Protonation-Induced Liquid–Liquid Phase Separation and Solid-Like Structural Transformation in Amine-Functional Polyelectrolytes

Natural polymers having charged groups (polyelectrolytes) form hierarchical structures on complexing with mono- or multivalent counterions. On the contrary, synthetic polyelectrolytes are generally known to undergo complexation with oppositely charged multivalent counter-(macro-)ions. Here, we demonstrate that the simple addition of monobasic acids to an aqueous solution of protonable amine-containing synthetic polyelectrolyte, polyethylenimine (PEI), induces microphase separation, much similar to the macrophase separation observed with the dibasic or tribasic acids. This results in structurally well-defined, metastable, liquid-like droplets of PEI hitherto not observed earlier. The phenomenon is primarily driven by the counterion condensation on charged amine groups and attractive interactions between hydrophobic moieties of the PEI chains. Remarkably, a pH/polarity gradient develops between the surface and the interior of these microdroplets. This leads to slow (∼ few days) sequential protonation (of the PEI chains) that propagates inward from the surface toward the interior of the droplets. The protonation front triggers a curvature change, leading to a morphological transition of the droplets to form anisotropic structures that self-assemble into micron-sized interconnected hollow fibers. Such a phenomenon is generic to other protonable-amine-functional polymers and depends on the valency of the counterion, which dictates the size of phase-separated structures. This study provides the first experimental evidence that even monovalent counterions can mediate complexation with like-charged polyelectrolytes, which drives their liquid–liquid phase separation and subsequent microstructural transformations into fibrils. This model system provides an opportunity for plausible integration of such spatiotemporal dynamics into the synthetic droplets designed to mimic biological systems.