Itaru Asano

Oil-in-Oil Emulsions Stabilized by Asymmetric Polymersomes Formed by AC + BC Block Polymer Co-Assembly

We demonstrate a facile route to asymmetric polymersomes by blending AC and BC block copolymers in oil-in-oil emulsions containing polystyrene (PS) and polybutadiene (PB) in chloroform (CHCl3). Polymersomes were prepared by mixing polystyrene-b-poly(ethylene oxide) (SO) and polybutadiene-b-poly(ethylene oxide) (BO) in the oil-in-oil emulsion, where the droplets and continuous phase are PS- and PB-rich, respectively. The polymersome structure was directly visualized using dye-labeled SO and BO with confocal fluorescence microscopy; SO and BO with a high O block fraction co-assemble to produce asymmetric polymersomes. As the O block is insoluble in both PS and PB, we infer that the detailed structure of the polymersomes is a bilayer in which the S and B blocks face the PS-inner and PB-outer phases, respectively, while the common O blocks form the core membrane. This structure is only observed for sufficiently long O blocks. It is remarkable that although all the polymers are soluble in CHCl3, such elaborate structures are created by straightforward co-assembly. These asymmetric polymersomes should provide robust bilayer membranes around emulsion droplets, leading to stable nanoscopic dispersions of two fluids.

Location and Influence of Added Block Copolymers on the Droplet Size in Oil-in-Oil Emulsions

We have investigated the effect of added polystyrene-b-poly(ethylene oxide) (SO) copolymer on the stability of oil-in-oil (O/O) emulsions containing polystyrene (PS) and poly(ethylene glycol) (PEG) in chloroform (CHCl3) and directly visualized the location of SO in the emulsions by using dye-labeled SO (SO*) with confocal laser scanning microscopy (CLSM). The emulsion formed by PS/PEG/CHCl3 = 14/6/80 (wt %) consisted of a droplet phase of PS in CHCl3 and a continuous phase containing PEG in CHCl3. SO*s with various molecular weights (Mn,SO) and volume fractions of the PS block in SO (fPS) were prepared via living anionic polymerization and subsequent end-esterification. The effect of SO on the droplet size in the emulsions was investigated as a function of both Mn,SO and fPS. Increasing Mn,SO and decreasing fPS were effective at reducing the droplet size down to less than 1 μm, which is 100 times smaller than in the absence of SO. The location of SO*s in the O/O emulsions was further investigated by CLSM. We found that the location of SO*s changed from the droplet interior to the liquid-liquid interface and then to the continuous phase with decreasing fPS. We discuss the possible mechanism in terms of the relation of SO* location to the droplet size.