Nanostructured organic-inorganic hybrid materials: Kinetic control of the texture

The organic-inorganic hybrid materials obtained by the sol-gel route consti tute a new class of solids prepared directly from a molecular precursor in mild conditions. These materials are better described as kinetically contro lled solids instead of thermodynamically unstable materials. The reproducib ility is observed only when the experimental conditions and the purity of c hemicals are controlled with the precision used in analytical measurements. The results presented here illustrate the importance of all the parameters involved during the hydrolytic polycondensation of silicon alkoxides. They clearly evidence the narrow connections between the macroscopic data which describe the texture of the solids (specific surface area, porosity) and t he parameters which control the kinetics of all the reactions involved in t he process: the nature of the solvent, nature and concentration of catalyst and reagents, and temperature. However, some trends could be observed: pol ycondensation at silicon is also highly dependent on the geometry of the mo lecular precursor, "rigid" or "flexible". Moreover, very interestingly the aging conditions (time and temperature) exhibit a drastic influence on the textural properties of the material whereas the level of condensation at si licon is not influenced. These results clearly show that polycondensation a t silicon and the texture are not controlled in the same steps of the overa ll process.