Spectroscopic study on the early stages of the polymerization of hybrid TEOS–RSi (OR′)3 sols

A spectroscopic study of sols composed of tetraethyl orthosilicate and alkyl alkoxysilanes was performed. The molar ratio range studied was 95–60 for tetraethyl orthosilicate and 5–40 for the alkyl alkoxysilanes. The 29Si Nuclear Magnetic Resonance high resolution study along the first 20 h of the polymerization allowed us to demonstrate that cross-condensation predominated over self-condensation in the tetraethyl orthosilicate–methyl triethoxysilane and tetraethyl orthosilicate–phenyl triethoxysilane sols, compared to tetraethyl orthosilicate–ethyl triethoxysilane in the conditions studied. These results were consistent with the small angle X-ray scattering results, where two types of growing olygomeric species were found: those with a fractal dimension value Df=2.3, corresponding to randomly branched olygomers, and the others with a fractal dimension value Df=1.8, corresponding to multiparticle diffusion-limited aggregates. Inspite of performing a complete Fourier transform infrared spectroscopy study from sols to gels, only slight differences were found in the spectra.