Time-resolved dynamic light scattering study on the dynamics of silica gels during gelation process

The dynamics and structure have been compared for two types of silica gels prepared from tetramethoxysilane in dimethylformamide under acidic and basic conditions. An in-situ time-resolved dynamic light scattering (TRDLS) technique was employed to investigate the changes of the dynamics during gelation process. In both cases, a clear power-law behavior in the time-intensity correlation function, g((2))(tau) -1 similar to tau(n-1), was observed at the gelation threshold, from which the critical exponent of viscoelasticity, n, was evaluated. The gelation of the acid system was slower, and the gelation time was a weak decreasing function of the initial monomer concentration, C. The resultant gel was found to be made of long polysiloxane chains slightly connected with each other, which was characterized by n approximate to 0.5. On the other hand, the gelation time of silica gels prepared with a basic catalyst was strongly dependent on C. The evaluated value n approximate to 0.73 indicates the presence of highly branched clusters with short strands. It is demonstrated that the TRDLS technique is a powerful tool to investigate gelation process of silica from both dynamic and structural points of view.