Photochemical strengthening of silica aerogels modified with coumarin groups

Silica aerogels were silated with coumarin functionalities to allow for strengthening through the photochemical dimerization of the coumarin groups. Coumarin modified silica gels were prepared by either co-polymerizing tetramethoxysilane (TMOS) with small amounts of 7-(3-triethoxysilylpropoxy)coumarin (TEPC) or by surface modifying the silica gel with 7-(3-trimethoxysilylpropoxy)coumarin (TMPC). Aerogels, prepared by supercritical carbon dioxide drying, were irradiated with ultraviolet light (365 nm) for varying lengths of time and the flexural strength of cylindrical samples was measured. Copolymerization of TEPC with TMOS resulted in aerogels that, while initially weaker than pure silica aerogels, showed a 41% increase in flexural strength with 20 min exposure to UV. In contrast, the surface modified aerogels, while stronger than pure silica aerogels, showed no influence on flexural strength with exposure to UV.