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 measurements revealed that this PEC appeared to be a soft solid gel. However, with progressive addition of up to 2.0 M NaBr salt, it surprisingly stiffened from around 10(3) to 10(5) Pa in modulus. By thermogravimetric analysis, we found that this counterintuitive salt-stiffening behavior can be ascribed to dehydration of the complex phase indicative of osmotic deswelling. Using small-angle X-ray scattering and cryogenic transmission electron microscopy, we were able to depict the structural evolution of this system during the salt-stiffening and water loss behaviors. Locally, polyelectrolyte chains assembled into tightly coiled clusters of spherical aggregates that loosened and expanded upon salt doping. During this process, salt drove the expulsion of water out of the complex phase, resulting in greater polymer content in the denser complex. At approximately 2.0 M NaBr, the PEC began to transform into a viscoelastic liquid, and polyelectrolyte chains rearranged into more homogenous ladder-like structures at 2.5 M NaBr. 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.