Effect of Urea Links in the Backbone of Polyimide Aerogels

Flexible, conformal polyimide (PI) aerogels with low density, good mechanical properties, and high surface areas have attracted much attention for many potential applications such as lightweight antenna substrates, insulating materials for launch vehicles, inflatable structures, aircraft, or space suits. Development and improvements to the fabrication of polyimide aerogel thin films have been reported over the last decade to meet the needs of many of these applications. However, most starting materials are expensive. In this research, we utilized commercially available, low-cost monomers including 4,4′-bis­(4-aminophenoxy)­propane (BAPP) and 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), and 4,4-methylene diphenyl diisocyanate (MDI) in fabricating polyimide (PI) and poly­(imide-urea) (PIU), which were then cross-linked with 1,3,5-tris­(4-aminophenoxy)­benzene (TAB). It was found that the addition of MDI into the PI chains not only maintained the flexibility of the aerogel films but also enhanced the film casting, allowing the production on pilot scale. With the capability of producing robust films at affordable cost, the application of the PIU aerogel films can be expanded to terrestrial goods such as winter clothing, or pipe wrapping, etc. In addition, the presence of only a small addition of the urea links in the polyimide chains in PIU aerogels led to lower shrinkage when compared to the corresponding PI aerogels, leading to lower density.