Mechanical Dissipation in Polymer-Grafted Nanoparticle Assemblies

Macroscopic assemblies of polymer-grafted nanoparticles (PGNs) (i.e. single component nanocomposites) are being assessed for a broad range of structural, separation, electronic, and optical technologies. Elucidating the relationship between PGN design (graft density, graft length, and nanoparticle size), hierarchical structure (composition, shape, arrangement, spacing, and distribution of organic and inorganic constituents), and processing (rheology, solubility, wettability, mesoscopic self-assembly) is crucial to optimize performance. In all cases however, mechanical robustness is paramount. Herein, we will discuss recent experiments and simulations to understand the impact of PGN design on entanglements among adjacent neighbors, and the subsequent impact on plasticity and failure at low and extreme deformation rates. These studies point toward an optimal PGN design at intermediate graft density that maximizes inter-canopy entanglements throughout the polymer region. This architecture affords sub-Tg, bulk energy dissipation processes within the polymer, such as cavitation and crazing, and enables the nanoparticle core to act as a low relaxation rate crosslink, creating a secondary mesoscale nodal network. The most favorable entanglement arrangement likely depends on strain rate, and whether maximum strength or energy dissipation is required. Future concepts are required to increase entanglement density, and thus decrease the canopy (polymer) volume fraction, as well as elucidate the impact of hierarchical structures on mechanical dissipation, such as those comprised of multiple and/or anisotropic PGNs.