Flower-like Micelles of Polyethylene Oxide End-Capped with Cholesterol

Abstract

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.

Block Sequence Effects on the Self-Assembly Behaviors of Polypeptide-Based Penta-Block Copolymer Hydrogels

“…Unlike K 15 –B 15 -EG 34 -B 15 –K 15 , the total length of the middle hydrophilic section of B 15 –K 15 -EG 34 -K 15 –B 15 is sufficiently long to form flower-like micelles with a hydrophobic B 15 core and a hydrophilic K 15 -EG 34 -K 15 shell (Figure c). This configuration has been reported in a cholesterol-capped PEG series with a similar block arrangement . However, the formation of flower-like micelles does not exclude the possible interconnection between micelles with the two hydrophobic B 15 blocks located at different cores of the adjacent micelles at high concentrations, forming percolation (or network) between micelles.…”

“…This configuration has been reported in a cholesterol-capped PEG series with a similar block arrangement. 54 However, the formation of flower-like micelles does not exclude the possible interconnection between micelles with the two hydrophobic B 15 blocks located at different cores of the adjacent micelles at high concentrations, forming percolation (or network) between micelles. Such a network leads to the gelation of the material.…”

Synthesis and Micellization of Bottlebrush Poloxamers

“…Micelle solutions were prepared via direct dissolution in aqueous buffer and were equilibrated at 37 °C for at least 24 h. The CMC a and hydrodynamic radii ( R h ) were determined via dynamic light scattering (DLS). DLS measures the diffusive relaxation of concentration fluctuations in dilute solution, which yields the particle size distribution by analyzing the correlation function via cumulant analysis or inverse Laplace techniques such as regularized positive exponential sum. − …”

Effect of Block Sequence on the Solution Self-Assembly of Symmetric ABCBA Pentablock Polymers in a Selective Solvent

“…In the case of ABCBA pentablocks (Figure a), most polymer chains adopt a loop conformation with the solvophobic blocks looping in the interior of the aggregate’s core, and given the very short A blocks, these aggregates resemble crew-cut micelles or semibald micelles observed in diblock polymer systems. On the other hand, in the spherical aggregates from CBABC pentablocks (Figure b), the loops are displayed at the exterior of the cores; these structures are similar to the so-called flower-like micelles, which have been obtained by using triblock polymers. − As the polymer volume fraction increases, the instability of these aggregates (due to the very short A blocks unable to cover the whole solvophobic domain) leads to the fusion of aggregates, a process that is limited by the diffusion of the aggregates. The fusion of these spherical aggregates gives place to flat disk-like micelles, which have been reported to be an intermediate structure between the spherical micelles and vesicles. , …”