Influence of temperature, salt and molecular weight on the dynamics of polyelectrolyte complexes.

Oppositely-charged polyelectrolytes can spontaneously associate into either solid-like complexes or liquid-like coacervates based on the external salt concentration. The linear viscoelasticity of polyelectrolyte complexes/coacervates (PEC) can be influenced by a variety of factors, such as temperature, salt and pH. Due to the insufficient chain length of polyelectrolyte, few works have reported the entanglement behavior of PEC and only reptation time (τ_rep) has been found. Here, we prepared five pairs of PEC with different molecular weight and matched polycation/polyanion chain length. Rheology experiments were carried out for all PEC pairs at different temperatures and salt concentrations. Time-temperature (TTS) and time-temperature-salt (TTSS) superpositions were achieved with good fit to sticky association theory. Relaxation times for polymer partnering (τ_b), entanglement (τ_e) and reptation (τ_rep) were revealed directly from the TTS data. All these characteristic lifetimes were slowed by the sticky dynamics of Pol⁺Pol^- pairs. We found that the relaxation kinetics of temperature showed Arrhenius dependence, whereas changing the salt concentration impacted the lifetime of Pol⁺Pol^- pairs, the number of stickers per chain and the volume fraction of polymer.