Growth process for fractal polymer aggregates formed by perfluorooctyltrimethoxysilane. Time-resolved small-angle X-ray scattering study

The acid-catalyzed condensation reaction of perfluorooctyltrimethoxysilane (PFOS) and n-octyltrimethoxysilane (OTMS) in ethanol has been followed by t ime-resolved synchrotron radiation small-angle X-ray scattering (SAXS) on a short time scale. SAXS curves for PFOS and OTMS have been interpreted as a rising from mass fractals with D-f = 2 (PFOS) and D-f = 1.7 (OTMS). The tim e dependence of the apparent radius of gyration, obtained from the Guinier plots, showed that the growth of fractal precursors occurs in a two-step pr ocess within 2 h for PFOS and within 1.5 h for OTMS, in which small cluster s involving monomers, dimers and trimers are formed in the initial step and formation of larger clusters occurs in the second step. Furthermore, it ha s been suggested that the hydrophobicity and lipophobicity of the bulky alk yl groups may also contribute to the formation of these giant aggregates.