Molecular Dynamics Simulations of Small and Sticky Nanoparticles in Polymer Nanocomposites

Large-scale molecular dynamics simulations indicate that the diffusion of small nanoparticles, where the diameter of the nanoparticle is of the same size as the Kuhn segment length and smaller than the entanglement tube diameter, is influenced by its affinity to the segments of the polymer matrix. In the simulations, this affinity is characterized by the desorption time, τ_des, which is defined as the characteristic time for a nanoparticle to desorb from a polymer segment. The sticky nanoparticle diffusion in a polymer melt can be represented as a nanoparticle with an effective radius, due to an adsorbed polymer layer, if its size is sufficiently large compared to matrix chains. While for longer matrix chains, there is decoupling of the diffusivity of the nanoparticle with the polymer matrix. We observe that the diffusion coefficient of small and sticky nanoparticles decreases as a function of polymer matrix degree-of-polymerization, N, and then the value saturate and becomes N independent. The crossover to N independence occurs at a higher N as τ_des increases. This behavior is akin to non-adsorbing nanoparticle of increasing size, where matrix entanglements begin to impede its diffusion as its size increases.