Dimensions of polyelectrolyte chains and concentration fluctuations in semidilute solutions of sodium-poly (styrene sulfonate) as measured by small-angle neutron scattering

We have measured the radius of gyration (R-g) of labeled polymer chains and the correlation length (xi) for semidilute solutions of sodium-poly(styrene sulfonate) using small-angle neutron scattering (SANS). This is the first measurement of both length scales as the system is advanced to an unstable phase boundary by adding barium chloride salt. The Rg decreases from 80 +/- 2 to 54 +/- 2 Angstrom as the salt concentration is increased. Simultaneously, xi is found to crossover from xi much less than R-g to xi much greater than R-g. This crossover signals the enhancement of concentration fluctuations. In addition to extracting these length scales a crossover from mean-field to Ising criticality is observed. This conclusion is made within the context of a mean-field model, which supports temperature and salt-induced phase separation. (C) 2001 Elsevier Science Ltd. All rights reserved.