The effect of added block copolymer on oil in oil emulsions

Oil-in-oil emulsions, formed by polymer A and polymer B in the presence of an organic solvent (A/solvent-in-B/solvent), are a unique class of emulsion, because both phases are composed of organic components. Here, the effects of PS-$b$-PEGs (PS: polystyrene and PEG: polyethylene glycol) on the stability of the oil-in-oil emulsions composed of PS/CHCl$_{3}$-in-PEG/CHCl$_{3}$ were studied by varying the molecular weight (20 \textless $M_{n}$ \textless 200 kg/mol) and the volume fraction (0.40 \textless $f_{PS}$ \textless 0.82) of the PS-$b$-PEG. We found that higher $M_{n}$ of PS-$b$-PEGs with low $f_{PS}$ were able to stabilize the emulsions with reduced droplet size. In particular, using a large PS-$b$-PEG (200 kg/mol, $f_{PS} \quad =$ 0.52), the emulsion was stable for more than 2 weeks, and the droplet size decreased to the nanoscale, around 300 nm in diameter, whereas the emulsion was unstable with large droplets (\textgreater 50 $\mu $m) without the PS-$b$-PEG. In order to reveal the mechanism of stabilization, dye-labeled PS-$b$-PEGs in the emulsions were directly monitored by fluorescence microscopy.