Insight into the Molecular Self-Assembly and Structural Organization of Mesoporous Hybrid Silica Films with Binary Composition

Despite the large amount of work reported on mesoporoushybridorganic-inorganic films, the organization of functionalitiesin the pores remains very poorly documented and understood. Comprehensionof the molecular self-assembling processes in hybrid mesoporous filmsis crucial to control the localization of functional groups and functionaldomains in the pores and, thus, the overall properties of the finalmaterial. This work is devoted to the fundamental understanding ofthe process at the molecular level using model precursors. Orderedmesoporous silica thin films are prepared by the co-condensation sol-gelprocess between hybrid and nonhybrid precursors. Self-assembly ofthe mesophase is achieved by the templating method using an ionicsurfactant. Bromoalkylsilanes with different chain lengths are chosenas the model molecules because of the weak interactions at the molecularlevel. The influence of several parameters (length of the alkyl chain,concentration, relative humidity) on the film mesostructure is evaluatedby small-angle X-ray scattering (SAXS), environmental ellipsometricspectroscopy (EEP), and transmission electron microscopy (TEM). Arelationship between those parameters and the mesophase arrangement,pore size distribution, porosity, and Young's modulus is established.Moreover, a simple method to estimate the average number of moleculesper pore and the number of pores per volume unit is proposed, bringingquantitative insight into the film organization at different scales.