Structure of Arylene-Bridged Polysilsesquioxane Xerogels and Aerogels

Arylene-bridged polysilsesquioxanes are an interesting class of porous materials prepared by sol-gel processing of ethoxysilane monomers in which there are two or more trialkoxysilyl groups positioned about an arylene bridging group. The majority of these materials are highly porous with surface areas as high as 1880 m²/g. In an effort to understand the nature of porosity in these materials, small-angle X-ray and neutron scattering were employed to characterize phenylene-, biphenylene-, and terphenylene-bridged polysilsesquioxanes. Phenylene-bridged polysilsesquioxane xerogels and aerogels were also compared to understand the effect of drying protocol on pore structure. The effect of catalyst concentration is also reported for the base-catalyzed system. In all cases studied here, we find evidence for domains in the nanometer range with distinct fractal character. We associate these domains with porosity rather than microphase separation of organic and inorganic moieties. The nature of this porosity depends on the bridging group in a systematic way, but is only weakly dependent on other synthetic parameters such as catalyst type, catalyst concentration, and drying protocol.