Synthesis and Properties of Step-Growth Polyamide Aerogels Cross-linked with Triacid Chlorides

Abstract

We report the first synthesis of step-growth aromatic polyamide (PA) aerogels made using amine end-capped polyamide oligomers cross-linked with 1,3,5-benzenetricarbonyl trichloride (BTC). Isophthaloyl chloride (IPC) or terephthaloyl chloride (TPC) were combined with <i>m</i>-phenylenediamine (mPDA) in <i>N</i>-methylpyrrolidinone (NMP) to give amine-capped polyamide oligomers formulated with up to 40 repeat units. Addition of the cross-linker, BTC, typically induces gelation in under 5 min. Solvent exchange of the resulting gels into ethanol followed by supercritical CO₂ drying gives colorless aerogels with densities ranging from 0.06 to 0.33 g/cm³, compressive moduli between 5 and 312 MPa, and surface areas as high as 385 m²/g. Dielectric properties were also measured in the X-band frequency range. It was found that relative dielectric constant decreased with density as seen with other aerogels with the lowest relative dielectric constant being 1.15 for aerogels with densities of 0.06 g/cm³. Because of their superior mechanical properties, these aerogels can be utilized in a number of aerospace related applications, such as insulation for rovers, habitats, deployable structures, and extravehicular activity suits, as well as low dielectric substrates for antennas and other electronics. Because of potentially lower cost relative to polyimide and other polymer aerogels, they also have potential for use in more terrestrial applications as well, such as insulation for refrigeration, building and construction, and protective clothing