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The Effect of Nanoparticle Shape on Polymer-Nanocomposite Rheology and Tensile Strength

Nanoparticles can influence the properties of polymer materials by a variety of mechanisms. With fullerene, carbon nanotube, and clay or graphene sheet nanocomposites in mind, we investigate how particle shape influences the melt shear viscosity $η$ and the tensile strength $τ$, which we determine via molecular dynamics simulations. Our simulations of compact (icosahedral), tube or rod-like, and sheet-like model nanoparticles indicate an order of magnitude increase in the viscosity $η$ relative to the pure melt. This finding evidently can not be explained by continuum hydrodynamics and we provide evidence that the $η$ increase in our model nanocomposites has its origin in chain bridging between the nanoparticles. We find that this increase is the largest for the rod-like nanoparticles and least for the sheet-like nanoparticles. Curiously, the enhancements of $η$ and $τ$ exhibit {\it opposite trends} with increasing chain length $N$ and with particle shape anisotropy. Evidently, the concept of bridging chains alone cannot account for the increase in $τ$ and we suggest that the deformability or flexibility of the sheet nanoparticles contributes to nanocomposite strength and toughness by reducing the relative value of the Poisson ratio of the composite. Our simulations point to a substantial contribution of nanoparticle shape to both mechanical and processing properties of polymer nanocomposites.