Abstract
Self-assembly of amphiphilic polymers into micelles is an archetypical example of a “self-confined” system due to the formation of micellar cores with dimensions of a few nanometers. In this work, we investigate the chain packing and resulting shape of Cn-PEOx micelles with semicrystalline cores using small/wide-angle X-ray scattering (SAXS/WAXS), contrast-variation small-angle neutron scattering (SANS), and nuclear magnetic resonance spectroscopy (NMR). Interestingly, the n-alkyl chains adopt a rotator-like conformation and pack into prolate ellipses (axial ratio ϵ ≈ 0.5) in the “crystalline” region and abruptly arrange into a more spheroidal shape (ϵ ≈ 0.7) above the melting point. We attribute the distorted spherical shape above the melting point to thermal fluctuations and intrinsic rigidity of the n-alkyl blocks. We also find evidence for a thin dehydrated PEO layer (≤1 nm) close to the micellar core. The results provide substantial insight into the interplay between crystallinity and molecular packing in confinement and the resulting overall micellar shape.
Highlights
The self-assembly of polymers and the resulting multitude of different nanostructures have found application in a range of fields.[1−3] Generally, association is driven by a reduction in the surface energy, in water notably by the hydrophobic effect.[4]other driving forces such as electrostatic interaction (“coacervation”)[5] or crystallization may be involved
We recently reported on the effect of core crystallization on the molecular exchange kinetics between Cn-PEOx micelles.[21,23]
The interfacial tension between PEO and alkanes is around 9−12 mN/m,76,77 which coincides much better with the interfacial tension between core and corona we found in the Gibbs−Thomson analysis
Summary
The self-assembly of polymers and the resulting multitude of different nanostructures have found application in a range of fields.[1−3] Generally, association is driven by a reduction in the surface energy, in water notably by the hydrophobic effect.[4]. Solid-state 13C NMR experiments were performed on a 50 vol % gel sample prepared from C28-hPEO5 in D2O. The relative width of the outer surface was fixed at σout = 0.1, based on previous studies.[15,19] All other model parameters are calculated in the following way: The molecular volume of an n-alkyl chain is calculated from its molecular weight and density, VM,Cn = MCn/dCn, which determines the core radius via NaggVM,Cn =. With the blob scattering added incoherently, the overall macroscopic scattering cross section is dΣ (Q ) dΩ φ Vmic
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