Block and gradient copoly(2-oxazoline) micelles: striking different on the inside

Herein, we provide a direct proof for differences in the micellar structure of amphiphilic diblock and gradient copolymers, thereby unambiguously demonstrating the influence of monomer distribution along the polymer chains on the micellization behavior[1]. The internal structure of amphiphilic block and gradient co poly(2-oxazolines) based on the hydrophilic poly(2-methyl-2-oxazoline) (PMeOx) and the hydrophobic poly(2-phenyl-2-oxazoline) (PPhOx) was studied in water and water-ethanol mixtures by Small-Angle X-ray Scattering (SAXS), Small-Angle Neutron Scattering (SANS), Static and Dynamic Light Scattering (SLS/DLS), and ¹H NMR spectroscopy. Contrast matching small angle neutron scattering (SANS) experiments revealed that block copolymers form micelles with a uniform density profile of the core. In contrast to popular assumption, the outer part of the core of the gradient copolymer micelles has a distinctly higher density than the middle of the core. We attribute the latter finding to back-folding of chains resulting from hydrophilic-hydrophobic interactions, leading to a new type of micelles that we refer to as micelles with a “bitterball-core” structure.
References
[1] S.K. Filippov,et.al. J. Phys. Chem. Let. 2017, 8, 3800-3804