Heterogeneous silica-polyimide aerogel-in-aerogel nanocomposites

• Unique structure of particulate silica aerogel embedded in fibrous polyimide aerogel. • Nanocomposite inherits hydrophobicity and temperature stability from silica aerogel. • Composite maintains superior mechanical and low dielectric property from PI aerogel. • Low dielectric constant and loss are stable at various frequencies from 10 -1 to 3 × 10 10 . 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 m 2 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 -10 6 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.