From SiO1.5CH3 to vitreous SiO2: A structural evolution study

The conversion of SiO_1.5CH₃ into SiO₂ glass by pyrolysis in air has been studied by combining density measurements, vibrational spectroscopy, positron annihilation spectroscopy, and nuclear magnetic resonance. The results show that a critical transition takes place between 400 and 500 °C and at 600 °C the organic features are completely removed. The conversion of the hybrid structure into a fully inorganic network takes place through the formation of metastable 3-membered rings that progressively evolve into more stable 5+ rings as the pyrolysis temperature increases. The evolution of the organic moieties is coupled with the formation of Si-OH and molecular water trapped in the network which are well visible between 500 and 800 °C. In such temperature regions, a clear evolution of the network density can be detected, though the amount of free volume is nearly constant (due to the presence of trapped H₂O). Only at 1250 °C, the network is comparable with a melt-derived glass.