A new route to monolithic methylsilsesquioxanes: Gelation behavior of methyltrimethoxysilane and morphology of resulting methylsilsesquioxanes under one-step and two-step processing

Polymerization of methyltrimethoxysliane (MTMS) results in methylsilsesquioxane (MSQ), which has found important applications in recent years including use as low-k dielectric materials in the semiconductor industry, superhydrophobic materials, monolithic columns, and hybrid matrixes for immobilizing proteins. For polycondensation of MTMS in ethanolic solutions, we report the sol-gel behavior under two different sets of conditions. First, we examined one-step polymerization over a wide range of pH and show that the initial pH is important in determining both the gelation behavior of MTMS-derived sots and the morphology of the resulting MSQ materials. In the one-step method, we obtained either transparent precipitates and/or macroscopically phase-separated resins when the pH was below the isoelectric point (IEP) of the silanols; either macroscopically phase-separated resins or macroporous monolithic gels with pH > IEP; and homogeneous solutions when the pH was close to the IEP. We also report on the use of a two-step catalysis method using an initial acid catalysis step followed by a base-catalyzed condensation step (denoted as B2), which is able to produce bimodal micro/meso or trimodal micro/meso/macroporous MSQ monoliths, depending on the specific conditions employed. The resulting materials are shown to be more resistant to exposure to base relative to macroporous silica. These results indicate that MSQ materials derived by the two-step processing method should be useful for the development of chromatographic stationary phases and as porous materials for protein entrapment.