Thermal-induced phase transitions in self-assembled mesostructured films studied by small-angle X-ray scattering

Two examples of phase transition in self-assembled mesostructured hybrid thin
films are reported. The materials have been synthesized using tetraethoxysilane
as the silica source hydrolyzed with or without the addition of methyltriethoxysilane.
The combined use of transmission electron microscopy, small-angle X-ray
scattering and computer simulation has been introduced to achieve a clear
identification of the organized phases. A structural study of the self-assembled
mesophases as a function of thermal treatment has allowed the overall phase
transition to be followed. The initial symmetries of mesophases in as-deposited
films have been linked to those observed in samples after thermal treatment.
The monodimensional shrinkage of silica films during calcination has induced a
phase transition from face-centered orthorhombic to body-centered cubic. In
hybrid films, instead, the phase transition has not involved a change in the unit
cell but a contraction of the cell parameter normal to the substrate.