The addition of nanoparticles to polymer composites has been shown to influence significantly their mechanical, optical, and electrical properties. However, nanoparticle aggregation often leads to a breakdown of the composite nanostructure and properties. For example, a dielectric breakdown was observed in polyethylene impregnated by silica nanoparticles when the filling fraction exceeded 3 vol %. One of the ways to prevent nanoparticle agglomeration is the addition of surfactants to the composites in order to increase their affinity to the polymer. Here we study the factors that affect silica nanoparticle dispersion by means of molecular dynamics (MD) simulations. All the molecular models used in this work are described by the united-atom approach, with the exception of the chemical modifications performed on the silica nanoparticle surface to include a hydroxylated layer, for which atomistic models are used. We examine both the hydroxylated surfaces (i.e. surfaces contain a high number of silanols) and the dehydroxylated surfaces (i.e. surfaces have a low coverage of hydroxyl groups).

Our results show that the binding energy between the nanoparticles decreased by 37.5% by adding the nonionic surfactant oleyl alcohol (C18H36O) to a concentration of 3.3%, which indicates much less overall interparticle agglomeration. To further quantify the results the specific heat (Cv) was calculated during the simulations. By studying the specific heat the optimal conditions for nanoparticle dispersion could be determined. It was concluded that for a filling fraction of 1.15% or less, the particles remain in a homogeneously dispersed state that confirms experimental findings. The transitional boundary between dispersed and clustered states is shifted to a larger filling fraction, showing that the range of dispersed states expands. The width of the transition region becomes narrower, indicating lower fluctuation level when the surfactant is present. In addition, the effects of nanoparticle filling fraction, surface silanol (Si-O-H) group coverage, and surfactant concentration on nanoparticle dispersion are also investigated.

Extended Abstract Status: Not Uploaded