(39e) Molecular Simulation Studies On the Rheological Properties of Silica Nanoparticles Embedded in a Polyethylene Melt

The addition of nanoparticles to polymer composites has been shown to significantly influence their mechanical, optical, and electrical properties. We use a combination of thermodynamics theory, Molecular Dynamics ( a coarse grained approach) and atomistics simulations to develop a multiscale model for Nanoparticle Composites. In our coarse-grained model, eight methylene groups of polyethylene are represented by one soft bead. The nanoparticles are 4nm in diameter and are modeled as spherical clusters of beads kept together by rigid harmonic bonds. The particles move in a simulation box of dimensions 30nmx30nmx30nm. Using this coarse-grained model, we explore the diffusion of particles in the melt and rheological properties of the polymeric fluids and nanoparticle composites under shear flow. Specifically we investigate the effects of polymer chain length, nanoparticle filling fraction and shear rate on viscosities and first normal stress difference N1. For all the polymer matrices, the shear viscosity consistently increases with filler concentration. The monotonic increase of viscosity with filler concentration is commonly observed in experimental systems. We also demonstrate that the addition of nanoparticle fillers leads to a more pronounced shear thinning behavior, as a result of increasing shear rates in the gaps between the filler particles. Moreover, the addition of particles leads to a reduction in the first normal stress difference, which is commonly equivalent to the elasticity of the composite. The results of our simulations are compared with an experimental analysis of a polyethylene filled with spherical silica nanoparticles. The steady-state shear viscosities of the simulated polymer-filler system agree with those seen in the experimental results. Also, results show that shear viscosities, zero-shear viscosities, and the rate of shear thinning are all seen to increase with filler content in both the experimental and simulated system.