(19d) Kinetic and Minimum Free Energy Pathways in Diblock Copolymer Micelle Exchange

Surfactant and polymer-based micelles are ubiquitous in both nature and industry. They exist naturally within our cells and are leveraged across many fields and applications, such as drug delivery and viscosity modification of oils and consumer products. Chain expulsion/insertion and micelle fission/fusion are the two most fundamental processes governing micelle aggregation and dynamics. The exact pathways for these processes, and how dominant they are in micelle formation and destruction, has long been debated. Most experimental results of chain exchange have been evaluated in the context of the Halperin and Alexander mechanism, where the core block is assumed to collapse upon entering the corona, leading to a N^2/3 scaling of the free energy barrier. Recent experiments and simulations, however, have reported linear scaling in N, calling into question the validity of the budding-like mechanism. We show that the minimum free energy pathway for chain exchange follows the Halperin and Alexander mechanism by employing state-of-the art enhanced sampling techniques in molecular dynamics simulations. In addition, we elucidate the true kinetic transition pathway by using forward flux sampling with Monte Carlo simulations of a single chain in an external field.