Correlation length of hydrophobic polyelectrolyte solu - tions

– The combination of two techniques (Small Angle X-ray Scattering and Atomic Force Microscopy) has allowed us to measure in reciprocal and real space the correlation length ξ of salt-free aqueous solutions of highly charged hydrophobic polyelectrolyte as a function of the polymer concentration Cp, charge fraction f and chain length N . Contrary to the classical behaviour of hydrophilic polyelectrolytes in the strong coupling limit, ξ is strongly dependent on f . In particular a continuous transition has been observed from ξ ∼ C −1/2 p to ξ ∼ C −1/3 p when f decreased from 100% to 35%. We interpret this unusual behaviour as the consequence of the two features characterising the hydrophobic polyelectrolytes: the pearl necklace conformation of the chains and the anomalously strong reduction of the effective charge fraction. Introduction. – Polyelectrolytes are polymers containing ionisable groups which, in a polar solvent like water, dissociate into charges tied to the polymer backbone and counter-ions spread in the solution. Polyelectrolytes are called hydrophobic when water is a poor solvent for the backbone. Their single chain properties (conformation and effective charge density) are very different from those of hydrophilic polyelectrolytes. Theories [1,2,3], simulations [4,5] and experiments [6, 7] are all consistent with a pearl-necklace description of the single chain where low dielectric constant nanoregions (the pearls) are connected by narrow elongated strings. Another important feature is the charge renormalisation. By osmometry Essafi et al. measured the effective charge fraction feff as a function of the chemical charge fraction f for partially sulfonated polystyrene (PSS) [8]. In a range of charge fractions where hydrophilic polyelectrolytes followed rather well the so-called Manning-Oosawa condensation theory [9,10], they found for PSS a further strong reduction of feff [11]. What happens for a large number of interacting chains in solution is still a largely open question. Indeed in the framework of the theoretical pearl-necklace model two semi-dilute (∗) Present address: Laboratoire de Physique des Solides, Université Paris Sud, Orsay, France (∗∗) E-mail: [email protected]