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Oppositely-charged polymers can undergo an associative phase separation process known as complex coacervation, which is driven by the electrostatic attraction between the two polymer species. This driving force for phase separation can be harnessed to drive self-assembly, via pairs of block copolyelectrolytes with opposite charge and thus favorable coulombic interactions. There are few predictions of coacervate self-assembly phase behavior due to the wide variety of molecular and environmental parameters, along with fundamental theoretical challenges. In this paper, we use recent advances in coacervate theory to predict the solution-phase assembly of diblock polyelectrolyte pairs for a number of molecular design parameters (charged block fraction, polymer length). Phase diagrams show that self-assembly occurs at high polymer, low salt concentrations for a range of charge block fractions. We show that we qualitatively obtain limiting results seen in the experimental literature, including the emergence of a high polymer-fraction reentrant transition that gives rise to a self-compatibilized homopolymer coacervate behavior at the limit of high charge block fraction. In intermediate charge block fractions, we draw an analogy between the role of salt concentration in coacervation-driven assembly and the role of temperature in χ -driven assembly. We also explore salt partitioning between microphase separated domains in block copolyelectrolytes, with parallels to homopolyelectrolyte coacervation.
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Tuning chain interaction entropy in complex coacervation using polymer stiffness, architecture, and salt valency
Oppositely-charged polyelectrolytes can undergo a liquid–liquid phase separation in a salt solution, resulting in a polymer-dense ‘coacervate’ phase that has found use in a wide range of applications from food science to self-assembled materials. Coacervates can be tuned for specific applications by varying parameters such as salt concentration and valency, polyelectrolyte length, and polyelectrolyte identity. Recent simulation and theory has begun to clarify the role of molecular structure on coacervation phase behavior, especially for common synthetic polyelectrolytes that exhibit high charge densities. In this manuscript, we use a combination of transfer matrix theory and Monte Carlo simulation to understand at a physical level how a range of molecular features, in particular polymer architecture and stiffness, and salt valency can be used to design the phase diagrams of these materials. We demonstrate a physical picture of how the underlying entropy-driven process of complex coacervation is affected by this wide range of physical attributes.
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- Award ID(s):
- 1654158
- PAR ID:
- 10048920
- Date Published:
- Journal Name:
- Molecular Systems Design & Engineering
- ISSN:
- 2058-9689
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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With the rise of green engineering, there is an increasing need to manufacture materials without relying on organic solvents. Using all-aqueous approaches mitigates the industrial safety and environmental concerns that are associated with volatile organic compounds, while enabling scalable and sustainable fabrication processes. Water-insoluble polyelectrolyte complexes (PECs) arise due to the electrostatic attraction between oppositely charged polyelectrolytes in solution. Notably, when salt is present, these rigid or glassy PECs can be transformed into malleable and liquid states, enabling researchers to process solid materials from the previously deemed unprocessable. The liquid PEC phase, also known as a polyelectrolyte complex coacervate, arises through liquid–liquid phase separation and offers a tunable viscosity to match the needs of the processing method. These coacervates exhibit adjustable rheological properties by varying parameters, including temperature, salt type, ionic strength, polymer ratio, and molecular weight. This tunability makes them attractive for applications ranging from coatings and adhesives to biomedical delivery systems. Notably, the transition between liquid and solid PECs is reversible, as removing salt ions restores the physical cross-links. Additionally, PECs exhibit exceptional stability in various organic solvents and solutions with extreme pH values, without requiring chemical cross-linking. However, the aqueous processing strategies and reversibility of PECs have yet to be fully explored. In this Account, we primarily focus on the well-studied PEC system composed of the strong polyelectrolytes poly(sodium 4-styrenesulfonate) and poly(diallyldimethylammonium chloride). First, we describe how salt concentration is a crucial parameter that enables the aqueous processing of coacervates via electrospinning, spin coating, bar casting, and 3D printing into fibers, coatings, membranes, and 3D structures. We also discuss the impact that processing conditions, like drying and quenching, have on the properties of solid materials, such as their porosity and mechanical strength. Next, we highlight reports that explore how the solubility mismatch between polyelectrolyte pairs and salt ions result in solid and liquid PECs that are nonstoichiometric, thereby exhibiting an overcompensation phenomenon or nonstoichiometry. How the mechanical behavior of a material changes as a function of temperature, i.e., their thermomechanical properties, as well as membrane separation performance are notably influenced by nonstoichiometry, even when the degree of nonstoichiometry is minimal. Interestingly, we are starting to see research reports in the literature on how post-treatment methods, including salt and heat annealing, previously applied to polyelectrolyte multilayer films, offer some transferability to bar-casted separation membranes, which warrants further research. We conclude with a forward-looking discussion that highlights the potential opportunities and challenges related to the future implementation of PEC-based materials.more » « less
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Complex coacervation is an associative, liquid–liquid phase separation that can occur in solutions of oppositely-charged macromolecular species, such as proteins, polymers, and colloids. This process results in a coacervate phase, which is a dense mix of the oppositely-charged components, and a supernatant phase, which is primarily devoid of these same species. First observed almost a century ago, coacervates have since found relevance in a wide range of applications; they are used in personal care and food products, cutting edge biotechnology, and as a motif for materials design and self-assembly. There has recently been a renaissance in our understanding of this important class of material phenomena, bringing the science of coacervation to the forefront of polymer and colloid science, biophysics, and industrial materials design. In this review, we describe the emergence of a number of these new research directions, specifically in the context of polymer–polymer complex coacervates, which are inspired by a number of key physical and chemical insights and driven by a diverse range of experimental, theoretical, and computational approaches.more » « less
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Oppositely charged polyelectrolytes can undergo a macroscopic, associative phase separation in solution, via a process known as complex coacervation. Significant recent effort has gone into providing a clear, physical picture of coacervation; most work has focused on improving the field theory picture that emerged from the classical Voorn–Overbeek theory. These methods have persistent issues, however, resolving the molecular features that have been shown to play a major role in coacervate thermodynamics. In this paper, we outline a theoretical approach to coacervation based on a transfer matrix formalism that is an alternative to traditional field-based approaches. We develop theoretical arguments informed by experimental observation and simulation, which serve to establish an analytical expression for polymeric complex coacervation that is consistent with the molecular features of coacervate phases. The analytical expression provided by this theory is in a form that can be incorporated into more complicated theoretical or simulation formalisms, and thus provides a starting point for understanding coacervate-driven self-assembly or biophysics.more » « less
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The phase behavior and chain conformational structure of biphasic polyzwitterion–polyelectrolyte coacervates in salted aqueous solution are investigated with a model weak cationic polyelectrolyte, poly(2-vinylpyridine) (P2VP), whose charge fraction can be effectively tuned by pH. It is observed that increasing the pH leads to the increase of the yielding volume fraction and the water content of dense coacervates formed between net neutral polybetaine and cationic P2VP in contrast to the decrease of critical salt concentration for the onset of coacervation, where the P2VP charge fraction is reduced correspondingly. Surprisingly, a single-molecule fluorescence spectroscopic study suggests that P2VP chains upon coacervation seem to adopt a swollen or an even more expanded conformational structure at higher pH. As the hydrophobicity of P2VP chains is accompanied by a reduced charge fraction by increasing the pH, a strong pH-dependent phase and conformational behaviors suggest the shift of entropic and enthalpic contribution to the underlying thermodynamic energy landscape and chain structural dynamics of polyelectrolyte coacervation involving weak polyelectrolytes in aqueous solution.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c7me00108h
