Counterion distribution in the coronal layer of polyelectrolyte diblock copolymer micelles

With a view to describe the structural arrangement of the tetramethylammonium (TMA(+)) counterions in simple salt-free aqueous poly(styrene-block-acrylic acid) [PS(20)-b-PA(85)] solutions, the partial structure factors pertaining 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 contrast matching in water. The copolymers self-assemble with an aggregation number similar to 100 into spherical micelles made of a PS-block core, surrounded 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 direction 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 the radius is very close to the one for the segments of the corona-forming blocks. Within a 10% error margin, all counterions are trapped in the coronal layer, and there is no indication for charge annealing effects and/or counterion migration toward the outer corona region.