Polymer Diffusion in Nanocomposites with Nanorods: Bridging the Gap between Nanosphere and Nanotube fillers

The tracer diffusion of deuterated polystyrene (dPS; 168-3200 kg/mol) is measured in polystyrene (650 kg/mol) nanocomposites containing phenyl-capped nanorods with a similar aspect ratio (AR $=$ 9) but different sizes, NR-short (TiO$_{2}$; $l=$43.1 nm and $d=$4.6 nm) and NR-long (SiO$_{2}$-[Ni(N$_{2}$H$_{4})_{3}$]Cl$_{2}$; $l=$371 nm and $d=$43 nm). For NR-long where $l$ \textgreater 2$R_{g}$, the diffusion coefficient initially decreases as nanorod volume fraction increases but then begins to increase for near the percolation threshold. In this system, $R$ \textless $R_{g}$ and the diffusion behavior is consistent with previous studies of carbon nanotubes (i.e., $l $\textgreater \textgreater 2$R_{g})$. However, for NR-short (i.e., $l$ \textless 2$R_{g})$, diffusion shows a monotonic slowing down as the volume fraction increases despite the small values of $R$/$R_{g}$. This behavior is similar to the slowing down observed for isotropic nanoparticles. These experiments demonstrate that not only radius but also length of the nanoparticle plays a key role in diffusion. Moreover, these results indicate that a comprehensive model for polymer dynamics should include the geometry of the nanoparticle relative to $R_{g}$.