Path-Dependent Morphologies in Oil/Water/Diblock Copolymer Mixtures

Amphiphilic block copolymer surfactants have been studied primarily in binary or pseudobinary systems, where they form a variety of dispersions, including micelles and vesicles, characterized by extremely low critical micelle concentrations (CMCs). In this study, a poly(1,2-butadiene-b-ethylene oxide) (OB) diblock copolymer (M n = 33 kg/mol, and 72 wt % PEO) was combined with water (W) and 1,5-cyclooctadiene (C) in binary (OB/W and OB/C) and ternary (OB/W/C and OB/C/W) mixtures, where the order of components indicates the mixing sequence. The resulting morphologies were characterized by small-angle X-ray scattering (SAXS) and cryogenic scanning electron microscopy (cryo-SEM). Addition of 1,5-cyclooctadiene to premicellized OB in water (OB/W) leads to swelling of the initially spherical hydrophobic micelle cores followed by micelle fusion leading to three-dimensional network structures. Combining water with premixed OB in 1,5-cyclooctadiene (OB/C) transforms a periodic sheet-like lamellar microstructure into a complex state of phase separation built on water-swollen poly(ethylene oxide) bilayers, which are manifested as uniformly sized small vesicles. Thus, two entirely different microstructures, which are stable for at least a month, have been prepared from a single three-component formulation. This pronounced nonequilibrium phase behavior, attributed to the non-ergodic character of amphiphilic block copolymers, may offer new processing strategies for the preparation of desirable self-assembled structures.