The
diffusion of monomerically thin nanorods in polymer melts is
studied by molecular dynamics simulations. We focus on the systems
where the chains are long enough to screen the hydrodynamic interactions,
in which case the diffusion coefficient D
∥ for the direction parallel to the rod decreases linearly with increasing
rod length l. In unentangled polymers, the diffusion
coefficient for the direction normal to the rod exhibits a crossover
from D
⊥ ∼ l
–2 to ∼l
–1 with increasing l, corresponding to a progressive
coupling of nanorod motion to the polymers. Accordingly, the rotational
diffusion coefficient D
R ≈ D
⊥
l
–2 ∼ l
–4 and then D
R ∼ l
–3 as l increases. In entangled polymers, D
⊥ and D
R are
suppressed for l larger than the entanglement mesh
size a. D
⊥ ∼ l
–3 and D
R ∼ l
–5 for l sufficiently
above a, in agreement with de Gennes’ rod
reptation model.