(713f) Transforming Polyelectrolyte/Multivalent Counterion Network Properties through a Marriage of Ionotropic Gelation and Coacervation

Polyelectrolyte/multivalent counterion self-assembly enables the generation of functional soft materials and complex fluids, including hydrogels and complex coacervates that find use in diverse fields ranging from drug delivery to bioprocessing and underwater adhesion. Throughout their various applications, these gels and coacervates exhibit distinct properties. Ionically crosslinked gels hold their shape but tend to be water-rich and highly permeable to small molecules. The polyelectrolyte/multivalent ion coacervates, on the other hand, are polymer-rich and much less permeable to solute diffusion; being viscoelastic fluids, however, they ultimately flow from their application sites and generally do not maintain their shapes over extended timescales. Here, we show how, through a combination of ionotropic gelation and coacervation of common commercial polymers, attractive properties of gels and coacervates can be combined into a single material. To prepare such gel/coacervate composites, mixtures of alginate (which gels upon complexing divalent cations) and polyphosphate (PP; which forms complex coacervates when mixed with the same divalent ions) are exposed to calcium chloride solutions, resulting in simultaneous calcium/alginate gelation and calcium/PP coacervation. When the PP concentration in the parent alginate/PP solution is low, the coacervate remains colloidally dispersed within the calcium alginate gel and has little impact on its mechanical and solute permeability properties. At high PP concentrations, however, the coacervate phase becomes continuous, and upon its formation, collapses the gel network into a material that is much stiffer and less permeable to low-molecular-weight solutes. In other words, the gel/coacervate composites obtained at these higher PP concentrations hold their shapes (like traditional hydrogels) while providing the polymer-rich structures and low solute permeabilities that are characteristic of coacervate phases. When these hybrid ionic networks are prepared through dropwise addition of aqueous alginate/PP mixtures into calcium chloride solutions, they offer a facile route to the generation of capsules for the long-term sustained release of small molecules (whose release profiles are much more extended than those of regular calcium alginate gels and can be readily tuned by adjusting the polymer compositions). Finally, early results on how such gel/coacervate composites can be prepared from other polyelectrolyte/multivalent counterion systems will also be briefly discussed.