Conformation of poly(styrenesulfonate) polyions in the presence of multivalent ions: Small-angle neutron scattering experiments

Small-angle neutron scattering (SANS) experiments are used to study fully c harged poly (styrenesulfonate) (PSS) polyions in the semidilute regime, in the presence of multivalent cations. PSSNa solutions with added Na+, Ca2+, or La3+ ions and PSSCa solutions with added Ca2+ ions are studied. The chai n conformation of the single chain is directly measured by the zero-average d contrast method. Fitting the scattered intensity with the wormlike chain model leads to the determination of the weight per unit length 1/alpha and of the persistence length l(p). The results can be separated in two regimes . For low ionic strength in PSSNa solutions, the chain remains wormlike, an d l(p) roughly varies as I-(1/3), I being the ionic strength calculated wit hout counterion condensation. The conformation seems to be ruled mostly by I for both multivalent and monovalent cations. For higher ionic strength in PSSNa solutions and for PSSCa solutions, the chain is no longer wormlike i f multivalent ions are added, the chain appears "thicker", and I is not the only parameter that controls the conformation. The phenomena are qualitati vely identical for both Ca2+ and La3+ although a phase separation occurs fo r La3+ contrary to Ca2+. The conformation changes may result from bridging phenomena between monomers or from counterions condensation, which leads to a more compact chain.