Solvent-Mediated Phase Transitions in Polyelectrolyte Complex Coacervates for Tunable Material Properties

In aqueous solutions, oppositely charged polyelectrolytes can undergo phase separation, forming either solid polyelectrolyte complexes (PECs) or liquid coacervates through electrostatic, hydrophobic, cation-π, or π-π interactions, depending on interaction strength. Solid PECs can be plasticized with salts, allowing extrusion into tubes, rods, or adhesive applications, known as saloplastics. Liquid coacervates, which retain 40–80% water, exhibit fluid-like properties, and their transition to a solid state would enable extrusion and molding, similar to saloplastics. However, evaporative drying is difficult to control and often results in brittle structures. Here, we explore an alternative approach using organic solvents to drive dehydration while preserving structural integrity. In this study, we formed liquid coacervates from two oppositely charged polyelectrolytes and introduced them to various organic solvents, including methanol, ethanol, and isopropyl alcohol (IPA). The coacervates underwent solvent-induced phase transitions, resulting in gel-like or glass-like states with significant volume reduction. This approach enabled direct molding of liquid coacervates into defined geometries, solidifying upon solvent exposure. To investigate the mechanisms underlying these transitions, we employed modulated Differential Scanning Calorimetry (DSC) to assess thermal properties and low-field Nuclear Magnetic Resonance (NMR) to probe molecular dynamics. Confocal microscopy and Transmission Electron Microscopy (TEM) provided structural insights into the microscale organization of the resulting phases. By systematically characterizing the compositional evolution of PEC coacervates under different solvent conditions, we demonstrate how solvent selection influences mechanical properties, offering a tunable platform for designing adaptive materials. Notably, water acts as a plasticizer, and reintroducing water to the solidified PECs restored the coacervate phase, highlighting the process's reversibility. This reversibility suggests potential applications in stimuli-responsive materials, self-healing polymers, and recyclable soft matter systems. These findings provide a novel perspective on solvent-mediated phase control in PEC coacervates, expanding their potential in materials science and engineering. By leveraging organic solvent interactions, we establish a pathway for fine-tuning the mechanical and structural properties of polyelectrolyte complexes, enabling advanced applications in biomedicine, coatings, adhesives, and sustainable materials.