Solution rheology of a strongly charged polyelectrolyte in good solvent

Solution rheology of poly(2-vinylpyridine) and a random copolymer of 60% N-methyl-2-vinylpyridinium iodide and 40% 2-vinylpyridine in N-methylformamide (NMF) was studied over wide ranges of concentration. The fraction of monomers bearing an effective charge f = 0.31 of these copolymers was measured using dielectric spectroscopy. This low fraction of effectively charged monomers suggests that the polyion disrupts the hydrogen-bonding alignment of NMF that makes the dielectric constant of the pure solvent anomalously large. NMF is a good solvent for neutral poly(2-vinylpyridine) but has a significant quantity of ionic impurities even when freshly distilled. The specific viscosity and relaxation time of dilute and semidilute unentangled solutions exhibit the scaling with concentration expected by theory if c_s ≈ 1 mM residual salt is present, identified from the change in slope of viscosity-concentration power laws from 5/4 to 1/2 at concentration c = 2c _s/f= 9.5 × 10^-3 M (moles of monomer per liter). The specific viscosity data obey the Fuoss law in the semidilute unentangled regime for c > 2c_s/f, where there are more free counterions than residual salt ions. The correlation length measured by the peak in small-angle X-ray scattering agrees quantitatively with that obtained from the viscosity of semidilute unentangled solutions with c > 2c_s/f, using the scaling model.