Comprehensive Dynamics in a Polyelectrolyte Complex Coacervate

The linear viscoelastic response, LVR, of a hydrated polyelectrolyte complex coacervate, PEC, was evaluated over a range of frequencies, temperatures, and salt concentrations. The PEC was a nearly stoichiometric blend of a quaternary ammonium poly([3-(methacrylamido)propyl]trimethylammonium chloride), PMAPTAC, and poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt), PAMPS, an aliphatic sulfonate, selected because they remain fully charged over the conditions of use. Narrow molecular weight distribution polyelectrolytes were prepared using fractionation techniques. A partially deuterated version of PMAPTAC was incorporated to determine its coil radius of gyration, R_g, within PECs using small-angle neutron scattering. Chain dimensions were determined to be Gaussian with a Kuhn length of 2.37 nm, which remained constant from 25 to 65 °C. The LVR for a series of matched molecular weight PECs, mostly above the entanglement threshold, exhibited crossovers of modulus versus frequency classically attributed to the reptation time, relaxation between entanglements, and the relaxation of a Kuhn length of units (the “monomer” time). The scaling for zero shear viscosity, η₀, versus chain length, N, was η₀ ∼ N^3.1, in agreement with “sticky reptation” theory. The lifetime and activation energy, E_p, of a pair between polyanion and polycation repeat units, Pol⁺Pol^-, were determined from diffusion coefficients of salt ions within the PEC. The activation energy for LVR of salt-free PECs was 2E_p, showing that the key mechanism limiting the dynamics of undoped PECs is pair exchange. An FTIR technique was used to distinguish whether SCN^- acts as a counterion or a co-ion within PECs. Doping of PECs with NaSCN breaks Pol⁺Pol^- pairing efficiently, which decreases effective cross-linking and decreases viscosity. An equation was derived that quantitatively predicts this effect.