Assembly of Branched Colloidal Nanocrystals in Polymer Films Leads to Enhanced Viscous Deformation Resistance

Progress in the integration of nanocrystals with polymers has enabled the creation of materials for applications ranging from photovoltaics to biosensing. However, controlling the nanocrystal segregation and aggregation in the polymer phase remains a challenging task, especially because nanocrystals tend to form amorphous clusters inside the polymer matrix. Here, we present the ability of octapod-shaped particles to overcome their strong entropy-driven tendency to aggregate disorderly and form instead centipede-like linear arrays that are randomly oriented and fully embedded in polystyrene films upon controlled solvent evaporation. This behavior cannot be entirely described by short-range van der Waals interactions between the octapods in the polymer solution. An important role here is played by the increment of the viscosity of the medium during the evaporation of the solvent, which prevents disaggregation of the chains once they are formed. We show that increasing the octapod loading in the blends does not impact the length of the linear arrays beyond a critical length, while it favors instead chain demixing to form self-segregated regions of parallel interlocked chains. Our experiments evidence that softening of the polymer matrix by ex situ heating of the films induces a tail-to-tail coupling of the preformed chains and leads to the formation of longer linear structures of octapods, up to 2 microm long. The presence of 1D arrays of octapods in free-standing polystyrene films improves the creep response by a remarkable 37%, owing to an octapod pinning effect of the polymer matrix.