Syntheses of Silica/Polystyrene-block-Poly(ethylene oxide) Films with Regular and Reverse Mesostructures of Large Characteristic Length Scales by Solvent Evaporation-Induced Self-Assembly

Silica/diblock films with various mesostructures of large characteristic length scales were synthesized through evaporation-induced self-assembly (EISA). The structure-directing agents used were amphiphilic polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymers of high molecular weights. The synthesis process began with a dilute homogeneous solution of a silica precursor and the diblock copolymer in a mixture of tetrahydrofuran (THF) and water. After this dilute solution was cast, THF preferentially evaporated; accordingly, the species in the depositing film increasingly concentrated and the solvent quality for the diblock progressively decreased. At some critical point, cooperative self-assembly of both the PSb-PEO diblock and the silicate started. Subsequently, liquid-crystalline mesophases were obtained. The present study indicates that silica/diblock films with different mesostructures can be synthesized by using one identical diblock; as the volume ratio of the diblock to silica increases, mesostructures change progressively from regular to inverted (reverse) via lamellar. For the silica/diblock films with regular mesophases, copolymer removal produces mesopores; highly ordered mesoporous silica films with different pore sizes result from using diblocks with different molecular weights. Particularly noteworthy is the ready formation of the silica/diblock multi-bilayer vesicular mesostructures. The present system is believed to be the first to use high glass transition temperature (Tg ≈373 K), PS-based amphiphilic diblock copolymers to prepare silica/diblock films with regular and reverse mesophases, as well as multi-bilayer vesicular mesostructures, through solvent evaporation-induced self-assembly.