(178s) Molecular Dynamics Studies On the Rheological Properties of Silica Nanoparticles Embedded Polyethylene Melt Using a Coarse-Grained Model

The addition of nanoparticles to polymer composites has been shown to significantly influence the mechanical, optical, and electrical properties. We report a coarse-grained molecular dynamics study of silica nanoparticle aggregation and segregation in a polyethylene melt. Firstly a multiscale simulation scheme was developed for such system, where parameters of the solid-fluid interaction are determined from the results of a separate atomistic model. In our coarse grained model, every eight methylene groups of polyethylene are represented by one soft bead. The nanoparticles of 4nm in diameter are modeled as spherical clusters of beads kept together by rigid harmonic bonds. The particles move in simulation box of dimensions 30nm x 30nm x 30nm, and the simulations are carried out for 100ns. Using this coarse-grained model, we explore the diffusion and rheological properties of polymeric fluids and nanoparticle composites under shear flow, and specifically we investigate the effects of polymer chain length, nanoparticle filling fraction and shear rate on viscosities η and first normal stress difference N₁. The shear viscosity is calculated from the tangential stress that is exerted on the walls. We 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 shear viscosity for longer molecules shows that, in Planar Couette Flow, shear thinning occurs independent of the molecular weight. 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. These results are qualitatively consistent with the experimental studies of nonlinear rheology of particle-filled polymer melts. In addition, we characterize the structure of the nano-composite using particle-particle radial distribution functions and the composite specific heat C_V. The dependence of the specific heat on the nanoparticle filling fraction exhibits a maximum occurs at about 3.6% wt, which we attribute to the nanoparticle agglomeration transition. At lower filling fractions the nanoparticles are in a dispersed state, and for filling fractions exceeding 3.6 wt%, the nanoparticles show a tendency to agglomerate. The polymer-mediated particle-particle forces exhibit a more repulsive character in the case of longer chains than in the case of the shorter ones. Moreover, the dispersed state of the nanocomposite becomes more stable as the molecular weight of polyethylene increases. This effect is most visible when the radius of gyration of the linear polymer exceeds the nanoparticle radius. The polymer gyration radius grows with nanoparticle filling fraction due to chain entanglement around the particles, which is typical in polymer reinforcement.