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 cs ≈ 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 = 2cs/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 > 2cs/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 > 2cs/f, using the scaling model.