Copolymer Vesicles: Thermodynamic Motifs of Self-Assembly and Shear-Induced Reorganization

In shear flow experiments, copolymer vesicles are seen to break up into 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)). Molecular dynamics simulations are used to probe the energetic and entropic motifs of vesiculation in solutions of diblock and hairpin-shaped triblock copolymers. Vesiculation pathways that involve intermediate structures, i.e., spheres/rods/lamellae for diblocks and perforated polyhedra for triblocks, are directed by hydrophobic interactions with little change in configurational entropy. Vesiculation is also seen to occur by water diffusion into spherical aggregates of diblock copolymers. Flow strength is characterized by the ratio R of the time scale of shape fluctuations at equilibrium to the inverse shear rate. For relatively low O(1) values of R, a spherical unilamellar vesicle deforms into a flow-aligned ellipsoidal bilayer. The dynamics of shape/orientation variations are characterized by limit cycles. For larger R values, highly elongated vesicles fragment into lamellae which could regroup into cylindrical micelles. Structure anisotropy results in shear-thinning and viscoelastic rheology. Equilibration of flow-induced structures after flow cessation results in the formation of new persistent morphologies, e.g., two vesicles connected by a lamellar bridge. Plausible mechanisms of such hysteretic behavior will be discussed.