Anomalous Small-Angle X-ray Scattering Study of Structure of Polymer Micelles Having Bromines in Hydrophobic Core

Anomalous small-angle X-ray scattering (ASAXS) near the absorption edge of bromine has been used to investigate the internal structure of polymer micelles comprising poly(4-bromostyrene)-block-poly(ethylene glycol)-block-poly(4-bromostyrene) (PBrS-b-PEG-b-PBrS) in aqueous solution. The ASAXS analysis revealed that the hydrophobic core composed of PBrS can be regarded as a homogeneous sphere with a radius of 10.2 nm and a decaying electron density at the surface. The aggregation number of micelle directly estimated on the basis of ASAXS analysis well agreed with that determined by field-flow fractionation coupled with multiangle light scattering, confirming the reliability of ASAXS analysis. The small-angle X-ray scattering (SAXS) profile from whole PBrS-b-PEG-b-PBrS micelle was further analyzed by using the result of ASAXS analysis. The micelles could be well described by a core-corona model in which the monomer concentration of PEG in the shell decreased as R-4.2/3, where R is the distance from the center of micelle. The index 4.2/3 is very close to that predicted by theory for star polymers (R-4/3), indicating that the PEG chains in the shell can be well described by the blob model in which chains are modeled as strings of blobs that extend radially. Thus, combination of ASAXS and conventional SAXS analyses permit to reveal precisely the internal structure and nature of the tethered chains of polymer micelles, and it could become a powerful tool for exploring internal structures of multiphase systems when they contain a probe atom for ASAXS.