CN108530673B - Linear polyimide aerogel and preparation method thereof - Google Patents

Linear polyimide aerogel and preparation method thereof Download PDF

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CN108530673B
CN108530673B CN201810461847.8A CN201810461847A CN108530673B CN 108530673 B CN108530673 B CN 108530673B CN 201810461847 A CN201810461847 A CN 201810461847A CN 108530673 B CN108530673 B CN 108530673B
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polyimide
aerogel
wet gel
polyimide aerogel
dianhydride
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CN108530673A (en
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程旭东
李聪聪
龚伦伦
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University of Science and Technology of China USTC
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    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/28—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof by elimination of a liquid phase from a macromolecular composition or article, e.g. drying of coagulum
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2201/00—Foams characterised by the foaming process
    • C08J2201/04—Foams characterised by the foaming process characterised by the elimination of a liquid or solid component, e.g. precipitation, leaching out, evaporation
    • C08J2201/048—Elimination of a frozen liquid phase
    • C08J2201/0482—Elimination of a frozen liquid phase the liquid phase being organic
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2205/00—Foams characterised by their properties
    • C08J2205/02—Foams characterised by their properties the finished foam itself being a gel or a gel being temporarily formed when processing the foamable composition
    • C08J2205/026—Aerogel, i.e. a supercritically dried gel
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2379/00—Characterised by the use of macromolecular compounds obtained by reactions forming in the main chain of the macromolecule a linkage containing nitrogen with or without oxygen, or carbon only, not provided for in groups C08J2361/00 - C08J2377/00
    • C08J2379/04—Polycondensates having nitrogen-containing heterocyclic rings in the main chain; Polyhydrazides; Polyamide acids or similar polyimide precursors
    • C08J2379/08—Polyimides; Polyester-imides; Polyamide-imides; Polyamide acids or similar polyimide precursors

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Abstract

The invention provides a polyimide aerogel which is prepared by freeze drying of polyimide wet gel or polyamide acid wet gel. The invention also provides a preparation method of the polyimide aerogel. The invention adopts a simple and cheap freeze drying technology, utilizes a sol-gel method, and finally prepares the linear polyimide aerogel through a specific solvent exchange step and a freeze drying method in the aging process after gelation. The preparation process provided by the invention is simple and easy to control, has low cost, is convenient to popularize and utilize, is suitable for large-scale industrial production, and greatly widens the limitation on the prior art; and the prepared 3D blocky polyimide aerogel has the characteristics of lower density, higher porosity, lower thermal conductivity, excellent mechanical property and the like.

Description

Linear polyimide aerogel and preparation method thereof
Technical Field
The invention relates to the technical field of high-performance porous materials, and relates to polyimide aerogel and a preparation method thereof. In particular to a linear polyimide aerogel and a preparation method thereof.
Background
Aerogel, also called xerogel, is a solid matter form, is a highly dispersed solid material which is formed by mutually agglomerating colloidal particles or high polymer molecules to form a nano porous network structure, and is filled with gaseous dispersion media in pores, is one of the solids with small density in the existing material, has the performances of ultralow density, high specific surface area, high porosity, excellent heat insulation and the like, and has wide application in the aspects of super heat insulation materials, sound insulation materials, particle detectors, low dielectric constant aerogel films, inertia targeting materials and the like. Such as X-ray, laser, heat insulation materials, energy storage devices, environmental protection, aerospace materials, bulletproof, ultra-light materials and other fields, and crack-free block aerogel is required for the application in aerospace heat insulation, transparent heat insulation windows, heat insulation containers and other fields.
There are many kinds of aerogels, including silicon-based, carbon-based, sulfur-based, metal oxide-based, or metal-based ones, and the common aerogel is silicon-based aerogel (SiO)2Etc.), has excellent heat-insulating property and environmental stability, is generally prepared by a sol-gel method, but is easy to crack gel due to the shrinkage generated in the gel aging and drying processes, so that the yield of the block aerogel is not high, and the method can only be carried out by using materials such as fiber cloth and the likeStrengthening and toughening seriously obstruct the commercialization process of the aerogel. The polymer aerogel is light in weight, flexible and easy to process, so that the application field of the polymer aerogel is continuously expanded, and common polymer aerogel materials comprise phenolic aldehyde (RF), Polystyrene (PS), Polyimide (PI) aerogel and the like. In recent years, with the extensive research on polyimide materials, the research on PI aerogels has attracted considerable attention.
Polyimide is an organic polymer with an imide ring in the main chain, has the advantages of good mechanical strength and high thermal stability, wear resistance and the like, and has unique advantages no matter being used as a structural material or a functional material. There are several dozen methods for synthesizing polyimides, of which the formation of polyimides by the reaction of dianhydride monomers (containing two anhydride groups) and diamine monomers (containing two amino groups) groups is the most commonly used method, which is generally divided into two steps: the first step is to form polyamic acid from dianhydride monomer and diamine monomer in an aprotic polar solvent such as N ', N ' -Dimethylformamide (DMF), N ' -dimethylacetamide (DMAc), or N-methylpyrrolidone (NMP), and the second step is to dehydrate amic acid groups in polyamic acid by heating or a chemical imidizing agent (typically acetic anhydride in combination with a tertiary amine such as pyridine, isoquinoline, or triethylamine) to form cyclic imide groups, thereby obtaining polyimide.
In 2006, Rhine and the like first react in solvent NMP to form polyamic acid by using a rigid dianhydride monomer and a diamine monomer as raw materials, then add a chemical imidization reagent, gradually increase the rigidity of a molecular chain with the increase of the degree of dehydration and cyclization of the polyamic acid, gradually reduce the solubility of the polyamic acid in the solvent to be gelled (to form polyimide wet gel), and then perform CO treatment on the gel to obtain the polyimide wet gel2Supercritical drying is carried out to obtain the polyimide aerogel, but the prepared polyimide aerogel has high shrinkage rate.
In 2010, the PI aerogel is synthesized by an isocyanate method by selecting PMDA and 4, 4-diphenylmethane diisocyanate (MDI) as raw materials at room temperature of American university of Soyspori. As a control, it is reacted with PMDA and 4, 4' -diaminodiphenylmethane (MDA) to generate a PAA solution, then subjected to chemical imidization treatment and thermal imidization treatment in sequence, and finally dried by supercritical carbon dioxide to obtain a linear PI aerogel.
However, the existing method for preparing the PI aerogel still has the following defects that the supercritical drying process is complex and tedious, the production efficiency is low, the product cost is high, and the large-scale, quick and efficient preparation of the aerogel is difficult to realize; in addition, the polyamic acid product can be rapidly degraded in the storage process, so that the molecular weight of the polyamic acid product is reduced, and the performance of the polyimide aerogel is finally influenced; in addition, the density of the cross-linked PI aerogel prepared by the prior art is generally high, which undoubtedly increases the cost of the aerogel as a heat insulation material, and simultaneously causes the defect of insufficient heat insulation performance, and the requirement of the aerogel heat insulation base material is difficult to achieve.
Therefore, how to find a simple preparation method to obtain the polyimide aerogel, so as to overcome the above defects and widen the popularization prospect of the polyimide aerogel becomes one of the focuses of extensive attention of many application researchers in the industry.
Disclosure of Invention
In view of the above, the technical problem to be solved by the present invention is to provide a polyimide aerogel and a preparation method thereof, and particularly to a linear polyimide aerogel and a preparation method thereof.
The invention provides a polyimide aerogel which is prepared by freeze drying of polyimide wet gel or polyamide acid wet gel.
Preferably, the polyimide aerogel is a linear polyimide aerogel;
the porosity of the polyimide aerogel is 85% -95%;
the average pore diameter of the polyimide aerogel is 10-50 nm;
the polyimide wet gel is obtained by chemical imidization treatment of a dianhydride monomer and a diamine monomer;
the polyamic acid wet gel is obtained by copolymerizing a dianhydride monomer and a diamine monomer.
Preferably, the density of the polyimide aerogel is 0.08-0.15 g/cm3；
The specific surface area of the polyimide aerogel is 400-800 m2/g；
The thermal conductivity of the polyimide aerogel is 0.024-0.030W/(m.K);
the thermal decomposition temperature of the polyimide aerogel is 610-648 ℃;
the polyimide aerogel is a polyimide aerogel block;
and freeze-drying the polyamic acid wet gel, and then performing thermal imidization treatment.
The invention provides a preparation method of polyimide aerogel, which comprises the following steps:
1) standing the polyimide wet gel, and then placing the polyimide wet gel in an organic solvent for replacement to obtain a replaced polyimide wet gel;
2) soaking the displaced polyimide wet gel obtained in the step into tert-butyl alcohol and/or cyclohexane for displacement again to obtain a semi-finished product;
3) and (4) freezing and drying the semi-finished product obtained in the step to obtain the polyimide aerogel.
Preferably, the standing time is 24-48 h;
the organic solvent comprises acetone;
the replacement is a plurality of replacements;
the times of the multiple replacement are 2-5 times;
and the time of each replacement in the multiple replacements is 6-24 hours.
Preferably, the replacing is performed several times;
the times of the plurality of times of replacement are 3-8 times;
the time of each replacement in the plurality of replacements is 6-24 hours;
the freeze drying comprises the steps of low-temperature freezing and vacuum drying;
the low-temperature freezing time is 4-6 h; the temperature of the low-temperature freezing is-30 to-5 ℃;
the temperature of the vacuum drying is-5-10 ℃; the vacuum drying time is 36-48 h.
Preferably, the polyimide wet gel is prepared by the following steps:
a) mixing a dianhydride monomer, a diamine monomer and a polar aprotic solvent, and then adding a chemical cross-linking agent for cross-linking to obtain a reaction solution;
b) and mixing the reaction solution, the dehydrating agent and the catalyst obtained in the step again to obtain sol, and pre-standing to obtain the polyimide wet gel.
Preferably, the dianhydride monomer includes one or more of 3,3 ', 4,4 ' -biphenyltetracarboxylic dianhydride, 4,4 ' -diphenyl ether dianhydride, 3 ', 4,4 ' -benzophenonetetracarboxylic dianhydride and pyromellitic dianhydride;
the diamine monomer comprises one or more of 4,4 '-diaminodiphenyl ether, 3, 4' -diaminodiphenyl ether, 2 '-dimethylbenzidine, 2-bis [4- (4-aminophenoxy) phenyl ] propane and 4, 4' -diaminodiphenylmethane;
the polar aprotic solvent comprises one or more of N-methylpyrrolidone, a mixed solution of tetrahydrofuran and methanol, N-dimethylformamide and N, N-dimethylacetamide;
the chemical cross-linking agent comprises 1,3, 5-tri (4-aminophenoxy) benzene and/or octa (aminophenyl) cage polysilsesquioxane;
the molar ratio of the dianhydride monomer to the diamine monomer is (1-1.05): 1;
the molar ratio of the polar aprotic solvent to the diamine monomer is (30-50): 1;
the molar ratio of the chemical crosslinking agent to the diamine monomer is 1: (40-50);
the crosslinking time is 30-60 min.
Preferably, the dehydrating agent comprises acetic anhydride;
the catalyst comprises pyridine;
the molar ratio of the dehydrating agent to the dianhydride monomer is 1: (100-150);
the molar ratio of the catalyst to the dianhydride monomer is 1: (100-150);
the pre-standing time is 2-4 h.
The invention also provides a preparation method of the polyimide aerogel, which comprises the following steps:
1') standing the polyamic acid wet gel, and then placing the polyamic acid wet gel in an organic solvent for replacement to obtain a replaced polyamic acid wet gel;
2') soaking the displaced polyamic acid wet gel obtained in the step into tert-butyl alcohol and/or cyclohexane for displacement again to obtain a semi-finished product;
3') freeze-drying the semi-finished product obtained in the step to obtain polyamic acid aerogel, and performing heat treatment to obtain polyimide aerogel;
the polyamic acid wet gel is prepared by the following steps:
A) mixing dianhydride monomer, diamine monomer and polar aprotic solvent to obtain reaction liquid, and standing to obtain polyamide acid wet gel.
The invention provides a polyimide aerogel which is prepared by freeze drying of polyimide wet gel or polyamide acid wet gel. The invention also provides a preparation method of the polyimide aerogel. Compared with the prior art, the preparation method disclosed by the invention aims at the practical problems that the existing polyimide aerogel is mostly prepared by adopting a supercritical carbon dioxide drying method, the process is complex and tedious, the equipment investment is large, the production efficiency is low, the product cost is higher, the large-scale, quick and efficient preparation of the aerogel is difficult to realize, and the like, and the polyamic acid as a process product can be quickly degraded in the storage process, so that the molecular weight of the polyamic acid is reduced, and the performance of the polyimide aerogel is finally influenced. And the prepared cross-linked PI aerogel also has the problems of high shrinkage rate and high density, which causes the cost of the aerogel as a heat insulation material to be increased, insufficient heat insulation performance and difficulty in meeting the requirements of the aerogel heat insulation base material. The invention also discovers that the prior art mostly adopts high-boiling point solvent, which is difficult to remove in the replacement process, and has adverse effects on the performance and processing of materials, and the removal process of the replaced low-boiling point solvent also has adverse effects on the environment, and the like.
The invention adopts a simple and cheap freeze-drying technology, and overcomes the technical bias that the existing polyimide aerogel can not have better performance and structure through the conventional freeze-drying technology. The linear polyimide aerogel is prepared by a sol-gel method, a specific solvent exchange step is carried out in the aging process after gelation, and a freeze drying method is carried out. The preparation process provided by the invention is simple and easy to control, has low cost, is convenient to popularize and utilize, is suitable for large-scale industrial production, and greatly widens the limitation on the prior art; and the prepared 3D blocky polyimide aerogel has the characteristics of lower density, higher porosity, lower thermal conductivity, excellent mechanical property and the like.
Experimental results show that the 3D blocky polyimide aerogel with the linear structure is successfully prepared by the freeze drying technology, the method is simple, the time is short, and the density of the polyimide aerogel is only 0.08-0.15 g/cm3The specific surface area can reach 800m2(ii)/g; the thermal conductivity is only 0.024-0.030W/(m.K), and the thermal decomposition temperature can reach 610-648 ℃.
Drawings
FIG. 1 is a schematic view of the process for preparing a polyimide aerogel according to example 1 of the present invention;
FIG. 2 is a photograph showing the appearance of 3D bulk polyimide aerogel prepared in example 1 according to the present invention;
FIG. 3 is a scanning electron micrograph of a 3D bulk polyimide aerogel prepared according to example 1 of the present invention;
FIG. 4 is a graph showing mechanical properties of a 3D bulk polyimide aerogel prepared in example 1 of the present invention;
fig. 5 is a thermogravimetric plot of the 3D bulk polyimide aerogel prepared in example 1 of the present invention.
Detailed Description
For a further understanding of the invention, preferred embodiments of the invention are described below in conjunction with the examples, but it should be understood that these descriptions are included merely to further illustrate the features and advantages of the invention and are not intended to limit the invention to the claims.
All of the starting materials of the present invention, without particular limitation as to their source, may be purchased commercially or prepared according to conventional methods well known to those skilled in the art.
All of the starting materials of the present invention are not particularly limited in their purity, and the present invention preferably employs purity levels commonly used in the art of analytically pure or aerogel materials.
The invention provides a polyimide aerogel which is prepared by freeze drying of polyimide wet gel or polyamide acid wet gel.
The polyimide wet gel of the present invention is preferably obtained by chemical imidization of a dianhydride monomer and a diamine monomer, and is not particularly limited in its source, and may be obtained from conventional polyimide wet gels known to those skilled in the art, may be prepared according to conventional preparation methods, or may be commercially available.
The source of the polyamic acid wet gel is not particularly limited, and the polyamic acid wet gel known to those skilled in the art can be prepared by a conventional preparation method or can be purchased commercially, and those skilled in the art can select and adjust the source according to the actual application, product quality and product performance.
The polyimide aerogel parameters are not particularly limited by the present invention, and the conventional parameters of the polyimide aerogel known to those skilled in the art can be used, and those skilled in the art can select and adjust the parameters according to the actual application, product quality and product performance, and the polyimide aerogel prepared by the freeze-drying technology of the present invention is particularly preferably a polyimide aerogel block having a macroscopic 3D morphology. The polyimide aerogel disclosed by the invention has a linear structure and is linear polyimide aerogel. The polyimide aerogel of the present invention has a low porosity, i.e., a low shrinkage (during freeze-drying), preferably 85% to 95%, more preferably 87% to 93%, and still more preferably 89% to 91%. The average pore diameter of the polyimide aerogel is preferably 10-50 nm, more preferably 15-45 nm, more preferably 20-40 nm, and more preferably 25-35 nm.
The density of the polyimide aerogel disclosed by the invention is preferably 0.08-0.15 g/cm3More preferably 0.09 to 0.14g/cm3More preferably 0.1 to 0.13g/cm3More preferably 0.1 to 0.12g/cm3. The specific surface area of the polyimide aerogel is preferably 400-800 m2(iv)/g, more preferably 450 to 750m2A concentration of 500 to 700m is more preferable2(iv)/g, more preferably 550 to 650m2(ii) in terms of/g. The thermal conductivity of the polyimide aerogel is preferably 0.024-0.030W/(m.K), more preferably 0.025-0.029W/(m.K), and more preferably 0.026-0.028W/(m.K). The thermal decomposition temperature of the polyimide aerogel is preferably 610-648 ℃, more preferably 615-643 ℃, more preferably 620-638 ℃ and more preferably 625-633 ℃.
The steps of the invention provide a polyimide aerogel, a simple and cheap freeze drying technology is adopted, the technical bias that the existing polyimide aerogel can not have better performance and structure through the conventional freeze drying technology is overcome, and a 3D macroscopic polyimide aerogel block with a linear structure is prepared from polyimide wet gel or polyamic acid wet gel by using a freeze drying method. The 3D blocky polyimide aerogel prepared by the invention has the characteristics of lower density, higher porosity, lower thermal conductivity, excellent mechanical property and the like, and the freeze drying technical process is simple and easy to control, low in cost, convenient to popularize and utilize and suitable for large-scale industrial production.
The invention also provides a preparation method of the polyimide aerogel, which comprises the following steps:
1) standing the polyimide wet gel, and then placing the polyimide wet gel in an organic solvent for replacement to obtain a replaced polyimide wet gel;
2) soaking the displaced polyimide wet gel obtained in the step into tert-butyl alcohol and/or cyclohexane for displacement again to obtain a semi-finished product;
3) and (4) freezing and drying the semi-finished product obtained in the step to obtain the polyimide aerogel.
The steps of the invention provide a method for preparing polyimide wet gel by chemical imidization and then preparing polyimide aerogel. The selection, proportion and parameters of the raw materials or products in the preparation method of the present invention, and the corresponding preferred principles, etc., correspond to the selection, proportion and parameters of the raw materials in the polyimide aerogel, and the corresponding preferred principles, etc., if not specifically noted, and are not described in detail herein.
The source of the polyimide wet gel is not particularly limited, the polyimide wet gel can be prepared by a conventional source of the polyimide wet gel known by a person skilled in the art or can be commercially available according to a conventional preparation method, and the person skilled in the art can select and adjust the polyimide wet gel according to the actual application condition, the product quality and the product performance, and in order to ensure the performance of the final product, complete and refine the preparation route,
the polyimide wet gel is preferably obtained by chemical imidization treatment of a dianhydride monomer and a diamine monomer, and is more preferably prepared by the following steps:
a) mixing a dianhydride monomer, a diamine monomer and a polar aprotic solvent, and then adding a chemical cross-linking agent for cross-linking to obtain a reaction solution;
b) and mixing the reaction solution, the dehydrating agent and the catalyst obtained in the step again to obtain sol, and pre-standing to obtain the polyimide wet gel.
The method comprises the steps of mixing a dianhydride monomer, a diamine monomer and a polar aprotic solvent, and then adding a chemical cross-linking agent for cross-linking to obtain a reaction solution.
The dianhydride monomer of the present invention is not particularly limited, and may be selected and adjusted according to the actual application, product quality and product performance by those skilled in the art, and preferably includes one or more of 3,3 ', 4, 4' -biphenyltetracarboxylic dianhydride (BPDA), 4,4 '-diphenyl ether dianhydride (ODPA), 3', 4,4 '-benzophenonetetracarboxylic dianhydride (BTDA) and pyromellitic dianhydride (PMDA), and more preferably 3, 3', 4,4 '-biphenyltetracarboxylic dianhydride (BPDA), 4, 4' -biphenyltetracarboxylic dianhydride (ODPA), 3 ', 4, 4' -benzophenonetetracarboxylic dianhydride (BTDA) or pyromellitic dianhydride (PMDA).
The diamine monomer is not particularly limited in the present invention, and may be selected and adjusted by those skilled in the art according to practical use, product quality, and product properties, and the diamine monomer used for preparing polyimide according to the present invention preferably includes one or more of 4,4 ' -diaminodiphenyl ether (ODA), 3,4 ' -diaminodiphenyl ether, 2 ' -Dimethylbenzidine (DMBZ), 2-bis [4- (4-aminophenoxy) phenyl ] propane (BAPP), and 4,4 ' -diaminodiphenylmethane (MDA), more preferably 4,4 ' -diaminodiphenyl ether (ODA), 3,4 ' -diaminodiphenyl ether, 2 ' -Dimethylbenzidine (DMBZ), 2-bis [4- (4-aminophenoxy) phenyl ] propane (BAPP), or 4, 4' -diaminodiphenylmethane (MDA).
The amount of the diamine monomer and the dianhydride monomer is not particularly limited in the present invention, and may be selected and adjusted by those skilled in the art according to the practical application, product quality and product performance, and the molar ratio of the dianhydride monomer to the diamine monomer in the present invention is preferably (1-1.05): 1, more preferably (1.01 to 1.04): 1, more preferably (1.02 to 1.03): 1.
the polar aprotic solvent is not particularly limited in the present invention, and may be selected and adjusted by those skilled in the art according to the practical application, the product quality and the product properties, and preferably includes one or more of N-methylpyrrolidone, a mixed solution of tetrahydrofuran and methanol, N-dimethylformamide and N, N-dimethylacetamide, more preferably N-methylpyrrolidone, a mixed solution of tetrahydrofuran and methanol, N-dimethylformamide or N, N-dimethylacetamide, and still more preferably N-methylpyrrolidone.
The amount of the polar aprotic solvent used in the present invention is not particularly limited, and may be any amount conventionally used for preparing polyimide, which is well known to those skilled in the art, and may be selected and adjusted by those skilled in the art according to the actual application, the product quality and the product performance, and the molar ratio of the polar aprotic solvent to the diamine monomer in the present invention is preferably (30-50): 1, more preferably (32 to 48): 1, more preferably (35-45): 1, more preferably (37 to 42): 1.
the chemical crosslinking agent is not particularly limited in the present invention, and may be a chemical crosslinking agent for preparing polyimide, which is well known to those skilled in the art, and can be selected and adjusted by those skilled in the art according to the actual application, product quality and product properties, and preferably includes 1,3, 5-tris (4-aminophenoxy) benzene (TAB) and/or octa (aminophenyl) cage polysilsesquioxane (OAPS), more preferably 1,3, 5-tris (4-aminophenoxy) benzene (TAB) or octa (aminophenyl) cage polysilsesquioxane (OAPS).
The amount of the chemical crosslinking agent used in the present invention is not particularly limited, and may be any amount conventionally used for preparing polyimide, which is well known to those skilled in the art, and can be selected and adjusted by those skilled in the art according to the actual application, the product quality and the product performance, and the molar ratio of the chemical crosslinking agent to the diamine monomer in the present invention is preferably 1: (40-50), more preferably 1: (42-48), more preferably 1: (44-46).
The crosslinking conditions are not particularly limited, and the chemical crosslinking process and parameters for preparing polyimide, which are well known to those skilled in the art, can be selected and adjusted by those skilled in the art according to the actual application condition, the product quality and the product performance, and the crosslinking time in the invention is preferably 30-60 min, more preferably 35-55 min, and more preferably 40-50 min.
The mixing procedure and parameters are not particularly limited in the present invention, and may be conventional mixing procedures and parameters well known to those skilled in the art, and those skilled in the art can select and adjust the mixing procedure according to the actual application, product quality and product performance, and the mixing method of the present invention is preferably stirring mixing. The mixing time of the invention is preferably 10-30 min, more preferably 12-28 min, and more preferably 15-25 min.
According to the invention, the reaction solution, the dehydrating agent and the catalyst obtained in the above steps are mixed again to obtain sol, and the polyimide wet gel is obtained after pre-standing.
The dehydrating agent of the present invention is not particularly limited, and may be a dehydrating agent for preparing polyimide, which is well known to those skilled in the art, and may be selected and adjusted by those skilled in the art according to practical use, product quality, and product properties, and preferably includes acetic anhydride.
The amount of the dehydrating agent used in the present invention is not particularly limited, and may be any amount conventionally used for preparing polyimide, which is well known to those skilled in the art, and may be selected and adjusted by those skilled in the art according to the actual application, the product quality and the product performance, and the molar ratio of the dehydrating agent to the dianhydride monomer in the present invention is 1: (100 to 150), more preferably 1: (110 to 140), more preferably 1: (120-130).
The catalyst of the present invention is not particularly limited, and may be selected and adjusted by those skilled in the art according to the actual application, product quality and product properties, and preferably includes pyridine.
The amount of the catalyst used in the present invention is not particularly limited, and may be selected and adjusted by those skilled in the art according to the practical application, product quality and product performance, and the molar ratio of the catalyst to the dianhydride monomer is 1: (100 to 150), more preferably 1: (110 to 140), more preferably 1: (120-130).
The pre-standing condition is not particularly limited, the chemical crosslinking process and parameters for preparing polyimide, which are well known to those skilled in the art, can be selected and adjusted by those skilled in the art according to the actual application condition, the product quality and the product performance, and the pre-standing time is preferably 2-4 hours, more preferably 2.2-3.8 hours, more preferably 2.5-3.5 hours, and more preferably 2.7-3.3 hours. The pre-standing according to the invention can be carried out in an open environment or under protective gas, vacuum or air-tight conditions, preferably in a sealed container.
The process and parameters of the remixing in the present invention are not particularly limited, and may be conventional mixing processes and parameters well known to those skilled in the art, and those skilled in the art may select and adjust the process according to the actual application, the product quality and the product performance, and the remixing in the present invention is preferably mixing with stirring. The remixing time is preferably 10-30 min, more preferably 12-28 min, and more preferably 15-25 min.
In order to further ensure the product performance, complete and refined reaction process, the preparation process of the polyimide wet gel specifically comprises the following steps:
(I) solution preparation
When preparing the solution, firstly, the diamine monomer is dissolved in the polar aprotic solvent, and the dianhydride monomer is added after the diamine monomer is completely dissolved by stirring. Stirring until the reactant becomes transparent to form a solution A;
the crosslinker is then dissolved in the polar aprotic solvent and stirred continuously to form solution B.
(II) preparation of Sol
And adding the solution B into the solution A, stirring to crosslink the solution B to form a homogeneous solution, continuously stirring, adding a dehydrating agent, and adding a catalyst to form sol C.
(III) gel
And continuously stirring the sol C for 10-30 min, uniformly pouring the sol C into a sealed container (mold) to wait for gelation, and pre-standing to obtain wet gel.
After the polyimide wet gel is prepared by the steps of the invention, the polyimide wet gel is stood and then is placed in an organic solvent for replacement to obtain the replaced polyimide wet gel
The standing condition is not particularly limited, the chemical crosslinking process and parameters for preparing polyimide, which are well known to those skilled in the art, can be selected and adjusted by those skilled in the art according to the actual application condition, the product quality and the product performance, and the standing time is preferably 24-48 hours, more preferably 27-45 hours, more preferably 30-42 hours, and more preferably 33-39 hours. The standing process is an aging process of the wet gel, the standing can be carried out in an open environment or under the conditions of protective gas, vacuum or air isolation, and the standing is preferably carried out in a sealed container.
The organic solvent is not particularly limited in the present invention, and may be any conventional organic solvent known to those skilled in the art, and those skilled in the art can select and adjust the organic solvent according to the actual application, the product quality and the product performance, and the organic solvent of the present invention preferably includes acetone.
The amount of the organic solvent used in the present invention is not particularly limited, and may be selected and adjusted by those skilled in the art according to the practical application, the quality of the product, and the product performance, in the conventional amount for preparing polyimide, which is well known to those skilled in the art.
The process and parameters for the replacement of the organic solvent are not particularly limited in the present invention, and may be replaced by conventional aerogel materials known to those skilled in the art, and those skilled in the art can select and adjust the process and parameters according to practical application, product quality and product performance, and the replacement in the present invention is preferably performed several times. The number of times of the multiple replacement is preferably 2-5 times, and more preferably 3-4 times. The time of each replacement in the multiple replacements is preferably 6-24 hours, more preferably 10-20 hours, and more preferably 14-16 hours.
According to the invention, the replaced polyimide wet gel obtained in the above step is immersed in tert-butyl alcohol and/or cyclohexane for replacing again to obtain a semi-finished product.
In the present invention, tertiary butanol and/or cyclohexane, more preferably tertiary butanol or cyclohexane, are selected in particular for the secondary substitution. The research of the invention considers that the freeze drying technology is to firstly carry out the related solvent freezing treatment and put the freeze drying technology in a vacuum environment to realize the temperature rise sublimation, so as to prevent the capillary pressure in the gas-liquid phase transformation link, and the related crystals of the solvent in a special pore channel have the expansion change to cause the crushing of the framework. In view of the above problems, the present invention is based on the difference in the actual expansion coefficient of each solvent, and a solvent having a low boiling point and a low vapor pressure is more suitable as a solvent for the freeze-drying process. Tert-butanol and/or cyclohexane are particularly chosen, which have a low boiling point, a high melting point and a low vapor pressure, so that volatilization is facilitated.
And in the subsequent drying treatment, the mass transfer resistance of the water vapor is mainly the flow resistance in the drying layer. And particularly, matched control measures are adopted, so that a relatively ideal drying rate can be realized even in a low-temperature environment, the collapse problem can be prevented, and the whole drying time can be shortened. More particularly, the tert-butanol and/or cyclohexane form special needle crystals, and after sublimation of such ice crystals, relatively many pores remain, which effectively control the actual mass transfer resistance, and which can maintain a desired rate at a lower temperature, thereby preventing the collapse of the framework therein. The comprehensive production cost can be controlled, and the time required by the drying process is reduced.
The process and parameters of the second replacement are not particularly limited, and those skilled in the art can select and adjust the process according to the actual application, the product quality and the product performance, and the second replacement in the invention is preferably performed several times. The number of times of the secondary replacement is preferably 3 to 8 times, more preferably 4 to 7 times, and still more preferably 5 to 6 times. The time of each replacement in the secondary replacement is preferably 6-24 h, more preferably 10-20 h, and more preferably 14-16 h. The immersion according to the invention is preferably carried out by dipping or soaking, more preferably soaking.
In the standing, replacing and replacing processes, the polyimide wet gel is subjected to an aging process in the aging process, the gel continues to perform polycondensation reaction, and a network structure continues to grow up, so that the strength of the skeleton structure of the gel can be enhanced.
Finally, freeze-drying the semi-finished product obtained in the step to obtain the polyimide aerogel.
The specific process of freeze-drying is not particularly limited in the present invention, and may be a conventional freeze-drying process of such aerogel, which is well known to those skilled in the art, and can be selected and adjusted by those skilled in the art according to the actual application, product quality and product performance, and the freeze-drying of the present invention preferably includes low-temperature freezing and vacuum drying steps.
The temperature of the low-temperature freezing is not particularly limited, the freezing temperature of the aerogel material prepared by a conventional freeze-drying method well known to a person skilled in the art can be selected and adjusted by the person skilled in the art according to the actual application condition, the product quality and the product performance, and the temperature of the low-temperature freezing is preferably-30 to-5 ℃, more preferably-25 to 0 ℃, more preferably-20 to-5 ℃, and more preferably-15 to-10 ℃. The time for low-temperature freezing is not particularly limited, and the freezing time for preparing the aerogel material by using a conventional freeze-drying method well known to those skilled in the art can be selected and adjusted by those skilled in the art according to the actual application condition, the product quality and the product performance, and the time for low-temperature freezing is preferably 4-6 hours, more preferably 4.2-5.8 hours, and more preferably 4.5-5.5 hours.
The temperature of the vacuum drying is not particularly limited, and the temperature of the aerogel material prepared by conventional freeze drying well known to those skilled in the art can be selected and adjusted by those skilled in the art according to the actual application condition, the product quality and the product performance, and the temperature of the vacuum drying is preferably-5-10 ℃, more preferably-3-8 ℃, and more preferably 0-5 ℃. The vacuum drying time is not particularly limited, and the freeze drying time for preparing the aerogel material by using a conventional freeze drying method well known to those skilled in the art can be selected and adjusted by those skilled in the art according to the actual application condition, the product quality and the product performance, and the vacuum drying time is preferably 36-48 hours, more preferably 38-46 hours, and more preferably 40-44 hours.
The steps of the invention provide a preparation method of polyimide aerogel, and in order to further ensure the performance of the product, complete and refine the preparation process, the specific preparation process of the polyimide aerogel can be as follows:
(I) solution preparation
When preparing the solution, firstly, the diamine monomer is dissolved in the polar aprotic solvent, and the dianhydride monomer is added after the diamine monomer is completely dissolved by stirring. Stirring until the reactant becomes transparent to form a solution A;
the crosslinker is then dissolved in the polar aprotic solvent and stirred continuously to form solution B.
(II) preparation of Sol
And adding the solution B into the solution A, stirring to crosslink the solution B to form a homogeneous solution, continuously stirring, adding a dehydrating agent, and adding a catalyst to form sol C.
(III) gel
And continuously stirring the sol C for 10-30 min, uniformly pouring the sol C into a sealed container (mold) to wait for gelation, and pre-standing to obtain wet gel.
(IV) aging, solvent replacement
And aging the gel block or the film in a sealed container for 24h to enable the gel to continue to carry out polycondensation reaction, replacing the gel in an organic solvent for 24h, and soaking the gel in a low-boiling-point solvent (tert-butyl alcohol or cyclohexane) of which the boiling point is 10 times that of the gel for solvent exchange to obtain the alcohol gel (semi-finished product) containing the tert-butyl alcohol and/or cyclohexane.
(V) drying
And (3) freezing the obtained alcogel at-30 ℃, and then freezing and drying the alcogel in a vacuum freeze dryer at different temperatures to obtain the polyimide aerogel.
The polyimide aerogel is prepared through the steps, and the application range of the polyimide aerogel is further widened, and the practicability and the adaptability of the preparation method are enhanced. The invention also provides a method for preparing polyimide aerogel by a thermal imidization method, which is characterized in that the thermal imidization and the chemical imidization are different from each other in the whole, chemical imidization is carried out, cross-linking agents, pyridine and other gels are required to be added into dianhydride and diamine, and then freeze drying is carried out; the thermal imidization may be carried out by adding a diamine dianhydride monomer to form a gel, followed by freeze-drying and heating, more preferably by vacuum stepwise heating.
The invention also provides a preparation method of the polyimide aerogel, which comprises the following steps:
1') standing the polyamic acid wet gel, and then placing the polyamic acid wet gel in an organic solvent for replacement to obtain a replaced polyamic acid wet gel;
2') soaking the displaced polyamic acid wet gel obtained in the step into tert-butyl alcohol and/or cyclohexane for displacement again to obtain a semi-finished product;
3') freeze-drying the semi-finished product obtained in the step to obtain polyamic acid aerogel, and performing heat treatment to obtain polyimide aerogel.
The method for preparing the polyimide aerogel by preparing the polyimide wet gel through the thermal imidization provided by the steps is provided by the invention. The selection, proportion and parameters of the raw materials or products in the above preparation method of the present invention, and the corresponding preferred principles, etc., correspond to the selection, proportion and parameters of the raw materials in the preparation of the polyimide wet gel by the polyimide aerogel or the chemical imidization, and the corresponding preferred principles, etc., if not specifically noted, and are not described herein again.
The steps of the invention provide a method for preparing polyamide acid wet gel through thermal imidization and then preparing polyimide aerogel. The selection, proportion and parameters of the raw materials or products in the preparation method of the present invention, and the corresponding preferred principles, etc., correspond to the selection, proportion and parameters of the raw materials in the polyimide aerogel, and the corresponding preferred principles, etc., if not specifically noted, and are not described in detail herein.
The present invention has no particular limitation on the source of the polyamic acid wet gel, and the polyamic acid wet gel can be prepared by a conventional preparation method or purchased commercially according to a conventional source of the polyamic acid wet gel known to those skilled in the art, and can be selected and adjusted according to the actual application condition, the product quality and the product performance, and in order to ensure the performance of the final product, complete and refine the preparation route,
the polyamic acid wet gel is preferably obtained by polymerizing a dianhydride monomer and a diamine monomer, and is more preferably prepared by the following steps:
A) mixing dianhydride monomer, diamine monomer and polar aprotic solvent to obtain reaction liquid, and standing to obtain polyamide acid wet gel.
The preparation method comprises the steps of mixing a dianhydride monomer, a diamine monomer and a polar aprotic solvent to obtain a reaction solution, and standing to obtain the polyamide acid wet gel.
The dianhydride monomer of the present invention is not particularly limited, and may be selected and adjusted according to the actual application, product quality and product performance by those skilled in the art, and preferably includes one or more of 3,3 ', 4, 4' -biphenyltetracarboxylic dianhydride (BPDA), 4,4 '-diphenyl ether dianhydride (ODPA), 3', 4,4 '-benzophenonetetracarboxylic dianhydride (BTDA) and pyromellitic dianhydride (PMDA), and more preferably 3, 3', 4,4 '-biphenyltetracarboxylic dianhydride (BPDA), 4, 4' -biphenyltetracarboxylic dianhydride (ODPA), 3 ', 4, 4' -benzophenonetetracarboxylic dianhydride (BTDA) or pyromellitic dianhydride (PMDA).
The diamine monomer is not particularly limited in the present invention, and may be selected and adjusted by those skilled in the art according to practical use, product quality, and product properties, and the diamine monomer used for preparing polyimide according to the present invention preferably includes one or more of 4,4 ' -diaminodiphenyl ether (ODA), 3,4 ' -diaminodiphenyl ether, 2 ' -Dimethylbenzidine (DMBZ), 2-bis [4- (4-aminophenoxy) phenyl ] propane (BAPP), and 4,4 ' -diaminodiphenylmethane (MDA), more preferably 4,4 ' -diaminodiphenyl ether (ODA), 3,4 ' -diaminodiphenyl ether, 2 ' -Dimethylbenzidine (DMBZ), 2-bis [4- (4-aminophenoxy) phenyl ] propane (BAPP), or 4, 4' -diaminodiphenylmethane (MDA).
The amount of the diamine monomer and the dianhydride monomer is not particularly limited in the present invention, and may be selected and adjusted by those skilled in the art according to the practical application, product quality and product performance, and the molar ratio of the dianhydride monomer to the diamine monomer in the present invention is preferably (1-1.05): 1, more preferably (1.01 to 1.04): 1, more preferably (1.02 to 1.03): 1.
the polar aprotic solvent is not particularly limited in the present invention, and may be selected and adjusted by those skilled in the art according to the practical application, the product quality and the product properties, and preferably includes one or more of N-methylpyrrolidone, a mixed solution of tetrahydrofuran and methanol, N-dimethylformamide and N, N-dimethylacetamide, more preferably N-methylpyrrolidone, a mixed solution of tetrahydrofuran and methanol, N-dimethylformamide or N, N-dimethylacetamide, and still more preferably N-methylpyrrolidone.
The amount of the polar aprotic solvent used in the present invention is not particularly limited, and may be any amount conventionally used for preparing polyimide, which is well known to those skilled in the art, and may be selected and adjusted by those skilled in the art according to the actual application, the product quality and the product performance, and the molar ratio of the polar aprotic solvent to the diamine monomer in the present invention is preferably (30-50): 1, more preferably (32 to 48): 1, more preferably (35-45): 1, more preferably (37 to 42): 1.
the mixing procedure and parameters are not particularly limited in the present invention, and may be conventional mixing procedures and parameters well known to those skilled in the art, and those skilled in the art can select and adjust the mixing procedure according to the actual application, product quality and product performance, and the mixing method of the present invention is preferably stirring mixing. The mixing time of the invention is preferably 10-30 min, more preferably 12-28 min, and more preferably 15-25 min.
The restating condition is not particularly limited, and conventional conditions for preparing polyimide, which are well known to those skilled in the art, can be selected and adjusted by those skilled in the art according to the actual application condition, the product quality and the product performance, and the restating time is preferably 2-4 hours, more preferably 2.2-3.8 hours, more preferably 2.5-3.5 hours, and more preferably 2.7-3.3 hours. The restating according to the invention can be carried out in an open environment or under protective gas, vacuum or in the absence of air, preferably in a sealed container.
After the polyamic acid wet gel is prepared in the steps, the polyamic acid wet gel is placed still and then is displaced in an organic solvent, so that the displaced polyamic acid wet gel is obtained.
The standing condition is not particularly limited, the chemical crosslinking process and parameters for preparing polyimide, which are well known to those skilled in the art, can be selected and adjusted by those skilled in the art according to the actual application condition, the product quality and the product performance, and the standing time is preferably 24-48 hours, more preferably 27-45 hours, more preferably 30-42 hours, and more preferably 33-39 hours. The standing process is an aging process of the wet gel, the standing can be carried out in an open environment or in a protective gas, vacuum or air isolation, and the standing is preferably carried out in a sealed container.
The organic solvent is not particularly limited in the present invention, and may be any conventional organic solvent known to those skilled in the art, and those skilled in the art can select and adjust the organic solvent according to the actual application, the product quality and the product performance, and the organic solvent of the present invention preferably includes acetone.
The amount of the organic solvent used in the present invention is not particularly limited, and may be selected and adjusted by those skilled in the art according to the practical application, the quality of the product, and the product performance, in the conventional amount for preparing polyimide, which is well known to those skilled in the art.
The process and parameters for the replacement of the organic solvent are not particularly limited in the present invention, and may be replaced by conventional aerogel materials known to those skilled in the art, and those skilled in the art can select and adjust the process and parameters according to practical application, product quality and product performance, and the replacement in the present invention is preferably performed several times. The number of times of the multiple replacement is preferably 2-5 times, and more preferably 3-4 times. The time of each replacement in the multiple replacements is preferably 6-24 hours, more preferably 10-20 hours, and more preferably 14-16 hours.
According to the invention, the replaced polyamic acid wet gel obtained in the step is soaked in tert-butyl alcohol and/or cyclohexane for replacing again to obtain a semi-finished product.
In the present invention, tertiary butanol and/or cyclohexane, more preferably tertiary butanol or cyclohexane, are selected in particular for the secondary substitution. The research of the invention considers that the freeze drying technology is to firstly carry out the related solvent freezing treatment and put the freeze drying technology in a vacuum environment to realize the temperature rise sublimation, so as to prevent the capillary pressure in the gas-liquid phase transformation link, and the related crystals of the solvent in a special pore channel have the expansion change to cause the crushing of the framework. In view of the above problems, the present invention is based on the difference in the actual expansion coefficient of each solvent, and a solvent having a low boiling point and a low vapor pressure is more suitable as a solvent for the freeze-drying process. Tert-butanol and/or cyclohexane are particularly chosen, which have a low boiling point, a high melting point and a low vapor pressure, so that volatilization is facilitated.
And in the subsequent drying treatment, the mass transfer resistance of the water vapor is mainly the flow resistance in the drying layer. And particularly, matched control measures are adopted, so that a relatively ideal drying rate can be realized even in a low-temperature environment, the collapse problem can be prevented, and the whole drying time can be shortened. More particularly, the tert-butanol and/or cyclohexane form special needle crystals, and after sublimation of such ice crystals, relatively many pores remain, which effectively control the actual mass transfer resistance, and which can maintain a desired rate at a lower temperature, thereby preventing the collapse of the framework therein. The comprehensive production cost can be controlled, and the time required by the drying process is reduced.
The process and parameters of the second replacement are not particularly limited, and those skilled in the art can select and adjust the process according to the actual application, the product quality and the product performance, and the second replacement in the invention is preferably performed several times. The number of times of the secondary replacement is preferably 3 to 8 times, more preferably 4 to 7 times, and still more preferably 5 to 6 times. The time of each replacement in the secondary replacement is preferably 6-24 h, more preferably 10-20 h, and more preferably 14-16 h. The immersion according to the invention is preferably carried out by dipping or soaking, more preferably soaking.
In the standing, displacing and replacing again processes, the polyamic acid wet gel is subjected to an aging process simultaneously in the process.
And finally, freeze-drying the semi-finished product obtained in the step to obtain polyamic acid aerogel, and performing heat treatment to obtain polyimide aerogel.
The specific process of freeze-drying is not particularly limited in the present invention, and may be a conventional freeze-drying process of such aerogel, which is well known to those skilled in the art, and can be selected and adjusted by those skilled in the art according to the actual application, product quality and product performance, and the freeze-drying of the present invention preferably includes low-temperature freezing and vacuum drying steps.
The temperature of the low-temperature freezing is not particularly limited, the freezing temperature of the aerogel material prepared by a conventional freeze-drying method well known to a person skilled in the art can be selected and adjusted by the person skilled in the art according to the actual application condition, the product quality and the product performance, and the temperature of the low-temperature freezing is preferably-30 to-5 ℃, more preferably-25 to 0 ℃, more preferably-20 to-5 ℃, and more preferably-15 to-10 ℃. The time for low-temperature freezing is not particularly limited, and the freezing time for preparing the aerogel material by using a conventional freeze-drying method well known to those skilled in the art can be selected and adjusted by those skilled in the art according to the actual application condition, the product quality and the product performance, and the time for low-temperature freezing is preferably 4-6 hours, more preferably 4.2-5.8 hours, and more preferably 4.5-5.5 hours.
The temperature of the vacuum drying is not particularly limited, and the temperature of the aerogel material prepared by conventional freeze drying well known to those skilled in the art can be selected and adjusted by those skilled in the art according to the actual application condition, the product quality and the product performance, and the temperature of the vacuum drying is preferably-5-10 ℃, more preferably-3-8 ℃, and more preferably 0-5 ℃. The vacuum drying time is not particularly limited, and the freeze drying time for preparing the aerogel material by using a conventional freeze drying method well known to those skilled in the art can be selected and adjusted by those skilled in the art according to the actual application condition, the product quality and the product performance, and the vacuum drying time is preferably 36-48 hours, more preferably 38-46 hours, and more preferably 40-44 hours.
In order to ensure the performance of the final aerogel and complete and optimize a reaction route, the heat treatment is preferably vacuum heat treatment, more preferably vacuum step-by-step gradient heat treatment, and the specific process can be as follows:
preferably in 3 gradients;
the temperature of the first gradient is preferably 20-100 ℃, more preferably 40-80 ℃, more preferably 50-70 ℃, and the temperature is kept constant for 2-6 hours, more preferably 3-5 hours after the temperature is increased;
the temperature of the second gradient is preferably 100-200 ℃, more preferably 120-180 ℃, more preferably 140-160 ℃, and the temperature is kept constant for 2-6 hours, more preferably 3-5 hours after the temperature is raised;
the temperature of the third gradient is preferably 200-300 ℃, more preferably 220-280 ℃, and more preferably 240-260 ℃.
The heating rate is preferably 5-10 ℃/min, more preferably 6-9 ℃/min, and more preferably 7-8 ℃/min.
The steps of the invention provide the linear polyimide aerogel and the preparation method thereof, a simple and cheap freeze drying technology is adopted, and the technical bias that the existing polyimide aerogel can not have better performance and structure through the conventional freeze drying technology is overcome. The linear polyimide aerogel is prepared by a sol-gel method, and through specific tert-butyl alcohol and/or cyclohexane and exchange steps thereof in an aging process after gelation and a freeze-drying method. The preparation process provided by the invention is simple and easy to control, has low cost, can be used for thermal imidization and chemical imidization, is convenient to popularize and utilize, is suitable for large-scale industrial production, and greatly widens the limitation on the prior art; in addition, the 3D blocky polyimide aerogel with a linear structure prepared by the invention has the advantages of lower density, good mechanical property and excellent heat-insulating property. Meanwhile, the intermediate of the preparation method provided by the invention can be stored for a long time, is easy to control and is environment-friendly, and particularly, the whole preparation process is short in time, which is only about 5 days.
Experimental results show that the 3D blocky polyimide aerogel with the linear structure is successfully prepared by the freeze drying technology, the method is simple, the time is short, and the density of the polyimide aerogel is only 0.08-0.15 g/cm3The specific surface area can reach 800m2(ii)/g; the thermal conductivity is only 0.024-0.030W/(m.K), the thermal decomposition temperature can reach 610-648 ℃, and the thermal insulation material is expected to be applied to the field of high-temperature thermal insulation.
In order to further illustrate the present invention, the following will describe a polyimide aerogel and a method for preparing the same in detail with reference to the following examples, but it should be understood that the examples are carried out on the premise of the technical solution of the present invention, and the detailed embodiments and specific procedures are given, only for further illustrating the features and advantages of the present invention, not for limiting the claims of the present invention, and the scope of the present invention is not limited to the following examples.
Example 1
First, 4,4 ' -aminodiphenyl ether (ODA, 8mmol) was dissolved in a solvent of 1-methyl-2-pyrrolidone (NMP,33ml) and stirred until it was completely dissolved, and then 3,3 ', 4,4 ' -biphenyltetracarboxylic dianhydride (BPDA, 8.15mmol) was added. Stirring until the reactant becomes transparent to form a solution A; then, the crosslinking agent 1,3, 5-tri (4-aminophenoxy) benzene (TAB) is dissolved in the solvent 1-methyl-2-pyrrolidone (NMP) to form a solution B. Wherein the molar ratio of the raw materials is ODA: BPDA ═ 1: 1.03, TAB: ODA ═ 1: 45. adding the solution B into the solution A, stirring to crosslink the solution B to form a homogeneous solution, continuously stirring for 10min, adding a dehydrating agent acetic anhydride (65mmmol), adding a catalyst pyridine (65mmol), wherein the molar ratio of acetic anhydride and pyridine to 3,3 ', 4, 4' -biphenyltetracarboxylic dianhydride (BPDA) is 8: 1, pouring the formed sol into a sealed container, and standing for 2 hours at room temperature to form gel.
And aging the gel block or the film in a sealed container for 24 hours to enable the gel to continuously carry out polycondensation reaction, continuously growing a network structure, enhancing the strength of the skeleton structure of the gel, replacing the gel in a pure acetone solution for 24 hours, soaking the gel in 10 times of tert-butyl alcohol to carry out solvent exchange, and repeating the exchange for four times. And freezing the obtained gel at the temperature of-25 ℃, and freeze-drying the gel in a vacuum freeze dryer at the temperature of 10 ℃ to obtain the 3D blocky polyimide aerogel with the linear structure.
Referring to fig. 1, fig. 1 is a schematic view of a process flow for preparing a polyimide aerogel according to example 1 of the present invention.
The 3D bulk polyimide aerogel prepared in example 1 of the invention was characterized.
Referring to fig. 2, fig. 2 is a photograph showing the appearance of the 3D bulk polyimide aerogel prepared in example 1 of the present invention.
Referring to fig. 3, fig. 3 is a scanning electron microscope image of the 3D bulk polyimide aerogel prepared in example 1 of the present invention.
As can be seen from FIG. 3, the polyimide aerogel prepared by the present invention has a linear structure, has a specific porous structure, and such pores have specific polymerization and are affected by a solvent during the gelation process, and are occupied by a solution before drying, because related freeze-drying is used, thereby avoiding capillary pressure generated during gas-liquid phase transition, and the porous structure is maintained to the maximum extent. It can be found by scanning electron microscopy that the gel interior exhibits a particular fiber network architecture.
The 3D bulk polyimide aerogel prepared in example 1 of the present invention was subjected to performance testing.
Referring to fig. 4, fig. 4 is a mechanical property curve of the 3D bulk polyimide aerogel prepared in example 1 of the present invention.
As can be seen from fig. 4, according to the compressive stress-strain curve, the compressive stress continuously increases as the compressive strain increases. The whole process is divided into three stages, the first stage is a contact stage, the stress is slowly increased, and the whole numerical value is small. The second phase is a linear phase, with the stress increasing linearly with strain. The third stage is the densification stage, where the stress increases slowly first and then rapidly. With the effect of increasing bulk density, the corresponding compressive strength is increased to some extent, and the strain associated with the yield point is reduced.
Selecting a linear range of materials, wherein the calculation formula is as follows:
σs＝Fs/S
in the formula: sigmasRepresenting the actual compressive strength (Mpa); s represents the actual compression range (mm) of the sample2)；FsRepresenting the corresponding load (N) of strain
Referring to fig. 5, fig. 5 is a thermogravimetric plot of the 3D bulk polyimide aerogel prepared in example 1 of the present invention.
As can be seen from fig. 5, the weight loss of 5% occurred during the first stage from room temperature to 200 ℃, which is considered to be caused by the removal of water in the air absorbed by the polyimide aerogel. With the increasing actual temperature, there is a slow weight loss, mainly water and tert-butanol solvent remaining in the voids. And in the second stage, the weight loss process is from 200 ℃ to 560 ℃, the process is mainly a polyimide aerogel network framework glass transition region, and related polymer chains of the gel are adjusted to a new motion state from initial freezing. And the mass loss process of the third stage is mainly a process from 560 ℃ to 650 ℃, the mass loss rate is about 25 percent, the C-N bond is broken, and the aerogel realizes the relevant decomposition. The mass loss at the end stage is mainly the process at 800 ℃, the mass loss rate is about 10 percent, and the aerogel is completely decomposed. Tonset represents the initial temperature of the polyimide weight loss and Tpeak represents the peak temperature of the weight loss.
The results showed that the 3D bulk polyimide aerogel prepared in example 1 of the present invention had a density of 0.015g/cm3The normal-temperature thermal conductivity is 0.024W/m.K, the thermal decomposition temperature is 642 ℃, and the compressive strength is 8.89 MPa.
Example 2
The aged aerogel obtained in example 1 was taken and subjected to solvent exchange in acetone for 24 times, followed by solvent exchange in cyclohexane for 6 hours each time. Freezing the sample in an environment of-25 ℃ to-5 ℃, and then carrying out freeze drying for 36h in a freeze drying device at 10 ℃ to obtain the polyimide aerogel with a linear structure.
The 3D bulk polyimide aerogel prepared in example 2 of the present invention was subjected to performance testing.
The results showed that the polyimide aerogel prepared in example 2 of the present invention had a density of 0.017g/cm3The thermal conductivity at normal temperature is 0.027W/m.K, the thermal decomposition temperature is 600 ℃, and the tensile strength is 9.77 MPa.
Example 3
This example embodiment differs from example 1 in that the diamine dianhydride monomer concentration was varied, the polar aprotic solvent was not changed, 9mmol of 4,4 ' -aminodiphenyl ether (ODA) was dissolved in 33ml of 1-methyl-2-pyrrolidone (NMP) and stirred until it was completely dissolved, and then 9.15mmol of 3,3 ', 4,4 ' -biphenyltetracarboxylic dianhydride (BPDA) was added.
Finally, the polyimide aerogel with a linear structure is obtained.
The 3D bulk polyimide aerogel prepared in example 3 of the present invention was subjected to performance testing.
The results showed that the polyimide aerogel prepared in example 3 of the present invention had a density of 0.021g/cm3The thermal conductivity at normal temperature is 0.030W/m.K, the thermal decomposition temperature is 605 ℃, and the tensile strength is 12.15 MPa.
While the present invention has been described in detail with reference to a linear polyimide aerogel and a method for preparing the same, the present invention is illustrated in the drawings by the following examples, which are provided only to help understand the method and the core concept of the present invention, including the best mode, and also to enable any person skilled in the art to practice the present invention, including making and using any devices or systems and performing any combination of the methods. It should be noted that, for those skilled in the art, it is possible to make various improvements and modifications to the present invention without departing from the principle of the present invention, and those improvements and modifications also fall within the scope of the claims of the present invention. The scope of the invention is defined by the claims and may include other embodiments that occur to those skilled in the art. Such other embodiments are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Claims (8)

1. The preparation method of the polyimide aerogel is characterized by comprising the following steps:
1) standing the polyimide wet gel, and then placing the polyimide wet gel in an organic solvent for replacement to obtain a replaced polyimide wet gel;
the organic solvent comprises acetone;
2) soaking the displaced polyimide wet gel obtained in the step into tert-butyl alcohol and/or cyclohexane for displacement again to obtain a semi-finished product;
3) freezing and drying the semi-finished product obtained in the step to obtain polyimide aerogel;
the porosity of the polyimide aerogel is 85% -95%;
the average pore diameter of the polyimide aerogel is 10-50 nm;
the specific surface area of the polyimide aerogel is 400~800m2/g；
The freeze drying comprises the steps of low-temperature freezing and vacuum drying;
the low-temperature freezing time is 4-6 h; the low-temperature freezing temperature is-30 to-5 ℃;
the temperature of the vacuum drying is-5-10 ℃; the vacuum drying time is 36-48 h.
2. The preparation method according to claim 1, wherein the polyimide aerogel has a thermal conductivity of 0.024 to 0.030W/(m-K);
the thermal decomposition temperature of the polyimide aerogel is 610-648 ℃;
the polyimide aerogel is a polyimide aerogel block.
3. The preparation method according to claim 1, wherein the standing time is 24-48 h;
the replacement is a plurality of replacements;
the times of the multiple replacement are 2-5 times;
and the time of each replacement in the multiple replacements is 6-24 hours.
4. The method of claim 1, wherein the re-substitution is a plurality of substitutions;
the times of the plurality of times of replacement are 3-8 times;
the time of each replacement in the plurality of replacements is 6-24 hours.
5. The preparation method according to claim 1, wherein the polyimide wet gel is prepared by the following steps:
a) mixing a dianhydride monomer, a diamine monomer and a polar aprotic solvent, and then adding a chemical cross-linking agent for cross-linking to obtain a reaction solution;
b) and mixing the reaction solution, the dehydrating agent and the catalyst obtained in the step again to obtain sol, and pre-standing to obtain the polyimide wet gel.
6. The method according to claim 5, wherein the dianhydride monomer comprises one or more of 3,3 ', 4,4 ' -biphenyltetracarboxylic dianhydride, 4,4 ' -diphenyl ether dianhydride, 3 ', 4,4 ' -benzophenonetetracarboxylic dianhydride, and pyromellitic dianhydride;
the diamine monomer comprises one or more of 4,4 '-diaminodiphenyl ether, 3, 4' -diaminodiphenyl ether, 2 '-dimethylbenzidine, 2-bis [4- (4-aminophenoxy) phenyl ] propane and 4, 4' -diaminodiphenylmethane;
the polar aprotic solvent comprises one or more of N-methylpyrrolidone, a mixed solution of tetrahydrofuran and methanol, N-dimethylformamide and N, N-dimethylacetamide;
the chemical cross-linking agent comprises 1,3, 5-tri (4-aminophenoxy) benzene and/or octa (aminophenyl) cage polysilsesquioxane;
the molar ratio of the dianhydride monomer to the diamine monomer is (1-1.05): 1;
the molar ratio of the polar aprotic solvent to the diamine monomer is (30-50): 1;
the molar ratio of the chemical crosslinking agent to the diamine monomer is 1: (40-50);
the crosslinking time is 30-60 min.
7. The production method according to claim 5, wherein the dehydrating agent comprises acetic anhydride;
the catalyst comprises pyridine;
the molar ratio of the dehydrating agent to the dianhydride monomer is 1: (100-150);
the molar ratio of the catalyst to the dianhydride monomer is 1: (100-150);
the pre-standing time is 2-4 h.
8. The preparation method of the polyimide aerogel is characterized by comprising the following steps:
1') standing the polyamic acid wet gel, and then placing the polyamic acid wet gel in an organic solvent for replacement to obtain a replaced polyamic acid wet gel;
the organic solvent comprises acetone;
2') soaking the displaced polyamic acid wet gel obtained in the step into tert-butyl alcohol and/or cyclohexane for displacement again to obtain a semi-finished product;
3') freeze-drying the semi-finished product obtained in the step to obtain polyamic acid aerogel, and performing heat treatment to obtain polyimide aerogel;
the polyamic acid wet gel is prepared by the following steps:
A) mixing a dianhydride monomer, a diamine monomer and a polar aprotic solvent to obtain a reaction solution, and standing to obtain polyamide acid wet gel;
the porosity of the polyimide aerogel is 85% -95%;
the average pore diameter of the polyimide aerogel is 10-50 nm;
the specific surface area of the polyimide aerogel is 400-800 m2/g；
The freeze drying comprises the steps of low-temperature freezing and vacuum drying;
the low-temperature freezing time is 4-6 h; the low-temperature freezing temperature is-30 to-5 ℃;
the temperature of the vacuum drying is-5-10 ℃; the vacuum drying time is 36-48 h.
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