Polyelectrolyte chain dimensions and concentration fluctuations near phase boundaries

We have measured the temperature (T) dependence of the correlation length (xi) for concentration fluctuations in aqueous solutions of sodium-poly(styrene sulfonate) with a fixed level of added barium chloride salt. Apparent critical behavior is observed upon lowering the temperature to precipitation phase boundaries that complements our earlier work on salt-dependent behavior. We interpret experimental deviations from xi(-2) versus T-1 as crossover from the mean field to the Ising universality class. We also measured the radius of gyration (R-g) of labeled chains and xi for semidilute polyelectrolyte solutions at low ionic strengths. We recovered the familiar result of xi scaling with polymer concentration (C-p) and degree of polymerization (N), such that xi=(73+/-9) (NCp-0.48+/-0.03)-C-0 [Angstrom], and using SANS high concentration labeling R-g=(400+/-28)C-p(-0.24+/-0.01) [Angstrom] (for N=577) and R-g=(2.8+/-2.1)N0.6+/-0.1 [Angstrom] (for C-p=206 gL(-1)), respectively. The indices recovered are in agreement with theoretical predictions for low ionic strength semidilute solutions. Such experiments offer insight into relatively unexplored phase behavior in charged macromolecular solutions. (C) 2003 American Institute of Physics.