Preparation and surface properties of silica-gel coating films containing branched-polyfluoroalkylsilane

Composite coating films crack-free and with smooth surfaces have been prepared by the sol-gel technique using branched chain polyfluoroalkylsilane (triethoxy[4,4-bis(trifluoromethyl)-5,5,6,6,7,7,7-heptafluoroheptyl]silane) (D3Et) and tetraethoxysilane (TEOS) as starting materials. Surface properties of the fluorinated coating films heated at various temperatures were examined in comparison with films derived from methyltrimethoxysilane (MTM) and TEOS, based on the measurements of infra-red spectra, thermal gravimetric changes and X-ray photoelectron spectra and contact angle of water and diiodo methane. It was found that the fluorosilane-containing films had thermal stability and surface energies, gamma(s), as small as polytetrafluoroethylene (PTFE) up to 400 degrees C and a large contact angle of water comparable to the films derived from MTM and TEOS. Large hydrophobicity of fluorosilane-containing films seems to be due to the dispersion component of the surface energy compared with methyltrimethoxysilane and TEOS films. We conclude from X-ray photoelectron spectra measurements that the surface energy of the fluorinated coating film originate from polyfluorocarbon molecules at the topmost film surface. (C) 1999 Elsevier Science B.V. All rights reserved.