Arborescent polystyrene-graft-poly(2-vinylpyridine) copolymers as unimolecular micelles: Solubilization studies

Solubilization of various polycyclic aromatic hydrocarbon (PAH) probes by arborescent polystyrene-graft-poly(2-vinylpyridine) unimolecular micelles of different structures in aqueous solutions was investigated by UV and fluorescence spectroscopies. The micelle-water partition coefficient Kw and the solubilization capacity κG both increased with the polystyrene content of the copolymers for all probes, but the increase was less significant for the less hydrophobic probes. The Kw values obtained correlated well with the octanol-water partition coefficient and the boiling point of the more hydrophobic probe molecules, but no clear trend was observed for κG. The results obtained suggest that highly hydrophobic probes are solubilized mainly in the non-polar polystyrene core. A significant portion of the least hydrophobic probes resides in the ionic shell, while probes of intermediate polarity appear to remain in the palisade (interfacial) region. The branching functionality of the micelles played an important role in the solubilization process: for copolymers of similar composition, the solubilization capacity and rate were lower for higher generations, presumably due to the more rigid structure of the molecules. The results obtained show that arborescent polystyrene-graft-poly(2-vinylpyridine) copolymers can be designed to solubilize compounds of different polarities at controllable rates.