Our results show that the potential energy of nanoparticles in the presence of polymers and surfactants has a crossover between clustered and dispersed states. The specific heat due to nanoparticle interaction fluctuations shows a clear maximum that can be used as an approximate indicator of the boundary between clustered and dispersed states. In the presence of surfactants, we find that the potential energy becomes less negative because the number of particle-particle contacts in the system is reduced, and the approximate boundary is shifted to a larger filling fraction (i.e. the range of dispersion is extended), indicating that surfactant molecules can reduce the tendency of nanoparticle aggregation. When the number of the hydrophobic particles is increased, while keeping the hydrophilic head fixed, there is no significant change in the potential energy plot and the approximate boundary between clustered and dispersed states. However when the number of the hydrophilic particles is increased, while keeping the hydrophobic head fixed, we see a greater tendency for the nanoparticles to be separated.
Nonequilibrium molecular dynamics (NEMD) simulations are then used to investigate the rheological behavior of flowing polymeric materials (our case study is oleyl alcohol, silica nanoparticles and polyethylene). We studied the influence of polymer-nanoparticle interaction, filling fraction and the presence of non-ionic surfactants on the rheology of the composites. Our results show that interfacial interactions between surfactants and nanoparticles play a critical role in the resulting viscosity of the material and in the clustering of nanoparticles inside the polymer matrix. Our results are in agreement with experimental findings that show that the viscosity decreases as a power law, whose exponent is correlated to the strength of the interaction surfactant-nanoparticle.