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 sols 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.

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The development of monolithic methylsilsesquioxane (MSQ) materials with bicontinuous meso-/ macroporous morphologies has proven to be advantageous for the fabrication of chromatographic stationary phases. However, it is currently very difficult to obtain suitable columns over a range of capillary sizes, owing to alterations in morphology in narrow diameter capillaries and material shrinkage in large-bore capillaries. Herein, we describe a new acid/acid two-step sol-gel processing method (A2) for fabricating MSQ bicontinuous monolithic capillary columns with sizes ranging from 20 to 530 µm. The effect of temperature on pore volume and pore size of the related bulk gel was characterized by mercury intrusion porosimetry. It was found that the pore morphologies are stable at 300 °C while pores contract significantly at 400 °C. The evolution of the MSQ chemical structure upon heat treatment was examined by infrared spectroscopy and thermogravimetric analysis. IR spectra demonstrate that both the unstable Si-C bonds (from SiCH 3 groups) and the opening of rings at temperatures of 400 °C are responsible for the densification of gel skeletons and the collapse of macropores. These results indicate that column fabrication should be done without a calcining step to avoid irreversible loss of desirable morphologies and chemical properties.

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