FT-Raman Spectroscopic Study of the Structural Evolution in Binary UV-Curable Vinyltriethoxysilane/Tetraethoxysilane Mixtures from the Sol to the Xerogel State

Abstract

Fourier transform Raman spectroscopy was used in order to study acid catalyzed hydrolysis and polycondensation processes from the sol via the gel until the xerogel state in binary mixtures of vinyltriethoxysilane (ViTEOS) and tetraethoxysilane (TEOS). Supplementary ²⁹Si-, ¹³C-MAS-NMR, and FT-IR data were considered. Intermediate hydrolysates, oligomers, and polycondensates were detected independently of the sample's physical state. Vibrational assignments are given for pure ViTEOS and its partially hydrolyzed intermediates. The corresponding mono-, di-, and tri-hydroxylated species are found to exhibit characteristic ν_sym(CSiO₃) vibrational modes in the 650–672 cm⁻¹ region. Subsequent oligomerization results in a progressive shift of the ν_sym(CSiO₃) bands towards lower wavenumbers. Condensation proceeds faster in ViTEOS–TEOS mixtures than in pure ViTEOS. Assignments for some early oligomeric species and some network connectivity are given. The Raman spectra suggest that the main building blocks of the evolving network are already present in the wet gels. Moreover, it is shown that residual ethoxy groups attached to the siloxane network are still present in xerogels, whereas no silanol groups are detected. The spectra demonstrate that the vinyl groups did not react under the applied conditions. This system has potential for the preparation of UV-curable abrasion-resistant coatings for plastic substrates.