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 crazing behavior, among other properties. Specifically, we create PGNs by attaching Kremer-Grest chains to spherical nanoparticles at different graft densities and chain lengths. 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. We analyze the melt state properties and also cool the monolayers below the glass transition and study the glassy freestanding monolayer film. As intuitively expected, grafts on adjacent PGNs are more interpenetrated in lower graft density systems. For entangled systems, we define and analyze the interparticle entanglements (in which grafts originating from two different particles are entangled with each other). 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. Under uniaxial deformation, the glassy PGN monolayers form a more regular craze structure than analogous homopolymer thin films, and the perforations in the film occur primarily in the interstitial areas of the particles. These qualitative features were also observed in recent experiments, and we analyze entanglement type and location in detail to further understand this behavior.

Entanglements and toughness in model polymer-grafted nanoparticle monolayers

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.