Solid-to-Liquid Phase Transition in Polyelectrolyte Complexes: Structural Evolution, Dynamics, and Phase Behavior

The physical states of polyelectrolyte complexes (PECs), formed by mixing together solutions of oppositely charged polyelectrolytes, can span from glassy solids to low viscosity liquids. Transformation between these two states can be readily achieved by altering salt concentration and temperature, but our understanding towards this process is still incomplete. To fill this gap, we here study a model PEC system comprising two controllably synthesized styrenic polyelectrolytes. We first employed rheology to determine phase and evaluate mechanical properties of this PEC under different conditions. Surprisingly, we detected a counterintuitive trend that in the solid regime, these complexes became stiffer as temperature and salt concentration increased. Above certain threshold, viscoelastic liquid responses appeared. Next, we used small-angle X-ray scattering and cryogenic electron microscopy to unveil the structural evolution of this PEC. Additionally, thermogravimetric analysis was adopted to quantitatively probe water composition and phase behavior. Together, structures, dynamics and phase behavior demonstrated excellent agreement and dictated an integrated mechanism for the physical responses of this PEC system during solid-to-liquid phase transition.