Mechanical stability of templated mesoporous silica thin films

A surfactant templated mesoporous silica thin film is essentially an air-silica composite, with mechanical properties dominated by the more rigid component: the silica mesopore walls. It is often convenient to assume that mesopore wall properties are those of fused silica. However, we measured substantial reductions in wall density and elastic modulus (Ewall), due to incomplete silica condensation. Elastic moduli (Efilm) of disordered (sponge-like) and ordered (honeycomb-like) thin films were measured by nanoindentation. Curve fits to the well-known modulus vs. film porosity (P) correlation, Efilm/Ewall = (1-P)n, gave a higher n = 2.5 for the sponge-like film, but a lower n = 2.2 for the honeycomb-like film, in contrast to theoretical expectations (n = 2 for sponge-like and n = 3 for honeycomb-like). Although properties such as the dielectric constant depend primarily on first-order structural information (P), the elastic modulus of structurally imperfect films may require second-order (pore wall thickness/diameter ratio) and third-order (pore connectivity) parameters to explain the data. The power law exponent n can vary continuously, depending on the details of the mesostructure.