New Routes of Molecular Assembly by Shearing and Equilibration of Block Copolymer Solutions

Non-equilibrium molecular dynamics simulations are employed to investigate morphology evolution of triblock copolymer nanovesicles in startup and cessation of uniform shear flow. Above shear rates an order of magnitude greater than the inverse of the time scale of vesicle shape fluctuations, the vesicle is stretched to a dumbbell shape with its membrane exhibiting pronounced nonuniformities in thickness and copolymer chain extension. Below a critical strain G*, equilibration of the elongated vesicle after flow stoppage results in the recovery of the parent structure. However, above G*, the dumbbell retains the memory of its gross morphology and copolymer chain configurations during equilibration. Consequently, a novel equilibrated shear-induced structure (NoESIS) in which two vesicular regions are connected by a dynamic molecular bridge is established. NoESIS is structurally different from bivesicle structures formed at equlibrium and stable to thermal perturbations. The molecular bridge formed during equilibration is able to shield additional polymer layers from the solvent, thereby providing a new, flow-mediated enthalpic route to copolymer assembly. Computations of the system’s thermodynamic markers along the flow cycle support this mechanism.