Raman spectra of oligomeric species obtained by tetraethoxysilane hydrolysis-polycondensation process

By varying the molar water alkoxide ratios R, (0.5 less than or equal to R less than or equal to 1.75), the progressive hydrolysis and polycondensation of tetraethoxysilane (TEOS) in acidic medium (HCl 0.03 M) were investigated. In order to appreciate the effects of the alcohol on the reactions involved, part of the solutions were prepared with different amounts of ethanol. The identification of the intermediate products obtained under these experimental conditions was carried out with the help of vibrational spectroscopies (Raman and infrared) combined with Si-29 nuclear magnetic resonance. In the case of the solution prepared without alcohol and for R = 0.5, the intensity of the Raman band characteristic of the monomer TEOS, appearing at 650 cm(-1) and assigned to the symmetric SiO4 stretch, was invariant for reaction times (t(R)) ranging from 24 to 123 h. This fact led us to maintain the t(R) parameter at 24 h for all the samples prepared without alcohol. However, the initial presence of ethanol, even in low quantity, in the systems studied (TEOS-H2O-HCl) induces more advanced hydrolysis-polycondensation reactions after a t(R) less than 24 h. The analysis of the solutions with 0.5 (l)ess than or equal to R less than or equal to 1 showed that traces of the monomer and its simple hydrolysis products were still detected. Concurrently, the intensity of the band characteristic of the dimer arising at 600 cm(-1) increased. For larger values of R (R greater than or equal to 1.25), the shift of this band from 600 to 530 cm(-1) revealed the presence of more condensed species such as trimers (570 to 550 cm(-1)), branched trimers and polymeric structures (550 to 530 cm(-1)).