Amphiphilic Block Copolymer Vesicles Under Shear Flow: A Novel Equlibrated Shear-Induced Structure

The phase behavior of block copolymer (bcp) solutions (Liu & Sureshkumar, Colloids and Interfaces, 7, 40 (2023); 8, 12 (2024); Liu et al., Colloids and Interfaces, 8, 29 (2024)) and morphology evolution of bcp vesicles under uniform shear flow are studied by molecular dynamics simulations. Flow strength is characterized by the Weissenberg number Wi defined as the ratio of the vesicle shape relaxation time to the inverse shear rate. Above a critical Wi ~ 10, triblock polymer vesicles undergo flow-alignment, stretching and breakup into lamellar fragments. Equilibration of the solution after flow cessation leads to the reorganization of the fragments into a dumbbell-shaped morphology in which two vesicular structures are connected by a dynamic molecular bridge. By quantifying the polymer-solvent interfacial area and individual molecular configurations, the evolution of certain energetic and entropic markers of the system is analyzed suggesting that the dumbbell-like, novel equilibrated shear-induced structure (NoESIS) lowers the system’s free energy. Implications of this inference to flow-mediated irreversible morphology transitions observed in amphiphilic solutions will be discussed (Vasudevan et al., Nature Materials, 9, 436 (2010)).