Flexible Polyimide Aerogels Derived from the Use of a Neopentyl Spacer in the Backbone

Polyimide aerogels have gained much attention in the last decade due to their highly porous structure with low density, low dielectric constant, good mechanical properties, and thermal stability. They are desirable for many aerospace and commercial applications including lightweight substrates for antennas and for aerospace vehicles, due to the low dielectric constant, approaching one as density decreases. However, with only aromatic diamines in the backbone, the aerogels are flexible only as very thin films, nominally 0.5 mm thick. A more flexible aerogel substrate would allow them to be used as conformal antennas, which would reduce drag and also save space in aerospace vehicles. In this research, we demonstrate that the flexibility can be increased by substituting the aromatic diamines typically used in polyimide aerogels with 1,3-bis(4-aminophenoxy)-2,2-dimethylpropane (BAPN), which contains a flexible neopentyl spacer group. Using the aliphatic diamine, BAPN leads to aerogels with a high bend radius even when the aerogels are 2–3 mm thick. Twenty different formulations of polyimide aerogels were synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and a combination of BAPN and 2,2′-dimethylbenzidine (DMBZ) and cross-linked with 1,3,5-triaminophenoxybenzene (TAB) to understand the effect of polymer concentration, oligomer chain length, and concentration of BAPN on the properties of the aerogels. Aerogels made using 50 mol % BAPN with n of 45 and 7 wt % polymer possessed the best combination of properties conducive to use as a flexible substrate for conformal antennas, including low density, good hydrophobicity, low shrinkage with high surface area, a 1 in. bend radius in aerogels up to 3 mm thickness, and a dielectric constant of 1.11.