Heterogeneous silica-polyimide aerogel-in-aerogel nanocomposites

Polymer aerogels are a promising, non-brittle alternative to silica aerogel, but are limited by their very poor high-temperature stability. Polyimide is widely known for its high heat-resistance, however, a high degree of volume shrinkage is common for polyimide aerogels after exposure to temperatures above 200 °C. Here, we present the aerogel-in-aerogel composites that comprise silica aerogel grains with a nanoparticulate microstructure embedded in a nanofibrous polyimide aerogel matrix. The mixing procedure and synthesis protocol were optimized to avoid excessive infiltration of the polyimide sol into the silica aerogel mesopores. The composites display a unique, heterogeneous structure with a high surface area, >600 m² g^-1, a low thermal conductivity down to 17.5 mW m^-1 K^-1, a very low dielectric constant (∼2.5 at 10^-1-10⁶ Hz, ∼1.2 at 8-12.5 GHz, ∼1.2 at 26.5-32 GHz) and dielectric loss (10^-3 to 10^-1 at all studied frequencies). The hydrophobic silica aerogel component contributes a high water resistance, with high water contact angles (>150°) and a low humidity uptake (<4 wt.% at 88% relative humidity), and a much-reduced volume shrinkage at high temperature. The polyimide component imparts excellent mechanical properties to the composites, including improved compressive modulus, compressive and bending strengths. The composite aerogels offer great potential for applications that require high mechanical strength, a low dielectric constant and/or thermal conductivity and/or high-temperature stability.