Neutron scattering estimates of the effect of charge on the micelle structure in aqueous polyelectrolyte diblock copolymer solutions

The dimension of spherical micelles of the diblock copolymer poly(styrene-block-acrylic acid) [PS(20)-b-PA(85)] was investigated as a function of the degree of ionization of the polyelectrolyte in the coronal layer. To describe the structural arrangement of the blocks, 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 water. The PS blocks form a densely packed spherical core with a radius 4.5 nm and an aggregation number similar to 100; the core structure does not depend on the corona charge to a significant degree. The extension of the PA chains in the coronal layer, and, hence, the micelle radius, are found to be highly sensitive to the degree of ionization. At full ionization, the PA chains are almost fully stretched with a density scaling proportional to the inverse second power of the radius away from the core. For lower charge fraction, the results are interpreted in terms of scaling theory for star-branched polyelectrolytes, including the effects of charge annealing. The dimension of the micelles was found to be controlled by the balance of the elastic, conformational, stretching forces and the osmotic pressure exerted by the counterions trapped within the corona.