The objectives of this dissertation are to gain a fundamental understanding of the assembly and properties of polyelectrolyte complex (PEC) hydrogels, harness the self-assembly of PEC hydrogels to mitigate the shortcomings of existing photocrosslinkable materials, and create PEC/covalent interpenetrating polymer network (IPN) hydrogels with precisely tuned microstructure and material properties. PEC hydrogels self-assemble swiftly upon mixing oppositely charged triblock polyelectrolytes and feature tunable mechanical properties, microstructural diversity, as well as self-healing attributes and responsiveness to salt and pH changes in their environment. Moreover, the nanoscale PEC domains that constitute the three-dimensional (3D) network in PEC hydrogels spontaneously encapsulate charged macromolecules (e.g., protein, drug, and nucleic acids). However, a few drawbacks limit their widespread biomedical applications. The electrostatically assembled 3D network results in their moderate shear strength, poor tensile strength, and uncontrolled swelling. We demonstrate that interpenetration of the PEC network with a covalently-linked network not only addressed the limitations of the PEC hydrogels but also contribute to synergistic improvements in the mechanical performance of the resulting IPN hydrogels. The unique attributes of PEC hydrogels – swift self-assembly in aqueous surroundings, rapid moduli recovery upon cessation of flow, and interim insolubility in water – were further harnessed to employ them as scaffoldings for photocrosslinkable materials. PEC hydrogels were compatible with four representative precursors (linear and 4-arm poly(ethylene glycol) acrylate, acrylamide, and gelatin methacryloyl (GelMA)) and their corresponding networks, which featured different molecular weights, polymer origins, crosslinking mechanisms, and molecular structures. Mixing of oppositely charged bPEs with photocrosslinkable precursors resulted in precursor-encapsulating PEC hydrogels (PEC+precursor hydrogels) that exhibited significantly higher viscosity and shear strength as compared to precursors solutions. The PEC+precursor hydrogels did not suffer from issues such as dilution, precursor deactivation, and unwanted flows that affect in situ crosslinking of photocrosslinkable hydrogels in wet environments. Moreover, the PEC/precursor IPN hydrogels produced by in situ crosslinking of the PEC+precursor hydrogels exhibited improved shear and tensile properties. Consequently, PEC+GelMA hydrogels were demonstrated as robust bioinks for extrusion-based 3D bioprinting at physiological temperatures (37 �C). The PEC+GelMA inks avoided undesirable secondary flow and produced a higher printing resolution, enabling printing of intricate multilayered constructs.

This dissertation aims to elucidate the behaviors of interfacially active molecules that self-assemble at fluid-fluid interfaces and stabilize them. Examples of such molecules include amphiphilic surfactant molecules that comprise hydrophobic (oil-loving) and hydrophilic (water-loving) moieties and therefore have a natural affinity for oil-water as well as water-air interfaces. Surfactant molecules are ever present in typical household soaps and detergents, and when dissolved in water, self-assemble at the air water interface as well as into multi-molecule aggregates known as micelles. These assemblies, in turn, facilitate foaming and form the basis of the cleaning actions of the soaps and detergents. The first part of this dissertation examines the role of micellar and interfacially self-assembled surfactants on the stability of thin foam films. Specifically, the correspondence between inter-micellar interactions in bulk solutions and in thin films, and the influence of these interactions on the average lifetime of foam films are investigated. I envision that my studies will inspire colloidal scientists to explore foam film studies as a simple but effective method for characterizing nanoscopic colloidal interactions and forces. In the second part of this dissertation, development of interfacially active polymers that stabilize water-water interfaces, designed specifically for stabilization of polyelectrolyte complex coacervates, is demonstrated. Complex coacervates form upon electrostatic complexation of oppositely charged macromolecules and their subsequent condensation into aqueous macromolecules-rich phase. This aqueous two-phase system has been demonstrated to exhibit unique capabilities to achieve dynamic spatial compartmentalization as well as spontaneous sequestering of biological molecules. Despite these exciting prospects, their use has been limited owing to our inability to stabilize coacervate droplets and prevent their macro-phase separation. In my research, I have developed block polymers and comb polymers that have successfully stabilized micron-sized coacervate droplets, leading to first demonstrations of stable coacervate emulsions. I have also demonstrated sequestration of proteins (enzymes) into the stabilized coacervate droplets, paving way for these emulsions to be employed as protein-based bioreactors with selective small molecule transport.

The phase behavior of particle-polyelectrolyte complexes was systematically studied using a model system comprising oppositely charged silica nanoparticles and poly(allylamine) hydrochloride (PAH) polycations. Phase behaviors of aqueous mixtures of silica nanoparticles and PAH were elucidated over a wide parameter space of particle and polyelectrolyte concentrations as well as solution pH. Trends in phase behaviors were analyzed to create a fundamental understanding of the fundamental properties that govern the complexation of these oppositely charged species.

This dissertation elucidates the significance of macromolecular design in two types of systems - (i) covalently-linked polyurethane networks and composites, and (ii) complexes of oppositely charged polymers (polyelectrolyte complexes) – unified by their macromolecular nature, yet distinguished by their design, governing principles, and applications. The first part of this dissertation highlights the profound effect of the chemistry of flexible polyurethane foams on their physico-mechanical and thermochemical properties; by harnessing these correlations, a simple strategy has been proposed for the facile classification of post-consumer-use flexible polyurethane foams for their chemical recycling. Following this, the upcycling of depolymerized polyurethane products (polyols) into lightweight high-strength organic-inorganic composites with superior thermal insulation and acoustic barrier performance has been discussed. The amenability of the compositing strategy with polyols from different sources, varying chemistries of the diisocyanate linker, and types of inorganic materials have been highlighted. This dual approach is a step toward enabling the circularity of post-consumer-use flexible polyurethane foam and informing the design of polyurethane-inorganic composites with unique properties. The second part of this dissertation delineates the phenomenon of polyelectrolyte complexation and examines the non-trivial role of salt ion valency and polyelectrolyte length asymmetry on complexation. Divalent ions have been shown to reverse the composition of PECs, favoring their sequestration in the complex phase, and hindering chain relaxation. Following this, the dramatic influence of polyelectrolyte length symmetry in controlling the polymer content of the complex phase has been discussed in the context of thermodynamics and the Voorn-Overbeek theory. Furthermore, length-asymmetry will be shown to alter the viscosity of the complex phase. These dual studies are aimed to serve as design guides for polyelectrolyte complexation occurring in environments (biological and medical systems, environmental and wastewater treatment applications) where little to no control can be exercised over the properties of the solution and polyelectrolytes.

A scalable strategy for the fabrication of high flexural strength (~15 MPa), lightweight (0.9-1.2 g/cm3) organic-inorganic composites has been demonstrated. These hybrid composites comprise naturally occurring aluminosilicate particles, polyols (upcycled or virgin), and isocyanate linkers. These composites exhibit superior flexural and compressive properties, sound attenuation, and thermal insulation performance as compared to commonly employed materials such as ordinary Portland cement (OPC) and drywall. The material properties of the composites have been optimized by a systematic variation of the inorganic (60-46 wt%) and excess isocyanate contents (10-24 wt%). Additionally, the effect of the changes in polymer content (20-40 wt%) and size of aluminosilicate particles (2–20 �m) on the flexural properties of the composite have been analyzed to ascertain the optimum polymer content and inorganic particle size. Through this optimization, composites with flexural strength comparable to OPC and ~5� as compared to drywall and thermal insulation properties superior to OPC (~2�) and drywall (~1.12�) were obtained. In addition, the transverse and longitudinal attenuations of the composites were examined after the systematic variation of the compositions. The transverse attenuations were measured to be up to ∼6.4� and ∼8.2� that of gypsum for the IG300 and NX9014 composites respectively. The longitudinal attenuations were measured to be up to ∼8.8� and ∼15� that of gypsum for the IG300 and NX9014 composites respectively. The mechanical and material property trends of the composites were supported by morphological analysis of their surfaces. The composites are also shown to exhibit enhanced flame-resistant properties and antibacterial properties, with the latter achieved by subtle modification of the inorganic component of the composites.

Coagulation and flocculation are important phenomena that find widespread applications in water treatment. Polyelectrolytes are charged macromolecules that have found relevance in this domain due to their proven efficiency and effectiveness. The objective of the thesis would be to review and emphasize the fundamental mechanisms on which both natural and synthetic polyelectrolyte coagulants and flocculants operate. Advances in understanding phase characteristics and structure of aggregated polyelectrolyte complexes post interaction with charged impurities are discussed. These would help elucidate the correlation between salient polyelectrolyte properties such as molecular weight, persistence length, charge density, and aggregated complex properties such as turbidity and size. This review aims to provide relevant insights that enable better coagulants and flocculants for water treatment systems through examples to ultimately correlate several concepts and develop a collective understanding,

Complex coacervate microdroplets can be stabilized by comb polyelectrolytes and used as encapsulants for biological molecules such as proteins. In this thesis, we aim to create complex coacervate microdroplet dispersions with narrow size dispersity distributions by optimizing the concentrations and the molecular weights of homopolyelectrolytes and comb polyelectrolytes. The particle size distribution of suspended droplets is analyzed using static light scattering and microscopy. Further, we study the effect of mechanical processing (centrifugation and continuous stirring) on the particle size distribution of the complex coacervate microdroplets. Additionally, we investigate the reusability and recyclability of the complex coacervate microdroplet dispersions. Our process can potentially advance the use of complex coacervate microdroplets as synthetic reactor media for continuous stirred-type bioreactors.

This dissertation delineates my research on the electrostatically driven self-assembly on a wide range of materials. These materials include zeolites where metal cations act as structure directing agents, polyelectrolyte nanoparticle complexes with tunable interparticle separation, stabilized polyelectrolyte complex coacervate microdroplet dispersions and complex coacervate protocells with encapsulated enzymes to model biomolecular condensates. In all these systems, coulombic interactions play a central role in governing the functionality of the resultant material in distinct ways. For zeolites, the valency of the metal cation influences their micropore topology and the co-precipitation of different structures can be accomplished by tuning the cation content. In the case of polyelectrolyte-nanoparticle systems, the charge ratio of these two components directs the average interparticle distance in the complex which can be effectively manipulated. The coacervate microdroplet dispersions were studied in high throughput, and the addition of comb-polyelectrolytes suppress their coalescence and provide long-term stability against high ionic screening. Enzymes spontaneously partition into these coacervate domains due to their heterogenous surface charge, and such stabilized coacervate microdroplets can serve as in vitro protocells models and can elucidate the reaction-diffusion kinetics of enzymatic reactions.

The addition of salt speeds up chain relaxation dynamics in polyelectrolyte complexes (PECs), and time-salt superposition (TSS) approaches to describe the linear viscoelastic response of PECs are well-established. However, TSS is carried out at fixed initial polyelectrolyte concentrations, and varying the initial polyelectrolyte concentration results in distinct TSS master curves. In this thesis, we show that accounting for the small ions that accompany the oppositely charged polyelectrolyte chains (designated as accompanying counterions) enables assimilation of these distinct TSS master curves into a single universal master curve. This approach, that we christen as time-ionic strength superposition (TISS), enables a unified description of the PEC viscoelastic response in terms of the solution ionic strength, that accounts for both the accompanying counterions and the added ions, and underlines the dynamic similarities between PECs and semi-dilute polymer solutions. The sticky electrostatic associations among the oppositely charged chains, however, contribute additional relaxation modes in the PECs. We demonstrate that the timescales of these additional relaxation modes are described quantitatively by a modified sticky Rouse model that accounts for the influence of solution ionic strength on the electrostatic screening and chain friction. We then investigate the effect of the cationic valency of the added salt on the composition and rheology of the PECs. A stronger screening of electrostatic interactions is observed with increasing cation valency, leading to higher concentrations of polyelectrolytes in the supernatant phase and lower concentrations in the complex phase. In addition, electrostatic bridging of the polyanion chains by the multivalent cation alters the chain relaxation process in PECs by competing with the electrostatic interactions between the oppositely charged chains, resulting in non-monotonic variations of PEC moduli with increasing ionic strength of the solution.