Small-angle scattering study of perfluorosulfonated ionomer solutions

A structural study of perfluorosulfonated ionomer (PFSI) solutions in polar solvents such as alcohols, amides, and water using both small-angle X-ray and neutron scattering is presented. Contrast variation methods have shown that the ionomer molecule particles form dense particles of constant scattering length. The study of dilute solutions allows one to evidence the rodlike structure of PFSI solutions. The radius of the cylindrical particles is determined from both the shape and the intensities of the scattering curves. These results are confirmed by the analysis of the asymptotic behavior at large angles of the scattering curves. Typical values for the radii are found to be around 20 Angstrom. The diameter of the rod is shown to be related to the polymer-solvent interfacial energy, whereas the dielectric constant of the solvent acts only on the local order. Mixtures of solvents have been used to check the evolution of the structure with the solvents, The effect of the counterion is also studied. The effect of added salt on the small-angle scattering spectra can be interpreted as due to a decrease of the local order through the disappearance of the interference term and the shape of the particles is not modified.