STRUCTURE AND DYNAMICS OF LINEAR FLEXIBLE POLYELECTROLYTES IN SALT-FREE SOLUTION AS SEEN BY LIGHT-SCATTERING

Two sets of light scattering experiments were performed on salt-free aqueous solutions of sodium poly(styrene sulfonate) (NaPSS): a molecular weight dependence study and a concentration dependence study (1, 2). In the first experiments, thirteen NaPSS samples of different molecular weight, ranging from M(w) = 5 000 to 1 200 000, were studied c = 10 - 45.6 g/L. Two diffusion coefficients, D-f (fast) and D-s (slow), were observed for all systems. The fast diffusive mode is a coupled polyion-counterion diffusion that is dominated by counterion dynamics. D-f is found to be independent of molecular weight. The slow diffusive mode reflects the dynamics of large, multi-chain domains in solution. In contrast to the fast mode, D-s is molecular weight dependent as D-s similar to c(-nu), where nu similar to M(w)(mu). This molecular weight dependence contradicts the isotropic models of de Gennes and Odijk (3, 4). Apparent domain dimensions are calculated from angular dependencies of D-s and the scattered light intensity. Domain size is molecular weight independent. In the second study, the concentration dependence of polyelectrolyte solution behavior was examined for three NaPSS samples (M(w), = 5 000, 100 000 and 1 200 000) from c = 0.01 - 45.6 g/L. A critical concentration c(cr) congruent to 0.5 g/L was established and found to be independent of M(w). Above c(cr) a strong polyion-counterion coupling exists and domains form in solution. D-f is independent of concentration, whereas D-s increases upon dilution. The excess intensity of scattered light (I) exhibits a power dependency on concentration with an exponent slightly greater than 1. The angular dependence of the normalized reciprocal scattering intensity (I(0)/I(theta)) weakens as the domain size diminishes with dilution. At c = c(cr) the angular dependence of ceases. Below c(cr) repulsive Coulombic interactions dominate solution structure. D-f decreases from high values and finally merges with D-s. I scales with concentration to a power much less than 1, and (I(0)/I(theta)) decreases with angle to form a broad minimum.