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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 PPO 29 PEO 26 , 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 PPO 29 PEO 26 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 PPO 29 PEO 26 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.

The origin of the coil-globule transition for water-soluble thermoresponsive polymers frequently used in nanomaterials remains elusive. Using polypropylene oxide as an example we demonstrate by means of atomistic molecular dynamics simulations that temperature-induced increase in the sequence length of monomers that are not hydrogen bonded to water drives the coilglobule transition. Longer chains statistically exhibit longer sequences which serve as nucleation sites for hydrophobic cluster formation facilitating chain collapse at lower temperature in agreement with experimental data.

Nanostructures self-assembled from natural or biocompatible macromolecules attract increasing attention due to their potential in nanomedical and technological applications. Self-assembly and structural properties of flowerlike micelles formed by cholesterol end-capped polyethylene oxide (PEO) have been investigated by contrast-variation small-angle neutron scattering, small-angle X-ray scattering, dynamic light scattering, and molecular dynamics (MD) simulations. Three molecular weights (MWs) of the middle PEO block, (6, 10, and 20 kg/mole) have been synthesized and examined individually. As expected, the critical micelle concentration increases with PEO block length and for the two higher MW polymer samples, flower-like micelles coexist with unimers. A core− two-shell model was applied to analyze the small-angle neutron and X-ray scattering data, showing that in all cases, the cholesterol core of micelles is about 24 Å in radius and practically free of water, while the PEO corona contains a denser inner shell with about 50% of water and a well-hydrated outer shell (>88%). MD simulations with the same number of cholesterol units in the core based on the experimental outcome revealed a somewhat ellipsoidal cholesterol core with an average radius ∼24 Å, inner PEO shell, and well-hydrated outer shell, consistent with the experimental analysis. For all micelles studied, the PEO block was found to be slightly extended (∼30%) compared to the free coil configuration, while the cholesterol core and inner PEO shell were found to be very similar implying comparable aggregation numbers, nearly independent of the PEO length. The polymer concentration was below the overlap limit, and we observe well-defined stable non-clustering flower-like micelles, which have a nice potential for biomedical applications. This study provides a universal approach to unambiguously identify the morphology of flower-like micelles with detailed internal structural and compositional information.

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