Polyelectrolyte Hydrogels Research Articles

A polyelectrolyte hydrogel coated loofah sponge evaporator based on Donnan effect for highly efficient solar-driven desalination

Solar desalination is considered an efficient way to alleviate the global shortage of freshwater resources. However, the salt accumulation on the surface of solar absorbers drastically reduces light absorption and steam evaporation efficiency, especially in highly concentrated brines. Herein, inspired by the salt rejection mechanisms of the mangrove trees, a sodium polyacrylate carbonized loofah (PAAS-CLF) was designed using the biomass porous loofah sponge as the matrix, modified with sodium polyacrylate (PAAS) hydrogel coating for solar-driven desalination. The loofah sponge coated PAAS hydrogel as a substrate could obtain a naturally high porosity. Moreover, relying on the negatively charged PAAS hydrogel rich in –COO– groups, it could confine Na+ to induce the Donnan effect, thus reducing the diffusion of salt ions into the water supply layer and avoiding salt accumulation fundamentally. The PAAS-CLF had high water evaporation rates of 1.83 kg m−2h−1 with an efficiency of 94.54% attained under 1 sun. More notably, the evaporator maintained stable water evaporation rates over 1.45 kg m−2h−1 in salt water with a broad range of salinity (35–200 g kg−1), and no salt accumulation was observed during the long-time evaporation. Therefore, this work provides an approach of the flexible application using environmentally friendly biomass sponge composite polyelectrolyte hydrogel for salt-resistant evaporator in solar desalination and concentrated brine treatment.

Multi-material 3D Printing of Mechanochromic Double Network Hydrogels for On-Demand Patterning.

Cephalopods can change their color and patterns by activating the skin chromatophores for camouflage. However, in the man-made soft material systems, it is greatly challenging to fabricate the color-change structure in the desired patterns and shapes. Herein, we employ a multi-material microgel direct ink writing (DIW) printing method to make mechanochromic double network hydrogels in arbitrary shapes. We prepare the microparticles by grinding the freeze-dried polyelectrolyte hydrogel and immobilize the microparticles in the precursor solution to produce the printing ink. The polyelectrolyte microgels contain mechanophores as the cross-linkers. We adjust the rheological and printing properties of the microgel ink by tailoring the grinding time of freeze-dried hydrogels and microgel concentration. The multi-material DIW 3D printing technique is utilized to fabricate various 3D hydrogel structures which could change into a colorful pattern in response to applied force. The microgel printing strategy shows great potential in the fabrication of the mechanochromic device with arbitrary patterns and shapes.

Carboxymethyl cellulose/sodium alginate beads incorporated with calcium carbonate nanoparticles and bentonite for phosphate recovery

Although anionic polyelectrolyte hydrogel beads offer attractive adsorption of cationic dyes, phosphate adsorption is limited by electrostatic interactions. In this work, carboxymethyl cellulose (CMC)/sodium alginate (SA) hydrogel beads were modified with calcium carbonate (CaCO3) and/or bentonite (Be). The compatibility between CaCO3 and Be was proven by the homogeneous surface, as shown in the scanning electron microscopic images. Fourier-transform infrared and X-ray diffraction spectra further confirmed the existence of inorganic filler in the hydrogel beads. Although CMC/SA/Be/CaCO3 hydrogel beads attained the highest methylene blue and phosphate adsorption capacities (142.15 MB mg/g, 90.31 P mg/g), phosphate adsorption was significantly improved once CaCO3 nanoparticles were incorporated into CMC/SA/CaCO3 hydrogel beads. The kinetics of MB adsorption by CMC/SA hydrogel beads with or without inorganic fillers could be described by the pseudo-second-order model under chemical interactions. The phosphate adsorption by CMC/SA/Be/CaCO3 hydrogel beads could be explained by the Elovich model due to heterogeneous properties. The incorporation of Be and CaCO3 also improved the phosphate adsorption through chemical interaction since Langmuir isotherm fitted the phosphate adsorption by CMC/SA/Be/CaCO3 hydrogel beads. Unlike MB adsorption, the reusability of these hydrogel beads in phosphate adsorption reduced slightly after 5 cycles.

An integrated Mg battery-powered iontophoresis patch for efficient and controllable transdermal drug delivery

Wearable transdermal iontophoresis eliminating the need for external power sources offers advantages for patient-comfort when deploying epidermal diseases treatments. However, current self-powered iontophoresis based on energy harvesters is limited to support efficient therapeutic administration over the long-term operation, owing to the low and inconsistent energy supply. Here we propose a simplified wearable iontophoresis patch with a built-in Mg battery for efficient and controllable transdermal delivery. This system decreases the system complexity and form factors by using viologen-based hydrogels as an integrated drug reservoir and cathode material, eliminating the conventional interface impedance between the electrode and drug reservoir. The redox-active polyelectrolyte hydrogel offers a high energy density of 3.57 mWh cm−2, and an optimal bioelectronic interface with ultra-soft nature and low tissue-interface impedance. The delivery dosage can be readily manipulated by tuning the viologen hydrogel and the iontophoresis stimulation mode. This iontophoresis patch demonstrates an effective treatment of an imiquimod-induced psoriasis mouse. Considering the advantages of being a reliable and efficient energy supply, simplified configuration, and optimal electrical skin-device interface, this battery-powered iontophoresis may provide a new non-invasive treatment for chronic epidermal diseases.

Mechanical deformation affects the counterion condensation in highly-swollen polyelectrolyte hydrogels.

Polyelectrolyte gels can generate electric potentials under mechanical deformation. While the underlying mechanism of such a response is often attributed to changes in counterion-condensation levels or alterations in the ionic conditions in the pervaded volume of the hydrogel, the exact molecular origins are largely unknown. By using all-atom molecular dynamics simulations of a polyacrylic acid hydrogel in explicit water as a model system, we simulate the uniaxial compression and uniaxial stretching of weakly to highly swollen (i.e., between 60-90% solvent content) hydrogel networks and calculate the microscopic condensation levels of counterions around the hydrogel chains. The counterion condensation under deformation is highly non-monotonic. Ionic condensation around the constituting chains of the deformed hydrogel tends to increase as the chains are stretched. This increase reaches a maximum and decreases as the chains are strongly stretched. The condensation around the collapsed chains of the hydrogel is weakly affected by the deformation. As a result, both compressing and stretching the model hydrogel lead to an overall increase in the counterion condensation. The effect vanishes for weakly swollen hydrogels, for which most ions are already condensed. The simulations with single, stretched polyelectrolyte chains show a qualitatively similar response, suggesting the effect of chain elongation on the ionic distribution throughout the hydrogel. Notably, this deformation-induced counterion condensation phenomenon does not occur in a polyelectrolyte solution at its critical concentration, indicating the role of hydrogel topology constraining the chain ends. Our results indicate that counterion condensation in a deforming polyelectrolyte hydrogel can be highly heterogeneous and exhibit a rich behaviour of electrostatic responses.

3D printed and stimulus responsive drug delivery systems based on synthetic polyelectrolyte hydrogels manufactured via digital light processing.

Hydrogels are three-dimensional hydrophilic polymeric networks absorbing up to and even more than 90 wt% of water. These superabsorbent polymers retain their shape during the swelling process while enlarging their volume and mass. In addition to their swelling behavior, hydrogels can possess other interesting properties, such as biocompatibility, good rheological behavior, or even antimicrobial activity. This versatility qualifies hydrogels for many medical applications, especially drug delivery systems. As recently shown, polyelectrolyte-based hydrogels offer beneficial properties for long-term and stimulus-responsive applications. However, the fabrication of complex structures and shapes can be difficult to achieve with common polymerization methods. This obstacle can be overcome by the use of additive manufacturing. 3D printing technology is gaining more and more attention as a method of producing materials for biomedical applications and medical devices. Photopolymerizing 3D printing methods offer superior resolution and high control of the photopolymerization process, allowing the fabrication of complex and customizable designs while being less wasteful. In this work, novel synthetic hydrogels, consisting of [2-(acryloyloxy) ethyl]trimethylammonium chloride (AETMA) as an electrolyte monomer and poly(ethylene glycol)-diacrylate (PEGDA) as a crosslinker, 3D printed via Digital Light Processing (DLP) using a layer height of 100 μm, are reported. The hydrogels obtained showed a high swelling degree q∞m,t ∼ 12 (24 h in PBS; pH 7; 37 °C) and adjustable mechanical properties with high stretchability (εmax ∼ 300%). Additionally, we embedded the model drug acetylsalicylic acid (ASA) and investigated its stimulus-responsive drug release behaviour in different release media. The stimulus responsiveness of the hydrogels is mirrored in their release behavior and could be exploited in triggered as well as sequential release studies, demonstrating a clear ion exchange behavior. The received 3D-printed drug depots could also be printed in complex hollow geometry, exemplarily demonstrated via an individualized frontal neo-ostium implant prototype. Consequently, a drug-releasing, flexible, and swellable material was obtained, combining the best of both worlds: the properties of hydrogels and the ability to print complex shapes.

Tuning the Mechanical Properties of Multiarm RAFT-Based Block Copolyelectrolyte Hydrogels via Ionic Cross-Linking for 3D Cell Cultures.

Hydrogels that serve as native extracellular matrix (ECM) mimics are typically naturally derived hydrogels that are physically cross-linked via ionic interactions. This means rapid gelation of synthetic polymers, which give control over the chemical and physical cues in hydrogel formation. Herein, we combine the best of both systems by developing a synthetic hydrogel with ionic cross-linking of block copolyelectrolytes to rapidly create hydrogels. Reversible addition-fragmentation chain-transfer (RAFT) polymerization was used to synthesize oppositely charged polyelectrolyte molecules and, in turn, modulate the mechanical property of stiffness. The mechanical stiffness of a range of 900-3500 Pa was tuned by varying the number of charged ionic groups, the length of the polymer arms, and the polymer concentration. We demonstrate the synthetic polyelectrolyte hydrogel as an ECM mimic for three-dimensional (3D) in vitro cell models using MCF-7 breast cancer cells.

Some aspects of the development of formalism of nonequilibrium thermodynamics for systems based on polyelectrolyte hydrogels

Показано, что формализм неравновесной термодинамики, основанныйна использовании функционалов, выражающих определенный принцип максимума илиминимума, для ряда частных задач может быть развит вне зависимости от адекватноститрактовки понятия "энтропия" в применении к системам, далеким от состояния равновесия.В частности, показано, что для неравновесных систем, поведение которых преимущественноопределяется движением низкомолекулярных ионов, функционалы, выполняющие ту жефункцию, что и термодинамические, могут быть получены как первые интегралыуравнений движения данных частиц. Преимуществом такого подхода является исключениенеобходимости обоснования адекватности использования понятия "энтропия". Получен явныйвид таких функционалов. Обсуждаются возможности их использования в прикладных целях

Undirected graphical model of adjacency matrix for dynamic elasticity in polyelectrolyte hydrogels

Molecular network plays a critical role in determining dynamic elasticity of soft condensed polymers, e.g., their varied cross-linking densities and end-to-end distances with changed Young's moduli. Although it has been studied for decades, the coupling relationship between vertices and edges of molecular networks is not well understood, mainly because the degree of the crosslinking points and asymmetry molecular networks have not been considered in the previously models. In this study, a graph theory was employed to formulate an undirected graphical model and describe the coupling between vertices and edges in molecular networks, based on which the dynamic elasticity of polyelectrolyte hydrogel was modeled. From the Kirchhoff graph theory and bead-spring model, the coupling relationship between vertices and edges was obtained using end-to-end distance and viscosity parameters. Combining the Watts–Strogatz model and kinetic probability, the coupling between vertices and edges for the polyelectrolyte network was studied. Furthermore, an adjacency matrix with eigenvalue, number of vertices and mean degree was proposed to formulate constitutive relationships including dynamic elasticity and stress-strain, according to rubber elasticity theory and Mooney-Rivlin model, respectively. The linking between the vertices and edges determines the network structure and dynamic elasticity of the polyelectrolyte hydrogel. Based on the graph theory, the vertices and edges are encoded by adjacency matrix, which is proposed to describe the dynamic elasticity of symmetric and asymmetric network structures using the crosslinking density and end-to-end distance. Finally, effectiveness of the undirected graphical model was verified using both finite element analysis and experimental results of polyelectrolyte hydrogels reported in literature.

Simulations Explain the Swelling Behavior of Hydrogels with Alternating Neutral and Weakly Acidic Blocks

We used computer simulations to study the equilibrium swelling of weak (pH-responsive) polyelectrolyte hydrogels as a function of pH and salt concentration in a supernatant solution. Our simulations were designed to represent recently synthesized gels composed of tetrapoly(acrylic acid) and tetrapoly(ethylene glycol) stars. To model the ionization equilibrium of the weak acid groups and the exchange of small ions with the reservoir, we applied the recently developed grand-reaction Monte Carlo method. We showed that the ionization of these gels as a function of the pH significantly deviates from the ideal Henderson–Hasselbalch equation due to two main contributions: (1) electrostatic interactions and (2) Donnan partitioning of small ions. The first contribution dominates in the gels composed of alternating neutral and acidic blocks, contrasting with our previous observations that both contributions are comparably strong in hydrogels composed of homogeneously distributed weak acid groups. We also critically examined the counterion condensation argument, previously invoked to explain why the experimentally observed swelling was lower than predicted by theory. Thus, a detailed analysis of the simulations allowed us to understand which of the above effects dominates in different systems and why, thereby allowing us to identify the correct physical origin of the deviations from ideal swelling. Such an understanding is important not only for correctly interpreting experimental measurements but also for designing polyelectrolyte gels tailored to exhibit specific swelling response to pH and salt concentration.

Water Desalination Using Polyelectrolyte Hydrogel: Gibbs Ensemble Modeling.

Polyelectrolyte hydrogels can absorb a large amount of water across an osmotic membrane as a result of their swelling pressure. On the other hand, the insoluble cross-linked hydrogel network enables dewatering under the influence of external (thermal and/or mechanical) stimuli. Moreover, from a thermodynamic perspective, a polyelectrolyte hydrogel is already an osmotic membrane. These properties designate hydrogels as excellent candidates for use in desalination, at the same time avoiding the use of expensive membranes. In this article, we present our recent theoretical study of polyelectrolyte hydrogel usage for water desalination. Employing a coarse-grained model and the Gibbs ensemble, we modeled the thermodynamic equilibrium between the coexisting gel phase and the supernate aqueous salt solution phase. We performed a sequence of step-by-step hydrogel swellings and compressions in open and closed systems, i.e., in equilibrium with a large and with a comparably small reservoir of aqueous solution. The swelling in an open system removes ions from the large reservoir, whereas the compression in a closed system decreases the salt concentration in the small reservoir. We modeled this stepwise process of continuous decrease of water salinity from seawater up to freshwater concentrations and estimated the energy cost of the process to be comparable to that of reverse osmosis.

Strong tough hydrogel solar evaporator with wood skeleton construction enabling ultra‐durable brine desalination

AbstractHydrogels, a highly versatile material platform, are poised to become a promising candidate for interfacial solar evaporators on account of their record‐high evaporation rates. Nevertheless, conventional hydrogels with loose crosslinking, low solid content, and isotropic network structure are plagued by fragile mechanical tolerance, impairing their application in severe seawater environments. Herein, we develop a skeleton‐construct polyelectrolyte hydrogel (SCPH) evaporator by a universal, simple strategy that maintains not only the original properties of hydrogels, but also endows a high mechanical strength. By virtue of this strategy, the SCPH increased its mechanical strength to 1 MPa, 500 times higher than the original hydrogel. Surprisingly, the strategy also endows the SCPH with some extra properties, such as a high salt rejection ratio, a high water‐pumping rate, and low thermal conductivity, which render the SCPH a perfect evaporator. Specifically, this SCPH material significantly improves the long‐term stability of evaporators, enabling the evaporator to be capable of evaporating for 30 days in 20 wt% brine at a stable evaporation rate, exceeding the longest evaporation time ever reported. In addition, it achieves an evaporation rate of up to 2.13 kg m−2 h−1, which is twice that of the currently existing polyelectrolyte hydrogel evaporators. The skeleton construction has the potential to significantly improve the efficiency, stability, and durability of evaporators, hence bringing interfacial desalination technology toward practical applications.image

PH-responsive hydrogels of carboxymethyl cellulose and polyethyleneimine for efficient removal of ionic dye molecules

A carboxymethyl cellulose (CMC)/polyethyleneimine (PEI) hybrid hydrogel was successfully prepared under green processing conditions. The CMC and PEI formed a polyelectrolyte complex, and the three-dimensional (3D) network structure was hardened by chemical crosslinking, resulting in suitable hydrogel properties. In the prepared CMC/PEI polyelectrolyte hydrogel, the surface charge was easily switched according to the pH change, and the resulting swelling and contraction was reversible. The pH sensitivity of the CMC/PEI hydrogel was effective in removing ionic dye contaminants, and as a result, it showed an excellent removal capacity of 319 mg/g for anionic acid orange (AO) and 129 mg/g for cationic methylene blue (MB). In addition, the CMC/PEI polyelectrolyte hydrogel maintained structural stability despite repeated changes in surface charge characteristics and shrinkage-swelling due to repeated pH conversion. As a result, in the reuse process through repeated adsorption and desorption, it showed an excellent reuse efficiency of more than 93% even after 10 reuse cycles.

Polyelectrolyte hydrogel: A versatile platform for mechanical-electric conversion and self-powered sensing

There is an urgent need to develop self-powered technologies for portable strain/pressure sensors. In this work, inspired by salinity-gradient electricity generation, we report the design and fabrication of a mechanical-electric conversion and self-powered sensing platform based on polyelectrolyte hydrogels. Using polyacrylic acid (PAA, whose -COOH group can ionize H+ in water) hydrogel as a model system, we observe that the compression deformation process of the hydrogel leads to a localized increase in the H+ concentration, that induces a directional H+ diffusion as a result of ion concentration difference. In contrast, the PAA chain (negatively charged), as a hydrogel backbone, is hindered from moving freely due to the locking of the hydrogel network. Such drastic difference between ionized H+ and fixed PAA chain defines an opportunity for converting mechanical energy into electricity by compressing the PAA hydrogel. The good elasticity of the PAA hydrogel allows us to repeatedly regenerate and fully recover the electrical signal and finally completes the recycle sensing (>1000 s). PAA hydrogels have also been shown to convert static or low frequency (∼0.05 Hz) pressure (0.05–5 N) to output direct voltage/current. In particular, the hydrogel can yield a voltage output up to ∼6 mV and a current of ∼2 μA when the pressure is 5 N. Taking full advantage of the special sensing principle, we have developed both self-powered and temperature-insensitive PAA hydrogel-based position recognizers and multi-dimensional sound detectors. Importantly, this mechanical-to-electric conversion concept is also applicable to other polyelectrolyte hydrogels. This simple and low-cost soft material system is promising for applications in sensing and energy harvesting.

Superlubricity of pH-responsive hydrogels in extreme environments.

Poly(acrylamide-co-acrylic acid) (P(AAm-co-AA)) hydrogels are highly tunable and pH-responsive materials frequently used in biomedical applications. The swelling behavior and mechanical properties of these gels have been extensively characterized and are thought to be controlled by the protonation state of the acrylic acid (AA) through the regulation of solution pH. However, their tribological properties have been underexplored. Here, we hypothesized that electrostatics and the protonation state of AA would drive the tribological properties of these polyelectrolyte gels. P(AAm-co-AA) hydrogels were prepared with constant acrylamide (AAm) concentration (33 wt%) and varying AA concentration to control the amount of ionizable groups in the gel. The monomer:crosslinker molar ratio (200:1) was kept constant. Hydrogel swelling, stiffness, and friction behavior were studied by systematically varying the acrylic acid (AA) concentration from 0–12 wt% and controlling solution pH (0.35, 7, 13.8) and ionic strength (I = 0 or 0.25 M). The stiffness and friction coefficient of bulk hydrogels were evaluated using a microtribometer and borosilicate glass probes as countersurfaces. The swelling behavior and elastic modulus of these polyelectrolyte hydrogels were highly sensitive to solution pH and poorly predicted the friction coefficient (µ), which decreased with increasing AA concentration. P(AAm-co-AA) hydrogels with the greatest AA concentrations (12 wt%) exhibited superlubricity (µ = 0.005 ± 0.001) when swollen in unbuffered, deionized water (pH = 7, I = 0 M) and 0.5 M NaOH (pH = 13.8, I = 0.25 M) (µ = 0.005 ± 0.002). Friction coefficients generally decreased with increasing AA and increasing solution pH. We postulate that tunable lubricity in P(AAm-co-AA) gels arises from changes in the protonation state of acrylic acid and electrostatic interactions between the probe and hydrogel surface.

Polyelectrolyte Hydrogels for Tissue Engineering and Regenerative Medicine.

Polyelectrolyte hydrogels are emerging materials for tissue engineering and regenerative medicine applications due to their tunable biochemical properties, electrical conductivity, biocompatibility and similar network structure to the extracellular matrix in mammalian bodies. In this review, representative polyelectrolyte hydrogels carrying anionic, cationic, ampholytic, zwitterionic and ionic liquid moieties are systemically cataloged to express their chemical structures and preparation strategies. Recent advance of polyelectrolyte hydrogels in tissue engineering and regenerative medicine for drug delivery, skin healing, bone regeneration, cardiac tissue repair and anti-biofouling coating are also highlighted. Eventually, the outlook and challenges of polyelectrolyte hydrogels and their biomedical material applications are also discussed to offer future directions.

A polyzwitterion-based antifouling and flexible bilayer hydrogel coating

A coating of implanted medical devices with antifouling ability, preferable mechanical property and stability in the physiological environment proves beneficial for their clinical use. Here we develop a flexible bilayer hydrogel coating with self-healing, antifouling, and lubricating functions. Our bilayer coating consists of two functional layers: i) an adhesive flexible polyelectrolyte hydrogel layer, ii) an antifouling and lubricous polyzwitterionic layer. The bilayer hydrogel coating is prepared in two steps dip-coating and in situ UV photopolymerization. Our results demonstrate that the prepared bilayer hydrogel coating has proper mechanical properties, favorable hydrophilicity, and lower surface friction. We demonstrate that the bilayer hydrogel coating maintains hydrophilicity under extreme condition (1 h sonicating or 5000 times repeating bending). The as-prepared bilayer hydrogel coating resists non-specific protein adsorption, inhibits cell and platelet adhesion, and prevents thrombosis. This work provides a reliable coating on the medical materials, showing a promising practical application in diversified implantable medical devices.

Facile fabrication of high performance zwitterionic P(NVP‐co‐SPE)/polyvinyl alcohol hydrogel polyelectrolyte for capacitor

AbstractPolyelectrolyte hydrogel used for supercapacitors have the roles of both electrolyte and separator, the morphology and the strength of gel polyelectrolyte (GPE) are the keys for ionic transport properties therein. Here, the P(NVP‐co‐SPE)/polyvinyl alcohol (PVA) IPN hydrogel was prepared by photopolymerization and thermal polymerization, respectively. The influences of preparation method on morphology, mechanical strength and electrochemical performances were studied. Photopolymerized IPN GPE (UV‐GPE) exhibits a special regular microphase separation structure, and therefore has outstanding strength‐toughness and better electrochemical performance. The tensile strength and the toughness increased intensively from the 6.2 MPa and 3.4 MJ m−3 of thermal polymerized IPN GPE (TH‐GPE) to 28.5 MPa and 24.9 MJ m−3, respectively. The maximum swelling ratio of UV‐GPE is increased from 327% of TH‐GPE to 630%. The ionic conductivity of UV‐GPE increased from 4.29 × 10−2 S/cm of TH‐GPE to 5.82 × 10−2 S/cm. UV‐GPE has higher response current and specific capacitance. It also has the obviously better charge–discharge symmetry and cycling stability, and the capacity retention rate is still 92.3% after 1000 cycles. Thus, it brings out the fast, eco‐economic photopolymerization method is an effective way to prepare high‐performance polyelectrolyte hydrogel.

Laponite-Doped Poly(acrylic acid-co-acrylamide) Hydrogels

Electroacoustic characterization of soft nanocomposites has provided unique insights into the microstructure of soft nanocomposites, including nanoparticle (NP)-doped hydrogels and polyelectrolyte hydrogels without NPs. An outstanding problem is how to interpret the electrokinetic sonic amplitude (ESA) of charged hydrogels bearing charged NPs because both components generate an acoustic response to the electrical forcing. To this end, we study a series of Laponite XLG-doped, neutralized poly(acrylic acid-co-acrylamide) hydrogels, drawing principally on the ESA, electrical conductivity, and linear viscoelastic rheology. The hydrogel charge density was varied by the fraction of acrylic acid monomer fAAc = 0–1 while maintaining the total monomer concentration ≈8 wt % with Laponite concentration ≈0.85 wt %. Upon comparison of data from this study to those in a recent benchmark study of charged hydrogels without NPs, Laponite doping increased the electroacoustic signal and ionic conductivity but decreased the hydrogel storage modulus. Mechanistic theoretical models predicting how the real part of the ESA (at low frequency) and ionic conductivity of polyelectrolyte hydrogels depend on fAAc were extended to Laponite-doped hydrogels, together furnishing an estimate of the partial molar volume of acrylamide (in polymer form) that is close to the value for pure acrylamide (based on its density and molecular weight). The generally lower storage modulus with Laponite doping contrasts with previous studies of Laponite-doped polyacrylamide and poly(acrylic acid) hydrogels and solutions. This seems to reflect the high degree of neutralization, which transforms an attraction between protonated carboxyl moieties and Laponite to an electrostatic repulsion. The hindering effects of polymerization and cross-linking on acrylic acid-co-acrylamide networks were also investigated by comparing the ESA and conductivity of hydrogels with their monomer solution counterparts. Systematically varying the ratio of charged to uncharged monomers, with and without chemical cross-linking, provides insights to benefit a broad range of technological applications for hydrogel nanocomposites.

Superior stretchable and tough P(AA‐co‐AAm)/silica hybrid hydrogels enabled by acid‐mediated radiation polymerization

AbstractThe emerging stretchable and tough hydrogels find their extraordinary contributions in widespread use. However, the applicability of existing tough hydrogels in harsh environments like highly acidic stomachs has remained a challenge. Here, we develop superior stretchable and tough P(AA‐co‐AAm)/SiO2 hybrid hydrogels through radiation‐induced polymerization and crosslinking in the presence of acids. The performance of the nanocomposite gels was optimized by altering fabrication variables including acid concentration, monomer composition and absorbed dose. The mechanic tests show that the protonated hybrid hydrogels displayed a high elongation at break (1919%), fracture strength (117 kPa), and compressive strength at 50% strain (126 kPa). These values are superior to those of conventional polyelectrolyte hydrogels and expect to significantly broaden the application. The proposed strategy is quite general and offers an alternative strategy to develop tough hydrogels.