Hydrogen-bonding polymer complexation: Coacervation interfered with gelation

Complexation between polymers of different species in a common solvent is a fundamental process in various applications such as functional polymeric membrane and drug delivery, yet the understanding of energetics, phase behaviors, and kinetics of polymer complexation is rather challenging partially due to the multi-component nature of the system. In this work, we investigate the hydrogen-bonding complexation of poly(acrylic acid) (PAA) and poly(vinylpyrrolidone) (PVPON) in different solvents, and identify a variety of phase states, including homogenous solution, solid precipitate, liquid–liquid separation, gelation, and kinetically trapped structures. Among different solvents, N-methyl-2-pyrrolidone (NMP) has a similar structure to the repeating unit of PVPON and the complexation in NMP almost has no heat, suggesting the process is driven by entropy. The PAA/PVPON-NMP system exhibits an upper critical solution temperature; specifically, as the temperature decreases, a homogeneous solution first becomes opalescent, then undergoes a liquid–liquid separation, and finally undergoes a liquid–gel separation. On the other hand, when the polymer concentration is above a threshold, a homogenous solution turns into a gel directly without involving liquid–liquid separation with decreasing temperature. Accordingly, we construct a temperature–concentration phase diagram that shows how the gelation interplays with coacervation. This work provides a broad vision on phase behaviors of polymer complexation.