Long-time stability of multimacroion domains in polyelectrolyte solutions

Long-time stability of multimacroion domains in polyelectrolyte solutions was investigated over time intervals ranging up to 37 months. Periodic static and dynamic light scattering measurements were performed on salt-free solutions of sodium poly(styrenesulfonate) and ionized samples of poly(acrylic acid) and poly(methacrylic acid). Extremely slow kinetic changes were observed. While the size of domains did not change over time, a slow decrease of the domain density (the difference between polyion concentrations inside and outside the domain) was observed. This decrease was reflected in the decrease of domain scattering, the increase of scattering by "free" polyions due to their release from domains and in the increase of domain diffusion coefficient due to increased drainability. The shape of time dependencies was always such that a stronger dependence was observed in the beginning with leveling off at longer times. Quantitatively, however, relatively large differences were observed from sample to sample. This was interpreted as due to different initial conditions given by different arrangement of polyions in domains according to the bulk structure and solution preparation. Results show that domains cannot be characterized as equilibrium dynamic fluctuations, but rather as long-lived associates of macroions. Performed scattering experiments on domains enabled for the first time to discriminate between tightly packed structures and loose structures with solvent inside. The infinite (or at least extremely long) life-times of loose domains suggest thermodynamic attraction between polyions. (C) 2002 American Institute of Physics.