Monolithic silsesquioxane materials with well-defined pore structure

In this article, monolithic porous silsesquioxane materials, which are derived by sol–gel from trialkoxysilanes with substituent groups such as trimethoxysilane (HTMS), methyltrimethoxysilane (MTMS), and vinyltrimethoxysilane (VTMS), are reviewed with a special emphasis on our recent works. Careful controls over fundamental synthetic parameters such as pH, amounts of water and solvent, and kind of solvent and additives play a crucial role in the formation of monolithic gels based on random polysiloxane networks. Crystalline/amorphous precipitation is otherwise observed when the formation of isolated species including polyhedral oligomeric silsesquioxanes dominates or if phase separation of the hydrophobic networks in aqueous media is not adequately controlled. In the successfully controlled system, pore size can be varied from a few tens of nanometers to a few tens of micrometers; porous materials such as transparent aerogels and hierarchically porous monoliths have been explored. In addition, unique properties derived from trialkoxysilanes such as reactivity of the pore surface and flexible mechanical properties are demonstrated. Possibilities in the silsesquioxane materials with controlled pore structures are discussed.