Conformation of a chain polyelectrolyte in solution with low molecular weight salt: small angle neutron scattering measurements

Small angle neutron scattering measurements have been carried out on the tetramethylammonium salt of the polystyrenesulfonic acid withDP_w=310 and 1060 in water solution with tetramethylammonium chloride with ionic strength between 0.02 M and 1.0 M. The scattering curves in the scattering vector range 0.05 nm^–1Q1.8nm^–1 have been fitted using the form factor of a worm-like chain of finite thickness. The conformational parameters mean square radius of gyration, statistic chain element, mass per unit length and mean square radius of the cross-section have been determined experimentally and used for describing the conformation of the coils. By these molecular weights and ionic strengths, excluded volume is not necessary to explain the conformation changes depending on the salt content of the solutions; relatively short coil molecules can be described in their unperturbed dimensions even in a thermodynamically good solvent: a change in the stiffness of the chain according to Odijk's theory succeeds in describing the conformation of the polyions. Together with a slow decrease of the coil dimension by increasing salt content, a transition at ionic strength 0.1–0.5 M between two different conformations has been observed. The conformation at lower ionic strength is characterized by higher stiffness of the chain and lower mass per unit length than the form at higher salt concentration.