Aqueous process for preparing polyamic acid gels, polymate gels, polyimide gels, and porous carbon materials.

Abstract

The present disclosure is directed to a method for forming polyamic acid and polyimide gels in water. The resulting polyamic acid and polyimide gels can be converted into aerogels, which can be further converted into carbon aerogels. Such carbon aerogels have the same physical properties as carbon aerogels prepared from polyimide aerogels obtained by conventional methods, i.e., organic solvent-based methods. The disclosed methods are advantageous in reducing or avoiding costs associated with the use and disposal of potentially toxic solvents and by-products. Gel materials prepared by the disclosed methods are suitable for use in environments involving electrochemical reactions, for example, as electrode materials in lithium-ion batteries.

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the benefit and priority of U.S. Provisional Patent Application No. 63 / 124,451, filed on December 11, 2020; U.S. Provisional Patent Application No. 63 / 124,458, filed on December 11, 2020; and U.S. Patent Application No. 17 / 546,761, filed on December 9, 2021, each of which is hereby incorporated by reference in its entirety.

[0002] The present disclosure generally relates to porous polyamic acid and polyimide gel materials, and aqueous processes for making them.

Background Art

[0003] Aerogels are solid materials containing a highly porous network of micro-, meso-, and macro-sized pores. Depending on the precursor material used and the processing performed, when the density of an aerogel is approximately 0.05 g / cc, the pores of the aerogel often occupy more than 90% of its volume. Aerogels are generally prepared by removing the solvent from the gel (a solid network containing the solvent) so that minimal gel shrinkage can be brought about by capillary forces at its pore walls, or not at all. Methods of solvent removal include, but are not limited to, supercritical drying (or drying using a supercritical fluid such that the low surface tension of the supercritical fluid replaces the high surface tension gelling solvent in the gel), exchange of solvent with a supercritical fluid, exchange of solvent with a fluid that will later be converted to a supercritical state, subcritical or near-critical drying, and sublimation of the frozen solvent in freeze-drying processes. See, for example, PCT Patent Application Publication WO2016127084A1. It should be noted that when dried under ambient conditions, gel shrinkage may occur due to solvent evaporation, potentially leading to the formation of a xerogel. Therefore, aerogel preparation via sol-gel processes or other polymerization processes typically proceeds in the following steps: dissolution of solute in solvent, addition of a catalyst or reagent to induce or accelerate the solute reaction, formation of a reaction mixture, formation of a gel (which may involve additional heating or cooling), and removal of the solvent from the gel by supercritical drying techniques or any other method without causing shrinkage or pore collapse.

[0004] Aerogels can be formed from inorganic materials, organic materials, or mixtures thereof. For example, when formed from organic materials such as phenol, resorcinol-formaldehyde (RF), phloroglucinol-furfuraldehyde (PF), polyacrylonitrile (PAN), polyimide (PI), polyurethane (PU), polyurea (PUA), polyamine (PA), polybutadiene, polydicyclopentadiene, and their precursors or polymer derivatives, the organic aerogel can be carbonized (e.g., by thermal decomposition) to form a carbon aerogel, which may have different or overlapping properties (e.g., pore volume, pore size distribution, morphology, etc.) depending on the precursor material and method used.

[0005] In recent years, efforts have been dedicated to the development and characterization of carbon aerogels as electrode materials with improved performance for applications in energy storage devices such as lithium-ion batteries (LIBs). Therefore, there is a demand for corresponding organic aerogels. Such organic aerogels are generally prepared in organic solvents. For example, polyimide aerogels are generally prepared by reacting diamines and tetracarboxylic dianhydrides in an organic solvent, followed by dehydration of the resulting polymeramic acid ("polyamic acid") to form a polyimide gel. For economic, safety, and environmental reasons, it would be desirable to perform such gelation using "green" chemical processes (i.e., using alternatives to traditional organic solvents). [Overview of the project]

[0006] This technology generally relates to methods for forming polyimide gels while minimizing or eliminating the use of harmful organic solvents. These methods generally involve providing or forming a polyamic acid and then imidizing the polyamic acid, with both formation and imidation carried out in water. In some embodiments, imidation is carried out chemically, for example, in the presence of a dehydrating agent. Surprisingly, according to this disclosure, it has been found that polyimide gels can be prepared in water. Particularly surprising was the discovery that not only did the reaction sequence proceed in water and without the destruction of the intuitively expected dehydrating agent (e.g., acid anhydrides such as acetic anhydride), but gelation was also very rapid. In other embodiments, imidation is carried out thermally, for example, by utilizing microwave heating for rapid thermal dehydration of the polyamic acid. Surprisingly, according to this disclosure, thermal dehydration has been found to occur rapidly under aqueous conditions. This method is advantageous in providing rapid gelation, thereby making it suitable for configuration in continuous processes such as the preparation of polyimide beads and environmentally friendly in the overall use of water-based solutions. This method is less costly to implement than conventional polyimide gelation methods because the byproducts from the reaction sequence are less toxic, have lower disposal costs, and avoid the use of expensive and potentially toxic organic solvents. Using the disclosed method, polyimide monoliths, micron-sized beads, or millimeter-sized beads can be formed. The polyimide gel can be converted to an aerogel, as can carbon aerogels. When the polyimide aerogel of this disclosure is converted to a carbon aerogel, the carbon aerogel has a nanostructure with properties similar to a carbide polyimide aerogel prepared by a conventional organic solvent-based process. This technique further generally covers methods for forming polyamic acid gels and aerogels. In some embodiments, polyamic acid gels and aerogels are directly converted to carbon gels or aerogels without intermediate conversion to polyimide gels or aerogels.Such methods are advantageous in avoiding additional conversions and reducing the overall complexity, time, and cost associated with the production of carbon aerogels, and can further reduce costs for, for example, the disposal of additional waste streams.

[0007] Accordingly, in one embodiment, a method for forming a polyimide aerogel is provided, the method comprising: providing an aqueous solution of a polyamic acid salt, wherein the polyamic acid salt comprises a polyamic acid containing a carboxylic acid group, the carboxylic acid group being bonded to a cationic species and substantially present as a carboxylate anion; imidizing the polyamic acid salt to form a polyimide gel; and drying the polyimide gel to form a polyimide aerogel.

[0008] In some embodiments, providing an aqueous solution of a polyamic acid salt includes providing a polyamic acid, adding the polyamic acid to water to form an aqueous suspension of the polyamic acid, and adding a base to the aqueous suspension of the polyamic acid to form an aqueous solution of the polyamic acid salt.

[0009] In some embodiments, the base is an alkali metal hydroxide, and the cationic species is an alkali metal cation. In some embodiments, the alkali metal hydroxide is lithium hydroxide, sodium hydroxide, or potassium hydroxide.

[0010] In some embodiments, the base is a non-nucleophilic amine and the cationic species is an ammonium cation. In some embodiments, the non-nucleophilic amine has a solubility of at least about 4 grams per liter of water at 20°C. In some embodiments, the non-nucleophilic amine is a tertiary amine. In some embodiments, the non-nucleophilic amine is selected from the group consisting of triethylamine, trimethylamine, tri-n-butylamine, N-methylpyrrolidine, N-methylpiperidine, diisopropylethylamine, and combinations thereof. In some embodiments, the non-nucleophilic amine is triethylamine or diisopropylethylamine.

[0011] In some embodiments, a non-nucleophilic amine is added in an amount sufficient to maintain substantially all of the polyamic acid in solution. In some embodiments, the molar ratio of the non-nucleophilic amine to the polyamic acid is about 2 to about 4, or about 2.2 to about 2.5.

[0012] In some embodiments, the polyamic acid includes tetracarboxylic acids selected from the group consisting of benzene-1,2,4,5-tetracarboxylic acid, [1,1'-biphenyl]-3,3',4,4'-tetracarboxylic acid, 4,4'-oxydiphthalic acid, 4,4'-sulfonyldiphthalic acid, 4,4'-carbonyldiphthalic acid, 4,4'-(propane-2,2-diyl)diphthalic acid, 4,4'-(perfluoropropane-2,2-diyl)diphthalic acid, naphthalene-1,4,5,8-tetracarboxylic acid, 4-(2-(4-(3,4-dicarboxyphenoxy)phenyl)propane-2-yl)phthalic acid, perylenetetracarboxylic acid, and combinations thereof.

[0013] In some embodiments, the polyamic acid comprises a C2-C6 alkylenediamine, in which one or more carbon atoms of the C2-C6 alkylene are optionally substituted with one or more alkyl groups. In some embodiments, the C2-C6 alkylenediamine is selected from the group consisting of ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, or combinations thereof. In some embodiments, the polyamic acid comprises a diamine selected from the group consisting of 1,4-phenylenediamine, 4,4'-methylenedianiline, 4,4'-diaminodiphenyl ether, or combinations thereof. In some embodiments, the polyamic acid comprises a diamine selected from the group consisting of 1,4-phenylenediamine, 4,4'-methylenedianiline, 4,4'-diaminodiphenyl ether, and combinations thereof.

[0014] In some embodiments, the concentration range of the polyamic acid salt in the solution is approximately 0.01 to approximately 0.3 g / cm³, based on the weight of the polyamic acid. 3 That is the case.

[0015] In some embodiments, the polyimide gel is in a monolithic form, and imidizing the polyamic acid involves adding a dehydrating agent to an aqueous solution of the polyamic acid to form a gelling mixture, the method further comprising pouring the gelling mixture into a mold to form a gel.

[0016] In some embodiments, the polyimide gel is in a monolithic form, and the imidation of the polyamic acid salt is carried out thermally, the method further comprising adding delta-gluconolactone to an aqueous solution of the polyamic acid salt to form a gelling mixture, pouring the gelling mixture into a mold and forming a gel from the gelling mixture, washing the resulting polyamic acid gel with water, and thermal imidizing the polyamic acid gel to form a polyimide gel, the thermal imidization comprising exposing the polyamic acid gel to irradiation at a microwave frequency.

[0017] In some embodiments, the polyimide gel is in the form of beads, and imidizing a polyamic acid salt involves adding a dehydrating agent to an aqueous solution of the polyamic acid salt to form a gelling mixture, the method further comprising adding the gelling mixture to a solution of a water-soluble acid in water to form polyimide gel beads, the addition of which includes dropping the gelling mixture into a solution of a water-soluble acid in water, spraying the gelling mixture under pressure into a solution of a water-soluble acid in water by passing it through one or more nozzles using pressure, or electrolyzing the gelling mixture into a solution of a water-soluble acid in water.

[0018] In some embodiments, the dehydrating agent is anhydride acetic acid.

[0019] In some embodiments, the water-soluble acid is a mineral acid or acetic acid.

[0020] In some embodiments, the polyimide gel is in bead form, and imidizing the polyamic acid salt comprises adding a dehydrating agent to an aqueous solution of the polyamic acid salt to form a gelling mixture, the method further comprising optionally adding the gelling mixture to a water-immiscible solvent containing an acid, the addition comprising dropping the gelling mixture into the water-immiscible solvent, spraying the gelling mixture under pressure into the water-immiscible solvent by passing it through one or more nozzles using pressure, or electrolyzing the gelling mixture into the water-immiscible solvent.

[0021] In some embodiments, the dehydrating agent is anhydride acetic acid.

[0022] In some embodiments, the optional acid is acetic acid.

[0023] In some embodiments, the method involves electrolyzing a gelling mixture by passing it through one or more needles at a voltage ranging from about 5 to about 60 kV.

[0024] In some embodiments, the polyimide gel is in bead form, and imidizing the polyamic acid involves adding a dehydrating agent to an aqueous solution of the polyamic acid to form a gelled mixture, the method further comprising combining the aqueous solution of the polyamic acid with a water-immiscible solvent containing a surfactant, and mixing the resulting mixture under high shear conditions.

[0025] In some embodiments, the polyimide gel is in bead form, and imidizing the polyamic acid salt involves chemical imidization, the method comprising combining an aqueous solution of the polyamic acid salt with a water-immiscible solvent containing a surfactant, mixing the resulting mixture under high shear conditions to form a metastable emulsion, and adding a dehydrating agent to the metastable emulsion.

[0026] In some embodiments, the water-immiscible organic solvent is a C5-C12 hydrocarbon. In some embodiments, the C5-C12 hydrocarbon is a mineral spirit.

[0027] In some embodiments, providing an aqueous solution of a polyamic acid salt involves dissolving a water-soluble diamine in water to form an aqueous diamine solution, adding a non-nucleophilic amine to the aqueous diamine solution, adding a tetracarboxylic dianhydride to the aqueous diamine solution, and stirring the resulting solution at a temperature in the range of about 15 to about 60°C for a period of about 1 to about 24 hours.

[0028] In some embodiments, providing an aqueous solution of a polyamic acid salt involves dissolving a water-soluble diamine in water to form an aqueous diamine solution, adding a tetracarboxylic dianhydride to the aqueous diamine solution, stirring the resulting suspension at a temperature in the range of about 15 to about 60°C for a period of about 1 to about 24 hours, adding a non-nucleophilic amine to the aqueous diamine solution, and stirring the resulting suspension at a temperature in the range of about 15 to about 60°C for a period of about 1 to about 24 hours.

[0029] In some embodiments, providing an aqueous solution of a polyamic acid salt involves simultaneously or intermittently adding a water-soluble diamine, a tetracarboxylic dianhydride, and a non-nucleophilic amine to water, and stirring the resulting mixture at a temperature in the range of about 15 to about 60°C for a period of about 1 to about 24 hours.

[0030] In some embodiments, a water-soluble diamine, a tetracarboxylic dianhydride, and a non-nucleophilic amine are added to water simultaneously. In some embodiments, a water-soluble diamine, a tetracarboxylic dianhydride, and a non-nucleophilic amine are added to water continuously.

[0031] In some embodiments, the resulting mixture is stirred at a temperature in the range of about 15 to about 25°C. In some embodiments, the resulting mixture is stirred at a temperature in the range of about 50 to about 60°C.

[0032] In some embodiments, the non-nucleophilic amine has a solubility of at least about 4 grams per liter of water at 20°C.

[0033] In some embodiments, the non-nucleophilic amine is a tertiary amine. In some embodiments, the non-nucleophilic amine is selected from the group consisting of triethylamine, trimethylamine, tri-n-butylamine, N-methylpyrrolidine, N-methylpiperidine, diisopropylethylamine, and combinations thereof. In some embodiments, the non-nucleophilic amine is triethylamine or diisopropylethylamine.

[0034] In some embodiments, the molar ratio of the non-nucleophilic amine to the diamine is approximately 2 to approximately 2.5.

[0035] In some embodiments, the tetracarboxylic dianhydride is selected from the group consisting of pyromellitic anhydride (PMDA), biphthalic dianhydride (BPDA), oxydiphthalic dianhydride (ODPA), perylenetetracarboxylic dianhydride, and combinations thereof.

[0036] In some embodiments, the diamine is a C2-C6 alkylenediamine, where one or more carbon atoms of the C2-C6 alkylene are optionally substituted with one or more alkyl groups. In some embodiments, the C2-C6 alkylenediamine is selected from the group consisting of ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, and combinations thereof. In some embodiments, the diamine is 1,3-phenylenediamine, 1,4-phenylenediamine, or a combination thereof. In some embodiments, the diamine is 1,4-phenylenediamine.

[0037] In some embodiments, the molar ratio of the tetracarboxylic dianhydride to the diamine is approximately 0.9 to approximately 1.1.

[0038] In another embodiment, a method for forming a polyamic acid aerogel is provided, the method comprising: providing an aqueous solution of a polyamic acid salt; acidifying the polyamic acid salt solution to form a polyamic acid gel; and drying the polyamic acid gel to form a polyamic acid aerogel.

[0039] In some embodiments, the polyamic acid gel is in a monolithic form, and acidifying the polyamic acid salt includes adding delta-gluconolactone to an aqueous solution of the polyamic acid salt to form a gelling mixture, and pouring the gelling mixture into a mold to form a gel.

[0040] In some embodiments, the polyamic acid gel is in the form of beads, and acidifying the polyamic acid salt involves adding an aqueous solution of the polyamic acid salt to a solution of a water-soluble acid in water to form polyamic acid gel beads, the addition of which includes dropping the aqueous solution of the polyamic acid salt into a solution of a water-soluble acid in water, spraying the aqueous solution of the polyamic acid salt under pressure into a solution of a water-soluble acid in water using pressure through one or more nozzles, or electrolyzing the aqueous solution of the polyamic acid salt into a solution of a water-soluble acid in water.

[0041] In some embodiments, the water-soluble acid is a mineral acid or acetic acid.

[0042] In some embodiments, the method involves electrolyzing an aqueous solution of polyamic acid by passing it through one or more needles at a voltage ranging from about 5 to about 60 kV.

[0043] In some embodiments, the polyamic acid gel is in the form of microbeads, and the method further comprises combining an aqueous solution of a polyamic acid salt with a water-immiscible solvent containing a surfactant, mixing the resulting mixture under high shear conditions to form an emulsion, and adding an organic acid to the emulsion.

[0044] In some embodiments, the water-immiscible organic solvent is a C5-C12 hydrocarbon. In some embodiments, the water-immiscible organic solvent is a mineral spirit.

[0045] In some embodiments, the organic acid is acetic acid.

[0046] In some embodiments, providing an aqueous solution of a polyamic acid salt includes providing a substantially pure form of polyamic acid, adding polyamic acid to water to form an aqueous suspension of polyamic acid, and adding a base to the aqueous suspension of polyamic acid to form an aqueous solution of a polyamic acid salt.

[0047] In some embodiments, the base is a non-nucleophilic amine.

[0048] In some embodiments, the non-nucleophilic amine has a solubility of at least about 4 grams per liter of water at 20°C.

[0049] In some embodiments, the non-nucleophilic amine is a tertiary amine. In some embodiments, the non-nucleophilic amine is selected from the group consisting of triethylamine, trimethylamine, tri-n-butylamine, N-methylpyrrolidine, N-methylpiperidine, diisopropylethylamine, and combinations thereof. In some embodiments, the non-nucleophilic amine is triethylamine or diisopropylethylamine.

[0050] In some embodiments, a non-nucleophilic amine is added in an amount sufficient to maintain substantially all of the polyamic acid in solution.

[0051] In some embodiments, the molar ratio of the non-nucleophilic amine to the polyamic acid is about 2 to about 4, or about 2.2 to about 2.5.

[0052] In some embodiments, the polyamic acid includes tetracarboxylic acids selected from the group consisting of benzene-1,2,4,5-tetracarboxylic acid, [1,1'-biphenyl]-3,3',4,4'-tetracarboxylic acid, 4,4'-oxydiphthalic acid, 4,4'-sulfonyldiphthalic acid, 4,4'-carbonyldiphthalic acid, 4,4'-(propane-2,2-diyl)diphthalic acid, 4,4'-(perfluoropropane-2,2-diyl)diphthalic acid, naphthalene-1,4,5,8-tetracarboxylic acid, 4-(2-(4-(3,4-dicarboxyphenoxy)phenyl)propane-2-yl)phthalic acid, perylenetetracarboxylic acid, and combinations thereof.

[0053] In some embodiments, the polyamic acid comprises a C2-C6 alkylenediamine, wherein one or more carbon atoms of the C2-C6 alkylene are optionally substituted with one or more alkyl groups. In some embodiments, the C2-C6 alkylenediamine is selected from the group consisting of ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, and combinations thereof.

[0054] In some embodiments, the polyamic acid comprises 1,3-phenylenediamine, 1,4-phenylenediamine, 4,4'-methylenedianiline, 4,4'-diaminodiphenyl ether, or a combination thereof. In some embodiments, the polyamic acid comprises a diamine selected from the group consisting of 1,4-phenylenediamine, 4,4'-methylenedianiline, 4,4'-diaminodiphenyl ether, and combinations thereof.

[0055] In some embodiments, the concentration range of the polyamic acid salt in the solution is approximately 0.01 to approximately 0.3 g / cm³, based on the weight of the polyamic acid. 3 That is the case.

[0056] In some embodiments, providing an aqueous solution of a polyamic acid salt involves dissolving a water-soluble diamine in water to form an aqueous diamine solution, adding a non-nucleophilic amine to the aqueous diamine solution, adding a tetracarboxylic dianhydride to the aqueous diamine solution, and stirring the resulting mixture at a temperature in the range of about 15 to about 60°C for a period of about 1 to about 24 hours.

[0057] In some embodiments, the resulting mixture is stirred at a temperature in the range of about 15 to about 25°C. In some embodiments, the resulting mixture is stirred at a temperature in the range of about 50 to about 60°C.

[0058] In some embodiments, providing an aqueous solution of a polyamic acid salt involves dissolving a water-soluble diamine in water to form an aqueous diamine solution, adding a tetracarboxylic dianhydride to the aqueous diamine solution, stirring the resulting mixture at a temperature in the range of about 15 to about 60°C for a period of about 1 to about 24 hours, adding a non-nucleophilic amine to the mixture, and stirring the resulting mixture at a temperature in the range of about 15 to about 60°C for a period of about 1 to about 24 hours.

[0059] In some embodiments, the resulting mixture is stirred at a temperature in the range of about 15 to about 25°C. In some embodiments, the resulting mixture is stirred at a temperature in the range of about 50 to about 60°C.

[0060] In some embodiments, providing an aqueous solution of a polyamic acid salt involves simultaneously or intermittently adding a water-soluble diamine, a tetracarboxylic dianhydride, and a non-nucleophilic amine to water, and stirring the resulting mixture at a temperature in the range of about 15 to about 60°C for a period of about 1 to about 24 hours.

[0061] In some embodiments, the resulting mixture is stirred at a temperature in the range of about 15 to about 25°C. In some embodiments, the resulting mixture is stirred at a temperature in the range of about 50 to about 60°C.

[0062] In some embodiments, a water-soluble diamine, a tetracarboxylic dianhydride, and a non-nucleophilic amine are added to water simultaneously. In some embodiments, a water-soluble diamine, a tetracarboxylic dianhydride, and a non-nucleophilic amine are added to water continuously.

[0063] In some embodiments, the non-nucleophilic amine has a solubility of at least about 4 grams per liter of water at 20°C.

[0064] In some embodiments, the non-nucleophilic amine is a tertiary amine. In some embodiments, the non-nucleophilic amine is selected from the group consisting of triethylamine, trimethylamine, tri-n-butylamine, N-methylpyrrolidine, N-methylpiperidine, and diisopropylethylamine. In some embodiments, the non-nucleophilic amine is triethylamine or diisopropylethylamine.

[0065] In some embodiments, the molar ratio of the non-nucleophilic amine to the diamine is approximately 2 to approximately 2.5.

[0066] In some embodiments, the tetracarboxylic dianhydride is selected from the group consisting of pyromellitic anhydride (PMDA), biphthalic dianhydride (BPDA), oxydiphthalic dianhydride (ODPA), perylenetetracarboxylic dianhydride, and combinations thereof.

[0067] In some embodiments, the diamine is a C2-C6 alkylenediamine, in which one or more carbon atoms of the C2-C6 alkylene are optionally substituted with one or more alkyl groups. In some embodiments, the C2-C6 alkylenediamine is selected from the group consisting of ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, and combinations thereof.

[0068] In some embodiments, the diamine is 1,4-phenylenediamine.

[0069] In some embodiments, the molar ratio of the tetracarboxylic dianhydride to the diamine is approximately 0.9 to approximately 1.1.

[0070] In another embodiment, a method for forming a monolithic polyimide aerogel, wherein the method is To provide an aqueous solution of a polyamic acid salt, wherein the polyamic acid salt comprises a polyamic acid containing a carboxylic acid group, the carboxylic acid group being bonded to a cationic species and substantially existing as a carboxylate anion, and the provision of the aqueous solution of the polyamic acid salt comprises providing a polyamic acid, adding the polyamic acid to water to form an aqueous suspension of the polyamic acid, and adding a base to the aqueous suspension of the polyamic acid to form an aqueous solution of the polyamic acid salt, wherein the base is a non-nucleophilic amine and the cationic species is an ammonium cation. Adding delta-gluconolactone to an aqueous solution of polyamic acid salt to form a gel mixture, Pouring the gelling mixture into a mold and turning the gelling mixture into a gel, The obtained polyamic acid gel is washed with water, The process involves forming a polyimide gel by thermal imidation of a polyamic acid gel, wherein the thermal imidation includes exposing the polyamic acid gel to microwave frequency irradiation. A method is provided which includes drying a polyimide gel to form a monolithic polyimide aerogel.

[0071] In another embodiment, a method for forming a polyimide aerogel in bead form, wherein the method is To provide an aqueous solution of a polyamic acid salt, wherein the polyamic acid salt comprises a polyamic acid containing a carboxylic acid group, the carboxylic acid group being bonded to a cationic species and substantially existing as a carboxylate anion, and the provision of the aqueous solution of the polyamic acid salt comprises providing a polyamic acid, adding the polyamic acid to water to form an aqueous suspension of the polyamic acid, and adding a base to the aqueous suspension of the polyamic acid to form an aqueous solution of the polyamic acid salt, wherein the base is a non-nucleophilic amine and the cationic species is an ammonium cation. The process involves adding a dehydrating agent to an aqueous solution of polyamic acid salt to form a gel mixture, Forming polyimide gel beads by adding a gelling mixture to a solution of water-soluble acid in water, wherein the addition includes dropping the gelling mixture into the solution of water-soluble acid in water, spraying the gelling mixture under pressure into the solution of water-soluble acid in water through one or more nozzles using pressure, or electrolyzing the gelling mixture into the solution of water-soluble acid in water. A method is provided which includes drying polyimide gel beads to form a polyimide aerogel in bead form.

[0072] In some embodiments, the dehydrating agent is anhydride acetic acid.

[0073] In some embodiments, the water-soluble acid is a mineral acid or acetic acid.

[0074] In another embodiment, a method for forming a polyimide aerogel in bead form, wherein the method is To provide an aqueous solution of a polyamic acid salt, wherein the polyamic acid salt comprises a polyamic acid containing a carboxylic acid group, the carboxylic acid group being bonded to a cationic species and substantially existing as a carboxylate anion, and the provision of the aqueous solution of the polyamic acid salt comprises providing a polyamic acid, adding the polyamic acid to water to form an aqueous suspension of the polyamic acid, and adding a base to the aqueous suspension of the polyamic acid to form an aqueous solution of the polyamic acid salt, wherein the base is a non-nucleophilic amine and the cationic species is an ammonium cation. The process involves adding a dehydrating agent to an aqueous solution of polyamic acid salt to form a gel mixture, Forming polyimide gel beads by adding a gelling mixture to a solution of a water-soluble acid in water, wherein the addition includes optionally adding the gelling mixture to a water-immiscible solvent containing the acid, and further includes dropping the gelling mixture into a water-immiscible solvent, spraying the gelling mixture under pressure through one or more nozzles into a water-immiscible solvent using pressure, or electrolyzing the gelling mixture into a water-immiscible solvent. A method is provided which includes drying polyimide gel beads to form a polyimide aerogel in bead form.

[0075] In some embodiments, the dehydrating agent is anhydride acetic acid.

[0076] In some embodiments, the optional acid is acetic acid.

[0077] In some embodiments, the method involves electrolyzing a gelling mixture by passing it through one or more needles at a voltage ranging from about 5 to about 60 kV.

[0078] In another embodiment, a method for forming a polyimide aerogel in bead form, wherein the method is To provide an aqueous solution of a polyamic acid salt, wherein the polyamic acid salt comprises a polyamic acid containing a carboxylic acid group, the carboxylic acid group being bonded to a cationic species and substantially existing as a carboxylate anion, and the provision of the aqueous solution of the polyamic acid salt comprises providing a polyamic acid, adding the polyamic acid to water to form an aqueous suspension of the polyamic acid, and adding a base to the aqueous suspension of the polyamic acid to form an aqueous solution of the polyamic acid salt, wherein the base is a non-nucleophilic amine and the cationic species is an ammonium cation. The process involves adding a dehydrating agent to an aqueous solution of polyamic acid salt to form a gel mixture, Combining a gelling mixture with a water-immiscible solvent containing a surfactant, The resulting mixture is mixed under high shear conditions, A method is provided which includes drying polyimide gel beads to form a polyimide aerogel in bead form.

[0079] In some embodiments, the dehydrating agent is anhydride acetic acid.

[0080] In some embodiments, the water-immiscible organic solvent is a C5-C12 hydrocarbon. In some embodiments, the C5-C12 hydrocarbon is a mineral spirit.

[0081] In another embodiment, a method for forming a polyimide aerogel in bead form, wherein the method is To provide an aqueous solution of a polyamic acid salt, wherein the polyamic acid salt comprises a polyamic acid containing a carboxylic acid group, the carboxylic acid group being bonded to a cationic species and substantially existing as a carboxylate anion, and the provision of the aqueous solution of the polyamic acid salt comprises providing a polyamic acid, adding the polyamic acid to water to form an aqueous suspension of the polyamic acid, and adding a base to the aqueous suspension of the polyamic acid to form an aqueous solution of the polyamic acid salt, wherein the base is a non-nucleophilic amine and the cationic species is an ammonium cation. Combining an aqueous solution of polyamic acid salt with a water-immiscible solvent containing a surfactant, The resulting mixture is mixed under high shear conditions to form a metastable emulsion, Adding a dehydrating agent to a metastable emulsion, A method is provided which includes drying polyimide gel beads to form a polyimide aerogel in bead form.

[0082] In some embodiments, the dehydrating agent is anhydride acetic acid.

[0083] In some embodiments, the water-immiscible organic solvent is a C5-C12 hydrocarbon. In some embodiments, the C5-C12 hydrocarbon is a mineral spirit.

[0084] In another embodiment, a method is provided for forming polyamic acid metal salt aerogels in bead form, the method comprising: providing an aqueous solution of an ammonium or alkali metal salt of polyamic acid; performing metal ion exchange, which includes adding a solution of polyamic acid to a solution containing a soluble metal salt to form polyamate metal salt gel beads; and drying the polyamic acid metal salt gel beads to form polyamic acid metal salt aerogel beads.

[0085] In some embodiments, the soluble metal salt includes major transition metals, rare earth metals, alkaline earth metals, or combinations thereof. In some embodiments, the soluble metal salt includes copper, iron, nickel, silver, calcium, magnesium, or combinations thereof. In some embodiments, the soluble metal salt includes lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or combinations thereof.

[0086] In some embodiments, adding a polyamic acid solution to a solution containing a soluble metal salt includes dropping an aqueous solution of polyamic acid into the solution of the soluble metal salt, spraying an aqueous solution of polyamic acid into the solution of the soluble metal salt under pressure through one or more nozzles, or electrolyzing an aqueous solution of polyamic acid into the solution of the soluble metal salt.

[0087] In some embodiments, the method involves electrolyzing a polyamate solution by passing it through one or more needles at a voltage ranging from about 5 to about 60 kV.

[0088] In some embodiments, drying the polyimide gel optionally includes washing or desolvent-changing the polyimide gel, optionally subjecting the washed or desolvent-changing polyimide gel to high-temperature conditions, optionally freeze-drying the washed or desolvent-changing polyimide gel, or optionally contacting the washed or desolvent-changing polyimide gel with supercritical fluid carbon dioxide.

[0089] In some embodiments, washing or solvent exchange is carried out using water, C1-C3 alcohols, acetone, acetonitrile, ether, tetrahydrofuran, toluene, liquid carbon dioxide, or a combination thereof.

[0090] In some embodiments, the method further comprises converting a polyimide aerogel to an isomorphic carbon aerogel, the conversion comprising thermally decomposing the polyimide aerogel in an inert atmosphere at a temperature of at least about 650°C.

[0091] In some embodiments, drying the polyamic acid gel optionally includes washing or desolvent-changing the polyamic acid gel, optionally subjecting the washed or desolvent-changing polyamic acid gel to high-temperature conditions, optionally freeze-drying the washed or desolvent-changing polyamic acid gel, or optionally contacting the washed or desolvent-changing polyamic acid gel with supercritical fluid carbon dioxide.

[0092] In some embodiments, washing or solvent exchange is carried out using water, C1-C3 alcohols, acetone, acetonitrile, ether, tetrahydrofuran, toluene, liquid carbon dioxide, or a combination thereof.

[0093] In some embodiments, the method further comprises converting a polyamic acid aerogel to an isomorphic carbon aerogel, the conversion comprising thermally decomposing the polyamic acid aerogel material in an inert atmosphere at a temperature of at least about 650°C.

[0094] In some embodiments, the method further comprises converting the polyamic acid aerogel to an isomorphic polyimide aerogel before thermal decomposition, and the conversion of the polyamic acid aerogel to a polyimide aerogel comprises thermal imidization of the polyamic acid aerogel.

[0095] In some embodiments, the method further comprises converting a polyamic acid metal salt aerogel to an isomorphic metal or metal oxide doped carbon aerogel, the conversion comprising thermally decomposing the polyamic acid aerogel in an inert atmosphere at a temperature of at least about 650°C.

[0096] In some embodiments, the method further comprises adding an electroactive material to an aqueous solution of a polyamic acid salt.

[0097] In some embodiments, the carbon aerogel has properties substantially similar to those of a carbon aerogel prepared by thermal decomposition of a corresponding polyimide aerogel prepared by a conventional non-aqueous method.

[0098] In some embodiments, the polyimide gel contains more than about 75% by volume of residual water.

[0099] In some embodiments, the polyamic acid gel contains more than about 75% by volume of residual water.

[0100] In some embodiments, the polyamic acid metal salt gel beads contain more than about 75% by volume of residual water.

[0101] In further embodiments, polyimide aerogels prepared by the methods disclosed herein are provided. In some embodiments, the polyimide aerogel is a solid 15 It contains terminal amine groups, as determined by N-NMR.

[0102] In another further embodiment, polyamic acid aerogels prepared by the methods disclosed herein are provided. In some embodiments, the polyamic aerogel is a solid 15 It contains terminal amine groups, as determined by N-NMR.

[0103] In a further embodiment, a carbon aerogel prepared by the method disclosed herein is provided.

[0104] In another further embodiment, a carbon aerogel comprising an electroactive material is provided, i.e., a carbon aerogel prepared by the method disclosed herein.

[0105] To provide an understanding of embodiments of this technology, the accompanying drawings, which are not necessarily drawn to scale, are referenced. The drawings are illustrative only and should not be construed as limiting the technology. The disclosures contained herein are illustrated in the accompanying drawings as examples, not as limitations. [Brief explanation of the drawing]

[0106] [Figure 1] Figure 1 is a flowchart summarizing several generalized pathways for forming an aerogel material according to non-limiting embodiments of the disclosed method. [Figure 2A] Figure 2A is a flowchart illustrating the process for preparing an alkali metal salt solution of a polyamic acid according to a non-limiting embodiment of the disclosed method. [Figure 2B] Figure 2B is a flowchart illustrating the process for preparing a solution of an ammonium salt of a polyamic acid according to a non-limiting embodiment of the disclosed method. [Figure 2C] Figure 2C is a flowchart illustrating three routes for the in situ preparation of solutions of ammonium salts of polyamic acids according to non-limiting embodiments of the disclosed method. [Figure 3] Figure 3 is a flowchart illustrating the process for preparing a polyimide aerogel monolith according to a non-limiting embodiment of the disclosed method. [Figure 4] Figure 4 is a flowchart illustrating another process for preparing a polyimide aerogel monolith according to a non-limiting embodiment of the disclosed method. [Figure 5] Figure 5 is a flowchart illustrating the process for preparing polyimide aerogel beads according to a non-limiting embodiment of the disclosed method. [Figure 6] Figure 6 is a flowchart illustrating another process for preparing polyimide aerogel microbeads according to a non-limiting embodiment of the disclosed method. [Figure 7]Figure 7 is a flowchart illustrating another process for preparing polyimide aerogel microbeads according to a non-limiting embodiment of the disclosed method. [Figure 8] Figure 8 is a flowchart illustrating the process for preparing a polyamic acid aerogel monolith according to a non-limiting embodiment of the disclosed method. [Figure 9A] Figure 9A is a flowchart illustrating the process for preparing polyamic acid aerogel beads according to a non-limiting embodiment of the disclosed method. [Figure 9B] Figure 9B is an illustration illustrating the formation of polyamic acid wet gel beads by a non-limiting embodiment of the disclosed method. [Figure 10] Figure 10 is a flowchart illustrating the process for preparing polyamic acid aerogel microbeads according to a non-limiting embodiment of the disclosed method. [Figure 11] Figure 11 is a flowchart illustrating the process for preparing metal polyamate aerogel beads according to a non-limiting embodiment of the disclosed method. [Figure 12] Figure 12 is a flowchart illustrating the process for preparing a carbon aerogel from a polyamic acid aerogel according to a non-limiting embodiment of the disclosed method. [Figure 13] Figure 13 is a flowchart illustrating a process for preparing a carbon aerogel from a polyamic acid or polyimide aerogel according to a non-limiting embodiment of the disclosed method. [Figure 14] Figure 14 is a flowchart illustrating a process for preparing a metal or metal oxide-doped carbon aerogel from a metal polyamate salt aerogel according to a non-limiting embodiment of the disclosed method. [Figure 15A] Figure 15A shows the solid-state 13C NMR spectrum of a polyimide aerogel monolith prepared at a target density of approximately 0.040 g / mL according to a non-limiting embodiment of the present disclosure. [Figure 15B]Figure 15B shows the solid-state 15N NMR spectrum of a polyimide aerogel monolith prepared at a target density equal to approximately 0.040 g / mL according to a non-limiting embodiment of the present disclosure. [Figure 16A] Figure 16A is a photograph of a polyimide aerogel monolith according to a non-limiting embodiment of the present disclosure. [Figure 16B] Figure 16B is a photograph of a carbide polyimide aerogel monolith according to a non-limiting embodiment of the present disclosure. [Figure 17A] Figure 17A is a high-magnification scanning electron microscope image of a polyimide aerogel according to a non-limiting embodiment of the present disclosure. [Figure 17B] Figure 17B is a high-magnification scanning electron microscope image of a carbide polyimide aerogel according to a non-limiting embodiment of the present disclosure. [Figure 17C] Figure 17C is a plot of the pore size distribution of a polyimide aerogel according to a non-limiting embodiment of the present disclosure, showing the pore size. [Figure 17D] Figure 17D is a plot of the pore size distribution of a carbide polyimide aerogel according to a non-limiting embodiment of the present disclosure, showing the pore size. [Figure 18A] Figure 18A shows a series of solid-state 13C NMR spectra obtained after different reaction times of polyimide aerogel monoliths according to non-limiting embodiments of the present disclosure. [Figure 18B] Figure 18B shows a series of solid-state 15N NMR spectra obtained after different reaction times of polyimide aerogel monoliths according to non-limiting embodiments of the present disclosure. [Figure 19A] Figure 19A is a plot of carbide yield versus reaction time for a series of polyimide aerogels according to non-limiting embodiments of the present disclosure. [Figure 19B] Figure 19B is a plot of total pore volume against pore volume measured by nitrogen sorption porosimetry, with respect to reaction time, for a series of polyimide aerogel monoliths and a series of carbide polyimide aerogel monoliths according to non-limiting embodiments of the present disclosure. [Figure 19C]Figure 19C shows plots of BET surface area versus reaction time for a series of polyimide aerogel monoliths and a series of carbide polyimide aerogel monoliths according to non-limiting embodiments of the present disclosure. [Figure 19D] Figure 19D shows plots of bulk density versus reaction time for a series of polyimide aerogel embodiments of the present disclosure and a series of carbide polyimide aerogel monoliths according to non-limiting embodiments of the present disclosure. [Figure 20A] Figure 20A is a scanning electron microscope image of a polyimide aerogel prepared by a conventional (reference) organic solvent method. [Figure 20B] Figure 20B is a scanning electron microscope image of a polyimide aerogel according to a non-limiting embodiment of the present disclosure. [Figure 21A] Figure 21A shows the solid-state 13C NMR spectrum of polyamic acid prepared from the reaction of 1,4-phenylenediamine (PDA) and pyromellitic dianhydride (PMDA) in N,N-dimethylacetamide. [Figure 21B] Figure 21B shows the solid-state 15N NMR spectrum of polyamic acid prepared from the reaction of PDA and PMDA in N,N-dimethylacetamide. [Figure 22A] Figure 22A shows the solid 15N NMR spectrum of the precipitate obtained after stirring PDA and PMDA in water for 24 hours. [Figure 22B] Figure 22B shows the solid-state 15N NMR spectrum of the reaction product obtained after stirring PDA and PMDA in water for 24 hours in the presence of triethylamine (TEA). [Figure 23A] Figure 23A is a histogram showing the size distribution of millimeter-sized carbide aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 23B] Figure 23B is a scanning electron microscope image of a skin of carbide aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 23C]Figure 23C is a plot of the pore size distribution for a polyimide aerogel according to a non-limiting embodiment of the present disclosure. [Figure 24] Figure 24 is a micrograph of a micron-sized polyimide gel bead according to a non-limiting embodiment of the present disclosure. [Figure 25] Figure 25 is a micrograph of a micron-sized polyimide xerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 26] Figure 26 shows the Fourier transform infrared attenuation total internal reflection (FTIR-ATR) spectrum of a polyimide xerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 27A] Figure 27A is a scanning electron microscope image of aerogel carbide beads according to a non-limiting embodiment of the present disclosure. [Figure 27B] Figure 27B is a scanning electron microscope image of the interior of a carbide aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 27C] Figure 27C is a scanning electron microscope image of the interior of a xerogel carbide bead according to a non-limiting embodiment of the present disclosure. [Figure 28] Figure 28 is a scanning electron microscope image of a cross-section near the surface of a xerogel carbide bead according to a non-limiting embodiment of the present disclosure. [Figure 29A] Figure 29A is a scanning electron microscope image of carbon xerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 29B] Figure 29B is a scanning electron microscope image of a cross-section of a carbon xerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 29C] Figure 29C is a scanning electron microscope image of a region near the surface of a carbon xerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 30] Figure 30 is a micrograph of silicon-doped polyimide gel beads according to a non-limiting embodiment of the present disclosure. [Figure 31A] Figure 31A is a micrograph of silicon-doped polyimide aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 31B] Figure 31B is a scanning electron microscope image of silicon-doped aerogel carbide beads according to a non-limiting embodiment of the present disclosure. [Figure 31C] Figure 31C is a scanning electron microscope image of silicon-doped aerogel carbide beads according to a non-limiting embodiment of the present disclosure. [Figure 32] Figure 32 is a micrograph of polyimide aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 33] Figure 33 is a micrograph of polyamic acid wet gel beads according to a non-limiting embodiment of the present disclosure. [Figure 34A] Figure 34A shows the FTIR spectrum of a polyimide aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 34B] Figure 34B shows the FTIR spectrum of polyamic acid aerogel beads according to a non-limiting embodiment of the present disclosure. [Figures 35A-35B] Figures 35A and 35B are scanning electron microscope images of the outer skin of carbon aerogel beads at two different magnifications according to a non-limiting embodiment of the present disclosure. [Figures 35C-35D] Figures 35C and 35D are scanning electron microscope images of the outer skin of carbon aerogel beads at two different magnifications according to non-limiting embodiments of the present disclosure. [Figure 36A] Figure 36A is a scanning electron microscope image of the interior of a carbon aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 36B] Figure 36B is a scanning electron microscope image of the interior of a carbon aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 37A] Figure 37A is a scanning electron microscope image of carbon aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 37B] Figure 37B is a scanning electron microscope image of the outer skin of a carbon aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 37C]Figure 37C is a scanning electron microscope image of the interior of a carbon aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 37D] Figure 37D is a plot showing the average diameter of carbon aerogel beads according to embodiments of the present disclosure. [Figure 38] Figure 38 is a plot showing the pore volume distribution according to pore size for carbon aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 39A] Figure 39A is a scanning electron microscope image of the outer skin of a carbon aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 39B] Figure 39B is a scanning electron microscope image of the interior of a carbon aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 39C] Figure 39C is a plot showing the pore volume distribution according to pore size for carbon aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 40A] Figure 40A is a scanning electron microscope image of carbon aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 40B] Figure 40B is a scanning electron microscope image of the outer skin of a carbon aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 40C] Figure 40C is a scanning electron microscope image of the interior of a carbon aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 41A] Figure 41A is a plot showing the average diameter of carbon aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 41B] Figure 41B is a plot showing the pore volume distribution according to pore size for carbon aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 42A] Figure 42A shows the FTIR spectrum of polyamic acid aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 42B] Figure 42B shows the FTIR spectrum of a polyimide aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 43A] Figure 43A is a scanning electron microscope image of an aggregate of carbon aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 43B] Figure 43B is a scanning electron microscope image of the outer skin of a carbon aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 43C] Figure 43C is a scanning electron microscope image of a destroyed carbon aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 43D] Figure 43D is a scanning electron microscope image of the interior of a carbon aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 44] Figure 44A is a scanning electron microscope image of an aggregate of carbon aerogel beads according to a non-limiting embodiment of the present disclosure. Figure 44B is a scanning electron microscope image of the outer skin of the carbon aerogel beads according to a non-limiting embodiment of the present disclosure. Figures 44C and 44D are scanning electron microscope images of the interior of the carbon aerogel beads according to a non-limiting embodiment of the present disclosure. [Figure 45] Figure 45A is a graph showing the average bead size in the polyamic acid wet gel stage according to electrospinning conditions in a non-limiting embodiment of this disclosure. Figure 45B is a graph showing the average bead size in the polyamic acid aerogel stage according to electrospinning conditions in a non-limiting embodiment of this disclosure. Figures 45C and 45D are graphs showing the average bead size in the carbon aerogel stage according to electrospinning conditions in a non-limiting embodiment of this disclosure. [Figure 46] Figures 46A and 48B are micrographs of carbon aerogel beads obtained by thermal decomposition of polyamic acid aerogel beads obtained under two different electrospinning conditions according to non-limiting embodiments of the present disclosure. [Figure 47] Figure 47 shows the FTIR spectrum of a polyimide aerogel monolith according to a non-limiting embodiment of the present disclosure. [Figure 48]Figures 48A and 48B are scanning electron microscope images of aggregates of carbon aerogel microbeads and their surfaces obtained by thermal decomposition of polyimide aerogel microbeads according to a non-limiting embodiment of the present disclosure, respectively. Figures 48C and 48D are scanning electron microscope images of aggregates of carbon aerogel microbeads and their surfaces obtained by thermal decomposition of polyamic acid aerogel microbeads according to a non-limiting embodiment of the present disclosure, respectively. [Figures 49A-49B] Figures 49A and 49B show the 13C and 15N solid-state NMR spectra of polyimide aerogel microbeads according to non-limiting embodiments of the present disclosure, respectively. [Figures 49C-49D] Figures 49C and 49D show the 13C and 15N solid-state NMR spectra of polyamic acid aerogel microbeads according to non-limiting embodiments of the present disclosure, respectively. [Figure 50] Figures 50A and 50B are scanning electron microscope images of aggregates of carbon aerogel microbeads obtained from the thermal decomposition of polyimide aerogel microbeads according to a non-limiting embodiment of the present disclosure, and their surfaces, respectively. Figures 50C and 50D are scanning electron microscope images of aggregates of carbon aerogel microbeads obtained from the thermal decomposition of polyamic acid aerogel microbeads according to a non-limiting embodiment of the present disclosure, and their surfaces, respectively. [Figure 51] Figures 51A and 51B are scanning electron microscope images of aggregates of carbon aerogel microbeads and their surfaces obtained by thermal decomposition of polyimide aerogel microbeads according to a non-limiting embodiment of the present disclosure, respectively. Figures 51C and 51D are scanning electron microscope images of aggregates of carbon aerogel microbeads and their surfaces obtained by thermal decomposition of polyamic acid aerogel microbeads according to a non-limiting embodiment of the present disclosure, respectively. [Figure 52]Figures 52A and 52B are scanning electron microscope images of aggregates of carbon aerogel microbeads and their surfaces obtained by thermal decomposition of polyimide aerogel microbeads according to a non-limiting embodiment of the present disclosure, respectively. Figures 52C and 52D are scanning electron microscope images of aggregates of carbon aerogel microbeads and their surfaces obtained by thermal decomposition of polyamic acid aerogel microbeads according to a non-limiting embodiment of the present disclosure, respectively. [Figure 53] Figures 53A and 54B are scanning electron microscope images of aggregates of carbon aerogel microbeads and their surfaces obtained by thermal decomposition of polyimide aerogel microbeads according to a non-limiting embodiment of the present disclosure, respectively. Figures 53C and 53D are scanning electron microscope images of aggregates of carbon aerogel microbeads and their surfaces obtained by thermal decomposition of polyamic acid aerogel microbeads according to a non-limiting embodiment of the present disclosure, respectively. [Figure 54] Figures 54A and 54B are scanning electron microscope images of aggregates of carbon aerogel microbeads obtained from the thermal decomposition of polyimide aerogel microbeads according to a non-limiting embodiment of the present disclosure, and their surfaces, respectively. Figures 54C and 54D are scanning electron microscope images of aggregates of carbon aerogel microbeads obtained from the thermal decomposition of polyamic acid aerogel microbeads according to a non-limiting embodiment of the present disclosure, and their surfaces, respectively. [Figure 55A] Figure 55A shows the FTIR spectrum of a polyamic acid obtained by the reaction of ODA and PMDA in N,N-dimethylacetamide. [Figure 55B] Figure 55B shows the FTIR spectrum of a polyimide aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 55C] Figure 55C shows the FTIR spectrum of a polyamic acid obtained by the reaction of MDA and PMDA in N,N-dimethylacetamide. [Figure 55D] Figure 55D shows the FTIR spectrum of a polyimide aerogel bead according to a non-limiting embodiment of the present disclosure. [Figure 56] Figures 56A and 56B are scanning electron microscope images of aggregates of carbon aerogel microbeads and their surfaces obtained by thermal decomposition of polyimide aerogel microbeads according to a non-limiting embodiment of the present disclosure, respectively. Figures 56C and 56D are scanning electron microscope images of aggregates of carbon aerogel microbeads and their surfaces obtained by thermal decomposition of polyimide aerogel microbeads according to a non-limiting embodiment of the present disclosure, respectively. [Figure 57] Figure 57A is a scanning electron microscope image of a millimeter-sized carbon aerogel bead obtained by thermal decomposition of silver polyamate aerogel beads according to a non-limiting embodiment of the present disclosure. Figure 57B is a scanning electron microscope image of the surface of a millimeter-sized carbon aerogel bead obtained by thermal decomposition of silver polyamate aerogel beads according to a non-limiting embodiment of the present disclosure. Figures 57C and 57D are scanning electron microscope images of the interior of a millimeter-sized carbon aerogel bead obtained by thermal decomposition of silver polyamate aerogel beads according to a non-limiting embodiment of the present disclosure, at two different magnifications. [Figure 58] Figure 58A is a scanning electron microscope image of a millimeter-sized carbon aerogel bead obtained by thermal decomposition of lanthanum polyamate aerogel beads according to a non-limiting embodiment of the present disclosure. Figure 58B is a scanning electron microscope image of the surface of a millimeter-sized carbon aerogel bead obtained by thermal decomposition of lanthanum polyamate aerogel beads according to a non-limiting embodiment of the present disclosure. Figures 58C and 58D are scanning electron microscope images of the interior of a millimeter-sized carbon aerogel bead obtained by thermal decomposition of lanthanum polyamate aerogel beads according to a non-limiting embodiment of the present disclosure, at two different magnifications. [Figure 59]Figure 59A is a scanning electron microscope image of a millimeter-sized carbon aerogel bead obtained by thermal decomposition of magnesium polyamate aerogel beads according to a non-limiting embodiment of the present disclosure. Figure 59B is a scanning electron microscope image of the surface of a millimeter-sized carbon aerogel bead obtained by thermal decomposition of magnesium polyamate aerogel beads according to a non-limiting embodiment of the present disclosure. Figures 59C and 59D are scanning electron microscope images of the interior of a millimeter-sized carbon aerogel bead obtained by thermal decomposition of magnesium polyamate aerogel beads according to a non-limiting embodiment of the present disclosure, at two different magnifications. [Modes for carrying out the invention]

[0107] Before describing some exemplary embodiments of this technology, it should be understood that this technology is not limited to the structural or process step details described below. Other embodiments of this technology are possible and can be practiced or implemented in various ways.

[0108] Generally, this technology relates to methods for forming polyamic acid and polyimide gels without using harmful organic solvents. These methods generally include providing an aqueous solution of a polyamic acid salt and 1) dehydrating the polyamic acid in the aqueous solution to form a polyimide gel and drying the polyimide gel to form a polyimide aerogel; 2) acidifying the aqueous solution of the polyamic acid salt to form a polyamic acid gel and drying the polyamic acid gel to form a polyamic acid aerogel; or 3) performing metal ion exchange to form a polyamic acid metal salt gel and drying the polyamic acid metal salt gel to form a polyamic acid metal salt aerogel. In some embodiments, these methods further include preparing an aqueous solution of a polyamic acid salt in situ, thermal imidizing the polyamic acid gel, converting the polyimide gel to a carbon gel, and converting the polyamic acid or polyamic acid metal salt gel to a carbon gel.

[0109] According to this disclosure, it has been surprisingly found that polyimide gels can be prepared in water with very rapid gelation of aqueous solutions of polyamic acids and their ammonium salts. It was concluded that hydrolysis of dehydrating agents (e.g., acetic anhydride) is slower than the dehydrating activity. The resulting polyimide gels can be converted into aerogels having nanostructures with properties similar to aerogels prepared by conventional organic solvent-based processes. According to this disclosure, it has been surprisingly found that polyamic acids can be prepared and gelled in water, and that these polyamic acid gels can be thermally dehydrated under aqueous conditions to provide corresponding polyimide gels. This method is advantageous in providing rapid gelation, thereby making it suitable for configuration in a continuous process for, for example, the preparation of polyimide beads. Furthermore, surprisingly, according to this disclosure, it has been found that polyamic acid gels can be directly thermally decomposed into carbon gels without the need for intermediate conversion to the corresponding polyimides. The disclosed method is more economically preferable than conventional methods for preparing polyimide and polyamic acid gel materials (e.g., expensive organic solvents are avoided and disposal costs are minimized), is "green" (i.e., beneficial from an environmental standpoint, as potentially toxic organic solvents are avoided and the generation of toxic by-products is minimized or eliminated), and is advantageous in potentially reducing the overall number of operations that need to be performed to provide carbon gel materials.

[0110] Accordingly, this specification provides methods for preparing polyamic acids, polyamic acid metal salts, and polyimide gels under aqueous conditions for converting polyamic acids to polyimides, and for converting polyamic acids, polyamic acid metal salts, and polyimide gel materials to corresponding carbon gel materials. Various methods are further described below in this specification.

[0111] definition The following definitions are provided for terms used in this disclosure. This application uses the following terms as defined below, unless the context of the text in which the terms appear requires a different meaning.

[0112] The articles “a” and “an” are used herein to refer to one or more (i.e., at least one) of the grammatical objects of an article. The term “about” as used throughout this specification is used to describe and represent small variations. For example, the term “about” may mean ±10% or less or ±5% or less, e.g., ±2% or less, ±1% or less, ±0.5% or less, ±0.2% or less, ±0.1% or less, or ±0.05% or less. All numbers are modified by the term “about,” whether explicitly stated or not. Naturally, values ​​modified by the term “about” include a specific value. For example, “about 5.0” must include 5.0.

[0113] In the context of this disclosure, the terms “framework” or “framework structure” refer to a network of interconnected oligomers, polymers, or colloidal particles that form a solid structure in a gel or aerogel. The polymers or particles constituting the framework structure typically have a diameter of about 100 angstroms. However, the framework structures of this disclosure may also include networks of interconnected oligomers, polymers, or colloidal particles of all diameter sizes that form a solid structure within a gel or aerogel.

[0114] As used herein, the term “aerogel” refers to a solid object comprising a framework of interconnected solid structures having a corresponding network of interconnected pores integrated within the framework, regardless of shape or size, and containing a gas such as air as a dispersed interstitial medium. Thus, regardless of the drying method used, aerogels are continuous non-fluid colloidal or polymer networks formed by being expanded by gas throughout their entire volume and by the removal of all the expanding agent from the corresponding wet gel. Unless otherwise specified, references to “aerogel” herein include any open-cell porous material that can be classified as aerogel, xerogel, cryogel, ambigel, microporous material, etc., regardless of the material (e.g., polyimide, polyamic acid, or carbon).

[0115] Generally, aerogels have the following physical and structural characteristics: (a) average pore diameter in the range of approximately 2 nm to approximately 100 nm, (b) porosity of approximately 60% or more, and (c) approximately 0 to approximately 100 nm. 2 / g or more, typically around 0 to 20, 0 to 100, or 100 to 1000m 2 It has one or more of the specific surface areas per gram. Typically, such properties are determined using nitrogen porosimetry and / or helium pycnometry. It can be understood that the inclusion of additives such as reinforcing materials or electrochemically active species, e.g., silicon, may reduce the porosity and specific surface area of ​​the resulting aerogel composite. Densification may also reduce the porosity of the resulting aerogel composite.

[0116] In some embodiments, the gel material may also be specifically referred to as a xerogel. As used herein, the term “xerogel” refers to a type of aerogel comprising a continuous non-fluid colloid or polymer network formed by the removal of all leavening agents from the corresponding gel without taking any precautions to avoid substantial volume loss or to delay compression. Xerogels generally have a compact structure. Xerogels undergo substantial volume loss during ambient pressure drying, generally 0–100 m, as measured by nitrogen sorption analysis. 2 / g, for example, approximately 0 to approximately 20mg 2 It has a surface area of ​​ / g.

[0117] As used herein, references to “conventional” or “organic solvent-based” methods for forming polyamic acid or polyimide gels refer to methods prepared in an organic solvent solution by condensing a diamine and a tetracarboxylic dianhydride to form a polyamic acid, and optionally dehydrating the polyamic acid to form a polyimide. See, for example, U.S. Patents 7,071,287 and 7,074,880 by Rhine et al., and U.S. Patent Application Publication 2020 / 0269207 by Zafiropoulos, et al.

[0118] As used herein, the terms “gelation” or “gel transition” refer to the formation of a wet gel from a polymer system, such as polyimide or polyamic acid, as described herein. At some point in a polymerization or dehydration reaction, as described herein, defined as the “gelation point,” the sol loses its fluidity. While not intended to be bound by any particular theory, the gelation point can be understood as the point at which the gelled solution exhibits resistance to flow. In this context, gelation proceeds from an initial sol state (e.g., a solution of an ammonium salt of polyamic acid) through a highly viscous dispersion state, until the dispersion solidifies, the sol gels (gelation point), and a wet gel (e.g., a polyimide or polyamic acid gel) is formed. The time required for a polymer in solution (e.g., an ammonium salt of polyamic acid or polyimide) to be converted into a gel in a form that can no longer flow is called the “phenomenological gelation time.” Formally, the gelation time is measured using rheology. At the gelation point, the elastic properties of the solid gel begin to prevail over the viscous properties of the fluid sol. The formal gelation time is close to the time when the real and imaginary components of the complex modulus of the gelled sol intersect. The two moduli are monitored as a function of time using a rheometer. Time begins counting from the moment the last component of the sol is added to the solution.For example, see the discussion of gelation in HHWinter'''Can the Gel Point of a Cross-linking Polymer Be Detected by the G'-G'''Crossover'Polym.Eng.Sci.,1987,27,1698-1702, S.-Y.Kim,D.-G.Choi and S.-M.Yang'''Rheological analysis of the gelation behavior of tetraethylorthosilane / vinyltriethoxysilane hybrid solutions'''Korean J.Chem.Eng.,2002,19,190-196, and M.Muthukumar'''Screening effect on viscoelasticity near the gel point'''Macromolecules,1989,22,4656-4658.

[0119] As used herein, the term “wet gel” refers to a gel in which the mobile interstitial phase within a network of interconnected pores consists primarily of a liquid phase such as a conventional solvent or water, a liquefied gas such as liquid carbon dioxide, or a combination thereof. Aerogels typically require the first generation of a wet gel, followed by a process and extraction to replace the mobile interstitial liquid phase within the gel with air or another gas. Examples of wet gels include, but are not limited to, alcohol gels, hydrogels, ketogels, carbon gels, and any other wet gels known to those skilled in the art.

[0120] As used herein, the term "alkyl" generally refers to a linear or branched saturated hydrocarbon group having 1 to 20 carbon atoms (i.e., C1 to C20). Typical alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, n-butyl, n-pentyl, and n-hexyl, while branched alkyl groups include, but are not limited to, isopropyl, sec-butyl, isobutyl, tert-butyl, isopentyl, and neopentyl. Alkyl groups may be unsubstituted or substituted.

[0121] As used herein, the term “alkenyl” generally refers to a hydrocarbon group having 1 to 20 carbon atoms (i.e., C1 to C20) and at least one unsaturated site, i.e., a carbon-carbon double bond. Examples include, but are not limited to, ethylene or vinyl, allyl, 1-butenyl, 2-butenyl, isobutyrenyl, 1-pentenyl, 2-pentenyl, 3-methyl-1-butenyl, 2-methyl-2-butenyl, 2,3-dimethyl-2-butenyl, etc. The alkenyl group may be unsubstituted or substituted.

[0122] As used herein, the term "alkynyl" generally refers to a hydrocarbon group having 1 to 20 carbon atoms (i.e., C1 to C20) and at least one carbon-carbon triple bond. Examples of alkynyl groups include, but are not limited to, ethynyl and propargyl. Alkynyl groups may be unsubstituted or substituted.

[0123] As used herein, the term "aryl" generally refers to an aromatic carbocyclic group having 6 to 20 carbon atoms (i.e., C6 to C20). Examples of aryl groups include, but are not limited to, phenyl, naphthyl, and anthracenyl. Aryl groups may be unsubstituted or substituted.

[0124] As used herein, the term "cycloalkyl" refers to a saturated carbocyclic group that can be monocyclic or bicyclic. The cycloalkyl group includes a ring having 3 to 7 carbon atoms (i.e., C3 - C7) as a monocyclic ring or 7 to 12 carbon atoms (i.e., C7 - C12) as a bicyclic ring. Examples of monocyclic cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl. The cycloalkyl group may be unsubstituted or substituted.

[0125] As used herein, and when applied to any of the preceding groups (alkyl, alkenyl, alkynyl, aryl, cycloalkyl, etc.), the term "substituted" means that one or more hydrogen atoms of the group are each independently replaced by a substituent. Typical substituents include -X, -R, -OH, -OR, -SH, -SR, NH2, -NHR, -N(R)2, -N + (R)3, -CX3, -CN, -OCN, -SCN, -NCO, -NCS, -NO, -NO2, -N3, -NC(=O)H, -NC(=O)R, -C(=O)H, -C(=O)R, -C(=O)NH2, -C(=O)N(R)2, -SO3-, -SO3H, -S(=O)2R, -OS(=O)2OR, -S(=O)2NH2, -S(=O)2N(R)2, -S(=O)R, -OP(=O)(OH)2, -OP(=O)(OR)2, -P(=O)(OR)2, -PO3, -PO3H2, -C(=O)X, -C(=S)R, -CO2H, -CO2R, -CO2-, -C(=S)OR, -C(=O)SR, -C(=S)SR, -C(=O)NH2, -C(=O)N(R)2, -C(=S)NH2, -C(=S)N(R)2, -C(=NH)NH2, and -C(=NR)N(R)2, including but not limited to, wherein each X is independently selected from F, Cl, Br, and I each time, and each R is independently selected from C1 - C 20 alkyl and C6 - C 20 aryl each time. When a group is described as "optionally substituted", the group may be independently substituted with one or more of the preceding substituents each time.

[0126] It should be understood that certain nomenclature rules may include various bonding configurations depending on the context. For example, a substituent is understood to be bidentate if it requires two bonding sites to the rest of the molecule. For instance, substituents identified as alkyl but requiring two bonding sites include forms such as -CH2-, -CH2CH2-, and -CH2CH(CH3)CH2-. Other nomenclature rules clearly indicate that a group is bidentate, such as "alkylene," "alkenylene," and "arylene." If a substituent is bidentate, it should be understood that it can be bonded in any directional configuration unless otherwise specified.

[0127] As used herein, the term “substantially” means, unless otherwise specified, a considerable degree of the characteristic, quantity, etc. being referred to, in relation to a particular context (e.g., substantially pure, substantially identical, etc.), e.g., more than about 95%, more than about 99%, more than about 99.9%, more than 99.99%, or even 100%.

[0128] Methods for forming polyimides, polyamic acids, metal salts of polyamic acids, and carbon aerogels The methods disclosed herein generally utilize polyamic acid and polyimide wet gels that can be prepared without the use of organic solvents. These water-based methods are advantageous in reducing material and waste disposal costs and mitigating potential safety and environmental hazards. Figure 1 provides a general, non-limiting overview of three options for preparing polyimide aerogels, polyamic acid aerogels, and polyamic acid metal salt aerogels, as well as their corresponding carbon aerogels, all of which are derived from aqueous solutions of polyamic salts.

[0129] Referring to Figure 1, in option 1, an aqueous solution of polyamic acid is imidized and dried to provide a polyimide (PI) aerogel in monolithic or bead form. Optionally, the PI aerogel may be thermally decomposed to form a corresponding carbon aerogel.

[0130] Referring further to Figure 1, in option 2, an aqueous solution of polyamic acid is acidified and dried to form a polyamic acid (PAA) aerogel as either a monolith or beads. The PAA aerogel may be converted to a PI aerogel by thermal imidation, or directly to a corresponding carbon aerogel by thermal decomposition.

[0131] Referring further to Figure 1, in option 3, an aqueous solution of polyamic acid is subjected to metal ion exchange to form a monolithic or bead-shaped PAA metal salt aerogel. Such a PAA metal salt aerogel can be directly thermally decomposed to form a corresponding metal or metal oxide-doped carbon aerogel.

[0132] Accordingly, one aspect of the present disclosure provides a method for preparing a polyimide aerogel. This method generally includes providing an aqueous solution of a salt of a polyamic acid, imidizing the polyamic acid to form a polyimide gel, and drying the polyimide gel to form a polyimide aerogel. References to aqueous solutions herein mean that the solution is substantially free of any organic solvent. As used herein in the context of organic solvents, the term “substantially free” means that no organic solvent has been intentionally added and that no organic solvent is present in trace amounts. For example, in certain embodiments, an aqueous solution may be characterized by having less than 1 vol% of an organic solvent, or less than 0.1 vol%, or less than 0.01 vol%, or even 0 vol% of an organic solvent.

[0133] In some embodiments, the polyamic acid is purchased or pre-prepared and dissolved in water in the presence of a base. In other embodiments, the polyamic acid is prepared in situ under aqueous conditions to directly form a polyamic acid solution.

[0134] Another aspect of the present disclosure provides a method for preparing a polyamic acid aerogel. This method generally includes providing an aqueous solution of a polyamic acid salt, acidifying the polyamic acid salt solution to form a polyamic acid gel, and drying the polyamic acid gel to form a polyamic acid aerogel.

[0135] In yet another aspect of the present disclosure, a method for preparing polyamic acid metal salt aerogels is provided. This method generally includes providing an aqueous solution of an ammonium or alkali metal salt of polyamic acid; performing a metal ion exchange, which involves adding a solution of the polyamic acid to the solution containing the soluble metal salt to form polyamate metal salt gel beads; and drying the polyamic acid metal salt gel beads to form polyamic acid metal salt aerogel beads.

[0136] In other embodiments, methods are provided for converting polyamic acid aerogels to isomorphic polyimide aerogels, as well as for converting various polyamic acid, polyimide, and polyamic acid metal salt aerogels to corresponding carbide aerogels. These methods and their individual steps are each discussed in detail below herein.

[0137] I. Provision of aqueous solutions of polyamic acid salts All disclosed methods share the common feature of providing an aqueous solution of a polyamic acid. Such solutions may be obtained by dissolving a pre-formed polyamic acid in water in the presence of a base, or by in situ preparation from polyamic acid precursors (diamines and tetracarboxylic dianhydrides) under aqueous conditions in the presence of a base. Each method is further described below herein.

[0138] Polyamic acid Polyamic acids are polymeric amides having repeating units comprising a carboxylic acid group, a carboxamide group, and an aromatic or aliphatic moiety containing a diamine and a tetracarboxylic acid from which the polyamic acid is derived. As defined herein, “repeating unit” is a portion of a polyamic acid (or corresponding polyimide) whose repetitions link together in a continuous manner along the polymer chain to form a complete polymer chain (excluding terminal amino groups or unreacted anhydride ends). Those skilled in the art will recognize that polyamic acid repeating units arise from the partial condensation of a carboxyl group of a tetracarboxylic dianhydride and an amino group of a diamine.

[0139] In some embodiments, the polyamic acid is any commercially available polyamic acid. In other embodiments, the polyamic acid is pre-formed ("pre-formed") and isolated, for example, prepared by the reaction of a diamine with a tetracarboxylic dianhydride in an organic solvent by a conventional synthetic method. In any case, whether purchased or prepared and isolated, a suitable polyamic acid is in substantially pure form. Pre-formed and isolated or commercially available polyamic acids may be in solid form, such as powder or crystalline form, or in liquid form.

[0140] A variety of suitable polyamic acid structures can exist. In some embodiments, the polyamic acid is of formula I: [ka] It has a structure represented by the formula, in which, Z is a group that connects the two terminal amino groups of the diamine. L is a group that connects to a carboxyl group. n is an integer representing the number of repeating units of the polyamic acid, and it determines the molecular weight of the polyamic acid.

[0141] In some embodiments, Z is aliphatic (e.g., alkyl, alkenyl, alkynyl, or cycloalkyl), as previously described herein. Thus, in some embodiments, the polyamic acid comprises an aliphatic diamine amide as a repeating unit. In some embodiments, the polyamic acid comprises an alkanediamine amide having 2 to 12 carbon atoms (i.e., C2 to C12) as a repeating unit. In some embodiments, the polyamic acid comprises a C2 to C6 alkanediamine amide as a repeating unit, including but not limited to ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, or 1,6-diaminohexane. In some embodiments, one or more carbon atoms of the C2 to C6 alkane of the diamine are substituted with one or more alkyl groups, such as methyl.

[0142] In some embodiments, Z is an aryl as described earlier herein. Thus, in some embodiments, the polyamic acid comprises an aryldiamine amide as a repeating unit. In some embodiments, the polyamic acid comprises an aryldiamine amide of phenylenediamine, diaminodiphenyl ether, or alkylenedianiline as a repeating unit. In some embodiments, the polyamic acid comprises an aryldiamine amide selected from the group consisting of 1,3-phenylenediamine, 1,4-phenylenediamine, 4,4'-diaminodiphenyl ether, 4,4'-methylenedianiline, and combinations thereof as a repeating unit. In some embodiments, the polyamic acid comprises an aryldiamine amide selected from the group consisting of 1,4-phenylenediamine, 4,4'-methylenedianiline, and 4,4'-diaminodiphenyl ether as a repeating unit. In some embodiments, the polyamic acid comprises an aryldiamine amide which is 1,4-phenylenediamine (PDA) as a repeating unit.

[0143] In some embodiments, L includes an alkyl group, a cycloalkyl group, an aryl group, or a combination thereof, as described earlier herein. In some embodiments, L includes an aryl group. In some embodiments, L includes a phenyl group, a biphenyl group, or a diphenyl ether group. In some embodiments, the polyamic acid includes, as a repeating unit, an amide of a tetracarboxylic acid selected from the group consisting of benzene-1,2,4,5-tetracarboxylic acid, [1,1'-biphenyl]-3,3',4,4'-tetracarboxylic acid, 4,4'-oxydiphthalic acid, 4,4'-sulfonyldiphthalic acid, 4,4'-carbonyldiphthalic acid, 4,4'-(propane-2,2-diyl)diphthalic acid, 4,4'-(perfluoropropane-2,2-diyl)diphthalic acid, naphthalene-1,4,5,8-tetracarboxylic acid, 4-(2-(4-(3,4-dicarboxyphenoxy)phenyl)propane-2-yl)phthalic acid, perylenetetracarboxylic acid, and combinations thereof. In some embodiments, the polyamic acid includes, as a repeating unit, an amide of a tetracarboxylic acid which is benzene-1,2,4,5-tetracarboxylic acid.

[0144] Polyamic acid salts While polyamic acids are generally insoluble in water, according to this disclosure, certain polyamic acid salts in which the carboxylic acid groups of the polyamic acid are bonded to cationic species and substantially exist as carboxylate anions have been found to have useful water solubility. "Substantially present as carboxylate anions" means that more than 95%, more than 99%, more than 99.9%, more than 99.99%, or even 100% of the free carboxylic acid groups present in the polyamic acid molecule are deprotonated (i.e., -CO2). -This means that it is in a state of ). Cationic species may be, for example, alkali metal cations or ammonium cations. Referring to Figures 2A and 2B, generally providing a polyamic acid salt in solution includes adding polyamic acid to water to form an aqueous suspension of polyamic acid, and adding a base to the aqueous suspension of polyamic acid to form an aqueous solution of polyamic acid salt. Polyamic acid is as previously described herein and may be purchased or prepared as described herein.

[0145] The base can vary. For example, in some embodiments, the base is an alkali metal hydroxide and the cation is an alkali metal ion. Referring to Figure 2A, a polyamic acid is suspended in water, and an alkali metal hydroxide is added to the suspension to obtain an aqueous solution of the alkali metal salt of the polyamic acid. Suitable alkali metal hydroxides include, but are not limited to, lithium hydroxide, sodium hydroxide, and potassium hydroxide.

[0146] The amount of alkali metal hydroxide added may vary, but is generally sufficient to react (e.g., neutralize or deprotonate) substantially all of the free carboxylic acid groups present in the polyamic acid, thereby causing substantially all of the polyamic acid to dissolve. As used herein in the context of neutralizing carboxylic acid groups, “substantially all” means that more than 95% of the carboxylic acid groups are neutralized, e.g., 99%, 99.9%, 99.99%, or even 100% of the carboxylic acid groups are neutralized. As used herein in the context of dissolving polyamic acid, “substantially all” means that more than 95% of the polyamic acid dissolves, e.g., 99%, 99.9%, 99.99%, or even 100% of the polyamic acid. In some embodiments, the molar ratio of alkali metal hydroxide to polyamic acid is about 0.1 to about 8, e.g., about 2 to about 8. In some embodiments, the molar ratio of alkali metal hydroxide to polyamic acid is about 2 to about 4, or about 2.2 to about 2.5.

[0147] The amount of water used will vary depending on the desired concentration, the scale of the solution formed, and the solubility of the polyamic acid salt in water. In some embodiments, the concentration range of the alkali metal salt of the polyamic acid in the solution is approximately 0.01 to approximately 0.3 g / cm³, based on the weight of the polyamic acid. 3 That is the case.

[0148] In some embodiments, the base is a non-nucleophilic amine base and the cation is an ammonium ion. Referring to Figure 2B, a polyamic acid is suspended in water, and a non-nucleophilic amine base is added to the suspension to obtain an aqueous solution of the ammonium polyamic acid salt. Typical non-nucleophilic amines are bulky, tertiary, or both, so that a proton can bond to the basic center, but alkylation, acylation, complexation, etc., are impossible or too slow to yield any practical results. Suitable non-nucleophilic amine bases include, but are not limited to, tertiary amines such as alkyl, cycloalkyl, and aromatic tertiary amines. As used herein in the context of amines, “tertiary” means that the amine nitrogen atom has three bonds or organic substituents attached to it. Generally, suitable non-nucleophilic amines will have a solubility in water of at least about 4 grams per liter at 20°C. Particularly suitable non-nucleophilic amine bases are water-soluble lower trialkylamines, including cyclic trialkylamines. In some embodiments, the non-nucleophilic amine base is selected from the group consisting of trimethylamine, triethylamine, tri-n-propylamine, tri-n-butylamine, N-methylpyrrolidine, N-methylpiperidine, diisopropylethylamine, and combinations thereof. In some embodiments, the non-nucleophilic amine base is triethylamine. In some embodiments, the non-nucleophilic amine base is diisopropylethylamine.

[0149] The amount of non-nucleophilic amine base added can vary, but is generally sufficient to react (e.g., neutralize or deprotonate) with substantially all free carboxylic acid groups present in the polyamic acid, thereby dissolving substantially all of the polyamic acid. In some embodiments, the non-nucleophilic amine is added in an amount sufficient to maintain substantially all of the polyamic acid in solution. In some embodiments, the molar ratio of the non-nucleophilic amine base to the polyamic acid is about 0.1 to about 8, for example, about 2 to about 8. In some embodiments, the molar ratio of the non-nucleophilic amine base to the polyamic acid is about 2 to about 4, or about 2.2 to about 2.5.

[0150] The amount of water used will vary depending on the desired concentration, the scale at which the solution is formed, and the solubility of the polyamic acid and / or non-nucleophilic amine base in water. In some embodiments, the concentration range of the ammonium salt of the polyamic acid in the solution is about 0.01 to about 0.3 g / cm³, based on the weight of the polyamic acid (i.e., the weight of the free acid). 3 That is the case.

[0151] Ammonium polyamate salt, in situ preparation In some embodiments, aqueous solutions of polyamic acid salts are prepared in situ, for example, by reacting a diamine with a tetracarboxylic dianhydride in the presence of a non-nucleophilic amine to provide an aqueous solution of ammonium polyamic acid salt. Generally, diamines can react with tetracarboxylic dianhydrides in the presence of a non-nucleophilic amine to form ammonium polyamic acid salts. In some embodiments, combinations of more than one diamine may be used. Combinations of diamines may be used to optimize the properties of the gel material. In some embodiments, a single diamine is used. Generally, diamines have considerable solubility in water. For example, a suitable diamine may have a solubility in water of at least about 0.1 g per 100 ml, at least about 1 g per 100 ml, or at least about 10 g per 100 ml at 20°C.

[0152] A non-limiting general reaction sequence is provided in Scheme 1. In some embodiments, the reaction generally occurs according to Scheme 1, and the reagents and products have structures according to the formula of Scheme 1.

[0153] Scheme 1 [ka] Referring to Scheme 1, Z, L, and n are as defined herein earlier with reference to Formula I, and the non-nucleophilic amine is a non-nucleophilic amine base as described herein earlier (for example, R1, R2, and R3 are alkyl, cycloalkylaryl, or a combination thereof). Suitable diamines, tetracarboxylic dianhydrides, and non-nucleophilic amines are further described below. The order of addition of individual reactants can vary, as can the structures of the reactants. Suitable reactant structures and reaction conditions, as well as the order of addition, are further described herein below.

[0154] Referring to Figure 2C, there are three common options for providing an aqueous solution of polyamic acid salt according to the general scheme 1.

[0155] Option 1 In some embodiments, providing an aqueous solution of a polyamic acid salt is possible. Dissolving a water-soluble diamine in water to form an aqueous diamine solution, Adding a non-nucleophilic amine to an aqueous diamine solution, Adding tetracarboxylic dianhydride to an aqueous diamine solution, The method includes stirring the resulting solution at a temperature in the range of approximately 15 to 60°C for a period of approximately 1 to 24 hours.

[0156] Referring to Figure 2C, Option 1, and Scheme 1, a water-soluble diamine is dissolved in water. The structure of the diamine can vary. In some embodiments, the diamine has a structure according to Formula II, where Z is an aliphatic (i.e., alkylene, alkenylene, alkylylene, or cycloalkylene) or aryl, as described earlier herein. In some embodiments, Z is an alkylene such as a C2-C12 alkylene or a C2-C6 alkylene. In some embodiments, the diamine is a C2-C6 alkanediamine such as ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, or 1,6-diaminohexane, but is not limited thereto. In some embodiments, the C2-C6 alkylene of the alkanediamine is substituted with one or more alkyl groups such as methyl.

[0157] In some embodiments, Z is an aryl. In some embodiments, the aryldiamine is 1,3-phenylenediamine, 1,4-phenylenediamine, or a combination thereof. In some embodiments, the diamine is 1,4-phenylenediamine (PDA).

[0158] Continuing to refer to Figure 2C, Option 1, and Scheme 1, a non-nucleophilic amine is added to the aqueous diamine solution. Suitable non-nucleophilic amines are described earlier herein. In some embodiments, the non-nucleophilic amine is selected from the group consisting of triethylamine, trimethylamine, tri-n-butylamine, N-methylpyrrolidine, N-methylpiperidine, diisopropylethylamine, and combinations thereof. In some embodiments, the non-nucleophilic amine is triethylamine. In some embodiments, the non-nucleophilic amine is diisopropylethylamine.

[0159] The amount of non-nucleophilic amine added can vary. In some embodiments, the molar ratio of non-nucleophilic amine to diamine is about 2 to about 4, or about 2 to about 3. In some embodiments, the molar ratio is about 2.0, about 2.1, about 2.2, about 2.3, about 2.4, or about 2.5 to about 2.6, about 2.7, about 2.8, about 2.9, or about 3.0. In some embodiments, the molar ratio of non-nucleophilic amine to diamine is about 2 to about 2.5. While we do not wish to be bound by any particular theory, in some exemplary embodiments, it is considered that at least a sufficient amount of amine is required to allow the neutralization of substantially all free carboxylic acid groups of the polyamic acid (i.e., to form a salt). According to this disclosure, in some embodiments, it has been observed that molar ratios less than 2.0 or less than 2.2 can result in the precipitation of the polyamic acid. Therefore, the molar ratio may require optimization for each set of reactants and conditions. In some embodiments, the molar ratio is selected to maintain the solubility of the polyamic acid. In some embodiments, the molar ratio is selected to avoid any precipitation of the polyamic acid.

[0160] Continuing to refer to Figure 2C, Option 1, and Scheme 1, a tetracarboxylic dianhydride is added. In some embodiments, one or more tetracarboxylic dianhydrides are added. Combinations of tetracarboxylic dianhydrides may be used to optimize the properties of the gel material. In some embodiments, a single tetracarboxylic dianhydride is added.

[0161] The structure of tetracarboxylic dianhydride can vary. In some embodiments, the tetracarboxylic dianhydride has a structure according to formula III, where L includes an alkylene group, a cycloalkylene group, an arylene group, or a combination thereof, as described earlier herein. In some embodiments, L includes an arylene group. In some embodiments, L includes a phenyl group, a biphenyl group, or a diphenyl ether group. In some embodiments, the tetracarboxylic dianhydride of formula III has a structure selected from one or more structures provided in Table 1. [Table 1]

[0162] In some embodiments, the tetracarboxylic dianhydride is selected from the group consisting of pyromellitic anhydride (PMDA), biphthalic acid dianhydride (BPDA), oxydiphthalic acid dianhydride (ODPA), benzophenonetetracarboxylic dianhydride (BTDA), ethylenediaminetetraacetic acid dianhydride (EDDA), 1,4,5,8-naphthalenetetracarboxylic dianhydride, and combinations thereof. In some embodiments, the tetracarboxylic dianhydride is PMDA.

[0163] The molar ratio of diamine to dianhydride can vary depending on the desired reaction time, reagent structure, and desired material properties. In some embodiments, the molar ratio is about 0.1 to about 10, for example, about 0.1, about 0.5, or about 1 to about 2, about 3, about 5, or about 10. In some embodiments, this ratio is about 0.5 to about 2. In some embodiments, this ratio is about 1 (i.e., stoichiometric), for example, about 0.9 to about 1.1. In certain embodiments, this ratio is about 0.99 to about 1.01.

[0164] Referring to Scheme 1, it is possible to form polyamic acids by reacting diamines and dianhydrides with each other in the presence of non-nucleophilic amines. Although we do not wish to be bound by theory, it is thought that polyamic acids, in the presence of non-nucleophilic amines, form ammonium salts of polyamic acids having the structure according to Formula IV, and the water solubility of these salts allows the ammonium salts of polyamic acids to remain in solution.

[0165] The molecular weight of polyamic acids can vary depending on the reaction conditions (e.g., concentration, temperature, reaction duration, properties of diamines and dianhydrides). The molecular weight is based on the number of repeating units of the polyamic acid, as indicated by the integer value "n" for the structure of Equation IV in Scheme 1. The specific molecular weight range of the polymer material produced by the disclosed method can vary. In general, the noted reaction conditions can vary and provide a gel with desired physical properties without specifically considering the molecular weight. In some embodiments, the molecular weight surrogate is provided in the viscosity of the ammonium polyamate solution, which is determined by variables such as temperature, concentration, molar ratio of reactants, and reaction time.

[0166] The molar ratio of diamine to dianhydride can vary depending on the desired reaction time, reagent structure, and desired material properties. In some embodiments, the molar ratio is about 0.1 to about 10, for example, about 0.1, about 0.5, or about 1 to about 2, about 3, about 5, or about 10. In some embodiments, this ratio is about 0.5 to about 2. In some embodiments, this ratio is about 1 (i.e., stoichiometric), for example, about 0.9 to about 1.1. In certain embodiments, this ratio is about 0.99 to about 1.01.

[0167] The molar ratio of a non-nucleophilic amine to a diamine or dianhydride determines the solubility of the polyamic acid. In some embodiments, the molar ratio of the non-nucleophilic amine to the diamine is about 2 to about 4, or about 2 to about 3. In some embodiments, the molar ratio is about 2.0, about 2.1, about 2.2, about 2.3, about 2.4, or about 2.5 to about 2.6, about 2.7, about 2.8, about 2.9, or about 3.0. While we do not wish to be bound by any particular theory, in some exemplary embodiments, it is considered that at least a sufficient amount of amine is required to allow for the neutralization of substantially all free carboxylic acid groups of the polyamic acid (i.e., to form a salt). According to this disclosure, in some embodiments, molar ratios less than 2.0 or less than 2.2 may result in the precipitation of an intermediate polyamic acid (e.g., due to evaporation losses of the non-nucleophilic amine). Therefore, the molar ratio may require optimization for each set of reactants and conditions. In some embodiments, the molar ratio is selected to maintain the solubility of the reactant (e.g., polyamic acid). In some embodiments, the molar ratio is adjusted to avoid any precipitation.

[0168] The reaction temperature can vary. A preferred range is generally about 10°C to about 100°C. In some embodiments, the reaction temperature is about 15 to about 60°C, for example, about 20, about 25, about 30, about 35, about 40, about 45, about 50, about 55, or about 60°C. In some embodiments, the temperature is about 15 to about 25°C. In some embodiments, the temperature is about 50 to about 60°C.

[0169] In some embodiments, as the temperature increases, polyimide gels may be produced with different pore size distributions and different structural properties. While we do not wish to be bound by theory, it is conceivable that in certain embodiments, properties such as pore size distribution and structural stiffness may vary with temperature, possibly as a result of the polyimide molecular weight, the degree of chemical crosslinking (if possible), and other factors that may exhibit temperature dependence.

[0170] The reaction is allowed to proceed for a certain period of time, generally until all available reactants (e.g., diamines and dianhydrides) have reacted with each other. The time required for a complete reaction can vary depending on the structure, concentration, and temperature of the reagents. In some embodiments, the reaction time is about 1 minute to about 1 week, for example, about 15 minutes to about 5 days, about 30 minutes to about 3 days, or about 1 hour to about 1 day. In some embodiments, the reaction time is about 1 hour to about 12 hours.

[0171] Option 2 In some embodiments, providing an aqueous solution of a polyamic acid salt is possible. Dissolving a water-soluble diamine in water to form an aqueous diamine solution, Adding tetracarboxylic dianhydride to an aqueous diamine solution, The resulting suspension is stirred at a temperature in the range of approximately 15 to 60°C for a period of approximately 1 to 24 hours. Adding a non-nucleophilic amine to an aqueous diamine solution, The method includes stirring the resulting suspension at a temperature in the range of approximately 15 to 60°C for a period of approximately 1 to 24 hours.

[0172] Referring to Figure 2C, Option 2, and Scheme 1, the water-soluble diamine is dissolved in water as previously described with respect to Option 1. However, in this embodiment, a tetracarboxylic dianhydride (as previously described herein with respect to Option 1) is added to the aqueous diamine solution to form a suspension. The relative amounts of the reactants may vary, as previously described with respect to Option 1.

[0173] In some embodiments, the suspension is stirred for a period ranging from about 1 hour to about 1 day, for example, from about 1 hour to about 12 hours.

[0174] The temperature at which the suspension is stirred can be various. A preferred range is generally from about 15 to about 60 °C, for example, about 20, about 25, about 30, about 35, about 40, about 45, about 50, about 55, or about 60 °C. In some embodiments, the temperature is from about 15 to about 25 °C. In some embodiments, the temperature is from about 50 to about 60 °C.

[0175] Referring to Figure 2C, Option 2, and Scheme 1, a non-nucleophilic amine is added. Suitable non-nucleophilic amines are described earlier in this specification. In some embodiments, the non-nucleophilic amine is selected from the group consisting of triethylamine, trimethylamine, tri-n-butylamine, N-methylpyrrolidine, N-methylpiperidine, diisopropylethylamine, and combinations thereof. In some embodiments, the non-nucleophilic amine is triethylamine. In some embodiments, the non-nucleophilic amine is diisopropylethylamine.

[0176] The amount of non-nucleophilic amine added can be various as described earlier with respect to Option 1. In some embodiments, the molar ratio of non-nucleophilic amine to diamine is from about 2 to about 2.5.

[0177] In some embodiments, the resulting mixture is stirred for a period ranging from about 1 hour to about 1 day, for example, from about 1 hour to about 12 hours.

[0178] The temperature at which the mixture is stirred can be various. A preferred range is generally from about 15 to about 60 °C, for example, about 20, about 25, about 30, about 35, about 40, about 45, about 50, about 55, or about 60 °C. In some embodiments, the temperature is from about 15 to about 25 °C. In some embodiments, the temperature is from about 50 to about 60 °C.

[0179] Option 3 In some embodiments, providing an aqueous solution of the polyamic acid salt is [[ID=Z4]]simultaneously or without interruption, adding a water-soluble diamine, a tetracarboxylic dianhydride, and a non-nucleophilic amine to water, and Stirring the resulting solution at a temperature in the range of about 15 to about 60 °C for a period in the range of about 1 hour to about 24 hours is included.

[0180] Referring to Figure 2C, Option 3, and Scheme 1, a water-soluble diamine, a tetracarboxylic dianhydride, and a non-nucleophilic amine are added either simultaneously or without interruption. The water-soluble diamine, the tetracarboxylic dianhydride, and the non-nucleophilic amine, as well as their relative amounts, are as described previously with respect to Options 1 and 2.

[0181] In some embodiments, the resulting mixture is stirred for a period in the range of about 1 hour to about 1 day, for example, in the range of about 1 hour to about 12 hours.

[0182] The temperature at which the mixture is stirred can vary. A suitable range is generally about 15 to about 60 °C, for example, about 20, about 25, about 30, about 35, about 40, about 45, about 50, about 55, or about 60 °C. In some embodiments, the temperature is about 15 to about 25 °C. In some embodiments, the temperature is about 50 to about 60 °C.

[0183] II. Method for Forming Polyimide Aerogel [[ID=*18]]As described previously herein, in one aspect of the present disclosure, a method for preparing a polyimide aerogel is provided, which includes providing an aqueous solution of a salt of a polyamic acid, imidizing the polyamic acid to form a polyimide gel, and drying the polyimide gel to form a polyimide aerogel. The polyimide gel and the corresponding aerogel can be in monolithic form or bead form. The salt of the polyamic acid can be an alkali metal salt or an ammonium salt. Various permutations for preparing a polyimide aerogel from such a polyamic acid salt solution are further described herein below.

[0184] A. Monolithic Polyimide Aerogel from an Aqueous Solution of a Salt of Polyamic Acid by Chemical Imidization In some embodiments, the polyimide gel and the corresponding aerogel are in monolithic form, and the salt is prepared as previously described with reference to Figure 2B or Figure 2C (options 1, 2, or 3), where the polyamic acid in aqueous solution is an ammonium salt. In such embodiments, the imidation may be chemical imidation, which may be the method generally described in Figure 3.

[0185] Referring to Figure 3, imidization of a polyamic acid salt involves adding a dehydrating agent to an aqueous solution of the polyamic acid salt to form a gelling mixture ("sol"), pouring the gelling mixture into a template, and forming a gel from the gelling mixture. Adding a dehydrating agent initiates and promotes imidization to form a polyimide wet gel from the ammonium polyamic acid salt. Non-limiting general reaction sequences are provided in Scheme 2. In some embodiments, the polyimide has a structure according to formula V as illustrated in Scheme 2, where L, Z, and n are each as previously described herein with respect to forming the ammonium polyamic acid salt of formula IV.

[0186] Scheme 2 [ka] The structure of the dehydrating agent can vary, but generally it is a reagent that is at least partially soluble in the reaction solution, reactive with the carboxylate group of the ammonium salt, effective in promoting the imidation of the carboxyl and amide groups of the polyamic acid, while having minimal reactivity with aqueous solutions. An example of a suitable class of dehydrating agents is carboxylic acid anhydrides such as acetic anhydride and propionic anhydride. In some embodiments, the dehydrating agent is acetic anhydride. Surprisingly, according to this disclosure, it has been found that the addition of acetic anhydride to an aqueous solution of an ammonium salt results in rapid gelation of the polyimide without the observation of the substantially expected hydrolysis of acetic anhydride by water. Any hydrolysis that occurs is not sufficient to compete with the function of acetic anhydride in polyimide formation.

[0187] In some embodiments, the amount of dehydrating agent may vary based on the amount of tetracarboxylic dianhydride. For example, in some embodiments, the dehydrating agent is present in various molar ratios with respect to tetracarboxylic dianhydride. The molar ratio of the dehydrating agent to tetracarboxylic dianhydride may vary depending on the desired reaction time, reagent structure, and desired material properties. In some embodiments, the molar ratio is about 2 to about 10, for example, about 2, about 3, about 4, or about 5 to about 6, about 7, about 8, about 9, or about 10. In some embodiments, this ratio is about 4 to about 5. In some embodiments, this ratio is 4.3.

[0188] The temperature at which the dehydration reaction can proceed can vary, but is generally below approximately 50°C, for example, between approximately 10°C and 50°C, or between approximately 15°C and 25°C.

[0189] Referring further to Figure 3, the gelling mixture can be poured into a mold and gelled. Generally, the resulting wet gel material can remain in the mold ("cast") for a certain period of time. The time required to completely gel the gelling mixture and form a wet gel can vary. This period can vary depending on many factors, such as whether aging of the material is desired, but is generally from a few hours to a few days.

[0190] The process of transferring a gelling mixture to a wet gel material may also include an aging step (also called curing) before drying. Aging the wet gel material after it has reached its gelation point can further strengthen the gel framework. For example, in some embodiments, the framework may be strengthened during aging. The duration of gel aging can be adjusted to control various properties within the corresponding aerogel material. This aging procedure may be useful in preventing potential volume loss and shrinkage during liquid-phase extraction of the wet gel material. Aging may involve keeping the gel stationary for a long period (before extraction), keeping the gel at a high temperature, or any combination thereof. Preferred temperatures for aging are typically around 10°C to about 200°C. Aging may also occur during solvent exchange, as described below herein. Aging of a wet gel material may also be referred to as "curing" and typically continues until liquid-phase extraction of the wet gel material.

[0191] The resulting wet gel monoliths may vary in size and shape. In some embodiments, the wet gel monoliths have a thickness of approximately 5 to 25 mm. In some embodiments, the monolith is in the form of a film, such as a film, having a thickness of approximately 50 microns to 1 mm.

[0192] Those skilled in the art will recognize that polyimide wet gels prepared by this method and other methods described herein have unreacted terminal amino groups at one or both ends of individual polymer chains. The percentage concentration of such amino groups in the polyimide wet gel will vary inversely to the average number of repeating units (i.e., molecular weight) present in the polyimide wet gel. In some embodiments, the terminal amino groups may react with a dehydrating agent to form, for example, terminal acetamides. The relative concentration of such terminal amines or amides is inversely proportional to the solid 15 It can be determined by methods known in the art, including but not limited to nuclear magnetic resonance spectroscopy such as N-NMR.

[0193] In some embodiments, the water content in the polyimide wet gel prepared as disclosed herein, prior to any solvent exchange or drying, is essentially the total amount of water initially used as the reaction solvent, and does not account for any evaporation or water produced or destroyed in the various reactions that occur during polyimide synthesis as previously described herein. Therefore, in some embodiments, the water content in the polyimide wet gel is about 0.07 to about 0.10 g / cm³. 3 Target density (T d In the case of formulations containing ), the amount varies from approximately 75% to approximately 83% by volume.

[0194] Referring further to Figure 3, following any aging, the resulting wet gel material can be demolded and washed or solvent-exchanged in a suitable secondary solvent to replace the primary reaction solvent (i.e., water) present in the wet gel. Such secondary solvents may be linear alcohols having one or more aliphatic carbon atoms, diols having two or more carbon atoms, or branched alcohols, cyclic alcohols, alicyclic alcohols, aromatic alcohols, polyols, ethers, ketones, cyclic ethers, or derivatives thereof. In some embodiments, the secondary solvent is water, C1-C3 alcohols (e.g., methanol, ethanol, propanol, isopropanol), acetone, tetrahydrofuran, ethyl acetate, acetonitrile, supercritical fluid carbon dioxide (CO2), or a combination thereof. In some embodiments, the secondary solvent is ethanol.

[0195] Once a wet gel monolith has been formed and processed, the liquid phase of the wet gel monolith can then be at least partially extracted from the wet gel material using an extraction method that includes processing and extraction techniques to form an aerogel material (i.e., "dry"). Liquid phase extraction plays a crucial role, among other factors, in manipulating the properties of the aerogel, such as porosity and density, as well as related properties such as thermal conductivity. Generally, an aerogel is obtained when the liquid phase is extracted from the wet gel in such a way that it causes low shrinkage in the porous network and framework of the wet gel. To provide an aerogel or xerogel, the wet gel can be dried using various techniques. In exemplary embodiments, the wet gel material can be dried at ambient pressure, under vacuum (e.g., by freeze-drying), under subcritical or supercritical conditions to form the corresponding dried gel (e.g., an aerogel such as a xerogel).

[0196] In some embodiments, it may be desirable to fine-tune the surface area of ​​the dry gel. If surface area fine-tuning is desired, the aerogel can be fully or partially converted into a xerogel with varying porosity. The high surface area of ​​an aerogel can be reduced by disrupting some of its pores. This can be done, for example, by immersing the aerogel in a solvent such as ethanol or acetone for a certain period of time, or by exposing them to solvent vapor. The solvent is then removed by drying under ambient pressure.

[0197] Aerogels are generally formed by removing a liquid mobile phase from a wet gel material at a temperature and pressure near or above the critical point of the liquid mobile phase. When the critical point is reached (near critical) or exceeded (supercritical, i.e., the pressure and temperature of the system are above the critical pressure and critical temperature, respectively), a new supercritical phase different from the liquid or vapor phase appears in the fluid. The solvent can then be removed without introducing any associated mass transfer limitations typically associated with liquid-vapor interfaces, capillary forces, or receding liquid-vapor boundaries. Further, the supercritical phase is generally more miscible with organic solvents and thus has the ability for better extraction. Co-solvents and solvent exchange are also commonly used to optimize the supercritical fluid drying process.

[0198] If evaporation or extraction occurs below the supercritical point, the capillary forces generated by liquid evaporation can cause shrinkage and pore collapse within the gel material. Maintaining the mobile phase near or above the critical pressure and temperature during the solvent extraction process reduces such negative effects of capillary forces. In certain embodiments of the present disclosure, the use of near-critical point conditions just below the critical point of the solvent system enables the production of aerogels or compositions with sufficiently low shrinkage and thus can produce a commercially viable end product.

[0199] To provide an aerogel, various techniques can be used to dry the wet gel. In an exemplary embodiment, the wet gel material can be dried at ambient pressure, subcritical conditions, or supercritical conditions.

[0200] Both room temperature and high temperature processes can be used to dry the gel material at ambient pressure. In some embodiments, a slow ambient pressure drying process can be used where the wet gel is exposed to air in an open container for a period sufficient to remove the solvent, e.g., in the range of several hours to several weeks, depending on the solvent, the amount of wet gel, the surface area exposed, the size of the wet gel, etc.

[0201] In another embodiment, the wet gel material is dried by heating. For example, the wet gel material can be heated in a convection oven for a certain period of time to evaporate most of the solvent (e.g., ethanol). After partial drying, the gel can be left at ambient temperature for a certain period of time, for example, several hours to several days, to allow it to dry completely. This drying method produces a xerogel.

[0202] In some embodiments, the wet gel material is dried by freeze-drying. "Freeze-drying" or "lyophilization" means a low-temperature process for solvent removal, which involves freezing the material (e.g., the wet gel material), reducing the pressure, and then removing the frozen solvent by sublimation. Since water is an ideal solvent for removal by freeze-drying and is the solvent in the method disclosed herein, freeze-drying is particularly suitable for aerogel formation from the disclosed polyimide wet gel material. This drying method produces cryogels that can be quite similar to aerogels.

[0203] Both supercritical and subcritical drying can be used to dry wet gel materials. In some embodiments, the wet gel material is dried under subcritical or supercritical conditions. In an exemplary embodiment of supercritical drying, the gel material may be placed in a high-pressure vessel for solvent extraction at supercritical CO2. After the removal of the solvent, for example, ethanol, the vessel may be held above the critical point of CO2 for a certain period, for example, about 30 minutes. Following supercritical drying, the vessel is reduced to atmospheric pressure. Generally, aerogels are obtained by this process.

[0204] In an exemplary embodiment of subcritical drying, the gel material is dried using liquid CO2 at a pressure ranging from about 800 psi to about 1200 psi at room temperature. This operation is faster than supercritical drying, and for example, the solvent (e.g., ethanol) can be extracted in about 15 minutes. Generally, aerogels are obtained by this process.

[0205] Several additional aerogel extraction techniques are known in the art and include a range of approaches in the use of supercritical fluids in drying aerogels, as well as ambient drying techniques. For example, Kistler (J. Phys. Chem. (1932) 36:52-64) describes a simple supercritical extraction process in which the gel solvent is maintained above its critical pressure and temperature, thereby reducing capillary forces due to evaporation and maintaining the structural integrity of the gel network. U.S. Patent No. 4,610,863 describes an extraction process in which the gel solvent is replaced with liquid carbon dioxide, and then extracted under conditions in which the carbon dioxide is in a supercritical state. U.S. Patent No. 6,670,402 teaches the extraction of a liquid phase from a gel via rapid solvent exchange by injecting supercritical (not liquid) carbon dioxide into an extractor that is preheated and prepressurized substantially above supercritical conditions, thereby producing an aerogel. U.S. Patent No. 5,962,539 describes a process for obtaining an aerogel from a polymer material in sol-gel form in an organic solvent by supercritical extraction of the fluid from the sol-gel by replacing the organic solvent with a fluid having a critical temperature below the polymer decomposition temperature. U.S. Patent No. 6,315,971 discloses a process for producing a gel composition, comprising drying a wet gel containing a gel solid and a desiccant, and removing the desiccant under sufficiently dry conditions to reduce the shrinkage of the gel during drying. U.S. Patent No. 5,420,168 describes a process by which a resorcinol / formaldehyde aerogel can be produced using a simple air-drying procedure. U.S. Patent No. 5,565,142 describes a drying technique for modifying the gel surface to be stronger and more hydrophobic so that the gel framework and pores can withstand collapse during ambient drying or subcritical extraction. Other examples of extracting the liquid phase from aerogel material can be found in U.S. Patents No. 5,275,796 and No. 5,395,805.

[0206] In some embodiments, extracting the liquid phase from a wet gel utilizes supercritical conditions for carbon dioxide, which include, for example, first substantially replacing the primary solvent present in the gel's pore network with liquid carbon dioxide, and then heating the wet gel (typically in an autoclave) above the critical temperature of carbon dioxide (approximately 31.06°C) and increasing the system pressure to a pressure higher than the critical pressure of carbon dioxide (approximately 1070 psig). The pressure around the gel material can be slightly varied to facilitate the removal of the supercritical carbon dioxide fluid from the gel. The carbon dioxide can be recirculated through the extraction system to facilitate the continuous removal of the primary solvent from the wet gel. Finally, the temperature and pressure are slowly returned to ambient conditions to produce a dry aerogel material. The carbon dioxide can also be pre-treated to a supercritical state before being injected into the extraction chamber. In other embodiments, extraction can be carried out using any preferred mechanism, for example, by changing the pressure, timing, and solvent discussed earlier.

[0207] B. Monolithic polyimide aerogel from aqueous solution of polyamic acid salts by thermal imidation In some embodiments, imidation may be thermal imidation, and this method may be the method generally described in Figure 4. Referring to Figure 4, in this embodiment, imidation of the ammonium polyamate salt is performed Adding delta-gluconolactone to an aqueous solution of polyamic acid salt to form a gel mixture, Pouring the gelling mixture into a mold and turning the gelling mixture into a gel, The obtained polyamic acid gel is washed with water, The process involves forming a polyimide gel by thermal imidation of a polyamic acid gel, wherein the thermal imidation includes exposing the polyamic acid gel to microwave frequency irradiation.

[0208] In an aqueous environment, DGL slowly reacts with water to form delta-gluconic acid (DGA; Formula 1), which helps at least initiate the acidification process for the gelation of polyamic acids. [ka]

[0209] The gelling mixture can be poured into a mold and gelled. Upon acidification, the polyamic acid becomes insoluble in an aqueous environment, forming a polyamic acid wet gel. In some embodiments, the ammonium polyamic acid salt has the structure of formula IV, and the polyamic acid gel has the structure of formula VI (Scheme 3), where L, Z, and n are as previously described herein, and the acid is DGA.

[0210] Scheme 3 [ka] The time required to completely gel a gel-forming solution (sol; e.g., polyamic acid) to form a wet gel can vary. Generally, gelation occurs in about 1.5 hours or less. Generally, the wet gel material can remain in the mold ("cast") for a certain period. This period can vary based on many factors, such as whether aging of the material, as previously described herein with respect to chemical imidation, is desired.

[0211] Next, referring to Figure 4, the obtained polyamic acid gel monolith is washed with water. The washing is carried out with sufficient time and in sufficient volume of water to remove any water-soluble by-products such as ammonium salts, DGA, or DGL, and other by-products from the formation of the ammonium polyamic acid salt solution.

[0212] Continuing with Figure 4, following the formation and washing of the polyamic acid wet gel monolith, the polyamic acid gel is dehydrated (i.e., imidized) using heat treatment (e.g., microwave exposure) to form the corresponding polyimide gel. Non-limiting general reaction sequences are provided in Scheme 4. In some embodiments, the polyimide has a structure according to formula V as illustrated in Scheme 4, where L, Z, and n are as previously described herein.

[0213] Scheme 4 [ka] Irradiation of wet gel materials with microwave frequency energy is one particularly suitable thermal treatment. Microwaves are low-energy electromagnetic waves with wavelengths ranging from 0.001 to 0.3 meters and frequencies ranging from 1,000 to 300,000 MHz. Typical microwave devices operate with microwaves at a frequency of 2450 MHz. The electric field component of microwaves is primarily responsible for heating, interacting with molecules through dipole rotation and ionic conduction. In dipole rotation, molecules constantly rotate back and forth, attempting to align their dipoles with the constantly oscillating electric field, and the friction between each rotating molecule generates heat. In ionic conduction, free ions or ionic species translate through space, attempting to align with the changing electric field. Similar to dipole rotation, the friction between these moving species generates heat. In both cases, the more polar and / or ionic the molecular species, the more efficient the heating rate. Compared to conventional heating based on slow heat conduction, microwave heating allows for rapid and efficient energy transfer. Therefore, microwave heating is particularly suitable for carrying out the thermal imidation reaction of the present invention. Generally, the microwave frequency irradiation is of sufficient power and duration to convert a substantial portion of the amide and carboxyl groups of the polyamic acid into imide groups. As used herein in the context of converting amide and carboxyl groups into imide groups, “a substantial portion” means that more than 90% of the amide and carboxyl groups, for example, 95%, 99%, or 99.9%, or 99.99%, or even 100%, are converted into imide groups.

[0214] Continuing with Figure 4, following the heating and formation of the polyimide gel monolith, the polyimide gel monolith is washed (solvent exchange) and dried as previously described herein for chemically imidized polyimide monolith to form a polyimide aerogel monolith.

[0215] C. Polyimide aerogel beads from aqueous solutions of ammonium salts of polyamic acid by chemical imidation (droplet method in aqueous solution) In some embodiments, the polyimide gel and the corresponding aerogel are in bead form, and the salt is prepared as previously described with reference to Figure 2B or Figure 2C (options 1, 2, or 3), where the polyamic acid in aqueous solution is an ammonium salt. In such embodiments, the imidation may be chemical imidation, and this method may be the method generally described in Figure 5. As used herein, the terms “beads” or “bead shape” generally mean including individual small units or pieces having a spherical shape. In some embodiments, the gel beads are substantially spherical. The beads are generally of uniform composition, and so each bead in a group of beads contains approximately the same amount of the same polyimide within the normal variables expected in the preparation of such beads. The size of the beads may vary depending on the desired properties and preparation method.

[0216] Referring to Figure 5, the ammonium polyamate salt is chemically imidized by adding a dehydrating agent to an aqueous solution of the ammonium polyamate salt to form a gelling mixture as previously described herein with respect to Figure 3. In some embodiments, the dehydrating agent is acetic anhydride. However, in this embodiment, instead of pouring the gelling mixture into a mold to form a monolith, this method involves adding the gelling mixture to a solution of water-soluble acid in water, or optionally adding the gelling mixture to a water-immiscible solvent containing the acid, before gelation, to form polyimide gel beads. Generally, the sol is rapidly added to complete the dropwise addition before the gelation of the sol occurs. The addition can be carried out by several different techniques, including dropping the gelling mixture into a solution of water-soluble acid in water, spraying the gelling mixture under pressure into a solution of water-soluble acid in water through one or more nozzles, or electro-misting the gelling mixture into a solution of water-soluble acid in water through one or more needles.

[0217] Referring to Figure 5, in some embodiments, the method involves adding a gelling mixture to a solution of a water-soluble acid in water. The water-soluble acid can be varied and may be, for example, an organic acid or a mineral acid. In some embodiments, the acid is a mineral acid such as hydrochloric acid, sulfuric acid, or phosphoric acid. In some embodiments, the acid is an organic acid. The organic acid can be varied but is typically a lower carboxylic acid, including but not limited to formic acid, acetic acid, or propionic acid. In some embodiments, the acid is acetic acid. The amount of acid present can vary but is typically about 10 to about 20 vol% in water. In some embodiments, the solution contains about 10 vol% or about 20 vol% of acetic acid.

[0218] The size of polyimide gel beads can vary based on the size of the droplets added to the solution of a water-soluble acid in water. In some embodiments, the gelling mixture is added as individual droplets (e.g., by dropping from a pipette or other suitable droplet-forming device, either manually or automatically). Polyimide gel beads produced from such droplets tend to be relatively large in diameter, for example, about 0.5 to about 10 mm, with diameters in the range of about 0.5, about 1, about 2, about 3, about 4, or about 5 to about 6, about 7, about 8, about 9, or about 10 mm. In some embodiments, the beads have sizes in the range of about 0.5 to about 5 mm in diameter.

[0219] Referring to Figure 5, in some embodiments, the gelling mixture is added by spraying to produce relatively small polyimide gel beads (e.g., on the order of microns). Spraying can be carried out using various aerosol formation techniques known in the art, such as pressurized gas-assisted aerosol formation or electro-spraying. In certain embodiments, spraying is electro-spraying. Generally, electro-spraying is carried out by passing a solution containing the gelling mixture through one or more needles into a bath of a solution of water-soluble acid in water, while simultaneously applying a voltage difference of about 5-60 kV between the bath and one or more needles. By this method, very fine droplets of the gelling mixture are introduced into the solution of water-soluble acid in water. Upon contact, the micron-sized droplets react with the acid to form a polyamic acid skin around the droplet, which gradually gels to form polyimide beads. While not wishing to be bound by theory, it is thought that the water-soluble acid protonates the carboxylate groups of the polyamic acid salt, forming an initial skin that is penetrated by a dehydrating agent, imidizing the polyamic acid salt within the droplet, and forming wet gel polyimide beads. In some embodiments, the beads have a diameter ranging from about 5 to about 200 microns, for example, in the range of about 5, 10, 20, 30, 40, or about 50 to about 60, 70, 80, 90, 100, or about 200 microns.

[0220] Continuing to refer to Figure 5, following the formation of polyimide gel beads by dropwise or spraying, the polyimide gel beads are aged, washed (solvent exchanged), and dried as previously described herein with respect to chemically imidized polyimide monoliths to form the corresponding polyimide aerogel beads.

[0221] D. Polyimide aerogel beads from aqueous solutions of ammonium salts of polyamic acid by chemical imidation (droplet method; water-immiscible solvent) Continuing to refer to Figure 5, in another embodiment, the gelling mixture is as previously described herein with respect to the aqueous droplet method. However, in this embodiment, instead of adding the gelling mixture as droplets into a solution of water-soluble acid in water, the method comprises adding the gelling mixture to an acid-containing, water-immiscible solvent to form polyimide gel beads. Generally, the sol is added rapidly to complete the dropwise addition before the gelation of the sol occurs.

[0222] The addition can be carried out by several different techniques, each as previously described herein, including dropping the gelling mixture into a water-immiscible solvent, spraying the gelling mixture under pressure into a water-immiscible solvent through one or more nozzles, or electro-spraying the gelling mixture into a water-immiscible solvent through one or more needles.

[0223] Water-immiscible solvents can vary. Suitable solvents include, but are not limited to, oils such as silicone oils or mineral oils, aliphatic hydrocarbons, aromatic hydrocarbons, and chlorinated hydrocarbons. In some embodiments, the solvent is an aliphatic or aromatic hydrocarbon with 5 to 12 carbon atoms (C5 to C12). In some embodiments, the solvent is hexane. In certain embodiments, the solvent is mineral spirits.

[0224] The optional acid can vary, but is typically a lower carboxylic acid, including but not limited to formic acid, acetic acid, or propionic acid. In some embodiments, the acid is acetic acid. The amount of acid present can vary, but if present, is typically about 10 to 20% by volume of the water-immiscible solvent. While we do not wish to be bound by theory, it is thought that the presence of acid during gelation may lead to the formation of an outer surface of beads with carboxyl groups that do not react to form imide groups, and the presence of such acidic groups on the outer surface may prevent the beads from coalescing.

[0225] In some embodiments, the gelling mixture is added as individual droplets (e.g., by dropping from a pipette or other suitable droplet-forming device, either manually or automatically). The polyimide gel beads produced from such droplets tend to be relatively large in diameter, for example, about 0.5 to about 10 mm, with diameters ranging from about 0.5, about 1, about 2, about 3, about 4, or about 5 to about 6, about 7, about 8, about 9, or about 10 mm. In some embodiments, the beads have a size ranging from about 0.5 to about 5 mm in diameter.

[0226] In some embodiments, the gelling mixture is added by spraying to produce relatively small polyimide gel beads (e.g., on the order of microns). Spraying can be carried out using various aerosol formation techniques known in the art, such as pressurized gas-assisted aerosol formation or electro-spraying. In certain embodiments, spraying is electro-spraying. Generally, electro-spraying is carried out by pumping a solution containing the gelling mixture through one or more needles into a bath of a solution of water-soluble acid in water, while simultaneously applying a voltage difference of about 5-60 kV between the bath and one or more needles. By this method, very fine droplets of the gelling mixture are introduced into the solution of water-soluble acid in water. Upon contact, the micron-sized droplets react with the acid to form a polyamic acid skin around the droplet, which gradually gels to form polyimide beads. While not wishing to be bound by theory, it is thought that the water-soluble acid protonates the carboxylate groups of the polyamic acid salt, forming an initial skin that is penetrated by a dehydrating agent, imidizing the polyamic acid salt within the droplet, and forming wet gel polyimide beads. In some embodiments, the beads have a diameter ranging from about 5 to about 200 microns, for example, in the range of about 5, 10, 20, 30, 40, or about 50 to about 60, 70, 80, 90, 100, or about 200 microns.

[0227] Continuing to refer to Figure 5, following the formation of polyimide gel beads by dropwise or spraying, the polyimide gel beads are aged, washed (solvent exchanged), and dried as previously described herein with respect to chemically imidized polyimide monoliths to form the corresponding polyimide aerogel beads.

[0228] E. Polyimide aerogel beads from aqueous solutions of ammonium salts of polyamic acid by chemical imidation (emulsion method 1) In some embodiments, the polyimide gel and the corresponding aerogel are in bead form, and the salt is prepared as previously described with reference to Figure 2B or Figure 2C (options 1, 2, or 3), when the polyamic acid in aqueous solution is an ammonium salt. In such embodiments, the imidation may be chemical imidation, and this method may be the method generally described in Figure 6. Referring to Figure 6, imidation of a polyamic acid involves adding a dehydrating agent to an aqueous solution of the polyamic acid to form a gelling mixture, as previously described herein. This method further includes combining the gelling mixture with a water-immiscible solvent containing a surfactant, and mixing the resulting mixture under high shear conditions.

[0229] Mixing two-phase mixtures under high shear conditions generally yields micron-sized polyimide beads. In some embodiments, a water-immiscible solvent and a surfactant are added to the aqueous gelling mixture.

[0230] A variety of water-immiscible solvents can be used. Suitable solvents include, but are not limited to, oils such as silicone oil or mineral oil, aliphatic hydrocarbons, aromatic hydrocarbons, and chlorinated hydrocarbons. In some embodiments, the solvent is a C5-C12 aliphatic or aromatic hydrocarbon. In some embodiments, the solvent is hexane. In certain embodiments, the solvent is mineral spirits.

[0231] Surfactants can vary. As used herein, the term “surfactant” refers to a substance that assists in the formation and stabilization of an emulsion by promoting the dispersion of hydrophobic and hydrophilic (e.g., oil and water) components. Preferred surfactants are generally nonionic and include, but are not limited to, polyethylene glycol esters of fatty acids, propylene glycol esters of fatty acids, polysorbates, polyglycerol esters of fatty acids, and sorbitan esters of fatty acids. Preferred surfactants have an HLB number in the range of about 0 to about 20. In some embodiments, the HLB number is about 3.5 to about 6. As will be understood by those skilled in the art, HLB is the hydrophilic-lipophilic balance of an emulsifier or surfactant, and is a measure of the degree to which an emulsifier or surfactant is hydrophilic or lipophilic. HLB values ​​can be determined by calculating values ​​in different regions of a molecule, as described by Griffin, William C. (1949), "Classification of Surface-Active Agents by HLB" (PDF), Journal of the Society of Cosmetic Chemists, 1(5):311-26 and Griffin, William C. (1954), "Calculation of HLB Values ​​of Non-Ionic Surfactants" (PDF), Journal of the Society of Cosmetic Chemists, 5(4):249-56, and by Davies, JT. (1957), "A quantitative kinetic theory of emulsion type, I. Physical chemistry of the emulsifying agent" (PDF), Gas / Liquid and Liquid / Liquid Interface, Proceedings of the International Congress of Surface Activity, pp.426-38.HLB values ​​can be determined according to the industry standard textbook, namely, '''The HLB SYSTEM, a time-saving guide to emulsifier selection''' ICI Americas Inc., published in 1976 and revised in March 1980.

[0232] Examples of suitable surfactants include, generally, polyoxyethylene sorbitan fatty acid esters; e.g., monolauryl and trilauryl, palmityl, stearyl, and oleyl esters; e.g., products of the type known as polysorbate and marketed under the trade name Tween®; polyoxyethylene fatty acid esters, e.g., polyoxyethylene stearate esters of the type known and marketed under the trade name Myrj®; polyoxyethylene ethers, e.g., those available under the trade name Brij®; and polyoxyethylene castor oil derivatives, e.g., those known and marketed as Cremophors®. Products of the following types include, but are not limited to: sorbitan fatty acid esters, for example, known and commercially available types such as Span® (e.g., Span 80); polyoxyethylene-polyoxypropylene copolymers, for example, known and commercially available types such as Pluronic® or Poloxamer®; glycerol triacetates; and monoglycerides and acetylated monoglycerides, for example, glycerol monodicocoate (Imwitor® 928), glycerol monocaprylate (Imwitor® 308), and mono and diacetylated monoglycerides. In some embodiments, one or more surfactants include commercially available polymeric surfactants of the type known by the trade name Hypermer® (Croda Industrial Chemicals, Edison, NJ, USA).

[0233] In some embodiments, one or more surfactants include Tween20, Tween80, Span20, Span40, Span60, Span80, or a combination thereof. In some embodiments, the surfactant is Span20, Tween80, or a mixture thereof. In some embodiments, one or more surfactants are Hypermer® B246SF. In some embodiments, one or more surfactants are Hypermer® A70.

[0234] The concentration of the surfactant can vary. In some embodiments, the surfactant, or a mixture of surfactants, is present in a water-immiscible solvent in an amount of about 1% to about 5% by weight, for example, about 1% by weight, about 2% by weight, about 3% by weight, about 4% by weight, or about 5% by weight.

[0235] Due to interfacial tension, spherical droplets in aqueous sol form are formed in a water-immiscible solvent. The droplets gel and strengthen over time in the water-immiscible solvent, e.g., mineral spirits. Stirring of the mixture is typically used to form an emulsion and / or to prevent droplet aggregation. For example, a mixture of aqueous gelling mixture and water-immiscible solvent can be stirred (e.g., agitated) to form an emulsion that may be stable or temporary. Exemplary embodiments of stirring to provide gel beads from the sol mixture and water-immiscible solvent include magnetic stirring (up to about 600 rpm), mechanical mixing (up to about 1500 rpm), and homogenization (i.e., mixing up to about 9000 rpm). In some embodiments, mixing is carried out under high shear conditions, for example, using a high shear mixer or homogenizer. A fluid is subjected to shear when one region of the fluid moves at a different velocity relative to an adjacent region. High-shear mixers (homogenizers) use a rotating impeller or high-speed rotor, or a series of such impellers or inline rotors, to create flow and shear in order to "act" on the fluid. The tip velocity (i.e., the velocity the fluid encounters at the outer diameter of the rotor) is higher than the velocity it encounters at the center of the rotor, and this velocity difference creates shear. Generally, higher shear results in smaller beads.

[0236] In some embodiments, an additional solvent, such as water or ethanol, can be added after gelation to produce smaller beads and reduce the aggregation of larger bead clusters.

[0237] The size of the wet gel beads can vary. In some embodiments, the wet gel beads have a diameter ranging from about 5 to about 500 microns, for example, about 5, about 10, about 20, about 30, about 40, or about 50 to about 60, about 70, about 80, about 90, about 100, about 200, about 300, about 400, or about 500 microns.

[0238] Continuing to refer to Figure 6, following the formation of polyimide gel beads, the polyimide gel beads are aged, washed (solvent exchanged), and dried as previously described herein with respect to chemically imidized polyimide beads from the droplet method to form the corresponding polyimide aerogel beads.

[0239] F. Polyimide aerogel beads from aqueous solutions of ammonium salts of polyamic acid by chemical imidation (emulsion method 2) In some embodiments, the polyimide gel and the corresponding aerogel are in bead form, and the salt is prepared as previously described with reference to Figure 2B or Figure 2C (options 1, 2, or 3), where the polyamic acid in aqueous solution is an ammonium salt. In such embodiments, the imidation may be chemical imidation, and this method may be the method generally described in Figure 7. Referring to Figure 7, this method comprises combining the gelling mixture with a water-immiscible solvent containing a surfactant, mixing the resulting mixture under high shear conditions to form a metastable emulsion, and adding a dehydrating agent to the metastable emulsion. This method differs from the emulsion method 1 previously described herein only in that it first forms a metastable emulsion of aqueous ammonium polyamic acid salt and a water-immiscible solvent, followed by the addition of a dehydrating agent.

[0240] The surfactant, water-immiscible solvent, and mixing conditions are as previously described herein with respect to emulsion method 1. In some embodiments, the water-immiscible organic solvent is a C5-C12 hydrocarbon. In some embodiments, the water-immiscible organic solvent is a mineral spirit. In some embodiments, the dehydrating agent is anhydride acetic acid.

[0241] II. Monolithic polyamic acids and polyimide aerogels from aqueous solutions of polyamic acid salts In another embodiment, a method for forming a polyamic acid aerogel in monolithic form is provided. This method generally includes providing an aqueous solution of a polyamic acid salt, acidifying the polyamic acid salt solution to form a polyamic acid gel, and drying the polyamic acid gel to form a polyamic acid aerogel. In some embodiments, acidifying the polyamic acid salt includes adding delta-gluconolactone to the aqueous solution of the polyamic acid salt to form a gelling mixture, as described with reference to Figure 4, and pouring the gelling mixture into a mold to form a gel. Thus, a polyamic acid gel monolith, as described with reference to Figure 4, can be a starting point for providing a polyamic acid aerogel monolith. In some embodiments, a polyamic acid aerogel monolith can be prepared from a corresponding polyamic acid gel monolith, as shown in Figure 8. Referring to Figure 8, as described earlier in this specification, the polyamic acid gel monolith is washed with water, solvent-exchanged, and dried to provide a polyamic acid aerogel monolith.

[0242] In some embodiments, the method further includes preparing a polyimide gel monolith from a polyamic acid gel monolith. Referring to Figure 8, thermal imidation (for example, by exposing the polyamic acid gel monolith to a temperature of about 300°C for a certain period of time) converts the polyamic acid gel monolith into the corresponding polyimide gel monolith.

[0243] In some embodiments, the method further includes preparing a polyimide aerogel monolith from a polyamic acid aerogel monolith. Referring to Figure 8, thermal imidation (for example, by exposing the polyamic acid gel monolith to a temperature of about 300°C for a certain period of time) converts the polyamic acid aerogel monolith into the corresponding polyimide aerogel monolith.

[0244] In some embodiments, the method further includes preparing polyimide aerogel monoliths from polyimide aerogel monoliths. Referring further to Figure 8, the polyimide gel monoliths are washed with water, solvent-exchanged, and dried to provide polyimide aerogel monoliths, each as previously described herein.

[0245] III. Polyamic Acid and Polyimide Aerogel Beads from Aqueous Solutions of Polyamic Acid Salts A. Droplet method In another embodiment, a method for forming polyamic acid aerogels in bead form is provided. In some embodiments, this method may be the method generally described in Figure 9A. Referring to Figure 9A, this method generally comprises providing an aqueous solution of a polyamic acid salt, acidifying the polyamic acid salt solution to form a polyamic acid gel, and drying the polyamic acid gel to form a polyamic acid aerogel. In some embodiments, acidifying the polyamic acid salt involves adding an aqueous solution of the polyamic acid salt to a solution of a water-soluble acid in water to form polyamic acid gel beads, as described with respect to Figure 5, and the addition includes dropping the aqueous solution of the polyamic acid salt into a solution of a water-soluble acid in water, spraying the aqueous solution of the polyamic acid salt under pressure through one or more nozzles into a solution of a water-soluble acid in water using pressure, or electrolyzing the aqueous solution of the polyamic acid salt into a solution of a water-soluble acid in water. Non-limiting illustrations of the processes that may occur during bead formation are provided in Figure 9B. While we don't want to be bound by theory, it is thought that water-soluble acids (e.g., acetic acid) protonate the carboxylate groups of polyamates, forming an initial skin that is penetrated by the water-soluble acid, and then protonate the carboxylate groups of the ammonium polyamate salt in the droplet, forming wet gel polyamate beads.

[0246] In some embodiments, the polyamic acid gel beads described with reference to Figure 5 are a starting point for providing the polyamic acid aerogel beads of Figure 9A. Referring to Figure 9A, the polyamic acid gel beads are washed with water, solvent-exchanged, and dried, as described earlier in this specification, to provide the polyamic acid aerogel beads.

[0247] In some embodiments, the method further includes preparing polyimide gel beads from polyamic acid gel beads. Referring to Figure 9A, thermal imidation (for example, by exposing polyamic acid gel beads to a temperature of about 300°C for a certain period of time) converts polyamic acid gel beads into the corresponding polyimide gel beads.

[0248] In some embodiments, the method further includes preparing polyimide aerogel beads from polyamic acid aerogel beads. Referring to Figure 9A, thermal imidation (for example, by exposing polyamic acid gel beads to a temperature of about 300°C for a certain period of time) converts polyamic acid aerogel beads into the corresponding polyimide aerogel beads.

[0249] In some embodiments, this method further includes preparing polyimide aerogel beads from polyimide aerogel beads. Referring further to Figure 9A, the polyimide gel beads are washed with water, solvent-exchanged, and dried to provide polyimide aerogel beads, each as previously described herein.

[0250] B. Emulsion method In another embodiment, a method for forming polyamic acid aerogels in bead form is provided. In some embodiments, this method may be the method generally shown in Figure 10. Referring to Figure 10, this method generally includes providing an aqueous solution of a polyamic acid salt, combining the aqueous solution of the polyamic acid salt with a water-immiscible solvent containing a surfactant, mixing the resulting mixture under high shear conditions to form an emulsion, and adding an organic acid to the emulsion.

[0251] Water-immiscible solvents can vary. Suitable solvents include, but are not limited to, oils such as silicone oils or mineral oils, aliphatic hydrocarbons, aromatic hydrocarbons, and chlorinated hydrocarbons. In some embodiments, the solvent is a C5-C12 aliphatic or aromatic hydrocarbon. In certain embodiments, the solvent is a mineral spirit.

[0252] The water-immiscible solvent includes surfactants as previously described herein. In some embodiments, the surfactant includes Tween20, Tween80, Span20, Span40, Span60, Span80, or a combination thereof. In some embodiments, the surfactant is Span20, Tween80, or a mixture thereof. In some embodiments, the surfactant is Hypermer® B246SF. In some embodiments, the surfactant is Hypermer® A70.

[0253] The concentration of the surfactant can vary. In some embodiments, the surfactant, or a mixture of surfactants, is present in a water-immiscible solvent in an amount of about 1% to about 5% by weight, for example, about 1% by weight, about 2% by weight, about 3% by weight, about 4% by weight, or about 5% by weight.

[0254] In some embodiments, the combination involves adding an aqueous solution of ammonium polyamate to an aqueous solution of ammonium polyamate containing a surfactant.

[0255] Mixing biphasic mixtures under high shear conditions generally yields micron-sized polyamic acid beads. The size of polyamic acid wet gel beads can vary. In some embodiments, the wet gel beads have sizes ranging from about 5 to about 500 microns in diameter, for example, about 5, about 10, about 20, about 30, about 40, or about 50 to about 60, about 70, about 80, about 90, about 100, about 200, about 300, about 400, or about 500 microns.

[0256] Continuing to refer to Figure 10, as previously described herein, polyamic acid gel beads are washed with water, solvent-exchanged, and dried to provide polyamic acid aerogel beads.

[0257] In some embodiments, the method further includes preparing polyimide gel beads from polyamic acid gel beads. Referring to Figure 10, thermal imidation (for example, by exposing polyamic acid gel beads to a temperature of about 300°C for a certain period of time) converts polyamic acid gel beads into the corresponding polyimide gel beads.

[0258] In some embodiments, the method further includes preparing polyimide aerogel beads from polyamic acid aerogel beads. Referring to Figure 10, thermal imidation (for example, by exposing polyamic acid aerogel beads to a temperature of about 300°C for a certain period of time) converts polyamic acid aerogel beads into the corresponding polyimide aerogel beads.

[0259] In some embodiments, this method further includes preparing polyimide aerogel beads from polyimide gel beads. Referring further to Figure 10, as each previously described herein, the polyimide gel beads are washed with water, the solvent is changed, and they are dried to provide polyimide aerogel beads.

[0260] IV. Polyamic acid metal salt aerogel beads from aqueous solutions of polyamic acid salts In another embodiment, a method for forming polyamic acid metal salt aerogels in bead form is provided. In some embodiments, this method may be the method generally shown in Figure 11. Referring to Figure 11, this method generally, To provide an aqueous solution of an ammonium or alkali metal salt of a polyamic acid, The process involves performing metal ion exchange, which includes adding a polyamate solution to a solution containing a soluble metal salt to form polyamate metal salt gel beads, This includes drying polyamic acid metal salt gel beads to form polyamic acid metal salt aerogel beads.

[0261] In some embodiments, the salt is prepared as previously described with reference to Figure 2A, Figure 2B, or Figure 2C. In some embodiments, the salt is an ammonium salt. In some embodiments, the salt is an alkali metal salt. This method involves performing metal ion exchange. Referring to Figure 11, the metal ion exchange involves adding a solution of polyamic acid to a solution containing a soluble metal salt. In some embodiments, the addition includes dropping an aqueous solution of polyamic acid into a solution of soluble metal salt, spraying an aqueous solution of polyamic acid into a solution of soluble metal salt under pressure through one or more nozzles, or electro-atomizing an aqueous solution of polyamic acid into a solution of soluble metal salt, with dropping, spraying, and electro-atomizing being as previously described herein, respectively. In certain embodiments, the method involves electro-atomizing a polyamic acid solution through one or more needles at a voltage in the range of about 5 to about 60 kV.

[0262] In some embodiments, the soluble metal salt includes major transition metals, rare earth metals, alkaline earth metals, or combinations thereof. In some embodiments, the soluble metal salt includes copper, iron, nickel, silver, calcium, magnesium, yttrium, or combinations thereof. In some embodiments, the soluble metal salt includes lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or combinations thereof.

[0263] While we do not wish to be bound by theory, it is thought that when droplets of an aqueous solution of an ammonium or alkali metal salt of polyamic acid come into contact with metal ions in the solution containing the soluble metal salt, they form an outer crust of insoluble polyamate metal salt, followed by the migration of ions from the soluble metal salt into the interior of the droplet, thus forming polyamate metal salt gel beads in which a significant portion of the polyamic acid carboxylate groups are bonded to the anions of the soluble metal salt.

[0264] Continuing to refer to Figure 11, as previously described herein, the obtained polyamic acid metal salt gel beads are aged, washed with water, solvent-exchanged, and dried to provide polyamic acid metal salt (polyamate) aerogel beads.

[0265] V. Formation of carbon aerogels from polyimide aerogels In some embodiments, polyimide aerogels (monolithic or beaded) as disclosed herein are thermally decomposed (e.g., carbonized) as illustrated in Figure 12, meaning that the polyimide aerogel is heated to a temperature and time sufficient to convert substantially all of the organic material into carbon. As used herein in the context of thermal decomposition, “substantially all” means that more than 95% of the organic material is converted into carbon, e.g., 99%, 99.9%, 99.99%, or even 100% of the organic material is converted into carbon. Thermal decomposition of a polyimide aerogel converts it into an isomorphic carbon aerogel, meaning that its physical properties (e.g., porosity, surface area, pore size, diameter, etc.) are substantially retained within the corresponding carbon aerogel. The time and temperature required for thermal decomposition can vary. In some embodiments, the polyimide aerogel is subjected to treatment temperatures in the range of approximately 650°C or higher, 800°C or higher, 1000°C or higher, 1200°C or higher, 1400°C or higher, 1600°C or higher, 1800°C or higher, 2000°C or higher, 2200°C or higher, 2400°C or higher, 2600°C or higher, 2800°C or higher, or between any two of these values, for the carbonization of the polyimide aerogel. Generally, the pyrolysis is carried out under an inert atmosphere to prevent combustion of the organic or carbon material. Preferred atmospheres include, but are not limited to, nitrogen, argon, or a combination thereof. In some embodiments, the pyrolysis is carried out under nitrogen.

[0266] VI. Formation of carbon aerogels from polyamic acid aerogels In some embodiments, polyamic acid aerogels (monolithic or bead-like) as disclosed herein are thermally decomposed as illustrated in Figure 13. Surprisingly, according to this disclosure, it has been found that polyamic acid aerogels can be directly converted to carbon aerogels (i.e., without a first imidation to provide polyimide aerogels). Thermal decomposition of polyamic acid aerogels converts them to isomorphic carbon aerogels. The time and temperature required for thermal decomposition can vary. In some embodiments, polyamic acid aerogels are subjected to treatment temperatures in the range of about 650°C or higher, 800°C or higher, 1000°C or higher, 1200°C or higher, 1400°C or higher, 1600°C or higher, 1800°C or higher, 2000°C or higher, 2200°C or higher, 2400°C or higher, 2600°C or higher, 2800°C or higher, or between any two of these values, for carbonization of the polyamic acid aerogel. Generally, pyrolysis is carried out under an inert atmosphere to prevent combustion of organic or carbon materials. Suitable atmospheres include, but are not limited to, nitrogen, argon, or combinations thereof. In some embodiments, pyrolysis is carried out under nitrogen.

[0267] Further reference to Figure 13, optionally, a polyamic acid aerogel can be thermally imidized as disclosed herein to first provide a polyimide aerogel, and then subsequently thermally decomposed to provide a carbon aerogel.

[0268] VII. Formation of metal or metal oxide-doped carbon aerogels from polyamic acid metal salt aerogels In some embodiments, polyamic acid metal salt aerogels (monolithic or beaded) as disclosed herein are thermally decomposed as illustrated in Figure 14. Thermal decomposition of polyamic acid metal salt aerogels converts them into isomorphic carbon aerogels. The time and temperature required for thermal decomposition can vary. In some embodiments, polyamic acid metal salt aerogels are subjected to treatment temperatures in the range of about 650°C or higher, 800°C or higher, 1000°C or higher, 1200°C or higher, 1400°C or higher, 1600°C or higher, 1800°C or higher, 2000°C or higher, 2200°C or higher, 2400°C or higher, 2600°C or higher, 2800°C or higher, or between any two of these values, for carbonization of the polyimide aerogel.

[0269] Upon thermal decomposition, the ions of the soluble metal salt present may form the corresponding metal oxide or sinter and form the corresponding metal, depending on the metal species and thermal decomposition conditions.

[0270] VIII. Formation of electroactive material-doped gels In some embodiments, an electroactive material can be used to dope any of the polyimide or polyamic acid gels and aerogels disclosed herein with silicon, such as silicon particles, to provide electroactive material-doped polyamic acid, polyimide, or carbon gel (wet gel, aerogel, monolith, or beads).

[0271] In the context of this disclosure, the term “silicon particles” refers to silicon or silicon-based materials having a particle size range suitable for use with polyimides or carbon gels, as disclosed herein. The silicon particles of this disclosure may be nanoparticles, e.g., two-dimensional or three-dimensional particles in the range of about 1 nm to about 150 nm. The silicon particles of this disclosure may be microparticles, e.g., micron-sized particles, having a maximum dimension, e.g., diameter, of substantially spherical particles in the range of about 150 nm to about 10 micrometers or more. For example, the silicon particles of this disclosure may have a maximum dimension, e.g., diameter, of substantially spherical particles in the range of about 10 nm, 50 nm, 100 nm, 150 nm, 200 nm, 500 nm, 1 micrometer, 1.5 micrometers, 2 micrometers, 3 micrometers, 5 micrometers, 10 micrometers, 20 micrometers, 40 micrometers, 50 micrometers, 100 micrometers, or in the range of any two of these values. In some embodiments, the particles are flat, fragmented shapes, e.g., small plates, having two dimensions, e.g., length and width, in a range of about 10 nm, 50 nm, 100 nm, 150 nm, 200 nm, 500 nm, 1 micrometer, 1.5 micrometers, 2 micrometers, 3 micrometers, 5 micrometers, 10 micrometers, 20 micrometers, 40 micrometers, 50 micrometers, 100 micrometers, or any two of these values. In some embodiments, the silicon particles may be monodisperse or substantially monodisperse. In other embodiments, the silicon particles may have a particle size distribution. In the context of this disclosure, the dimensions of the silicon particles are provided based on the median of the particle size distribution, i.e., D50. The silicon particles of this disclosure include silicon wire, crystalline silicon, amorphous silicon, silicon alloy, and silicon oxide (SiO₂). x), coated silicon, for example, carbon-coated silicon, and any combination of the silicon particle materials disclosed herein. In some embodiments, the silicon particles may be substantially planar flakes, i.e., having a flat, fragmented shape which may also be referred to as plate-like. For example, a particle has two substantially flat main surfaces connected by a subsurface that defines the thickness between the main surfaces. In other embodiments, the particles of silicon or other electroactive material may be substantially spherical, cubic, obroidal, elliptical, disc-shaped, or toroidal.

[0272] Silicon particles can be produced by various techniques, including electrochemical reduction and mechanical milling, i.e., grinding. Grinding can be carried out using wet or dry processes. In dry grinding processes, powder is added to a container along with a grinding medium. The grinding medium typically includes balls or rods of zirconium oxide (yttrium stabilized), silicon carbide, silicon dioxide, quartz, or stainless steel. The particle size distribution of the resulting ground material is controlled by the energy applied to the system and by matching the particle size of the starting material to the size of the grinding medium. However, dry grinding is an inefficient and energy-intensive process. Wet grinding is similar to dry grinding but involves the addition of a grinding liquid. The advantage of wet grinding is that the energy consumption to obtain the same results is 15-50% lower than in the case of dry grinding. A further advantage of wet grinding is that the grinding liquid can protect the ground material from oxidation. It has also been found that wet grinding can produce finer particles and result in less particle aggregation.

[0273] Wet grinding can be carried out using a wide variety of liquid components. In exemplary embodiments, the grinding liquid or components contained therein are selected to reduce or eliminate chemical functionalization on the surface of silicon particles during or after grinding. In other embodiments, the grinding liquid or components contained therein are selected to provide desired surface chemical functionalization of particles, e.g., silicon particles, during or after grinding. Components contained therein may also be selected to control the chemical reactivity or crystalline morphology of particles, e.g., silicon particles. In exemplary embodiments, components contained therein may be selected based on their compatibility or reactivity with downstream materials, processing steps, or uses of particles, e.g., silicon particles. For example, components contained therein may be compatible with, useful for, or identical to liquids or solvents used in processes for forming or manufacturing organic or inorganic aerogel materials. In yet another embodiment, the grinding solution may be selected so that the grinding solution or components contained therein react with an organic or inorganic aerogel material to form a coating on the surface of silicon particles or on intermediate species such as aliphatic or aromatic hydrocarbons, or to cross-link or form cross-functional compounds.

[0274] The solvent or solvent mixture used for grinding may be selected to control the chemical functionalization of the particles during or after grinding. Using silicon as an example, but without being bound by theory, grinding silicon in an alcohol-based solvent such as isopropanol can functionalize the surface of the silicon, allowing alkyl surface groups, such as isopropyl, to be covalently bonded to the surface of the silicon particles. Upon exposure to air, the alkyl groups can be converted to the corresponding alkoxides through oxidation, as demonstrated by FTIR-ATR analysis. In exemplary embodiments, grinding may be carried out in polar aprotic solvents such as DMSO, DMF, NMP, DMAC, THF, 1,4-dioxane, diglyme, acetonitrile, water, or any combination thereof.

[0275] Electroactive material (e.g., silicon) particles can be incorporated into polyamic acids, polyimides, or carbon gels disclosed herein by several methods. Generally, electroactive material (e.g., silicon) particles are incorporated during a sol-gel process. In one non-limiting embodiment, electroactive material (e.g., silicon) particles are dispersed in a polyamic acid sol before imidation. In some embodiments, electroactive material (e.g., silicon) particles are dispersed in a solvent, such as water or a polar aprotic solvent, before combination with a polyimide precursor. In some embodiments, electroactive material (e.g., silicon) particles are dispersed in a polyamic acid sol during the imidation process. In some embodiments, the electroactive material is added to an aqueous solution of a polyamic acid salt. In some embodiments, the electroactive material is silicon.

[0276] IX. Properties of polyimides, polyamic acids, and carbon aerogels In some embodiments, aerogels such as those disclosed herein (e.g., polyamic acids, polyimides, or carbon aerogels) may take the form of a monolith. As used herein, the term “monolith” refers to an aerogel material in which the majority (by weight) of the aerogel contained therein is in the form of a macroscopic, single, continuous, self-supporting object. Monolithic aerogel materials include aerogel materials that are initially formed to have a clearly defined shape but can subsequently crack, shatter, or segment into non-self-repeating objects. For example, an irregular mass can be considered a monolith. Monolithic aerogels may take the form of a self-supporting structure or the form of a reinforced material having fibers or interpenetrating foams.

[0277] In other embodiments, the aerogels of the Disclosure (e.g., polyamic acid, polyimide, or carbon aerogel) may be in the form of particulate matter, such as beads or particles, from, for example, the crushing of a monolithic material or from a preparation method intended for bead formation. Aerogels in particulate matter form may have a variety of particle sizes. For spherical particles (e.g., beads), particle size is the diameter of the particle. For irregular particles, the term particle size refers to the maximum dimension (e.g., length, width, or height). Particle size may vary depending on the physical form, the preparation method, and any subsequent physical steps performed. In some embodiments, aerogels in particulate matter form may have particle sizes ranging from about 1 micrometer to about 10 millimeters. For example, aerogels in the form of microparticles are approximately 1 micrometer, 2 micrometers, 3 micrometers, 4 micrometers, 5 micrometers, 6 micrometers, 7 micrometers, 8 micrometers, 9 micrometers, 10 micrometers, 15 micrometers, 20 micrometers, 25 micrometers, 30 micrometers, 35 micrometers, 40 micrometers, 45 micrometers, 50 micrometers, 60 micrometers, 70 micrometers, and 80 micrometers. The aerogel may have particle sizes ranging from about 90 micrometers, about 100 micrometers, about 200 micrometers, about 300 micrometers, about 400 micrometers, about 500 micrometers, about 600 micrometers, about 700 micrometers, about 800 micrometers, about 900 micrometers, about 1 millimeter, about 2 millimeters, about 3 millimeters, about 4 millimeters, about 5 millimeters, about 6 millimeters, about 7 millimeters, about 8 millimeters, about 9 millimeters, about 10 millimeters, or in the range between any two of these values. In some embodiments, the aerogel may have particle sizes ranging from about 5 micrometers to about 100 micrometers, or from about 5 to about 50 micrometers. In some embodiments, the aerogel may have particle sizes ranging from about 1 to about 4 millimeters.

[0278] Aerogels such as those disclosed herein have density. As used herein, the term “density” refers to a measured mass per unit volume of an aerogel material or composition. The term “density” generally refers to the true density or skeletal density of an aerogel material, as well as the bulk density of an aerogel composition. Density is typically expressed in kg / m³. 3 or g / cm 3 The density of the polyimide or carbon aerogel skeleton may be determined by methods known in the art, including but not limited to helium pycnometry. The bulk density of the polyimide or carbon aerogel may be determined by methods known in the art, including but not limited to: Standard Test Method for Dimensions and Density of Preformed Block and Board-Type Thermal Insulation (ASTM C303, ASTM International, West Conshohocken, Pa.); Standard Test Methods for Thickness and Density of Blanket or Batt Thermal Insulations (ASTM C167, ASTM International, West Conshohocken, Pa.); or Determination of the apparent density of preformed pipe insulation (ISO 18098, International Organization for Standardization, Switzerland). In the context of this disclosure, unless otherwise specified, density measurements are obtained according to the ASTM C167 standard. In some embodiments, the polyimide or carbon aerogel disclosed herein is present in a concentration of about 0.01 to about 0.3 g / cm³. 3 It has bulk density.

[0279] Aerogels such as those disclosed herein have a pore size distribution. As used herein, the term “pore size distribution” refers to the statistical distribution or relative quantity of each pore size within a sample volume of a porous material. A narrower pore size distribution refers to a relatively large proportion of pores within a narrow range of pore sizes. In some embodiments, a narrow pore size distribution may be desired, for example, to optimize the amount of pores that can surround electrochemically active species and to maximize the use of available pore volume. Conversely, a broader pore size distribution refers to a relatively small proportion of pores within a narrow range of pore sizes. Thus, the pore size distribution is typically measured in relation to pore volume and recorded as the unit size of the full width at half maximum of the major peak in a pore size distribution chart. The pore size distribution of a porous material can be determined by methods known in the art, including, but not limited to, surface area, skeleton density, and porosimetry, from which the pore size distribution can be calculated. Preferred methods for determining such characteristics include, but are not limited to, measurements such as gas adsorption / desorption (e.g., nitrogen), helium pycnometry, and mercury porosimetry. Unless otherwise specified, the pore size distribution measurements reported herein are obtained by nitrogen sorption analysis. In certain embodiments, the polyimides or carbon aerogels of this disclosure have a relatively narrow pore size distribution.

[0280] Aerogels such as those disclosed herein have pore volume. As used herein, the term “pore volume” refers to the total volume of pores in a sample of porous material. Specifically, pore volume is measured as the volume of voids in a porous material, typically in cubic centimeters (cm³) per gram. 3The volume is recorded as ( / g or cc / g). The pore volume of porous materials can be determined by methods known in the art, including but not limited to surface area and porosity analysis (e.g., nitrogen porosimetry, mercury porosimetry, helium pycnometry, etc.). In certain embodiments, the polyimides or carbon aerogels of this disclosure have relatively large pore volumes in the range of about 1 cc / g or more, 1.5 cc / g or more, 2 cc / g or more, 2.5 cc / g or more, 3 cc / g or more, 3.5 cc / g or more, 4 cc / g or more, or between any two of these values. In other embodiments, the polyimides or carbon aerogels and xerogels of the Disclosure have pore volumes of approximately 0.03 cc / g or more, 0.1 cc / g or more, 0.3 cc / g or more, 0.6 cc / g or more, 0.9 cc / g or more, 1.2 cc / g or more, 1.5 cc / g or more, 1.8 cc / g or more, 2.1 cc / g or more, 2.4 cc / g or more, 2.7 cc / g or more, 3.0 cc / g or more, 3.3 cc / g or more, 3.6 cc / g or more, or in a range between any two of these values.

[0281] In some embodiments of this disclosure, the aerogel (monolith or beads of polyamic acid, polyimide, or carbon aerogel or xerogel) may include a fibrous form. In the context of this disclosure, the term “fibrous form” refers to a structural form of a nanoporous material (e.g., carbon aerogel) that includes struts, rods, fibers, or filaments.

[0282] In some embodiments, carbon aerogels produced by any of the disclosed methods have properties substantially similar to those of carbon aerogels prepared by thermal decomposition of the corresponding polyimide aerogel prepared by a conventional non-aqueous method.

[0283] In some embodiments, the polyimide gel prepared by any of the disclosed methods contains more than about 75% by volume of residual water before any solvent exchange or drying.

[0284] In some embodiments, the polyimide aerogel prepared by any of the disclosed methods is a solid 15 It contains terminal amine groups, as determined by N-NMR. In some embodiments, the polyamic acid aerogel prepared by any of the disclosed methods is a solid. 15 It contains terminal amine groups, as determined by N-NMR.

[0285] All methods described herein may be carried out in any preferred order, unless otherwise indicated herein or unless otherwise clearly contradicted by the context. The use of any examples or illustrative language provided herein (e.g., "etc.") is intended solely to better describe the materials and methods and does not limit their scope unless otherwise claimed. Nothing in this specification should be construed as indicating that any unclaimed element is essential for carrying out the disclosed materials and methods.

[0286] It will be readily apparent to those skilled in the art that suitable modifications and adaptations to the compositions, methods, and uses described herein can be made without departing from the scope of any embodiment or its applicability. The compositions and methods provided are illustrative and are not intended to limit the scope of the claimed embodiments. All of the various embodiments, applicabilities, and options disclosed herein can be combined in all variations. The scope of the compositions, formulations, methods, and processes described herein includes all actual or potential combinations of the embodiments, applicabilities, options, examples, and preferences herein.

[0287] Although the techniques described herein have been explained with reference to specific embodiments, these embodiments should be understood as merely illustrative examples of the principles and applications of the techniques. It will be apparent to those skilled in the art that various modifications and changes can be made to the methods and apparatus of the techniques without departing from the spirit and scope of the techniques. Accordingly, the techniques are intended to include modifications and changes that fall within the scope of the appended claims and their equivalents.

[0288] Throughout this specification, any reference to “one embodiment,” “a particular embodiment,” “one or more embodiments,” or “embodiment” means that any particular feature, structure, material, or property described in relation to an embodiment is included in at least one embodiment of this art. Therefore, the appearance of phrases such as “in one or more embodiments,” “in a particular embodiment,” “in one embodiment,” or “in an embodiment” in various places throughout this specification does not necessarily refer to the same embodiment of this art. Furthermore, any particular feature, structure, material, or property may be combined in any preferred manner in one or more embodiments. All scopes referenced herein are inclusive.

[0289] Aspects of this technology will be described more fully with reference to the following examples. Before describing some exemplary embodiments of this technology, it should be understood that this technology is not limited to the structural or process step details described below. Other embodiments of this technology are possible and can be practiced or executed in various ways. The following examples are given to illustrate certain aspects of this technology and should not be construed as limiting them. [Examples]

[0290] The present invention can be further illustrated by the following non-limiting examples describing methods.

[0291] Example 1. Water-based preparation of polyimides and carbon aerogel monoliths using in-situ-formed polyamic acids from 1,4-phenylenediamine (PDA) and pyromellitic dianhydride (PMDA) in water. 1,4-Phenylenediamine (PDA; 1.66 g, 15.3 mmol) was dissolved in 100 mL of water. Triethylamine (TEA; 3.4 g, 4.69 mL, 33.7 mmol, 2.2 equivalents relative to PDA) was added to the solution, followed by solid pyromellitic dianhydride (PMDA; 3.34 g, 15.3 mmol). This solution was stirred at room temperature for 5 days. At the end of this period, 4.3 equivalents of acetic anhydride (6.7 g, 6.19 mL, 65.8 mmol) were added to the solution. The expected target density T of the aerogel was determined. d is 0.040 g / cm³ 3 The new solution was divided into cylindrical molds and gelled within approximately 3 minutes. The resulting gel was aged in the mold for 1 day. After 24 hours, the wet gel was removed from the mold by injecting ethanol into the mold, and the wet gel was washed three times with ethanol. The resulting wet gel was treated with supercritical fluid (SCF) carbon dioxide to form a polyimide aerogel monolith (Example 1A). The polyimide aerogel was then solidified. 13 C and 15 The properties were characterized by N NMR (Figures 15A and 15B, respectively). In addition to the resonance of the imide (-(C=O)-N-(C=O)-) functional group at approximately 176 ppm, the solid 15 The N NMR spectra also showed the presence of amide functional groups (approximately 133 ppm) and free amine-related groups (approximately 54.3 ppm). Notably, the resonance at approximately 54.3 ppm was either very low in intensity or completely absent in the corresponding spectra of the same diamine and dianhydride polyimides when prepared by conventional organic solvent-based methods (i.e., in N,N-dimethylacetamide, as in Reference Example 8). Photographs of the polyimide aerogel are provided in Figure 16A.

[0292] These aerogel samples were carbonized under nitrogen at 1050°C to provide corresponding carbon aerogel monoliths (Example 1B). A photograph of aerogel 1B is provided in Figure 16B. Scanning electron microscope images of the samples from Examples 1A and 1B are provided in Figures 17A and 17B, respectively. Figures 17C and 17D show the pore size distributions of the samples from Examples 1A and 1B, respectively. The properties of the aerogels from Examples 1A and 1B are provided in Table 2 below. [Table 2]

[0293] Examples 2-7. Water-based preparation of polyimide aerogel monoliths using in situ-formed polyamic acids from PDA and PMDA in water. Polyimide aerogel samples were prepared using the procedure of Example 1, except that the length of time the PDA-PMDA reaction was allowed to proceed before the addition of acetic anhydride was varied (from 1 hour to 4 days). The polyimide aerogel was then solidified as shown in the stacked spectra of Figures 18A and 18B, respectively. 13 C and 15 Each sample was characterized by N NMR (from top to bottom: samples from Examples 1, 7, 6, 5, 4, 3, and 2). The spectral characteristics were similar regardless of the PDA-PMDA reaction time. The acceptable reaction times before gelation to polyimide gel and the corresponding imide-to-amide group ratios in the aerogels obtained from Examples 1-7 are provided in Table 3 below. The imide-to-amide group ratio is for solids. 15 The result was obtained by integrating the N NMR spectrum (Figure 18B). [Table 3]

[0294] As shown in Table 3, all aerogels in Examples 1–7 contained 4.36–6.83 more imide groups than amide groups. The calculated ratios were based on the ratio of integrated peak intensities in Figure 18B, multiplied by the response coefficient.

[0295] Each sample of polyimide aerogel was carbonized as shown in Example 1. The carbonization yield remained constant at 40–42% throughout the reaction time (Figure 19A).

[0296] Figures 19B–19D provide plots of pore size distribution, surface area, and bulk density against reaction time for the polyimides and corresponding carbon aerogel monoliths of Examples 1–7, respectively. As shown in Figures 19A–19D, the properties of the polyimides and carbon aerogels remained similar to each other over the reaction time.

[0297] Example 8. Preparation of PMDA-PDA polyimide monoliths in organic solvents (reference example) The reference polyimide aerogel monolith is subjected to the same target density (T) as in Example 1. d = 0.040 g / cm³ 3 The solution was prepared using the following method, however, N,N-dimethylacetamide was used as the solvent instead of water, and the PDA-PMDA coupling reaction was carried out for 3 hours, after which triethylamine and anhydride acetic acid were added.

[0298] Scanning electron microscope images of a reference polyimide aerogel sample are provided in Figure 20A (Figure 20B), along with comparative images of the material sample from Example 1A. As illustrated in Figures 20A and 20B, Example 1A of the present invention (Figure 20B) had a fibrous structure similar to that of a material prepared by a conventional method (Example 8, Figure 20A).

[0299] Example 9. Water-based preparation of millimeter-sized PMDA-PDA polyimide aerogel beads using pre-formed and isolated polyamic acids from PMDA and PDA. Millimeter-sized polyimide gel beads were prepared by gelling an aqueous triethylammonium salt solution of polyamic acid prepared from a previously formed solid polyamic acid. 13 C and 15The N NMR spectra (products of the PDA-PMDA coupling reaction in a 1:1 molar ratio in N,N-dimethylacetamide as the solvent) are shown in Figures 21A and 21B, respectively. The solid polyamic acid was isolated by slowly adding the reaction mixture to water. 15 Note that the N NMR spectrum includes a weak resonance at 175 ppm, attributed to a small amount of polyimide, and a low-intensity resonance at 48.2 ppm, attributed to the terminal amine. For comparison, Figure 22A shows the solid of a 1:1 mol / mol mixture of PDA and PMDA stirred in water for 24 hours. 15 The N NMR spectra are shown. Resonances for various forms of unreacted aromatic amines are shown up to 52.5 and 50.4 ppm. Also, Figure 22B shows the solid mixture of PDA and PMDA reacted in water in the presence of TEA for 24 hours (as shown in Examples 1-7). 15 The N NMR spectrum is shown. The expected product was the triethylammonium salt of the polyamic acid, which was isolated by adding the reaction mixture to acetone. Triethylammonium resonance appeared at 55.0 ppm.

[0300] Solid polyamic acid was suspended in 100 mL of water and dissolved by adding triethylamine (3.41 g, 4.7 mL, 2.2 mol excess relative to the polyamic acid repeating units). To this solution, anhydride acetate (6.73 g, 6.22 mL, 4.3 mol excess relative to the polyamic acid repeating units) was added, and the fresh solution was vigorously stirred with a magnetic stirrer for approximately 1.5 minutes. The target density of the sol was 0.045 g / cm³. 3This gelled in approximately 3 minutes. Two minutes after the addition of anhydride acetic acid, the sol was dropwise added to 100 mL of a receiving solution consisting of hexane:acetic acid (90:10 v / v). Millimeter-sized beads formed from each droplet of sol entering the receiving solution were retained in the solution for 12 hours. They were then washed four times with 100 mL of water for 2 hours each time and dried with supercritical fluid CO2. Based on IR spectroscopy, the aerogel beads contained polyimide. The beads were then thermally decomposed at 1050°C under liquid nitrogen. Photographs of the resulting carbon aerogel bead aggregates were taken with a digital camera and the images were analyzed using the ImageJ software package. From the histogram (Figure 23A), the average diameter of the carbon aerogel beads was calculated to be 2.16 ± 0.09 mm.

[0301] Under an electron microscope, the beads had a denser skin, and their interiors consisted of interconnected short fibers (Figure 23B), similar to their monolithic counterparts (Figures 17A, 17B, and 20B). The carbide yield was 32.6% w / w. The pore size distribution in the range of 1.7–300 nm was relatively broad, with a maximum value at approximately 41 nm (Figure 23C). The properties of the carbon aerogel beads of Example 9 are summarized in Table 4 below. [Table 4]

[0302] Example 10. Water-based preparation of micron-sized PMDA-PDA polyimide aerogel beads using pre-formed and isolated polyamic acids from PMDA and PDA. Micron-sized polyimide gel beads, 0.07 g / cm³ 3The target density was achieved by gelation of an aqueous triethylammonium salt solution of polyamic acid in an emulsion. Solid polyamic acid (isolated as a product of the PDA-PMDA coupling reaction in N,N-dimethylacetamide as a solvent) (see Figures 21A and 21B) was dissolved in a mixture of 50 g of water and 3.45 g of triethylamine (TEA; mol / mol ratio of TEA to PMDA 2.2:1). After stirring for 3 hours, anhydride acetate (6.73 g, mol / mol ratio of 4.25 to PMDA) was added, and the mixture was stirred for 30 seconds. At the end of this period, the sol was poured into the immiscible phase under shear at 3800 rpm using a Ross mixer. The immiscible phase was prepared by mixing 9.7 g of surfactant (Hypermer® B246SF, HLB of 6) in 500 mL of hexane. The sol was added to the hexane phase in a v / v ratio of 1:8. Gelation occurred in 3.2 minutes at room temperature. After stirring under high shear for 8 minutes, the mixture was removed from the Ross mixer and aged for 35 minutes. Water (300 mL) was added, and the two-phase mixture was briefly stirred. The hexane layer was removed by decanting or using a separatory funnel. The gel beads were recovered from the aqueous phase by vacuum filtration, ensuring they did not dry on a filter, and aged in ethanol at 68°C. Three ethanol solvent changes were performed before drying. Micrographs of the gel beads after aging in ethanol are provided in Figure 24.

[0303] The gel beads were divided into two batches and dried either using supercritical CO2 or in an oven at 68°C. The former batch (Example 10A) is referred to herein as aerogel beads, and the latter (Example 10B) is referred to as xerogel beads. The aerogel beads were 465–516 m 2 It had a surface area of ​​ / g.

[0304] As shown in the micrograph in Figure 25, the polyimide xerogel beads had diameters of 2–15 microns. The FTIR-ATR spectra of these xerogel beads (Figure 26) were 1714 and 1775 cm⁻¹. -1 (C=O) and 1367cm-1 It contained characteristic features of polyimide having a (CN) band.

[0305] Carbonization of the beads was carried out at 1050°C for 2 hours under nitrogen using a ramp rate of 3°C per minute. Scanning electron microscope images of the carbonized aerogel beads are provided in Figures 27A and 27B. In cross-section (Figure 27B), the carbon aerogel beads showed a mesoporous fibrous microstructure. In contrast, a carbon xerogel of the same formulation (Example 10B) showed a more compact microstructure with smaller pores (Figure 27C).

[0306] Example 11. Water-based preparation of PMDA-PDA polyimide aerogel beads using pre-formed and isolated polyamic acids and various surfactants. Polyimide gel beads were prepared as shown in Example 10, except that magnetic rod stirring was used instead of high-shear mixing. Various surfactants and their mixtures were used in an immiscible phase at a concentration of 2 g / 100 mL, as shown in Table 5 below. [Table 5]

[0307] The polyimide beads had a diameter of 100–200 microns. Carbonization of the beads was carried out at 1050°C for 2 hours under nitrogen using a ramp rate of 3°C per minute.

[0308] SEM images of the interior of xerogel carbide beads prepared in hexane using Span80 / Tween80 (Example 11A) are provided in Figure 28, which show that these beads have a less porous shell compared to the core.

[0309] A series of SEM images (Example 11D) of xerogel carbide beads prepared in hexane using Span20 are provided in Figures 29A–29C. These beads exhibited unique morphologies with a mesoporous core and a denser shell. These core / shell morphologies may be advantageous for specific applications.

[0310] Carbide xerogel beads (Example 11C) prepared using Span80 were condensed together and fractured during the process. The fragments statically adhering to the beads were observed using a low-resolution SEM.

[0311] Not all surfactants resulted in spherical beads. For example, using a mixture of Span65 and Tween80 in hexane (Example 11B) yielded elongated or irregularly shaped carbonized particles, while Span80 in silicone oil (Example 11C) resulted in aggregates of small, condensed beads.

[0312] Polyimide gels prepared in the presence of Hypermer® B246SF surfactant (Ex. 11E) are strong, and different spherical beads are obtained after carbonization, which may be advantageous for certain applications.

[0313] The weight loss due to carbonization of polyimide beads at 1050°C is shown in Table 6. [Table 6]

[0314] In a further example, polyimide gel beads were prepared as described above, but without a surfactant, and silicone oil was used as the immiscible phase. The FTIR-ATR spectra showed that they were imidized, at 1368 cm⁻¹. -1 The band is characteristic of the CN stretch resonance of the imide. Carbonization of the beads was carried out at 1050°C for 2 hours under nitrogen using a ramp rate of 3°C per minute. The surface area of ​​these carbonized xerogel beads is 41.5 m². 2 It was / g.

[0315] Example 12. Water-based preparation of polyimide aerogel microbeads using in-situ-formed polyamic acid from PMDA and PDA in water. Polyimide gel beads, approximately 0.088 g / cm³ 3 The target density was achieved by gelling an aqueous triethylammonium salt solution of polyamic acid in an emulsion. For this purpose, PDA (1.68 g, 1:1 mol / mol ratio to PMDA) was mixed with 50 g of water and 3.72 g of triethylamine (2.37:1 mol / mol ratio to PMDA) for 1 hour. PMDA (3.38 g, 0.0155 mol) was added to the mixture and stirred at room temperature for 1 to 4 days. Acetyl anhydride (6.73 g, 4.25 mol / mol ratio to PMDA) was added to the obtained triethylammonium salt solution of polyamic acid and the mixture was stirred for 30 seconds. At the end of this period, the sol was poured into the immiscible phase under high shear using a Ross mixer at 1500 rpm. The immiscible phase was prepared by dissolving 7.5 g of the surfactant Hypermer® B246SF (HLB 6) in 500 mL of hexane or cyclohexane. The sol was added to the immiscible phase in a ratio of 1:8 v / v, and the mixture was stirred for 15 minutes. Gelation occurred in 3.5 minutes at room temperature. The mixture was removed from the Ross mixer, and the hexane phase was decanted. Water (500 mL) was added to the gel beads. After a brief stir, the aqueous layer was separated by decanting. The gel beads were placed in ethanol, and the aggregates were dispersed for 1 minute by probe sonication. The beads were further treated as in Example 10, with three ethanol exchanges at 68°C before drying.

[0316] Example 13. Water-based preparation of silicon-doped polyimide aerogel microbeads using in situ-formed polyamic acids from PMDA and PDA. Polyimide gel beads, approximately 0.088 g / cm³ 3The target density was achieved by gelling a polyamic acid triethylammonium salt emulsion. For this purpose, 1.68 g of PDA (1:1 mol / mol ratio to PMDA) was added to a mixture of 50 g of water and 3.72 g of triethylamine (2.37:1 mol / mol ratio to PMDA), and the solution was stirred for 1 hour. 3.38 g of PMDA (0.0155 mol) was added to the mixture, and the fresh solution was stirred at room temperature for 4 days. Using 1.78 g of zirconia medium and a FlackTek centrifugal mixer, 1.78 g of silicon powder (4.10 mol / mol ratio to PMDA) was dispersed in 10 g of the resulting polyamic acid triethylammonium salt solution for 5 minutes. The silicon powder had a particle size of 178 nm. The silicon dispersion was added to the remainder of the polyamic acid solution, and the mixture was stirred for 5 minutes. To this solution, anhydrous acetic acid (6.73 g, 4.25 mol / mol ratio relative to PMDA) was added, and the mixture was stirred for 30 seconds. At the end of this period, the silicon dispersion was poured into the immiscible phase at a ratio of 1:8 v / v under high shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 7.3 g of surfactant (Hypermer® B246SF, HLB 6) in 600 mL of hexane or cyclohexane. Gelation occurred in 3.5 minutes at room temperature. The mixture was removed from the Ross mixer after 16 minutes and stirred with 300 mL of water for 2 hours. A stable emulsion was formed at this point. The emulsion was broken by adding 1 L of ethanol, the gel beads were separated and treated as in Example 12.

[0317] Example 14. Water-based preparation of silicon-doped polyimide xerogel microbeads using pre-formed and isolated polyamic acids from PMDA and PDA. Polyimide gel beads, approximately 0.10 g / cm³ 3The target density was achieved by gelation of an aqueous triethylammonium salt solution of polyamic acid in an emulsion. Using a probe sonicator, 0.946 g of silicon powder (1.61 mol / mol ratio relative to PMDA) was sonicated in 46 g of water for 2 minutes. 4.68 g of triethylamine (2.2 mol / mol ratio relative to PMDA) was added to the silicon dispersion, and the mixture was stirred for 5 minutes. Solid polyamic acid (6.8 g; pre-prepared and isolated from the condensation reaction of PDA and PMDA in N,N-dimethylacetamide) was added to this dispersion, and the mixture was stirred for 2 hours. At the end of this period, 9.12 g of anhydride acetic acid (4.25 mol / mol ratio relative to PMDA) was added, and the mixture was mixed for 10 seconds. The viscous dispersion was rapidly poured into an immiscible phase consisting of 7 g of surfactant (Hypermer® B246SF, HLB 6) dissolved in 500 mL of hexane. An aqueous solution of the triethylammonium salt of polyamic acid was added to the hexane phase in a 1:8 v / v ratio. Gelation occurred in 2 minutes at room temperature. After stirring at 4500 rpm for 6 minutes, the mixture was removed from the Ross mixer and allowed to stand for 60 minutes. Water (300 mL) was added and briefly mixed. The hexane layer was separated by decanting, and the gel beads were processed as described in Example 12. The spherical wet gel beads had diameters ranging from 5 to 60 microns (Figure 30). Silicon particles were clearly visible and randomly dispersed within the wet gel beads. After processing and drying the beads in an oven, the beads shrank as expected for xerogels, while maintaining their spherical shape.

[0318] Similarly, silicon-doped polyimide gel beads are prepared according to the method described in the previous paragraph at a concentration of 0.08 g / cm³. 3 The target density was achieved by gelling a polyamic acid solution. After drying the gel beads in a 68°C oven, the silicon-doped polyimide xerogel beads were determined to be 1.51 m³ by nitrogen sorption analysis. 2 Surface area and 0.028 cm² / g 3 The pore volume per gram is shown.

[0319] Example 15. Water-based preparation of silicon-doped polyimide aerogel microbeads using pre-formed and isolated polyamic acids from PMDA and PDA. The polyimide gel was prepared as described in Example 14, but at a target density of approximately 0.08 g / cc. The silicon particles were clearly visible within the wet gel beads and were randomly dispersed, similar to those in Figure 30. After drying using supercritical CO2 extraction, the silicon-doped aerogel beads exhibited particle sizes ranging from 10 to 70 microns, as shown in Figure 31A. The surface area of ​​these aerogel beads was 328.48 m². 2 The pore volume is 1.92 cm³ / g. 3 The concentration was / g. Carbonization of the beads was carried out at 1050°C for 2 hours under nitrogen using a ramp rate of 3°C per minute. SEM images of the outer surface and inner core of these carbon aerogel beads (Figures 31B and 31C, respectively) showed a nearly spherical shape with a porous fibrous internal structure containing silicon flakes dispersed throughout the carbon matrix.

[0320] Example 16. Water-based preparation of polyimide aerogel microbeads using pre-formed and isolated polyamic acids from PMDA and PDA having diisopropylethylamine as a non-nucleophilic amine. Polyimide aerogel beads were prepared as described in Example 15, except that diisopropylethylamine was used as the non-nucleophilic amine instead of triethylamine. Polyimide aerogel beads were prepared with and without silicon doping. Micrographs of the undoped polyimide aerogel beads are shown in Figure 32.

[0321] Example 17. Water-based preparation of polyamic acid aerogel beads, polyimide aerogel beads, and corresponding carbon aerogel beads from aqueous solutions of triethylammonium salts of pre-formed and isolated polyamic acids added to aqueous acetic acid / acetic anhydride. Starting with pre-formed and isolated solid polyamic acid obtained from the reaction of stoichiometric amounts of 1,4-phenylenediamine and pyromellitic dianhydride in N,N-dimethylacetamide, polyamic acid and polyimide beads were prepared. 13 1C NMR and 15 The N NMR spectra are shown in Figure 21A and Figure 21B, respectively.

[0322] Solid polyamic acid (5 g) was suspended in 20 mL of water and dissolved by adding triethylamine (3.4 g, 4.7 mL, 2.2 mol excess relative to the polyamic acid repeating units). The nominal target density (Td) of the solution was 0.2024 (5 g of polyamic acid per 24.7 mL of total liquid volume).

[0323] The portion of the triethylammonium viscous solution obtained from polyamic acid was sprayed into a solution of acetic acid and anhydride acetic acid (20 / 20 / 80 by volume) in water using compressed gas assistance to form sub-millimeter polyamic acid beads (Example 17A). Photographs of the resulting wet gel beads taken with an optical microscope are shown in Figure 33.

[0324] Sub-millimeter polyamic acid wet gel beads were solvent-exchanged with water (3×) and microwaved in water (3×30 seconds and 2×20 seconds). The resulting polyimide beads were solvent-exchanged with ethanol and dried with supercritical fluid (SCF)CO₂ to provide polyimide aerogel beads (Example 17B). Infrared spectra are provided as Figure 34A, which are 1778, 1726, and 1377 cm⁻¹. -1 It shows a peak, demonstrating at least partial imidization.

[0325] Another portion of a viscous solution of the triethylammonium salt of polyamic acid was added dropwise to a solution of acetic acid and anhydride acetic acid in water (20 / 20 / 80 volume ratio) using a disposable pipette to form millimeter-sized polyamic acid wet gel beads (Example 17C). The resulting polyamic acid wet gel beads were desolvent-changed with ethanol and dried over SCF CO2 to provide millimeter-sized polyamic acid aerogel beads. The infrared spectrum of the aerogel beads of Example 17C is provided as Figure 34B, which shows that the aerogel beads consisted mainly of polyamic acid. A portion of the polyamic acid wet gel beads was carbonized under N2 at 1050°C to yield the corresponding carbon aerogel beads (Example 17D).

[0326] A portion of a millimeter-sized polyamic acid wet gel bead was solvent-exchanged with water (3×) and microwaved in water (3×30 seconds and 2×20 seconds). The resulting polyimide wet gel beads were solvent-exchanged with ethanol and dried with SCF CO2 to provide polyimide aerogel beads (Example 17E). A portion of the polyimide aerogel bead was carbonized under N2 at 1050°C to yield the corresponding carbon aerogel beads (Example 17F). Physical characterization data for Examples 17C to 17F are provided in Table 7. [Table 7]

[0327] Scanning electron microscope (SEM) images of the carbon bead skin of Example 17D at two different magnifications are provided as Figures 35A and 35B. Scanning electron microscope (SEM) images of the carbon bead skin of Example 17F at two different magnifications are provided as Figures 35C and 35D. SEM images of the interior of the carbon beads of Example 17D and 17F at high magnification are provided as Figures 36A and 36B, respectively, showing that both bead samples appear to consist of entangled nanofoils and nanoribbons, respectively. After carbonization, the skins of the microwave-treated beads (Example 17F; Figures 35C and 35D) appeared more similar internally (Figure 36B), while the skins of the unmicrowave-treated beads (Example 17D; Figures 35A and 35B) consisted of a denser crust with fewer openings.

[0328] Example 18. Water-based preparation of polyamic acid aerogel beads and corresponding carbon aerogel beads from aqueous solutions of triethylammonium salts of pre-formed and isolated polyamic acids added to hexane / acetic acid solution. Polyamic acid beads were prepared starting with pre-formed and isolated solid polyamic acid obtained from the reaction of 1,4-phenylenediamine and pyromellitic dianhydride in N,N-dimethylacetamide. 13 1C NMR and 15N NMR spectra are shown in Figures 21A and 21B, respectively. The target density was 0.0478. Polyamic acid (5 g) was dissolved in 100 mL of water by adding triethylamine (3.4121 g, 4.70 mL, 2.2 mol excess relative to the polyamic acid repeating units). The solution of the triethylammonium salt of polyamic acid was added dropwise to 100 mL of hexane:AcOH (90:10 v / v) using a large plastic pipette to form polyamic acid beads. The polyamic acid gel beads were solvent-exchanged with ethanol and dried over SCF CO2 to yield polyamic acid aerogel beads. The polyamic acid aerogel beads were carbonized under N2 at 1050°C to yield the corresponding carbon aerogel beads. SEM micrographs of a single bead are shown in Figure 37A, with the skin and interior shown in Figures 37B and 37C, respectively. The average diameter of the carbon aerogel beads was 2.1 mm (Figure 37D). The properties of the carbon aerogel beads are provided in Table 8, and the pore size distribution is provided in Figure 38. [Table 8]

[0329] Example 19. Water-based preparation of polyamic acid aerogel beads and corresponding carbon aerogel beads from aqueous solutions of triethylammonium salts of pre-formed and isolated polyamic acids added to aqueous acetic acid solution. Polyamic acid aerogel beads were prepared starting with pre-formed and isolated solid polyamic acid obtained from the reaction of 1,4-phenylenediamine and pyromellitic dianhydride in N,N-dimethylacetamide. 13 1C NMR and 15The N NMR spectra are shown in Figures 21A and 21B, respectively. Polyamic acid (5 g) was suspended as a solid powder in 30 mL of water and dissolved by adding triethylamine (3.4121 g, 4.70 mL, 2.2 mol excess relative to the polyamic acid repeating units). The target density was 0.144. The aqueous triethylammonium salt solution of polyamic acid was added dropwise to 100 mL of 20% aqueous acetic acid using a large plastic pipette to form polyamic acid wet gel beads. The beads were solvent-exchanged with ethanol and dried over SCF CO2 to obtain polyamic acid aerogel beads. The polyamic acid beads were carbonized under N2 at 1050°C to obtain the corresponding carbon aerogel beads. SEM images of the skin and interior of the carbon aerogel beads are shown in Figures 39A and 39B, respectively. The average diameter of the beads was 2.6 mm. The properties of carbon aerogel beads are provided in Table 9, and the pore size distribution is provided in Figure 39C. [Table 9]

[0330] Example 20. Water-based preparation of polyamic acid aerogel beads and corresponding carbon aerogel beads from aqueous solutions of triethylammonium salts of pre-formed and isolated polyamic acids added to aqueous acetic acid / acetic anhydride solution. Polyamic acid aerogel beads were prepared starting with pre-formed and isolated solid polyamic acid obtained from the reaction of 1,4-phenylenediamine and pyromellitic dianhydride in N,N-dimethylacetamide. 13 1C NMR and 15N NMR spectra are shown in Figures 21A and 21B, respectively. Solid polyamic acid (5 g) was suspended in 30 mL of water and dissolved by adding triethylamine (3.4121 g, 4.70 mL, 2.2 mol excess relative to the polyamic acid repeating units). The target density was 0.144. An aqueous solution of the triethylammonium salt of polyamic acid was added dropwise to 100 mL of H2O:AcOH:AcOAc (80:20:20 v / v / v) using a large plastic pipette to form gel beads. The gel beads were solvent-exchanged with ethanol and dried over SCF CO2 to yield polyamic acid aerogel beads. The polyamic acid aerogel beads were carbonized under N2 at 1050°C to yield the corresponding carbon aerogel beads. SEM images of a single bead are shown in Figure 40A, with the skin and interior shown in Figures 40B and 40C, respectively. The average diameter of the beads was 2.2 mm (Figure 41A). The pore size distribution is provided in Figure 41B. The properties of the carbon aerogel beads are provided in Table 10. [Table 10]

[0331] Example 21A. Water-based preparation of polyamic acid aerogel beads from aqueous in-situ preparation triethylammonium salt solution of polyamic acid electrolyzed in aqueous hydrochloric acid solution. 1,4-phenylenediamine (PDA, 66.30 g, 60.27 mL, 0.6131 mol) was dissolved in 1 L of water in a 2 L beaker. Dissolution was assisted by gentle heating (86-87°F). When the solution was cooled to below 83°F, the PDA began to precipitate. In such cases, the PDA was redissolved by heating above 84°F. Triethylamine (TEA: 148.89 g, 205.2 mL, 1.4713 mol, 2.4 mol excess) was added to the solution, and the mixture was stirred for about 5 minutes. During stirring, the 2 L beaker was tightly covered with copper foil held in place with several rubber bands. Then, pyromellitic dianhydride (PMDA; 133.70 g, 79.58 mL, 0.6130 mol) was added to the solution under vigorous stirring. The orange solution was stirred at room temperature for 1-3 days, gradually becoming very viscous.

[0332] Alternatively, PMDA (133.70 g, 79.58 mL, 0.6130 mol) was added all at once to the PDA solution as a white solid under vigorous stirring. The temperature of the mixture rose to 105–110°F. The texture and color of the solid suspension changed over time. The color changed from an initial purplish hue to gray. Vigorous stirring of the suspension was continued for 18–24 hours. At the end of this period, TEA (148.89 g, 205.2 mL, 1.4713 mol, 2.4 mol excess) was added to the suspension. The temperature of the solution rose significantly, all the solid material dissolved, and a dark orange solution was obtained. Vigorous stirring was continued for 24–72 hours. The orange solution became very viscous. During stirring, a 2 L beaker was covered with copper foil held in place by several rubber bands.

[0333] Regardless of the preparation method, the target density (T) for polyamic acid aerogel beads dThe initial density was 0.166 g / mL ((66.30 + 133.70) g / (1000 + 205.2) L). The actual target density (considering the volumes of PDA and PMDA) was (66.30 + 133.70) g / (1000 + 205.2 + 60.27 + 79.58) mL = 200 g / 1345.05 mL = 0.149 g / mL. The target density for the thermoimidized aerogel beads was 0.147 g / mL.

[0334] At the end of the stirring period, a viscous orange aqueous triethylammonium salt solution of polyamic acid was electrolyzed into aqueous HCl (20% v / v) using a 20-needle (22 gauge) spray head. The volume of the HCl receiving bath was 4 L per L of electrolyzed solution. The flow rate was adjusted to 1.5 mL / min per needle or 30 mL / min for the entire spray head using a dual-barrel syringe pump. The voltage difference between the needle and the receiving bath was set to 8 kV. The distance between the needle tip and the recovery bath was 15 cm. When droplets of the aqueous triethylammonium salt solution of polyamic acid struck the surface of the HCl receiving bath, solid gel beads were formed and settled at the bottom of the bath. The beads remained in the HCl receiving bath for 24 hours. The beads were then washed four times with water (2 gallons per wash per L of electrolyzed solution) and three times with ethanol (6 L per wash per L of electrolyzed solution). Next, the beads were dried with SCF CO2.

[0335] The process described above was performed twice. The total amount of acceptable polyamic acid aerogel beads combined from both runs was 292.92 g. The yield from PDA+PMDA (400 g) was 73.23 wt%. The tap density of the polyamic acid aerogel beads was 0.149 g cm³. -3 The IR spectrum of the polyamic acid aerogel beads is shown in Figure 42A.

[0336] Example 21B. Thermal conversion of polyamic acid aerogel beads to corresponding polyimide aerogel beads. The polyamic acid aerogel bead portion (99.99 g) of Example 21A was imidized in air at 300°C for 4 hours to form polyimide aerogel beads (receiver: 78.64 g, yield 78.64 wt%). Tap density: 0.151 g cm³ -3 The IR spectrum of the imidized beads is shown in Figure 42B.

[0337] Example 21C. Thermal decomposition conversion of polyamic acid aerogel beads to carbon aerogel beads. The polyamic acid aerogel bead portion (192.92 g) of Example 21A was directly carbonized at 1050°C for 2 hours under flowing N2 to form the corresponding carbon aerogel beads (receiver: 76.17 g, yield 39.48 wt%). Tap density: 0.155 g cm³ -3 Figure 43A shows an aggregate of carbon aerogel beads derived from the thermal decomposition of electrolyzed polyamic acid aerogel beads. Figure 43B is a high-magnification SEM of the skin of a single bead. Figure 43C shows a cross-section of a single bead, and Figure 43D shows a high-magnification image of the interior.

[0338] Example 21D. Thermal decomposition conversion of polyimide aerogel beads to carbon aerogel beads. Thermoimidized beads (e.g., 21B; 78.64 g) were carbonized under flowing N2 at 1050°C for 2 hours to produce carbon aerogel beads. Acceptance: 37.68 g. Yield from polyimide aerogel beads: 47.91 wt% per unit weight. Yield from polyamic acid aerogel beads (starting at 99.99 g): 37.68 wt% per unit weight. Yield from polyamic acid aerogel beads (calculated): 37.70 wt% per unit weight. Tap density: 0.144 g cm³ -3Figure 44A shows an aggregate of carbon aerogel beads derived from polyimide aerogel beads, which were also derived from the thermal imidization of polyamic acid aerogel beads. Figure 44B is a high-magnification SEM of the skin of one bead. Figures 44C and 44D show SEM images of the interior of one bead at two different magnifications. The actual total yield of carbon aerogel from PDA and PMDA was 26.46 wt% per weight. The calculated percent yield based on the partial yield along the process was 28.26 wt% per weight. The physical properties of electro-sprayed polyamic acid aerogel beads, their corresponding imidized aerogel beads, and the two carbonized versions by direct carbonization of polyamic acid aerogel beads and imidized derivatives are shown in Table 11. [Table 11]

[0339] Example 22. Several factors controlling the bead size of polyamic acid aerogel beads obtained from an aqueous in-situ prepared triethylammonium salt solution of polyamic acid electrolyzed in an aqueous hydrochloric acid solution, and the corresponding carbon aerogel beads. An aqueous solution of triethylammonium salt of PDA / PMDA polyamic acid is prepared as described in Example 21A, with the same target density (T dThe solution was prepared in water at a concentration of 0.166 g / mL. The solution was separated into several parts and electrolyzed into aqueous hydrochloric acid solution under various conditions, as shown in Example 21A. The parameters varied included the electrolyzation voltage of the triethylammonium salt of PDA / PMDA polyamic acid, the flow rate, and the viscosity of the aqueous solution. The wet gel polyamic acid beads were washed with water and ethanol and dried to polyamic acid aerogel beads using SCF CO2. The polyamic acid aerogel beads were carbonized to carbon aerogel beads at 1050°C under liquid nitrogen, as shown in Example 21A. The size of the beads was measured from images taken through an optical microscope: (a) in the wet gel state, (b) after drying to polyamic acid aerogel, and (c) after carbonization to carbon aerogel beads.

[0340] Using a low-viscosity (66.7 cP) aqueous solution of triethylammonium salt of polyamic acid and the same 20-needle spray head with 22-gauge needles as in Example 21A, Figure 45A shows the variation in the average bead size of wet gel polyamic acid beads with respect to flow rate and electro-spray voltage. Figure 45B shows the variation in the average size of polyamic acid aerogel beads with respect to flow rate and electro-spray voltage, and Figure 45C shows the variation in the average size of the corresponding carbon aerogel beads with respect to flow rate and electro-spray voltage. All other things being equal, Figures 45A, 45B, and 45C all show that the sizes of the polyamic acid gel and aerogel beads are approximately equal, but the size of the corresponding carbon aerogel beads is smaller.

[0341] Using a high-viscosity (309 cP) aqueous solution of triethylammonium polyamic acid and the same 20-needle spray head with 22-gauge needles as in Example 21A, Figure 45D shows the variation in the average size of carbon aerogel beads in response to flow rate and electro-spray voltage. Of the parameters investigated in Figures 45A to 45D, electro-spray voltage was the most important factor influencing bead size.

[0342] In addition to the electro-aerogel voltage, another factor that plays a role in bead size is the needle gauge. Figure 46A shows carbon aerogel beads prepared in a low-viscosity solution electro-aerogel through a 22-gauge needle at 30 kV and a flow rate of 2.5 mL per minute. Figure 46B shows smaller carbon aerogel beads under the same conditions, however, electro-aerogel through a 28-gauge needle. The effect of needle gauge on bead size is directly applicable to atomization using compressed gas assistance, as shown in Example 17.

[0343] Example 23. Water-based preparation of polyamic acid aerogel beads from pre-formation by acidification by delta-gluconolactone hydrolysis and aqueous solution of triethylammonium salt of isolated solid polyamic acid. Polyamic acid (5 g; from a 1:1 molar ratio PDA / PMDA reaction) was added to 100 mL of water. Triethylamine (3.41 g, 2.2 mol excess relative to the monomer repeating units) was added to the suspension, and the mixture was stirred until the polyamic acid was completely dissolved. Delta-gluconolactone (DGL; 6.59 g, 0.037 mol, 2.4 mol excess relative to the monomer repeating units) was added to the triethylammonium salt solution of polyamic acid, and the mixture was stirred until the DGL was dissolved. Approximately 15 minutes before the gelation point, which was about 1.5 hours at room temperature, the solution was added dropwise to an aqueous solution of acetic acid (20% v / v). The droplets sank into the solution. At higher volume percentage ratios of acetic acid, e.g., above 35% v / v, the droplets initially floated. The resulting millimeter-sized beads were aged in an acetic acid receiving solution for 24 hours, then washed twice with water, the solvent was changed with ethanol, and they were dried with supercritical fluid CO2 to provide polyamic acid aerogel beads.

[0344] Example 24. Water-based preparation of polyamic acid and polyimide monolithic gels from pre-formation by acidification by delta-gluconolactone hydrolysis and aqueous solution of triethylammonium salt of isolated solid polyamic acid. Polyamic acid (5 g; from a 1:1 molar ratio reaction of PDA / PMDA) was added to 50 mL of water. Triethylamine (3.41 g, 2.2 mol excess relative to the monomer repeating units) was added to the suspension, and the mixture was stirred until the polyamic acid was completely dissolved. Delta-gluconolactone (DGL: 6.59 g, 0.037 mol, 2.4 mol excess relative to the monomer repeating units) was added to the aqueous triethylammonium salt solution of polyamic acid, and the mixture was stirred until the DGL was dissolved. The resulting solution was divided into molds and gelled at room temperature. The gelling time was approximately 15 minutes. In particular, in a more diluted solution, for example, using the same amounts of polyamic acid and triethylamine, but with 100 mL of water, the gelling time was approximately 1.5 hours (see Example 23). The resulting wet gel was aged in the mold for 24 hours. Then, the molded wet gel was covered with water in the mold and microwaved for 4 × 10⁻⁶ seconds. The microwave-treated wet gels were washed with water in their molds, demolded with ethanol, and washed four times with ethanol, each time remaining in ethanol for 24 hours. The wet gels were dried with supercritical CO2. FTIR analysis of the aerogel samples showed that polyamic acid had been quantitatively converted to polyimide (Figure 47).

[0345] The following six examples were prepared via a high-shear emulsion gelation method using in-situ-forming triethylammonium salt solutions of polyamic acids, employing various methods for mixing PDA, PMDA, and TEA. A further scope of these examples was to compare the properties of polyimide (PI), polyamic acid (PAA), and the corresponding carbon aerogels from the same aqueous triethylammonium salt solutions of polyamic acids.

[0346] Example 25. Preparation of emulsion bases for micron-sized polyimide and polyamic acid aerogel beads from aqueous in situ triethylammonium salt solution of polyamic acid (sequential addition of PDA, TEA, and PMDA at room temperature). Each is 0.079 g / cm³. 3and 0.094 g / cm³ 3 Micron-sized polyimide and polyamic acid gel beads with a target density equal to 1 / 25 were prepared by gelling an aqueous triethylammonium salt solution of polyamic acid in an emulsion. For this purpose, PDA (27.94 g) was dissolved in 808 g of water. Triethylamine (TEA: 62.87 g, 86.66 mL, mol / mol ratio of 2.4:1 relative to PDA or PMDA) was added to the solution and the mixture was stirred for 5 minutes. PMDA (56.36 g, 0.26 mol, mol / mol ratio of 1:1 relative to PDA) was added to the mixture and the mixture was stirred at room temperature for 24 hours. The obtained aqueous triethylammonium salt solution of polyamic acid had a viscosity equal to 9.5 cP at room temperature and was bisected.

[0347] Acetate anhydride (56.71 g, 52.41 mL, 4.3 mol / mol ratio relative to PMDA in polyamic acid) was added to the first half of the aqueous triethylammonium salt solution of polyamic acid, and the resulting sol was magnetically stirred for 60 seconds. At the end of this period, the sol was poured into the immiscible phase under shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 8 g of surfactant (Hypermer® H70) in 800 mL of mineral spirits. The sol was added to the mineral spirits phase in a v / v ratio of 1:2. Gelation in a small amount of sol set aside as a control occurred at room temperature 3–4 minutes after the addition of acetic anhydride. After stirring under high shear for 4–5 minutes, the mixture was removed from the Ross mixer and allowed to stand for 1–3 hours. The mineral spirits layer was decanted. Gel beads were recovered using filtration under reduced pressure, and these were solvent-changed three times with ethanol. Gel beads that have been replaced (washed) with ethanol are dried using supercritical CO2 and are called PI aerogel beads.

[0348] Another half of the aqueous triethylammonium salt solution of polyamic acid was poured into the immiscible phase under shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 30 g of surfactant (Hypermer® H70) in 1600 mL of hexane. The aqueous solution was added to the hexane phase in a v / v ratio of 1:4. The mixture was stirred under high shear at 3000 rpm for 4 minutes to establish a metastable emulsion. At the end of this period, acetic acid was added to the emulsion in a v / v ratio of 25% relative to hexane, and the mixture was stirred for 2 minutes using a Ross mixer at 3000 rpm. The mixture was then removed from the Ross mixer and allowed to stand for 1-3 hours. The hexane layer was decanted. Gel beads were recovered using filtration under reduced pressure, and these were solvent-exchanged three times with ethanol. The gel beads exchanged (washed) with ethanol were dried using supercritical CO2 and referred to as PAA aerogel beads.

[0349] Carbonization of PI and PAA aerogel beads was carried out at 1050°C for 2 hours under fluid nitrogen using a ramp rate of 3°C per minute. The carbon aerogel beads from PI and PAA are referred to as C-PI and C-PAA, respectively. Data for all aerogel beads from this example are provided in Table 12. Figure 48A shows a low-magnification SEM image of the C-PI aerogel bead. Figure 48B shows a high-magnification SEM image of the surface of the C-PI aerogel bead. Figure 48C shows a low-magnification SEM image of the C-PAA aerogel bead. Figure 48D shows a higher-magnification SEM image of the surface of the C-PAA aerogel bead. [Table 12]

[0350] Example 26. Preparation of emulsion bases for micron-sized polyimide and polyamic acid aerogel beads from aqueous in situ triethylammonium salt solution of polyamic acid (sequential addition of PDA, TEA, and PMDA at 50-60°C) Each is 0.079 g / cm³. 3 and 0.094 g / cm³3 Micron-sized polyimide and polyamic acid gel beads with a target density equal to were prepared via gelation of an aqueous triethylammonium salt solution of polyamic acid in emulsion. For this purpose, PDA (27.94 g) was dissolved in 808 g of water. Triethylamine (TEA: 62.87 g, 86.66 mL, mol / mol ratio of 2.4:1 relative to PDA or PMDA) was added to the solution and the mixture was stirred for 5 minutes. PMDA (56.36 g, 0.26 mol, mol / mol ratio of 1:1 relative to PDA) was added to the mixture and the solution was stirred at 50-60°C for 24 hours. At the end of this period, the obtained aqueous triethylammonium salt solution of polyamic acid was cooled back to room temperature, where it had a viscosity equal to 9.2 cP at room temperature and was bisected.

[0351] Acetate anhydride (56.71 g, 52.41 mL, 4.3 mol / mol ratio relative to PMDA in polyamic acid) was added to the first half of the aqueous triethylammonium salt solution of polyamic acid, and the resulting sol was magnetically stirred for 60 seconds. At the end of this period, the sol was poured into the immiscible phase under shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 8 g of surfactant (Hypermer® H70) in 800 mL of mineral spirits. The sol was added to the mineral spirits phase in a v / v ratio of 1:2. Gelation in a small amount of sol set aside as a control occurred at room temperature 3–4 minutes after the addition of acetic anhydride. After stirring under high shear for 4–5 minutes, the mixture was removed from the Ross mixer and allowed to stand for 1–3 hours. The mineral spirits layer was decanted. Gel beads were recovered using filtration under reduced pressure, and these were solvent-changed three times with ethanol. Gel beads that have been replaced (washed) with ethanol are dried using supercritical CO2 and are called PI aerogel beads.

[0352] Another half of the aqueous triethylammonium salt solution of polyamic acid was poured into the immiscible phase under shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 30 g of surfactant (Hypermer® H70) in 1600 mL of hexane. The aqueous solution was added to the hexane phase in a v / v ratio of 1:4. The mixture was stirred under high shear at 3000 rpm for 4 minutes to establish a metastable emulsion. At the end of this period, acetic acid was added to the emulsion in a v / v ratio of 25% relative to hexane, and the mixture was stirred for 2 minutes using a Ross mixer at 3000 rpm. The mixture was then removed from the Ross mixer and allowed to stand for 1-3 hours. The hexane layer was decanted. Gel beads were recovered using filtration under reduced pressure, and these were solvent-exchanged three times with ethanol. The gel beads exchanged (washed) with ethanol were dried using supercritical CO2 and referred to as PAA aerogel beads.

[0353] Carbonization of PI and PAA aerogel beads was carried out at 1050°C for 2 hours under fluid nitrogen using a ramp rate of 3°C per minute. The carbon aerogel beads from PI and PAA are referred to as C-PI and C-PAA, respectively. Data for all aerogel beads from this example are provided in Table 13. Figures 49A and 49B show the solid PI aerogel microbeads. 13 C and 15 The N NMR spectra are shown. The imide-to-amide group ratio was 1.25, indicating that the conversion of amide groups to imides was greater than 50%. The resonance at 55.4 ppm was attributed to triethylammonium. Figures 49C and 49D show the solid state of the PAA aerogel beads. 13 C and 15The N NMR spectra are shown for each. The imide-to-amide group ratio was 3.48. The resonance at 55.7 ppm was attributed to triethylammonium. Figure 50A shows a low-magnification SEM image of the C-PI aerogel beads. Figure 50B shows a high-magnification SEM image of the surface of the C-PI aerogel beads. Figure 50C shows a low-magnification SEM image of the C-PAA aerogel beads. Figure 50D shows a higher-magnification SEM image of the surface of the C-PAA aerogel beads. [Table 13]

[0354] Example 27. Preparation of emulsion bases of micron-sized polyimides and polyamic acid aerogel beads from aqueous in situ triethylammonium salt solutions of polyamic acid by solid suspension method 1. Each is 0.079 g / cm³. 3 and 0.094 g / cm³ 3 Micron-sized polyimide and polyamic acid gel beads with a target density equal to 15.6 cP were prepared via gelation of an aqueous triethylammonium salt solution of polyamic acid in emulsion. For this purpose, PDA (27.94 g) was dissolved in 808 g of water. PMDA (56.36 g, 0.26 mol, 1:1 mol / mol ratio to PDA) was added to the PDA solution, and the resulting solid suspension was stirred magnetically or mechanically at room temperature for 24 hours. At the end of this period, triethylamine (TEA: 62.87 g, 86.66 mL, 2.4:1 mol / mol ratio to PDA or PMDA) was added to the solid suspension, and the resulting solution was stirred at room temperature for a further 24 hours. The resulting aqueous triethylammonium salt solution of polyamic acid had a viscosity equal to 15.6 cP at room temperature and was bisected.

[0355] Acetate anhydride (56.71 g, 52.41 mL, 4.3 mol / mol ratio relative to PMDA in polyamic acid) was added to the first half of the aqueous triethylammonium salt solution of polyamic acid, and the resulting sol was magnetically stirred for 60 seconds. At the end of this period, the sol was poured into the immiscible phase under shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 8 g of surfactant (Hypermer® H70) in 800 mL of mineral spirits. The sol was added to the mineral spirits phase in a v / v ratio of 1:2. Gelation in a small amount of sol set aside as a control occurred at room temperature 3–4 minutes after the addition of acetic anhydride. After stirring under high shear for 4–5 minutes, the mixture was removed from the Ross mixer and allowed to stand for 1–3 hours. The mineral spirits layer was decanted. Gel beads were recovered using filtration under reduced pressure, and these were solvent-changed three times with ethanol. Gel beads that have been replaced (washed) with ethanol are dried using supercritical CO2 and are called PI aerogel beads.

[0356] Another half of the aqueous triethylammonium salt solution of polyamic acid was poured into the immiscible phase under shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 30 g of surfactant (Hypermer® H70) in 1600 mL of hexane. The aqueous solution was added to the hexane phase in a v / v ratio of 1:4. The mixture was stirred under high shear at 3000 rpm for 4 minutes to establish a metastable emulsion. At the end of this period, acetic acid was added to the emulsion in a v / v ratio of 25% relative to hexane, and the mixture was stirred for 2 minutes using a Ross mixer at 3000 rpm. The mixture was then removed from the Ross mixer and allowed to stand for 1-3 hours. The hexane layer was decanted. Gel beads were recovered using filtration under reduced pressure, and these were solvent-exchanged three times with ethanol. The gel beads exchanged (washed) with ethanol were dried using supercritical CO2 and referred to as PAA aerogel beads.

[0357] Carbonization of PI and PAA aerogel beads was carried out at 1050°C for 2 hours under fluid nitrogen using a ramp rate of 3°C per minute. Carbon aerogel beads from PI and PAA are referred to as C-PI and C-PAA, respectively. Data for all aerogel beads from this example are provided in Table 14. Figure 51A shows a low-magnification SEM image of C-PI aerogel beads. C-PI beads aggregate and form large clumps together with fragments. Figure 51B shows a high-magnification SEM image of the surface of C-PI aerogel beads. Figure 51C shows a low-magnification SEM image of C-PAA aerogel beads. Figure 51D shows a higher-magnification SEM image of the surface of C-PAA aerogel beads. The surface of the beads is formed by a denser polymer with fewer pores. [Table 14]

[0358] Example 28. Preparation of emulsion bases of micron-sized polyimides and polyamic acid aerogel beads from triethylammonium salt solutions of aqueous in situ preparation of polyamic acids by solid suspension method 2. Each is 0.079 g / cm³. 3 and 0.094 g / cm³ 3 Micron-sized polyimide and polyamic acid gel beads with a target density equal to 16.4 cP were prepared via gelation of an aqueous triethylammonium salt solution of polyamic acid in an emulsion. For this purpose, PDA (27.94 g, 0.26 mol) and PMDA (56.36 g, 0.26 mol, 1:1 mol / mol ratio to PDA) were continuously added to 808 g of water. The resulting solid suspension was stirred magnetically or mechanically at room temperature for 24 hours. At the end of this period, triethylamine (TEA: 62.87 g, 86.66 mL, 2.4:1 mol / mol ratio to PDA or PMDA) was added to the solid suspension, and the resulting solution was stirred at room temperature for a further 24 hours. The resulting aqueous triethylammonium salt solution of polyamic acid had a viscosity equal to 16.4 cP at room temperature and was bisected.

[0359] Acetate anhydride (56.71 g, 52.41 mL, 4.3 mol / mol ratio relative to PMDA in polyamic acid) was added to the first half of the aqueous triethylammonium salt solution of polyamic acid, and the resulting sol was magnetically stirred for 60 seconds. At the end of this period, the sol was poured into the immiscible phase under shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 8 g of surfactant (Hypermer® H70) in 800 mL of mineral spirits. The sol was added to the mineral spirits phase in a v / v ratio of 1:2. Gelation in a small amount of sol set aside as a control occurred at room temperature 3–4 minutes after the addition of acetic anhydride. After stirring under high shear for 4–5 minutes, the mixture was removed from the Ross mixer and allowed to stand for 1–3 hours. The mineral spirits layer was decanted. Gel beads were recovered using filtration under reduced pressure, and these were solvent-changed three times with ethanol. Gel beads that have been replaced (washed) with ethanol are dried using supercritical CO2 and are called PI aerogel beads.

[0360] Another half of the aqueous triethylammonium salt solution of polyamic acid was poured into the immiscible phase under shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 30 g of surfactant (Hypermer® H70) in 1600 mL of hexane. The aqueous solution was added to the hexane phase in a v / v ratio of 1:4. The mixture was stirred under high shear at 3000 rpm for 4 minutes to establish a metastable emulsion. At the end of this period, acetic acid was added to the emulsion in a v / v ratio of 25% relative to hexane, and the mixture was stirred for 2 minutes using a Ross mixer at 3000 rpm. The mixture was then removed from the Ross mixer and allowed to stand for 1-3 hours. The hexane layer was decanted. Gel beads were recovered using filtration under reduced pressure, and these were solvent-exchanged three times with ethanol. The gel beads exchanged (washed) with ethanol were dried using supercritical CO2 and referred to as PAA aerogel beads.

[0361] Carbonization of PI and PAA aerogel beads was carried out at 1050°C for 2 hours under fluid nitrogen using a ramp rate of 3°C per minute. The carbon aerogel beads from PI and PAA are referred to as C-PI and C-PAA, respectively. Data for all aerogel beads from this example are provided in Table 15. Figure 52A shows a low-magnification SEM image of the C-PI aerogel bead. Figure 52B shows a high-magnification SEM image of the surface of the C-PI aerogel bead. Figure 52C shows a low-magnification SEM image of the C-PAA aerogel bead. Figure 52D shows a higher-magnification SEM image of the surface of the C-PAA aerogel bead. [Table 15]

[0362] Example 29. Preparation of emulsion bases of micron-sized polyimides and polyamic acid aerogel beads from aqueous in situ triethylammonium salt solutions of polyamic acid by solid suspension method 3. Each is 0.080 g / cm³. 3 and 0.094 g / cm³ 3 Micron-sized polyimide and polyamic acid gel beads with a target density equal to were prepared by gelling an aqueous triethylammonium salt solution of polyamic acid in an emulsion. For this purpose, PDA (16.76 g, 0.15 mol), PMDA (33.81 g, 0.15 mol, 1:1 mol / mol ratio to PDA), and triethylamine (TEA: 37.72 g, 51.99 mL, 2.4:1 mol / mol ratio to PDA or PMDA) were continuously added to 485 g of water. The resulting solution was stirred at room temperature for 24 hours. The obtained aqueous triethylammonium salt solution of polyamic acid had a viscosity equal to 317 cP at room temperature.

[0363] Acetate anhydride (23.74 g, 21.94 mL, 4.2 mol / mol ratio to PMDA or PDA in polyamic acid) was added to a portion (200 g) of an aqueous triethylammonium salt solution of polyamic acid, and the resulting sol was magnetically stirred for 60 seconds. At the end of this period, the sol was poured into the immiscible phase under shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 4.5 g of surfactant (Hypermer® H70) in 400 mL of mineral spirits. The sol was added to the mineral spirits phase in a v / v ratio of 1:2. Gelation in a small amount of sol set aside as a control occurred 3 minutes after the addition of acetic anhydride at room temperature. After stirring under high shear for 4-5 minutes, the mixture was removed from the Ross mixer and allowed to stand for 1-3 hours. The mineral spirits layer was decanted. The gel beads were recovered using filtration under reduced pressure, and these were subjected to three solvent changes with ethanol. The gel beads that had been washed with ethanol were dried using supercritical CO2 and are referred to as PI aerogel beads.

[0364] Another portion (200 g) of the aqueous triethylammonium salt solution of polyamic acid was poured into the immiscible phase under shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 8.6 g of surfactant (Hypermer® H70) in 800 mL of hexane. The aqueous solution was added to the hexane phase in a v / v ratio of 1:4. The mixture was stirred under high shear at 3000 rpm for 2 minutes to establish a metastable emulsion. At the end of this period, acetic acid was added to the emulsion in a v / v ratio of 25% relative to hexane, and the mixture was stirred for 1 minute using a Ross mixer at 3000 rpm. The mixture was then removed from the Ross mixer and allowed to stand for 1–3 hours. The hexane layer was decanted. Gel beads were recovered using filtration under reduced pressure, and these were solvent-changed three times with ethanol. The gel beads, which have been replaced (washed) with ethanol, are dried using supercritical CO2 and are called PAA aerogel beads.

[0365] Carbonization of PI and PAA aerogel beads was carried out at 1050°C for 2 hours under fluid nitrogen using a ramp rate of 3°C per minute. The carbon aerogel beads from PI and PAA are referred to as C-PI and C-PAA, respectively. Data for all aerogel beads from this example are provided in Table 16. Figure 53A shows a low-magnification SEM image of the C-PI aerogel beads. There is debris between the beads. Figure 53B shows a high-magnification SEM image of the surface of the C-PI aerogel beads. Figure 53C shows a low-magnification SEM image of the C-PAA aerogel beads. Figure 53D shows a higher-magnification SEM image of the surface of the C-PAA aerogel beads. The surface of all beads in this example shows some texture. [Table 16]

[0366] Example 30. Preparation of emulsion bases of micron-sized polyimides and polyamic acid aerogel beads from aqueous triethylammonium salt solutions of pre-formed and isolated polyamic acids. Each is 0.078 g / cm³. 3 and 0.093 g / cm³ 3 Micron-sized polyimide and polyamic acid gel beads with a target density equal to were prepared via gelation of a pre-prepared aqueous triethylammonium salt solution of polyamic acid in emulsion. For this purpose, solid polyamic acid (30 g, previously isolated after polymerization of PDA and PMDA in N,N-dimethylacetamide as solvent) was suspended in 291 g of water. The polyamic acid was dissolved by adding triethylamine (TEA: 22.63 g, 31.19 mL, with a mol / mol ratio of TEA to PMDA or PDA of 2.4:1 in the polyamic acid). After stirring at room temperature for 24 hours, the obtained aqueous triethylammonium salt solution of polyamic acid was divided into two equal parts.

[0367] Acetate anhydride (20.41 g, 18.86 mL, 4.2 mol / mol ratio to PMDA or PDA in polyamic acid) was added to the first half of the aqueous triethylammonium salt solution of polyamic acid, and the resulting sol was magnetically stirred for 60 seconds. At the end of this period, the sol was poured into the immiscible phase under high shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 4.5 g of surfactant (Hypermer® H70) in 400 mL of mineral spirits. The sol was added to the mineral spirits phase in a v / v ratio of 1:2. Gelation in a small amount of sol set aside as a control occurred at room temperature 3–4 minutes after the addition of acetic anhydride. After stirring under high shear for 4–5 minutes, the mixture was removed from the Ross mixer and allowed to stand for 1–3 hours. The mineral spirits layer was decanted. The gel beads were recovered using filtration under reduced pressure, and these were subjected to three solvent changes with ethanol. The gel beads that had been washed with ethanol were dried using supercritical CO2 and are referred to as PI aerogel beads.

[0368] Another half of the aqueous triethylammonium salt solution of polyamic acid was poured into the immiscible phase under high shear using a Ross mixer at 3000 rpm. The immiscible phase was prepared by dissolving 6.5 g of surfactant (Hypermer® H70) in 650 mL of hexane. The aqueous solution was added to the hexane phase in a v / v ratio of 1:4. The mixture was stirred at 3000 rpm for 2 minutes under high shear to establish a metastable emulsion. At the end of this period, acetic acid was added to the emulsion in a v / v ratio of 25% relative to hexane, and the mixture was stirred at 3000 rpm using a Ross mixer for 1 minute. The mixture was then removed from the Ross mixer and allowed to stand for 1-3 hours. The hexane layer was decanted. Gel beads were recovered using filtration under reduced pressure, and these were solvent-exchanged three times with ethanol. The gel beads exchanged (washed) with ethanol were dried using supercritical CO2 and referred to as PAA aerogel beads.

[0369] Carbonization of PI and PAA aerogel beads was carried out at 1050°C for 2 hours under fluid nitrogen using a ramp rate of 3°C per minute. The carbon aerogel beads from PI and PAA are referred to as C-PI and C-PAA, respectively. Data for all aerogel beads from this example are provided in Table 17. Figure 54A shows a low-magnification SEM image of the C-PI aerogel bead. Figure 54B shows a high-magnification SEM image of the surface of the C-PI aerogel bead. Figure 54C shows a low-magnification SEM image of the C-PAA aerogel bead. Figure 54D shows a higher-magnification SEM image of the surface of the C-PAA aerogel bead. [Table 17]

[0370] Example 31. Preparation of an emulsion base for micron-sized polyimide aerogel beads from the reaction of 4,4'-oxydianiline (ODA) with PMDA and aqueous triethylammonium salt solution of isolated polyamic acid. Micron-sized polyimide gel beads, 0.054 g / cm³ 3 The polyamic acid was prepared by gelling a pre-prepared aqueous triethylammonium salt solution in an emulsion to a target density equal to . For this purpose, solid polyamic acid (10 g, previously isolated after polymerization of 4,4'-oxydianiline (ODA) and PMDA in N,N-dimethylacetamide as a solvent) was suspended in 150 g of water. The polyamic acid was dissolved by adding triethylamine (TEA, 5.81 g, 8.01 mL, with a mol / mol ratio of TEA to PMDA or ODA of 2.4:1 in the polyamic acid). After stirring at room temperature for 24 hours, the obtained aqueous triethylammonium salt solution of polyamic acid was treated as follows.

[0371] Acetate anhydride (11.64 g, 10.77 mL, 4.3 mol / mol ratio relative to PMDA or ODA in polyamic acid) was added to an aqueous triethylammonium salt solution of polyamic acid, and the resulting sol was magnetically stirred for 30 seconds, at which point the sol became viscous. At the end of this period, the sol was poured into the immiscible phase under high shear using a Ross mixer at 4000 rpm. The immiscible phase was prepared by dissolving 8 g of surfactant (Hypermer® H70) in 600 mL of mineral spirits. The sol was added to the mineral spirits phase in a v / v ratio of approximately 1:3. Gelation in a small amount of sol set aside as a control occurred at room temperature within 1 minute of the addition of acetic anhydride. After stirring under high shear for 2 minutes, the mixture was removed from the Ross mixer and allowed to stand for 1-3 hours. The mineral spirits layer was decanted. The gel beads were recovered using filtration under reduced pressure, and these were subjected to three solvent changes with ethanol. The gel beads that had been washed with ethanol were dried using supercritical CO2 and are referred to as PI aerogel beads.

[0372] Carbonization of PI aerogel beads was carried out at 1050°C for 2 hours under fluid nitrogen using a ramp rate of 3°C per minute. Carbon aerogel beads are referred to as C-PI. Data for all aerogel beads from this example are provided in Table 18. Figure 55A shows the IR spectrum of polyamic acid obtained by reacting ODA and PMDA in N,N-dimethylacetamide. Figure 55B shows the IR spectrum of PI aerogel derived from emulsion. Figure 56A shows a low-magnification SEM image of C-PI aerogel beads. Figure 56B shows a higher-magnification SEM image of the surface of C-PAA aerogel beads. [Table 18]

[0373] Example 32. Preparation of an emulsion base for micron-sized polyimide aerogel beads from the reaction of 4,4'-methylenedianiline (MDA) with PMDA and aqueous triethylammonium salt solution of isolated polyamic acid. Micron-sized polyimide gel beads, 0.054 g / cm³ 3 The target density was achieved by gelling a pre-prepared aqueous triethylammonium salt solution of polyamic acid in an emulsion. For this purpose, solid polyamic acid (10 g, previously isolated after polymerization of 4,4'-methylenedianiline (MDA) and PMDA in N,N-dimethylacetamide as a solvent) was suspended in 150 g of water. The polyamic acid was dissolved by adding triethylamine (TEA: 5.84 g, 8.05 mL, with a mol / mol ratio of TEA to PMDA or MDA of 2.4:1 in the polyamic acid). After stirring at room temperature for 24 hours, the obtained aqueous triethylammonium salt solution of polyamic acid was treated as follows.

[0374] Acetate anhydride (11.7 g, 10.81 mL, 4.3 mol / mol ratio relative to PMDA or MDA in polyamic acid) was added to an aqueous triethylammonium salt solution of polyamic acid, and the resulting sol was magnetically stirred for 50 seconds. At the end of this period, the sol was poured into the immiscible phase under high shear using a Ross mixer at 4000 rpm. The immiscible phase was prepared by dissolving 8 g of surfactant (Hypermer® H70) in 600 mL of mineral spirits. The sol was added to the mineral spirits phase in a v / v ratio of approximately 1:3. Gelation in a small amount of sol set aside as a control occurred at room temperature 1.5 minutes after the addition of acetic anhydride. After stirring under high shear for 3 minutes, the mixture was removed from the Ross mixer and allowed to stand for 1–3 hours. The mineral spirits layer was decanted. Gel beads were recovered using filtration under reduced pressure, and these were solvent-changed three times with ethanol. Gel beads that have been replaced (washed) with ethanol are dried using supercritical CO2 and are called PI aerogel beads.

[0375] Carbonization of PI aerogel beads was carried out at 1050°C for 2 hours under fluid nitrogen using a ramp rate of 3°C per minute. Carbon aerogel beads are referred to as C-PI. Data for all aerogel beads from this example are provided in Table 19. Figure 55C shows the IR spectrum of the polyamic acid obtained by reacting MDA and PMDA in N,N-dimethylacetamide. Figure 55D shows the IR spectrum of the PI aerogel derived from the emulsion. Figure 56C shows a low-magnification SEM image of the C-PI aerogel beads. Figure 56D shows a higher-magnification SEM image of the surface of the C-PAA aerogel beads. The surfaces of the ODA-PMDA and MDA-PMDA beads were fairly dense. [Table 19]

[0376] Example 33. Preparation of metal polyamate salt aerogel beads from pre-formation of PDA and PMDA and aqueous salt solutions of isolated polyamic acids, and conversion to metal or metal oxide doped carbon aerogels. Millimeter-sized metal polyamate gel beads were prepared by adding a pre-prepared soluble aqueous salt solution of polyamic acid to a solution containing suitable metal ions. Generally, solid polyamic acid (pre-isolated after polymerization of PDA and PMDA in N,N-dimethylacetamide as a solvent) was dissolved in water using a base. Suitable bases include, but are not limited to, sodium hydroxide, ammonium hydroxide, tetrabutylammonium hydroxide, triethylamine, and diisopropylethylamine. For example, in one embodiment, polyamic acid (20 g) was suspended in 150 mL of water and dissolved by adding solid sodium hydroxide (NaOH; 4.9 g, in a 2:1 mol / mol ratio to either PMDA or PDA in the polyamic acid). The resulting solution was separated into five equal parts. Each part was added dropwise using a disposable pipette to five separate aqueous metal salt solutions, each prepared with a metal salt in a 4:1 molar ratio to the monomer repeating units in the corresponding sodium polyamate solution. The volume of each metal salt solution was 80 mL. The metals examined were iron (Fe), nickel (Ni), silver (Ag), magnesium (Mg), and lanthanum (La) in the form of salts: FeCl3, Ni(II) acetate tetrahydrate, AgNO3, MgCl2, and La(III) acetate hydrate. Other salts later qualitatively included were dysprosium and yttrium (as DyCl3 and YCl3).

[0377] When droplets of aqueous sodium polyamate solution were placed in each metal salt receiving solution, corresponding metal polyamate beads were formed. After addition was complete, the beads were aged in the receiving bath for 24 hours. Subsequently, the beads were washed with water (4 times, 4 hours each time) and ethanol (4 times, 4 hours each time) and dried with SCF CO2. Portions of each batch of beads were thermally decomposed at 850°C for 2 hours under liquid nitrogen gas. Table 20 summarizes the properties of the metal polyamate aerogels and their carbide counterparts. Each type of metal polyamate bead is abbreviated by the chemical symbol of the corresponding metal, and the corresponding carbide material is abbreviated by the chemical symbol of the metal preceded by "C-". For SEM purposes, metal polyamate salts are abbreviated as "M PAate", where M is the chemical symbol of the corresponding metal.

[0378] Figure 57A shows an SEM image of a carbide-silver polyamate bead. Figure 57B shows the surface of the carbide-silver polyamate bead at a higher magnification. Figures 57C and 57D show the interior of the silver polyamate bead at two different magnifications.

[0379] Figure 58A shows an SEM image of a carbide-la polyamate bead. Figure 58B shows the surface of the carbide-la polyamate bead at a higher magnification. Figures 58C and 58D show the interior of the La polyamate bead at two different magnifications.

[0380] Figure 59A shows an SEM image of a Mg carbide polyamate bead. Figure 59B shows the surface of the Mg carbide polyamate bead at a higher magnification. Figures 59C and 59D show the interior of the Mg polyamate bead at two different magnifications. The SEM image data in Table 20 and Figures 57-57 show that the chemical identity of the metal ions affects the morphology and material properties of both aerogels and carbon aerogels. [Table 20]

[0381] Summary of results Overall, the internal structure of the polyamic acid beads of this disclosure consisted of entangled nanofoils, while the internal structure of the monoliths and beads prepared in water and chemically imidized with acetic anhydride depended on the target density. At lower target densities, the interior consisted of interconnected short nanofibers similar to those observed in synthesis in organic solvents. At higher target densities, the morphology would be considered a combination of the two extremes. For example, at lower resolution electron microscopy, the structure might appear fibrous, while at higher resolution, it might appear as entangled nanofoils. Chemically, aerogels at lower target densities prepared by imidization with acetic anhydride in water consisted of more imides than amides, while as the target density increased, the ratio of the two functional groups shifted in favor of amides. Solid 15 N NMR is a tool that can quantify the imide-to-amide ratio in aerogels and can also distinguish between unreacted amines and residual ammonium ions at the ends of the aerogel polymer backbone due to unreacted polyamates. While we do not wish to be bound by theory, it is thought that both the chemical composition and morphology of aerogels are related to the solubility of acetic anhydride in water. Furthermore, the skin of polyamic acid beads formed in acetic acid alone had an appearance closer to the interior, while the skin of beads formed in acetic acid further containing acetic anhydride was dense and lacked character. Finally, when polyamate salts are gelled with metal ions in aqueous solution, beads with internal morphologies that vary depending on the metal are obtained. Some embodiments of the present invention are shown below. [Aspect 1] A method for forming a polyimide aerogel, wherein the method is To provide an aqueous solution of a polyamic acid salt, wherein the polyamic acid salt comprises a polyamic acid containing a carboxylic acid group, and the carboxylic acid group is bonded to a cationic species and substantially exists as a carboxylate anion, The process involves imidizing the aforementioned polyamic acid salt to form a polyimide gel, The method comprising drying the polyimide gel to form the polyimide aerogel. [Aspect 2] To provide the aqueous solution of the polyamic acid salt, To provide polyamic acid, Adding the polyamic acid to water to form an aqueous suspension of the polyamic acid, The method according to embodiment 1, comprising adding a base to the aqueous suspension of the polyamic acid to form the aqueous solution of the polyamic acid salt. [Aspect 3] The method according to embodiment 2, wherein the base is an alkali metal hydroxide and the cationic species is an alkali metal cation. [Aspect 4] The method according to embodiment 3, wherein the alkali metal hydroxide is lithium hydroxide, sodium hydroxide, or potassium hydroxide. [Aspect 5] The method according to embodiment 2, wherein the base is a non-nucleophilic amine and the cationic species is an ammonium cation. [Aspect 6] The method according to embodiment 5, wherein the non-nucleophilic amine has a solubility of at least about 4 grams per liter of water at 20°C. [Aspect 7] The method according to embodiment 5 or 6, wherein the non-nucleophilic amine is a tertiary amine. [Aspect 8] The method according to any one of embodiments 5 to 7, wherein the non-nucleophilic amine is selected from the group consisting of triethylamine, trimethylamine, tri-n-butylamine, N-methylpyrrolidine, N-methylpiperidine, diisopropylethylamine, and combinations thereof. [Aspect 9] The method according to any one of embodiments 5 to 8, wherein the non-nucleophilic amine is triethylamine or diisopropylethylamine. [Aspect 10] The method according to any one of embodiments 5 to 9, wherein the non-nucleophilic amine is added in an amount sufficient to maintain substantially all of the polyamic acid in solution. [Aspect 11] The method according to any one of embodiments 5 to 10, wherein the molar ratio of the non-nucleophilic amine to the polyamic acid is about 2 to about 4, or about 2.2 to about 2.5. [Aspect 12] The method according to any one of embodiments 1 to 11, wherein the polyamic acid comprises a tetracarboxylic acid selected from the group consisting of benzene-1,2,4,5-tetracarboxylic acid, [1,1'-biphenyl]-3,3',4,4'-tetracarboxylic acid, 4,4'-oxydiphthalic acid, 4,4'-sulfonyldiphthalic acid, 4,4'-carbonyldiphthalic acid, 4,4'-(propane-2,2-diyl)diphthalic acid, 4,4'-(perfluoropropane-2,2-diyl)diphthalic acid, naphthalene-1,4,5,8-tetracarboxylic acid, 4-(2-(4-(3,4-dicarboxyphenoxy)phenyl)propane-2-yl)phthalic acid, perylenetetracarboxylic acid, and combinations thereof. [Aspect 13] The method according to any one of embodiments 1 to 12, wherein the polyamic acid comprises a C2-C6 alkylenediamine, and one or more carbon atoms of the C2-C6 alkylene are optionally substituted with one or more alkyl groups. [Aspect 14] The method according to embodiment 13, wherein the C2-C6 alkylenediamine is selected from the group consisting of ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, and combinations thereof. [Aspect 15] The method according to any one of embodiments 1 to 12, wherein the polyamic acid comprises 1,3-phenylenediamine, 1,4-phenylenediamine, 4,4'-methylenedianiline, 4,4'-diaminodiphenyl ether, or a combination thereof. [Aspect 16] The method according to embodiment 15, wherein the polyamic acid comprises a diamine selected from the group consisting of 1,4-phenylenediamine, 4,4'-methylenedianiline, 4,4'-diaminodiphenyl ether, and combinations thereof. [Aspect 17] The concentration range of the polyamic acid salt in the solution is approximately 0.01 to approximately 0.3 g / cm³, based on the weight of the polyamic acid. 3 The method according to any one of embodiments 1 to 16. [Aspect 18] The method according to any one of embodiments 5 to 17, wherein the polyimide gel is in a monolithic form, and imidizing the polyamic acid comprises adding a dehydrating agent to the aqueous solution of the polyamic acid to form a gelling mixture, and the method further comprises pouring the gelling mixture into a mold to form a gel. [Aspect 19] The polyimide gel is in a monolithic form, the imidation of the polyamic acid salt is carried out thermally, and the method is The process involves adding delta-gluconolactone to the aqueous solution of the polyamic acid salt to form a gel mixture, Pouring the gelling mixture into a mold and turning the gelling mixture into a gel, The obtained polyamic acid gel is washed with water, The method according to any one of embodiments 5 to 17, further comprising forming the polyimide gel by thermal imidation of the polyamic acid gel, wherein the thermal imidation includes exposing the polyamic acid gel to irradiation at a microwave frequency. [Aspect 20] The method according to any one of embodiments 5 to 17, wherein the polyimide gel is in the form of beads, and imidizing the polyamic acid comprises adding a dehydrating agent to the aqueous solution of the polyamic acid to form a gelling mixture, the method further comprises adding the gelling mixture to a solution of a water-soluble acid in water to form the polyimide gel beads, the addition comprising dropping the gelling mixture into the solution of the water-soluble acid in water, spraying the gelling mixture under pressure into the solution of the water-soluble acid in water by passing it through one or more nozzles using pressure, or electrolyzing the gelling mixture into the solution of the water-soluble acid in water. [Aspect 21] The method according to embodiment 18 or 20, wherein the dehydrating agent is anhydride acetic acid. [Aspect 22] The method according to embodiment 20 or 21, wherein the water-soluble acid is a mineral acid or acetic acid. [Aspect 23] The method according to any one of embodiments 5 to 17, wherein the polyimide gel is in the form of beads, and imidizing the polyamic acid comprises adding a dehydrating agent to the aqueous solution of the polyamic acid to form a gelling mixture, the method further comprises optionally adding the gelling mixture to a water-immiscible solvent containing an acid, the addition comprising dropping the gelling mixture into the water-immiscible solvent, spraying the gelling mixture under pressure into the water-immiscible solvent by passing it through one or more nozzles using pressure, or electrolyzing the gelling mixture into the water-immiscible solvent. [Aspect 24] The method according to embodiment 23, wherein the dehydrating agent is anhydrous acetic acid. [Pattern 25] The method according to embodiment 23 or 24, wherein the optionally selected acid is acetic acid. [Aspect 26] The method according to any one of embodiments 20 to 25, wherein the method comprises electrolyzing the gelling mixture by passing it through one or more needles at a voltage in the range of about 5 to about 60 kV. [Aspect 27] The method according to any one of embodiments 5 to 17, wherein the polyimide gel is in the form of beads, and imidizing the polyamic acid salt comprises adding a dehydrating agent to the aqueous solution of the polyamic acid salt to form a gelling mixture, and the method further comprises combining the gelling mixture with a water-immiscible solvent containing a surfactant, and mixing the obtained mixture under high shear conditions. [Aspect 28] The polyimide gel is in the form of beads, and the imidation of the polyamic acid salt includes chemical imidation, and the method is Combining the aqueous solution of the polyamic acid salt with a water-immiscible solvent containing a surfactant, The obtained mixture is mixed under high shear conditions to form a metastable emulsion, The method according to any one of embodiments 5 to 17, comprising adding a dehydrating agent to the metastable emulsion. [Aspect 29] The method according to embodiment 27 or 28, wherein the water-immiscible organic solvent is a C5-C12 hydrocarbon. [Aspect 30] The method according to any one of embodiments 27 to 29, wherein the water-immiscible organic solvent is a mineral spirit. [Aspect 31] To provide an aqueous solution of polyamic acid salts, Dissolving a water-soluble diamine in water to form an aqueous diamine solution, Adding a non-nucleophilic amine to the aqueous diamine solution, Adding a tetracarboxylic dianhydride to the aqueous diamine solution, The method according to embodiment 1, comprising stirring the obtained solution at a temperature in the range of about 15 to about 60°C for a period of about 1 to about 24 hours. [Aspect 32] To provide an aqueous solution of polyamic acid salts, Dissolving a water-soluble diamine in water to form an aqueous diamine solution, Adding a tetracarboxylic dianhydride to the aqueous diamine solution, The obtained suspension is stirred at a temperature in the range of approximately 15 to approximately 60°C for a period of approximately 1 to approximately 24 hours. Adding a non-nucleophilic amine to the aqueous diamine solution, The method according to embodiment 1, further comprising stirring the obtained suspension at a temperature in the range of about 15 to about 60°C for a period of about 1 to about 24 hours. [Aspect 33] To provide an aqueous solution of polyamic acid salts, Simultaneously or intermittently, water is added to water, water-soluble diamine, tetracarboxylic dianhydride, and non-nucleophilic amine. The method according to embodiment 1, comprising stirring the obtained mixture at a temperature in the range of about 15 to about 60°C for a period of about 1 hour to about 24 hours. [Aspect 34] The method according to any one of embodiments 31 to 34, wherein the non-nucleophilic amine has a solubility of at least about 4 grams per liter of water at 20°C. [Aspect 35] The method according to any one of embodiments 31 to 34, wherein the non-nucleophilic amine is a tertiary amine. [Aspect 36] The method according to any one of embodiments 31 to 35, wherein the non-nucleophilic amine is selected from the group consisting of triethylamine, trimethylamine, tri-n-butylamine, N-methylpyrrolidine, N-methylpiperidine, diisopropylethylamine, and combinations thereof. [Aspect 37] The method according to any one of embodiments 31 to 36, wherein the non-nucleophilic amine is triethylamine or diisopropylethylamine. [Aspect 38] The method according to any one of embodiments 31 to 37, wherein the molar ratio of the non-nucleophilic amine to the diamine is about 2 to about 2.5. [Aspect 39] The method according to any one of embodiments 31 to 38, wherein the tetracarboxylic dianhydride is selected from the group consisting of pyromellitic anhydride (PMDA), biphthalic acid dianhydride (BPDA), oxydiphthalic acid dianhydride (ODPA), perylenetetracarboxylic dianhydride, and combinations thereof. [Aspect 40] The method according to any one of embodiments 31 to 38, wherein the diamine is a C2-C6 alkylenediamine, and one or more carbon atoms of the C2-C6 alkylene are optionally substituted with one or more alkyl groups. [Aspect 41] The method according to embodiment 40, wherein the C2-C6 alkylenediamine is selected from the group consisting of ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, and combinations thereof. [Aspect 42] The method according to any one of embodiments 31 to 41, wherein the diamine is 1,3-phenylenediamine, 1,4-phenylenediamine, or a combination thereof. [Aspect 43] The method according to embodiment 42, wherein the diamine is 1,4-phenylenediamine. [Aspect 44] The method according to any one of embodiments 30 to 43, wherein the molar ratio of the tetracarboxylic dianhydride to the diamine is about 0.9 to about 1.1. [Aspect 45] A method for forming a polyamic acid aerogel, wherein the method is To provide an aqueous solution of polyamic acid salt, Acidifying the polyamic acid solution to form a polyamic acid gel, The method comprising drying the polyamic acid gel to form the polyamic acid aerogel. [Aspect 46] The method according to embodiment 45, wherein the polyamic acid gel is in a monolithic form, and acidifying the polyamic acid salt includes adding delta-gluconolactone to the aqueous solution of the polyamic acid salt to form a gelling mixture, pouring the gelling mixture into a mold, and forming the gelling mixture into a gel. [Aspect 47] The method according to embodiment 45, wherein the polyamic acid gel is in the form of beads, and acidifying the polyamic acid salt comprises adding the aqueous solution of the polyamic acid salt to a solution of a water-soluble acid in water to form the polyamic acid gel beads, and the addition comprises dropping the aqueous solution of the polyamic acid salt into the solution of the water-soluble acid in water, spraying the aqueous solution of the polyamic acid salt under pressure into the solution of the water-soluble acid in water by passing it through one or more nozzles using pressure, or electrolyzing the aqueous solution of the polyamic acid salt into the solution of the water-soluble acid in water. [Aspect 48] The method according to embodiment 46 or 47, wherein the water-soluble acid is a mineral acid or acetic acid. [Aspect 49] The method according to any one of embodiments 46 to 48, wherein the method comprises electrolyzing the aqueous solution of the polyamic acid salt by passing it through one or more needles at a voltage in the range of about 5 to about 60 kV. [Aspect 50] The polyamic acid gel is in the form of microbeads, and the method is Combining the aforementioned aqueous solution of polyamic acid salt with a water-immiscible solvent containing a surfactant, The obtained mixture is mixed under high shear conditions to form an emulsion, The method according to embodiment 45, further comprising adding an organic acid to the emulsion. [Aspect 51] The method according to embodiment 50, wherein the water-immiscible organic solvent is a C5-C12 hydrocarbon. [Aspect 52] The method according to embodiment 50 or 51, wherein the water-immiscible organic solvent is a mineral spirit. [Aspect 53] The method according to any one of embodiments 50 to 52, wherein the organic acid is acetic acid. [Aspect 54] To provide the aqueous solution of the polyamic acid salt, To provide a substantially pure form of polyamic acid, Adding the polyamic acid to water to form an aqueous suspension of the polyamic acid, The method according to any one of embodiments 45 to 53, comprising adding a base to the aqueous suspension of the polyamic acid to form the aqueous solution of the polyamic acid salt. [Aspect 55] The method according to embodiment 54, wherein the base is a non-nucleophilic amine. [Aspect 56] The method according to embodiment 54 or 55, wherein the non-nucleophilic amine has a solubility of at least about 4 grams per liter of water at 20°C. [Aspect 57] The method according to embodiment 55 or 56, wherein the non-nucleophilic amine is a tertiary amine. [Aspect 58] The method according to any one of embodiments 55 to 57, wherein the non-nucleophilic amine is selected from the group consisting of triethylamine, trimethylamine, tri-n-butylamine, N-methylpyrrolidine, N-methylpiperidine, diisopropylethylamine, and combinations thereof. [Aspect 59] The method according to embodiment 58, wherein the non-nucleophilic amine is triethylamine or diisopropylethylamine. [Aspect 60] The method according to any one of embodiments 55 to 59, wherein the non-nucleophilic amine is added in an amount sufficient to maintain substantially all of the polyamic acid in solution. [Aspect 61] The method according to any one of embodiments 55 to 60, wherein the molar ratio of the non-nucleophilic amine to the polyamic acid is about 2 to about 4, or about 2.2 to about 2.5. [Aspect 62] The method according to any one of embodiments 55 to 61, wherein the polyamic acid comprises a tetracarboxylic acid selected from the group consisting of benzene-1,2,4,5-tetracarboxylic acid, [1,1'-biphenyl]-3,3',4,4'-tetracarboxylic acid, 4,4'-oxydiphthalic acid, 4,4'-sulfonyldiphthalic acid, 4,4'-carbonyldiphthalic acid, 4,4'-(propane-2,2-diyl)diphthalic acid, 4,4'-(perfluoropropane-2,2-diyl)diphthalic acid, naphthalene-1,4,5,8-tetracarboxylic acid, 4-(2-(4-(3,4-dicarboxyphenoxy)phenyl)propane-2-yl)phthalic acid, perylenetetracarboxylic acid, and combinations thereof. [Aspect 63] The method according to any one of embodiments 55 to 62, wherein the polyamic acid comprises a C2-C6 alkylenediamine, and one or more carbon atoms of the C2-C6 alkylene are optionally substituted with one or more alkyl groups. [Aspect 64] The method according to embodiment 63, wherein the C2-C6 alkylenediamine is selected from the group consisting of ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, and combinations thereof. [Aspect 65] The method according to any one of embodiments 55 to 62, wherein the polyamic acid comprises 1,3-phenylenediamine, 1,4-phenylenediamine, 4,4'-methylenedianiline, 4,4'-diaminodiphenyl ether, or a combination thereof. [Aspect 66] The method according to embodiment 65, wherein the polyamic acid comprises a diamine selected from the group consisting of 1,4-phenylenediamine, 4,4'-methylenedianiline, 4,4'-diaminodiphenyl ether, and combinations thereof. [Aspect 67] The concentration range of the polyamic acid salt in the solution is approximately 0.01 to approximately 0.3 g / cm³, based on the weight of the polyamic acid. 3 The method according to any one of embodiments 55 to 66. [Pattern 68] To provide an aqueous solution of polyamic acid salts, Dissolving a water-soluble diamine in water to form an aqueous diamine solution, Adding a non-nucleophilic amine to the aqueous diamine solution, Adding a tetracarboxylic dianhydride to the aqueous diamine solution, The method according to any one of embodiments 45 to 53, comprising stirring the obtained mixture at a temperature in the range of about 15 to about 60°C for a period of about 1 hour to about 24 hours. [Aspect 69] To provide an aqueous solution of polyamic acid salts, Dissolving a water-soluble diamine in water to form an aqueous diamine solution, Adding a tetracarboxylic dianhydride to the aqueous diamine solution, The obtained suspension is stirred at a temperature in the range of approximately 15 to approximately 60°C for a period of approximately 1 to approximately 24 hours. Adding a non-nucleophilic amine to the suspension, The method according to any one of embodiments 45 to 53, comprising stirring the obtained mixture at a temperature in the range of about 15 to about 60°C for a period of about 1 hour to about 24 hours. [Aspect 70] To provide an aqueous solution of polyamic acid salts, Simultaneously or intermittently, water is added to water, water-soluble diamine, tetracarboxylic dianhydride, and non-nucleophilic amine. The method according to any one of embodiments 45 to 53, comprising stirring the obtained mixture at a temperature in the range of about 15 to about 60°C for a period of about 1 hour to about 24 hours. [Aspect 71] The method according to any one of embodiments 68 to 70, wherein the non-nucleophilic amine has a solubility of at least about 4 grams per liter of water at 20°C. [Aspect 72] The method according to any one of embodiments 68 to 71, wherein the non-nucleophilic amine is a tertiary amine. [Aspect 73] The method according to any one of embodiments 68 to 72, wherein the non-nucleophilic amine is selected from the group consisting of triethylamine, trimethylamine, tri-n-butylamine, N-methylpyrrolidine, N-methylpiperidine, and diisopropylethylamine. [Aspect 74] The method according to embodiment 73, wherein the non-nucleophilic amine is triethylamine or diisopropylethylamine. [Aspect 75] The method according to any one of embodiments 68 to 74, wherein the molar ratio of the non-nucleophilic amine to the diamine is about 2 to about 2.5. [Aspect 76] The method according to any one of embodiments 68 to 75, wherein the tetracarboxylic dianhydride is selected from the group consisting of pyromellitic anhydride (PMDA), biphthalic dianhydride (BPDA), oxydiphthalic dianhydride (ODPA), perylenetetracarboxylic dianhydride, and combinations thereof. [Aspect 77] The method according to any one of embodiments 68 to 75, wherein the diamine is a C2-C6 alkylenediamine, and one or more carbon atoms of the C2-C6 alkylene are optionally substituted with one or more alkyl groups. [Aspect 78] The method according to embodiment 77, wherein the C2-C6 alkylenediamine is selected from the group consisting of ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, and combinations thereof. [Aspect 79] The method according to any one of embodiments 68 to 78, wherein the diamine is 1,4-phenylenediamine. [Aspect 80] The method according to any one of embodiments 68 to 79, wherein the molar ratio of the tetracarboxylic dianhydride to the diamine is about 0.9 to about 1.1. [Aspect 81] A method for forming a polyamic acid metal salt aerogel in bead form, wherein the method is To provide an aqueous solution of an ammonium or alkali metal salt of a polyamic acid, The process involves performing metal ion exchange, which includes adding a solution of the polyamic acid salt to a solution containing a soluble metal salt to form polyamate metal salt gel beads, The method comprising drying the polyamic acid metal salt gel beads to form the polyamic acid metal salt aerogel beads. [Aspect 82] The method according to embodiment 81, wherein the soluble metal salt comprises a main group transition metal, a rare earth metal, an alkaline earth metal, or a combination thereof. [Aspect 83] The method according to embodiment 82, wherein the soluble metal salt comprises copper, iron, nickel, silver, calcium, magnesium, or a combination thereof. [Aspect 84] The method according to embodiment 82, wherein the soluble metal salt includes lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or a combination thereof. [Aspect 85] The method according to any one of embodiments 81 to 84, wherein adding the polyamic acid solution to a solution containing a soluble metal salt includes dropping the aqueous solution of the polyamic acid into the solution of the soluble metal salt, spraying the aqueous solution of the polyamic acid into the solution of the soluble metal salt under pressure through one or more nozzles, or electrolyzing the aqueous solution of the polyamic acid into the solution of the soluble metal salt. [Aspect 86] The method according to embodiment 85, wherein the method comprises electrolyzing the polyamic acid solution by passing it through one or more needles at a voltage in the range of about 5 to about 60 kV. [Aspect 87] Drying the polyimide gel Optionally, the polyimide gel may be washed or its solvent replaced. The method according to any one of embodiments 1 to 44, comprising: subjecting the polyimide gel to high-temperature conditions; freeze-drying the polyimide gel; or contacting the polyimide gel with supercritical fluid carbon dioxide. [Pattern 88] The method according to embodiment 87, wherein the washing or solvent exchange is carried out using water, C1-C3 alcohols, acetone, acetonitrile, ether, tetrahydrofuran, toluene, liquid carbon dioxide, or a combination thereof. [Aspect 89] The method according to embodiment 87 or 88, further comprising converting the polyimide aerogel into an isomorphic carbon aerogel, wherein the conversion includes thermal decomposition of the polyimide aerogel in an inert atmosphere at a temperature of at least about 650°C. [Aspect 90] Drying the polyamic acid gel Optionally, the polyamic acid gel may be washed or its solvent replaced. The method according to any one of embodiments 45 to 81, comprising: subjecting the polyamic acid gel to high-temperature conditions; freeze-drying the polyamic acid gel; or contacting the polyamic acid gel with supercritical fluid carbon dioxide. [Aspect 91] The method according to embodiment 90, wherein the washing or solvent exchange is carried out using water, C1-C3 alcohols, acetone, acetonitrile, ether, tetrahydrofuran, toluene, liquid carbon dioxide, or a combination thereof. [Aspect 92] The method according to embodiment 90 or 91, further comprising converting the polyamic acid aerogel into an isomorphic carbon aerogel, wherein the conversion includes thermal decomposition of the polyamic acid aerogel material in an inert atmosphere at a temperature of at least about 650°C. [Aspect 93] The method according to embodiment 92, further comprising converting the polyamic acid aerogel to an isomorphic polyimide aerogel before the thermal decomposition, wherein the conversion of the polyamic acid aerogel to a polyimide aerogel includes thermal imidization of the polyamic acid aerogel. [Aspect 94] The method according to any one of embodiments 82 to 86, further comprising converting the polyamic acid metal salt aerogel into an isomorphic metal or metal oxide doped carbon aerogel, wherein the conversion includes thermal decomposition of the polyamic acid aerogel in an inert atmosphere at a temperature of at least about 650°C. [Aspect 95] The method according to any one of embodiments 1 to 44, further comprising adding an electroactive material to the aqueous solution of the polyamic acid salt. [Aspect 96] The method according to any one of embodiments 45 to 81, further comprising adding an electroactive material to the aqueous solution of the polyamic acid salt. [Aspect 97] The method according to any one of embodiments 82 to 86, further comprising adding an electroactive material to the aqueous solution of the ammonium polyamate or alkali metal salt. [Aspect 98] The method according to any one of embodiments 89 or 92-94, wherein the carbon aerogel has properties substantially similar to those of a carbon aerogel prepared by thermal decomposition of a corresponding polyimide aerogel prepared by a conventional non-aqueous method. [Aspect 99] The method according to any one of embodiments 1 to 44, wherein the polyimide gel contains more than about 75% by volume of residual water. [Aspect 100] The method according to any one of embodiments 45 to 81, wherein the polyamic acid gel contains more than about 75% by volume of residual water. [Aspect 101] The method according to any one of embodiments 82 to 86, wherein the polyamic acid metal salt gel beads contain more than about 75% by volume of residual water. [Aspect 102] A polyimide aerogel prepared by the method described in any one of embodiments 1 to 44, 87, or 88. [Aspect 103] solid 15 A polyimide aerogel according to embodiment 102, comprising terminal amine groups as determined by N-NMR. [Aspect 104] A polyamic acid aerogel prepared by the method described in any one of embodiments 45-81, 90, or 91. [Aspect 105] solid 15 A polyamic acid aerogel according to embodiment 104, comprising terminal amine groups as determined by N-NMR. [Aspect 106] A carbon aerogel prepared by the method described in any one of embodiments 89, 92, 93, or 94. [Aspect 107] A carbon aerogel comprising an electroactive material prepared by the method described in any one of embodiments 95 to 97.

Claims

1. A method for forming a polyimide aerogel, wherein the method is To provide an aqueous solution of a polyamic acid salt, wherein the polyamic acid salt comprises a polyamic acid containing a carboxylic acid group, the carboxylic acid group being bonded to an alkali metal cation and substantially existing as a carboxylate anion, and the provision of the aqueous solution of the polyamic acid salt comprises providing the polyamic acid, adding the polyamic acid to water to form an aqueous suspension of the polyamic acid, and adding an alkali metal hydroxide to the aqueous suspension of the polyamic acid to form an aqueous solution of the polyamic acid salt, wherein the provision of the aqueous solution of the polyamic acid salt comprises providing the polyamic acid, and the provision of the aqueous solution of the polyamic acid salt comprises adding the polyamic acid to water to form an aqueous suspension of the polyamic acid salt. The process involves imidizing the aforementioned polyamic acid salt to form a polyimide gel, The method comprising drying the polyimide gel to form the polyimide aerogel.

2. The method according to claim 1, wherein the alkali metal hydroxide is lithium hydroxide, sodium hydroxide, or potassium hydroxide.

3. The method according to claim 1, wherein the polyamic acid comprises a tetracarboxylic acid selected from the group consisting of benzene-1,2,4,5-tetracarboxylic acid, [1,1'-biphenyl]-3,3',4,4'-tetracarboxylic acid, 4,4'-oxydiphthalic acid, 4,4'-sulfonyldiphthalic acid, 4,4'-carbonyldiphthalic acid, 4,4'-(propane-2,2-diyl)diphthalic acid, 4,4'-(perfluoropropane-2,2-diyl)diphthalic acid, naphthalene-1,4,5,8-tetracarboxylic acid, 4-(2-(4-(3,4-dicarboxyphenoxy)phenyl)propane-2-yl)phthalic acid, perylenetetracarboxylic acid, and combinations thereof.

4. The method according to claim 1, wherein the polyamic acid comprises a C2-C6 alkylenediamine, and one or more carbon atoms of the C2-C6 alkylene are optionally substituted with one or more alkyl groups.

5. The method according to claim 1, wherein the polyamic acid comprises 1,3-phenylenediamine, 1,4-phenylenediamine, 4,4'-methylenedianiline, 4,4'-diaminodiphenyl ether, or a combination thereof.

6. The method according to claim 1, wherein the polyimide gel is in a monolithic form, and imidizing the polyamic acid salt involves adding a dehydrating agent to the aqueous solution of the polyamic acid salt to form a gelling mixture, and the method further comprises pouring the gelling mixture into a mold to form a gelling mixture.

7. The polyimide gel is in a monolithic form, the imidation of the polyamic acid salt is carried out thermally, and the method is The process involves adding delta-gluconolactone to the aqueous solution of the polyamic acid salt to form a gel mixture, Pouring the gelling mixture into a mold and turning the gelling mixture into a gel, The obtained polyamic acid gel is washed with water, The method according to claim 1, further comprising forming the polyimide gel by thermal imidation of the polyamic acid gel, wherein the thermal imidation includes exposing the polyamic acid gel to irradiation at a microwave frequency.

8. The method according to claim 1, wherein the polyimide gel is in the form of beads, and imidizing the polyamic acid comprises adding a dehydrating agent to the aqueous solution of the polyamic acid to form a gelling mixture, the method further comprises adding the gelling mixture to a solution of a water-soluble acid in water to form the polyimide gel beads, the addition comprising dropping the gelling mixture into the solution of the water-soluble acid in water, spraying the gelling mixture under pressure into the solution of the water-soluble acid in water by passing it through one or more nozzles using pressure, or electrolyzing the gelling mixture into the solution of the water-soluble acid in water.

9. The method according to claim 1, wherein the polyimide gel is in the form of beads, and imidizing the polyamic acid comprises adding a dehydrating agent to the aqueous solution of the polyamic acid to form a gelling mixture, the method further comprises optionally adding the gelling mixture to a water-immiscible solvent containing an acid, the addition comprising dropping the gelling mixture into the water-immiscible solvent, spraying the gelling mixture under pressure into the water-immiscible solvent by passing it through one or more nozzles using pressure, or electrolyzing the gelling mixture into the water-immiscible solvent.

10. The method according to claim 9, wherein the method comprises electrolyzing the gelling mixture by passing it through one or more needles at a voltage in the range of about 5 to about 60 kV.

11. The method according to claim 1, wherein the polyimide gel is in the form of beads, and imidizing the polyamic acid salt involves adding a dehydrating agent to the aqueous solution of the polyamic acid salt to form a gelling mixture, and the method further comprises combining the gelling mixture with a water-immiscible solvent containing a surfactant, and mixing the obtained mixture under high shear conditions.

12. The polyimide gel is in the form of beads, and the imidation of the polyamic acid salt includes chemical imidation, and the method is Combining the aqueous solution of the polyamic acid salt with a water-immiscible solvent containing a surfactant, The obtained mixture is mixed under high shear conditions to form a metastable emulsion, The method according to claim 1, comprising adding a dehydrating agent to the metastable emulsion.

13. To provide an aqueous solution of polyamic acid salts, Dissolving a water-soluble diamine in water to form an aqueous diamine solution, Adding a non-nucleophilic amine to the aqueous diamine solution, Adding a tetracarboxylic dianhydride to the aqueous diamine solution, The method according to claim 1, comprising stirring the obtained solution at a temperature in the range of 15 to 60°C for a period of about 1 hour to about 24 hours.

14. To provide an aqueous solution of polyamic acid salts, Dissolving a water-soluble diamine in water to form an aqueous diamine solution, Adding a tetracarboxylic dianhydride to the aqueous diamine solution, The obtained suspension is stirred at a temperature in the range of 15 to 60°C for a period of about 1 to 24 hours. Adding a non-nucleophilic amine to the aqueous diamine solution, The method according to claim 1, further comprising stirring the obtained suspension at a temperature in the range of 15 to 60°C for a period of about 1 hour to about 24 hours.

15. To provide an aqueous solution of polyamic acid salts, Simultaneously or intermittently, water is added to water, water-soluble diamine, tetracarboxylic dianhydride, and non-nucleophilic amine. The method according to claim 1, comprising stirring the obtained mixture at a temperature in the range of 15 to 60°C for a period of about 1 hour to about 24 hours.

16. A method for forming a polyamic acid aerogel, wherein the method is To provide an aqueous solution of polyamic acid salt, Acidifying the polyamic acid solution to form a polyamic acid gel, The method comprising drying the polyamic acid gel to form the polyamic acid aerogel.

17. A method for forming a polyimide aerogel in bead form, wherein the method is To provide an aqueous solution of a polyamic acid salt, wherein the polyamic acid salt comprises a polyamic acid containing a carboxylic acid group, the carboxylic acid group is bonded to a cationic species, and more than 95% of the carboxylic acid group exists as a carboxylate anion. Forming a polyimide gel by imidizing the polyamic acid salt, wherein the imidization includes adding a dehydrating agent to an aqueous solution of the polyamic acid salt to form a gel mixture, Adding the gelling mixture to a water-immiscible solvent, wherein the addition includes dropping the gelling mixture into the water-immiscible solvent, spraying the gelling mixture into the water-immiscible solvent under pressure by passing it through one or more nozzles using pressure, or electrolyzing the gelling mixture into the water-immiscible solvent, The method comprising drying the polyamic acid gel to form the polyamic acid aerogel.

18. The method according to claim 1, 16, or 17, further comprising converting the polyimide aerogel into an isomorphic carbon aerogel, wherein the conversion includes thermal decomposition of the polyimide aerogel in an inert atmosphere at a temperature of at least 650°C.

19. The method according to claim 1, 16, or 17, further comprising adding an electroactive material to the aqueous solution of the polyamic acid salt.

20. The method according to claim 1, 16, or 17, wherein the polyimide gel contains more than 75% by volume of residual water.

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