Cross-Sectional Effects on Nanorod Diffusion in Polymer Melts

The diffusion of anisotropic nanoparticles, such as nanorods, in complex polymer environments is relevant to several scientific and industrial topics, including nanocomposites, and drug delivery. Despite recent advancements in understanding the dynamics of thin nanorods in polymer melts through scaling analyses and simulations, systematic investigations into the cross-sectional effects of nanorods on diffusion processes remain limited. In this study, coarse-grained molecular dynamics (CGMD) simulations were performed to explore the translational and rotational dynamics of a single nanorod in unentangled and entangled linear polymer melts. To introduce cross-sectional effects, we increased the nanorod diameter d, and analyzed the rotational diffusion for spinning D_RS, which is distinct from the rotational diffusion for end-overend tumbling D_RT. We conducted quantitative analyses to examine the scaling behavior of various diffusion coefficients with respect to the nanorod diameter, categorizing nanorods as either ‘thin’ (with diameters smaller than the tube diameter) or ‘thick’ (with diameters larger than the tube diameter). Our findings reveal a significant crossover in the scaling of the translational diffusion coefficient as d increases, while the scaling of the rotational diffusion coefficient remains almost constant. Hopping mechanisms contribute to nanorod dynamics in entangled melts, exhibiting different variations of translational and rotational dynamics with d. We finally unveiled that, the coupling between nanorod diffusion and polymer chain fluctuations enhances translational-rotational correlation at short time intervals but diminishes in the Fickian regime.