One-step room-temperature synthesis of fibrous polyimide aerogels from anhydrides and isocyanates and conversion to isomorphic carbons

Abstract

Monolithic polyimide aerogels (PI-ISOs) have been prepared by drying wet-gels synthesized via a rather underutilized room-temperature reaction of pyromellitic dianhydride (PMDA) with 4,4′-methylene diphenyl diisocyanate (MDI). The reaction is followed up to the gelation point by liquid 13C-NMR in DMSO-d6 and it proceeds through a seven-member ring intermediate that collapses to the imide by expelling CO2. PI-ISOs are characterized comparatively with aerogels referred to as PI-AMNs, obtained via the classic reaction of PMDA and 4,4′-methylenedianiline (MDA). The two materials are chemically identical, they show similar degrees of crystallinity (30–45%, by XRD) and they both consist of similarly sized primary particles (6.1–7.5 nm, by SANS). By N2-sorption porosimetry they contain both meso- and macroporosity and they have similar BET surface areas (300–400 m2 g−1). Their major difference, however, is that PI-AMNs are particulate while PI-ISOs are fibrous. The different morphology has been attributed to the rigidity of the seven-member ring intermediate of PI-ISOs. PI-AMNs shrink significantly during processing (up to 40% in linear dimensions), but mechanically are much stronger materials than PI-ISOs of the same density. Upon pyrolysis at 800 °C both PI-ISO and PI-AMN are converted to porous carbons; PI-AMNs loose their nanomorphology and more than 2/3 of their surface area, as opposed to PI-ISOs, which retain both. Etching with CO2 at 1000 °C increases the BET surface area of both PI-AMN (to 417 m2 g−1) and PI-ISO (to 1010 m2 g−1), and improves the electrical conductivity of the latter by a factor of 70.