Molecular Dynamics\nStudy of the Role of Nanoparticle\nAssemblies on Polymer Nanocomposite Rheology

We perform coarse-grained molecular\ndynamics simulations to critically\ninvestigate the factors determining the nanoparticle (NP) morphology\nand their effects on the rheological properties of polymer nanocomposites\n(PNCs). Our results unequivocally verify experimental conjectures\nthat (i) the structure of PNCs with well-dispersed NPs and small NP\nclusters does not depend on the preparation method, but others, such\nas percolated NP morphologies, do depend on it and (ii) percolated\nNP networks provide higher mechanical reinforcement than any other\nparticle assemblies at the same filler volume fraction. Going beyond\nthese facts, our results suggest a new mechanism for mechanical reinforcement,\nespecially in PNCs with well-dispersed NPs. In particular, we find\nin our model that well-dispersed NP states occur due to a balance\nbetween the interparticle repulsion (which tends to push the NPs apart)\nand filling constraints at large enough NP loadings (which tend to\npush the NPs closer). Under these conditions, the NPs become localized\nabout “equilibrium” positions naturally leading to a\nsolid-like mechanical response. While current understanding only supports\nthe formation of percolated networks (either a pure NP network or\none where the NPs are bridged by polymer chains) as being the source\nof mechanical reinforcement, this new mechanism could potentially\nprovide a basis to understand some currently unexplained experimental\nresults.