Polyelectrolyte-based soft materials have been engineered for applications across food science, consumer products, healthcare, and oil recovery. This thesis explores the formation and control of microstructure in two important subclasses of these materials: (1) coacervates (Chapters 2-4), which are sticky Rouse liquids formed by associative phase separation in polyelectrolyte solutions; and (2) chemically-crosslinked hydrogels (Chapter 5), which are permanent, water-swelled networks that form during polymerization.
Coacervates are chiefly understood at equilibrium, but their functionality often depends their non-equilibrium behavior. Herein, we investigate the formation and destabilization of non-equilibrium coacervate structures. We examine the influence of mixing flows on coacervate coarsening in a model chemistry of poly(acrylic acid sodium salt) (PAA) and poly(allylamine hydrochloride) (PAH). Leveraging a combination of microfluidics, microscopy, and image analysis techniques, we find that flow conditions can induce formation of microscale aggregates or “precipitates,” and discover and characterize their interfacial relaxation. Our observations reveal that non-equilibrium structural control in coacervates can be achieved orthogonally to chemical structure design.
Inspired by this concept, we demonstrate spontaneous formation of core-shell droplets in the PAA-PAH system—i.e., with a single coacervate—through careful control of the mixing approach. These all-water, core-shell droplets remain metastable without surfactant. Using image classification machine learning, we discover which compositions, PAA:PAH ratios, and flow conditions favor core-shell formation over generic droplets. Our proposed mechanisms for core-shell formation and stabilization are distinctive within the broader context of multiple emulsions.
Finally, we develop a phase field polymer dynamics model for coacervates. We use it to offer insights into coacervate spinodal decomposition, which has proved challenging to explore experimentally due to the fast dynamics. Taken in concert, our works on coacervates establish the fundamental principles necessary to predict and control their microstructure formation beyond equilibrium.
The final chapter addresses one overarching challenge in hydrogel design: the tradeoffs between optimizing for different performance properties. We explain simultaneous superlubricity and mechanical robustness in a polyacrylamide hydrogel by using a reaction-diffusion model to predict the surface microlayer’s structure. The model is a promising tool for designing gel surface properties, and could readily be modified to other chemistries.
