Self-Healing Lamellar Silsesquioxane Thin Films

Silsesquioxanes are widely used inorganic-organic hybrid materials because of their high thermal and chemical stability and diverse organic functions. Self-healing of cracks formed in polymeric siloxane networks of silsesquioxanes is crucial for many applications. In this study, we report the design of self-healing silsesquioxane [(O3/2Si-R-SiO3/2)n] thin films by the self-assembly process. Lamellar-structured thin films were prepared by hydrolysis and polycondensation of bis-alkoxysilane precursors [(EtO)3Si-R-Si(OEt)3, where R = C2H4, C2H2, or C6H4] in the presence of a surfactant, followed by spin-coating on substrates. These films exhibit spontaneous and rapid healing of micrometer-scale cracks even under mild conditions (at room temperature and 50-60% relative humidity). Compared with the conventional silica-based lamellar thin films prepared using tetraethoxysilane and the surfactant, a significant enhancement of the crack-healing ability is evident. This can be attributed to the higher flexibility of the silsesquioxane networks and the higher swelling ratio of the lamellar silsesquioxane with moisture. Furthermore, the film hardness and adhesion to the substrate were greatly improved by adding a bis-alkoxysilane precursor with a long bridging organic group for interlamellar cross-linking. These results will lead to the development of self-healing silsesquioxane materials for practical applications.