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A variety of experimental techniques (multinuclear NMR, Raman, fluorescence polarization, small-angle neutron scattering, viscosity, turbidity, static and dynamic light scattering experiments) have been employed to investigate the nature of the sol-gel transition of tetramethoxysilicate, Si(OCH,), (TMOS). These experiments probe changes in structural and dynamical properties at the macroscopic and microscopic levels in the course of the sol-gel transition. The experimental results are compared with recent theories for the gelation process. The experiments show that no drastic change in structure occurs at the gelation threshold of TMOS. The formed silica network exhibits a self-similar structure, and the gross features of the sol-gel transition of TMOS can be described within the framework of percolation theory. The underlying growth process might be classified as reaction-limited cluster-cluster growth. However, the detailed chemical structure and reactivity of the reactants, e.g., the time-dependent functionality of the monomers during the hydrolysis step, also play an important role and have to be taken into account for a more quantitative theoretical description of this gelation process.

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