-interpenetrating network structure comprises the following steps:

step (1), preparation of PAAs powder: n, N-dimethylformamide is taken as a solvent, and the condensation polymerization reaction is carried out on 4,4' -diaminodiphenyl ether and pyromellitic dianhydride in a molar ratio of 1.01 in a low-temperature constant-temperature reaction bath at 0 ℃, wherein the specific process comprises the following steps: 135g of N, N-dimethylformamide and 7.14 g of 4,4' -diaminodiphenyl ether were placed in a three-necked flask, followed by 7.86 g pyromellitic anhydride in an ice-water bath, and 6 h was stirred. Then 7.86 g triethylamine was slowly added dropwise with a syringe, and 3h was further stirred to obtain a water-soluble polyamic acid salt solution having a solid content of 10%. And (3) precipitating the solution in deionized water, washing the solution for multiple times by using the deionized water, and then putting the solution into a 50 ℃ drying oven for drying to obtain water-soluble polyamic acid salt powder.

Step (2), preparing PAI/BMI nano fibers: the PAI/BMI spinning solution is prepared by taking N, N-dimethylformamide as a solvent, and the specific process is as follows: first, 10.50 g of PAI powder was dissolved in 17.46 g of N, N-dimethylformamide to prepare a PAI solution having a concentration of 35%, and after dissolving by stirring, 2.04 g of BMI (2,2-bis [4- (4-maleimidophenoxy) phenyl ] propane) powder (having a concentration of 6.8%) was added, followed by stirring at room temperature for 12 h to obtain a spinning solution. And then, spinning by using an electrostatic spinning machine to obtain the PAI/BMI nanofiber membrane, wherein the spinning conditions are as follows: the voltage is 24 kV, the distance from the spray head to the receiving roller is 18 cm, the receiving speed is 140 r/min, the translation speed is 300 mm/min, and the bolus injection speed is 0.08 mm/min.

Step (3), PAI/BMI/PAAs-1:1 aerogel preparation: 30 ml deionized water, 0.225g PAAs powder and 0.225g triethylamine are taken and evenly mixed, then 0.225g PAI/BMI short nano fiber is dispersed in triethylamine aqueous solution of PAAs by adopting a homogenizer, the rotating speed of the homogenizer is 10000 r/min, the homogenizing time is 10 min, and finally, even fiber dispersion solution is formed. Then, liquid nitrogen is adopted for quick freezing and drying, the drying temperature is-80 ℃, the vacuum degree is 1 Pa, and the drying time is 60 h.

Step (4), PAI/BMI/PI-1:1 aerogel preparation: and (3) placing the freeze-dried aerogel in a tube furnace filled with nitrogen, heating to 220 ℃ at the heating rate of 3 ℃/min, preserving the heat for 60 min to initiate BMI thermal polymerization, then heating to 300 ℃, preserving the heat for 60 min to ensure that the BMI thermal polymerization is fully thermal imidized to obtain the polyimide composite aerogel with the nanofiber reinforced and semi-interpenetrating network structure, and naming the polyimide composite aerogel as PAI/BMI/PI-1:1.

Example 2

step (1), preparation of PAAs powder: same as in step (1) of example 1.

Step (2), preparing PAI/BMI nano-fiber: same as in step (2) of example 1.

Step (3), preparing PAI/BMI/PAAs-2:1 aerogel: 30 ml deionized water, 0.225g PAAs powder and 0.225g triethylamine are taken to be uniformly mixed, then 0.1125 g PAI/BMI short nano fiber is dispersed in the triethylamine water solution of PAAs by adopting a homogenizer, the rotation speed of the homogenizer is 10000 r/min, the homogenizing time is 10 min, and finally, a uniform fiber dispersion solution is formed. Then, liquid nitrogen is adopted for quick freezing and drying, the drying temperature is-80 ℃, the vacuum degree is 1 Pa, and the drying time is 60 h.

Step (4), PAI/BMI/PI-2:1 aerogel preparation: the same procedure as in step (4) of example 1.

Example 3

step (1), preparation of PAAs powder: same as in step (1) of example 1.

Step (2), preparing PAI/BMI nano-fiber: same as in step (2) of example 1.

Step (3), PAI/BMI/PAAs-1:2 aerogel preparation: 30 ml deionized water, 0.225g PAAs powder and 0.225g triethylamine are taken to be uniformly mixed, then 0.450 g PAI/BMI short nano fiber is dispersed in the triethylamine water solution of PAAs by adopting a homogenizer, the rotation speed of the homogenizer is 10000 r/min, the homogenization time is 10 min, and finally, a uniform fiber dispersion solution is formed. Then, the mixture is quickly frozen and dried by adopting liquid nitrogen, the drying temperature is-80 ℃, the vacuum degree is 1 Pa, and the drying time is 60 h.

Step (4), PAI/BMI/PI-1:2 aerogel preparation: the same procedure as in step (4) of example 1.

Example 4

Step (1), preparation of PAAs powder: same as in step (1) of example 1.

Step (2), PAI nano-fiber preparation: the PAI spinning solution without a cross-linking agent is prepared by taking N, N-dimethylformamide as a solvent, and the specific process is as follows: a PAI solution having a concentration of 35% was first prepared by dissolving 10.50 g of PAI powder in 17.46 g of N, N-dimethylformamide, and then stirring at room temperature for 12 h to give a spinning solution. And then, spinning by using an electrostatic spinning machine to obtain the PAI nanofiber membrane, wherein the spinning conditions are as follows: the voltage is 24 kV, the distance from the spray head to the receiving roller is 18 cm, the receiving speed is 140 r/min, the translation speed is 300 mm/min, and the bolus injection speed is 0.08 mm/min.

Step (3), PAI/PAAs-1:1 aerogel preparation: 30 ml deionized water, 0.225g PAAs powder and 0.225g triethylamine are taken and evenly mixed, then 0.225g PAI nano fiber membrane is scattered and dispersed in triethylamine water solution of PAAs by a homogenizer, the rotating speed of the homogenizer is 10000 r/min, the homogenizing time is 10 min, and finally, even fiber dispersion solution is formed. Then, liquid nitrogen is adopted for quick freezing and drying, the drying temperature is-80 ℃, the vacuum degree is 1 Pa, and the drying time is 60 h.

Step (4), preparing PAI/PI-1:1 aerogel: the same procedure as in (4) of example 1.

Example 5

Step (1), preparation of PAAs powder: same as in step (1) of example 1.

Step (2), preparation of PAAs aerogel: 30 ml deionized water, 0.225g PAAs powder and 0.225g triethylamine are uniformly mixed, and then liquid nitrogen is adopted for quick freezing and drying, wherein the drying temperature is-80 ℃, the vacuum degree is 1 Pa, and the drying time is 60 h.

Step (3), preparing PI aerogel: the same procedure as in step (4) of example 1.

FIG. 1 is a schematic diagram of a preparation process of a polyimide composite aerogel with a nanofiber reinforced and semi-interpenetrating network structure, which comprises the steps of preparing PAI/BMI nanofibers through electrostatic spinning, homogenizing the nanofibers in a triethylamine aqueous solution of PAAs, freeze drying and heat treatment.

FIG. 2 is a scanning electron microscope image of PAI/BMI/PI-1:1 aerogel, and FIG. 2A is a macroscopic scanning electron microscope image, and it can be seen from the image that it presents a hierarchical multi-layer pore structure, mainly composed of secondary pores and smaller micropores formed by interlacing of macropores between sheets and nanofibers; fig. 2B, C, D are high power scanning electron microscope images, and due to hydrogen bonding between PI, PAI and BMI and cross-linking of BMI, a large number of connection points fused together are formed between fibers and PI matrix, so that the aerogel is endowed with a unique network structure, namely, PAI/BMI nanofibers are longitudinally distributed between PI sheets (fig. 2B), and simultaneously wrapped on PI sheets (fig. 2C), and connection points are formed between PAI/BMI nanofibers (fig. 2D).

FIG. 3 is a scanning electron microscope image of PAI/BMI/PI-2:1 and PAI/BMI/PI-1:2 aerogel, and FIG. A, B is a scanning electron microscope image of PAI/BMI/PI-2:1 aerogel, when the amount of nanofibers added is small, it is not enough to support the whole material, and the aerogel takes on a cellular network structure; FIG. 3238 Zxft 3238 is a scanning electron microscope image of PAI/BMI/PI-3262 Zxft 3262 aerogel, when the amount of the added nanofibers is large, the degree of order of the lamellar structure is significantly reduced, and a large amount of the fibers are free.

FIG. 4 is a scanning electron micrograph of PAI/PI-1:1 and PI aerogel, FIG. 4A, B is a scanning electron micrograph of PAI/PI-1:1 aerogel, and from FIG. 4A it can be seen that PAI/PI-1:1 aerogel exhibits similar nanofiber reinforcement structure as PAI/BMI/PI-1:1 aerogel, but from FIG. 4B it is found that PAI/PI-1:1 aerogel without BMI crosslinker forming no junctions between fibers. Fig. 4C and D are scanning electron micrographs of PI aerogel, and since no nanofibers are added, it is difficult to form a good and stable three-dimensional porous structure by itself, and the PI sheet layer presents a disordered structure.

FIG. 5 is a graph of the porosity of PAI/BMI/PI-2:1, PAI/BMI/PI-1:1 and PAI/BMI/PI-1:2 aerogels. PAI/BMI/PI-2:1 has a porosity of 94.4497, PAI/BMI/PI-1:1 has a porosity of 94.9206, PAI/BMI/PI-1:2 has a porosity of 92.5708, indicating that reasonable addition of nanofibers can effectively improve the pore structure of aerogels and increase porosity, but when more nanofibers are added, the porosity decreases. PAI/BMI/PI-1:1 has the best porosity, which shows that the air conditioner can hold more still air and has excellent warm-keeping performance.

FIG. 6 is a plot of compressive stress strain curves of PAI/BMI/PI-1:1 aerogel at different strains. When the compressive strain of the aerogel reaches 80%, the stress reaches 85kpa, and the aerogel can be recovered to the initial state after the external force is removed, so that the PAI/BMI/PI-1:1 aerogel with the fiber-reinforced semi-interpenetrating network structure has excellent mechanical properties and can meet the requirements of actual application.

FIG. 7 is a graph of compressive stress strain curves for PAI/BMI/PI-2:1, PAI/BMI/PI-1:1, and PAI/BMI/PI-1:2 aerogels at 80% strain. As can be seen from the figure, the compressive mechanical properties of the PAI/BMI/PI aerogel are gradually improved along with the increase of the addition amount of the PAI/BMI nano fibers, and the PAI/BMI nano fibers are added to promote the improvement of the mechanical properties of the polyimide composite aerogel.

FIG. 8 is a histogram of Young's modulus for PAI/BMI/PI-2:1, PAI/BMI/PI-1:1 and PAI/BMI/PI-1:2 aerogels. As can be seen from the figure, the Young's modulus of the aerogel gradually increased with increasing nanofiber content, indicating the reinforcing effect of PAI/BMI nanofibers on the aerogel. However, when the content of the nanofibers is excessively added, the aerogel has high rigidity and poor flexibility, and is difficult to bend, deform and recover in the using process, so that the actual taking effect is limited.

FIG. 9 is a histogram of the density of PAI/BMI/PI-2:1, PAI/BMI/PI-1:1 and PAI/BMI/PI-1:2 aerogels. As the material for clothing, on the basis of ensuring certain mechanical properties, the material also needs to have low density and light weight, and the density and the weight of the aerogel are gradually increased along with the increase of the content of the nano fibers, so that the effect of clothing is influenced. PAI/BMI/PI-1:1 aerogels have relatively low densities (26 mg/cm) ^-3 ) From the physical picture enclosed in fig. 9, it can be seen that the aerogel was freely resting on green bristlegrass, demonstrating its light texture.

FIG. 10 is a graph of a PAI/BMI/PI-1:1 aerogel in compression recovery and in curved folds. After the aerogel is compressed, external force is removed, the aerogel can be rapidly recovered to an initial state, the original appearance is kept and is not damaged, and the aerogel has excellent mechanical properties. In addition, PAI/BMI/PI-1:1 aerogel can be bent and folded, external force is removed, and the aerogel returns to a flat state, so that the aerogel shows good flexibility, thereby showing the advantage of taking the aerogel. In a word, the PAI/BMI/PI-1:1 aerogel has ultra-light weight, good mechanical property and flexibility, and can be practically applied as a material for thermal wear.

FIG. 11 is a graph of compressive stress strain curves for PAI/BMI/PI-1:1, PAI/PI-1:1, PI aerogels at 80% strain. It can be seen from the figure that the compression performance of the pure PI aerogel is the worst. When PAI nano-fiber is added, the mechanical property of the PAI/PI-1:1 aerogel is improved to a small extent. When PAI/BMI nano-fibers are added, the cross-linking agent BMI in the nano-fibers is thermally initiated to enable the aerogel to form a semi-interpenetrating network structure, the PAI/BMI/PI-1:1 aerogel mechanical properties are remarkably improved, and the promotion effect of the cross-linking agent BMI on the mechanical properties is proved.

FIG. 12 is a graph of tensile stress strain curves for PAI/BMI/PI-1:1, PAI/PI-1:1, PI aerogels. Also, pure PI aerogels have the worst tensile properties. The tensile property of the PAI/PI-1:1 aerogel is obviously improved by adding the PAI nano fibers. When PAI/BMI nano-fiber is added, a large number of connection points which are fused into a whole are formed among fibers and between fibers and a PI matrix by hydrogen bonding action among PI, PAI and BMI and cross-linking action of BMI, so that the tensile stress and strain of the PAI/BMI/PI-1:1 aerogel are remarkably improved, and the PAI/BMI nano-fiber has the most outstanding tensile property, the tensile strength is 160kpa, and the strain is 25%.

FIG. 13 is a graph of the thermal conductivity coefficients of PAI/BMI/PI-2:1, PAI/BMI/PI-1:1, and PAI/BMI/PI-1:2 aerogels at room temperature. The addition of the nano-fibers can effectively improve the pore structure of the aerogel and improve the heat preservation performance, but when the nano-fibers are added more, the porosity is reduced and the heat preservation performance is reduced. The PAI/BMI/PI-1:1 aerogel has the lowest thermal conductivity of 30.06 mw ^-1 k ^-1 The good pore structure of aerogel behind this benefit from nanofiber reinforcing, the porous structure that nanofiber interlude entanglement formed between the lamella has effectively prolonged the motion route of air molecule to reduce the thermal conductivity of aerogel, reach good cold-proof effect.

FIG. 14 is a graph of thermal conductivity of PAI/BMI/PI-1:1 aerogel at different temperatures. The thermal conductivity at-40 ℃ is only 24.16mw ^-1 k ^-1 The result shows that the fabric has excellent heat preservation performance under the low temperature condition. When the temperature is raised to 200 ℃, the thermal conductivity is 48.58 mw ^-1 k ^-1 To remainCan show good thermal insulation performance, and shows that the polyimide composite aerogel obtained by the method is a good thermal insulation material.

FIG. 15 is a plot of storage modulus, loss modulus, and damping ratio for PAI/BMI/PI-1:1 aerogel. The storage modulus, loss modulus and damping ratio of the PAI/BMI/PI-1:1 aerogel are almost constant in a larger temperature range of-100-200 ℃, which shows that the polyimide composite aerogel obtained by the patent has stable viscoelasticity mechanical properties in a wider temperature range.

FIG. 16 is a photograph of an infrared thermal image of PAI/BMI/PI-1:1 and cotton cloth of the same thickness under cryogenic conditions. Under the condition of low temperature, in the range of 5 s, the surface temperature of cotton cloth is rapidly reduced from 2 ℃ to-1.6 ℃, and when the time is prolonged to 120 s, the surface temperature is reduced to-4 ℃, however, the surface temperature of PAI/BMI/PI-1:1 aerogel is kept unchanged at 2 ℃. The infrared thermal imaging image shows that compared with the conventional textile, the PAI/BMI/PI-1:1 aerogel in the patent has good thermal insulation performance.

Claims

1. A preparation method of polyimide composite aerogel with a nanofiber reinforced and semi-interpenetrating network structure is characterized by comprising the following steps:

(1) Preparation of water-soluble Polyamic Acid Salt (PAAs) powder: dissolving diamine monomers and dicarboxylic anhydride monomers in a solvent, dropwise adding triethylamine solution after uniformly stirring, pouring the mixed solution into deionized water to precipitate, washing for multiple times, and drying in an oven to obtain polyamic acid salt powder;

(2) Preparation of PAI/BMI nanofibers: preparing a polyamide imide (PAI) spinning solution loaded with a cross-linking agent Bismaleimide (BMI), and preparing a PAI/BMI nanofiber membrane through electrostatic spinning equipment;

(3) Preparing a polyimide composite aerogel precursor (PAI/BMI/PAAs) with a nanofiber reinforced and semi-interpenetrating network structure: scattering the PAI/BMI nano-fiber membrane in a triethylamine aqueous solution of PAAs into a uniform dispersion liquid by using a homogenizer, quickly freezing by using liquid nitrogen, and drying by using a freeze dryer to obtain a sample;

(4) Preparing polyimide composite aerogel (PAI/BMI/PI) with a nanofiber reinforced and semi-interpenetrating network structure: and (4) putting the sample obtained in the step (3) into a carbonization furnace filled with nitrogen, and performing thermal imidization treatment by a gradient heating method to finally obtain the nanofiber reinforced polyimide composite aerogel with the semi-interpenetrating network structure.

2. The method for preparing the nanofiber reinforced, semi-interpenetrating network structure polyimide composite aerogel according to claim 1, wherein in step (1), the diamine monomer is 4,4' -diaminodiphenyl ether or p-phenylenediamine, the dianhydride monomer is pyromellitic dianhydride or biphenyltetracarboxylic dianhydride, and the solvent is N, N-dimethylformamide or N, N-dimethylacetamide.

3. The preparation method of the nanofiber reinforced polyimide composite aerogel with the semi-interpenetrating network structure according to claim 1, wherein the total mass fraction of the diamine monomer and the dicarboxylic anhydride monomer in the step (1) is 8-12%, the molar ratio of the diamine monomer to the dicarboxylic anhydride monomer is 1:1-1, 1.02, and the mass ratio of the dicarboxylic anhydride to the triethylamine is 1:1-1:2.

4. The method for preparing the nanofiber reinforced polyimide composite aerogel with the semi-interpenetrating network structure according to claim 1, wherein the mass concentration of PAI in the spinning solution in the step (2) is 30-35%, and the mass concentration of BMI (bismaleimide) as a crosslinking agent is 5-8%.

5. The method for preparing the nanofiber reinforced, semi-interpenetrating network structure polyimide composite aerogel according to claim 1, wherein the cross-linking agent Bismaleimide (BMI) in step (2) is a bifunctional compound with maleimide as an active terminal group, wherein the five-membered ring of maleimide contains a polymerizable double bond, and the type of bismaleimide comprises N, N ' -vinyl bismaleimide, N ' -tetramethylene bismaleimide, N ' -cyclohexane bismaleimide, N ' - (4,4 ' -methylenediphenyl) bismaleimide, and 2,2-bis [4- (4-maleimidophenoxy) phenyl ] propane.

6. The method for preparing the polyimide composite aerogel with the nanofiber reinforced and semi-interpenetrating network structure according to claim 1, wherein the conditions of the electrostatic spinning in the step (2) are as follows: the voltage is 20-24 kV, the distance from the spray head to the receiving roller is 15-18 cm, the receiving speed is 100-140 r/min, the translation speed is 300-500 mm/min, and the bolus injection speed is 0.04-0.10 mm/min.

7. The method for preparing the nanofiber reinforced polyimide composite aerogel with the semi-interpenetrating network structure according to claim 1, wherein the mass ratio of the water-soluble polyamic acid salt to the PAI/BMI nanofibers is 2:1-1:2, the rotation speed of a homogenizer is 8000-12000 r/min, the homogenizing time is 5-20 min, and the freeze-drying time is 24-60 h.

8. The method for preparing the polyimide composite aerogel with the nanofiber-reinforced and semi-interpenetrating network structure according to claim 1, wherein the gradient temperature raising method in the step (4) is as follows: heating to 220 ℃ at a heating rate of 2-10 ℃/min, preserving heat for 1-2 h, and heating to 300 ℃ at a heating rate of 2-10 ℃/min, preserving heat for 1-3h.

9. The polyimide composite aerogel prepared by the preparation method of the nanofiber reinforced polyimide composite aerogel with the semi-interpenetrating network structure as claimed in any one of claims 1 to 8.