Structural ordering of self-assembled alkylene-bridged silsesquioxanes probed by X-ray diffraction experiments

Powder X-ray diffraction experiments have been performed to analyse the structural order in alkylene bridged silsesquioxanes. These organic–inorganic hybrids were obtained by the hydrolysis–condensation of a series of six α,ω-bis[triethoxysilyl(propyl)ureido]alkylenes associating hydrophobic alkylene units with various chain length and H-bonding urea groups. The sol–gel hydrolysis in ethanol with a stoichiometric amount of water and using fluoride anion as catalyst only afforded amorphous bridged silsesquioxanes. Conversely, the hydrolysis with a large excess of water under acidic catalysis produced materials as layered sheets. A lamellar periodicity on the molecular scale level was observed for longer alkylene chain (Cn C8–C12). Under similar reaction conditions, the hydrolysis of the short alkylene chain precursor (C6) led to an amorphous solid. The medium to long range arrangement of the organic fragment in the solids is attributable to a combination of the hydrogen bonding interactions between the urea groups and of the van der Waals interactions between the long alkylene chains. In excess water, the hydrophobic nature of the long alkylene chains prevails and favours the formation of the intermolecular H-bonds between the urea groups leading to ordered assemblies of the organic substructures in the silsesquioxane network.