Mechanism of Escape of a Single Chain from a Diblock Copolymer Micelle

We investigate the dependence of the free energy trajectory for chain expulsion from a diblock copolymer micelle in a selective solvent on core chain length through dissipative particle dynamics simulations and umbrella sampling. The free energy barrier scales linearly with the core block length of the expelled tracer chain for N_core= 4-12, consistent with experiments. The simulations further reveal that the core chain undergoes a "hyperstretching" mechanism near the transition state, where the core block partially stretches through the corona to allow monomers further from the chain junction to remain shielded in the micelle core. As the junction extends past the transition state, it becomes more favorable for the chain to be fully expelled, and the monomer furthest from the junction exits the micelle core, allowing the core block to escape from the micelle and collapse upon entering the solvent. We propose a simple model to describe this process of chain expulsion, which provides an effective description of the simulation results.