A critical Assessment of Linear Viscoelasticity and Time-Temperature-Salt and Other Superpositions in Polyelectrolyte Coacervates

The validity of time-salt, time-pH, and time-hydration superpositions in the linear viscoelasticity of polyelectrolyte coacervates is assessed for a variety of polyelectrolytes. The comparison shows the frequent, but not universal, success of time-salt superposition, and master curves that are in many cases similar to those for neutral polymers, including Rouse and reptation theories. However, in some cases, solid-like, as opposed to fluid-like, response is observed at low frequencies, especially at low salt concentrations. Some coacervates seem to fit “sticky diffusion” theory reasonably well, wherein relaxation is controlled by the breakage rate of ion pairs; the dependence of the “sticker” lifetime on salt concentration has been explored but is not well understood as yet. It is also possible that local relaxation is not controlled by breakage of ion pairs, but by cooperative, “glassy,” relaxation of monomers, salt ions, and water molecules. A compilation and comparison of different data sets and suggested formulas for rheological time constants are presented, and some suggestions are given for future directions.