Influence of asymmetric ratio of amphiphilic diblock copolymers on one-step formation and stability of multiple emulsions

Multiple emulsions have attracted significant interests for fundamental study and practical applications. However, the preparation of stable multiple emulsions is difficult in general because the emulsification often involves a two-step process as well as two kinds of surfactants are needed to stabilize the two thermodynamically unstable interfaces. Recently, we described a one-step inversion approach for fabricating multiple emulsions with long-term stability by using a precisely defined amphiphilic copolymer, poly(ethylene glycol)-b-polystyrene (PEG-b-PS), as sole surfactant. Herein we examine the correlation of the nature of this block copolymer with its emulsification performance in more details. Our result shows that the asymmetric ratio, namely the ratio of block length between PS and PEG of the synthesized diblock copolymer, has a great influence on the catastrophic phase inversion as well as the type and stability of the resulting emulsions. The symmetric block copolymer with the asymmetric value close to 1 can lead to stable multiple emulsions because they might have the highest surface covering density not only at the oil-in-water (O/W) normal emulsion interface, but also at the water-in-oil (W/O) inverse emulsion interface. For highly asymmetric block copolymer like PEG45-b-PS150, only W/O emulsion is obtained as the longer PS block will increase the hydrophobicity of the polymer and then it will be preferentially wetted by oil. However, for asymmetric PEG45-b-PS6 block copolymer with shorter PS block, it leads to very different catastrophic phase inversion depending on whether the micelles are formed in the aqueous phase.