Structure-property behavior 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 so lutions of triethoxysilane and methyldiethoxysilane. Measurements of elasti c 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 cond ensation taking place in the xerogel network, as shown by solid-state nucle ar magnetic resonance (NMR). The stiffness increase was shown to correspond to higher glass transition temperature and larger density and refractive i ndex. Different increasing rates of the physical and mechanical characteris tics were observed for films and rods during aging. This behavior was relat ed to the different loss rate of the pore liquid, which affects the network evolution in the wet gel.