QUASI-ELASTIC LIGHT-SCATTERING STUDY ON SOLUTIONS OF LINEAR FLEXIBLE POLYELECTROLYTES AT LOW IONIC STRENGTHS

Dynamic light scattering measurements have been performed on aqueous s olutions of linear poly(ethyleneimine), poly(2-vinylpyridine), and sod ium poly(styrenesulfonate), all flexible polyelectrolytes, at several polymer concentrations above the overlap concentration c and over a w ide NaCl concentration range. A change in the shape of the intensity c orrelation functions (ICFs) of the scattered light from a monomodal to a bimodal decay is observed with an increase of the polyelectrolyte c oncentration and a decrease of the NaCl concentration. The decay rates of the monomodal ICFs and the fast decay rates of the bimodal ICFs ar e related and can be explained by the cooperative diffusion in a trans ient polyelectrolyte network. The slow mode of the bimodal decay can b e removed by filtration, disappears for some solutions spontaneously a fter a given time, and, for solutions of linear poly(ethyleneimine), e xhibits hysteresis depending on the way the NaCl concentration has bee n established. It is concluded that the slow mode is related to cluste rs of polyelectrolyte chains which are apparently formed in solutions with a high polyelectrolyte concentration and low concentrations of ad ded NaCl.