Influence of Grafted Polymers on Translational and Rotational Diffusion of Au Nanorods

Nanoparticles are often grafted with polymers to tailor their solubility in organic solvents and improve dispersion within polymer nanocomposites. Polymer nanocomposites incorporating noble metal nanorods, in particular, exhibit distinctive optical absorption and scattering properties, and the diffusion coefficients of polymer-grafted nanorods are key parameters for designing such materials. Using a combination of depolarized dynamic light scattering (DDLS) measurements and dissipative particle dynamics (DPD) simulations, we studied how attaching polymers to Au nanorod (AuNR) surfaces affects their translational and rotational diffusion in solution. We grafted poly(methyl acrylate) (PMA) from AuNR surfaces using a new surface-initiated RAFT procedure that preserved nanorod dimensions and resulted in polymer brushes with controlled thicknesses and high grafting densities. We compared the diffusion coefficients of these particles to those synthesized by grafting thiol-terminated polymers to the surface, and found much slower diffusion in the former – indicative of greater drag, a larger hydrodynamic size and thus a thicker surface layer of polymer.  DPD simulations employing a quaternion-based rigid-body description of AuNR dynamics reproduced these trends and provided molecular-level insight into the relationship between brush structure and particle mobility. These results establish a foundation for using DDLS to quantitatively link grafted polymer structure to rotational dynamics, enabling rational design of AuNR-containing nanocomposites for applications in coatings, optics, and sensing.