Nanoconfinement Release Toughens Polymer-Grafted Nanoparticle Assemblies through Better Interdigitation and Entanglements

Polymer-grafted nanoparticles (PGNs) in matrix-free nanocomposites offer unique opportunities for highly loaded nanocomposites and superior mechanical performance compared to neat polymers. However, increasing Young's modulus with high nanoparticle volume fractions generally reduces toughness. This study uses coarse-grained molecular dynamics simulations to examine how grafted chain length, grafting density, and nanoparticle size affect the high strain rate mechanical performance of glassy PGN systems. Young's modulus generally increases with the inorganic volume fraction but deviates across grafting densities due to steric hindrance near the PGN core, causing stiffening. Sparsely grafted PGNs demonstrate superior toughness due to the release of nanoconfinement in the polymer brush. This reduction in confinement enables high interdigitation, facilitating effective inter-PGN entanglements that drive strain hardening and enhance toughness. Finally, two primary fracture mechanisms, disentanglement and chain scission, are attributed to enabling sustained energy dissipation during large deformations, promoting PGN toughness.