Relation between Microstructure and Flexibility of Doubly Cross-Linked Organic–Inorganic Aerogels

Novel hybrid aerogels composed of aliphatic hydrocarbon chains connected by siloxane, namely, doubly cross-linked aerogels (DCLAs), have been proposed as the most flexible and applicable type of aerogel available. To unravel the intrinsic origin of the flexibility of DCLAs, two kinds of aerogel, polyvinylpolymethylsiloxane (PVPMS) and polyvinylpolysilsequioxane (PVPSQ), were theoretically examined. First, reactive molecular dynamics (RMD) simulations were conducted for polymerization, and three (with 72%, 80%, and 89% reaction ratios (RRs)) and four (with 45%, 52%, 57%, and 69% RRs) models were constructed for the PVPMS and PVPSQ aerogels, respectively. Deformation simulations were then conducted to measure the mechanical response and variations of the microstructure. The simulation results show that the structures of most of the small rings, which are a majority in DCLAs with a low reaction ratio, are self-bridging in the same polymer chain. Such self-bridged rings are less restricted owing to the soft local network with hydrocarbon chains. This might be the main reason why DCLAs show good deformability at a lower degree of reaction. In addition, we found that PVPSQ aerogel possesses Q³ silicon, which can bridge the polymer chains more, and thus generate more complicated and stiffer networks than PVPMS at a high reaction ratio. In contrast, the absence of Q³ silicon in PVPMS reduces the growth of fragile networks consisting of multiple polymer chains and consequently maintains the flexibility even at a high reaction ratio. Therefore, the super flexibility of PVPMS is attributed to the microstructure in which two-dimensional (2D)-like polymer chains are connected using a 2D network of siloxane connections without creating Q⁴ and Q³ stiff silicon. This study demonstrated that a theoretical investigation using RMD simulations instead of costly experiments is a powerful tool to explore novel DCLAs for developing advanced materials for practical applications.