Origin of Flexibility of Organic-Inorganic Aerogels: Insights from Atomistic Simulations
S Urata and AT Kuo and H Murofushi, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 20555-20563 (2018).
Silica aerogel has a variety of excellent properties, but the mechanical brittleness inhibits the practical applications. Recently, many experimental efforts have been made to improve the compressibility and bendability of aerogels by hybridization with organic materials; however, the reason of the flexibility has not yet been well understood. To identify the intrinsic origins of the flexibility of organic- inorganic hybrid aerogels, polymerization and mechanical responses of tetramethoxysilane (TMOS), methyltrimethoxysilane (MTMS), and 1,2-bis (methyldiethoxysilyl) ethane (BMDEE) polymers were investigated by using reactive molecular dynamics simulations. As a result, cyclic compressive deformation simulations successfully reproduce the experimental results that TMOS is substantially fragile, whereas MTMS and BMDEE are easy to be reshaped. Detailed structure analyses showed that Si-O-Si-O rings in TMOS are collapsed by compressive deformation, whereas any kind of ring structure in BMDEE is maintained even after large compression. Tetrahedral SiO4-based network structure (Q(4)) in TMOS is found to be the source of the brittleness. On the contrary, the absence of Q(4) silicones and the presence of ethylene units, which provide rotatable dihedrals, in BMDEE allow it to deform without disrupting the microscale network. The insightful information provided by the theoretical investigation in atomistic scale is essential to design new composite aerogels.
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