Aqueous polyimide processes
The present disclosure is directed to methods of forming polyamic acid and polyimide gels in water. The resulting polyamic acid and polyimide gels may be converted to aerogels, which may further be converted to carbon aerogels. Such carbon aerogels have the same physical properties as carbon aerogels prepared from polyimide aerogels obtained according to conventional methods, i.e., organic solvent-based. The disclosed methods are advantageous in reducing or avoiding costs associated with use and disposal of potentially toxic solvents and byproducts. Gel materials prepared according to the disclosed methods are suitable for use in environments involving electrochemical reactions, for example as an electrode material within a lithium-ion battery.
1. A method of forming a polyimide aerogel, the method comprising:
providing an aqueous solution of a polyamic acid salt, the polyamic acid salt comprising a polyamic acid including carboxylic acid groups, wherein the carboxylic acid groups are associated with an alkali metal cation and greater than 95% of the carboxylic acid groups are present as carboxylate anions, wherein providing the aqueous solution of the polyamic acid salt comprises:
providing a polyamic acid;
adding the polyamic acid to water to form an aqueous suspension of the polyamic acid; and
adding an alkali metal hydroxide to the aqueous suspension of the polyamic acid to form the aqueous solution of the polyamic acid salt;
imidizing the polyamic acid salt to form a polyimide gel; and
drying the polyimide gel to form the polyimide aerogel.
2. The method of claim 1 , wherein the alkali metal hydroxide is lithium hydroxide, sodium hydroxide, or potassium hydroxide.
3. The method of claim 1 , wherein the polyimide gel is in bead form, and wherein imidizing the polyamic acid salt comprises adding a dehydrating agent to the aqueous solution of the polyamic acid salt to form a gelation mixture; the method further comprising adding the gelation mixture to a solution of a water-soluble acid in water to form the polyimide gel beads, wherein adding comprises dripping the gelation mixture into the solution of the water soluble acid in water, spraying the gelation mixture under pressure through one or more nozzles into the solution of the water-soluble acid in water using pressure; or electrospraying the gelation mixture into the solution of the water soluble acid in water.
4. The method of claim 3 , wherein the dehydrating agent is acetic anhydride.
5. The method of claim 4 , wherein the water-soluble acid is a mineral acid or is acetic acid.
6. The method of claim 1 , wherein the polyimide gel is in bead form, and wherein imidizing the polyamic acid salt comprises adding a dehydrating agent to the aqueous solution of the polyamic acid salt to form a gelation mixture, the method further comprising combining the gelation mixture with a water-immiscible solvent comprising a surfactant; and mixing the resulting mixture.
7. The method of claim 1 , wherein the polyimide gel is in bead form, and wherein imidizing the polyamic acid salt comprises chemical imidization, the method comprising:
combining the aqueous solution of the polyamic acid salt with a water-immiscible solvent comprising a surfactant;
mixing the resulting mixture to form an emulsion; and adding a dehydrating agent to the emulsion.
8. The method of claim 1 , further comprising adding an electroactive material to the aqueous solution of the polyamic acid salt.
9. The method of claim 1 , wherein drying the polyimide gel comprises:
optionally, washing or solvent exchanging the polyimide gel; and
subjecting the polyimide gel to elevated temperature conditions, lyophilizing the polyimide gel, or contacting the polyimide gel with supercritical fluid carbon dioxide.
10. The method of 9 , further comprising converting the polyimide aerogel to an isomorphic carbon aerogel, the converting comprising pyrolyzing the polyimide aerogel under inert atmosphere at a temperature of at least about 650° C.
11. A method of forming a polyimide aerogel in bead form, the method comprising:
providing an aqueous solution of a polyamic acid salt, the polyamic acid salt comprising a polyamic acid including carboxylic acid groups, wherein the carboxylic acid groups are associated with a cationic species and greater than 95% of the carboxylic acid groups are present as carboxylate anions;
imidizing the polyamic acid salt to form a polyimide gel, wherein the imidizing comprises adding a dehydrating agent to the aqueous solution of the polyamic acid salt to form a gelation mixture;
adding the gelation mixture to a water-immiscible solvent, wherein adding comprises dripping the gelation mixture into the water-immiscible solvent, spraying the gelation mixture under pressure through one or more nozzles into the water-immiscible solvent using pressure, or electrospraying the gelation mixture into the water-immiscible solvent; and
drying the polyimide gel to form the polyimide aerogel.
12. The method of claim 11 , wherein the dehydrating agent is acetic anhydride.
13. The method of claim 11 , further comprising an acid.
14. The method of claim 13 , wherein the acid is acetic acid.