Inorganic nanoparticles with polymers grafted to their surface, known as polymer-grafted nanoparticles (PGNs), are a means to create functional materials with a controllable nanoscale structure. For instance, a thin film of PGNs on a surface can self assemble into a hexagonally packed structure with mechanically robust and precise spacing, leading to interesting optical properties, improved dielectric breakdown strength, or other desirable features. The type of ordering, interparticle spacing, and other structural details, as well as the resulting material properties, depend on parameters that can be controlled during PGN synthesis, including particle size, the type of polymer used, graft length, and grafting density. We use coarse-grained molecular dynamics simulations to show how these factors interact to determine the interparticle spacing, polymer conformations, and amount of polymer entanglements between nearby particles, and how this relates to the film’s mechanical properties. We consider moderate to high graft densities such that particles can form a stable, well ordered monolayer on a slightly attractive surface. We find that decreasing graft density leads to greater interpenetration of canopies on adjacent particles, a greater number of interparticle entanglements per chain, and thus greater mechanical toughness, both for PGN monolayer melts on a surface and for freestanding glassy films. Furthermore, simulations of crazing of these monolayers show a qualitatively similar behavior to experiments; relative to homopolymers of the same length, the craze structure of PGN films is more uniform and the films are significantly tougher, which we attribute to the entanglement structure and polymer attachment to the nanoparticles. We hope that by relating synthetic parameters to entanglements and to mechanical properties, we can provide insights to improve the rational design of robust, well ordered materials.

Structure and properties of polymer grafted nanoparticle monolayers from coarse-grained simulations

ACS Fall 2021

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