Increasing Viscosity of a Rodlike Polyelectrolyte Solution with a Monovalent Salt

The behavior of a synthetic self-assembled rodlike polyelectrolyte is investigated both in salt-free solutions and in the presence of monovalent salt. The inherent rigidity and rodlike nature of poly­(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT) in salt-free aqueous solutions results in a nematic liquid-crystalline transition at a 1.0 wt % concentration. Specific viscosity η_sp of PBDT scales with concentration (c) as η_sp ∼ c^1.2 in semidilute salt-free solutions, which deviates from the behavior of most semidilute polyelectrolyte solutions that follow Fuoss law (η_sp ∼ c^0.5). Additionally, PBDT solutions containing 30 mM NaCl reveal a crossover concentration (c > c_x) and show up to a greater than 20-fold increase in η_sp compared with salt-free PBDT solutions at the same concentration. Oscillatory rheology experiments display a rise in modulus at all frequencies for PBDT solutions containing 10 mM NaCl and above, suggesting an increase in rod–rod association strength. X-ray scattering studies in salt-free semidilute PBDT solutions reveal polyelectrolyte correlation length (ξ) scaling as ε ∼ c^{–0.43±0.03}, which is a weaker dependence than typical scaling (ξ ∼ c^–1/2) due to the balance of electrostatic and associative intermolecular interactions. We propose that PBDT can serve as a synthetic model system for studying the solution behavior of associative rodlike polyelectrolytes.