Crosslinking 3D Assemblies of Silica Nanoparticles (Aerogels) by Surface-Initiated Free Radical Polymerization of Styrene and Methylmethacrylate
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Quasi-stable, ultra-low density, three-dimensional assemblies of nanoparticles are referred to as aerogels. Aerogels are open-cell foams derived from supercritical fluid (SCF) drying of wet gels. Their large internal void space is responsible for low dielectric constants, low thermal conductivities and high acoustic impedance. At the same time though those materials are fragile and impractical for high load applications. The fragility problem has been addressed by casting a thin conformal polymer coating over the entire internal porous surface of the nanostructure.1 That process is referred to as crosslinking. The coating connects chemically skeletal nanoparticles and renders interparticle necks wider. Thus, the internal void space is not compromised significantly, while the flexural strength of a typical monolith is increased by 300Ã for a nominal increase in density only by a factor of three. A major issue, however, is the fact that current preparation procedures for crosslinked aerogels involve several solvent exchange steps, which are expensive and must be eliminated before crosslinked aerogels become commercially viable. In order to eliminate solvent exchange steps, the reagents for the crosslinking process should be included in the sol, which in turn means that the crosslinking chemistry should be deconvoluted from the gelation chemistry (an ionic process). In this context, there are several chemistries involving nanoparticle surface modification for making core-shell structures. These methods span the entire range from layer-by-layer electrostatic assembly of oppositely charged materials,2-6 to atom transfer radical polymerization,7 and direct free radical polymerization of olefins from surface-bound initiators such as peroxides,8 and AIBN.9,10 by comparison, free-radical initiators have received the least attention as silica surface modifiers, while all literature examples concern asymmetric peroxide and AIBN derivatives, which are attached on silica only from one side. Such monodentate free-radical initiators would not work for our purposes, because upon homolytic cleavage they would produce one surface-bound radical, which is the desirable outcome, but they would also release a second radical in the mesopores. The polymer formed in the solution filling the mesopores will have to be removed, and that introduces more solvent exchange steps. For our purposes, we need bidentate free-radical initiators that will attach themselves on silica from both sides. In that regard, we report here the synthesis of AIBN analogue as well as its incorporation into silica, and the surface initiated polymerization of styrene and methylmethacrylate to yield conformal polymer coatings on the mesoporous surfaces of typical base-catalyzed aerogels.