LIGHT-SCATTERING ON SEMIDILUTE POLYELECTROLYTE SOLUTIONS - IONIC-STRENGTH AND POLYELECTROLYTE CONCENTRATION-DEPENDENCE

Semidilute solutions of the flexible highly charged polyelectrolyte so dium poly(styrenesulfonate) have been studied with dynamic light scatt ering over a wide polyelectrolyte concentration range. To perform a sy stematic study of the slow mode filters with a relatively large pore s ize of 0.65 mu m were used. Double-exponential correlation functions w ere measured. At the lowest polyelectrolyte concentrations the correla tion functions became single exponential. The apparent fast and slow d iffusion coefficients were determined for three ionic strengths and on e molar mass. The fast diffusion coefficient depends on the polyelectr olyte concentration and on the ionic strength but becomes polyelectrol yte concentration independent at a certain high polyelectrolyte concen tration. This transition concentration depends on the ionic strength, In the semidilute regime the fast diffusion coefficient follows a powe r law, with a power law exponent that depends on the ionic strength. T he slow diffusion coefficient depends on the polyelectrolyte concentra tion and on the ionic strength over the whole concentration range stud ied. Transition concentrations in the static scattering intensity as a function of the polyelectrolyte concentration are also seen in the co ncentration dependence of the slow diffusion coefficient. The slow mod e is interpreted as caused by aggregates or domains that form in semid ilute and concentrated solutions. The dimensions of these aggregates, as determined from the angular dependence of the reciprocal normalized scattering intensity, decrease with decreasing polyelectrolyte concen tration and increasing ionic strength. Conventional light microscopy s hows directly the existence of aggregates.