(703a) Molecular Dynamics Studies on the Dispersion of Silica Nanoparticles in 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. However when nanoparticles aggregate, they lose their nanoscale size and corresponding properties. The addition of surfactants prevents the agglomeration of nanoparticles in nanoparticle-based nanocomposites. We report a coarse-grained molecular dynamics study of silica (SiO₂) nanoparticle aggregation and segregation in a polyethylene melt in the presence and absence of oleyl alcohol (C₁₈H₃₆O). We developed a multiscale simulation scheme for such system, where parameters of the coarse-grained model 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 parameters of the potentials are fitted to the radial distribution functions and chain dynamics obtained from atomistic molecular dynamics simulations. 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 30nmx30nmx30nm, and the simulations are carried out for 100ns.

In this work, we explore the effects of nanoparticle filling fraction, polymer chain length, and surfactant concentration on nanoparticle aggregation. We characterize the structure of the nano-composite using particle-particle radial distribution functions and the composite specific heat c_V. With no surfactant added, the dependence of the specific heat on the filling fraction exhibits a maximum at a filling fraction of 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. We also show that 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.

The addition of oleyl alcohol (a nonionic surfactant) reduces the effective attraction between the nanoparticles, further weakening the agglomeration, which was monitored via particle-particle contribution to the potential energy and the specific heat c_V. Our results show that, for a surfactant concentration of greater or equal than 6.4 wt%, the particles remain in a homogeneously dispersed state, which is consistent with experimental findings.