Salt-Induced Contraction of Polyelectrolyte Diblock Copolymer Micelles

Abstract

To describe the effect of salt on the structural arrangement of the blocks in aqueous poly(styrene-block-acrylic acid) [PS(20)-b-PA(85)] solutions, the partial structure factors pertaining to PS−PS and PA−PA density correlations, as well as the composition structure factor were obtained with small-angle neutron scattering and contrast matching in the water. The copolymers self-assemble with an aggregation number ∼100 into spherical micelles made of a PS-block core, surrounded by a coronal layer formed by the PA blocks. The addition of salt has no effect on the size of the core and the aggregation number. At full corona charge and minimal screening conditions, the PA chains are almost fully stretched in the radial direction away from the core. With increasing salt concentration, the micelle contracts and the corona chain statistics can be described with a two-region density-scaling 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 the two different density-scaling regimes comply with theory for osmotic star-branched polyelectrolytes in the salt-dominated regime. For low fractional corona charge, the density scaling is determined by charge annealing effects. Here, the addition of salt does not affect the density scaling, but the 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 one that can be covered by variation in pH.