Subphase pH Effect on Surface Micelle of Polystyrene-b-poly(2-vinylpyridine) Diblock Copolymers at the Air-Water Interface

The subphase pH effect on the surface micelles of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymers was investigated by the π−A isotherm and the morphology of the Langmuir−Blodgett (LB) film. Ionization of P2VP block significantly affects the π−A isotherm of the surface micelles at the air−water interface. At high pH, where the degree of ionization is low, the π−A isotherm shows a more expanded form and high transition surface pressure reflecting the strong tendency of P2VP blocks spreading on the subphase surface. As the subphase pH is lowered, the transition surface pressure decreases while the transition region is more extended, indicating the facile equilibrium between the flotation and submergence of the P2VP blocks upon surface pressure change. At pH 1.8 at which the 2VP units are completely ionized, the ionized P2VP blocks submerge into the subphase even at low surface pressure, and the transition behavior is not observed. The π−A isotherm behavior can be understood considering the balance of the solubility and the electrostatic repulsion of the ionized P2VP chains. Atomic force microscopy images of the LB films of the PS-b-P2VP surface micelles show isolated circular micelles at high pH. As the subphase pH decreases, the intermicellar distance becomes shorter and the micelles eventually contact each other to form a laced network of circular micelles. The association behavior of surface micelles and the π−A isotherms at low pH are strongly dependent on the ionic strength of the subphase. The linear association of the surface micelles at low pH appears to result from the balance of the hydrophobic attraction among the floated PS cores and the electrostatic repulsion among the submerged P2VP chains. On the other hand, the average aggregation number of each surface micelle is independent of subphase pH, which indicates that the aggregation of the block copolymers to form surface micelles is likely to take place before the P2VP blocks are ionized.