Polymer nano-encapsulation of templated mesoporous silica monoliths with improved mechanical properties

Macroporous (1–5 μm) monolithic silica aerogels consisting of both random but also ordered mesoporous walls have been synthesized via an acid-catalyzed sol–gel process from tetramethoxysilane (TMOS) using a triblock co-polymer (Pluronic P123) as a structure-directing agent and 1,3,5-trimethylbenzene (TMB) as a micelle-swelling reagent. Pluronic P123 was removed by Soxhlet extraction, and materials in monolithic form were obtained by extracting the pore filling solvent with liquid CO2, which eventually was taken out supercritically. Although these monoliths are more robust than base-catalyzed silica aerogels of similar density, nevertheless, the mechanical properties can be improved dramatically by letting an aliphatic di-isocyanate (Desmodur N3200) react with the silanols on the macro- and mesoporous surfaces. As it turns out, the polymer fills the mesopores and coats conformally the macropores of templated samples, so that BET surface areas decrease dramatically, from 550–620 m2 g−1 to <5 m2 g−1. By comparison, polymer nano-encapsulation of non-templated acid-catalyzed aerogels preserves a large fraction of their mesoporous surface area, and BET values decrease from 714 m2 g−1 to 109 m2 g−1. Finally, since polymer nano-encapsulation preserves the macroscopic physical dimensions of the monoliths before drying, comparative analysis of the physical dimensions against XRD data of native versus polymer nano-encapsulated samples provides evidence that upon drying macropores (micron size regime) shrink less than mesopores (nanometer size regime).