Polymerization-Induced Formation of Ordered Complex Coacervate Hydrogels

Complex coacervation is an electrostatically driven liquid–liquid phase separation process in which oppositely charged polyelectrolytes associate and release counterions to form water-rich polymer-dense domains. When charged block copolymers are used, this association can further generate ordered coacervate hydrogels with periodic nanostructures such as hexagonal, gyroid, and lamellar phases. These water-rich ordered materials are attractive as model protocellular compartments and ion-conductive soft materials. Nevertheless, most ordered coacervate hydrogels are prepared by mixing pre-synthesized polyelectrolytes. Such static mixing obscures a central aspect of complex coacervation: how a growing charged species transitions from a monomeric salt-like species to a polymeric counterion capable of driving phase separation and ordering.

Here, we address this question by using polymerization-induced self-assembly (PISA) to form ordered complex coacervate hydrogels during chain growth. A cationic ABA triblock copolymer, AOA80, was employed as a structured polyelectrolyte platform, while anionic P(AMPS) chains were generated from AMPS monomers by aqueous RAFT polymerization. To establish a reference pathway, AOA80 was first mixed with pre-synthesized P(AMPS) of controlled chain lengths, and static model mixtures representing intermediate conversion states were prepared by combining P(AMPS) with residual AMPS monomer. These experiments define the critical chain length and conversion at which AMPS-derived species begin to drive coacervate ordering. AMPS polymerization was then carried out directly in the presence of AOA80, and the structural evolution was monitored by in situ SAXS. This work aims to reveal how chain growth, charge balance, and assembly pathway govern the emergence of ordered complex coacervate hydrogels