Nanostructured surfaces

Fluorination of silylated silicas
Kuzema P., Bolbukh Yu., Tertykh V.
Chuiko Institute of Surface Chemistry of NAS of Ukraine.
17 General Naumov Str., Kyiv 03164, Ukraine.
E-mail: coralchance@gmail.com
Surface modification with fluorine is a well known reaction which allows changing the chemical and physical properties of silica, namely, viscosity, density, and solubility in different media. Fluorinated silicas are widely used in technological and industrial fields such as metallurgy, optics, electronics, and catalysis. Among the variety of fluorinating techniques, surface modification using c-C4F8 rf-plasma was found to be highly efficient in hydrophobization of some silicate minerals as well as amorphous silica. The presence of surface hydrophobic groups in combination with groups active in polymerization and structuring processes should ensure a good dispersion of active filler in polymer, however, it is not clearly understood how fluorination will affect the surface layer of silylated silica. The aim of this work is to investigate the changes in surface layer of nanodispersed silica with grafted vinyl and hydride groups after fluorination with c-C4F8 under rf-plasma conditions.
Fluorination of fumed silica with various grafted functional groups was performed in rf-plasma conditions under 100 mTorr c-C4F8 at 25°C, during 15 and 60 min. The silicas subjected to fluorination were: either pristine or modified with triethoxysilane, vinyltriethoxysilane, and trichlorovinylsilane. Surface chemistry of the fluorinated silica samples was characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption mass spectrometry. The hydrophobicity of the nanoparticles surface after plasma treatment was evaluated using calorimetric study.
The surface layer structure of the silylated silicas after plasma fluorination was found to be defined by the reactivity of the pre-grafted groups. It was revealed that for hydride- and vinylsilylated silicas a long term (about 60 min) plasma treatment leads to formation of polymeric film at the surface, whereas at facile (about 15 min) treatment the single fragments with C:F ratio close to 1:1 are attached to surface as in the case of pristine silica fluorination. Both the nature of pre-grafted groups capable to be involved in polymerization reaction and their concentration effect on polymeric film structure. Thus, at surface groups concentration of about 0.5 mmol/g fluorine-enriched polymeric chains are formed, while at twice as little concentration the chains with unsaturated bonds are generated.