Humidity-driven deformation of ordered mesoporous silica films

The mechanical response of two ordered mesoporous silica films on silicon substrates was investigated by in situ grazing-incidence small-angle X-ray scattering during change of relative humidity. Films were prepared by using well known approaches of evaporation-induced self-assembly (EISA) of block-copolymer templated silica precursor sols. A thick film (≈500 nm) was prepared by dip coating and a thin film (≈100 nm) was prepared by spin coating. Both films exhibited a distorted 2D-hexagonal arrangement of cylindrical mesopores. While for the thin film these pores were highly oriented within the film plane, the thick film was much more disordered with respect to the in-plane orientation. The evaluation of the pore lattice strain as a consequence of changing relative humidity reveals a similar magnitude of deformation and similar elastic properties in the out-of-plane direction for the two samples. However, the in-plane strains are completely different. While the thin film is essentially unstrained within the plane of the film as expected for a thin film on a rigid substrate, there seems to be a considerable in-plane strain for the thick film. This result underlines the importance of film thickness and film quality when aiming at using such systems for bioinspired bilayer actuators