Anisotropic and Non-Gaussian Diffusion of Thin Nanorods in Polymer Networks

Understanding the confined diffusion dynamics of anisotropic-shaped nanoparticles in polymer networks is of great importance for many applications. By performing molecular dynamics simulations, we demonstrate here that the translational diffusion of nanorods in cross-linked polymer networks is anisotropic in directions parallel and perpendicular to the rod main axis. The parallel component of the translational diffusion couples with the dynamics of only the surrounding polymer monomers, showing a decrease with rod length as D-parallel to similar to L-1. The displacements in the parallel direction, relative to the rod body frame, exhibit a Gaussian distribution. For the direction normal to the rod, the diffusion coefficient transitions from D-perpendicular to similar to L-2 to similar to L-3. Accordingly, the rotational diffusion coefficient exhibits a crossover from D-R similar to L-4 to similar to L-5 as D-perpendicular to and D-R both are suppressed by the constraints of either chemical cross-links or topological entanglements when L exceeds twice the network strand fluctuation distance or the entanglement tube diameter. A time-varied and size-dependent non-Gaussianity is observed for both the transverse motion and rotation. For the transverse motion, the displacement distributions exhibit a "fat" exponential tail, which does not vanish even at the long-time Brownian stage due to the presence of the intermittent tilting events. For the rotation of short rods, the non-Gaussianity stems from the relatively slowly relaxing polymer network environment. This source of non-Gaussianity gradually disappears as L increases. Instead, intermittent tilting becomes increasingly important, but its contribution to the exponential tail of rotational displacement distributions is less significant than that to transversely translational non-Gaussianity. Our findings may inspire the preparation of high-performance polymer nanocomposites as well as the novel and rational design of rod-like nanoparticle-based drug delivery systems.