The persistence length of sodium sulfonated polystyrene (PSSNa) is measured by small angle neutron scattering in the semidilute regime for two polymer concentrations $c$ as a function of the added salt concentration $c_s$. The results suggest that this persistence length $l_p$ depends only on ${c+2c}_s$. This questions the role of the counterion condensation at the nanometric scale. Moreover, a variation $l_p\sim ({c+2c}_s{)}^{-1/3\mathrm{}}$ is found. $l_p$ could be simply related to the distance between ions, which is the relevant length for concentrated solutions.

• Received 16 May 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.5218