Structural analysis of hexagonal mesoporous silica films produced from triblock-copolymer-structuring sol-gel

The increasing interest in mesoporous materials in various fields of application has motivated the investigation of novel high-technology thin films with pore size, structure and organisation controlled by the presence of directing agents. The design of optical coatings made by this method is described in this work. Silica thin films with 1d-hexagonal mesoporous structure were deposited on silicon and glass substrates from a silicate precursor solution, containing a non-ionic triblock copolymer (EO₁₀₆PO₇₀EO₁₀₆:F127) as structuring agent. 70 to 700 nm thick films, with an excellent optical quality, were prepared by single dipcoating at a constant withdrawal rate. The surfactant was removed from the coatings by various treatments (i.e. thermal treatment, washing, ultra-sonic bath or sohxlet extraction). XRD investigations showed that shrinkage or collapsing of the film network can take place upon removal of the surfactant. Profiles of the relative amounts of surfactant, present in the pores, were measured by Rutherford Back Scattering. Refractive index, thickness and porosity of untreated and treated coatings were deduced from ellipsometry measurements. Each treated film exhibits an uni-axial anisotropy. The highest porosity (46%) and the lowest structural degradation are obtained for samples pre-treated at 400 K before extraction of the surfactant by sohxlet technique, as a result of the rigidification of the silica matrix.