Cross-Linking 3D Assemblies of Nanoparticles into Mechanically Strong Aerogels by Surface-Initiated Free-Radical Polymerization

Skeletal nanoparticles of porous low-density materials formally classified as aerogels are cross-linked by surface-initiated polymerization (SIP) using a new surface-confined bidentate free-radical initiator structurally related to azobisisobutyronitrile (AIBN). Methylmethacrylate, styrene, and divinylbenzene are introduced in the mesopores, and upon heating at 70 °C, all mesoporous surfaces throughout the entire skeletal framework are coated conformally with a 10−12 nm thick polymer layer indistinguishable spectroscopically from the respective commercial bulk materials. The amount of polymer incorporated in the structure is controlled by the concentration of the monomer in the mesopores, and albeit an up to a 3-fold increase in bulk density (up to 0.6−0.8 g cm −3 ) and a decrease in the porosity even down to 40%, the materials remain mesoporous with average pore diameters increasing from 20 nm in the native samples to 41 and 62 nm in PMMA and polystyrene cross-linked samples, respectively. The new materials combine hydrophobicity with vastly improved mechanical properties in terms of strength, modulus, and toughness relative to their native (non-cross-linked) counterparts. The effect of polymer accumulation on the modulus has been also simulated numerically. Being able to use SIP for cross-linking 3D assemblies of nanoparticles comprising the skeletal framework of typical aerogels paves the way for the deconvolution of cross-linking from gelation (a free-radical versus an ionic process, respectively), so that ultimately all gelation and cross-linking reagents can be included together in one pot, leading to great process simplification. The mechanical properties of the new materials render them appropriate for anti-ballistic applications (e.g., armor).