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.

This article reports on the use of the Neumann triangle method combined with a focused ion beam sample preparation technique and atomic force microscopy (NT-FIB-AFM) to measure interfacial tension ratios in partially wetted ternary and quaternary immiscible polymer blends prepared by melt processing. It is shown that PS/PP/HDPE, PS/PCL/PP, PLLA/PCL/PS, PMMA/PS/PP, and a quaternary blend system comprised of HDPE/PP/PS/PMMA all display a partial wetting morphology with a three-phase line of contact and that the interfacial tension ratios obtained by the NT-FIB-AFM approach compare well with results obtained by the classical breaking thread method. The HDPE/PP/PS/PMMA quaternary blend, in particular, is quite unique and displays a partial wetting morphology with spectacular PS/PMMA composite droplets located at the HDPE/PP interface. Furthermore, all of the above data generated for the ternary and quaternary systems also satisfy the Laplace equation. When 1% of an SEB diblock copolymer is added to the PS/PP/HDPE system, the Neumann triangle method reveals that the PS/HDPE interfacial tension decreases from 4.2 ( 0.6 to 3.3 ( 0.4 mN/m, with an estimated apparent areal density of 0.19 ( 0.07 molecule/nm 2 of copolymer at the PS/HDPE interface. The results presented in this paper show that it is possible to generate complex morphologies demonstrating partial wetting for a wide range of polymer blend systems with a relatively simple experimental approach. Furthermore, it allows the measurement of the interfacial tension ratios of a matrix-dispersed phase blend system examined in situ after melt processing. The apparent areal density of a copolymer interfacial modifier can also be estimated. This is an important result, since it is still a challenge to measure the variation of the interfacial tension as a function of the copolymer areal density in multiphase polymer blends.

Hydrogels embedded with networks of fully interconnected pores were prepared with microporous polylactide (PLA) molds obtained by extracting the polystyrene (PS) phase in melt-processed cocontinuous blends of PLA and PS. Quiescent annealing of the blends prior to the PS extraction allowed control over the average pore diameter from ∼1 to ∼500 μm for the PLA molds. Solutions of agar or alginate were injected within the molds and gelled in situ. Porous gels were obtained by extracting the PLA molds and X-ray microtomography was employed to characterize their microstructure. Water removal/uptake cycles were fully reversible with very fast kinetics. Freeze-drying yielded ultraporous materials without modification of the macroscopic dimensions, and rehydration yielded back porous hydrogels. It was possible to scale up the technique by using extrusion and injection molding equipment. This versatile new method allows extensive control over the gels’ porosity parameters and the use of various gel chemistries.

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