Ultrathin films containing polymer-grafted nanoparticles (PGNs) show promise for use in hybrid electronics and high energy density materials. In this work, we use a coarse-grained model to simulate a hexagonally packed monolayer of PGNs adsorbed on a smooth surface that is attractive to both nanoparticles and polymer chains. We find that decreasing graft density at the same graft length increases interpenetration of the polymer-grafted layers, as expected. We quantify both overall and interparticle entanglements (between polymers grafted to different PGNs). While the higher graft density particles have a higher overall number of entanglements per chain, the lower graft density particles have higher interparticle entanglements per chain due to their increased chain interpenetration. Finally, we apply uniaxial tensile deformation to the monolayers; the peak stress occurs at lower strain values for higher graft density particles, which is attributed to the relatively lower number of interparticle entanglements. Our analysis provides a molecular picture of how decreasing graft density leads to better interpenetration, increased interparticle entanglements, and increased toughness of PGN monolayers, though it can lead to slightly less uniformly spaced monolayers, in agreement with previous experimental observations. These tradeoffs are crucial to understand for the design of robust, well-ordered inorganic−organic hybrid films.

We analyze the canopy structure and entanglement network of isolated polymer-grafted nanoparticles (PGNs) adsorbed on a surface. As expected, increasing the monomer-surface adsorption strength causes the polymer chains to spread out to increase contact with the surface, leading to a canopy shape that is in qualitative agreement with recent experimental results. We compare height profiles and other structural features of four PGN systems to show the separate and combined effects of increasing chain length and graft density. At moderate graft density and low surface attraction strength, nearby PGN canopies interpenetrate substantially and their combined shape is similar to that of a single PGN canopy. At high graft density or surface interaction, the interparticle spacing increases significantly. We use a geometrical primitive path analysis to calculate average entanglement properties including canopy-canopy entanglements between pairs of PGNs. The longer chain systems are well entangled at both graft densities considered, and we find that as the monomer-surface interaction strength is increased (and the interparticle distance increases), entanglements between the two PGNs are reduced. We find that the number of inter-PGN entanglements per chain is slightly larger at the lower graft density, likely because steric constraints at high graft density tend to reduce interparticle entanglements.

The deformation behavior of neat, glassy polymer-grafted nanoparticle (PGN) monolayer films is studied using coarse-grained molecular dynamics simulations and experiments on polystyrene-grafted silica. In both the simulations and experiments, apparent crazing behavior is observed during deformation. The PGN systems show a relatively more uniform, perforated sheet craze structure and significantly higher strain at break than reference homopolymers of the same length. Short chain, unentangled PGN monolayers are also simulated for comparison; these are brittle and break apart without crazing. The entangled PGN simulations are analyzed in detail for systems at both high and moderate graft density. Stress−strain curves show three distinct regions: yielding and strain localization, craze widening, and strain hardening preceding catastrophic failure. The PGN stress−strain behavior appears more similar to that of longer chain, highly entangled homopolymer films than to the reference homopolymer films of the same length as the graft chains, suggesting that the particles effectively add additional entanglement points. The moderate graft density particles have higher strain-to-failure and maximum stress than the high graft density particles. We suggest this increased robustness for lower graft density systems is due to their increased interpenetration of graft chains between neighboring particles, which leads to increased interparticle entanglements per chain.

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