Direct verification of the core-shell structure of shell cross-linked micelles in the solid state using X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is used to probe the nanostructure of shell cross-linked micelles in the solid state. A novel ABC triblock copolymer was synthesized via atom transfer radical polymerization (ATRP) comprising quaternized nitrogen atoms in the relatively hydrophilic A block, cross-linkable hydroxy groups in the middle B block, and neutral nitrogen atoms in the relatively hydrophobic C block. Two types of micelles were prepared: conventional micelles with the C block in the core in aqueous media and inverted micelles with the A block in the core in nonaqueous media. Efficient shell cross-linking of both types of micelles was achieved using divinyl sulfone (DVS), which reacts with the hydroxy groups in the central B block at room temperature. Although the XPS sampling depth of 2-10 nm is comparable to the dimensions of the micelle layers, and despite the probability of some micelle deformation in the absence of solvent, analysis of the high-resolution N 1s spectra provided the first direct evidence for the expected core-shell nature of SCL micelles in the solid state.