Molecular Dynamics Simulations of Polymer-Grafted Nanoparticle Monolayers

Inorganic nanoparticles with polymers grafted to their surface, known as hairy nanoparticles or polymer-grafted nanoparticles (PGNs) are a means to create functional materials with a controllable nanoscale structure. A thin film of PGNs on a surface can self assemble into a mechanically robust hexagonally packed structure with a precise spacing, leading to interesting optical properties, improved dielectric breakdown strength, or other desirable features. To guide design of future materials, one needs to connect the molecular-scale structure and resulting material properties to parameters that can be controlled during PGN synthesis, including particle size, graft length, and grafting density. Here, we use coarse-grained molecular dynamics (MD) simulations to show how these factors interact to set the interparticle spacing, interpenetration and entanglements of polymers on adjacent particles, and mechanical properties of PGN monolayers. Specifically, we graft Kremer-Grest chains of N=160 (lightly entangled) to spherical nanoparticles of 10 times the monomer diameter. We consider a series of high to moderate graft densities such that nanoparticle surfaces are sterically protected from directly interacting. Lower graft density leads to greater interpenetration of canopies on adjacent particles and thus a greater number of interparticle entanglements per chain, which corresponds to greater mechanical toughness, both for PGN monolayer melts on a smooth surface and for freestanding glassy films. We compare the crazing behavior of glassy PGN films under deformation to that of analogous homopolymers, finding that the PGNs lead to a more regular craze structure.