Salt-induced contraction of polyelectrolyte diblock copolymer micelles

To describe the effect of salt on the structural arrangement of the blocks in aqueous poly(styrene-bloch-acrylic acid) [PS(20)-b-PA(85)] solutions, th e partial structure factors pertaining to PS-PS and PA-PA density correlati ons, as well as the composition structure factor were obtained with small-a ngle neutron scattering and contrast matching in the water. The copolymers self-assemble with an aggregation number similar to 100 into spherical mice lles made of a PS-block core, surrounded by a coronal layer formed by the P A blocks. The addition of salt has no effect on the size of the core and th e aggregation number. At full corona charge and minimal screening condition s, the PA chains are almost fully stretched in the radial direction away fr om the core. With increasing salt concentration, the micelle contracts and the corona chain statistics can be described with a two-region density-scal ing model. In the inner coronal region the statistics is unaffected by the salt, whereas for larger radial distances the scaling is similar to neutral polymer stars. Both the micelle radii and the crossover distance between t he two different density-scaling regimes comply with theory for osmotic sta r-branched polyelectrolytes in the salt-dominated regime. For low fractiona l corona charge, the density scaling is determined by charge annealing effe cts. Here, the addition of salt does not affect the density scaling, but th e micelle nevertheless contracts and eventually precipitates at high ionic strength. Despite the high salt concentrations required to compete with the salinity in the coronal layer generated by the counterions coming from the polyelectrolyte blocks, the range in micelle dimension is similar to the o ne that can be covered by variation in pH.