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

Abstract

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