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 nadic 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 m2 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 (Dm = 2.34 ± 0.03) turning to closed-packed surface fractal objects (DS = 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 50 J 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 °C); hence, they are reasonable multifunctional candidate materials for further exploration as thermal/acoustic insulation at elevated temperatures. © 2011 American Chemical Society