Fluorometric and Ultraviolet−Visible Absorption Study of Poly(methacrylic acid) Shells of High-Molar-Mass Block Copolymer Micelles

Polyelectrolyte behavior of the inner part of poly(methacrylic acid) shell and microscopic properties of the interface between the hydrophobic core and the hydrophilic shell in polystyrene-block-poly(methacrylic acid) micelles were studied by UV−vis spectroscopy and by the steady-state and time-resolved fluorometry using two fluorescent probes, 5-(N-octadecanoyl)aminofluorescein (OAF) and 5-(N-dodecanoyl)aminofluorescein (DAF). Two almost identical copolymer samples of polystyrene-block-poly(methacrylic acid), PS-PMA and PS-DPA-PMA, were used in the study, except that the latter was fluorescently tagged between blocks by a pending p-vinyl-9,10-diphenylanthracene group (DPA), which allows also for studying the OAF binding in the interfacial region by means of nonradiative energy transfer from DPA to fluorescein head-groups of either OAF or DAF. Pronounced shifts in pK a of both OAF and DAF, which were observed after binding to polymeric micelles, are explained by a competition of two effects. The first effect is caused by the association processes of amphiphilic probes in the bulk aqueous solution, and the second one is due to an additional energy that is needed for the proton dissociation from the fluorescein −OH group in the anionic polyelectrolyte shell and for the consequent proton transfer into the bulk (the latter is similar to that described by Fromherz et al.).