Understanding the Dynamics of Complex Nanoparticle and Polymer Solutions Using Molecular Simulations

Understanding nanoparticle dynamics in polymer solutions holds significance for drug delivery and enhanced oil recovery applications. Deviation from the generalized Stokes-Einstein relation occurs when nanoparticle and polymer sizes are comparable. We employ hybrid molecular dynamics-multiparticle collision dynamics (MD--MPCD) simulations to investigate nanoparticle dynamics in semidilute solutions of ring and linear polymers. Nanoparticle diffusivities agrees with predictions from a polymer coupling theory [Cai, Panyukov, and Rubinstein, Macromolecules 44, 7853 (2011)], indicating coupling to segmental relaxations for both polymer architectures. Short-time nanoparticle dynamics exhibit subdiffusive behavior, deviating from coupling theory, instead closely tracking polymer subdiffusive exponents. The strong coupling of nanoparticle dynamics to polymer center-of-mass motions holds for both architectures. We also explore the impact of ring polymer stiffness on nanoparticle dynamics, observing deviations from coupling theory predictions and a vanishing coupling between nanoparticle dynamics and polymer center-of-mass motions with increased stiffness. In our subsequent study, we delve into the dynamics of polymers grafted onto spherical nanoparticles. Mean-square displacements of monomers near the grafting surface show an intermediate plateau, signifying confined dynamics akin to neutron spin-echo experiment reports. This confined dynamics disappears beyond a specific radial distance from the nanoparticle surface, dependent on polymer grafting density. We demonstrate that this dynamical confinement transition adheres to theoretical predictions for the critical distance associated with the structural transition from concentrated brush regime to semidilute brush regime.