An NMR study of MeTMS based coatings filled with colloidal silica

Coatings were prepared by mixing MeTMS and an aqueous colloidal silica. Mixing of an MeTMS hydrolysis mixture with the aqueous colloidal silica is only possible, without flocculation of the colloidal silica particles, within a certain time window. Si-29 NMR was used to follow the hydrolysis/condensation reactions of MeTMS, whereas H-1 NMR was used to monitor the reaction of the MeTMS monomers and oligomers with the silica surface in the coating liquid. The reaction of MeTMS with the surface of the SiO2 particles is determined by the oligomer size. Typical SiO2 surface coverage is less than 3 molecules/nm(2) (approximately one monolayer). Large MeTMS oligomers and/or cyclic species do not react with the SiO2 surface. These species are probably too apolar to react or absorb at the SiO2 surface. Flocculation of the aqueous colloidal silica occurs due to the low polarity of the hydrolysis mixture. The water content and the degree of condensation of the MeTMS determine the width of the time window.