SANS from Salt-Free Aqueous Solutions of Hydrophilic and Highly Charged Star-Branched Polyelectrolytes.

Scattering functions of sodium sulfonated polystyrene (NaPSS) star-branched polyelectrolytes with high sulfonation degrees were measured from their salt-free aqueous solutions, using the Small Angle Neutron Scattering (SANS) technique. Whatever the concentration , they display two maxima. The first, of abscissa ₁*, is related to a position order between star cores and scales as ₁* ∝ . The second, of abscissa ₂*, is also observed in the scattering function of a semi-dilute solution of NaPSS linear polyelectrolytes. In the dilute regime ( < , non-overlapping stars), peak abscissa does not depend on concentration and is just an intramolecular characteristic associated with the electrostatic repulsion between arms of the same star. In the semi-dilute regime, due to the star interpenetration, the scattering function ⁻ through the peak position, reflects repulsion between arms of the same star or of different stars. The threshold between these distinct -dependencies of ₂ in the dilute and semi-dilute regimes is estimated as . Just as simple is the measurement of the geometrical radius of the star obtained from the ₁ value at * through the relation 2 = 2π/₁*. By considering NaPSS stars of the same functionality with different degrees of polymerization per arm , we find scaling linearly with , suggesting an elongated average conformation of the arms. This is in agreement with theoretical predictions and simulations. Meanwhile the value of ₂* measured in the dilute regime does not allow any inhomogeneous counterion distribution inside the stars to be revealed.