Polymer directed aggregation and dispersion of anisotropic nanoparticles

Abstract

The aggregation and dispersion of two anisotropic nanoparticles (NPs), cubes and tetrahedrons, in a polymer matrix are studied in this work using coarse-grained molecular dynamics simulations. We present the phase diagrams of NP–polymer composites, depicting microscopically phase-separated, dispersed, and bridged cubes and tetrahedrons in a polymer matrix, which depend on the interaction between the NPs and polymer (ε_np), along with the NPs’ volume fraction (ϕ). The microscopic phase separation occurs at very low ε_np, where NPs self-organize into multidimensional structures, depending on ϕ. In particular, for tetrahedrons, a cross-over from an ordered spherical aggregate to a disordered sheet-like aggregate is observed with increasing ϕ. In the case of cubes, a transition from cubic array → square column → square array (sheet) is identified with increasing ϕ. The clusters of NPs are characterized by their asphericity and principal radii of gyration. The free energy profile for a structured assembly is estimated, which clearly shows that the successful assembly of NPs is energetically favorable at a lower temperature. However, there exists an energy barrier for the successful assembly of all the NPs in the system. At intermediate ε_np, a transition from a clustered state to a state comprising dispersed cubes and tetrahedrons in a polymer matrix is observed. At higher ε_np, a further transition takes place, where gas-like dispersed NPs form a liquid-like aggregate via polymer layers. Therefore, the findings in this work illustrate that the effective interaction between anisotropic NPs in a polymer matrix is very diverse, which can generate multidimensional structured assemblies, with the disordered clustering, dispersion, and bridging-induced aggregation of NPs.