Polyimide Aerogels by Ring-Opening Metathesis Polymerization (ROMP)

Polyimide aerogel monoliths are prepared by ring-opening metathesis polymerization (ROMP) of a norbornene end-capped diimide, bis-NAD, obtained as the condensation product of radic anhydride with 4,4'-methylenedianiline. The density of the material was varied in the range of 0.13-0.66 g cm(-3) by varying the concentration of bis-NAD in the sol. Wet gels experience significant shrinkage, relative to their molds (28%-39% in linear dimensions), but the final aerogels retain high porosities (50%-90% v/v), high surface areas (210-632 m(2) g(-1), of which up to 25% is traced to micropores), and pore size distributions in the mesoporous range (20-33 nm). The skeletal framework consists of primary particles 16-17 nm in diameter, assembling to form secondary aggregates (by SANS and SEM) 60-85 nm in diameter. At lower densities (e.g., 0.26 g cm(-3)), secondary particles are mass fractals (D-m = 2.34 +/- 0.03) turning to closed-packed surface fractal objects (D-S = 3.0) as the bulk density increases (>= 0.34 g cm(-3)), suggesting a change in the network-forming mechanism from diffusion-limited aggregation of primary particles to a space-filling bond percolation model. The new materials combine facile one-step synthesis with heat resistance up to 200 C, high mechanical compressive strength and specific energy absorption (168 MPa and 50J g(-1), respectively, at 0,39 g cm(-3) and 88% ultimate strain), low speed of sound (351 m s(-1) at 0.39 g cm(-3)) and styrofoam-like thermal conductivity (0.031 W m(-1) K-1 at 0.34 g cm(-3) and 25 degrees C); hence, they are reasonable multifunctional candidate materials for further exploration as thermal/acoustic insulation at elevated temperatures.