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 c, they display two maxima. The first, of abscissa q₁*, is related to a position order between star cores and scales as q₁* ∝ c^1/3. The second, of abscissa q₂*, is also observed in the scattering function of a semi-dilute solution of NaPSS linear polyelectrolytes. In the dilute regime (c < c*, non-overlapping stars), peak abscissa does not depend on concentration c 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 c threshold between these distinct c-dependencies of q₂* in the dilute and semi-dilute regimes is estimated as c*. Just as simple is the measurement of the geometrical radius R of the star obtained from the q₁* value at c* through the relation 2R = 2π/q₁*. By considering NaPSS stars of the same functionality with different degrees of polymerization per arm N_a, we find R scaling linearly with N_a, suggesting an elongated average conformation of the arms. This is in agreement with theoretical predictions and simulations. Meanwhile the value of q₂* measured in the dilute regime does not allow any inhomogeneous counterion distribution inside the stars to be revealed.