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 basi c conditions. An in-situ time-resolved dynamic light scattering (TRDLS) tec hnique was employed to investigate the changes of the dynamics during gelat ion process. In both cases, a clear power-law behavior in the time-intensit y correlation function, g((2))(tau) -1 similar to tau(n-1), was observed at the gelation threshold, from which the critical exponent of viscoelasticit y, n, was evaluated. The gelation of the acid system was slower, and the ge lation time was a weak decreasing function of the initial monomer concentra tion, C. The resultant gel was found to be made of long polysiloxane chains slightly connected with each other, which was characterized by n approxima te to 0.5. On the other hand, the gelation time of silica gels prepared wit h a basic catalyst was strongly dependent on C. The evaluated value n appro ximate to 0.73 indicates the presence of highly branched clusters with shor t strands. It is demonstrated that the TRDLS technique is a powerful tool t o investigate gelation process of silica from both dynamic and structural p oints of view.