Novel Self-Assembling Close-Packed Droplet Array at the Interface in Ternary Polymer Blends

This paper reports, for the first time, on the self-assembly of polystyrene (PS) droplets into a perfectly segregated close-packed droplet array at a high-density polyethylene/polypropylene (HDPE/PP) interface during melt processing. Remarkably, even when highly concentrated at the interface, the PS remains in droplet form and does not coalesce into a uniform layer at the interface. This highly organized, Pickering-type emulsion microstructure is induced by the partial wetting of the HDPE/PP interface by the PS droplets. It is shown that the affinity of the PS droplets for the HDPE/PP interface can be controlled by the addition of a 1,4-hydrogenated styrene−(ethylene−butylene) (SEB) diblock copolymer. A focused ion beam/atomic force microscopy technique is used to show that, depending on the concentration of SEB copolymer, the PS droplets migrate from the PP side to the HDPE side of the HDPE/PP interface due to the selective location of the SEB copolymer at the HDPE/PS interface. The close-packed droplet array at the interface is accentuated during annealing due to a “sweep and grab” effect induced by the coarsening of the HDPE/PP cocontinuous microstructure. It is shown that this phenomenon of assembled droplets, as well as the migration of the droplets at the interface, can be thermodynamically predicted via spreading coefficient theory. The special case of interfacial coalescence of the PS droplets in these systems was also studied by varying the time of quiescent annealing. In the unmodified blend, the coalescence rate is significantly higher for the PS droplets located at the HDPE/PP interface compared to the ones in the bulk phases. The addition of 1% SEB copolymer results in an even higher coalescence rate early in the quiescent annealing procedure. However, coalescence rapidly decreases and falls to virtually zero after 60 min due to the saturation of the HDPE/PS interface by the SEB copolymer. It is shown that when interfacially modified PS droplets are located at the HDPE/PP interface, they also serve to completely stabilize the cocontinuous HDPE/PP network from further coalescence. This work opens new perspectives in generating novel complex microstructures in polymer blends.