Structure-property behaviour during aging of sol-gel-derived silica modified with Si-H and Si-CH3 groups

Unsupported films and thin rods were prepared by the sol-gel method from solutions of triethoxysilane and methyldiethoxysilane. Measurements of elastic modulus, density, glass transition temperature, and refractive index were performed at different aging times. Results showed a large increase of the elastic modulus with aging time, which was related to the progressive condensation taking place in the xerogel network, as shown by solid-state nuclear magnetic resonance (NMR). The stiffness increase was shown to correspond to higher glass transition temperature and larger density and refractive index. Different increasing rates of the physical and mechanical characteristics were observed for films and rods during aging. This behavior was related to the different loss rate of the pore liquid, which affects the network evolution in the wet gel.