Coupling of nanoparticle dynamics to polymer center-of-mass motion in semidilute polymer solutions

TL;DR

This study explores nanoparticle motion in semidilute polymer solutions, revealing a new coupling mechanism with polymer center-of-mass motion that affects subdiffusive behavior beyond existing theories.

Contribution

It uncovers an additional coupling mechanism influencing nanoparticle subdiffusion, extending current understanding of nanoparticle-polymer dynamics.

Findings

Nanoparticles exhibit subdiffusive behavior before transitioning to diffusion.

Long-time diffusivities agree with existing coupling theory predictions.

An additional coupling mechanism affects nanoparticle subdiffusive exponents.

Abstract

We investigate the dynamics of nanoparticles in semidilute polymer solutions when the nanoparticles are comparably sized to the polymer coils using explicit- and implicit-solvent simulation methods. The nanoparticle dynamics are subdiffusive on short time scales before transitioning to diffusive motion on long time scales. The long-time diffusivities scale according to theoretical predictions based on full dynamic coupling to the polymer segmental relaxations. In agreement with our recent experiments, however, we observe that the nanoparticle subdiffusive exponents are significantly larger than predicted by the coupling theory over a broad range of polymer concentrations. We attribute this discrepancy in the subdiffusive regime to the presence of an additional coupling mechanism between the nanoparticle dynamics and the polymer center-of-mass motion, which differs from the polymer relaxations that control the long-time diffusion. This coupling is retained even in the absence of many-body hydrodynamic interactions when the long-time dynamics of the colloids and polymers are matched.