Session Y03: Polyelectrolyte Complexation II

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The ion pairing responsible for the spontaneous formation of polyelectrolyte complex coacervates, PECs, may be partially reversed by adding small ions (“salt”). As pairs between positive, Pol⁺, and negative, Pol^-, polyelectrolyte repeat units are broken, the material becomes significantly more fluid-like. These “sticky interactions” may be reformed simply by washing out the salt ions with water. Quantitative equilibrium expressions may be derived for the distribution of salt ions inside and outside the PECs. The equilibrium is a balance between entropy (difference in ion density inside versus outside the PEC phase) and enthalpic effects such as ion specificity. Entropy may be calculated using the Donnan equilibrium, and enthalpy may be measured using sensitive calorimetry. This talk will cover a range of PEC/salt systems to illustrate a consistent picture for the rôle of ions in complexed polyelectrolytes.   

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The alternate adsorption of polycations and polyanions on a substrate yields multilayer structures that have highly adaptable properties. These polyelectrolyte multilayers (PEMs) have promising applications in batteries, drug-delivery and protective coating systems. The three major components of PEMs are polymer, water and salt ions, all of which largely influence the PEM properties. In this talk, we discuss the influence of salt and polyelectrolyte on the water content of the PEM- specifically, the water associated with the polycation-polyanion (intrinsic) ion pairs, “i”. Attempts to quantify i for polyelectrolyte systems are few and complex. Here, using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and ellipsometry, we determine values of i for poly(diallyldimethylammonium)/polystyrene sulfonate PEMs. These results provide an understanding of water and ion-pair relationships and aim to provide a universal technique for all PEM systems.   

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Although an important design parameter in complex coacervation, the molecular mass dependence of the interfacial tension far from the critical point has yet to be studied in detail. Here we perform careful experiments supplemented with coarse-grained molecular dynamics simulations to elucidate this dependence. Subsequently, we derive the dependence and show that it is independent of the functional form of the non-ideal interactions assuming that (1) the concentration of polymer is the supernatant is negligible, a good assumption far from the critical point, and (2) that the non-ideal interactions have no explicit molecular mass dependence.   

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Complex coacervates are promising materials for use as underwater adhesives, inspired by biological systems of sand-castle worms and mussels. The performance of such adhesives depends on the wetting behavior of coacervates on solid surfaces. We analyze the wetting transition in systems where the polyelectrolyte adsorption is driven by electrostatic (electrowetting) and by nonelectrostatic forces (i.e. hydrogen bonding, hydrophobicity, coordination, etc.).   

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It is largely accepted that the physical properties of polyelectrolyte complexes and multilayers are influenced by temperature, ionic strength, polyelectrolyte or salt type, pH, and water content, among other considerations. A growing body of knowledge points to the idea that many of these features ultimately control the number of polycation-polyanion “intrinsic” ion pairs and the lifetime of the intrinsic ion pair. This talk will discuss how the glass transition temperature and rheological properties relate to intrinsic ion pairing. Methods to quantify the number or fraction of intrinsic ion pairs in solid polyelectrolyte complexes and multilayers are discussed, along with a new method to estimate the number of water molecules at the intrinsic ion pair. Results suggest that the glass transition is related to the number of water molecules at the intrinsic ion pair. With regard to dynamic mechanical properties of solid complexes, time-temperature-water superpositioning is applied and a free-volume relationship is proposed. Results are complemented by all-atom molecular dynamics simulations.   

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Polyelectrolyte complex micelles (PCMs) are widely used in the delivery of hydrophilic payloads. PCM attributes are strongly dependent on the size and chemical structure of each polymer block. Neutral blocks drive nanoscale phase separation while charged blocks control micelle core size and stability. An understanding of physical property behavior controlled by block size, chemistry, and salt conditions is crucial when designing for use in dynamic or biological environments and provide a greater understanding of the physics of polyelectrolyte assembly. In this work, we use small angle x-ray scattering, light scattering, and electron microscopy to determine scaling behaviors of micelle shape, size, and stability for commonly used polyelectrolytes.<div id="fpCE_version" style="display:none">8.5.1</div>   

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Complex coacervate core micelles (C3Ms) have been investigated in a wide range of applications owing to stimuli-responsiveness and hydrophilic cores. Compared to structure and morphology, much less is known about the kinetics of C3Ms, yet the fundamental understanding is essential for micelle stability, reproducibility, and stimuli-responsiveness. In this study, we investigated the equilibrium exchange dynamics of C3Ms characterized by time-resolved small-angle neutron scattering (TR-SANS). A pair of well-defined poly(ethylene oxide-b-allyl glycidyl ether) (PEO-PAGE) was synthesized and functionalized with charged moieties including guanidinium (G), ammonium (A), and sulfonate (S); one with a normal PEO, and the other with a fully deuterated PEO. C3Ms were prepared by simple mixing of two oppositely charged hPEO-PAGE (or dPEO-PAGE) solutions. Salt concentration dependence and polymer concentration dependence of the molecular exchange rate revealed that the coacervate core dynamics is crucial. Furthermore, a combination of the charged moieties significantly tunes the core dynamics, and thus the exchange rate.   

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Incorporation of complex coacervate domains as physical crosslinks in polymer network leads to multi-responsive hydrogels with versatile functionalities. We investigate the relaxation dynamics of the hydrogels as a function of salt concentration, accompanied by the structural characterizations. Using the hydrogels constructed by mixing oppositely charged ABA triblock copolymers of equal block lengths in aqueous media, we observed that the dynamic moduli spectra are well superposed with varying salt concentration by the time-salt superposition (TSS). The successful superposition indicates the rheological simplicity of coacervate hydrogels and allows a deeper understanding of the hydrogel rheology within the framework of polyelectrolyte chain exchanges. Based on a model combining the sticky Rouse-like diffusion in coacervate phase and the activated chain pull-out from the coacervate core, we rationalize the observed dependencies of relaxation dynamics on the length and ionic group of charged end block as well as the salt concentration. The results will be discussed in conjunction with the current understanding of the polyelectrolyte complexes and the practical implications such as the injectability and self-healing in hydrogels.   

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Polymerized ionic liquids (PILs) are a subset of polyelectrolytes with numerous interesting properties, including solubility in organic solvents for certain ion pairs. In our study, two oppositely charged PILs were used to form complex coacervates in different organic solvents, i.e. 2,2,2-trifluoroethanol (TFE) and hexafluoro-2-propanol (HFIP). In these two media with relatively low dielectric constants (27 for TFE, 16.7 for HFIP) compared to aqueous systems (80 for water), coacervate phase diagrams were constructed via UV-Vis and ¹⁹F-NMR spectroscopy. Notably, the critical salt concentrations required for complete miscibility were significantly different in the two solvents, while ‘positive’ tie lines—corresponding to higher concentration of small ions in the coacervate phases than in the supernatant phases—were found in both systems. This study provides insight into the effect of low dielectric solvents on complex coacervation, which has not been widely studied due to the limited solubility of most polyelectrolytes in these media.   

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The liquid-liquid phase separation of aqueous solutions of polyelectrolyte and salt is responsible for fascinating phenomena ranging from membraneless organelles to targeted drug delivery. Recently, Ali et al. (2019) demonstrated lower critical solution temperature (LCST) liquid-liquid phase separation in these systems, which has not previously been modeled or observed. Adhikari et al. (2019) demonstrated that LCST could be modeled with solvent-polymer and dipole-dipole interactions if the temperature-dependence of water's dielectric constant is incorporated. Here, we show that electrostatic correlations based on a Bjerrum length that increases with temperature is sufficient to model LCST in a symmetric polyelectrolyte and salt solution, using both Voorn-Overbeek and a liquid state theory. We show qualitative agreement with the experimental trends in the binodal, but found that the observed widening of the binodal with polymer on polymer-temperature axis requires a negatively sloped tie line, contrary to experimental findings. We also demonstrate UCST and LCST as in Ye et al. (2020) and suggest further validation experiments.   

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In this experimental work, we explored the interfacial properties of membraneless polyelectrolyte complex coacervate droplets prepared in a controlled saline environment via emulsion macrophase transfer. We achieved stability against droplet coarsening in a variety of polyelectrolyte coacervates, demonstrating the generality of this phenomenon. When subjected to a compressional force, rather than coalescing, the droplets deformed and remained stable in non-spherical shapes suggesting an elastic interface. Rheological measurements show that there is only a little change in the viscoelastic properties of the complexes upon stabilization, preserving the liquid-like properties of these complexes. These droplets were stable for over 2 months at lab conditions and were thermally stable at elevated temperatures way beyond physiological conditions. The absence of an external membrane on these droplets conserved their excellent biomolecular diffusion and partitioning capabilities, promoting their application as artificial bioreactors mimicking cells. Some of these properties were explored using enzyme cascade reactions localized within the coacervate droplets.   

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Nonequilibrium behavior of aqueous polyelectrolytes is critical to many applications, yet engineering control is largely limited to equilibrium phase behavior. Here, we investigate the mechanism of polyelectrolyte complex (PEC) growth and its sensitivity to flow processing. Microfluidic mixing of poly(acrylic acid) and poly(allylamine) salt solutions reveals that PECs grow by coalescence at high salt and low polyelectrolyte concentration, and elsewhere by diffusion-limited droplet aggregation to form a rough microstructure. This transition is flow rate-dependent, consistent with macroscale observations that aggregates form at some concentrations only when the polyelectrolytes are vortex- rather than pipette-mixed. Aggregate roughness can also be time-dependent and relaxes via internal coalescence. Importantly, coacervate-like viscoelasticity does not preclude an aggregated, precipitate-like microstructure. These results suggest that colloidal and mass transport effects underly sensitivity of PEC formation and relaxation to various mixing parameters, replacing the conventional concept of a state diagram with disparate precipitate and coacervate regions, and opening the door to controlling PEC formation and nonequilibrium structure.   

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Polyelectrolyte complexes (PEC) hydrogels form when oppositely charged block polyelectrolyte chains spontaneously associate and self-assemble in aqueous media. The promising potential of PEC hydrogels in drug delivery and bioadhesion applications requires the establishment of elaborate fundamental mappings interrelating the hydrogel mesoscale structure, relaxation processes and bulk material properties. In this presentation, we will discuss our investigations on hybrid hydrogels comprising interpenetrating PEC and covalent networks. Minimal influence of incorporation of the covalent network on the equilibrium hierarchical structure of PEC networks will be demonstrated, conserving the gel’s ability to encapsulate biomolecules and other charged cargo. At the same time, we will highlight marked improvements in the shear and the tensile strengths of the PEC hydrogels upon incorporation of the covalent network, even as a minor component, in the hybrid hydrogels. The decoupling of hydrogel structure and rheology, along with improved resistance to salt and controllable swelling, will be argued to further broaden the utility of the PEC hydrogels in diverse biomedical applications.