Dynamics of Nanorods in Polymer Melts

The dynamics of nanorods in a polymer matrix is critical to the manipulation of the positions and orientations of nanorods and thus the functions of nanorod polymer composites. We perform molecular dynamics simulations of thin nanorods in polymer melts, where the rod diameter is equal to the monomer size. The simulations demonstrate a large anisotropy between the diffusion in the easy direction along the rod axis and that in the hard direction perpendicular to the axis. The diffusion coefficient D_‖ in the easy direction is inversely proportional to the rod length L in both unentangled and entangled polymers, as the rod only experiences the monomeric viscosity. The diffusion coefficient D_⊥ in the hard direction first scales as L^-2 and then as L^-1 with increasing L in unentangled polymers, suggesting that the corresponding viscosity increases linearly with L before saturating at the melt viscosity. D_⊥ for sufficiently large L in entangled polymers is strongly suppressed due to the topological constraints. The same viscosity as in the hard direction dictates the rotation. The rotational diffusion coefficient D_R first scales as L^-4 and then as L^-3 as L increases in unentangled polymers. Rotational diffusion is also strongly suppressed for sufficiently large L in entangled polymers.