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.
C. Chidambareswarapattar et al., "One-step Room-temperature Synthesis of Fibrous Polyimide Aerogels from Anhydrides and Isocyanates and Conversion to Isomorphic Carbons," Journal of Materials Chemistry, Royal Society of Chemistry, Jan 2010.
The definitive version is available at http://dx.doi.org/10.1039/c0jm01844a
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