Low-density, hydrophobic, highly flexible ambient-pressure-dried monolithic bridged silsesquioxane aerogels

Abstract

Silica aerogels are nanostructured, porous solids with an open pore structure. Their unique properties are of great interest to researchers in the field of science and technology. However, practical applications of silica aerogel monoliths have been limited because of their fragility, hydrophilicity, and the necessity to use costly supercritical fluid (SCF) drying in the production. For many applications, a flexible material is most desirable, and in this study, we report the synthesis of a highly flexible bridged silsesquioxane aerogel (BSA) monolith containing C–C, C–O and C–S bonds. The synthesis of the aerogels began with a click reaction of vinyltriethoxysilane (VTES) and 2,2′-(ethylenedioxy)diethanethiol (EDDET), followed by an acid–base sol–gel process and ambient pressure drying. The mechanical properties of the resulting BSAs were characterised by nitrogen adsorption–desorption, scanning electron microscopy (SEM), contact angle and thermal conductivity measurements, and unidirectional compression testing. The BSAs showed high flexibility, with a Young's modulus of compression that increased from 0.11 to 1.31 MPa as the density of the aerogels increased from 0.125 to 0.237 g cm⁻³. Additionally, the aerogels were hydrophobic, with a contact angle as high as 142°. They had low thermal conductivity (0.037 W m⁻¹ K⁻¹) and exhibited good absorption and recyclability for organic liquids. All of these characteristics are critical for the practical application of aerogels, especially in energy-saving insulation and oil/chemical clean-up practices.