Synthesis and characterization of mesoporous silica monoliths with polystyrene homopolymers as porogens

In this work, we explored the preparation of mesoporous silica materials by using polystyrene (PS) homopolymers as the porogens. First, a series of polystyrene (PS) homopolymers with various molecular weights were synthesized via atom transfer radical polymerization (ATRP). The single ends of these PS homopolymers were then functionalized with hydroxyl groups via copper(I)-catalyzed cycloaddition reaction, and these can then react with 3-isocyanatopropyltriethoxysilane to afford the triethoxysilane-terminated PS homopolymers. In the presence of the triethoxysilane-terminated PS homopolymers, the sol-gel process of tetraethoxysilane (TEOS) was carried out to afford a series of organic-inorganic silica gels. These silica gels were then used as precursors to obtain the mesoporous silica materials via the removal of PS microdomains with pyrolysis at elevated temperatures. Transmission electron microscopy (TEM) showed that all the silica materials displayed spherical or cylindrical nanopores with the size of the nanopores being 10-30 nm. The mesoporous structure was further evidenced by the measurement of specific surface areas with nitrogen sorption experiments. It was found that the specific surface areas can be adjusted in terms of the contents and molecular weights of the PS homopolymers. The generation of nanopores in the silica materials is ascribed to the formation of the PS microdomains in the organic-inorganic silica gels as revealed by small angle X-ray scattering (SAXS). It is proposed that the functionalization of the PS chain ends confined the reaction-induced phase separation of the PS homopolymers on the nanometer scale in the organic-inorganic silica gels. The approach reported in this work was in marked contrast to the utilization of amphiphiles as the porogens of the mesoporous silica.