Cross-linking Amine-Modified Silica Aerogels with Epoxies: Mechanically Strong Lightweight Porous Materials

Abstract

The mesoporous surfaces of TMOS-derived silica aerogels have been modified with amines by copolymerization of TMOS with APTES. The amine sites have become anchors for cross-linking the nanoparticles of the skeletal backbone of the aerogel by attachment of di-, tri-, and tetra-functional epoxies. The resulting conformal coatings increase the density of the native aerogels by a factor of 2−3 but the strength of the resulting materials may increase by more than 2 orders of magnitude. Processing variables such as the amount of APTES used to make the gels, the epoxy type and concentration used for cross-linking, and the cross-linking temperature and time were varied according to a multivariable design-of-experiments (DOE) model. It was found that while elastic modulus follows a similar trend with density, maximum strength is attained neither at the maximum density nor at the highest concentration of −NH2 groups, suggesting surface saturation effects. Aerogels cross-linked with the trifunctional epoxide always show improved strength compared with aerogels cross-linked with the other two epoxides under identical conditions. Solid 13C NMR studies show residual unreacted epoxides, which condense with one another by heating cross-linked aerogels at 150 °C.