Simulating the miscibility of nanoparticles and polymer melts

Abstract

While the miscibility and spatial dispersion of nanoparticles (NPs) in a polymer melt critically affects the properties of the resulting nanocomposite, little simulation work exists on understanding this critical issue. We use isothermal–isobaric ensemble simulations and show that larger NPs disperse more easily than small NPs, implying the relative dominance of NP–polymer attractions over depletion-induced inter-NP attractions. Similarly, polymer chain length only plays a secondary role, probably because the entropic, depletion-induced inter-NP attractions only occur over length scales comparable to the correlation length in the melt, namely the segment size, σ. Importantly, no NP self-assembly is observed, and the only transition that occurs for polymer systems with large enough NPs (σ_NP ≥ 6σ) is of a purely, first-order solid–fluid type. This result follows from the fact that the range of effective attractions between the NPs, δ = σ/σ_NP, is short enough to preclude a vapor–liquid transition. This finding is given more weight since an equivalent sticky sphere model can reproduce the essence of our simulations. The observed behavior is captured by an effective two-body, polymer-mediated, inter-NP interaction potential, a surprising result in light of conventional wisdom in this field which implies the importance of many body effects.