Polystyrene/poly(2-vinylpyridine) heteroarm star copolymer micelles in aqueous media and onion type micelles stabilized by diblock copolymers

Micellizatian of polystyrene/poly(2-vinylpyridine) heteroarm star copolymer s, PSnPVPn, in aqueous media has been studied by a combination of light sca ttering and fluorescence techniques. Three copolymer samples differing in n umbers of arms and their length form micelles with compact hydrophobic core s and polyelectrolyte poly(2-vinylpyridine) shells. The association number decreases with increasing number of water-soluble arms and their length. Mi cellization equilibrium is kinetically frozen in aqueous media. The micella r cores are frozen and the behavior of the micellar solutions is controlled by the polyelectrolyte behavior of the polyelectrolyte shell. In the case of the present heteroarm stars with ca. 10(1) arms, the soluble and insolub le arms are strongly segregated and the micellar cores are formed of pure p olystyrene as with micelles formed by linear diblock copolymers. As concern s the sample with several tens of arms, a significant steric hindrance, tog ether with a low association number (ca. 4), does not allow for a complete segregation of polystyrene and the poly(2-vinylpyridine) arms. The micellar cores formed by the very hairy star copolymer with several tens of arms co ntain a fraction of poly(2-vinylpyridine) arms as show by fluorescence meas urements with solubilized pyrene. Alkalimetric titration of an acidic mixtu re of PSnPVPn micelles with a linear poly(2-vinylpyridine)-block-poly(ethyl ene oxide) copolymer, PVP-PEO, yields fairly; monodisperse spherical onion- skin micelles. The onion-skin micelle has a compact spherical PS core deriv ing from the parent PS,PVP, micelle, a fairly thin and compact middle layer formed by PVP both from the parent micelle and PVP-PEO copolymer, and a pr otective PEO shell. The formation of onion-skin micelles is fully reversibl e and occurs suddenly and very rapidly at pH Values higher than 4.8. The "o nions" dissociate immediately into PSnPVPn micelles and PVP-PEO copolymer b elow pH 4.8. Evaluation of the alkalimetric titration data shows that only ca. 40% of the PVP units in the micellar shells are protonized (even in str ongly acidic solutions that contain a large surplus of a strong acid, e.g., in 0.1 M HCl). This observation is in agreement with indirect fluorometric data which indicate that in the case of st weak polyelectrolyte, such as p oly(2-vinylpyridine), the inner layer of the polyelectrolyte shell close to the core is not ionized.