Molecular Simulations of the Motion of Polymer-Tethered Nanoparticles in Unentangled Polymer Melts

Polymer-tethered nanoparticles are commonly added to a polymer matrix to yield superior material properties. Critical to the fabrication and processing of such a composite is the mobility of polymer-tethered nanoparticles. We study the motion of polymer-tethered nanoparticles in unentangled polymer melts using molecular simulations, which offer a precise control of the grafted chain (tail) length N_tail and the number Z of tails per particle. For loosely-grafted particles with small Z, the diffusion coefficient decreases with increasing N_tail, exhibiting a crossover from particle-dominated to tail-dominated diffusion. If the diffusion is tail-dominated, there are two sub-diffusive regimes in the mean squared displacement 〈△r²(t)〉of particles before diffusion. The sub-diffusion at small t arises from the dynamical coupling of the particle and the melt chains, while the one at large t results from the participation of the particle in the dynamics of the tails. For densely-grafted particles with large Z, 〈△r²(t)〉can be approximated as that of a larger particle. The friction coefficient of the tails is smaller than the prediction based on Rouse dynamics of tails. These results suggest that the particle and tails move as one object with the tails hydrodynamically coupled to each other.