Calcium induced shrinking of NaPA chains: A SANS investigation of single chain behavior

Calcium ions were used to induce a shrinking process with sodium polyacrylate coils in aqueous 0.01 M NaCl. By means of light scattering, four solutions with promising intermediates were selected for a detailed investigation by small-angle neutron scattering. All intermediates were half as large in size as the corresponding unperturbed dimensions but still had radii of gyration that were 45% larger than the hydrodynamic radius. The resulting SANS curves showed power laws typical for objects with a sharp boundary. They were compared with various model curves. From these models, a mixture of spheres and dumbbells resulted in theoretical curves that agreed best with experiments. The sphere size of single spheres and spheres in dumbbells was roughly 10 run. Two spheres on a dumbbell had an averaged distance larger by almost an order of magnitude. The results are considered to be compatible with pearl necklace-like transition states of shrinking polyelectrolyte chains.