Hydrophobic monolithic aerogels by nanocasting polystyrene on amine-modified silica

Abstract

We describe a three-dimensional core–shell structure where the core is the assembly of nanoparticles that comprises the skeletal framework of a typical silica aerogel, and the shell is polystyrene. Specifically, the mesoporous surfaces of silica were first modified with amines by co-gelation of tetramethylorthosilicate (TMOS) and 3-aminopropyltriethoxysilane (APTES). Next, styrene moieties were attached to the amines by reaction with p-chloromethylstyrene. Finally, dangling styrene moieties were crosslinked by a free-radical polymerization process initiated by AIBN and styrene, p-chloromethylstyrene or 2,3,4,5-pentafluorostyerene introduced in the mesopores. Polystyrene crosslinked aerogels are mechanically strong, lightweight (0.41–0.77 g cm⁻³), highly porous materials (they consist of ca. 63% empty space, with a BET surface areas in the range of 213–393 m² g⁻¹). Their thermal conductivity (0.041 W m⁻¹ K⁻¹) is comparable to that of glass wool. Hydrophobicity, however, is the property that sets the new material apart from analogous polyurea and epoxy crosslinked aerogels. The contact angles of water droplets on disks cut from larger monoliths are >120°. (By comparison, the contact angle with polyurea crosslinked aerogels is only ca. 60°.) Polystyrene crosslinked aerogel monoliths float on water indefinitely, while their polyurea counterparts absorb water and sink within minutes.