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 Si-29 solid state nuclear magnetic resonance spectroscopies, thermal analyses, X-ray diffraction, N-2 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.