Aqueous process for preparing polyamic acid and polyamic acid-related gel materials

Abstract

The present invention relates to a method for forming polyamic acid, polyamic acid metal salt, and polyimide gel under aqueous conditions using a water-soluble carbonate or bicarbonate. These gels can be converted into aerogels or xerogels, and further into carbon aerogels or xerogels. Such carbon aerogels or xerogels have the same physical properties as carbon aerogels or xerogels prepared from polyimide aerogels obtained according to conventional methods, i.e., organic solvent-based methods.

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims priority and benefit to U.S. Provisional Patent Application No. 63 / 352,571, filed on June 15, 2022, entitled "Aqueous Process For Preparing Polyamic Acids and Polyamic Acid Related Gel Materials", the entire disclosure of which is hereby incorporated by reference in its entirety.

[0002] The present disclosure generally relates to an aqueous process for preparing an aqueous solution of a salt of a polyamic acid, as well as methods for converting such materials into porous polyamic acids, polyimides, and corresponding porous carbon materials.

Background Art

[0003] An aerogel is a solid material that includes a porous network of interconnected micro-, meso-, and macro-sized pores. Depending on the precursor materials used and the processes performed, the pores of an aerogel can often occupy a proportion of more than 90% of the volume. An aerogel is generally prepared by removing the solvent from a wet gel (a solid network prepared by a sol-gel process containing a solvent) in a manner that can minimize or completely suppress shrinkage due to capillary forces in the pore walls of the wet gel. Methods of solvent removal include, but are not limited to, supercritical drying (i.e., drying using a supercritical fluid such that the high surface tension gelling solvent in the gel is replaced with a supercritical fluid of low surface tension), solvent exchange with a supercritical fluid, solvent exchange with a fluid that is later converted to the supercritical state, subcritical or near-critical point drying, and sublimation of the frozen solvent in a freeze-drying process. See, for example, PCT Patent Application Publication No. WO2016127084A1. It should be noted that when drying under ambient conditions, gel shrinkage may occur with solvent evaporation, and a xerogel may be formed. Thus, the preparation of an aerogel by the sol-gel method (or optionally a similar process) generally proceeds in the following series of steps: dissolution of monomers, or a mixture of monomers, or gel precursors in a solvent, addition of a catalyst or reagent to induce or facilitate the reaction of the monomers or gel precursors, formation of a reaction mixture, formation of a wet gel (which may include additional heating or cooling steps), and removal of the solvent by supercritical drying techniques or any other method that does not cause shrinkage or pore collapse of the gel.

[0004] Aerogels can be formed from inorganic materials, organic materials, or mixtures thereof. When formed from organic materials, the organic aerogels are carbonizable (e.g., by pyrolysis at high temperature in an inert atmosphere) and can form carbon aerogels, which may have different or overlapping properties (e.g., pore volume, pore size distribution, morphology, etc.) from the corresponding organic aerogels depending on the precursor materials and methodologies used. Examples of organic materials that can be carbonized include phenol-aldehyde polymers such as resorcinol-formaldehyde (RF) and phloroglucinol-furfuraldehyde (PF), polyolefins such as polyacrylonitrile (PAN), selected polyimides (PI), polyurethanes (PU), polyurea (PUA), polyamides (PA), polydicyclopentadiene, precursors or any polymer derivatives thereof, and combinations thereof, but are not limited thereto.

[0005] In recent years, efforts have been made to focus on the development and characterization of carbon aerogels, xerogels, and ambigels as electrode materials with improved performance for applications in energy storage devices such as lithium-ion batteries (LIB). Therefore, the corresponding organic aerogels are in demand. 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 and then dehydrating the polyamic acid, which is the primary polymer product obtained by the reaction, to form a polyimide gel. For economic, safety, and environmental reasons, it would be desirable to perform the formation and gelation of such polyamic acid-based gel materials using "green" chemical processes (i.e., using environmentally friendly alternatives in place of conventional organic solvents and / or organic reagents). SUMMARY OF THE INVENTION

[0006] This technology generally relates to a method for forming polyamic acids and related gel materials (e.g., polyimide wet gels, aerogels, xerogels, and ambigels) while (a) minimizing or eliminating the use of harmful organic solvents and (b) minimizing the use of organic reagents. Specifically, this technology relates to using water-soluble carbonates or bicarbonates for solubilization of polyamic acids to form polyamic acids and polyimide gels in water. Optionally, the gels thus formed may then be converted into polyamic acid aerogels, polyimide gels, and aerogels, as well as the corresponding carbon aerogels, xerogels, and / or other gel materials.

[0007] The disclosed method generally involves preparing an aqueous solution of a polyamic acid salt in the presence of a water-soluble carbonate or bicarbonate from the reaction of a water-soluble diamine and a tetracarboxylic dianhydride. Alternatively, an aqueous solution of a polyamic acid salt may be prepared by dissolving an acid in water in the presence of a water-soluble carbonate or bicarbonate from a pre-formed polyamic acid. Surprisingly, according to the present disclosure, such water-soluble carbonates or bicarbonates (including, but not limited to, carbonates and bicarbonates of certain alkali metals, ammonia, and guanidine) result in the formation of the corresponding polyamic acid salts (salts of polyamic acids, or simply referred to as polyamic acid salts), and further, upon imidization, the resulting polyimide was found to be mechanically strong and often optically transparent. In particular, the resulting polyimide had substantially the same physical properties as polyimides prepared by conventional (i.e., organic solvent-based) methods. In contrast, using an amine base with reduced nucleophilicity (e.g., triethylamine) under the same conditions usually results in wet gels, which are generally translucent or opaque and weak with respect to their physical strength.

[0008] Accordingly, in one aspect, a method for preparing an aqueous solution of polyamic acid salt is provided. In a general non-limiting aspect, the method includes mixing a water-soluble diamine, a water-soluble carbonate or bicarbonate, and a tetracarboxylic dianhydride in water and reacting the components to provide a solution of polyamic acid salt. The order of addition of each of the reaction components (i.e., the water-soluble diamine, the water-soluble carbonate or bicarbonate, and the tetracarboxylic dianhydride) can vary.

[0009] In some aspects, mixing the reaction components includes dissolving the water-soluble diamine in water to form an aqueous diamine solution, adding a salt of the water-soluble carbonate or bicarbonate to the aqueous diamine solution, adding the tetracarboxylic dianhydride to the aqueous solution of the diamine and the water-soluble carbonate or bicarbonate to form a solution, and stirring the solution at a temperature in the range of about 4 °C to about 60 °C for a period in the range of about 1 hour to about 4 days.

[0010] In some aspects, the water-soluble carbonate or bicarbonate includes lithium, sodium, potassium, ammonium, alkylammonium, guanidinium, or combinations thereof.

[0011] In some aspects, the water-soluble carbonate or bicarbonate is selected from the group consisting of lithium carbonate, lithium bicarbonate, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, and combinations thereof.

[0012] In some aspects, the water-soluble carbonate or bicarbonate is selected from the group consisting of guanidinium carbonate, ammonium carbonate, and ammonium bicarbonate. In some aspects, the water-soluble carbonate or bicarbonate is guanidinium carbonate.

[0013] In some aspects, the water-soluble carbonate or bicarbonate is a carbonate, and the molar ratio of the carbonate to the diamine is about 1 to about 1.4.

[0014] In some aspects, the water-soluble carbonate or bicarbonate is a bicarbonate, and the molar ratio of the bicarbonate to the diamine is about 2 to about 2.8.

[0015] In some embodiments, the molar ratio of the tetracarboxylic dianhydride to the diamine is from about 0.9 to about 1.1.

[0016] In some embodiments, the tetracarboxylic dianhydride is selected from the group consisting of pyromellitic dianhydride (PMDA), biphenyl dianhydride (BPDA), oxydiphthalic dianhydride (ODPA), benzophenone tetracarboxylic dianhydride (BTDA), ethylenediaminetetraacetic dianhydride (EDDA), 1,4,5,8-naphthalenetetracarboxylic dianhydride, and combinations thereof. In some embodiments, the tetracarboxylic dianhydride is pyromellitic dianhydride (PMDA). In some embodiments, the tetracarboxylic dianhydride is benzophenone tetracarboxylic dianhydride (BTDA).

[0017] In some embodiments, the water-soluble diamine is a C2-C6 alkylene diamine, and one or more of the carbon atoms of the C2-C6 alkylene are optionally substituted with one or more alkyl groups. In some embodiments, the C2-C6 alkylene diamine is selected from the group consisting of 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, and ethylenediamine.

[0018] In some embodiments, the water-soluble diamine is 1,3-phenylenediamine, 4,4'-methylenedianiline, or a combination thereof. In some embodiments, the water-soluble diamine is selected from the group consisting of 1,4-phenylenediamine, 1,3-phenylenediamine, and combinations thereof. In some embodiments, the water-soluble diamine is 1,4-phenylenediamine. In some embodiments, the water-soluble diamine is 1,3-phenylenediamine.

[0019] In some embodiments, the concentration range of the polyamic acid salt in the aqueous solution is from about 0.01 to about 0.3 g / cm based on the weight of the polyamic acid 3 is.

[0020] In some embodiments, the molar ratio of the tetracarboxylic dianhydride to the diamine is from about 0.9 to about 1.1.

[0021] In some embodiments, the method further includes adding an electroactive material to an aqueous solution of the polyamic acid salt. In some embodiments, the electroactive material includes carbon, graphite, silicon, tin, sulfur, Prussian blue, lithium metal phosphate, lithium mixed metal phosphate, or lithium metal fluorophosphate. In some embodiments, the electroactive material includes lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, combinations thereof, or any one or more precursors thereof. In some embodiments, the electroactive material is silicon. In some embodiments, the electroactive material is lithium iron phosphate.

[0022] In some embodiments, the method further includes forming a polyamic acid aerogel, where forming the polyamic acid aerogel includes acidifying the polyamic acid salt solution to form a polyamic acid gel and drying the polyamic acid gel to form the polyamic acid aerogel.

[0023] In some embodiments, drying the polyamic acid gel optionally includes washing or solvent-exchanging the polyamic acid gel, subjecting the optionally washed or solvent-exchanged polyamic acid gel to high temperature conditions, freeze-drying the optionally washed or solvent-exchanged polyamic acid gel, or contacting the optionally washed or solvent-exchanged polyamic acid gel with supercritical fluid carbon dioxide.

[0024] In some embodiments, washing or solvent-exchanging is performed using water, a C1-C4 alcohol, acetone, acetonitrile, ether, tetrahydrofuran, toluene, liquid carbon dioxide, or combinations thereof.

[0025] In yet another embodiment, a polyamic acid aerogel prepared by the method disclosed herein is provided.

[0026] In some embodiments, the polyamic acid aerogel contains a residual amount of a water-soluble carbonate or bicarbonate.

[0027] In some embodiments, the method further includes forming a polyimide aerogel, where forming the polyimide aerogel includes imidizing a polyamic acid salt to form a polyimide gel and drying the polyimide gel to form the polyimide aerogel.

[0028] In some embodiments, imidizing the polyamic acid salt includes adding a gelation initiator to an aqueous solution of the polyamic acid salt to form a gelation mixture and gelling the gelation mixture. In some embodiments, the gelation initiator is a carboxylic acid anhydride. In some embodiments, the gelation initiator is acetic anhydride.

[0029] In some embodiments, drying the polyimide gel includes optionally washing or solvent-exchanging the polyimide gel, subjecting the polyimide gel to high-temperature conditions, freeze-drying the polyimide gel, or contacting the polyimide gel with supercritical fluid carbon dioxide.

[0030] In some embodiments, the washing or solvent-exchanging is performed using water, a C1-C4 alcohol, acetone, acetonitrile, ether, tetrahydrofuran, toluene, liquid carbon dioxide, or a combination thereof.

[0031] In a further embodiment, a polyimide acid aerogel prepared by the method disclosed herein is provided.

[0032] In some embodiments, the polyimide acid aerogel contains a residual amount of a water-soluble carbonate or bicarbonate.

[0033] In some embodiments, the method further comprises converting a polyamic acid aerogel and / or a polyimide aerogel as disclosed herein into a graphitic carbon aerogel, wherein converting comprises pyrolyzing the aerogel in an inert atmosphere at a temperature of at least about 500°C.

[0034] In a further embodiment, there is provided a carbon aerogel prepared by the method disclosed herein.

[0035] In some embodiments, the carbon aerogel comprises an electroactive material.

[0036] In some embodiments, the carbon aerogel has substantially the same properties as a carbon aerogel prepared by pyrolyzing a corresponding polyimide aerogel prepared by conventional non-aqueous methods.

[0037] In another embodiment, there is provided a method for preparing an aqueous solution of a polyamic acid salt, the method comprising providing a suspension of a pre-formed polyamic acid in water and adding a water-soluble carbonate or bicarbonate to the suspension in an amount sufficient to completely dissolve the polyamic acid to form an aqueous solution of the polyamic acid salt. Such an aqueous solution of the polyamic acid salt may be further processed according to any of the above embodiments.

[0038] The present disclosure includes, but is not limited to, the following embodiments.

[0039] Embodiment 1: A method for preparing an aqueous solution of a salt of a polyamic acid, the method comprising providing a polyamic acid and mixing the polyamic acid with a water-soluble carbonate or bicarbonate in water, thereby providing a solution of the salt of the polyamic acid.

[0040] Embodiment 2: The method according to Embodiment 1, wherein the water-soluble carbonate or bicarbonate comprises a lithium, sodium, potassium, ammonium, or guanidinium cation.

[0041] Aspect 3: The method according to aspect 1 or 2, wherein the water-soluble carbonate or bicarbonate is selected from the group consisting of lithium carbonate, lithium bicarbonate, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, ammonium carbonate, ammonium bicarbonate, guanidinium carbonate, and combinations thereof.

[0042] Aspect 4: A method for preparing an aqueous solution of a salt of polyamic acid, the method comprising mixing a water-soluble diamine, a water-soluble carbonate or bicarbonate, and a tetracarboxylic dianhydride in water, reacting the components, and providing a solution of the salt of polyamic acid.

[0043] Aspect 5: The mixing includes dissolving a water-soluble diamine in water to form an aqueous diamine solution, adding the water-soluble carbonate or bicarbonate to the aqueous diamine solution, adding a tetracarboxylic dianhydride to the aqueous solution of the diamine and the water-soluble carbonate or bicarbonate to form a solution, and stirring the solution at a temperature in the range of about 4 to about 60 °C for a period in the range of about 1 hour to about 4 days.

[0044] Aspect 6: The method according to aspect 4, wherein the mixing includes dissolving a water-soluble diamine in water to form an aqueous diamine solution, adding a tetracarboxylic dianhydride to the aqueous diamine solution to form a suspension, stirring the suspension at a temperature in the range of about 4 to about 60 °C for a period in the range of about 1 hour to about 4 days, adding the water-soluble carbonate or bicarbonate to the suspension, and stirring the suspension at a temperature in the range of about 4 to about 60 °C for a period in the range of about 1 hour to about 4 days to provide an aqueous solution of the salt of polyamic acid.

[0045] Aspect 7: The method according to aspect 4, wherein the mixing includes adding a water-soluble diamine, a tetracarboxylic dianhydride, and a water-soluble carbonate or bicarbonate to water simultaneously or in rapid succession, and stirring the resulting mixture at a temperature in the range of about 4 to about 60 °C for a period in the range of about 1 hour to about 4 days to provide an aqueous solution of the polyamic acid salt.

[0046] Aspect 8: The method according to any one of Aspects 4 to 7, wherein the water-soluble carbonate or bicarbonate contains a lithium, sodium, potassium, ammonium, or guanidinium cation.

[0047] Aspect 9: The method according to any one of Aspects 4 to 8, wherein the water-soluble carbonate or bicarbonate is selected from the group consisting of lithium carbonate, lithium bicarbonate, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, ammonium carbonate, ammonium bicarbonate, guanidinium carbonate, and combinations thereof.

[0048] Aspect 10: The method according to any one of Aspects 4 to 9, wherein the water-soluble carbonate or bicarbonate is a carbonate, and the molar ratio of the water-soluble carbonate to the diamine is about 1 to about 1.4, or the water-soluble carbonate or bicarbonate is a bicarbonate, and the molar ratio of the water-soluble bicarbonate to the diamine is about 2 to about 2.8.

[0049] Aspect 11: The method according to any one of Aspects 4 to 10, wherein the molar ratio of the tetracarboxylic dianhydride to the diamine is about 0.9 to about 1.1.

[0050] Aspect 12: The method according to any one of Aspects 4 to 11, wherein the tetracarboxylic dianhydride is selected from the group consisting of biphenyltetracarboxylic dianhydride (BPDA), benzophenonetetracarboxylic dianhydride (BTDA), oxydiphthalic dianhydride (ODPA), naphthyltetracarboxylic dianhydride, perylenetetracarboxylic dianhydride, and pyromellitic dianhydride (PMDA).

[0051] Aspect 13: The method according to any one of Aspects 4 to 12, wherein the diamine is 1,3-phenylenediamine, 1,4-phenylenediamine, or a combination thereof.

[0052] Aspect 14: The method according to any one of Aspects 4 to 13, wherein the diamine is 1,4-phenylenediamine.

[0053] Aspect 15: The concentration range of the polyamic acid salt in the aqueous solution is about 0.01 to about 0.3 g / cm based on the weight of the polyamic acid. 3 The method according to any one of Aspects 4 to 14, wherein the concentration range of the polyamic acid salt in the aqueous solution is about 0.01 to about 0.3 g / cm based on the weight of the polyamic acid.

[0054] Aspect 16: The method according to any one of Aspects 1 to 15, wherein the water-soluble carbonate or bicarbonate is guanidinium carbonate, and the solution of the salt of the polyamic acid exhibits thixotropic behavior.

[0055] Aspect 17: The method according to any one of Aspects 1 to 16, further comprising adding an electroactive material to the aqueous solution of the polyamic acid salt.

[0056] Aspect 18: The method according to Aspect 17, wherein the electroactive material comprises carbon, graphite, silicon, sulfur, Prussian blue, lithium iron phosphate, a combination thereof, or one or more precursors thereof.

[0057] Aspect 19: The method according to any one of Aspects 1 to 18, further comprising forming a polyamic acid aerogel, wherein forming the polyamic acid aerogel comprises acidifying the polyamic acid salt solution to form a polyamic acid wet gel, and drying the polyamic acid wet gel to form the polyamic acid aerogel.

[0058] Aspect 20: The method according to Aspect 19, wherein drying the polyamic acid wet gel optionally comprises washing or solvent-exchanging the polyamic acid wet gel, subjecting the polyamic acid wet gel to high-temperature conditions, freeze-drying the polyamic acid wet gel, or contacting the polyamic acid wet gel with supercritical fluid carbon dioxide.

[0059] Aspect 21: The method according to Aspect 19 or 20, further comprising converting the polyamic acid aerogel into a carbon aerogel having the same shape, wherein the conversion comprises pyrolyzing the polyamic acid aerogel material at a temperature of at least about 650 °C in an inert atmosphere.

[0060] Aspect 22: Further comprising forming a polyimide aerogel, wherein forming the polyimide aerogel comprises imidizing the polyamic acid salt to form a polyimide wet gel, and drying the polyimide wet gel to form the polyimide aerogel, the method according to any one of Aspects 1 to 18.

[0061] Aspect 23: The method according to Aspect 22, wherein imidizing the polyamic acid salt comprises adding a gelation initiator to an aqueous solution of the polyamic acid salt to form a gelation mixture, and gelling the gelation mixture.

[0062] Aspect 24: The method according to Aspect 23, wherein the gelation initiator is acetic anhydride.

[0063] Aspect 25: Drying the polyimide wet gel optionally comprises washing or solvent-exchanging the polyimide wet gel, subjecting the polyimide wet gel to high temperature conditions, freeze-drying the polyimide wet gel, or contacting the polyimide wet gel with supercritical fluid carbon dioxide, the method according to any one of Aspects 22 to 24.

[0064] Aspect 26: The method according to Aspect 20 or 25, wherein the washing or solvent-exchanging is carried out using water, C1-C4 alcohol, acetone, acetonitrile, ether, tetrahydrofuran, toluene, liquid carbon dioxide, or a combination thereof.

[0065] Aspect 27: Further comprising converting the polyimide aerogel into a carbon aerogel having the same shape, the conversion comprising pyrolyzing the polyimide aerogel at a temperature of at least about 650 °C in an inert atmosphere, the method according to any one of Aspects 22 to 26.

[0066] Aspect 28: A polyamic acid salt prepared by the method according to any one of Aspects 1 to 18.

[0067] Aspect 29: A polyamic acid aerogel prepared by the method according to Aspect 19 or 20.

[0068] Aspect 30: A polyimide aerogel prepared by the method according to any one of Aspects 22 to 26.

[0069] Aspect 31: The polyamic acid aerogel according to Aspect 29, comprising the residual amount of the water-soluble carbonate or bicarbonate.

[0070] Aspect 32: The polyamic acid aerogel according to Aspect 30, comprising the residual amount of the water-soluble carbonate or bicarbonate.

[0071] Aspect 33: A carbon aerogel prepared by the method according to Aspect 21 or 27.

[0072] To provide an understanding of the aspects of the present technology, reference is made to the accompanying drawings, which are not necessarily drawn to scale. The drawings are merely illustrative and should not be construed as limiting the present technology. The disclosure described herein is shown in the accompanying drawings by way of illustration and not limitation.

Brief Description of the Drawings

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Mode for Carrying Out the Invention

[0074] Before describing some exemplary aspects of the present technology, it is to be understood that the present technology is not limited to the details of the configurations or process steps described in the following description. The present technology can be in other aspects and can be implemented or executed in various ways. Generally, the present technology relates to a method of forming polyamic acid gels and polyimide gels without using harmful organic solvents.

[0075] In one aspect, the method generally includes providing a polyamic acid, mixing the polyamic acid with a water-soluble carbonate or bicarbonate in water, thereby providing a solution of a salt of the polyamic acid. In other aspects, the method generally includes preparing a polyamic acid salt in situ. In such aspects, the method generally includes mixing a water-soluble diamine, a water-soluble carbonate or bicarbonate, and a tetracarboxylic dianhydride in water, reacting the components, and providing a solution of the polyamic acid salt. In some aspects, the method further includes converting the polyamic acid salt into a polyamic acid wet gel or aerogel, a polyimide wet gel or aerogel, or the corresponding carbon aerogel.

[0076] The disclosed method is economically preferable over conventional methods for preparing polyimide and polyamic acid gel materials (e.g., expensive organic solvents are avoided and disposal costs are minimized), "green" (i.e., potentially toxic organic solvents and reagents are avoided or minimized, and the production of toxic by-products is minimized or eliminated, which is beneficial from an environmental perspective), and advantageous in potentially reducing the overall number of operations that have to be performed to provide a carbon gel material.

[0077] The disclosed method generally relies on the use of water-soluble carbonates or bicarbonates, including but not limited to alkali metals, ammonium, or guanidinium carbonate or bicarbonate, which neutralize the polyamic acid upon formation and provide charge-compensating cations for the resulting polyamic acid salt. Without wishing to be bound by theory, carbonates and bicarbonates in particular are thought to be advantageous in that the roles of neutralizing species and cation provider are separated. Specifically, the carbonate or bicarbonate reacts with the formed polyamic acid and is converted to CO2 (leaving the reaction mixture as a gas), leaving the polyamic acid salt with the cations (e.g., alkali metal, ammonium, or guanidinium cations) introduced with the carbonate or bicarbonate. Further, such carbonates or bicarbonates have buffering activity and maintain the pH of the solution within a desired range.

[0078] Surprisingly, according to the present disclosure, a solution of a polyamic acid salt produced from the reaction of 1,4-phenylenediamine and pyromellitic dianhydride in water reaches a higher viscosity at a lower target density (Td) value in a carbonate solution, in contrast to the viscosity achieved using triethylamine (abbreviated as Et3N or TEA) under similar conditions, which is generally considered a non-nucleophilic amine base. Here too, without wishing to be bound by any particular theory, some residual and non-negligible nucleophilicity of Et3N is likely at least partially involved in the resulting low viscosity by converting some of the PMDA to side reactions, and thus may limit the length of the polyamic acid polymer. Another non-binding theory is that the hydrolysis of Et3N produces a high concentration of hydroxide ions, whereas in contrast, the buffering effect of the carbonate / bicarbonate / carbonic acid / CO2 system maintains a lower pH value (lower concentration of hydroxide ions). Further, as noted above, the protonation of the bicarbonate and carbonate bases produces H2CO3 (which decomposes to CO2 + H2O), completely separating the reaction mixture as gas and leaving the cations of the carbonate or bicarbonate as charge-balancing cations in the polyamic acid salt. Without wishing to be bound by any particular theory, it appears that polymerization proceeds to a relatively high molecular weight polymer in the absence of significant side reactions that interfere with polymer growth (such as reactions that can occur with bases such as triethylamine). Desirably, the lithium, sodium, potassium, ammonium, and guanidinium salts of polyamic acid, for example, those produced from PDA and PMDA, are very water-soluble, and the corresponding carbonates and bicarbonates are readily commercially available and inexpensive. Even more surprisingly, polyimide gels prepared from such polyamic acids by chemical imidization under aqueous conditions appear to be physically similar (transparency, color, texture) to the corresponding polyimide gels obtained from conventional organic solvent processes (e.g., in an organic solvent such as DMAC). Specifically, the gels were transparent, ranging from pale yellow to amber, and had a plastic feel.

[0079] Even more surprisingly, according to the present disclosure, a specific solution of a polyamic acid salt produced from the reaction of a polyamic acid and guanidinium carbonate in water, or a specific solution of a polyamic acid salt produced from the reaction of a specific diamine, tetracarboxylic dianhydride, and guanidinium carbonate in water, has been found to exhibit thixotropic behavior. Thixotropy is a time-dependent shear thinning property. Specifically, a particular gel or fluid that is viscous under static conditions becomes less viscous (e.g., flows) when subjected to a specific stress (e.g., oscillation, agitation, or shear stress). Then, when the stress is removed, it takes a certain amount of time to return to a more viscous state. Some thixotropic fluids return to the gel state almost instantaneously, while others require a much longer time to return to a solid or nearly solid state. Thixotropic behavior is preferred for the processing of certain materials at high flow rates (i.e., shear rates). Such flow behavior is applicable to three-dimensional (3D) printing, in which case a viscous solution flows through an injection needle with a small force, creating a high shear situation inside the needle, but then the solution almost immediately recovers its high viscosity after exiting the needle to form a solid structure.

[0080] Accordingly, provided herein are methods for preparing polyamic acid salt solutions, polyamic acid gels, and polyimide gels under aqueous conditions. Further provided are methods for converting polyamic acids to polyimides under aqueous conditions, and methods for converting polyamic acid and polyimide gel materials to corresponding carbon gel materials. Each of the various methods is further described hereinbelow.

[0081] Definitions Regarding the terms used in the present disclosure, the following definitions are provided. In this application, the terms defined below are used unless a different meaning is required in the context of the text in which the term appears.

[0082] The articles "a" and "an" are used herein to refer to one or more (i.e., at least one) of the grammatical objects of the article. The term "about" used throughout this specification is used to describe and account for minor variations. For example, the term "about" can refer to ±10% or less, or ±5% or less, such as ±2% or less, ±1% or less, ±0.5% or less, ±0.2% or less, ±0.1% or less, or ±0.05% or less. All numerical values in this specification, whether or not explicitly indicated, shall be assumed to be modified by the term "about". Of course, the specific value is included in the value modified by the term "about". For example, "about 5.0" should include 5.0.

[0083] With respect to the present disclosure, the term "framework" or "framework structure" refers to a network of interconnected oligomers, polymers, or colloidal particles that form the solid structure of a gel or aerogel. The polymers or particles that make up the framework structure typically have a diameter of about 100 angstroms. However, the framework structures of the present disclosure can also include networks of interconnected oligomers, polymers, or colloidal particles of all diameter sizes that form a solid structure within the gel or aerogel.

[0084] As used herein, the term "aerogel" refers to a solid that has a corresponding network of interconnected pores incorporated within a framework and contains a gas such as air as a dispersed interstitial medium, regardless of its shape or size. Thus, regardless of the drying method used, an aerogel is an open non-fluid colloidal network or polymer network formed by expanding it with a gas throughout its entire volume and removing all swelling agents (e.g., solvents) without volume reduction or compression from the corresponding wet gel. References to "aerogel" in this specification include any continuous cellular porous material that can be classified as an aerogel, xerogel, cryogel, ambigel, microporous material, etc., regardless of the material (e.g., polyimide, polyamic acid, or carbon), unless otherwise specified.

[0085] Generally, an aerogel has one or more of the following physical and structural properties: (a) an average pore diameter in the range of about 2 nm to about 100 nm, (b) a porosity of about 60% or more, (c) a specific surface area of about 1, about 10 or about 20 to about 100 or about 1000 m 2 / g. Usually, such properties are measured using nitrogen adsorption porosimetry and / or helium pycnometry. It can be understood that including additives such as reinforcing materials or electrochemically active species, such as silicon or lithium iron phosphate, may reduce the porosity or specific surface area of the resulting aerogel composite material. Densification can also reduce the porosity of the resulting aerogel composite.

[0086] In some embodiments, the gel material is sometimes specifically referred to as a xerogel. As used herein, the term "xerogel" refers to a type of aerogel that includes an open non-fluid colloidal network or polymer network formed by removing all swelling agents from the corresponding wet gel without taking precautions to avoid substantial volume reduction or delay compression. Xerogels generally include a compact structure. Xerogels undergo significant volume reduction during atmospheric drying, and usually the porosity is about 40% or less.

[0087] As used herein, the terms "gelation" or "gel transition" refer to forming a wet gel from a polymer system, such as the polyimides or polyamic acids described herein. At some point during the reaction described herein with respect to gelation (defined as the "gelation point"), the sol loses its fluidity. For the present invention, gelation proceeds from an initial sol state (e.g., a solution of a salt of polyamic acid), through a high-viscosity dispersion state, until the dispersion solidifies and the sol gels (gelation point) to obtain a wet gel (e.g., a polyimide or polyamic acid gel). In particular, such definitions of gelation and the gelation point are simplified and do not consider the possibility of fluidity under stress, such as the thixotropic behavior of the specific gels described herein.

[0088] The time required for a polymer solution (e.g., an aqueous solution of a polyamic acid salt) to change into a non-flowable gel 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 dominate over the viscous properties of the fluid sol. The formal gelation time is close to the time at which the real and imaginary components of the complex elastic modulus of the gelling sol cross. The two elastic moduli are monitored as a function of time using a rheometer. The time count starts at the instant when the last component of the sol is added to the solution. See, for example, the discussion of gelation in H.H. Winter “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.

[0089] As used herein, the term “wet gel” refers to a gel in which the mobile interstitial phase within a network of interconnected pores is mainly composed 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 formation of an initial wet gel, followed by treatment 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, alcogels, hydrogels, ketogels, carbogels, and any other wet gels known to those skilled in the art.

[0090] As used herein, the "conventional" or "organic solvent-based" method for forming a polyimide gel material refers to a method of preparing a polyamic acid solution in an organic solvent by condensation of a diamine and a tetracarboxylic dianhydride, followed by dehydration of the polyamic acid to form a polyimide gel. See, for example, U.S. Pat. Nos. 7,071,287 and 7,074,880 to Rhine et al., and U.S. Patent Application Publication No. 2020 / 0269207 to Zafiropoulos et al.

[0091] As used herein, the term "alkyl" generally refers to a straight-chain or branched saturated hydrocarbon group having from 1 to 20 carbon atoms (i.e., C1-C20). Representative 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. The alkyl group can be unsubstituted or substituted.

[0092] As used herein, the term "alkenyl group" generally refers to a hydrocarbon group having from 1 to 20 carbon atoms (i.e., C1-C20) and having 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, isobutenyl, 1-pentenyl, 2-pentenyl, 3-methyl-1-butenyl, 2-methyl-2-butenyl, 2,3-dimethyl-2-butenyl, and the like. The alkenyl group can be unsubstituted or substituted.

[0093] As used herein, the term "substantially" means, unless otherwise specified, a large degree of a reference property, quantity, etc. relevant to a particular context, e.g., greater than about 95%, greater than about 99%, greater than about 99.9%, greater than 99.99%, or 100% (e.g., substantially pure, substantially the same, etc.).

[0094] Methods for forming polyimide, polyamic acid, metal salts of polyamic acid, and carbon aerogel The methods disclosed herein generally utilize polyamic acid and polyimide wet gels, which can be prepared without using an organic solvent and without using an organic base (e.g., an amine). The reference herein to the preparation of "organic base-free" polyamic acid and polyimide wet gels means that carbon-based alkaline substances such as amines are not utilized either for solubilization of preformed polyamic acid in water or for in situ solubilization of polyamic acid when formed (i.e., by reaction of diamine and tetracarboxylic dianhydride). To avoid doubt, the reference to "organic base" does not include carbonates and bicarbonates, and further does not include carbonates and bicarbonates containing nitrogen-containing cationic species (such as ammonium or guanidinium).

[0095] The reference herein to an aqueous solution means that the solution is substantially free of organic solvent. The term "substantially free of" as used herein with respect to an organic solvent means that the organic solvent is not intentionally added and that the organic solvent is present in an amount not exceeding trace amounts. For example, in certain embodiments, the aqueous solution can be characterized as containing less than 1 volume % of organic solvent, or less than 0.1 volume %, or less than 0.01%, or even 0 volume % of organic solvent. These water-based methods are advantageous for reducing the processing costs of materials and waste and for reducing potential safety and environmental hazards.

[0096] Preparation of polyamic acid and polyimide gel materials under aqueous conditions Provided herein is a method for preparing polyamic acid and polyimide gel materials under aqueous conditions. This method generally involves preparing an aqueous solution of a polyamic acid salt without using an organic base, and subsequently converting the polyamic acid salt into a polyamic acid gel or aerogel material, a polyimide gel or aerogel material, or the corresponding carbon aerogel material. Each of these materials and the corresponding method(s) are further described hereinbelow.

[0097] Polyamic acid and polyamic acid salts This specification provides a method for preparing an aqueous solution of a polyamic acid salt. A polyamic acid is a polymeric amide having repeating units including 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 used herein, a "repeating unit" is a portion of a polyamic acid (or the corresponding polyimide) such that, upon repetition, the repeating units are continuously linked along the polymer chain to produce a complete polymer chain (excluding terminal amino groups or unreacted anhydride ends). One of ordinary skill in the art will recognize that the repeating units of a polyamic acid result from the partial condensation of the carboxyl groups of a tetracarboxylic dianhydride and the amino groups of a diamine.

[0098] In one aspect, the method includes providing a polyamic acid and mixing the polyamic acid with a water-soluble carbonate or bicarbonate in water, thereby providing a solution of a salt of the polyamic acid. In such an aspect, the polyamic acid is a preformed polyamic acid, which can be either a commercially purchased material or a material prepared from suitable diamines and tetracarboxylic dianhydrides according to conventional known techniques (such as preparation in an organic solvent solution). For in situ synthesized polyamic acids, suitable preformed polyamic acids are as described hereinbelow. Suitable water-soluble carbonates or bicarbonates are further described hereinbelow.

[0099] Alternatively, polyamic acid may be prepared in situ. Thus, in another embodiment, an aqueous solution of polyamic acid salt is prepared by reacting a water-soluble diamine with a tetracarboxylic dianhydride in the presence of a water-soluble carbonate or bicarbonate. Generally, the diamine is reacted with the tetracarboxylic dianhydride in the presence of the above carbonate or bicarbonate to form a polyamic acid salt. Thus, this method involves mixing a water-soluble diamine, a water-soluble carbonate or bicarbonate, and a tetracarboxylic dianhydride in water and reacting the components to provide a solution of polyamic acid salt. The polyamic acid salt contains an anionic carboxylic acid group charge-compensated by a cation derived from the carbonate or bicarbonate, and the polyamic acid salt is water-soluble. Each component (e.g., water-soluble diamine, tetracarboxylic dianhydride, water-soluble carbonate or bicarbonate, etc.) utilized in this method will be further described below.

[0100] The order of addition of the various components can vary widely. For example, in some embodiments, the mixing involves dissolving the water-soluble diamine in water to form an aqueous diamine solution, adding the water-soluble carbonate or bicarbonate to the aqueous diamine solution, adding the tetracarboxylic dianhydride to the aqueous solution of the diamine and the water-soluble carbonate or bicarbonate to form a solution, and stirring the 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 4 days.

[0101] In some embodiments, the mixing involves dissolving the water-soluble diamine in water to form an aqueous diamine solution, adding the tetracarboxylic dianhydride to the aqueous diamine solution to form a suspension, stirring the suspension at a temperature in the range of about 15 to about 60 °C for a period in the range of about 1 minute to about 24 hours, adding the water-soluble carbonate or bicarbonate to the suspension, and stirring the suspension 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 4 days to provide an aqueous solution of polyamic acid salt.

[0102] In some embodiments, the mixing comprises adding a water-soluble diamine, a tetracarboxylic dianhydride, and a water-soluble carbonate or bicarbonate to water simultaneously or in rapid succession, and stirring the resulting mixture at a temperature in the range of about 15 °C to about 60 °C for a period in the range of about 1 hour to about 4 days to provide an aqueous solution of a polyamic acid salt.

[0103] A non-limiting general reaction sequence is shown in Scheme 1. In some, the reaction is generally carried out according to Scheme 1, and the reagents and products have structures according to the formulas of Scheme 1. Scheme 1. Formation of an aqueous solution of a salt of a polyamic acid by reaction of monomers in the presence of a water-soluble carbonate or bicarbonate

Chemical formula

[0104] The diamines disclosed herein are generally referred to as "water-soluble diamines." As used herein, the term "water-soluble diamine" means that the diamine has a significant solubility in water such that a synthetically useful concentration of the diamine is obtained under the conditions utilized in the disclosed methods. For example, diamines suitable for use in the disclosed methods can have a solubility of at least about 0.01 g per 100 mL, 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 of water at 20 °C.

[0105] In some embodiments, combinations of multiple diamines 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.

[0106] As shown in Scheme I, the structure of the diamine can vary widely. In some embodiments, the diamine has a structure according to Formula I, where Z is aliphatic (i.e., alkylene, alkenylene, alkynylene, or cycloalkylene) or aryl, each as described above herein. In some embodiments, Z is alkylene such as C2-C12 alkylene or C2-C6 alkylene. In some embodiments, the diamine is a C2-C6 alkanediamine such as, but not limited to, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, and ethylenediamine. In some embodiments, the C2-C6 alkylene of the alkanediamine is substituted with one or more alkyl groups such as methyl.

[0107] In some embodiments, Z is aryl. In some embodiments, the aryl diamine is 1,3-phenylenediamine, methylenedianiline, 1,4-phenylenediamine (PDA), or combinations thereof. In some embodiments, the diamine is 1,3-phenylenediamine. In some embodiments, the diamine is 1,4-phenylenediamine (PDA).

[0108] Continuing as shown in Scheme 1, a tetracarboxylic dianhydride is added. In some embodiments, a plurality of tetracarboxylic dianhydrides are added. To optimize the properties of the gel material, a combination of tetracarboxylic dianhydrides may be used. In some embodiments, a single tetracarboxylic dianhydride is added. The structure of the tetracarboxylic dianhydride can vary widely. In some embodiments, the tetracarboxylic dianhydride has a structure according to Formula II, where L includes an alkylene group, a cycloalkylene group, an arylene group, or combinations thereof, each as described above 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 II has a structure selected from one or more of the structures shown in Table 1.

Table 1

[0109] In some embodiments, the tetracarboxylic dianhydride is selected from the group consisting of pyromellitic dianhydride (PMDA), biphenyl dianhydride (BPDA), oxydiphthalic dianhydride (ODPA), benzophenone tetracarboxylic dianhydride (BTDA), ethylenediamine tetraacetic dianhydride (EDDA), 1,4,5,8-naphthalenetetracarboxylic dianhydride, and combinations thereof. In some embodiments, the tetracarboxylic dianhydride is PMDA.

[0110] The methods disclosed herein utilize a water-soluble carbonate or bicarbonate. The water-soluble carbonate or bicarbonate can vary widely. As used herein, the term "water-soluble" with respect to a salt means that the carbonate or bicarbonate has a substantial solubility in water such that anions of the carbonate or bicarbonate at a synthetically useful concentration are obtained under the conditions utilized in the disclosed methods. For example, a water-soluble carbonate or bicarbonate suitable for use in the disclosed methods can have a solubility 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 of water at 20 °C.

[0111] As used herein, the term "carbonate or bicarbonate" refers to an alkaline substance that includes anions of the carbonate or bicarbonate, and in particular excludes alkaline substances that contain a carbon-hydrogen covalent bond (i.e., organic bases including, but not limited to, alkylamines, arylamines, and heteroaromatic amines). A water-soluble carbonate or bicarbonate suitable for use in the disclosed methods can be further described as being non-nucleophilic, which means that the carbonate or bicarbonate does not participate in chemical reactions by donating an electron pair other than as a proton acceptor.

[0112] In certain embodiments, the water-soluble carbonate or bicarbonate is a carbonate. In other certain embodiments, the water-soluble carbonate or bicarbonate is a bicarbonate. Continuing as shown in Scheme 1, the water-soluble carbonate or bicarbonate has the general formula M2CO3 or MHCO3, where M is a cationic species having a valence of +1.

[0113] In some embodiments, the cationic species M comprises, or is, an ammonium ion, a guanidinium ion, or an alkali metal ion. In some embodiments, the cationic species M comprises lithium, sodium, potassium, ammonium, guanidinium, or combinations thereof. In some embodiments, the cationic species M is lithium. In some embodiments, the cationic species M is sodium. In some embodiments, the cationic species M is potassium. In some embodiments, the cationic species M is ammonium (NH4 + ). In some embodiments, the cationic species M is guanidinium (NH2-C(=NH2 + )-NH2).

[0114] Particularly preferred water-soluble carbonates and bicarbonates include alkali metal carbonates and bicarbonates. In some embodiments, the water-soluble carbonate or bicarbonate is selected from the group consisting of lithium carbonate, lithium bicarbonate, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, and combinations thereof. In some embodiments, the water-soluble carbonate or bicarbonate is selected from the group consisting of lithium carbonate, lithium bicarbonate, sodium carbonate, sodium bicarbonate, potassium carbonate, and potassium bicarbonate.

[0115] In some embodiments, the water-soluble carbonate or bicarbonate is selected from the group consisting of ammonium carbonate, ammonium bicarbonate, guanidinium carbonate, and combinations thereof.

[0116] In some embodiments, the water-soluble carbonate or bicarbonate is guanidinium carbonate. As described herein with respect to the above and below examples, certain polyamic acid salt solutions prepared using guanidinium carbonate are thixotropic. This thixotropic behavior is desirable, for example, in 3D printing applications. For example, a 3D printed structure can be prepared by needle deposition of a thixotropic solution described herein (e.g., an aqueous guanidinium polyamate), where the needle is moved in a predetermined manner as the polymer solution exits the needle to form a desired pattern. The resulting structure can be treated as previously described in the literature to form either a polymer or a carbon aerogel that retains the pressed structure.

[0117] The amount of water-soluble carbonate or bicarbonate added can vary and can depend, for example, on the stoichiometry of the particular salt used. For example, one of ordinary skill in the art will recognize that it depends on the charge associated with the particular anion species (carbonate or bicarbonate) present in the salt. For example, sodium bicarbonate (NaHCO3) supplies one equivalent of base (bicarbonate ion, HCO3 - ) that can react with one proton each, and also supplies one equivalent of sodium ions for each molar equivalent of sodium bicarbonate. In contrast, sodium carbonate (Na2CO3) supplies two equivalents of base (carbonate ion, CO3 2- ) that can react with two equivalents of protons from each repeating unit of the polyamic acid, and supplies two equivalents of sodium ions for each molar equivalent of sodium carbonate.

[0118] The amount of the water-soluble carbonate or bicarbonate may be expressed as a molar ratio to another reaction component (e.g., diamine). The molar ratio of the water-soluble carbonate or bicarbonate to the diamine may need to be optimized according to 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. In some embodiments, the molar ratio of the water-soluble carbonate or bicarbonate to the diamine ranges from about 1 to about 4, or from about 2 to about 3. In some embodiments, the molar ratio is from about 1.0, about 1.1, about 1.2, about 1.3, about 1.4, or about 1.5 to about 1.6, about 1.7, about 1.8, about 1.9, or about 2.0. In some embodiments, the molar ratio of the water-soluble carbonate or bicarbonate to the diamine is from about 2.0 to about 2.6, for example, about 2.0, about 2.1, about 2.2, about 2.3, about 2.4, about 2.5, or about 2.6. Without wishing to be bound by any particular theory, in some exemplary embodiments, it is believed that at least sufficient base is required to enable the neutralization (i.e., salt formation) of substantially all of the free carboxylic acid groups of the polyamic acid. In some embodiments, the amount of the water-soluble carbonate or bicarbonate utilized is an amount that neutralizes substantially all of the carboxylic acid groups present in the polyamic acid formed during the reaction.

[0119] In some embodiments, the water-soluble salt is a carbonate such as lithium carbonate, sodium carbonate, potassium carbonate, ammonium carbonate or guanidinium carbonate, and the molar ratio of the carbonate ion to the diamine is from about 1.0 to about 1.3.

[0120] In some embodiments, the water-soluble carbonate or bicarbonate is a bicarbonate such as lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, or ammonium bicarbonate, and the molar ratio of the bicarbonate ion to the diamine is from about 2.0 to about 2.6.

[0121] In some embodiments, the amount of water-soluble carbonate or bicarbonate present can be expressed relative to the carboxylic acid groups of the polyamic acid formed during the reaction or otherwise present in the reaction mixture. In some embodiments, the water-soluble carbonate or bicarbonate is a bicarbonate such as lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, or ammonium bicarbonate, and the molar ratio of bicarbonate ions to the carboxylic acid groups of the polyamic acid is about 2.0. In some embodiments, the water-soluble carbonate or bicarbonate is a carbonate such as lithium carbonate, sodium carbonate, potassium carbonate, or ammonium carbonate, and the molar ratio of carbonate ions to the carboxylic acid groups of the polyamic acid is about 1.0.

[0122] The relative amounts of diamine and dianhydride present may be expressed as a molar ratio. 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 from about 0.1 to about 10, for example, about 0.1, about 0.5, or from about 1 to about 2, about 3, about 5, or about 10. In some embodiments, this ratio is from about 0.5 to about 2. In some embodiments, this ratio is about 1 (i.e., stoichiometric), for example, from about 0.9 to about 1.1. In certain embodiments, this ratio is from about 0.99 to about 1.01.

[0123] The molecular weight of the polyamic acid can vary based on the reaction conditions (e.g., concentration, temperature, reaction time, nature of the diamine and dianhydride, etc.). The molecular weight is based on the number of polyamic acid repeat units, as indicated by the value of the integer "n" for the structure of Formula III in Scheme 1. The specific molecular weight range of the polymeric material produced by the disclosed methods can vary. Generally, without particular consideration of the molecular weight, the above reaction conditions can be varied in various ways to provide a gel having the desired physical properties. In some embodiments, a surrogate value for the molecular weight is provided by the viscosity of the polyamic acid salt solution determined by variables such as temperature, concentration, molar ratio of reactants, reaction time, etc.

[0124] The temperature at which the reaction is carried out can be various. A suitable range is generally from about 4 °C to about 100 °C. In some embodiments, the reaction temperature is 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.

[0125] The reaction is allowed to proceed for a certain period of time, generally until all available reactants (e.g., diamine and dianhydride) react with each other. The time required until the reaction is complete can vary depending on the structure, concentration, and temperature of the reagents. In some embodiments, the reaction time is from about 1 minute to about 1 week, for example, from about 15 minutes to about 5 days, from about 30 minutes to about 3 days, or from about 1 hour to about 1 day. In some embodiments, the reaction time is from about 1 hour to about 12 hours.

[0126] The concentration of the polyamic acid salt in the aqueous solution can be various. For example, in some embodiments, the concentration range of the polyamic acid salt in the aqueous solution is from about 0.01 to about 0.3 g / cm 3 is.

[0127] In some embodiments, one or more additional materials such as electroactive materials, conventional polymer fillers, carbon materials, etc. may be doped into the polyamic acid, polyimide, or carbon gel and aerogel materials disclosed herein. Such dopants or their precursors may be introduced at various stages and in various ways. In some embodiments, the dopant is introduced during or immediately after the formation of the polyamic acid salt solution. Thus, in some embodiments, the method further comprises adding a dopant to an aqueous solution of the polyamic acid salt or a solution of a water-soluble diamine.

[0128] In some embodiments, the dopant is an electroactive material or a precursor thereof. Suitable electroactive materials include graphite, silicon, such as silicon particles, tin, or Prussian blue, lithium iron phosphate, lithium manganese iron phosphate, and the like, combinations thereof, other anode and / or cathode active materials, or one or more precursors of any of these, but are not limited thereto. In some embodiments, the electroactive material or its precursor is in particulate form. In some embodiments, the dopant is a particulate material in the nanometer or micron range (i.e., having two-dimensional or three-dimensional particles in the range of about 1 nm to about 10 micrometers or more).

[0129] In some embodiments, the dopant in particulate form includes silicon, such as silicon, silicon wire, crystalline silicon, amorphous silicon, silicon alloy, silicon oxide (SiOx), silicon carbide, silicon nitride, coated silicon (e.g., carbon-coated silicon), and any combination of the silicon particle materials disclosed herein.

[0130] In some embodiments, the dopant in particulate form includes carbon, graphite, tin, sulfur, nickel, manganese, cobalt, iron, vanadium, manganese, or combinations thereof. For example, in some embodiments, the dopant in particulate form is lithium iron phosphate or a manganese or vanadium variant thereof.

[0131] Dopant particles (e.g., lithium iron phosphate) may be incorporated into polyamic acid, polyimide, or carbon gel in several ways as disclosed herein. Generally, electroactive dopant particles (e.g., lithium iron phosphate) are incorporated during the sol-gel process. In one non-limiting embodiment, electroactive dopant particles (e.g., lithium iron phosphate) are dispersed in a polyamic acid sol prior to imidization. In some embodiments, electroactive dopant particles (e.g., lithium iron phosphate) are dispersed in a solvent, such as water, or an aprotic polar solvent, prior to mixing with the polyimide precursor. In some embodiments, electroactive dopant particles (e.g., lithium iron phosphate) are dispersed in a polyamic acid sol during the imidization process. In some embodiments, electroactive dopant particles (e.g., lithium iron phosphate) are added to an aqueous solution of a polyamic acid salt. In other embodiments, precursors of electroactive materials (e.g., materials containing sulfur or sulfur itself, or nickel, manganese, cobalt, iron, vanadium, manganese, phosphate, Prussian blue, etc.) may be introduced at various stages including but not limited to during the preparation of the polyamic acid salt.

[0132] In some embodiments, residual lithium, sodium, potassium, ammonium, or guanidinium ions from, for example, water-soluble carbonates or bicarbonates are retained in the polyamic acid, polyimide, or carbon gel material.

[0133] Polyamic Acid and Polyimide Gels In some embodiments, the method further includes converting an aqueous solution of a polyamic acid salt to the corresponding polyamic acid gel. Generally, the method of converting a polyamic acid salt solution to the corresponding polyamic acid gel includes acidifying the polyamic acid salt solution to convert the polyamic acid salt to polyamic acid and phase-separating the polyamic acid as a gel. This acid is also referred to as a gelation initiator. The acidification to form polyamic acid generally follows Scheme 2.

[0134] The acidification methods can vary widely. For example, in some embodiments, a polyamic acid salt solution is added to an acid solution, and the acidification of the polyamic acid salt solution is rapid. Alternatively, the polyamic acid salt solution may be acidified by adding an acid to the polyamic acid salt solution. In some embodiments, the polyamic acid salt solution may be acidified gradually or slowly using conditions or techniques known to those skilled in the art. For example, in one non-limiting embodiment, an acid precursor is used. An acid precursor is a substance that can be gradually converted to an acid, for example, by hydrolysis. One such suitable acid precursor substance is acetic anhydride, which produces acetic acid in the presence of water. Scheme 2. Formation of polyamic acid gel by reaction of polyamic acid salt with acid [Chemical formula]

[0135] In some embodiments, the polyamic acid wet gel prepared as disclosed herein, or the corresponding aerogel described below, contains residual carbonate or bicarbonate(s). Generally, the residual amount is trace, but the carbonate or bicarbonate, and / or the associated counter cations (e.g., alkali metal ions, guanidinium ions, etc.) can be detected by analytical methods known to those skilled in the art.

[0136] Subsequently, the obtained polyamic acid gel material may be dried to form a polyamic acid aerogel. The methods for acidification and formation of the polyamic acid gel material are described, for example, in International Patent Application Publication No. WO2022125835, the entire content of which is incorporated herein by reference. The drying method for forming the corresponding aerogel will be further described herein below.

[0137] In some embodiments, this method further includes forming a polyimide aerogel from an aqueous solution of a polyamic acid salt. Generally, this method includes imidizing the polyamic acid salt to form a polyimide gel and drying the polyimide gel to form a polyimide aerogel. Methods for imidizing an aqueous solution of a polyamic acid salt are described, for example, in International Patent Application No. PCT / US2021 / 062706, which is incorporated herein by reference in its entirety, and suitable methods are also further described herein below. A drying method for forming the corresponding polyimide aerogel is further explained herein below.

[0138] In some embodiments, imidizing the polyamic acid salt includes thermally imidizing the corresponding polyamic acid. Irradiation of a wet gel polyamic acid material with microwave frequency energy is one particularly suitable heat treatment. Compared to conventional heating that relies on slow heat conduction, microwave heating enables rapid and efficient energy transfer. Thus, microwave heating is particularly suitable for performing the thermal imidization reaction of the present invention. Generally, irradiation at microwave frequencies is carried out with sufficient power and time to convert a substantial portion of the amide groups and carboxyl groups of the polyamic acid to imide groups. As used herein in connection with converting amide groups and carboxyl groups to imide groups, "substantial portion" means that more than 80%, such as 85%, 90%, 95%, 99%, or 99.9%, or 99.99%, or even 100% of the amide groups and carboxyl groups are converted to imide groups.

[0139] In other embodiments, imidizing the polyamic acid salt includes performing chemical imidization, which involves adding a gelation initiator to an aqueous solution of the salt of the polyamic acid to form a gelling mixture (“sol”), and gelling the gelling mixture (e.g., in a mold, or poured onto a sheet, or in various other forms such as beads). In such embodiments, a gelation initiator is added to initiate and facilitate imidization to form a polyimide wet gel from the polyamic acid salt. A non-limiting general reaction sequence is shown in Scheme 3. In some embodiments, the polyimide has a structure according to Formula V, as shown in Scheme 3, where L, Z, and n are as described hereinabove with respect to the formation of the polyamic acid salt of Formula III. Scheme 3. Conversion of the salt of polyamic acid to polyimide

Chemical formula

[0140] The structure of the gelation initiator can be various, but generally, it is a reagent that has minimal reactivity with the aqueous solution, is at least partially soluble in the reaction solution, reacts with the carboxylic acid groups of the polyamic acid salt, and is effective in promoting the imidization of the carboxyl and amide groups of the polyamic acid. An example of a suitable type of gelation initiator is carboxylic acid anhydrides such as acetic anhydride and propionic anhydride. In some embodiments, the gelation initiator is acetic anhydride.

[0141] In some embodiments, the amount of the gelation initiator can vary based on the amount of the tetracarboxylic dianhydride or the polyamic acid. For example, in some embodiments, the gelation initiator is present in various molar ratios with the tetracarboxylic dianhydride. In some embodiments, the gelation initiator is present in various molar ratios with the polyamic acid. The molar ratio of the gelation initiator to the tetracarboxylic dianhydride or the polyamic acid can vary differently depending on the desired reaction time, reagent structure, and desired material properties. In some embodiments, the molar ratio is from about 2 to about 10, for example, about 2, about 3, about 4, or from about 5 to about 6, about 7, about 8, about 9, or about 10. In some embodiments, this ratio is from about 2 to about 5.

[0142] The temperature at which the gelation reaction proceeds can vary widely, but is generally less than about 50°C, for example, about 10 to about 50°C, or about 15 to about 25°C.

[0143] The above gelation conditions (both acidification and imidization) are general and are not intended to limit the method of performing gelation. For example, one skilled in the art will recognize various permutations for preparing monoliths or beads (including microbeads). For example, in this specification, a method of forming a monolith by pouring a gelation mixture into a mold, a method of forming beads of various sizes by dropping or spraying a polyamic acid salt solution into an acidic receiving solution, or a method of forming micron-sized beads of a polyamic acid or polyimide gel in an emulsion is contemplated. Further, in this specification, a method of forming a polyamic acid metal salt gel by contacting a polyamic acid salt solution with specific metal ions in a solution is also contemplated (see, for example, Scheme 4). As shown in Scheme 4, certain metal salts (e.g., alkaline earth metal salts, d-block element salts, p-block element salts, lanthanide metal salts, actinide metal salts) form metal polyamic acid gels having the desired properties. These additional gelation methods (i.e., metal polyamic acid salts, bead monoliths) are described, for example, in International Patent Application Publication No. WO2022 / 125835 by Leventis et al., which is hereby incorporated by reference herein with respect to the disclosure of the formation of polyamic acids, polyimides, and metal polyamic acid gels from aqueous solutions. Scheme 4. Formation of Metal Polyamic Acid Salt Gel

Chemical formula

[0144] One skilled in the art would recognize that the polyimide wet gel prepared according to the method described herein has unreacted terminal amino groups at one or both ends of the individual polymer chains. The proportion of the concentration of such amino groups in the polyimide wet gel varies inversely with the average number of repeating units (i.e., molecular weight) present in the polyimide wet gel. In some embodiments, the terminal amino groups can react with a gelation initiator (e.g., acetic anhydride) to form terminal amides such as acetamide. The relative concentration of such terminal amines or amides may be determined according to methods known in the art, including but not limited to nuclear magnetic resonance spectroscopy such as solid 15N-NMR.

[0145] In some embodiments, the water content of the polyimide wet gel prepared as disclosed herein is, prior to solvent exchange or drying, essentially the total amount of water initially used as the reaction solvent, and evaporation or water generated or destroyed in the various reactions occurring during the polyimide synthesis described above herein is not considered.

[0146] In some embodiments, the polyimide wet gel prepared as disclosed herein, or the corresponding aerogel described below herein, contains residual carbonate(s) or bicarbonate(s). Generally, the residual amount is trace, but the carbonate or bicarbonate, and / or the associated counter cations (e.g., alkali metal ions, guanidinium ions, etc.) can be detected by analytical methods known to those skilled in the art.

[0147] Polyamic acid, polyimide, and metal polyamic acid salt aerogels As described above in this specification, in some embodiments, this method further includes converting the polyamic acid salt to an aerogel material via the corresponding polyamic acid, polyimide, or metal polyamic acid wet gel. Generally, the formation of an aerogel involves drying the wet gel in one or more steps. In some embodiments, the wet gel (polyamic acid, polyimide, or metal polyamic acid) is aged. After aging, the resulting wet gel material is recovered (e.g., removed from a mold), first washed with water or solvent exchanged to remove unreacted organic salts or acids, and then the primary reaction solvent (i.e., water) present in the wet gel may be replaced with a suitable secondary solvent. Such a secondary solvent needs to be miscible with supercritical fluid carbon dioxide (CO2) and includes linear alcohols having one or more aliphatic carbon atoms, diols having two or more carbon atoms, or branched alcohols, cyclic alcohols, cycloaliphatic alcohols, aromatic alcohols, polyols, ethers, ketones, cyclic ethers, or their derivatives. In some embodiments, the secondary solvent is water, a C1-C4 alcohol (e.g., methanol, ethanol, propanol, isopropanol, or n-, iso-, or sec-butanol), acetone, tetrahydrofuran, ethyl acetate, acetonitrile, supercritical fluid carbon dioxide (CO2), or a combination thereof. In some embodiments, the secondary solvent is ethanol.

[0148] When a wet gel is formed and processed, an extraction method including processing and extraction techniques can then be used to at least partially extract the liquid phase of the wet gel from the wet gel material to form an aerogel material (i.e., “dry”). Liquid phase extraction plays an important role in designing properties of aerogels such as porosity and density, and related properties such as thermal conductivity, among other factors. Generally, an aerogel is obtained when the liquid phase is extracted from a wet gel in a manner that causes low shrinkage in the porous network and solid framework of the wet gel. The wet gel can be dried using various techniques to provide an aerogel or xerogel. In an exemplary aspect, the wet gel material can be dried at ambient pressure, under vacuum (e.g., by freeze-drying), under subcritical conditions, or under supercritical conditions to form the corresponding dry gel (e.g., an aerogel such as a xerogel).

[0149] In some aspects, it may be desirable to reduce the surface area of the dry gel. When a reduction in surface area is desired, the aerogel can be completely or partially converted to a xerogel having various porosities. The high surface area of the aerogel can be reduced by forcibly collapsing some of the 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 the aerogel to solvent vapor. Subsequently, the solvent is removed by drying at ambient pressure.

[0150] Aerogels are typically 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. Thereafter, the solvent can be removed without introducing the liquid-vapor interface, capillary forces, or any associated mass transfer limitations typically associated with the receding liquid-vapor boundary. Furthermore, the supercritical phase generally has higher miscibility with organic solvents, thus enabling better extraction. Co-solvents and solvent exchange are also commonly used to optimize the supercritical fluid drying process.

[0151] If evaporation or extraction occurs below the supercritical point, capillary forces generated by the evaporation of the liquid 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 the adverse effects of such capillary forces. In certain embodiments of the present disclosure, using near-critical conditions just below the critical point of the solvent system may enable the production of aerogels or compositions with sufficiently low shrinkage, thus producing a commercially viable final product.

[0152] 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.

[0153] Both room temperature processes and high temperature processes can be used to dry the gel material at ambient pressure. In some embodiments, depending on the solvent, the amount of wet gel, the exposed surface area, the size of the wet gel, etc., a slow atmospheric pressure drying process can be used in which the wet gel is exposed to air in an open container for a period sufficient to remove the solvent, for example, a period ranging from several hours to several weeks.

[0154] In another aspect, 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 to dry completely over a period of time, e.g., from several hours to several days. Xerogel is produced by this drying method. In particular, according to the present disclosure, when a monolithic wet gel is dried, cracks occur, but the wet gel in bead form retains its spherical shape even from a solution with a low target density Td (e.g., Td = 0.05 g cm 3 ) as was found.

[0155] In some aspects, the wet gel material is dried by freeze drying. "Freeze drying" or "lyophilizing" means a low-temperature process for removing a solvent, which involves freezing the material (e.g., the wet gel material), reducing the pressure, and then removing the frozen solvent by sublimation. Water is an ideal solvent for removal by freeze drying, and since water is the solvent in the methods disclosed herein, freeze drying is particularly suitable for aerogel formation from the disclosed polyimide wet gel materials. This drying method produces a cryogel that may closely resemble an aerogel.

[0156] Both supercritical drying and subcritical drying can be used to dry the wet gel material. In some aspects, the wet gel material is dried under subcritical or supercritical conditions. In an exemplary aspect of supercritical drying, the gel material can be placed in a high-pressure vessel to extract the solvent with supercritical CO2. After removing the solvent, e.g., ethanol, the vessel can be held above the critical point of CO2 for a certain period of time, e.g., about 30 minutes. After supercritical drying, the vessel is depressurized to atmospheric pressure. Generally, aerogels are obtained by this process.

[0157] In an exemplary embodiment of subcritical drying, a gel material is dried using liquid CO2 at a pressure in the range of about 800 psi to about 1200 psi at room temperature. This operation is faster than supercritical drying. For example, a solvent (e.g., ethanol) can be extracted in about 15 minutes. Generally, aerogels are obtained by this process.

[0158] Several additional aerogel extraction techniques are known in the art, including various approaches using supercritical fluids for drying aerogels, as well as atmospheric pressure drying techniques. For example, U.S. Patent No. 6,670,402 teaches producing an aerogel by injecting supercritical (not liquid) carbon dioxide into an extractor preheated and pre-pressurized to substantially supercritical conditions or above, thereby extracting the liquid phase from the gel via rapid solvent exchange.

[0159] In some embodiments, supercritical conditions of CO2 are used for extraction of the liquid phase from the wet gel.

[0160] Formation of carbon aerogels from polyamic acid or polyimide aerogels In some embodiments, this method further includes converting a polyamic acid or polyimide aerogel into a homologous carbon aerogel, and the conversion includes pyrolyzing each aerogel under suitable conditions. Thus, in some embodiments, this method further includes pyrolyzing (e.g., carbonizing) the polyamic acid or polyimide aerogel disclosed herein, which means heating the aerogel at a temperature and for a time sufficient to convert substantially all of the organic material to carbon. As used herein, "substantially all" with respect to pyrolysis means that more than 80% by weight of the organic substance is converted to carbon, for example, 80%, 85%, 90%, or more, for example, up to 99%, 99.9%, 99.99%, or 100% by weight of the organic substance is converted to carbon. When the aerogel is pyrolyzed, the aerogel is converted into a homologous carbon aerogel, which means that the physical properties (e.g., porosity, surface area, pore diameter, diameter, etc.) are substantially retained within the corresponding carbon aerogel. The time and temperature required for pyrolysis can vary widely. In some embodiments, the polyimide aerogel is subjected to a treatment temperature 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 in a range between any two of these values to carbonize the aerogel. Generally, pyrolysis is performed under an inert atmosphere to prevent combustion of the organic or carbon material. Suitable atmospheres include, but are not limited to, nitrogen, argon, or combinations thereof. In some embodiments, pyrolysis is performed under nitrogen.

[0161] In some embodiments, the aerogel is the metal polyamic acid salt aerogel described above herein. When such a metal polyamic acid salt aerogel is pyrolyzed, the resulting carbon aerogel may contain (i.e., be doped with) the corresponding metal, metal oxide, metal carbide, or combinations thereof. The species present depend on the pyrolysis conditions such as temperature and reducing atmosphere, and the particular metal ions.

[0162] Properties of Aerogel The aerogel disclosed in this specification has a density. As used herein, the term "density" refers to a measure of the mass per unit volume of an aerogel material or composition. The term "density" generally refers to the true density or skeletal density of the aerogel material and the bulk density of the aerogel product. Density is usually reported as kg / m 3 or g / cm 3 The skeletal density of an aerogel (polyamic acid, polyimide, metal salt of polyamic acid, or carbon) may be determined by methods known in the art, including but not limited to helium pycnometry. The bulk density of an aerogel (polyamic acid, polyimide, or carbon) may be measured 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). For the purposes of this disclosure, unless otherwise specified, density measurements are obtained in accordance with ASTM C167. In some embodiments, the aerogel (polyamic acid, polyimide, metal salt of polyamic acid, or carbon) disclosed herein has a bulk density of from about 0.01 to about 1, such as from about 0.1 to about 0.3 g / cm 3 of.

[0163] The aerogels disclosed herein have a pore size distribution. As used herein, the term "pore size distribution" refers to the statistical distribution or relative amount of each pore size within the sample volume of a porous material. A narrower pore size distribution indicates that a relatively large proportion of the pores are within a narrow range of pore sizes. In some embodiments, for example, a narrow pore size distribution may be desirable in optimizing the amount of pores that can surround electrochemically active species and maximizing the use of the available pore volume. Conversely, a wider pore size distribution indicates that a relatively small proportion of the pores are within a narrow range of pore sizes. Thus, the pore size distribution is typically measured as a function of pore volume and is recorded as the unit size of the full width at half maximum of the main peak in a pore size distribution chart. The pore size distribution of a porous material may be determined by methods known in the art. Suitable methods for determining the pore size distribution include, but are not limited to, gas adsorption / desorption (e.g., nitrogen) measurements, mercury porosimetry, etc. The reported pore size distribution measurements herein are obtained by nitrogen adsorption analysis, unless otherwise specified. In certain embodiments, the aerogels of the present disclosure (e.g., polyamic acid, polyimide, polyamic acid metal salt, or carbon) have a relatively narrow pore size distribution.

[0164] The aerogel materials disclosed herein have a pore volume. As used herein, the term "pore volume" refers to the total volume of pores within a sample of a porous material. The pore volume is specifically measured as the volume of voids within the porous material and is typically in cubic centimeters per gram (cm 3It is recorded as ( / g or cc / g). For example, the pore volume of the porous material may be measured by methods known in the art, including but not limited to combinations with nitrogen porosimetry, mercury porosimetry, or helium pycnometry and bulk density measurements. In certain embodiments, the aerogels (polyamic acid, polyimide, or carbon) of the present disclosure have a relatively large pore volume 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 within the range between any two of these values. In other embodiments, the aerogels and xerogels (polyamic acid, polyimide, or carbon) of the present disclosure have a pore volume of about 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 within the range between any two of these values.

[0165] In some embodiments of the present disclosure, the aerogel material (polyamic acid, polyimide, or carbon, aerogel or xerogel) may include a fibrous form. As used herein, the term "fibrous form" refers to the structural form of a nanoporous material (e.g., carbon aerogel) including struts, rods, fibers, or filaments.

[0166] In some embodiments, the carbon aerogel produced by any of the disclosed methods has substantially the same properties as a carbon aerogel prepared by pyrolyzing a corresponding polyimide aerogel prepared by a conventional non-aqueous method. See, for example, U.S. Pat. Nos. 7,071,287 and 7,074,880 to Rhine et al.

[0167] In some embodiments, the carbon aerogel materials of the present disclosure, such as those derived from polyamic acid, polyamic acid metal salts, or polyimide, can have a residual "heteroatom" (i.e., non-carbon atom) nitrogen content of at least about 1 wt% as measured by elemental analysis. For example, the carbon aerogel material can have a residual nitrogen content of at least about 1 wt% and up to about 10 wt%. In some embodiments, the residual nitrogen content is about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, or about 10 wt%.

[0168] All of the methods described herein can be performed in any suitable order, unless otherwise indicated herein or clearly contradicted by the context. All examples provided herein, or the use of exemplary language (e.g., "such as"), are for the purpose of clearly explaining the materials and methods only and are not intended to limit the claims, unless otherwise stated. No language in this specification should be construed as indicating that any non-claimed element is essential for the practice of the disclosed materials and methods.

[0169] It will be readily apparent to those skilled in the relevant art that suitable modifications and adaptations to the compositions, methods, and uses described herein can be made without departing from the scope of any aspect thereof. The provided configurations and methods are exemplary and are not intended to limit the scope of the claimed aspects. All of the various aspects and options disclosed herein can be combined in any variation. The scope of the compositions, formulations, methods, and processes described herein includes all actual or potential combinations of the aspects, options, examples, and preferences described herein.

[0170] Although the technology of this specification has been described with reference to specific embodiments, it should be understood that these embodiments merely illustrate the principles and applications of the technology. It will be apparent to those skilled in the art that various modifications and changes can be made to the method and apparatus of the technology without departing from the spirit and scope of the technology. Therefore, the technology is intended to include modifications and variations within the scope of the appended claims and their equivalents.

[0171] Throughout this specification, when reference is made to "one embodiment", "a particular embodiment", "one or more embodiments", or "an embodiment", it means that the particular features, structures, materials, or characteristics described in connection with that embodiment are included in at least one embodiment of the technology. Thus, the phrases such as "in one or more embodiments", "in a particular embodiment", "in one embodiment", or "in an embodiment" used in various places in this specification do not necessarily refer to the same embodiment of the technology. Further, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. Any range recited herein is inclusive.

[0172] The embodiments of the technology are more fully illustrated by reference to the following examples. Before explaining some exemplary embodiments of the technology, it should be understood that the technology is not limited to the details of the configurations or process steps described in the following description. The technology can be other embodiments and can be implemented or executed in various ways. The following examples are described to illustrate particular embodiments of the technology and should not be construed as limiting them.

Examples

[0173] The present invention can be further illustrated by the following non-limiting examples that describe the method.

[0174] Example 1: Preparation of polyimide aerogel in water using Li2CO3 (target density 0.05 g / cc) as a base, and thermal decomposition to the corresponding carbon aerogel p-Phenylenediamine (3.315 g, 0.03065 mol) was dissolved in 200 mL of deionized water. Lithium carbonate (2.718 g, 1.2 mol excess) was added and dissolved in the same aqueous solution. Pyromellitic dianhydride (6.685 g) was suspended in the same solution. After several minutes, no solid remained in the suspension. Gas evolution (CO2) was observed at the initial stage of the reaction. The viscous yellow solution was stirred for up to 4 days. Aliquots were taken every 24 hours and the viscosity was measured with a rheometer. The values were 76.6 cP on the first day, 74.2 cP on the second day, 71.0 cP on the third day, and 70.8 cP on the fourth day.

[0175] On the second day, 50 g aliquots of the viscous solution were removed and acetic anhydride (1.471 g, 2.0 mol excess relative to the monomer repeating units contained in the aliquot) was added. The solution obtained after adding acetic anhydride was stirred for about 30 seconds and then transferred to a cylindrical mold. The solution gelled in less than 5 minutes. The resulting gel was transparent, light brown, and rubbery, and very similar to gels obtained by conventional routes (i.e., prepared in organic solvents). The gels were aged in their molds at room temperature for 24 hours, then washed with ethanol (4 times) and dried with supercritical fluid CO2 to obtain the corresponding monolithic polyimide aerogels. These aerogels were designated as Td005-Li2-d2-AA2. The monoliths were pyrolyzed to the corresponding carbon aerogels at 1050 °C while flowing nitrogen and were named C-Td005-Li2-d2-AA2. Material property data for both Td005-Li2-d2-AA2 and C-Td005-Li2-d2-AA2 are shown in Tables 2 and 3.

[0176] On the second day, another 50 g aliquot of the viscous solution was removed, and acetic anhydride (1.471 g, 2.0 molar excess relative to the monomer repeating units contained in the aliquot) was added. The solution obtained after adding acetic anhydride was stirred for about 30 seconds and then transferred to a cylindrical mold. The solution gelled in less than 5 minutes. The resulting gel was transparent, light brown, and rubbery, and very similar to the gels obtained by conventional routes (i.e., prepared in organic solvents). The gel in the mold was aged at 68 °C for 24 hours, then cooled to room temperature, removed from the mold, washed with ethanol (4 times), and dried with supercritical fluid CO2 to obtain the corresponding monolithic polyimide aerogel. These aerogels are designated as Td005-Li2-d2-AA2-68. Figure 1A shows the solid 15N NMR spectrum of the polyimide aerogel obtained by aging the wet gel at 68 °C. 15 Figure 1B shows the solid 15N NMR spectrum of the polyimide aerogel obtained by aging the wet gel at 25 °C.

[0177] The selected monolith was pyrolyzed to the corresponding carbon aerogel at 1050 °C while flowing nitrogen, and this was named C-Td005-Li2-d2-AA2-68. The material property data for both Td005-Li2-d2-AA2-68 and C-Td005-Li2-d2-AA2-68 are shown in Tables 2 and 3.

[0178] On the fourth day, a 50 g aliquot of the viscous solution was removed, and acetic anhydride (3.163 g, 4.3 molar excess relative to the monomer repeating units contained in the aliquot) was added. The solution obtained after adding acetic anhydride was stirred for about 30 seconds and then transferred to a mold. The solution gelled in about 1 minute. The resulting gel was transparent, light brown, and rubbery, and very similar to gels obtained by conventional routes (i.e., prepared in organic solvents). The gel samples were aged in their molds for 24 hours, then washed with ethanol (4 times) and dried with supercritical fluid CO2 to obtain the corresponding polyimide aerogels. These aerogel monoliths are designated as Td005-Li2-d4. The selected monoliths were pyrolyzed to the corresponding carbon aerogels at 1050 °C while flowing nitrogen and named C-Td005-Li2-d4. Material property data for both Td005-Li2-d4 and C-Td005-Li2-d4 are shown in Tables 2 and 3.

[0179] A portion of the remaining viscous yellow solution on the fourth day was dropped into an aqueous hydrochloric acid solution (30% concentrated HCl in deionized water). Polyamic acid beads were instantly formed. These beads were aged in the receiving HCl solution for 24 hours, then washed with water (4 times, 12 hours each time, using 1 L of water each time), similarly washed with ethanol, and dried with supercritical fluid CO₂ to obtain the corresponding polyamic acid aerogel beads. These aerogel beads are designated as PAA-beads-Td005-Li2-d4. A portion of these beads was pyrolyzed to the corresponding carbon aerogel beads at 1050 °C while flowing nitrogen, and this was named C-PAA-beads-Td005-Li2-d4. The material property data of both PAA-beads-Td005-Li2-d4 and C-PAA-beads-Td005-Li2-d4 are shown in Tables 2 and 3. It is noted that the lithium residue amounts by elemental analysis in both PAA-beads-Td005-Li2-d4 and C-PAA-beads-Td005-Li2-d4 (0.02% and 0.05% weight / weight, respectively) are much less than the lithium residue amounts in Td005-Li2-d4 and C-Td005-Li2-d4 (1.45% and 1.52% weight / weight, respectively). The decrease in the lithium amount in the aerogel beads was considered to be due to the water washing of the wet gel beads.

[0180] On the fourth day, a portion of the remaining viscous yellow solution was treated with acetic anhydride as described above, and the gelling material was cast onto an aluminum sheet to form a thin film. The film was tough and flexible. A photographic image of the film is shown in Figure 2. As shown in Figure 2, one end of the film is 50 microns thick and the other end is 120 microns thick.

[0181] Example 2: Preparation of polyimide aerogel in water using Li₂CO₃ (target density 0.1 g / cc) as a base, and pyrolysis to the corresponding carbon aerogel A polyimide gel was prepared in the same manner as in Example 1, but with the target density doubled. p-Phenylenediamine (6.63 g, 0.06131 mol) was dissolved in 200 mL of deionized water. Lithium carbonate (5.435 g, 1.2 mol excess) was added and dissolved in the same aqueous solution. Pyromellitic dianhydride (13.37 g, 0.06130 mol) was suspended in this solution. After a few minutes, no solid remained in the suspension. At the initial stage of the reaction, a large amount of gas evolution (CO2) was observed. The viscous yellow solution was stirred for 2 days. Aliquots were taken every 24 hours and the viscosity of the aliquots was measured with a rheometer. The values were: Day 1: 422 cP, Day 2: 348 cP. On the second day, 3.43 mol excess acetic anhydride with respect to the monomer repeating unit was added to the very viscous solution (amount of acetic anhydride: 21.512 g). The resulting solution was stirred for about 30 seconds and then divided into molds. The solution gelled in less than 1 minute. The gel was very dark in color but still transparent and rubbery and was equivalent to the gels obtained in organic solvents by the classical route. The gels were aged in their molds for 24 hours and then washed with ethanol (4 times) and dried with supercritical fluid CO2. These aerogel monoliths were designated as Td01-Li2-d2-AA3.43. The selected monoliths were pyrolyzed to the corresponding carbon aerogels at 1050 °C while flowing nitrogen and were named C-Td01-Li2-d2-AA3.43. Material property data for both Td01-Li2-d2-AA3.43 and C-Td01-Li2-d2-AA3.43 are shown in Tables 2 and 3.

[0182] Example 3: Preparation of polyimide aerogels in water using Li2CO3 as a base at other target densities, and pyrolysis to the corresponding carbon aerogels Polyimide gels, aerogels, and carbon aerogels were prepared at target densities (Td) of 0.02 and 0.085 in the same manner as in Example 2 by appropriately adjusting the monomer concentration in the reaction mixture.

[0183] When Td = 0.085, p-phenylenediamine (5.64 g, 0.0521 mol) was dissolved in 200 mL of deionized water. Lithium carbonate (4.620 g, 1.2 mol excess) was added and dissolved in the same aqueous solution. Pyromellitic dianhydride (11.36 g, 0.0521 mol) was suspended in the same solution. After several minutes, no solid remained in the suspension. At the initial stage of the reaction, a large amount of gas evolution (CO2) was observed. The viscous yellow solution was stirred for 2 days. On the second day, the viscosity was reconfirmed with a rheometer: 126.7 cP. On the second day, acetic anhydride in an amount 4.3 mol in excess relative to the monomer repeating unit was added to the very viscous solution (amount of acetic anhydride: 22.87 g). The new solution after adding acetic anhydride was stirred for about 30 seconds and then divided into molds. The solution gelled in less than 1 minute. The gel was dark but still transparent and rubbery, similar to the gels obtained in organic solvents by the classical route. The gels were aged in those molds at room temperature for 24 hours. Then, they were washed with ethanol (4 times) and dried with supercritical fluid CO2 to obtain the corresponding monolithic polyimide aerogels. These aerogel monoliths are designated as Td0085-Li2-d2. The selected monoliths were pyrolyzed to the corresponding carbon aerogels at 1050 °C while flowing nitrogen, and this was named C-Td0085-Li2-d2. Material property data for both Td0085-Li2-d2 and C-Td0085-Li2-d2 are shown in Tables 2 and 3.

[0184] When Td = 0.02, p-phenylenediamine (1.33 g, 0.0123 mol) was dissolved in 200 mL of deionized water. Lithium carbonate (1.087 g, 1.2 mol excess) was added and dissolved in the same aqueous solution. Pyromellitic dianhydride (2.674 g, 0.0123 mol) was suspended in the same solution. After several minutes, no solid remained in the suspension. The evolution of gas (CO2) was not significant. The pale yellow solution was stirred for 2 days. On the second day, the viscosity was reconfirmed with a rheometer: 2.7 cP. On the second day, acetic anhydride in an amount 4.3 mol in excess with respect to the monomer repeating unit was added to the solution (amount of acetic anhydride: 5.399 g). The new solution after adding acetic anhydride was stirred for about 30 seconds and then divided into molds. Gelation occurred over 1 hour. The gels were pale yellow, transparent, and jelly-like. The gels were aged in their molds at room temperature for 7 days. During that time, a very small amount of bleeding occurred and the gels could be removed from the molds. Then, these were washed with ethanol (4 times) and dried with supercritical fluid CO2 to obtain the corresponding monolithic polyimide aerogels. These aerogel monoliths are designated as Td002-Li2-d2-age7. The selected monoliths were pyrolyzed to the corresponding carbon aerogels at 1050 °C while flowing nitrogen and named this as C-Td002-Li2-d2-age7. Material property data for both Td002-Li2-d2-age7 and C-Td002-Li2-d2-age7 are shown in Tables 2 and 3.

Table 2

Table 3

[0185] Example 4. Polyimide aerogels in water at Td 0.05 g / cc using NaHCO3 as the base, and pyrolysis to the corresponding carbon aerogels. p-Phenylenediamine (3.315 g, 0.03065 mol) was dissolved in 200 mL of deionized water. Sodium bicarbonate (5.666 g, 2.2 mol excess) was added and dissolved in the same aqueous solution. Pyromellitic dianhydride (6.685 g, 0.03065 mol) was suspended in the same solution. After several minutes, no solid remained in the suspension. Gas evolution (CO2) was observed at the initial stage of the reaction. The viscous yellow solution was stirred for 3 days. Aliquots were taken every 24 hours and the viscosity of the aliquots was measured with a rheometer. The values were 43.2 cP on the first day, 44.7 cP on the second day, and 44.8 cP on the third day. On the fourth day, 50 g of the viscous solution was separated and acetic anhydride in an amount 4.3 mol in excess with respect to the monomer repeating units contained in that amount was added thereto (amount of acetic anhydride: 3.120 g). The resulting solution was stirred for about 30 seconds and then cast into molds. The solution gelled in about 1 minute. The gel was transparent but light brown and rubbery and was equivalent to the gels obtained in organic solvents by the classical route. The gels were aged in their molds for 24 hours and then washed with ethanol (4 times) and dried with supercritical fluid CO2.

[0186] The remainder of the viscous yellow solution on the third day was added dropwise to an aqueous hydrochloric acid solution (30% concentrated HCl in deionized water). Beads were formed instantaneously. The beads were aged in the receiving HCl solution for 24 hours and then washed with water (4 times, 12 hours each time, using 1 L of water each time). The beads were then washed with ethanol in the same manner and dried with supercritical fluid CO2. The material property data of Td005-NaH-d3 and C-Td005-NaH-d3 (monolith and beads) are shown in Figure 3 (Table 4).

[0187] Example 5. Polyimide aerogel in water at Td 0.05 g / cc using ammonium carbonate as a base, and thermal decomposition to the corresponding carbon aerogel. p-Phenylenediamine (3.315 g, 0.03065 mol) was dissolved in 200 mL of deionized water. Ammonium carbonate (3.535 g, 1.2 mol excess) was added and dissolved in the same aqueous solution. Pyromellitic dianhydride (6.685 g, 0.03065 mol) was suspended in the same solution. After several minutes, no solid remained in the suspension. At the initial stage of the reaction, gas evolution (CO2) was observed. The viscous yellowish solution was stirred for 1 day. The viscosity was measured with a rheometer. The value was 205 cP. Then, acetic anhydride in an amount 4.3 mol in excess relative to the monomer repeating unit was added to the solution (amount of acetic anhydride: 13.563 g). The new solution after adding acetic anhydride was stirred for about 30 seconds and then cast into molds. The solution gelled in about 1 minute. The gel was transparent but light brown and rubbery, similar to the gels obtained in organic solvents by the classical route. The gels were aged at room temperature in those molds for 24 hours. Subsequently, these were washed with ethanol (4 times) and dried with supercritical fluid CO2 to obtain the corresponding monolithic polyimide aerogels. These aerogel monoliths were designated as Td005-Am2-d1. The selected monoliths were pyrolyzed to the corresponding carbon aerogels at 1050 °C while flowing nitrogen and named this C-Td005-Am2-d1. Material property data for both Td005-Am2-d1 and C-Td005-Am2-d1 are shown in Figure 4 (Table 5).

[0188] Example 6. Polyimide aerogels in water at a target density of 0.05 g / cc using guanidinium carbonate as the base, and pyrolysis to the corresponding carbon aerogels. p-Phenylenediamine (PDA; 3.32 g, 0.0307 mol) was dissolved in 200 mL of deionized water. Guanidinium carbonate (6.63 g, 1.2 mol excess) was added and dissolved in the PDA aqueous solution. Pyromellitic dianhydride (PMDA; 6.69 g, 0.0307 mol) was suspended in the solution. Gas evolution (CO2) was observed at the initial stage of the reaction. Solid PMDA did not remain in the suspension within a few minutes of addition. The solution rapidly (within a few minutes) became viscous. The viscous solution was divided into two, and acetic anhydride (13.48 g, 4.3 mol excess relative to the monomer repeating unit) was added to the solution 2 or 3 hours after the start of the reaction to gelate it. After adding acetic anhydride, the solution was stirred for a short time and it was observed that gelation occurred within a few seconds. The resulting monolithic gel was yellow and opaque. The monolithic gel was aged at either room temperature (25 °C) or 68 °C for 24 hours, divided into smaller pieces, washed 4 times with ethanol, and dried with supercritical fluid CO2 to obtain the corresponding monolithic aerogels with a foamy appearance. These aerogel monoliths were titled Td005-G2-1.2-2_hours and Td005-G2-1.2-3_hours depending on whether gelation with acetic anhydride was induced 2 or 3 hours after the start of the reaction.

[0189] Another set of aerogel monoliths titled Td005-G2-1.0-2_hours and Td005-G2-1.0-3_hours was prepared in the same manner except that only 1 equivalent of guanidinium carbonate relative to the monomer was used. Samples of aerogel monoliths prepared using 1 equivalent of guanidinium carbonate and aged at room temperature (25 °C) or 68 °C for 24 hours were analyzed by solid 15 13C NMR. Figures 5A and 5B are the overlays of the solid 15It is an N NMR spectrum. As shown in Figure 5A, aging at 68 °C promotes imidization (a larger peak at 177 ppm), while the spectrum in Figure 5B shows that aging at room temperature results in more amide than imide groups (132 ppm). Furthermore, as shown continued in Figure 5B, upon aging at room temperature, the sample contained residual polyamic acid guanidinium salt, as indicated by the guanidinium cation resonance at 74 ppm.

[0190] The selected monolith was pyrolyzed to the corresponding carbon aerogel at 1050 °C while flowing nitrogen, which was denoted with the prefix C. The material characterization data are shown in Figures 6 (Table 6) and 7, which are SEM micrographs of the carbonized samples. As shown in Figure 7, at high magnification (50,000×), the walls of the larger pores providing a foamy appearance consist of intertwined nanofibers.

[0191] A similar solution of PDA + PMDA polyamic acid guanidinium salt (Td = 0.05) was reacted for 24 hours. At that point, the solution (denoted as Td005 - G2 - 1.0 - 24_hours) became so viscous that it stopped flowing. However, the solution flowed when stirred. To further test this phenomenon, solutions of guanidinium carbonate and PDA + PMDA polyamic acid guanidinium salt with different target densities were prepared in the same way. The linear viscoelastic response of the polyamic acid solution was measured using a cone (60 mm diameter, 2° inclination) and plate geometry on a temperature - controlled Peltier plate (Figure 8). Specifically, Figure 8 shows the different target densities (Td, where Td is the total solid (PDA + PMDA) concentration in the sol (g / cm 3) (referring to ) is a graph showing the storage modulus and loss modulus against the oscillatory stress for the oscillatory amplitude sweep of the polyamic acid guanidinium solution. As shown in Figure 8, in the oscillatory mode and at low shear stress, some polyamic acid guanidinium solutions exhibit solid-like behavior (G’>G’’) including a storage modulus (G’) and a loss modulus (G’’) that are independent of the oscillatory stress. However, when the critical stress called the yield stress is exceeded, the storage modulus G’ decreases rapidly and the system flows. As the polyamic acid guanidinium concentration decreases, as the polymer network becomes more dispersed, the plateau modulus becomes lower and the yield stress becomes lower. Continuing as shown in Figure 8, at a sufficiently low polymer concentration for a solution with 2% solids (Td = 0.2), the polymer solution no longer forms a leached gel and flows at all observed stresses. Furthermore, it has been observed that some solutions in which the polyamic acid guanidinium concentration slightly exceeds the critical gel concentration do not gel when the polymer is limited to a low molecular weight. In addition to the concentration of polyamic acid guanidinium, the yield stress is also sensitive to temperature, and as shown in Figure 9, the lower the temperature, the higher the critical stress required to initiate flow.

[0192] When the experiment was completed, the solution immediately returned to solid behavior. Cycles of shear amplitude values below and then above the yield stress show a rapid and reproducible switch from solid (tan(δ)<1) to liquid-like (tan(δ)>1) viscoelastic response as shown in Figure 10, where tan(δ) is the G’’ / G’ ratio. As shown in Figure 10, for the polyamic acid guanidinium solution (Td = 0.05; ratio of monomer to guanidinium carbonate 1:2), the material undergoes shear thinning (thixotropy) with increasing shear rate.

[0193] Figure 11 is a graph showing shear rate versus viscosity for the steady-state rotational sweep of various aqueous polyamic acid solutions. As shown in Figure 11, for the guanidinium carbonate-based polyamic acid solution, a decrease in viscosity (shear thinning) with increasing shear rate (thixotropy) was observed. Continuing as shown in Figure 11, the monovalent metal cation Na +For the polyamic acid aqueous solution formed with (Na2CO3, Td = 0.5), thixotropic behavior is not observed. Li + , K + , and other monovalent metal cations such as ammonium also did not result in thixotropic behavior of the corresponding polyamic acid solution (data not shown). Gels formed by exposing the polyamic acid solution to solutions containing polyvalent cations also did not show thixotropic viscoelastic responses, but rather, they formed gels (data not shown). Without wishing to be bound by theory in this regard, it is thought that thixotropic behavior requires cations that can dynamically interact (e.g., via hydrogen bonding) with multiple polymer chains (e.g., guanidinium).

[0194] Example 7. Polyimide aerogel in water using other water-soluble carbonates as bases Other common water-soluble carbonates not included in the above specific examples include lithium bicarbonate (LiHCO3), sodium carbonate (Na2CO3), potassium carbonate (K2CO3), potassium bicarbonate (KHCO3), and ammonium bicarbonate (NH4HCO3). Polyamic acid aqueous solutions were formed for each of these according to the procedure of the above examples.

[0195] Example 8. Molecular weight measurement of polyamic acid salts produced using sodium carbonate as the base According to the foregoing examples (i.e., from PDA-PMDA in aqueous solution), a sodium poly(amic acid) salt solution was prepared using sodium carbonate as the base. Reference poly(amic acid) salt solutions were prepared using conventional processes (dimethylformamide as the solvent, triethylamine as the base, precipitation and washing with acetone) and previous aqueous processes (water as the solvent, triethylamine as the base). The precipitated polymer was dissolved in water and a solution of Na2CO3 (1.2 molar equivalents of Na2CO3: 5% solids PAA containing monomer). Gel permeation chromatography (GPC) was used to analyze the reference and inventive poly(amic acid) solutions for the molecular weight of the poly(amic acid) polymer, using a previous poly(amic acid) salt of known molecular weight as the molecular weight standard (Figure 12). The poly(amic acid) was eluted in water using 0.2 M Na2SO4, and the molecular weight was calculated from the peak elution time using a poly(amic acid) polymer having a previously measured molecular weight. As shown in Figure 12, the product of the disclosed carbonate / water process (black solid line) exhibits a faster elution time (higher molecular weight) compared to the reference TEA water process (gray solid line), and overlaps the molecular weight peak distribution (black dotted line) of a higher molecular weight sample prepared in DMF according to the conventional method. Generally, the GPC results show that the molecular weight peaks and distributions are equivalent for the products of the solvent-based conventional method and the disclosed inventive carbonate-based method, and that the molecular weight peaks for the products derived from both methods are in the range of about 20,000 - 30,000 g / mol. In contrast, the GPC results for the product of the previous aqueous process (water as the solvent, triethylamine as the base) show a very low product molecular weight (2,000 - 8,000 g / mol).

[0196] The rheology analysis of the solution was carried out (viscosity measurement; measured by continuous flow at 1 rad / sec). Specifically, the viscosities of the above three kinds of sodium poly(amic acid) salt solutions (conventional DMF method, redissolution; previous water-TEA method; and the disclosed water-carbonate method) were measured and plotted against the respective molecular weights measured by GPC (Figure 13). As shown in Figure 13, from the viscosities of equivalent solutions (same polymer concentration and same Na2CO3 concentration), a power-law relationship was shown and the GPC analysis results were confirmed.

Claims

1. A method for preparing a polyimide aerogel, wherein the method comprises: A water-soluble diamine, a water-soluble carbonate or bicarbonate, and a tetracarboxylic dianhydride are mixed in water. The above components are reacted to provide an aqueous solution of a polyamic acid salt. The method includes imidizing the polyamido salt to form a polyimide wet gel, wherein imidizing the polyamido salt includes adding a gelling initiator to an aqueous solution of the polyamido salt to form a gelling mixture, and gelling the gelling mixture, and the method also includes The preparation method comprising drying the polyimide wet gel to form the polyimide aerogel.

2. The aforementioned mixture A water-soluble diamine is dissolved in water to form an aqueous diamine solution. The water-soluble carbonate or bicarbonate is added to the aqueous diamine solution, A tetracarboxylic dianhydride is added to the aqueous solution of the diamine and the water-soluble carbonate or bicarbonate to form a solution. The method according to claim 1, comprising stirring the solution at a temperature in the range of about 4 to about 60°C for a period of about 1 hour to about 4 days.

3. The aforementioned mixture A water-soluble diamine is dissolved in water to form an aqueous diamine solution. A suspension is formed by adding tetracarboxylic dianhydride to the aforementioned aqueous diamine solution. The suspension is stirred at a temperature in the range of approximately 4 to approximately 60°C for a period of approximately 1 hour to approximately 4 days. The water-soluble carbonate or bicarbonate is added to the suspension. The method according to claim 1, comprising stirring the suspension at a temperature in the range of about 4 to about 60°C for a period of about 1 hour to about 4 days to provide an aqueous solution of the polyamic acid salt.

4. The aforementioned mixture A water-soluble diamine, a tetracarboxylic dianhydride, and a water-soluble carbonate or bicarbonate are added to water simultaneously or rapidly and sequentially. The method according to claim 1, comprising stirring the obtained mixture at a temperature in the range of about 4 to about 60°C for a period of about 1 hour to about 4 days to provide an aqueous solution of the polyamidate.

5. The method according to claim 1, wherein the water-soluble carbonate or bicarbonate comprises lithium, sodium, potassium, ammonium, or guanidinium cation.

6. The method according to claim 1, wherein the water-soluble carbonate or bicarbonate is selected from the group consisting of lithium carbonate, lithium bicarbonate, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, ammonium carbonate, ammonium bicarbonate, guanidinium carbonate, and combinations thereof.

7. The water-soluble carbonate or bicarbonate is a carbonate, and the molar ratio of the water-soluble carbonate to the diamine is about 1 to about 1.4, or The method according to claim 1, wherein the water-soluble carbonate or bicarbonate is a bicarbonate, and the molar ratio of the water-soluble bicarbonate to the diamine is about 2 to about 2.

8.

8. The method according to claim 1, wherein the molar ratio of the tetracarboxylic dianhydride to the diamine is about 0.9 to about 1.

1.

9. The method according to claim 1, wherein the tetracarboxylic dianhydride is selected from the group consisting of biphthalic acid dianhydride (BPDA), benzophenone tetracarboxylic dianhydride (BTDA), oxydiphthalic acid dianhydride (ODPA), naphthalyl tetracarboxylic dianhydride, perylene tetracarboxylic dianhydride, and pyromellitic acid dianhydride (PMDA).

10. The method according to claim 1, wherein the diamine is 1,3-phenylenediamine, 1,4-phenylenediamine, or a combination thereof.

11. The method according to claim 1, wherein the diamine is 1,4-phenylenediamine.

12. The concentration range of the polyamic acid salt in the aqueous solution is approximately 0.01 to approximately 0.3 g / cm³, based on the weight of the polyamic acid. ３ The method according to claim 1.

13. The method according to claim 1, wherein the water-soluble carbonate or bicarbonate is guanidinium carbonate, and the solution of the salt of the polyamic acid exhibits thixotropic behavior.

14. The method according to claim 1, further comprising adding an electroactive material to an aqueous solution of the polyamid salt.

15. The method according to claim 14, wherein the electroactive material comprises carbon, graphite, silicon, sulfur, Prussian blue, lithium iron phosphate, a combination thereof, or any one or more precursors thereof.

16. The method according to claim 1, wherein the gelling initiator is acetic anhydride.

17. Drying the polyimide wet gel Optionally, the polyimide-wetted gel may be washed or the solvent changed. The method according to claim 1, comprising subjecting the polyimide wet gel to high-temperature conditions, freeze-drying the polyimide wet gel, or contacting the polyimide wet gel with supercritical fluid carbon dioxide.

18. The method according to claim 17, wherein the washing or solvent exchange is carried out using water, C1-C4 alcohols, acetone, acetonitrile, ether, tetrahydrofuran, toluene, liquid carbon dioxide, or a combination thereof.

19. The method according to claim 1, further comprising converting the polyimide aerogel into an isomorphic carbon aerogel, wherein the conversion includes thermally decomposing the polyimide aerogel at a temperature of at least about 650°C in an inert atmosphere.

20. A polyimide aerogel prepared by the method described in any one of claims 1 to 18.

21. The polyimide aerogel according to claim 20, comprising a residual amount of the water-soluble carbonate or bicarbonate.

22. A carbon aerogel prepared by the method described in claim 19.

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