Dependence of SBA-15 formation on the block copolymer concentration in the course of synthesis with precursor containing ethylene glycol residues

Abstract

The formation of silica is governed by two parallel processes triggered by the addition of a precursor to a solution of P123 block copolymer. One process is sol–gel synthesis, while the other is the transformation of an initial micellar phase consisting of spherical micelles of P123 into a hexagonal mesophase, which serves as a template. The gelation of the reaction mixture terminates all transformations, thus making it possible to examine the phase state of the block copolymer at the moment of the sol–gel transition. The systematic study of systems with different P123 concentrations has shown that the structure, morphology, and porosity of the material is determined by the ratio between the rates of the aforementioned processes. A material with the structure of SBA-15 containing hexagonally packed cylindrical mesopores is formed at a block copolymer content of 10 wt %. As the P123 concentration is reduced, the rate of the transformations of its structures decreases relative to the rate of the sol–gel process. Analysis of electron micrographs has revealed that the material being formed contains, initially, irregular short rodlike mesopores rather than cylindrical ones; then, as the P123 concentration is further decreased, amorphous silica arises in the material. The role of their templates is played by intermediate structures formed during the transformation of the P123 micellar phase into the hexagonal mesophase. Advantages of the SBA-15 synthesis with the precurosr containing ethylene glycol residues are the good reproducibility, one-pot procedure, no addition of acid and organic solvent or heating, and the formation of bimodal monolithic material containing both meso- and macropores.