Eve F. Fabrizio

Hydrophobic monolithic aerogels by nanocasting polystyrene on amine-modified silica

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.

Polymer nanoencapsulated rare earth aerogels: chemically complex but stoichiometrically similar core–shell superstructures with skeletal properties of pure compounds

Rare earth (RE) aerogels combine the typical high porosity of aerogels with useful electrical, magnetic, optical and catalytic properties of the skeletal framework. RE aerogels were prepared by supercritical fluid CO2 drying of wet gels, which in turn were obtained via a modification of literature procedures involving epichlorohydrine-induced gelation of ethanolic solutions of the hydrated chlorides. But even more so than their silica counterparts, RE aerogels are fragile materials. This problem is addressed by using the innate hydroxyl functionality of the mesoporous surfaces as the focal point for casting a conformal polyurethane/polyurea layer over their entire inorganic framework, thus preserving most of the mesoporosity of the native network (70% v/v after vs. 94% v/v before applying the polymer layer) and a significant portion of the mesoporous surface area (156 ± 19 m2 g−1 after vs. 368 ± 14 m2 g−1 before casting the polymer). Detailed chemical analysis shows that RE aerogels are far from pure oxides. For example, the RE metal content (Pr to Lu) is in the range of 58.0 ± 2.3% w/w, vs. 85.4–87.9% in the pure oxides. RE aerogels contain also carbonate, chloride and organic products from the gelation process. Despite their chemical complexity, however, both native and polymer encapsulated RE sol–gel materials are stoichiometrically similar, and by using the magnetic susceptibility as a probe, it is found that physical properties depending on the atomic number (AN) of the RE core element vary linearly with those of pure RE compounds. Therefore, from an applications design perspective RE sol–gel materials themselves can be treated as pure compounds. By analogy, similar types of core–shell structures and the associated benefits should be possible for all sol–gel materials.