CN107698794B

CN107698794B - Preparation method of cross-linked polyimide aerogel

Info

Publication number: CN107698794B
Application number: CN201610640949.7A
Authority: CN
Inventors: 刘韬; 李文静; 刘斌; 张丽娟; 周玉贵; 张凡; 赵英民
Original assignee: Aerospace Research Institute of Materials and Processing Technology
Current assignee: Aerospace Research Institute of Materials and Processing Technology
Priority date: 2016-08-08
Filing date: 2016-08-08
Publication date: 2020-12-25
Anticipated expiration: 2036-08-08
Also published as: CN107698794A

Abstract

The invention provides a preparation method of a cross-linked polyimide aerogel, which adopts a sol-gel method, namely, a dianhydride and a diamine are used as monomers to synthesize polyamide acid, a triamino monomer is used as a cross-linking agent, a polyimide wet gel is formed by solvent thermal imidization, and a supercritical drying process is combined to prepare the cross-linked polyimide aerogel material. The method has the characteristics of wide applicability, cheap and easily-obtained raw materials, simple reaction process, low overall cost and the like, and the obtained material has a nanoscale multistage microstructure.

Description

Preparation method of cross-linked polyimide aerogel

Technical Field

The invention relates to a preparation method of a cross-linked polyimide aerogel, belonging to the technical field of aerogel preparation.

Background

The rapid development of space technology has increasingly stringent requirements for highly effective insulation materials. Aerogel materials are more and more widely regarded with the characteristics of light weight, super heat insulation, super insulation and the like, and gradually become one of the first-choice varieties of high-efficiency heat-proof/insulation materials. The polyimide aerogel is an organic aerogel with high temperature resistance, high flame retardance, high insulation, high thermal insulation, low dielectric constant and dielectric loss, has good mechanical property and flexibility compared with pure inorganic aerogel, and has low thermal conductivity, and the prepared film has good flexibility, can be bent and folded, and has very wide application prospect in the aspects of folding spacecrafts, aerospace craft thermal control systems, aerospace suits and the like.

The polyimide aerogel with a linear structure is prepared by American Aspen company, and the material has good thermal stability and good mechanical property, and the thermal decomposition temperature is 560 ℃, and is higher than the mechanical property of common organic polymer reinforced silicon aerogel. However, this material has a large shrinkage rate during supercritical drying, and is not suitable for industrial production. In order to improve the stability of the polyimide aerogel pore structure and reduce the linear shrinkage rate of the material, researchers have developed the research work of the polyimide aerogel with the cross-linked structure. US patent No. 9109088B 2 discloses a method for preparing a cross-linked polyimide aerogel, which comprises preparing anhydride-terminated acid amide sol from excessive dianhydride monomer and diamine monomer, adding cross-linking agent, i.e., polyamino monomer, performing chemical imidization (pyridine catalysis, acetic anhydride as a dehydrating agent) to form polyimide wet gel, and combining a supercritical drying process to prepare the cross-linked polyimide aerogel material, wherein the method is a common method for preparing the cross-linked polyimide aerogel at present, such as patents CN104693477A and CN 105384950A. However, the method has inherent defects that pyridine and acetic anhydride as catalysts for chemical imidization are required to be added before the sol system is gelled, and at the moment, the acetic anhydride as a dehydrating agent and unreacted amino of a cross-linking agent can quickly generate end capping reaction, so that the whole reaction system can not form a complete cross-linking network, finally formed gel is linear wet gel formed by precipitation due to the fact that the solubility of the polymer in a solution is reduced after imidization, and the structure of the gel is similar to that of physical lap joint. Therefore, how to realize a true cross-linked structure has become a bottleneck problem restricting the development of the polyimide aerogel material.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide a preparation method of a cross-linking type polyimide aerogel material, which has the advantages of wide application range, simple process, convenient operation and suitability for industrial production.

The technical solution of the invention is as follows: a preparation method of a cross-linked polyimide aerogel is realized by the following steps:

firstly, preparing a polyamic acid solution;

uniformly mixing an aromatic diamine monomer and an excessive aromatic dianhydride monomer in an organic solvent for condensation polymerization reaction to obtain polyamide acid solution with end capped by anhydride groups after the reaction is finished;

the aromatic diamine and the aromatic dianhydride are not particularly limited in the present invention, and 4,4' -diaminoanisole (ODA), 2' -dimethyl-4, 4' -Diaminobiphenyl (DMBZ), etc. which are commonly used in condensation polymerization reaction, may be used as the aromatic diamine; as the aromatic dianhydride, 3',4,4' -biphenyltetracarboxylic dianhydride, 3',4,4' -diphenylethertetracarboxylic dianhydride, 3',4,4' -benzophenonetetracarboxylic dianhydride, etc., which are commonly used in the condensation polymerization reaction, can be used.

The kind of the organic solvent is not particularly limited in the present invention, and may be N-methylpyrrolidone (NMP), N-Dimethylformamide (DMF), N-dimethylacetamide (DMAc), Dimethylsulfoxide (DMSO), etc., which are generally used in condensation polymerization.

The molar ratio of the aromatic dianhydride to the aromatic diamine is 1: (0.4-0.85); in the condensation polymerization reaction step, the time is 8-32 hours, and the temperature is 0-40 ℃.

The condensation polymerization reaction of the present invention is well known in the art, and those skilled in the art can select raw materials and process parameters according to the specific situation.

Second, a crosslinked polyamic acid wet gel is prepared,

adding a crosslinking agent triamino monomer into the polyamic acid solution prepared in the first step, uniformly mixing at room temperature, standing, and after a reaction system is gelled, heating and aging to obtain crosslinked polyamic acid wet gel;

the kind of the cross-linking agent is not particularly limited in the present invention, and may be triamino monomers commonly used for preparing polyimide aerogel, such as 1,3, 5-tris (amino) benzene, 1,3, 5-tris (aminophenoxy) benzene (TAB), 2, 6-bis (4 '-aminophenyl) -4- (4' -aminophenyl) pyridine (TAPP), and the like.

The amount of the cross-linking agent is determined based on the amount of the aromatic dianhydride, and the molar ratio of the total amount of amino groups (the total amount of amino groups in the aromatic diamine and the cross-linking agent) to the total amount of anhydride groups (the total amount of anhydride groups in the aromatic dianhydride) is equal. The invention determines that the molar ratio of aromatic dianhydride to aromatic diamine is 1: (0.4-0.85), on the premise of ensuring the molar ratio of amino groups to anhydride groups, if the content of the aromatic diamine is too much, the dosage of the cross-linking agent is too small, the density of cross-linking points in the prepared polyamic acid wet gel is too small, and in the third step of imidization through solvent heat, the polyamic acid wet gel can be dissolved in the solvent again, so that a polyimide wet gel cannot be formed, and a cross-linking type polyimide aerogel cannot be formed; if the aromatic diamine content is too low, too much crosslinking agent is used, which results in too rapid gelation of the sol system and is not easy to handle. Within the range required by the invention, the less the aromatic diamine is used, the more the cross-linking agent is used, the density of cross-linking points in the polyimide aerogel is increased along with the increase of the cross-linking agent, the molecular cohesion is increased, the linear shrinkage rate of the aerogel is increased along with the increase of the molecular cohesion, and the density of a sample tends to increase; the increase of the crosslinking degree also increases the proportion of micropores in the pore structure, so that the specific surface area tends to be increased; the increase of the crosslinking degree can limit the movement of molecular chains and simultaneously improve the stability of molecular structures, so that the glass transition temperature and the thermal stability of the aerogel tend to increase.

The method provided by the invention can be used for heating and aging the polyamic acid wet gel so as to promote the perfection of the crosslinking network of the wet gel and improve the strength of the wet gel. The aging temperature is 40-80 ℃, and the aging time is not less than 24 hours. If the aging temperature is too low, the effect of promoting the incomplete condensation polymerization reaction of the amino and the anhydride groups in the gel system is not obvious; the aging temperature is too high, so that the solvent in the wet gel can be promoted to be greatly volatilized, and the shrinkage and the structural damage of the wet gel are caused; the aging time is too short, so that the amino group and the anhydride group which are not completely reacted in the gel system can not be subjected to full polycondensation reaction; too long an aging time also results in a large amount of volatilization of the solvent inside the wet gel, and also reduces the preparation efficiency.

Thirdly, imidizing the crosslinked polyamic acid wet gel obtained in the second step by solvothermal under a sealing condition to obtain polyimide wet gel;

the imidization time is not less than 8 hours by solvothermal, and the temperature is 100-250 ℃. The time for imidization by solvothermal is too short to facilitate complete imidization; the time is too long, which is not beneficial to improving the preparation efficiency; the temperature is too low to be beneficial to complete imidization; at too high a temperature, the solvent will boil violently, destroying the structure of the wet gel.

The heat of a solvent is a well-known technique,the solvothermal preparation technique was originally adopted by Bibby et al in the synthesis of zeolites in non-aqueous systems, using a non-aqueous solvent (NH)₃、C₂H₅OH、CO₂、C₆H₆) As a reaction medium, a chemical reaction is carried out under high temperature and high pressure conditions. The invention uses the solvent thermal technology for reference in the imidization process, the principle is consistent with the known technology, but the solvent used by the solvent thermal technology in the invention is the same as the solvent used for preparing the polyamic acid solution in the first step, and the solvent is ensured to be completely immersed in the polyamic acid wet gel.

And fourthly, carrying out solvent replacement and supercritical drying on the polyimide wet gel to obtain the cross-linked polyimide aerogel.

Solvent substitution and supercritical drying are well known in the art, the solvent may be common ethanol or acetone, and the solvent and process may be selected by those skilled in the art according to the specific circumstances.

Compared with the prior art, the invention has the beneficial effects that:

(1) according to the invention, the intermediate product (polyamic acid wet gel) with a cross-linking network is prepared, and then is imidized by solvent heat to be converted into the polyimide wet gel in a cross-linking state, so that the problem that the polyimide aerogel with a real cross-linking structure cannot be obtained in the prior art is solved;

(2) the method has the characteristics of wide applicability, cheap and easily-obtained raw materials, simple reaction process, low overall cost and the like, and the obtained material has a nanoscale multistage microstructure.

Drawings

FIG. 1 is a flow chart of the present invention;

FIG. 2 is an infrared spectrum of a polyimide aerogel prepared in example 1 of the present invention;

FIG. 3 is a scanning electron micrograph of a polyimide aerogel prepared in example 1 according to the present invention;

fig. 4 is a TGA curve of the polyimide aerogel prepared in example 1 of the present invention and the polyimide aerogel prepared in comparative example 1.

Detailed Description

The present invention will be described in detail with reference to the following examples and accompanying drawings.

Example 1

The preparation method is shown in figure 1 and is realized by the following steps:

1. 2.94g (10mmol) of 3,3',4,4' -biphenyltetracarboxylic dianhydride and 1.70g (8.5mmol) of 4,4' -diaminoanisole were dissolved in 93mL of NMP and subjected to condensation polymerization at 25 ℃ for 24 hours to obtain an anhydride group-terminated polyamic acid solution.

2. Adding 0.40g (1mmol) of crosslinking agent 1,3, 5-tri (aminophenoxy) benzene (TAB) into the polyamic acid solution system, stirring for 5min at 25 ℃, pouring into a mold, standing, heating to 60 ℃ after the reaction system gels, and aging for 24h to obtain the crosslinked polyamic acid wet gel.

3. And (3) adding a small amount of NMP into the mold in the step (2) to cover the surface of the polyamic acid wet gel with NMP, sealing the mold, and performing solvent thermal imidization for 18h at 220 ℃ to obtain the polyimide wet gel.

4. Taking the polyimide wet gel obtained in the step 3 out of the mold, repeatedly soaking the polyimide wet gel in ethanol for 3 times, and then performing supercritical CO (carbon monoxide)₂Drying at 40 deg.C under 15MPa for 8 hr to obtain cross-linked polyimide aerogel.

The data of the results of the performance test of the crosslinked polyimide aerogel obtained in this example are shown in Table 1, the IR spectrum is shown in FIG. 2, the SEM photograph is shown in FIG. 3, and the TGA curve is shown in FIG. 4.

As can be seen from the spectrum of FIG. 2, at 1367cm^-1、1715cm^-1And 1775cm^-1The characteristic absorption peak of polyimide appears at 1660cm^-1And 1535cm^-1The characteristic absorption peak of the polyamic acid does not appear, which proves that the polyamic acid can be completely imidized by solvent heat treatment; as can be seen from the scanning electron micrograph of fig. 3, this example successfully prepared a cross-linked polyimide aerogel of the expected structure, the microstructure of the aerogel is in the shape of intertwined nanofibers, the diameter of the fibers is between 10 and 50nm, and the volume (porosity) occupied by air in the aerogel is 95%; from the TGA curves of example 1 and comparative example 1 shown in FIG. 4, the cross-linked polyimide aerogel prepared by the method of the present invention is compared with the conventional oneCompared with the method, the method has higher thermal stability.

Example 2

The conditions and procedures for producing the crosslinked polyimide aerogel were the same as those of example 1 except that the mass of 4,4' -diaminoanisole added was 1.40g (7mmol), the volume of NMP was 95mL, and the mass of the crosslinking agent 1,3, 5-tris (aminophenoxy) benzene (TAB) was 0.80g (2mmol), and the data of the results of the performance test of the crosslinked polyimide aerogel are shown in Table 1.

Example 3

The conditions and procedures for producing the crosslinked polyimide aerogel were the same as in example 1 except that the mass of 4,4' -diaminoanisole added was 1.10g (5.5mmol), the volume of NMP was 97mL, and the mass of 1,3, 5-tris (aminophenoxy) benzene (TAB) as the crosslinking agent was 1.20g (3mmol), and the data of the results of the performance test of the crosslinked polyimide aerogel are shown in Table 1.

Example 4

The conditions and procedures for producing the crosslinked polyimide aerogel were the same as those of example 1 except that the mass of 4,4' -diaminoanisole added was 0.80g (4mmol), the volume of NMP was 99mL, and the mass of 1,3, 5-tris (aminophenoxy) benzene (TAB) as a crosslinking agent was 1.60g (4mmol), and the data of the results of the performance test of the crosslinked polyimide aerogel are shown in Table 1.

Comparative example 1

The cross-linked polyimide aerogel is prepared by adopting the traditional method

1. 2.94g (10mmol) of 3,3',4,4' -biphenyltetracarboxylic dianhydride and 1.70g (8.5mmol) of 4,4' -diaminoanisole were dissolved in 93mL of NMP and amidation reaction was carried out at 25 ℃ for 24 hours to obtain an anhydride group-terminated polyamic acid solution.

2. Adding 0.40g (1mmol) of cross-linking agent 1,3, 5-tri (aminophenoxy) benzene (TAB) into the system, stirring for 5min at 25 ℃, adding 7mL (80mmol) of imidization catalyst pyridine and 8mL (80mmol) of dehydrating agent acetic anhydride, stirring for 1min, pouring into a mold, standing, after the reaction system gels, aging for 24h at 25 ℃ to obtain the polyimide wet gel.

3. Taking the polyimide wet gel obtained in the step 2) out of the mould, repeatedly soaking the polyimide wet gel for 3 times by using ethanol, and then carrying out supercritical treatmentBoundary CO²Drying at 40 deg.C under 15MPa for 8 hr to obtain polyimide aerogel.

The data of the test results of the properties of the polyimide aerogel prepared in comparative example 1 are shown in Table 1, and the TGA curve is shown in FIG. 4.

The specific surface area (sample was vacuum degassed at 80 ℃ for 10 hours before the test), the thermal conductivity at room temperature, the DSC (temperature rising rate of 20 ℃/min, nitrogen flow rate of 100mL/min, temperature range: 100 to 360 ℃) and the TGA (temperature rising rate of 20 ℃/min, nitrogen flow rate of 100mL/min, temperature range: 100 to 800 ℃) test results of examples 1 to 4 and comparative example 1 are shown in Table 1.

TABLE 1

From the performance data of the example in table 1, it can be seen that the cross-linked polyimide aerogel prepared by the method of the present invention has a higher upper limit temperature in use because the glass transition temperature and the 5% thermal weight loss temperature of the cross-linked polyimide aerogel exceed the polyimide aerogel prepared by the conventional method by more than 40 ℃ while maintaining good thermal insulation performance and low density.

The performance differences of examples 1 to 4 are the results of different degrees of internal crosslinking of the crosslinked polyimide aerogel, and as the content of the crosslinking agent is increased, the density of internal crosslinking points of the polyimide aerogel is increased, the density of the sample tends to be increased, the specific surface area tends to be increased, and the glass transition temperature and the thermal stability of the sample tend to be increased.

The invention has not been described in detail and is in part known to those of skill in the art.

Claims

1. The preparation method of the cross-linked polyimide aerogel is characterized by comprising the following steps of:

firstly, preparing a polyamic acid solution;

second, a crosslinked polyamic acid wet gel is prepared,

2. The method for preparing a crosslinked polyimide aerogel according to claim 1, wherein the method comprises the following steps: the molar ratio of the aromatic dianhydride to the aromatic diamine in the first step is 1: (0.4-0.85).

3. The method for preparing a crosslinked polyimide aerogel according to claim 1, wherein the method comprises the following steps: in the second step, the cross-linking agent adopts a triamino monomer, the dosage of the cross-linking agent is based on the dosage of the aromatic dianhydride, and the molar ratio of the total amount of amino groups to the total amount of anhydride groups is equal.

4. The method for preparing a crosslinked polyimide aerogel according to claim 1, wherein the method comprises the following steps: and in the second step, heating and aging the polyamic acid wet gel at 40-80 ℃ for not less than 24 hours.

5. The method for preparing a crosslinked polyimide aerogel according to claim 1, wherein the method comprises the following steps: and in the third step, the imidization is carried out through solvothermal for not less than 8 hours at the temperature of 100-250 ℃.

6. The method for preparing a crosslinked polyimide aerogel according to claim 1, wherein the method comprises the following steps: the solvent used for solvothermal operation in the third step must be the same as the solvent used for preparing the polyamic acid solution in the first step, and the solvent is such that the wet gel of polyamic acid is completely immersed.

7. The method for preparing a crosslinked polyimide aerogel according to claim 1, wherein the method comprises the following steps: the aromatic diamine in the first step is 4,4' -diaminoanisole (ODA) or 2,2' -dimethyl-4, 4' -Diaminobiphenyl (DMBZ); the aromatic dianhydride is 3,3',4,4' -biphenyl tetracarboxylic dianhydride, 3',4,4' -diphenyl ether tetracarboxylic dianhydride or 3,3',4,4' -benzophenone tetracarboxylic dianhydride.

8. The method for preparing a crosslinked polyimide aerogel according to claim 1, wherein the method comprises the following steps: the organic solvent in the first step is N-methylpyrrolidone (NMP), N-Dimethylformamide (DMF), N-dimethylacetamide (DMAc) or dimethyl sulfoxide (DMSO).

9. The method for preparing a crosslinked polyimide aerogel according to claim 1, wherein the method comprises the following steps: the condensation polymerization reaction in the first step is carried out for 8-32 hours at a temperature of 0-40 ℃.