Kinetics of phase separation and mesoscale structure formation in polyelectrolyte complex coacervates

The thermodynamics of liquid-liquid phase separation in oppositely-charged polyelectrolyte complex solutions has attracted interest in recent years, where quantitative phase diagrams are comparable to theory. However, the kinetics of phase separation in such systems and the mechanism of growth of mesoscale structures remains largely unexplored, yet critical to potential applications, such as membranes. We reported small-angle light scattering (SALS) experimental results on a model system where spinodal decomposition [1] was observed when the system was quenched into the unstable region on a known temperature-composition phase diagram [2,3]. Using a new SALS setup and optical microscopy, we quantify the kinetics of phase separation in a well-defined sodium poly(acrylic acid)/quaternary poly(N,N-dimethylamimoethyl methacrylate chloride) system. The growth rate of domain sizes, coarsening and late-stage kinetics are observed and differ from conventional neutral polymer systems. Without a guiding theory, we compare the observed mechanisms with other water-soluble systems, and discuss the connection of phase growth rate and coarsening mechanisms to diffusion behavior, with focus on charged polymers in the context of associative phase separation.

​​​​​​[1] S. Ali, Y. Mao, and V.M. Prabhu, Rev. Sci. Instrum. 93, 044104 (2022)

[2] S. Ali, M. Bleuel, and V.M. Prabhu, ACS Macro Lett. 8, 289 (2019).

[3] Y. Ma, S. Ali, and V.M. Prabhu, Macromolecules 54, 11338 (2021).