Structural and microstructural evolution during pyrolysis of hybrid polydimethylsiloxane-titania nanocomposites

The evolution during pyrolysis of hybrid polydimethylsiloxane-titania nanocomposites has been studied as a function of the ratio between polysiloxane and titania phases. The xerogels, prepared by the sol–gel process starting from diethoxydimethylsilane and titanium isopropoxide, have been heated under argon atmosphere and the evolution with temperature has been followed by infrared and 29 Si solid state nuclear magnetic resonance spectroscopies, thermal analyses, X-ray diffraction, N2 sorption measurements and scanning electron microscopy. Below 800◦ C, the polymer-to-ceramic conversion takes place at different temperatures with changing the titania content. The stability of Si C bonds in polydimethylsiloxane networks depends on the metal oxide amount. The high reactivity of titanium atoms towards the Si C bonds produces Si C bond cleavage with mild thermal treatments and in the case of 30 mol% TiO2 , leads to the ceramization of the hybrid nanocomposite at 500◦ C. Decreasing the titania load, a shift towards higher temperatures to complete the polymer-to-ceramic conversion is observed. The structural rearrangement of the siloxane moiety produces mesoporous and microporous materials, depending on the composition; in the case of 10 and 20 mol% TiO2 content, the samples present high specific surface area up to 1200◦ C. The crystallization process begins at 1000◦ C and the phase evolution depends on the composition. The phase analysis obtained from XRD spectra shows that different crystalline oxide and oxycarbide phases develop during the thermal process, as a function of the amount of available carbon, ultimately leading to the preferential crystallization of titanium carbide. Between 1000 and 1600◦ C the amorphous silicon oxycarbide phase undergoes a continuous structural evolution caused by the decrease of carbon content in the phase, leading to almost pure silica at 1600◦ C.