Mechanisms of Chain Exchange in Diblock Copolymer Micelles

Abstract

We investigate the mechanism of chain exchange in diblock copolymer micelles using two distinct yet complementary simulation techniques. First, enhanced sampling method is combined with coarse-grained molecular dynamics to compute a two-dimensional free energy surface for the chain expulsion process in the strong segregation regime. To facilitate chain expulsion, a distance-based collective variable is biased, and the end-to-end distance of the core block is also biased to ensure sufficient sampling of chain conformations. The free energy surface reveals a bimodal distribution of chain conformations along the effective reaction coordinate. The minimum free energy pathway qualitatively aligns with the Halperin-Alexander budding-like mechanism. The free energy barrier along this pathway is calculated for core block lengths ranging from $N_\textrm{core}=4$-$100$, and the barrier is shown to scale as $\beta\Delta F_\textrm{barr} \sim N_\textrm{core}^{2/3}$, consistent with the Halperin-Alexander prediction for a globular transition state. Notably, the free energy surface also reveals a nearly degenerate alternative pathway in which the chain escapes by extending out bead-by-bead, in agreement with previous simulations. We also study the case of a dense copolymer melt, where the core-block shrinks but does not collapse into a dry compact globule in the opposite phase. To examine the kinetic pathway, a simplified model is introduced in which a single chain escapes from a planar interface within a mean-field background. Forward flux sampling calculations yield a linear scaling of the barrier, $\beta\Delta F_\textrm{barr} \sim N_\textrm{core}$, in agreement with experimental observations and prior simulations. Moreover, analysis of successful escape trajectories highlights an extended chain conformation at the transition state.