Monolithic Hierarchical Fractal Assemblies of Silica Nanoparticles Cross-Linked with Polynorbornene via ROMP: A Structure–Property Correlation from Molecular to Bulk through Nano

Monolithic hierarchical fractal assemblies of silica nanoparticles are referred to as aerogels, and despite an impressive collection of attractive macroscopic properties, fragility has been the primary drawback to applications. In that regard, polymer-cross-linked silica aerogels have emerged as strong lightweight nanostructured alternatives rendering new applications unrelated to aerogels before, as in ballistic protection, possible. In polymer-cross-linked aerogels skeletal nanoparticles are connected covalently with a polymer. However, the exact location of the polymer on the elementary structure of silica and, therefore, critical issues, such as how much is enough, have remained ambiguous. To address those issues, the internal nanoporous surfaces of silica wet-gels were modified with norbornene (NB) by cogelation of tetramethyl orthosilicate (TMOS) with a newly synthesized derivative of nadic acid (<b>Si-NAD</b>: <i>N</i>-(3-triethoxysilylpropyl)-5-norbornene-2,3-dicarboximide). As inferred by both rheological and liquid <sup>29</sup>Si NMR data, <b>Si-NAD</b> reacts more slowly than TMOS, yielding a TMOS-derived skeletal silica network surface-derivatized with NB via monomer-cluster aggregation. Then, ring-opening metathesis polymerization (ROMP) of free NB in the nanopores engages surface-bound NB moieties and bridges skeletal nanoparticles either through cross-metathesis or a newly described stitching mechanism. After solvent exchange and drying with supercritical fluid CO<sub>2</sub> into aerogels (bulk densities in the range 0.27–0.63 g cm<sup>–3</sup>, versus 0.20 g cm<sup>–3</sup> of the native network), the bridging nature of the polymer is inferred by a >10-fold increase in mechanical strength and a 4-fold increase in the energy absorption capability relative to the native samples. The cross-linking polymer was freed from silica by treatment with HF, and it was found by GPC that it consists of a long and a short component, with around 400 and 10 monomer units, respectively. No evidence (by SAXS) was found for the polymer coiling up into particles, consistent with the microscopic similarity (by SEM) of both native and cross-linked samples. Most importantly, the polymer does not need to spill over higher aggregates for greatly improved mechanical strength; mechanical properties begin improving after the polymer coats primary particles. Extremely robust materials are obtained when the polymer fills most of the fractal space within secondary particles.