Role of soft interactions in enhanced diffusivity of polymer-grafted nanoparticles in heterogeneous environments

The dynamic behavior of dilute solutions of polymers is well described by the colloidal model or the Zimm model. In denser systems, the colloidal description fails as interactions with the environment become important. Here, we investigate the behavior of polymer-grafted nanoparticles (PGNPs), where the grafting to the nanoparticle surface causes a high self-concentration of the polymer chains. When dispersed in semidilute polymer solutions, the grafted polymer chains compress and their dynamics are confined by neighboring chains. This change in grafted polymer dynamics arises from the soft repulsive interaction between PGNPs and surrounding polymer chains. This soft interaction profile also affects the dynamics on longer length and time scales. Whereas hard sphere diffusivity decouples from bulk predictions in polymer solutions when the particle becomes comparably sized to the polymer, the diffusivity of soft PGNPs decouples from solution viscoelasticity even when they are larger than the surrounding polymer. Furthermore, the PGNP dynamics strongly depend on the ratio of free to grafted polymer molecular weights. These findings indicate that soft interactions allow PGNPs to diffuse faster than hard spheres in heterogeneous materials.