Counterion distribution in the coronal layer of polyelectrolyte diblock copolymer micelles

With a view to describe the structural arrangement of the tetramethylammoni um (TMA(+)) counterions in simple salt-free aqueous poly(styrene-block-acry lic acid) [PS(20)-b-PA(85)] solutions, the partial structure factors pertai ning to PS-PS, PS-TMA, and TMA-TMA density correlations were obtained with small-angle neutron scattering and isotopic labeling of the counterion. The contributions to the scattering related to the PA-blocks are blanked by co ntrast matching in water. The copolymers self-assemble with an aggregation number similar to 100 into spherical micelles made of a PS-block core, surr ounded by a coronal layer formed by the PA-blocks. At the present level of ionization (50%), the PA chains are almost fully stretched in the radial di rection away from the core. A comparison of the counterion involved partial structure factors with the previously reported relevant structure factors pertaining to the PA-blocks shows that the counterion distribution along th e radius is very close to the one for the segments of the corona-forming bl ocks. Within a 10% error margin, all counterions are trapped in the coronal layer, and there is no indication for charge annealing effects and/or coun terion migration toward the outer corona region.