Molecular Mechanism of Chain Expulsion from a Diblock Copolymer Micelle

The self-assembly of diblock copolymers in solution results in micelles with tunable properties relevant for numerous applications, including oil-based lubrication and drug delivery. Recent experiments studying single chain exchange, the relaxation mechanism governing the dynamics of micelles in solution around equilibrium, have resulted in controversy surrounding the molecular mechanism of this process. We will present results of coarse-grained dissipative particle dynamics simulations that characterize the trajectory of chain expulsion and furnish insights into the core-block conformations during expulsion from a micelle. Using umbrella sampling, we computed the free energy trajectory for the expulsion of a single chain of varying core block length from an isolated micelle. We find that the chain undergoing expulsion adopts a 'hyperstretched' configuration as it nears the transition state, allowing some monomers to remain in the micelle core until the chain is fully expelled. We will provide a simple model to describe this proposed mechanism of chain expulsion, which captures the scaling behavior of our simulations and is consistent with previous experimental results.