Polymer-Silica Nanocomposite Aerogels with Enhanced Mechanical Properties Using Chemical Vapor Deposition (CVD) of Cyanoacrylates

Abstract

Aerogels were structurally modified using chemical vapor deposition (CVD) of cyanoacrylate monomers to afford polycyanoacrylate-aerogel nanocomposites. Silica aerogels are low density, high surface area materials whose applications are limited by their fragility. Cyanoacrylate CVD allowed us to deposit a film of organic polymer throughout fragile porous monoliths within hours. Our experiments have shown that polymerization of the cyanoacrylate monomers was initiated by the adsorbed water on the surface of the silica permitting the nanocomposites structures to be formed with little or no sample preparation. We found that the strength of the polycyanoacrylate-aerogel nanocomposites increased thirty two-fold over the untreated aerogels with only a three-fold increase in density and an eight-fold decrease in surface area. Along with the improvement in mechanical properties, the aerogels became less hydrophilic than un-modified aerogels. Polycyanoacrylate-coated aerogels were placed directly into water and did not suffer catastrophic fragmentation as observed with un-modified silica aerogels. INTRODUCTIONSilica aerogels are supercritically-dried gels composed of aggregated silica nanoparticles surrounded by continuous, open cell mesopores. Aerogels are attractive for a variety of applications due to their low density, high porosity and vast surface areas [1]. This includes applications where weight is a concern, such as insulation and comet particle traps [2]. Aerogels have also been utilized for catalysis supports, high energy physics experiments, and separations media [3]. However, the utility of aerogels is limited by their mechanical fragility. Silica aerogels are expected to be weaker than non-porous silica due to their large volume fraction of air [4]. However, aerogels' fragile nature arises from the silica network having relatively weak connectivity between constituent silica nanoparticles. This deficiency provides an abundance of defects to initiate failure. As a consequence aerogels may catastrophically crack from capillary forces when wet with water or undergo brittle failure when subjected to slight mechanical stress during handling or processing [5].In response to their mechanical deficiency, a considerable effort has been directed at improving aerogels' mechanical properties without diminishing their attractive physical attributes. Most efforts have focused on wet chemical processing such as Ostwald ripening and surface silation [6,7]. Recently, silica-organic polymer composites were prepared by growing macromolecules, such as polyurethanes, from the surface of gels before supercritical drying [8].

Fabrication of Mechanically Strong Silica Aerogels with the Thermally Induced Phase Separation (TIPS) Method of Poly(methyl methacrylate)

“…Surface modification of the alcogel with reactive groups allows silicates [ 10 ], polyureas [ 14 ], polyurethanes [ 15 , 16 , 17 ], epoxies [ 18 ], polystyrene [ 19 ], and polymethacrylates [ 20 , 21 , 22 ] to be grown from or attached to the particles reinforcing the aerogel upon drying. Chemical vapor deposition and polymerization of cyanoacrylates [ 23 , 24 , 25 ] or silane monomers [ 26 ] can also be used to strengthen aerogels after supercritical drying. In one recent case, covalent attachment of polydimethylsiloxanes during supercritical drying was used to strengthen silica aerogels [ 27 ].…”