Robust urethane-bridged silica aerogels available for water-carved aerosculptures

Molecular bridged silica aerogels have received extensive interest in recent years due to their easily controlled mechanical properties varying with different organosilane precursors, and without any further post-modification. In this work, a series of novel and robust molecular bridged silica aerogels were prepared by using a symmetrical urethane-bridged monomer, named N,N-bis(propyltriethoxysilyl)carbamide (bPTSCA), which was synthesized via a solvent-free exothermic reaction between 3-isocyanatopropyltriethoxysilane (IPTES) and 3-aminopropyltriethoxysilane (APTES) for several minutes at normal temperature and pressure. The influence of catalysts on the obtained gels and aerogels was studied in detail. It was found that NH4F aqueous solution showed the most effective catalysis capability, and the corresponding aerogels were intact with slight shrinkages. The mechanical strength of these aerogels could reach as high as 100 times more than that of the native silica aerogels (unbridged), and the strength could be easily adjusted from a rigid region to an elastic region simply by varying the sample density. Moreover, these aerogels possessed an interesting character of regional shrinking when encountering with water, which made it possible to fabricate artistic aerosculptures with water or other polar solvents as a graver.