The hydrolysis and condensation reactions of three trifunctional silicon
alkoxides, R-Si(OEt)3, with various R alkyl chains (R =
CH3, C2H5, C8H17),
was followed by 29Si and 1H Nuclear Magnetic
Resonance. The condensation reactions are faster for methyltriethoxysilane
and the degree of condensation of the system is higher. The other two
systems have similar behaviors during the first hours, with lowest degree of
condensation. The condensation reactions slow down for the
octyltriethoxysilane, due to the presence of the long alkyl chains which
prevent condensation reactions between oligomeric species. The influence of
the addition of tetraethoxysilane (TEOS) was studied: for all three systems,
the degree of condensation of the network is higher, indicating the role of
cross-linking played by TEOS.