ELECTROSTATIC AND SCREENING EFFECTS ON THE DYNAMIC ASPECTS OF POLYELECTROLYTE SOLUTIONS

We present a quasielastic light scattering (QELS) study of a synthetic polyelectrolyte compound: poly(paraphenylene vinylene) derivative dissolved in methanol. The dynamical aspects of these solutions are found to be strongly dependent on whether or not we have added an ordinary electrolyte (salt for instance). For salt-free solutions we measured two different, and a priori independent, diffusion coefficients, DC and DS, attributed, respectively, to cooperative and large scale motions. The small value of the hydrodynamical correlation length ξH, deduced from DC, reflects the large interpenetration of polyions in salt-free solutions. The chains are, consequently, highly expanded even though not rod-like in shape. This size, ξH, does not depend on the polymer concentration cp for the range scanned. The addition of even a small amount of sodium chloride progressively modifies the solution's behavior. First, we measured only one diffusion coefficient when the salt concentration c s exceeded a certain threshold. The effective diffusion coefficient measured for these solutions depends on both the ionic strength and the polymer concentration. The type of variation of Def versus cp and cs, is compared to previous work on either polyelectrolytes and ionomers materials or on micelles. The set of data are also qualitatively compared to theoretical considerations such as the charged spheres approach and to the flexible chain model with electrostatic contributions. © 1990 American Institute of Physics.