The structure of porous silica–polysiloxane hybrid materials

Abstract

Porous organic–inorganic hybrid materials are prepared by means of a sol–gel process where tetraethoxysilane (TEOS) is hydrolyzed in a water–tetrahydrofuran (THF) mixture containing the reactive polar polysiloxane PS124. A porous material consisting of silica and PS124 domains is obtained after evaporation of the solvents. Such materials can be deposited as thin layers at the surface of electrodes and various transducers for the design of chemical sensors. A very important property of the materials is the accessibility to the organic domains where the active species are immobilized. Thus, the key parameters of the structure have been investigated by means of different techniques, solid state NMR, electron microscopy, nitrogen adsorption and small angle scattering. Solid state 29Si NMR shows the high condensation of the silica and the reaction of the PS124 during the sol–gel process. The porous volume mainly consists of macropores in interstices between large aggregated particles (0.3–1 μm). The size distribution of the pores displays only few micropores and no mesopores. The values of the specific areas of the three interfaces separating silica and the external medium, the PS124 polymer and the external medium and silica and PS124 have been determined by means of an original method combining nitrogen adsorption, X-ray and neutron small angle scattering measurements. The silica–exterior interfacial area is vanishingly small and the PS124–exterior interfacial area is much larger than that of the silica–PS124 interface. This means complete wetting of the silica by PS124 and a rough outer surface of the PS124 domains.