The ionic strength dependence of the structure and dynamics of polyelectrolyte solutions as seen by light scattering: The slow mode dilemma

Abstract

The ionic strength dependence of the structure and dynamics of polyelectrolytesolutions was investigated by static and dynamic light scattering. Narrow molecular weight distribution sodium poly(styrenesulfonate) (NaPSS) standards in aqueous NaCl solutions were chosen as model systems. The study covers moderate polymer concentrations (∼0.5–50 g/L) where interparticle interactions rather than single‐polyion properties are dominant. Two diffusive modes characterized as fast and slow were detected in most cases. Scattering amplitudes of these modes were evaluated from light scattering data, which was not done in previous investigations. Amplitudes were normalized by the scattering of a benzene standard. Attention was paid mainly to the occurrence of the slow diffusive mode, which is interpreted as the dynamics of large multichain domains. Experimental results show that the occurrence of the slow mode is not connected with any kind of a sharp transition at some critical conditions. The amplitude of the slow mode (scattering intensity associated with the slow mode) changes smoothly as a function of the ratio of the polymer concentration to the solution ionic strength. At sufficiently high polymer concentrations, the slow mode can even persist until theta conditions are reached (4.17 M NaCl at 25 °C). It was shown that the absolutescattering intensity associated with the slow mode increases upon decreasing ionic strength, which is a direct proof that the structures responsible for the slow mode (domains) really form in solution and cannot be associated with the presence of some impurities, stable aggregates due to the polymer incomplete solubility, etc. Multiangle dynamic light scattering data show that dimensions of the domains increase upon decreasing ionic strength, too. The fast and slow diffusive modes are always widely separated on the time scale and the ionic strength dependence of diffusion coefficient cannot be characterized as a ‘‘splitting of diffusion coefficient.’’