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 charged 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 chain conformation of the single chain is directly measured by the zero-averaged 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, and l(p) roughly varies as I-(1/3), I being the ionic strength calculated without 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 if multivalent ions are added, the chain appears "thicker", and I is not the only parameter that controls the conformation. The phenomena are qualitatively identical for both Ca2+ and La3+ although a phase separation occurs for 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.