title: Metastability of large aggregates and viscosity, and stability of the pearl necklace conformation after organic solvent treatment of aqueous hydrophobic polyelectrolyte solutions.
abstract: Aggregates-a phenomenon still not understood-as well as the pearl-necklace-like chain conformation in aqueous solutions of hydrophobic polyelectrolytes are addressed here, using treatment by an organic solvent. The second appear to be at equilibrium in water. The first appear to be metastable, and surprisingly associated with higher zero shear viscosity. The hydrophobic polyelectrolyte is poly(styrene-co-sodium styrenesulfonate) (PSS), and the solution treatment is to first add to water an organic solvent, THF, which is then evaporated and replaced by the same amount of water. To investigate polyelectrolyte solutions as a function of THF treatment, we use small angle neutron scattering in the semidilute regime, viscosimetry in the dilute and semidilute regimes (unentangled), and osmometry in the similar semidilute regime (the contribution of the counterions being dominant). First, the structure, namely, the scattering from all chains, is characterized by a maximum ("polyelectrolyte peak"). Its position, amplitude, and scattered intensity at zero angle depend, at a given sulfonation rate of PSS, on the solvent quality through the added amount of organic solvent (THF). This dependence is very pronounced when the sulfonation rate is low (more hydrophobic polyelectrolyte) and is canceled when the sulfonation rate is high (more hydrophilic polyelectrolyte). Second, the viscosity of the polyelectrolyte solutions decreases with THF treatment for the hydrophobic polyelectrolytes. Third, osmometry shows no noticeable increase of the effective charge with THF treatment. It is proposed that the large scale aggregates, especially in the case of very hydrophobic polyelectrolytes, disappear irreversibly with THF treatment, while the pearl-necklace conformation of the chain remains as in its initial state. Parallel test measurements for a fully hydrophilic polyelectrolyte, poly(sodium-2-acrylamido-2-methylpropanesulfonate)-co-(acrylamide) (P(AMAMPS)), at different sulfonation rates, show no evolution of the structure with THF treatment in the aqueous solution. The same behavior is highlighted by viscosimetry.
authors.affiliation: Institut National de Recherche et d'Analyse Physico-Chimique, Pôle Technologique de Sidi Thabet , 2020 Sidi Thabet, Tunisie.
authors.forename: Wafa
authors.initials: W
authors.lastname: Essafi
authors.forename: Wifek
authors.initials: W
authors.lastname: Raissi
authors.forename: Amira
authors.initials: A
authors.lastname: Abdelli
authors.forename: François
authors.initials: F
authors.lastname: Boué