Ultraviscosity In Entangled Polyelectrolyte Complexes And Coacervates

The spontaneous association of oppositely charged polyelectrolytes is an example of liquid-liquid phase separation. The resulting hydrated polyelectrolyte complexes or coacervates, both termed "PECs", display a wide range of viscosities. In addition to the usual dependence of viscosity on molecular weight and volume fraction expected for condensed neutral polymers, PECs also contain dense charge pairing between positive, Pol(+), and negative, Pol(-), repeat units. These "stickers" slow polymer chain dynamics on multiple length scales. Pol(+)Pol(-) charge pairs may be broken by the addition of salt to solutions contacting PECs, reducing viscosity ("saloplasticity"). Here, the dynamics of matched pairs of a polycation, poly(methacryloylaminopropyltrimethylammonium chloride), and polyanion, sodium poly(methacrylate), with molecular weights considerably above the entanglement concentration, were measured as a function of temperature and salt concentration. The dynamics of NaCl ions in PECs were also determined and correlated to the segmental relaxation times, which control viscosity. A suite of relaxation times corresponding to ion, monomer, Pol(+)Pol(-) pair exchange, entanglement, and reptation was determined or estimated. The zero-shear viscosity, eta(0), was found to be an unusually strong function of molecular weight, with the scaling eta(0) similar to M-5. A polymer coil size, measured by small-angle neutron scattering, was used in concert with new quantitative expressions to provide a good fit of theory to experiment for this unusual scaling.