Asymmetric Polymersomes from an Oil-in-Oil Emulsion: A Computer Simulation Study.

Asymmetric vesicles are valuable for understanding and mimicking cell and practical biomedicine applications. Recently, a very straightforward methodology for fabricating asymmetric polymersome was developed by Lodge's group through the coassembly of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) and polybutadiene-b-poly(ethylene oxide) (PB-b-PEO) block copolymers at the interface of a polystyrene/polybutadiene/chloroform (PS/PB/CHCl3) emulsion. However, the in-depth microscopic mechanism for the formation of asymmetric polymersomes remains unclear. To address this issue, in this article, the coassembly process for the formation of the asymmetric polymersomes in Asano's experimental system was systematically investigated by employing a dissipative particle dynamics (DPD) simulation. Our results definitely demonstrate the formation of the asymmetric polymersomes such as that in the experiments and that the bilayer formed through the folding and crossing of the PEO blocks. Besides, from the microscopic view, three stages can be discerned in the formation process: (1) the formation of micelles, (2) the micelle diffusion to the interface, and (3) the micelle rearrangement at the interface to form an asymmetric polymersome. Meanwhile, the incompatibility among PS, PB, and PEO is proven to be the main driving force for asymmetric polymersome formation. Moreover, the effects of the order of addition of copolymers and the volume fraction of PEO blocks on the structure of the asymmetric polymersomes are also investigated. We find that the formation process is affected severely by the order of addition, and adding PS-b-PEO first can make the asymmetric bilayer more perfect. Not only that, but perfect asymmetric polymersomes can be formed only when the volume fraction of PEO (fPEO) is greater than 0.55. We believe the present work can extend the knowledge of the self-assembly of asymmetric polymersomes, especially with respect to the self-assembly mechanism.