New insights on the solution behavior and self-assembly of polystyrene/poly(2-vinylpyridine) ‘hairy’ heteroarm star copolymers with highly asymmetric arms in polar organic and aqueous media

The solution behavior and self-assembly of a ‘hairy’ heteroarm star copolymer polystyrene/poly(2-vinylpyridine) containing 20 fairly short polystyrene and 20 long poly(2-vinylpyridine) arms was studied in 1,4-dioxane–methanol mixtures and in acidic aqueous solutions. The copolymer forms reversible micelles in 1,4-dioxane–methanol mixtures. Since the conformation of the unimolecular heteroarm star (unimer) with collapsed insoluble PS arms and stretched soluble PVP arms resembles the spherical core/shell micelle, the solubility of the copolymer in the selective solvent is fairly high and the multimolecular micelles are formed by only few heteroarm stars. Both heteroarm stars (in pure 1,4-dioxane) and micellar systems (composed of both micelles and unimolecular heteroarm stars (unimers) in 1,4-dioxane–methanol mixtures) may be transferred in acidic aqueous media by dialysis. Polyelectrolyte behavior of unimer stars was studied in detail in acidic aqueous media. Static and dynamic light scattering measurements show that heteroarm stars interact electrostatically over long distances in low ionic strength solutions. Experimental data indicate that the charged stars may form temporary shell interacting (electrostatically stabilized) aggregates at elevated concentrations.