Chakkaravarthy Chidambareswarapattar

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-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.

Robust monolithic multiscale nanoporous polyimides and conversion to isomorphic carbons

This paper describes a generalizable approach to the synthesis of monolithic multiscale micro/meso/macro-porous polymers with potential use in catalysis, gas separations and gas storage. The model system is based on hyperbranched polyimides synthesized via an unconventional route from dianhydrides and triisocyanates. Simulations reproducing experimental observables (e.g., XRD, skeletal densities) indicate that relatively small hyperbranched oligomers pack into inherently microporous primary nanoparticles. The latter phase-separate, react through their dangling surface functionality and form robust, monolithic, meso/macro-porous networks. Macroporosity is controlled mainly by the concentration of the reactants; mesoporosity by the chemical identity of the monomers that directs the way particles fill space, e.g., as strings of beads versus globular clusters. Microporosity, being an inherent property of the molecular network, survives pyrolysis and is transferred along with meso and macroporosity to multiscale nanoporous carbons. The materials described herewith are classified as polyimide and carbon aerogels.