SI-29 NMR AND VISCOSITY STUDY OF THE SOL-GEL TRANSITION AND EVOLUTION AFTER THE GEL TIME

In order to study the sol to gel transition and the early stage of aging, Si-29 NMR and viscosity measurements were performed on acid-catalyzed TEOS in excess of water, where evaporation is hindered. NMR spectra evolution was followed over an interval running from 0.5t(g) to 4t(g) (t(g) is gelation time). The difference between static spectra and magic angle spinning (MAS) spectra, analyzed in terms of anisotropy of the chemical shift, allowed evaluation of the gel weight-fraction. Its evolution appeared to be quite slow. A kinetic model of post-gelation aggregation accounts for this time dependance. NMR measurements were not very accurate close to the transition. Rheology experiments over a frequency range from 10(-2) Hz to 100 Hz were performed in this region. Scalar percolation transition was observed within a very narrow interval.