(59b) Structure and Dynamics of Polyelectrolyte Complexes in Solid-to-Liquid Phase Transition

When oppositely charged polyelectrolytes mix together in water, they tend to phase separate into a polymer-poor supernatant phase and a polymer-rich phase known as polyelectrolyte complexes (PECs), driven by the electrostatic attraction and the entropy gain from the release of counterions. Depending on the balance of water, polymer, and solution salinity, the physical states of PECs can span from glass-like solids to low viscosity liquids. Multiple studies have demonstrated that the transition between these two states can be readily achieved through changing the salt concentration. However, how PECs evolve during this process has been rarely studied. To fill this gap, we herein investigate the dynamics and structural evolution of PECs during this phase transition. We first employed aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization technique to develop a model PEC system, whose morphology would shift from glassy solid-like state to viscoelastic liquids by fine-tuning the solution salt concentration. Next, to unveil the structural evolution, we utilized small-angle X-ray scattering (SAXS) and optical microscopy to comprehensively study structural information at different length scales. We also probed the dynamics of PECs by rheology. Oscillatory shear rheology measurements was employed to examine the linear viscoelastic response and chain relaxation behaviors under the effect of salt. Salt in PECs play a role similar to temperature in polymer melts, allowing the access of relaxation behavior in a wide timescale through time-salt superposition. These findings reveal how PECs change between two distinct states, thereby enhancing our capabilities in predicting and controlling the properties of this intriguing polymeric material.