Solid-to-Liquid Phase Transition in Polyelectrolyte Complexes

Strongly interacting polyelectrolyte complexes (PECs) are known to form solid precipitates that can transform into liquid droplets upon the addition of salt to break intrinsic ionic associations. However, the origin of this phase transition and the molecular details of what constitutes a solid complex remain poorly understood. Here, we study comprehensively the salt-driven solid-to-liquid phase transition of a model symmetric PEC system formed by two styrenic polyelectrolytes, from the perspectives of dynamics, phase behavior, and internal structures. In the salt-free state, rheological and thermogravimetric measurements revealed that this PEC appeared to be a soft solid comprising 90 % water by weight. However, with progressive addition of up to 2.0 M NaBr salt, it surprisingly stiffened from around 103 to 105 Pa in modulus and expelled water into the supernatant. Using small-angle X-ray scattering and cryogenic transmission electron microscopy, we determined that the counterintuitive salt- stiffening and water loss behaviors can be ascribed to the structural evolution of polyelectrolyte chains in the complex phase, in which salt doping loosened tightly coiled and highly solvated clusters of spherical aggregates and exposed otherwise-hidden hydrophobic domains. At an approximate 2.5 M NaBr threshold, the PECs transformed into a viscoelastic liquid, and polyelectrolyte chains rearranged into more homogenous ladder-like structures. In the liquid regime, further salt addition enabled faster chain relaxation and slightly softened the materials. The breadth of material properties accessed in this versatile, charge-driven system gives new predictive insights into how to harness better ionic and chemical attributes toward physical performance in functional PEC materials.