Self-crosslinkable poly(urethane urea)-reinforced silica aerogels

Mechanically reinforced organic-inorganic hybrid silica aerogels are produced from simultaneous hydrolysis and condensation reactions of silane precursors - tetraethoxy silane (TEOS) and aminopropyltriethoxysilane (APTES) - and silane-modified polyurethane urea molecules each carrying multiple (>= 3) reactive silane groups. In this manner, the post-gelation crosslinking reactions are avoided, the amount of polymer introduced into the aerogel structures is controlled, and the chain length between two crosslink points is tailored. The long chain polymer molecules introduce a certain degree of flexibility to the hybrid aerogel structures. The morphology, compressive properties, and surface area are obtained respectively using scanning electron microscopy, Instron tensile testers, and Brunauer-Emmett-Teller (BET) surface area analysis. The data on solid state C-13 and Si-29 NMR spectra reveal chemical reactions of the silane-modified polymers with the silica particle networks. Small angle X-ray scattering (SAXS) data are used to determine the fractal dimension of the silica networks. It is found that the self-crosslinkable multifunctional polyurethane urea chains form coatings on the silica networks and produce large enhancements in compressive modulus although with increases in shrinkage and bulk density.