Morphology Evolution of Block Copolymer Assemblies under Shear Flow

Robust thermodynamic self-assembly of amphiphilic block copolymers in solution into soft nanostructures such as micelles, bilayers and vesicles finds applications in synthesis of biomimetic materials, medical diagnostics, gene therapy, detergency and catalysis. Under equilibrium conditions, experiments show the coexistence of rodlike micelles, lamellae and vesicles (L. Chen, et al., J. Phys. Chem. B, 103, 9488 (1999)). Further, under shear flow, vesicles could break up into bilayer (lamellar) fragments which further organize into wormlike micelles (Heinrich et al., Biophys. J., 76, 2056 (1999); Zheng et al., J. Phys. Chem. B, 104, 5263 (2000)). We adapt a coarse-grained molecular dynamics simulation framework, previously developed to study self-assembly and flow-structure interactions in surfactant solutions [Sangwai & Sureshkumar, Langmuir 27, 6628 (2011); Sambasivam et al., Phys. Rev. Lett. 114, 158302 (2015); Dhakal & Sureshkumar, J. Chem. Phys. 143, 024905 (2015)], to investigate the phase behavior (Liu & Sureshkumar, Colloids and Interfaces, 7, 40 (2023)) and fluid dynamics of solutions containing block copolymer assemblies. Perturbation of equilibrium states by uniform shear flow, especially near phase boundaries, leads to morphology transitions, e.g., vesicle to lamella, lamella to rod. Flow-mediated interactions among morphologies lead to formation of new non-equilibrium structures such as flow-aligned nano-sheets which render pronounced viscoelasticity to the fluid.