The emergence of a low-frequency elastic plateau in the rheology of coacervates of oppositely charged polyelectrolytes

A unique low-frequency storage modulus plateau has sometimes been observed in the rheological measurements of polyelectrolyte complexes, depending on salt concentration, pH, and other conditions, suggesting a novel liquid-to-soft solid transition in these materials . While this transition is important in the applications of coacervates as membranes, adhesives, or drug delivery agents, the origin of the transition remains unexplained. We here report a systematic experimental and theoretical study of the transition, the former by using a series of coacervates formed by a polycation (poly diallyl dimethyl ammonium chloride), PDADMAC, with molecular weights of 100kDa, 200kDa, and 400kDa with a polyanion solution (poly styrene sulfonate), PSS, with molecular weights of 70k and 200k Dalton, and multiple sodium chloride concentrations below 3M. We find that the emergence of the plateau is sensitive to molecular weights of both polyions and to salt concentration. Brownian dynamics simulations are used to explore the source of this low-frequency plateau in terms of formations of local clusters or ionomer-like structures at a critical chain length and in at sufficiently low dielectric constant environment.