From ‘Green’ Aerogels to Porous Graphite by Emulsion Gelation of Acrylonitrile

Abstract

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-ofinitiator 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, 13 C 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.

Air-oxidation of phenolic resin aerogels: backbone reorganization, formation of ring-fused pyrylium cations, and the effect on microporous carbons with enhanced surface areas

“…of the aerogel network, 23 the phenolic monomer-toformaldehyde ratio for PF, RF and FPOL was set at 1 : 2 mol mol À1 . On the other hand, since T bears two -CHO groups, it can play the role of the crosslinker itself, thus the POL : T mol ratio was set at 2 : 1.…”

Polydicyclopentadiene aerogels grafted with PMMA: I. Molecular and interparticle crosslinking

“…However, hinting at the importance of nanostructured matter in dissipating forces, that hypothesis has been supported by a wide variety of aerogels from all major polymeric classes, including polyurea, 9 polyimides, 10 aramids, 11 polyurethane, 12 polybenzoxazine, 13 and acrylic polymers via emulsion gelation in water. 14 All mechanically strong polymeric aerogels have been prepared via a bottom-up approach.…”

“…Formation of such three-dimensional networks of colloidal particles is more oen possible with multifunctional small-molecule monomers capable of molecular-level crosslinking. 14 Based on the above, polymerization of dicyclopentadiene (DCPD) via Ring Opening Metathesis Polymerization (ROMP) 16 yields bulk crosslinked polymer with excellent mechanical properties suitable even for armor. 17 ROMP is a "living" process, induced with Ru-(Grubbs') or W-and Mo-based (Schrock's) catalysts.…”

Tuning the rheological properties of sols for low-density aerogel coating applications

“…4b). 23 Specifically NB additions in excess of 10 wt. % NB lead to considerable shrinkage if the standard two-step solvent exchange drying procedure is used.…”