Auto-organization of nanostructured organic–inorganic hybrid xerogels prepared from planar precursors by sol–gel processing

Abstract

Nanostructured hybrid materials were prepared by sol–gel hydrolysis-polycondensation of 1,3,5-tris(p-triethoxysilylphenyl)benzene PR having a rigid structure and 1,3,5-tris(p-trimethoxysilylpropylphenyl)benzene PF having a planar core connected to three short and flexible arms. Both precursors presented nine directions for solid formation. Their organization was evaluated by birefringence measurements (micrometre scale) and X-ray diffraction studies (nanometre scale). The influence of the planar geometry and the rigidity or flexibility of the organic groups has been studied as well as the effect of changes in experimental conditions. It has been shown that the organization of these solids was less influenced by the experimental parameters than in the case of linear precursors. In the case of the rigid precursor PR, the solids obtained XR had a high intensity of birefringence (7.5–8 × 10⁻³) and the gels presented cracks whatever the experimental conditions (nucleophile or acid catalysis). In contrast the flexible precursor PF led to solids XF with a lower birefringence (6.2–6.8 × 10⁻³) and the morphology of the solids in the cells depended on the concentration of catalyst. Interestingly the birefringence appeared without cracks with a lower amount of catalyst and isotropic circular cross-shaped nodules were observed in this case whatever the temperature. The X-ray diffraction diagrams clearly showed that these solids presented a better nanometre scale organization than the gel formed from 1,3,5-tris(trimethoxysilyl)benzene. These observations show that the auto-organization of hybrid materials appears induced by weak van der Waals type interactions as a general phenomenon which is not limited to linear and mesogenic precursors.