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

We investigate how nanoparticle shape influences the melt shear viscosity $\eta$ and the tensile strength $\tau$, with a focus on fullerene, carbon nanotube, and clay sheet nanocomposites. We simulate model nanoparticle dispersions of icosahedral, tube or rod-like, and sheet-like nanoparticles, all at a volume fraction $\approx 0.05$. Our results indicate an order of magnitude increase in the viscosity $\eta$ relative to the pure melt. This finding can not be explained by continuum hydrodynamics and we provide evidence that the $\eta$ increase 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 $\eta$ and $\tau$ 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 $\tau$ 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.