Nanoparticle Geometrical Effect on Structure, Dynamics and Anisotropic Viscosity of Polyethylene Nanocomposites

Addition of nanoparticles into a polymer matrix can significantly alter its structure, dynamics as well as viscosity. In this paper, we study the structural, dynamical and viscous behaviors of polyethylene (PE) matrices under the influence of five differently shaped nanoparticles: bucicyball, graphene, nanodiamond (ND), X-shaped and Y-shaped junctions, at fixed volume fraction (4 vol %). These nanoparticles have different surface-area-to-volume ratios, arranged as graphene, X-shaped junction, Y-shaped junction, buckyball, and ND, from the largest to the smallest. In turn, different interaction energies between nanoparticles and PE matrices are enabled according to their surface-area-to-volume ratios. The graphene sheet is expected to have the strongest interaction with the PE matrix in accord with its largest surface-area-to-volume ratio. The interaction between NDs and their PE matrix is the smallest, due to their truncated octahedron shapes and the smallest surface-area-to-volume ratio. However, the graphene sheets tend to aggregate at the PE melting temperature (450 K), lowering their interactions with the PE matrix. Because of this interplay, the interactions between nanoparticles and polymer matrices can be tailored through the shapes (also surface-area-to-volume ratios) of nanoparticles as well as their dispersions. The polymer chains are found to be densely packed around these nanopartides in the range of 2 nm, except NDs, due to their strong interactions with PE matrices. Thus, these nanoparticles are found to be able to nucleate polymer entanglements around their surfaces and to increase the underlying entanglement densities of PE matrices. Both the polymer chain relaxation and anisotropic viscosity of PE nanocomposites are shown to be greatly affected by oriented nanoparticles. Our simulation results indicate that the surface-area-to-volume ratio of nanoparticles plays the dominated role in the structural, dynamical and viscous properties of PE nanocomposites.