posted on 2021-09-30, 21:30authored byBehrad Kangarlou, Rasika Dahanayake, Ian J. Martin, Dennis Ndaya, Chun-Ming Wu, Rajeswari M. Kasi, Elena E. Dormidontova, Mu-Ping Nieh
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 flower-like 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.