Transition State of Single Chain Expulsion from a Diblock Copolymer Micelle

Abstract

The presence of a selective solvent induces self-assembly of block copolymers into a myriad of micellar nanostructures, offering great versatility for utilization in a wide range of technological applications such as viscosity modification and drug delivery. In order to fully harness their use in practical application, it is necessary to gain a comprehensive understanding of the mechanisms underlying micellization and equilibration of block copolymer micelles. The process of single chain exchange holds significant importance in equilibration of block copolymer micelles. While existing techniques offer insights into the ensemble behavior of chain exchange, the molecular-level details of the process remain insufficiently understood. To address this, a simulation framework combining dissipative particle dynamics with umbrella sampling to study chain exchange in diblock copolymer micelles in dilute solution was introduced. In this thesis, umbrella sampling was employed to probe the free energy trajectory of single chain expulsion from a diblock copolymer micelle. Using dissipative particle dynamics simulations, a biasing potential was applied to hold the chain at various distances from the micelle center-of-mass and the weighted histogram analysis method was utilized to extract the free energyprofile. By capturing the full free energy landscape of chain expulsion, this approach diverges from previous methods, providing access to the experimentally unobservable expulsion mechanism. The investigation focuses on exploring the dependence of the free energy barrier on the interaction energy between the core block and the solvent, or the core block length of the expelled chain. It was found that there is a monotonic increase in the free energy barrier for chain expulsion as either the interaction energy or the block length of the expelled chain increases, aligning with experimental results. Interesting, the effect of the core block length of the expelled chain was independent of the micelle characteristics. Examination of the radius of gyration of the core block during expulsion revealed a remarkable feature of the transition state: the core block exhibited partial stretching, allowing specific core beads to remain within the micelle core until the chain was completely expelled. This stretching mechanism effectively minimized unfavorable contacts by ensuring that only a fraction of the core block was exposed to the solvent at any given point along the expulsion trajectory, challenging previous models of chain exchange. Finally, a model consistent with the scaling behavior observed in both simulation and experimental data was put forward, offering an alternative perspective on the process of single chain exchange.