Deformation,\nRupture, and Morphology Hysteresis of\nCopolymer Nanovesicles in Uniform Shear Flow

Copolymer nanovesicles are used extensively in chemical\nprocesses\nand biomedical applications in which they are subjected to dynamic\nflow environments. Flow-induced vesicle deformation, fragmentation,\nand reorganization modify the energetic (e.g., polymer–solvent\ninterfacial area) and entropic (e.g., copolymer chain configuration)\ncontributions to the solution free energy. Equilibration of a deformed\nmorphology by flow cessation could reorganize the system into a self-assembled\nstate, which is different from the parent structure through a local\nfree energy minimization pathway. We perform nonequilibrium molecular\ndynamics simulations to investigate morphology evolution in uniform\nshear flow of a unilamellar nanovesicle formed by the self-assembly\nof amphiphilic triblock copolymers in an aqueous solution. Flow strength\nis characterized by the Weissenberg number <i>Wi</i>, defined\nas the ratio of the time scale of vesicle shape fluctuations to the\ninverse shear rate. For <i>Wi</i> < 10, a spherical vesicle\ndeforms into a flow-aligned ellipsoidal bilayer executing tank-treading\nmotion. For <i>Wi</i> > 10, pronounced variations in\nbilayer\nthickness and polymer extension manifest along the contour of the\nelongated vesicle, which breaks up into lamellar fragments. Below\na critical strain, the deformed vesicle upon flow cessation returns\nto its initial spherical morphology. However, for larger strains,\nstructure reorganization after flow stoppage results in the formation\nof a Novel Equilibrated Shear-Induced Structure (NoESIS) in which\ntwo vesicles are connected by a dynamic molecular bridge that can\naccommodate additional layers of copolymers, leading to a reduction\nin the polymer–solvent interface area. Mechanisms of morphology\nhysteresis are explored via an analysis of the thermodynamic markers.