High concentration crossovers of polyelectrolyte solutions

In a few decades, several characteristic regimes and subregimes have been theoretically presented as functions of concentration C and contour length for flexible polyelectrolyte solutions. However, experimentally only the dilute and semidilute regimes have been well acknowledged. In this study, solutions of sodium poly(styrene sulfonate) are studied at various concentrations ranging from the semidilute regime to the bulk through the concentrated regime using small-angle and intermediate-angle x-ray scattering techniques. We have observed a single scattering peak being characteristic of polyelectrolyte solutions until C=4.1 mol/l. Above C=4.9 mol/l, such a characteristic peak disappears and instead, another new peak appears at higher q’s. In the plot of the maximum position qmax vs C, two crossover concentrations are found at C**=1.2 mol/l and C***=4.5 mol/l. The former is a continuous crossover from qmax∼C1/2 to qmax∼C1/4 which can be assigned to the crossover of the semidilute-to-concentrated regime, while the latter is a discontinuous crossover which may be assigned to that of the concentrated-to-swollen regime. In the swollen regime the characteristics of polyelectrolyte solutions are lost. The discontinuous crossover concentration C*** is about half the bulk concentration Cbulk=8.4 mol/l.