Early stages of the silicon alkoxide sol/gel polymerization as detected by dynamic light scattering

There has been much interest in the early stages of the polymerization of silicon alkoxides in view of the novel solutions offered by this technique to various problems in materials science. It has been generally agreed that continuous growth characterizes the monomer/oligomer/sol-gel stages, e.g., by the Ostwald ripening mechanism. Dynamic light scattering measurements provide evidence that this is indeed the case for acidic polymerization of Si(OCH₃)₄. However, under basic polymerization conditions we observe that following an initial fast growth stage, there is a substantial shortening of the correlation times in the light scattering experiments, suggesting a reduction in size of the sol particles, prior to gelation. The phenomenon seems to be general: we observed it with Si(OCH₃)₄ at various water/silane ratios, with Si(OCH₂CH₃)₄ and with Si(OCH₂CH₃)₃C₆H₅. We interpret these observations as indicating intra-particle polymerization causing densification, contraction and gradual collapse.