From “Green” Aerogels to Porous Graphite by Emulsion Gelation of Acrylonitrile


Porous carbons, including carbon (C-) aerogels, are technologically important materials, while polyacrylonitrile (PAN) is the main industrial source of graphite fiber. Graphite aerogels are synthesized herewith pyrolytically from PAN aerogels, which in turn are prepared first by solution copolymerization in toluene of acrylonitrile (AN) with ethylene glycol dimethacrylate (EGDMA) or 1,6-hexanediol diacrylate (HDDA). Gelation is induced photochemically and involves phase-separation of “live” nanoparticles that get linked covalently into a robust 3D network. The goal of this work was to transfer that process into aqueous systems and obtain similar nanostructures in terms of particle sizes, porosity, and surface areas. That was accomplished by forcing the monomers into (micro)emulsions, in essence inducing phase-separation of virtual primary particles before polymerization. Small angle neutron scattering (SANS) in combination with location-of-initiator control experiments support that monomer reservoir droplets feed polymerization in ∼3 nm radius micelles yielding eventually large (∼60 nm) primary particles. The latter form gels that are dried into macro-/mesoporous aerogels under ambient pressure from water. PAN aerogels by either solution or emulsion gelation are aromatized (240 °C, air), carbonized (800 °C, Ar), and graphitized (2300 °C, He) into porous structures (49-64% v/v empty space) with electrical conductivities >5× higher than those reported for other C-aerogels at similar densities. Despite a significant pyrolytic loss of matter (up to 50-70% w/w), samples shrink conformally (31-57%) and remain monolithic. Chemical transformations are followed with CHN analysis, 13C NMR, XRD, Raman, and HRTEM. Materials properties are monitored by SEM and N 2-sorption. The extent and effectiveness of interparticle connectivity is evaluated by quasi-static compression. Overall, irrespective of the gelation method, PAN aerogels and the resulting carbons are identical materials in terms of their chemical composition and microstructure. Although cross-linkers EGDMA and HDDA decompose completely by 800 °C, surprisingly their signature in terms of different surface areas, crystallinity, and electrical conductivities is traced in all the pyrolytic products. © 2011 American Chemical Society.



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