Macroporous Monolithic Methylsilsesquioxanes Prepared by a Two-Step Acid/Acid Processing Method

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 SiCH3 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.