Molecular Dynamics Study of the Role of Nanoparticle Assemblies on Polymer Nanocomposite Rheology

We perform coarse-grained molecular dynamics simulations to critically investigate the factors determining the nanoparticle (NP) morphology and their effects on the rheological properties of polymer nanocomposites (PNCs). Our results unequivocally verify experimental conjectures that (i) the structure of PNCs with well-dispersed NPs and small NP clusters does not depend on the preparation method, but others, such as percolated NP morphologies, do depend on it and (ii) percolated NP networks provide higher mechanical reinforcement than any other particle assemblies at the same filler volume fraction. Going beyond these facts, our results suggest a new mechanism for mechanical reinforcement, especially in PNCs with well-dispersed NPs. In particular, we find in our model that well-dispersed NP states occur due to a balance between the interparticle repulsion (which tends to push the NPs apart) and filling constraints at large enough NP loadings (which tend to push the NPs closer). Under these conditions, the NPs become localized about “equilibrium” positions naturally leading to a solid-like mechanical response. While current understanding only supports the formation of percolated networks (either a pure NP network or one where the NPs are bridged by polymer chains) as being the source of mechanical reinforcement, this new mechanism could potentially provide a basis to understand some currently unexplained experimental results.