Effects of dimethyldiethoxysilane addition on the Sol-Gel transition behavior of tetraethoxysilane

Abstract

Solutions of tetraethoxysilane (TEOS) and dimethyldiethoxysilane (DDS) were prepared by mixing the solution of TEOS and DDS dissolved in ethanol and the solution of water and HCl (a catalyst). Effects of DDS addition were analyzed during the sol-gel transition. First of all, the gelation model and the kinetic model were combined to analyze the gelation time and calculate the conversion when the molar ratio of TEOS to DDS was changed. The Macosko``s recursive model and the Assink-Kay``s statistical model were modified by introducing the ring formation parameter, $ρ_κ$ because NMR spectra showed the ring structure peaks. The experimental result of gelation time could not be predicted without the ring formation parameter. Hence, this parameter turned out to be essential to predict the gelation time. The theoretical gelation time was obtained by calculating the growth of molecular weight with time using a modified recursive technique. The extents of hydrolysis and condensation were also calculated as a function of time from the kinetic schemes developed by Assink-Kay and Pouxviel-Boilot. The modified model combined with the kinetic model of Assink-Kay and could predict very well the experimental data of tetramethoxysilane (TMOS) reported by Kelt and Armstrong. When applying the modified model to the mixed system of TEOS and DDS, the gelation time decreased until a minimum value is reached and then increased with the DDS content. Additionally, the prehydrolysis method was effective to make homogeneous solution when the reaction rate of silicon alkoxide was slow. DDS with the fast reaction rate was added to investigate hydrolysis and condensation process of TEOS-DDS mixed system by varying the input content (0~67% in mole fraction) and the addition-time of DDS. $^29Si-NMR$ technique, viscometry, infrared spectroscopy, and thermogravimetric analysis were used for this study. The mixtures of TEOS and DDS showed the faster gelation time than pure TEOS sol but they ha...