Relating Entanglements and Toughness in Model Polymer-Grafted Nanoparticles

Polymer-grafted nanoparticles (PGNs) are a means to create precisely structured inorganic-organic hybrid materials. The graft length and graft density are key parameters that control interparticle spacing and other structural and mechanical properties. To guide materials design, we use coarse-grained molecular dynamics (MD) simulations to relate these parameters to structure, entanglements, and mechanical properties. We consider moderate to high graft density PGNs, which do not have large bare surface regions and are stable in the melt state in a hexagonally packed monolayer on a smooth attractive surface. As intuitively expected, grafts on adjacent PGNs are more interpenetrated in lower graft density systems. We define the interparticle entanglements (involving grafts originating from two different particles) and analyze these using both a topological and a time averaging method. We find that lower graft density (increased interpenetration) leads to increased interparticle entanglements per chain and increased toughness in both the melt and glassy state. The relationship between entanglement type and location and toughness will also be discussed.