Isocyanate-crosslinked silica aerogel monoliths: preparation and characterization

Polymerization of di- and tri-isocyanates can be templated onto the mesoporous surface of a preformed network of sol–gelderived silica nanoparticles, resulting in a conformal crosslinked coating that renders the interparticle neck zone wider. Upon drying, these crosslinked networks yield aerogels which are up to � 3· more dense than native aerogels based on the underlying silica framework, but also up to 10· less hygroscopic and they may take more than 300· the force to break. These results have been obtained with one-step based-catalyzed sol–gel silica networks, as well as with gels derived through a two-step process involving an acid-catalyzed sol and a based-catalyzed gel. Furthermore, it has been also found that crosslinking increases the dielectric constant only by � 35% relative to values reported in the literature for native silica aerogels of about the same porosity. Chemical investigations into the polymerization reaction have shown that the process of crosslinking involves reaction of the isocyanate with: (a) AOH groups at the surface of silica to form carbamate; and (b) adsorbed water, to form an amine and carbon dioxide. This amine then reacts with additional isocyanates resulting in polymer chain extension and bridging of particles with urethane-terminated polyurea.