One-step Room-temperature Synthesis of Fibrous Polyimide Aerogels from Anhydrides and Isocyanates and Conversion to Isomorphic Carbons


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-d 6 and it proceeds through a seven-member ring intermediate that collapses to the imide by expelling CO 2. 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 N 2-sorption porosimetry they contain both meso- and macroporosity and they have similar BET surface areas (300-400 m 2 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 CO 2 at 1000 °C increases the BET surface area of both PI-AMN (to 417 m 2 g -1) and PI-ISO (to 1010 m 2 g -1), and improves the electrical conductivity of the latter by a factor of 70. © 2010 The Royal Society of Chemistry.



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