Smoother Surfaces Enhance Diffusion of Nanorods in Entangled Polymer Melts

Abstract

Coarse-grained molecular dynamics simulations are used to study the diffusion of thin nanorods in entangled polymer melts for varying nanorod length and roughness. While prior studies observed a nanorod parallel diffusion constant scaling inversely with rod length D ∥ ∼ l −1 , here, we show that this scaling is not universal and depends sensitively on the nanorod surface roughness. We observe D ∥ ∼ l −k , where k < 1 and decreases with decreasing surface roughness. The weaker scaling is driven by the non-Gaussian diffusion of nanorods due to the emergence of an intermittent hopping process that becomes more pronounced with decreasing roughness at the monomer scale. Analysis shows that the mean hop size grows for smoother rods but shows little to no variation with rod length. The mean hopping frequency shows no dependence on either rod length or roughness, suggesting it originates from the polymer melt environment. Our results show that the small-scale features of the nanorod surface strongly influence the large-scale and long-time transport of nanorods in polymer matrices, creating new material design opportunities for precisely engineered nanocomposites.

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

“…For spherical nanoparticles of diameter ( d NP ) smaller than the polymer characteristic length scales, the SE relationship underestimates the nanoparticle diffusivity because the particle dynamics is only coupled with the relaxations of local segments of polymer chains of size on the order of d NP . ,, While when the particle size is lightly larger than the mesh size of polymer entanglements ( d NP ≈ 1–10 d T ), the particles are still able to move faster than the SE estimation since they can transport through the local fluctuations of entanglements (the constraint release mechanism) , or by hopping when a gate (loop) between two neighboring confinement cages fluctuates to become large enough to slip around the particle (the hopping mechanism). − Likewise, the diffusion of rod-shaped particles in polymer systems should also depend on the geometric parameters of the rods with respect to the length scales of the polymers. Indeed, it has been examined by both experiments and simulations and found that the diffusivity of NRs in polymer melts is higher than that predicted by the continuum theory; ,,,,− however, the shape anisotropy makes their behavior quite different from that of spherical particles.…”