Transparent, Highly Insulating Polyethyl- and Polyvinylsilsesquioxane Aerogels: Mechanical Improvements by Vulcanization for Ambient Pressure Drying

Silica aerogels are unique porous materials possessing high visible-light transparency and low thermal conductivity. However, the practical applications are limited due to the native fragility of silica, and a lot of research focuses on the improvement of mechanical properties by organic–inorganic hybridization, and so forth. Here, the first synthesis of polyethylsilsesquioxane (PESQ; CH3CH2SiO1.5) and polyvinylsilsesquioxane (PVSQ; CH2═CHSiO1.5) aerogels is reported. The resultant PESQ and PVSQ aerogels obtained through a two-step acid–base sol–gel reaction in a surfactant-based solution exhibit visible-light transmittance and flexibility against compression without collapsing. The microstructural variations of these aerogels are systematically investigated by positron annihilation lifetime spectroscopy (PALS) in order to clarify the differences in properties derived from substituent groups. Furthermore, a post cure on the PVSQ wet gel using a radical initiator induces polymerization of vinyl groups in the solid network, resulting in mechanically reinforced aerogels with higher compressive modulus and resilience. This chemical modification, similar to vulcanization in silicone rubber materials, helps to produce xerogels with comparable properties to those of aerogels via ambient pressure drying. Since the resultant xerogel obtained from the vulcanization of PVSQ shows sufficiently low thermal conductivity of 15.3 mW m–1 K–1, these novel polysilsesquioxane materials are promising for transparent aerogels/xerogels superinsulators.