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

Abstract

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.