Molecular Structure and Co-solvent Distribution in PPO-PEO and Pluronic Micelles

The structure and properties of micelles formed by diblock and triblock copolymers containing polypropylene oxide (PPO) and polyethylene oxide (PEO) in aqueous solutions are affected by chain architecture and have important implications for applications, e.g., in the biomedical area. Using atomistic molecular dynamics simulations, we investigate and compare the molecular structure of diblock copolymer PPO29PEO26, Pluronic L64, and reverse Pluronic 17R4 micelles formed by block copolymers of the same length and composition but different distributions of PPO and PEO blocks in pure aqueous solution or with 5% (by volume) added co-solvents. We show that while the diblock copolymer forms a tightly packed mostly spherical micelle, Pluronic L64 micelles are non-spherical and contain 10-18% (by volume) water in the loosely packed PPO core partially interpenetrated by the PEO block. Reverse Pluronic 17R4 micelles are rather small but relatively well-packed. Addition of 5% (by volume) alcohol to aqueous micelle solutions results in a minimal change in the case of ethanol, while addition of butanol or hexanol leads to an increase of water content in the core and alcohol accumulation at the core-corona interface for the PPO29PEO26 micelle. For L64 micelles, alcohol makes micelles more spherical but enhances defects, e.g., concentrates water in the core center or enhances PEO penetration, depending on the aggregation number. For 17R4 micelles, butanol and especially hexanol penetrate into the micelle core, swelling it. Even stronger core swelling occurs upon addition of 5% (by volume) isobutyric acid to aqueous solution of PPO29PEO26 micelles. We show that the extent of co-solvent penetration and its distribution within the micelles strongly depend on co-solvent hydrophobicity, the capability of hydrogen bond formation with the polymer and micelle architecture, factors that can affect micelle properties and performance in practical applications.