Association of oppositely charged polyelectrolytes via electrostatic interaction can be used as a route to design and construction of polyelectrolyte materials with desired properties and performances. Depending on the conditions for complexation, this association can result in either soluble complex formation, or phase separation including liquid-liquid (complex coacervation) and liquid-solid (precipitation) separation. Complex coacervation initially leads to the formation of dispersed polymer-rich coacervate droplets, which can further coalesce to generate two macroscopic distinct phases, namely supernatant phase and coacervate phase.
This work mainly focuses on the design and construction of complex coacervates to achieve desired properties and performances based on the understanding of the phase behavior of polyelectrolyte systems and the mechanisms beyond the functionalities of the coacervate materials. To demonstrate the importance of selection of polyelectrolyte pairs to the phase behavior of polyelectrolyte systems and the properties of complex coacervates, polycations including branched polyethyleneimine (BPEI), poly(allylamine hydrochloride) (PAH) and poly(diallyldimethylammonium chloride) (PDAC), as well as polyanions including polyacrylic acid (PAA), poly(4-styrenesulfonic acid) (SPS), and polyvinyl sulfonate (PVS), of varying charge density, molecular weight and functional groups were utilized. Moreover, the impact of complexation conditions including varying ionic strength, solvent polarity and mixing ratios on the phase behavior and coacervate properties was also systematically investigated.
The efficiency and selectivity of solute uptake was compared for the coacervates composed of different pairs of polyelectrolytes formed at different conditions, suggesting the importance of intermolecular interactions on the solute encapsulation capability of coacervates. Specifically, for the sequestration of small molecules, the presence of short-range intermolecular interactions between the polymer and the solute, including π – π stacking, cation – π interaction, hydrogen bonding force, enables high sequestration efficiency into the complex coacervate phase, while with long-distance electrostatic interaction along, the encapsulation efficiency of the solute is limited. An increase in hydrophobicity of the environment within the coacervate droplets can promote the sequestration efficiency of small organic molecules which favors hydrophobic environment. Moreover, this work also investigated the encapsulation of proteins such as bovine serum albumin (BSA) and alkaline phosphatase (ALP) via complex coacervation as a function of mixing sequence, mixing ratio, and overall polyelectrolyte concentration, providing insights for the design of complex coacervation process to enable tunable encapsulation efficiency of proteins. Circular dichroism spectra of the protein encapsulated coacervate samples confirmed the preservation of the secondary structure of BSA during the complex coacervation process.
Rheological studies of complex coacervates composed of different polyelectrolyte pairs were used to demonstrate the impact of presence of small organic molecules, change in ionic strength, and selection of polyelectrolyte pairs on the rheological response of coacervates. The presence of methylene blue (MB) and bromothymol blue (BtB) in the coacervate phase can efficiently impair the ionic bonds between PDAC and SPS, due to the displacement of the intrinsic ionic pairs and further steric hinderance induced by the dyes, resulting in a decrease in the overall magnitude of storage ("G'" ) and loss ("G''" ) moduli and an increase in the loss tangent ("tanδ" ). Moreover, an increase in the ionic strength leads to a decrease in the overall magnitude of storage ("G'" ) and loss ("G''" ) moduli and a transition from solid-like behavior to liquid-like behavior, because the screening effect on the charges weakens ionic crosslinks between polyelectrolyte chains within the coacervates. The viscoelastic response of the coacervates is strongly dependent on the type of polyelectrolytes, where coacervates formed with strong polyelectrolytes exhibit more solid-like behavior than the coacervates formed with weak polyelectrolytes.
Application of coacervates, including wastewater purification, efficient drug loading and controlled drug release, protection of active components, and membrane-free microreactors are discussed. Complex coacervates composed of weak polyelectrolytes can serve as an efficient protective system of encapsulated proteins against the denaturation induced by extreme low or high pH, high temperature and exposure to urea, as well as against heavy metal contamination, providing new insights and methods to issues of maintaining stability and activity of proteins. Complex coacervates can also serve as membrane-free synthetic microreactors for immobilization and stabilization of enzymes with enhanced enzymatic activity.