Résumé
A wide range of hybrid organic-inorganic silicas can be synthetized through sol-gel chemistry (figure 1). The structural organization of the material can be tuned thanks to the organic parts that exhibit diverse self-assembly properties depending on (i) the nature of the organic moieties: phenylene versus alkylene core, urea versus thiourea bridging units (ii) the synthesis conditions, notably the solvent (acidic versus NH4F catalyst) .Hybrids silicas are expected to combine the mechanical, thermal, and chemical stability of silica with the solution processing the flexibility and extra functionalization brought by the organic moieties. However, their mechanical properties have been barely studied as compared to silica glasses, which have has driven a particular interest during the two last decades, and for which high pressure experiments revealed a purely elastic regime up to 10 GPa, followed by volumetric plastic deformation above this pressure [2].Thanks to in situ high pressure spectroscopic studies achieved in diamond anvil cells, the mechanical behavior of various organic inorganic silicas has been followed as a function of pressure. In particular, for urea or thiourea bridged silsequioxanes, vibrational studies coupled to ab-initio simulations show that the rigidity yielded by the inorganic polymerization is counterbalanced by the presence of the intermolecular H bond network. In a large range of pressures, these hybrid materials have a reversible behavior, and thus behave as molecular springs. We also demonstrate that the pressure behavior of these molecular springs is sensitive to the conformation of H bonds (cyclic versus linear) and to the constraints imposed by the covalent inorganic network.