(361a) 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 molecular model for Nanoparticle Composites. In our coarse-grained model, eight methylene groups of polyethylene are represented by one soft bead. The nanoparticles are modeled as spherical clusters of beads kept together by rigid harmonic bonds. The particles move in a simulation box of dimensions 30nmx30nmx16nm. 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 dimension and filling fraction on viscosities. Over the range of shear rates studied, the shear viscosity is strongly dependent on the shear rate. These trends are quantified by fitting the power-law behavior. For all the polymer matrices, the shear viscosity consistently increases with filler concentration, and increases up to 2 orders of magnitude with respect to the viscosity of pure polymer. 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. The results of our simulations are compared with an experimental analysis of a polyethylene filled with spherical silica nanoparticles of various sizes. The viscosity of polymer nanocomposite is sensitive to the size of the filler particles. We found that decreasing the size of spherical filler particles while keeping the filler volume fraction constant leads to an increase of the viscosity. This is mainly due to the aggregation of small nanoparticles reduces the chain movements and slows down the relaxation of polymer, as a result, less material that flows, leading the melt viscosity to increase. For these polymer composites where fillers are in the nanometer scale, Einstein and Batchelor equations greatly under predict the data. These findings qualitatively agree with the experimental studies reported in the literature.