Polyimide aerogels with three-dimensional cross-linked structure are made using linear oligomeric segments of polyimide, and linked with one of the following into a 3D structure: trifunctional aliphatic or aromatic amines, latent reactive end caps such as nadic anhydride or phenylethynylphenyl amine, and silica or silsesquioxane cage structures decorated with amine. Drying the gels supercritically maintains the solid structure of the gel, creating a polyimide aerogel with improved mechanical properties over linear polyimide aerogels.
Lightweight, low-density structures are desired for acoustic and thermal insulation for aerospace structures, habitats, astronaut equipment, and aeronautic applications. Aerogels are a unique material for providing such properties because of their extremely low density and small pore sizes. However, plain silica aerogels are brittle. Reinforcing the aerogel structure with a polymer (X-Aerogel) provides vast improvements in strength while maintaining low density and pore structure. However, degradation of polymers used in cross-linking tends to limit use temperatures to below 150 °C. Organic aerogels made from linear polyimide have been demonstrated, but gels shrink substantially during supercritical fluid extraction and may have lower use temperature due to lower glass transition temperatures.
The purpose of this innovation is to raise the glass transition temperature of all organic polyimide aerogel by use of tri-, tetra-, or poly-functional units in the structure to create a 3D covalently bonded network. Such cross-linked polyimides typically have higher glass transition temperatures in excess of 300–400 °C. In addition, the reinforcement provided by a 3D network should improve mechanical stability, and prevent shrinkage on supercritical fluid extraction. The use of tri-functional aromatic or aliphatic amine groups in the polyimide backbone will provide such a 3D structure.
Alternatively, cross-linking of the polyimide can be carried out by capping long-chain oligomers with latent reactive end caps (nadic anhydride or phenylethynylphenylamine, for example). After supercritical fluid extraction, the cross-linking is carried out on a postcure of the dried gels. In another embodiment, polyimide or polyamic acid can be capped with trialkoxy silanes, which can be co-reacted with small amounts of tetraalkoxy silanes to form regions of covalently bonded silica crosslinks. Alternately, polysilsesquioxane cages decorated with surface amines can be co-reacted with anhydride capped polyimide/polyamic acid.
Cross-linked polyimide aerogels with their high porosity, combined with higher strength, have excellent thermal as well as sound-insulating qualities. In addition, their high specific surface area (e.g. on the order of 200–1,000 m2/g) should make them well suited for numerous applications, including as adsorbent beds for chemical separations, and as platforms for solid-state sensors.
This work was done by Mary Ann B. Meador of Glenn Research Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Materials categor y.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18486-1.