Semidilute and concentrated solutions of a solvophobic polyelectrolyte in nonaqueous solvents

We study the behavior of a solvophobic polyelectrolyte in a series of polar organic solvents of various quality and polarity. Small-angle X-ray scattering is used to measure the semidilute correlation length ξ as a function of polymer concentration. In polar solvents of good quality ξ varies as c -1/2 in the whole concentration range, as expected. Contrastingly, in acetonitrile, a polar solvent of poor quality, the scaling exponent is found to change from -1/2 to -1/7, in qualitative agreement with the predictions of the pearl necklace model of Dobrynin and Rubinstein. This behavior is attributed to the crossover between string and bead-colloid controlled regimes of the isotropic transient network. At yet higher concentrations a gel-like phase is found attributed to bead interpenetration. It is thought to be driven by the small Debye-Huckel screening length at these concentrations, in agreement with recent molecular dynamics simulations of Micka et al. The crossover concentration for both string/bead-colloid and colloid/ gel regimes can be adjusted by mixing solvents of varying dielectric constant and quality. The effect of added of salt and temperature is also studied.