Dimethylamino- and trimethylammonium-tipped oxyethylene-oxybutylene diblock copolymers and their use as structure-directing agents in the preparation of mesoporous silica

Oxyethylene-oxybutylene diblock copolymers with a dimethylamino tip to the hydrophilic block were prepared by anionic polymerization, using aminoalcohol initiators in a dry aprotic medium. Conversion of a high percentage of the alcohol to alkoxide minimized the formation of unwanted by-products, but slightly broadened the molar mass distribution of the copolymers. Trimethylammonium-tipped copolymers were prepared by quaternization with methyl iodide. The copolymers were utilised as structure-directing agents in the preparation of silica under acidic conditions, where all the copolymers carry a positive charge, and subsequent removal of the copolymers by calcination gave mesoporous materials. The silica mesostructure depended on the relative sizes of the polymer blocks, changing from body-centred cubic to three-dimensional hexagonal to two-dimensional hexagonal on decreasing the proportion of hydrophilic units. The overall silica mesostructure dimensions, as determined from X-ray analysis, increased with an increase in micelle size, and the mesopore dimensions, as determined from nitrogen adsorption analysis, increased with an increase in the size of the core hydrophobic block. A simple model is suggested in which the hydrophilic chains are embedded in the silica walls in the uncalcined silica, generating micropores on calcination, whilst the hydrophobic chains form a separate microphase, giving rise to mesopores on calcination. The effect of charged tips, as compared with equivalent nonionic diblock copolymers, is to expand the dimensions of the mesostructures obtained and to generate higher surface areas in the calcined silica products.