Pyrolysis study of fluorinated sol-gel silica

Fluorinated silica gels at various fluorine content were prepared via sol-gel by hydrolysis of 3,3,3-trifluoropropyltrimethoxysilane and tetraethoxysilane mixtures. The gels, of nominal stoichiometry Si(CH2CH2CF3)(X)O(2-X/2) (X=0.1-1), were characterized by FT-IR, X-ray photoelectron spectroscopy (XPS) and N-2 adsorption analysis. The thermal stability of the fluorinated samples was investigated by coupling thermogravimetric measurements with mass spectrometric and gas chromatographic analyses of the evolved gaseous species. The chemical reactions occurring in the gel matrices during heating were siloxane chain rearrangements involving condensation between residual hydroxyl and ethoxyl groups in the 100-350degreesC temperature range, whereas the thermal decomposition of the fluoroalkyl groups were observed at higher temperatures (450-600degreesC). The release of the fluoroalkyl moieties also involved C-F/Si-O bond exchanges inside the siloxane chains, with gas-phase evolution of different fluorinated silicon units.