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Polyelectrolyte Gels: Volume II

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A special issue of Gels (ISSN 2310-2861).

Deadline for manuscript submissions: closed (28 February 2021) | Viewed by 42430

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Prof. Dr. Ferenc Horkay

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Section on Quantitative Imaging and Tissue Sciences, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
Interests: to understand the fundamental principles that govern molecular interactions and define structural hierarchy in complex synthetic and biopolymer systems, such as biological tissues, gels, soft materials, self-assemblies and functional nanostructures; to develop multiscale characterization approaches by combining microscopic (small angle scattering, static and dynamic light scattering, atomic force microscopy, etc.) and macroscopic (osmotic pressure measurements, rheology, etc.) methods probing the static and dynamic properties of gels and polymer solutions over a broad range of length and time scales
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Dear Colleagues,

This Special Issue on “Polyelectrolyte Gels” is dedicated to recent developments from theoretical and fundamental aspects to the synthesis, characterization, and applications of polyelectrolyte networks and gels. Within this context, a broad range of subjects, including structure and dynamics, molecular modeling and simulation, and applications will be discussed.

Polyelectrolytes are complex polymer systems of which properties reflect chain connectivity, electrostatic effects, and other molecular interactions over multiple length scales. Owing to the complexity of the interactions, an understanding of these materials has been slow to develop despite the importance of charged polymers, both to biology and to materials science. In living systems, many biopolymers are highly-charged macromolecules. In their natural environment, they are exposed to different ions. Synthetic polyelectrolyte gels have a variety of applications in material science and engineering, including materials for energy storage, separation, and drug delivery. Polyelectrolyte-based materials in which the constituents are organized across multiple length scales have the potential to address a wide range of technological challenges. The precise control of molecular and supramolecular architecture enables novel applications in biomedicine. Progress in this field requires an interdisciplinary effort to accomplish a more detailed understanding of the structure and interactions that define the behavior of complex polyelectrolyte systems, and makes it possible to tailor the properties of these materials.

Since it is impossible to cover all aspects of polyelectrolyte gel science in one issue, this Special Issue will contain only a few representative examples, illustrating the complexity of the polyelectrolyte problem. It is hoped that the topics will stimulate new research and discoveries in the field of polyelectrolyte networks and gels.

Prof. Dr. Ferenc Horkay
Guest Editor

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Keywords

Synthetic and Biological Polyelectrolytes
Theory of Polyelectrolyte Networks and Gels
Synthesis and Characterization
Structure–Property Relationships
Dynamic Properties
Modeling
Applications

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Polyelectrolyte Gels in Gels (14 articles)

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Research

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15 pages, 2613 KiB

Open AccessArticle

Enhanced Mechanical Properties by Ionomeric Complexation in Interpenetrating Network Hydrogels of Hydrolyzed Poly (N-vinyl Formamide) and Polyacrylamide

by Joseph M. Scalet, Tiffany C. Suekama, Jeayoung Jeong and Stevin H. Gehrke

Gels 2021, 7(3), 80; https://doi.org/10.3390/gels7030080 - 29 Jun 2021

Cited by 9 | Viewed by 2790

Abstract

Tough hydrogels were made by hydrolysis of a neutral interpenetrating network (IPN) of poly (N-vinyl formamide) PNVF and polyacrylamide (PAAm) networks to form an IPN of polyvinylamine (PVAm) and poly (acrylic acid) (PAAc) capable of intermolecular ionic complexation. Single network (SN) PAAm and SN PNVF have similar chemical structures, parameters and physical properties. The hypothesis was that starting with neutral IPN networks of isomeric monomers that hydrolyze to comparable extents under similar conditions would lead to formation of networks with minimal phase separation and maximize potential for charge–charge interactions of the networks. Sequential IPNs of both PNVF/PAAm and PAAm/PNVF were synthesized and were optically transparent, an indication of homogeneity at submicron length scales. Both IPNs were hydrolyzed in base to form PVAm/PAAc and PAAc/PVAm IPNs. These underwent ~5-fold or greater decrease in swelling at intermediate pH values (3–6), consistent with the hypothesis of intermolecular charge complexation, and as hypothesized, the globally neutral, charge-complexed gel states showed substantial increases in failure properties upon compression, including an order of magnitude increases in toughness when compared to their unhydrolyzed states or the swollen states at high or low pH values. There was no loss of mechanical performance upon repeated compression over 95% strain. Full article

(This article belongs to the Special Issue Polyelectrolyte Gels: Volume II)

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14 pages, 2060 KiB

Open AccessArticle

Swelling and Mechanical Properties of Polyacrylamide-Derivative Dual-Crosslink Hydrogels Having Metal–Ligand Coordination Bonds as Transient Crosslinks

by Louis Debertrand, Jingwen Zhao, Costantino Creton and Tetsuharu Narita

Gels 2021, 7(2), 72; https://doi.org/10.3390/gels7020072 - 15 Jun 2021

Cited by 6 | Viewed by 3362

Abstract

Hydrogels that have both permanent chemical crosslinks and transient physical crosslinks are good model systems to represent tough gels. Such “dual-crosslink” hydrogels can be prepared either by simultaneous polymerization and dual crosslinking (one-pot synthesis) or by diffusion/complexation of the physical crosslinks to the chemical network (diffusion method). To study the effects of the preparation methods and of the crosslinking ratio on the mechanical properties, the equilibrium swelling of the dual-crosslink gels need to be examined. Since most of these gels are polyelectrolytes, their swelling properties are complex, so no systematic study has been reported. In this work, we synthesized model dual-crosslink gels with metal–ligand coordination bonds as physical crosslinks by both methods, and we proposed a simple way of adding salt to control the swelling ratio prepared by ion diffusion. Tensile and linear rheological tests of the gels at the same swelling ratio showed that during the one-pot synthesis, free radical polymerization was affected by the transition metal ions used as physical crosslinkers, while the presence of electrostatic interactions did not affect the role of the metal complexes on the mechanical properties. Full article

(This article belongs to the Special Issue Polyelectrolyte Gels: Volume II)

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18 pages, 2838 KiB

Open AccessArticle

Neutralization and Salt Effect on the Structure and Mechanical Properties of Polyacrylic Acid Gels under Equivolume Conditions

by Yui Tsuji, Mitsuhiro Shibayama and Xiang Li

Gels 2021, 7(2), 69; https://doi.org/10.3390/gels7020069 - 9 Jun 2021

Cited by 2 | Viewed by 3421

Abstract

The effects of neutralization and salt on the structure and mechanical properties of polyacrylic acid (PAA) gels under equivolume conditions were investigated by small-angle X-ray scattering (SAXS) measurements and tensile tests. We attained the equivolume condition by immersing a piece of PAA gel sample in an ion reservoir containing linear PAA, NaOH, and NaCl at prescribed concentrations (post-ion-tuning). The volume fraction of the linear polymer was set to be the same as that of the gel so as to satisfy the iso-osmotic pressure at the reference state. Various types of reservoirs were prepared by adding NaOH and/or NaCl with different concentrations to the reference reservoir, followed by immersing a PAA gel piece. In the SAXS measurements, a scattering peak appeared, and the scattering intensity at q = 0 decreased by neutralization, while the addition of salt increased the scattering intensity. On the other hand, Young’s modulus measured with the tensile test decreased with neutralization; however, it scarcely changed with the addition of salt. The newly developed equivolume post-ion-tuning technique may serve as a new standard scheme to study polyelectrolyte gels. Full article

(This article belongs to the Special Issue Polyelectrolyte Gels: Volume II)

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13 pages, 2742 KiB

Open AccessArticle

Systematic Modification of the Glass Transition Temperature of Ion-Pair Comonomer Based Polyelectrolytes and Ionomers by Copolymerization with a Chemically Similar Cationic Monomer

by Guodong Deng, Timothy D. Schoch and Kevin A. Cavicchi

Gels 2021, 7(2), 45; https://doi.org/10.3390/gels7020045 - 13 Apr 2021

Cited by 6 | Viewed by 3252

Abstract

Ion-pair comonomers (IPCs) where both the anion and cation contain polymerizable functional groups offer a route to prepare polyampholyte, ion-containing polymers. Polymerizing vinyl functional groups by free-radical polymerization produces bridging ion-pairs that act as non-covalent crosslinks between backbone segments. In particular the homopolymerization of the IPC vinyl benzyl tri-n-octylphosphonium styrene sulfonate produces a stiff, glassy polymer with a glass transition temperature (T_g) of 191 °C, while copolymerization with a non-ionic acrylate produces microphase separates ionomers with ion-rich and ion-poor domains. This work investigates the tuning of the T_g of the polyelectrolyte or ion-rich domains of the ionomers by copolymerizing with vinyl benzyl tri-n-octylphosphonium p-toluene sulfonic acid. This chemically similar repeat unit with pendant rather than bridging ion-pairs lowers the T_g compared to the polyelectrolyte or ionomer containing only the IPC segments. Rheological measurements were used to characterize the thermomechanical behavior and T_g of different copolymers. The T_g variation in the polyelectrolyte vs. weight fraction IPC could be fit with either the Gordon–Taylor or Couchman–Karasz equation. Copolymerization of IPC with a chemically similar cationic monomer offers a viable route to systematically vary the T_g of the resulting polymers useful for tailoring the material properties in applications such as elastomers or shape memory polymers. Full article

(This article belongs to the Special Issue Polyelectrolyte Gels: Volume II)

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21 pages, 8722 KiB

Open AccessEditor’s ChoiceArticle

Ionotropic Gelation Fronts in Sodium Carboxymethyl Cellulose for Hydrogel Particle Formation

by William N. Sharratt, Carlos G. Lopez, Miriam Sarkis, Gunjan Tyagi, Róisín O’Connell, Sarah E. Rogers and João T. Cabral

Gels 2021, 7(2), 44; https://doi.org/10.3390/gels7020044 - 12 Apr 2021

Cited by 21 | Viewed by 4479

Abstract

Hydrogel microparticles (HMPs) find numerous practical applications, ranging from drug delivery to tissue engineering. Designing HMPs from the molecular to macroscopic scales is required to exploit their full potential as functional materials. Here, we explore the gelation of sodium carboxymethyl cellulose (NaCMC), a model anionic polyelectrolyte, with Fe³⁺ cations in water. Gelation front kinetics are first established using 1D microfluidic experiments, and effective diffusive coefficients are found to increase with Fe³⁺ concentration and decrease with NaCMC concentrations. We use Fourier Transform Infrared Spectroscopy (FTIR) to elucidate the Fe³⁺-NaCMC gelation mechanism and small angle neutron scattering (SANS) to spatio-temporally resolve the solution-to-network structure during front propagation. We find that the polyelectrolyte chain cross-section remains largely unperturbed by gelation and identify three hierarchical structural features at larger length scales. Equipped with the understanding of gelation mechanism and kinetics, using microfluidics, we illustrate the fabrication of range of HMP particles with prescribed morphologies. Full article

(This article belongs to the Special Issue Polyelectrolyte Gels: Volume II)

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9 pages, 1360 KiB

Open AccessArticle

Polyelectrolyte Gels Formed by Filamentous Biopolymers: Dependence of Crosslinking Efficiency on the Chemical Softness of Divalent Cations

by Katrina Cruz, Yu-Hsiu Wang, Shaina A. Oake and Paul A. Janmey

Gels 2021, 7(2), 41; https://doi.org/10.3390/gels7020041 - 8 Apr 2021

Cited by 7 | Viewed by 2474

Abstract

Filamentous anionic polyelectrolytes are common in biological materials. Some examples are the cytoskeletal filaments that assemble into networks and bundled structures to give the cell mechanical resistance and that act as surfaces on which enzymes and other molecules can dock. Some viruses, especially bacteriophages are also long thin polyelectrolytes, and their bending stiffness is similar to those of the intermediate filament class of cytoskeletal polymers. These relatively stiff, thin, and long polyelectrolytes have charge densities similar to those of more flexible polyelectrolytes such as DNA, hyaluronic acid, and polyacrylates, and they can form interpenetrating networks and viscoelastic gels at volume fractions far below those at which more flexible polymers form hydrogels. In this report, we examine how different types of divalent and multivalent counterions interact with two biochemically different but physically similar filamentous polyelectrolytes: Pf1 virus and vimentin intermediate filaments (VIF). Different divalent cations aggregate both polyelectrolytes similarly, but transition metal ions are more efficient than alkaline earth ions and their efficiency increases with increasing atomic weight. Comparison of these two different types of polyelectrolyte filaments enables identification of general effects of counterions with polyelectrolytes and can identify cases where the interaction of the counterions and the filaments exhibits stronger and more specific interactions than those of counterion condensation. Full article

(This article belongs to the Special Issue Polyelectrolyte Gels: Volume II)

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9 pages, 1640 KiB

Open AccessArticle

Microcapillary Reactors via Coaxial Electrospinning: Fabrication of Small Poly(Acrylic Acid) Gel Beads and Thin Threads of Biological Cell Dimensions

by Susan K. Kozawa, Audrey Lord, Jonah J. Scott-McKean, Anne Y. Walker, Alberto C. S. Costa and Gary E. Wnek

Gels 2021, 7(2), 37; https://doi.org/10.3390/gels7020037 - 30 Mar 2021

Cited by 2 | Viewed by 2810

Abstract

Poly(acrylic acid) (PAA) bulk gels and threads, typically derived via free-radical polymerization, are of interest as anionic polyelectrolyte mimics of cellular cytosol and as models for early protocells. The thread dimensions have been limited by the diameters of readily-available glass or plastic capillaries, and threads with diameters of less than 50 µm have been difficult to achieve. Here, we report a useful approach for achieving crosslinked, partially neutralized PAA, namely poly(acrylate), gel threads with diameters of a few microns when dry. This technique utilizes coaxial electrospinning to effectively produce capillaries (shells) of polystyrene loaded with a gel-forming precursor mixture composed of 3 M acrylic acid, methylene-bisacrylamide, potassium persulfate and 2.2 M NaOH in the core, followed by thermally-induced polymerization and then the removal of the polystyrene shell. Relatively long (up to 5 mm), continuous PAA threads with thicknesses of 5–15 µm are readily obtained, along with a multitude of PAA gel particles, which result from the occasional break-up of the fluid core prior to gel formation during the electrospinning process. The threads and beads are of the sizes of interest to model ancient protocells, certain functional aspects of excitable cells, such as myocytes and neurons, and various membraneless organelles. Full article

(This article belongs to the Special Issue Polyelectrolyte Gels: Volume II)

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Review

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43 pages, 13513 KiB

Open AccessReview

Polyelectrolyte Gels: Fundamentals, Fabrication and Applications

by Nisal Wanasingha, Pramod Dorishetty, Naba K. Dutta and Namita Roy Choudhury

Gels 2021, 7(3), 148; https://doi.org/10.3390/gels7030148 - 18 Sep 2021

Cited by 20 | Viewed by 6093

Abstract

Polyelectrolyte gels are an important class of polymer gels and a versatile platform with charged polymer networks with ionisable groups. They have drawn significant recent attention as a class of smart material and have demonstrated potential for a variety of applications. This review begins with the fundamentals of polyelectrolyte gels, which encompass various classifications (i.e., origin, charge, shape) and crucial aspects (ionic conductivity and stimuli responsiveness). It further centralises recent developments of polyelectrolyte gels, emphasising their synthesis, structure–property relationships and responsive properties. Sequentially, this review demonstrates how polyelectrolyte gels’ flourishing properties create attractiveness to a range of applications including tissue engineering, drug delivery, actuators and bioelectronics. Finally, the review outlines the indisputable appeal, further improvements and emerging trends in polyelectrolyte gels. Full article

(This article belongs to the Special Issue Polyelectrolyte Gels: Volume II)

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20 pages, 494 KiB

Open AccessEditor’s ChoiceReview

Theory of Charged Gels: Swelling, Elasticity, and Dynamics

by Di Jia and Murugappan Muthukumar

Gels 2021, 7(2), 49; https://doi.org/10.3390/gels7020049 - 21 Apr 2021

Cited by 29 | Viewed by 4700

Abstract

The fundamental attributes of charged hydrogels containing predominantly water and controllable amounts of low molar mass electrolytes are of tremendous significance in biological context and applications in healthcare. However, a rigorous theoretical formulation of gel behavior continues to be a challenge due to the presence of multiple length and time scales in the system which operate simultaneously. Furthermore, chain connectivity, the electrostatic interaction, and the hydrodynamic interaction all lead to long-range interactions. In spite of these complications, considerable progress has been achieved over the past several decades in generating theories of variable complexity. The present review presents an analytically tractable theory by accounting for correlations emerging from topological, electrostatic, and hydrodynamic interactions. Closed-form formulas are derived for charged hydrogels to describe their swelling equilibrium, elastic moduli, and the relationship between microscopic properties such as gel diffusion and macroscopic properties such as elasticity. In addition, electrostatic coupling between charged moieties and their ion clouds, which significantly modifies the elastic diffusion coefficient of gels, and various scaling laws are presented. The theoretical formulas summarized here are useful to adequately capture the essentials of the physics of charged gels and to design new hydrogels with specified elastic and dynamical properties. Full article

(This article belongs to the Special Issue Polyelectrolyte Gels: Volume II)

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17 pages, 815 KiB

Open AccessReview

Ion-Induced Volume Transition in Gels and Its Role in Biology

by Matan Mussel, Peter J. Basser and Ferenc Horkay

Gels 2021, 7(1), 20; https://doi.org/10.3390/gels7010020 - 18 Feb 2021

Cited by 11 | Viewed by 3358

Abstract

Incremental changes in ionic composition, solvent quality, and temperature can lead to reversible and abrupt structural changes in many synthetic and biopolymer systems. In the biological milieu, this nonlinear response is believed to play an important functional role in various biological systems, including DNA condensation, cell secretion, water flow in xylem of plants, cell resting potential, and formation of membraneless organelles. While these systems are markedly different from one another, a physicochemical framework that treats them as polyelectrolytes, provides a means to interpret experimental results and make in silico predictions. This article summarizes experimental results made on ion-induced volume phase transition in a polyelectrolyte model gel (sodium polyacrylate) and observations on the above-mentioned biological systems indicating the existence of a steep response. Full article

(This article belongs to the Special Issue Polyelectrolyte Gels: Volume II)

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Other

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18 pages, 943 KiB

Open AccessPerspective

On the Determination of Mechanical Properties of Aqueous Microgels—Towards High-Throughput Characterization

by Ingrid Haga Oevreeide, Renata Szydlak, Marcin Luty, Husnain Ahmed, Victorien Prot, Bjørn Helge Skallerud, Joanna Zemła, Małgorzata Lekka and Bjørn Torger Stokke

Gels 2021, 7(2), 64; https://doi.org/10.3390/gels7020064 - 31 May 2021

Cited by 7 | Viewed by 3820

Abstract

Aqueous microgels are distinct entities of soft matter with mechanical signatures that can be different from their macroscopic counterparts due to confinement effects in the preparation, inherently made to consist of more than one domain (Janus particles) or further processing by coating and change in the extent of crosslinking of the core. Motivated by the importance of the mechanical properties of such microgels from a fundamental point, but also related to numerous applications, we provide a perspective on the experimental strategies currently available and emerging tools being explored. Albeit all techniques in principle exploit enforcing stress and observing strain, the realization differs from directly, as, e.g., by atomic force microscope, to less evident in a fluid field combined with imaging by a high-speed camera in high-throughput strategies. Moreover, the accompanying analysis strategies also reflect such differences, and the level of detail that would be preferred for a comprehensive understanding of the microgel mechanical properties are not always implemented. Overall, the perspective is that current technologies have the capacity to provide detailed, nanoscopic mechanical characterization of microgels over an extended size range, to the high-throughput approaches providing distributions over the mechanical signatures, a feature not readily accessible by atomic force microscopy and micropipette aspiration. Full article

(This article belongs to the Special Issue Polyelectrolyte Gels: Volume II)

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