Polymer Network Research Articles

Randomly cross-linked amphiphilic copolymer networks of n-butyl acrylate and N,N-dimethylacrylamide: synthesis and characterization

Abstract Five randomly cross-linked amphiphilic copolymer networks (ACPN) were prepared via the free radical cross-linking copolymerization of the hydrophobic n-butyl acrylate (BuA) and the hydrophilic N,N-dimethylacrylamide (DMAAm), in the presence of a small amount (5 mol% with respect to the sum of BuA plus DMAAm monomers) of the hydrophobic 1,6-hexanediol diacrylate (HexDA) cross-linker, initiated by 4,4ʹ-azobis(4-cyanovaleric acid) in 1,4-dioxane at a 10 % w/v total monomer concentration. The five ACPNs differed in their BuA content, fixed at 10, 30, 50, 70 and 90 mol%. The two homopolymer networks, BuA and DMAAm, were also prepared using the same polymerization method. Thus, this study involved a total of seven polymer networks, forming a homologous series with BuA contents ranging from 0 to 100 mol%. These networks were characterized in terms of their degrees of swelling in tetrahydrofuran (THF) and water, their mechanical properties in water, and their adhesion to human skin. The degrees of swelling (DS) of the networks in THF were higher than those in water because THF is a non-selective solvent, whereas water is selective for the hydrophilic DMAAm units. The DSs in THF increased with the network content in the less polar BuA units, while the opposite was true for the DSs in water which decreased with the content in the hydrophobic BuA units from 11 (0 mol% BuA) down to 1.1 (100 mol% BuA). A maximum in the elastic modulus was observed for the hydrogel with 50 mol% BuA, reflecting the opposing effects of polymer composition in soft polymer (polyBuA) content and hydrogel water content. In contrast, the tensile strain at break increased monotonically with the hydrogel BuA content, reaching a remarkable ∼900 % for the hydrogel with 90 mol% BuA. Finally, the adhesion of the ACPNs, both in their dried and hydrated states, onto human skin was explored. The adhesion of the hydrated samples onto skin was stronger than that of the dried ones. The hydrated ACPN with 30 mol% BuA exhibited the strongest adhesion onto skin, attributable to the best combination of a rather high content in polar DMAAm units (70 mol%), and a rather low aqueous DS (∼2.5), with the low DS value causing only a small dilution in the DMAAm units participating in the polar interactions with skin. The present work demonstrates that, even in this synthetically simple ACPN system, the multiple effects of ACPN composition on a certain property, in some cases opposing and in some other cooperating, lead to a rather complicated behavior. In particular, as the BuA content increases, some properties display maxima (elastic modulus, stress at break and fracture energy of hydrated ACPNs, and adhesion of hydrated ACPNs onto skin) while some other properties exhibit monotonic increases (strain at break of hydrated ACPNs, and adhesion of dried ACPNs onto skin). Thus, the optimal ACPN for a particular application will highly depend on the property best-serving the particular application, e.g., the ACPNs with 30, 50 and 90 mol% BuA for strongest wet adhesion to skin, stiffest hydrogel response, and highest hydrogel extensibility and toughness, respectively.

Strain‐Stiffening, Robust yet Compliant Ionic Elastomer from Highly Entangled Polymer Networks and Metal–Oxygen Interactions

Strain‐Stiffening, Robust yet Compliant Ionic Elastomer from Highly Entangled Polymer Networks and Metal–Oxygen Interactions

Poroviscoelastic relaxations and rate-dependent adhesion in gelatin.

Hydrogels, polymeric networks swollen with water, exhibit time/rate-dependent adhesion due to their poroviscoleastic constitution. In this study, we conducted probe-tack experiments on gelatin and investigated the influence of dwelling times and unloading rates on pull-off forces and work of adhesion. We utilized in situ contact imaging to monitor separation kinematics and interfacial crack velocities. We found that the crack velocities scaled nonlinearly with the unloading rate, in a power law with an exponent of 0.8 and were independent of dwelling time. At maximum unloading rates corresponding to subsonic interfacial crack speeds, we observed an order of magnitude enhancement in the apparent work of adhesion. The enhancement of adhesion and the crack velocities were related by a power law with an exponent of 0.39. The maximum vertical extension during unloading, a measure of crack opening, exhibited linear correlation with the enhancement of adhesion. Both correlations were in line with the rate-dependent work of fracture modeled for viscoelastic solids (e.g., Persson and Brener model). We explored the links between dwelling times corresponding to varying degrees of poroelastic diffusion and the adhesion. We found 40% additional enhancement in adhesion at the highest unloading rate. This enhancement is due to the unbalanced osmotic pressure, also known as the suction effect. The influence of dwelling times on adhesion was negligible for the interfacial cracks propagating slower than the diffusive time scales. These results identify viscoelastic relaxations as the dominant mechanism governing the rate-dependent enhancement of adhesion, and hence pave the way for tuning rate-dependent adhesion in soft multiphasic materials.

Dynamic covalent polymer network enabled bio‐based vitrimers with excellent substrate bonding

AbstractIn this article, we report the synthesis of dynamic covalent polymer network (DCPN)‐enabled vitrimers derived from gum rosin—a pine tree extract. The concept of Diels Alder (DA) thermo‐reversibility has been used to introduce self‐healing/dynamic covalent ability into the polymer systems. For this purpose, the diene moieties were incorporated in the gum rosin‐based polymer network, and the bismaleimides served as the dienophiles. The thermo‐reversible crosslinks formed between the diene and dienophile moieties can enhance the service life of the materials by improving the durability, mechanical as well as chemical properties of the materials. Upon damage, the thermo‐reversible DA bonds de‐crosslink resulting in smaller chain segments, enabling the material to flow to the site of damage and seal the fractured edges. The formation of DA crosslinks has been confirmed by using Fourier transform‐infrared spectroscopic technique. Scanning electron microscopy analysis was used to verify the self‐healing ability of the resin matrix. The healing process was studied after manual scratches were made. The polymers' ability to heal multiple times is investigated using differential scanning calorimetry. The adhesion strength of the self‐healing material has been examined on variety of substrates such as glass, steel, and concrete. The materials exhibited superior adhesion strength on glass substrate.

Solar-Driven Drum-Type Atmospheric Water Harvester Based on Bio-Based Gels with Fast Adsorption/Desorption Kinetics.

Sorption-based atmospheric water harvesting is an attractive technology for exploiting unconventional water sources. A critical challenge is how to facilitate fast and continuous collection of potable water from air. Here, a bio-based gel (CAL gel), resulting from the integration of a whole biomass-derived polymer network with lithium chloride is reported. A fast adsorption/desorption kinetics, with a water capture rate of 1.74kg kg-1 h-1 at 30% relative humidity and a desorption rate of 1.98kg kg-1 h-1, was simultaneously realized in one piece of CAL gel, because of its strong hygroscopicity, hydrophilic network, abundant water transport channels, photothermal conversion ability, and ∼200-μm-thick self-supporting bulky structure caused by multicomponent synergy. A solar-driven, drum-type, tunable, and portable harvester is designed that can harvest atmospheric water within a brief time. Under outdoor conditions, the harvester with CAL gels operates 36 switches (180°) per day realizes a water yield of 8.96kg kggel -1 (18.87g kgdevice -1). This portable harvester highlights the potential for fast and scalable atmospheric water harvesting in extreme environments. This article is protected by copyright. All rights reserved.

Lysine-Triggered Polymeric Hydrogels with Self-Adhesion, Stretchability, and Supportive Properties

Hydrogels, recognized for their flexibility and diverse characteristics, are extensively used in medical fields such as wearable sensors and soft robotics. However, many hydrogel sensors derived from biomaterials lack mechanical strength and fatigue resistance, emphasizing the necessity for enhanced formulations. In this work, we utilized acrylamide and polyacrylamide as the primary polymer network, incorporated chemically modified poly(ethylene glycol) (DF-PEG) as a physical crosslinker, and introduced varying amounts of methacrylated lysine (LysMA) to prepare a series of hydrogels. This formulation was labeled as poly(acrylamide)-DF-PEG-LysMA, abbreviated as pADLx, with x denoting the weight/volume percentage of LysMA. We observed that when the hydrogel contained 2.5% w/v LysMA (pADL2.5), compared to hydrogels without LysMA (pADL0), its stress increased by 642 ± 76%, strain increased by 1790 ± 95%, and toughness increased by 2037 ± 320%. Our speculation regarding the enhanced mechanical performance of the pADL2.5 hydrogel revolves around the synergistic effects arising from the co-polymerization of LysMA with acrylamide and the formation of multiple intermolecular hydrogen bonds within the network structures. Moreover, the acid, amine, and amide groups present in the LysMA molecules have proven to be instrumental contributors to the self-adhesion capability of the hydrogel. The validation of the pADL2.5 hydrogel’s exceptional mechanical properties through rigorous tensile tests further underscores its suitability for use in strain sensors. The outstanding stretchability, adhesive strength, and fatigue resistance demonstrated by this hydrogel affirm its potential as a key component in the development of robust and reliable strain sensors that fulfill practical requirements.

Stereoscopic Polymer Network for Developing Mechanically Robust Flexible Perovskite Solar Cells with an Efficiency Approaching 25.

Flexible perovskite solar cells (pero-SCs) have the potential to overturn the application scenario of silicon photovoltaic technology. However, their mechanical instability severely impedes their practical applicability, and the corresponding intrinsic degradation mechanism remains unclear. In this study, we systematically analyzed the degradation behavior of flexible pero-SCs under mechanical stress and observed that the structural failure firstly occurs in the polycrystal perovskite film, then extend to interfaces. To suppress the structural failure, we employed pentaerythritol triacrylate, a crosslinked molecule with three stereoscopic crosslink sites, to establish a three-dimensional polymer network in both the interface and bulk perovskite. This network reduced the Young's modulus of the perovskite and simultaneously enhanced the interfacial toughness. As a result, the formation of cracks and delamination, which occur under a high mechanical stress, was significantly suppressed in the flexible pero-SC, which consequently retained 92% of its initial power conversion efficiency (PCE) after 20000 bending cycles. Notably, the flexible device also showed a record PCE of 24.9% (certified 24.48%). This article is protected by copyright. All rights reserved.

Swelling Behaviors of Natural Rubber/Solvent Systems Based on the Extended Modified Double Lattice Model

AbstractSwelling experiments are conducted on nonfiller natural rubber using four solvents (toluene, cyclohexane, tetrahydrofuran (THF), and methylethylketone (MEK)) over temperatures from 10 to 70 °C. Toluene, cyclohexane, and THF, classified as effective solvents, show swelling ratios between 3 and 7, influenced by the crosslink density of the rubber. MEK, however, has a lower ratio of 1.5 to 2. Temperature has a minor impact on swelling compared to the crosslink density. The study evaluates the Extended Modified Double Lattice (EMDL) model for its mixing contribution in polymer network swelling, aiming to improve the Flory–Hüggins (FH) model. The superiority of EMDL above FH is in the boundary condition at the unvulcanized state, the former aligning its interaction energy with values from solvent activities in primary linear polymer/solvent solutions, unlike the FH model. The EMDL model also accounts for oriented interactions in polar solvents through a secondary lattice, linking specific interaction energy with solvent dipole moments. The study observes a nonlinear correlation between crosslinking density and sulfur amount, proposing a nonrandom mixing at lower sulfur concentrations. This model shows strong alignment with experimental data, suggesting that replacing the FH model's mixing contribution with the EMDL model could improve results with minimal additional complexity.

Nested non-covalent interactions expand the functions of supramolecular polymer networks

Supramolecular polymer networks contain non-covalent cross-links that enable access to broadly tunable mechanical properties and stimuli-responsive behaviors; the incorporation of multiple unique non-covalent cross-links within such materials further expands their mechanical responses and functionality. To date, however, the design of such materials has been accomplished through discrete combinations of distinct interaction types in series, limiting materials design logic. Here we introduce the concept of leveraging “nested” supramolecular crosslinks, wherein two distinct types of non-covalent interactions exist in parallel, to control bulk material functions. To demonstrate this concept, we use polymer-linked Pd2L4 metal–organic cage (polyMOC) gels that form hollow metal–organic cage junctions through metal–ligand coordination and can exhibit well-defined host-guest binding within their cavity. In these “nested” supramolecular network junctions, the thermodynamics of host-guest interactions within the junctions affect the metal–ligand interactions that form those junctions, ultimately translating to substantial guest-dependent changes in bulk material properties that could not be achieved in traditional supramolecular networks with multiple interactions in series.

Polymerized Network-Based Artificial Peroxisome Reprogramming Macrophages for Photoacoustic Imaging-Guided Treatment of Rheumatoid Arthritis.

Artificial peroxisomes (AP) with enzyme-mimetic catalytic activity and recruitment ability have drawn a great deal of attention in fabricating protocell systems for scavenging reactive oxygen species (ROS), modulating the inflammatory microenvironment, and reprogramming macrophages, which is of great potential in treating inflammatory diseases such as rheumatoid arthritis (RA). Herein, a macrophage membrane-cloaked Cu-coordinated polyphthalocyanine-based AP (CuAP) is prepared with a macrocyclic conjugated polymerized network and embedded Cu-single atomic active center, which mimics the catalytic activity and coordination environment of natural superoxide dismutase and catalase, possesses the inflammatory recruitment ability of macrophages, and performs photoacoustic imaging (PAI)-guided treatment. The results of both in vitro cellular and in vivo animal experiments demonstrated that the CuAP under ultrasound and microbubbles could efficiently scavenge excess ROS in cells and tissues, modulate microenvironmental inflammatory cytokines such as interleukin-1β, tumor necrosis factor-α, and arginase-1, and reprogram macrophages by polarization of M1 (proinflammatory phenotype) to M2 (anti-inflammatory phenotype). We believe this study offers a proof of concept for engineering multifaceted AP and a promising approach for a PAI-guided treatment platform for RA.

Multifunctional coating films of gelatin/d-glucose/agarose reinforced by Piper L. leaf extract: Physicochemical properties and mandarin preservation

Eco-friendly, active, non-toxic coatings have recently been considered as a feasible approach for enhancing the storability of post-harvest produce, which deal with global problem from environmental pollutions of chemical uses and petro-based plastics. In this interest, the present work is to the development of multifunctional coating films through blending gelatin, agarose, d-glucose, and Piper betle L. extract before being applied for preserving ‘Duong’ mandarin. As compared to single addition of agarose, the co-supplement of agarose and d-glucose was considered to be more efficient in revitalizing the mechanical strength and surface hydrophobicity of the neat gelatin, which highlighted a vital role of d-glucose in the polymeric network. Interestingly, gelatin/d-glucose/agarose formulated with Piper betle L. extract showed greater mechanical strength, flexibility, UV-shielding, solubility, antibacterial and antioxidant activities than gelatin/d-glucose/agarose coating films. Considerably, the antibacterial property of such active films/coatings was found to even better after being aged for 30 days at the room temperature (25 °C). Based on initial screening, the gelatin/agarose/d-glucose coating formulation included with 2 % (w/w) Piper betle L. extract presenting the best mechanical strength (2.46 MPa), flexibility (31.03 %), and antioxidant activity (IC50 = 37.45 μg/mL) along with the adequate tested bacterial inhibition, was applied for ready-to-eat mandarin preservation. The significant improvement in visual appearance and peel color was recorded for the mandarin coated with gelatin/agarose/d-glucose/Piper betle L. extract (2 %, w/w) after 17-d storage at 20 °C and 64 %RH, which exhibited the lowest mass loss and total soluble solids.

Deciphering the factors influencing electric field mediated polymerization and depolymerization at the solution–solid interface

Strong and oriented electric fields are known to influence structure as well as reactivity. The strong electric field (EF) between the tip of a scanning tunneling microscope (STM) and graphite has been used to modulate two-dimensional (2D) polymerization of aryl boronic acids where switching the polarity of the substrate bias enabled reversible transition between self-assembled molecular networks of monomers and crystalline 2D polymer (2DP) domains. Here, we untangle the different factors influencing the EF-mediated (de)polymerization of a boroxine-based 2DP on graphite. The influence of the solvent was systematically studied by varying the nature from polar protic to polar aprotic to non-polar. The effect of monomer concentration was also investigated in detail with a special focus on the time-dependence of the transition. Our experimental observations indicate that while the nucleation of 2DP domains is not initiated by the applied electric field, their depolymerization and subsequent desorption, are a consequence of the change in the polarity of the substrate bias within the area scanned by the STM tip. We conclude that the reversible transition is intimately linked to the bias-induced adsorption and desorption of the monomers, which, in turn, could drive changes in the local concentration of the monomers.

Synthesis of PDMS Chain Structure with Introduced Dynamic Covalent Bonding for High‐Performance Rehealable Tactile Sensor Application

AbstractIn addressing the increasing demand for wearable sensing systems, the performance and lifespan of such devices must be improved by enhancing their sensitivity and healing capabilities. The present work introduces an innovative method for synthesizing a healable disulfide bond contained in a polydimethylsiloxane network (PDMS−SS) that incorporates ionic salts, which is designed to serve as a highly effective dielectric layer for capacitive tactile sensors. Within the polymer network structure, the cross‐linking agent pentaerythritol tetrakis 3‐mercaptopropionate (PTKPM) forms reversible disulfide bonds while simultaneously increasing polymer softness and the dielectric constant. The incorporation of dioctyl sulfosuccinate sodium salt (DOSS) significantly improves the capacitance and sensing properties by forming an electrical double‐layer through interactions between the electrode charge and salt ions at the contact interface. The developed polymer material‐based tactile sensor shows a strong response signal at low pressure (0.1 kPa) and maintains high sensitivity (0.175 kPa−1) over a wide pressure range (0.1–10 kPa). It also maintains the same sensitivity over 10 000 repeated applications of external pressure and is easily self‐healed against mechanical deformation due to the dynamic disulfide covalent bonding, restoring ≈95% of its detection capacity.

Rugged Island-Bridge Inorganic Electronics Mounted on Locally Strain-Isolated Substrates.

Various strain isolation strategies that combine rigid and stretchable regions for stretchable electronics were recently proposed, but the vulnerability of inorganic materials to mechanical stress has emerged as a major impediment to their performance. We report a strain-isolation system that combines heteropolymers with different elastic moduli (i.e., hybrid stretchable polymers) and utilize it to construct a rugged island-bridge inorganic electronics system. Two types of prepolymers were simultaneously cross-linked to form an interpenetrating polymer network at the rigid-stretchable interface, resulting in a hybrid stretchable polymer that exhibited efficient strain isolation and mechanical stability. The system, including stretchable micro-LEDs and microheaters, demonstrated consistent operation under external strain, suggesting that the rugged island-bridge inorganic electronics mounted on a locally strain-isolated substrate offer a promising solution for replacing conventional stretchable electronics, enabling devices with a variety of form factors.

A Rational Design of Bio-Derived Disulfide CANs for Wearable Capacitive Pressure Sensor.

Classic approaches to integrate flexible capacitive sensor performance are to on-demand microstructing dielectric layers and to adjust dielectric material compositions via the introduction of insoluble carbon additives (to increase sensitivity) or dynamic interactions (to achieve self-healing). However, the sensor's enhanced performances often come with the increased material complexity, discouraging its circular economy. Herein, we introduce a new intrinsic self-healable, closed-loop recyclable dielectric layer material, a fully nature-derived dynamic covalent poly(disulfide) decorated with rich H bonding and metal-catechol complexations. The polymer network possesses a mechanically ductile character with an Arrhenius-like dependent viscoelasticity. The assembled capacitive pressure sensor is able to achieve a sensitivity up to 9.26 kPa-1, fast response/recovery times of 32/24ms, and could deliver consistent signals of continuous consecutive cycles even after being self-healed or closed-loop recycled for real-time detection of human motions. This is expected to be of high interest for current capacitive sensing research to move towards a life-like, high performance and circular economy direction. This article is protected by copyright. All rights reserved.

Bulk transparent supramolecular glass enabled by host–guest molecular recognition

Supramolecular glass is a non-covalently cross-linked amorphous material that exhibits excellent optical properties and unique intrinsic structural features. Compared with artificial inorganic/organic glass, which has been extensively developed, supramolecular glass is still in the infancy stage, and itself is rarely recognized and studied thus far. Herein, we present the development of the host–guest molecular recognition motifs between methyl-β-cyclodextrin and para-hydroxybenzoic acid as the building blocks of supramolecular glass. Non-covalent polymerization resulting from the host–guest complexation and hydrogen bonding formation enables high transparency and bulk state to supramolecular glass. Various advantages, including recyclability, compatibility, and thermal processability, are associated with dynamic assembly pattern. Short-range order (host–guest complexation) and long-range disorder (three dimensional polymeric network) structures are identified simultaneously, thus demonstrating the typical structural characteristics of glass. This work provides a supramolecular strategy for constructing transparent materials from organic components.

Functional Soft Materials with Resistance to Liquid Leakage for Thermal Energy Storage

AbstractFunctional soft materials have great potential commercial applications in thermal energy storage, which are required to have a long life, good flexibility, and resistance to liquid leakage. Herein, a composite hydrogel with thermal storage properties is prepared through coupling molecular self‐assembly and in situ polymerization. Hydrophobic stearic acid (SA), as a thermal storage phase change material (PCM), is dispersed in polyacrylamide (PAM) hydrogel network in the form of oil–water emulsion (O/W). The PAM polymer network with good flexibility physically limits high‐frequency collision between SA PCM droplets. This unique design avoids demulsification in phase change emulsion so that the as‐prepared hydrogel composite can resist liquid leakage. The PAM hydrogel network plays the role of heterogeneous nucleation, resulting in the super‐cooling of SA emulsion at only 0.3 °C. On the other hand, SA PCM droplets in as‐prepared soft material do not directly contact with liquid water, so melting/crystallization process is independent of water. As a result, the soft material exhibits a thermal storage density of up to 99.3 J g−1. The present study is an important step toward designing soft energy storage materials.

Effect of Cross-Link Homogeneity on the High-Strain Behavior of Elastic Polymer Networks

Effect of Cross-Link Homogeneity on the High-Strain Behavior of Elastic Polymer Networks

Unleashing the potential of Cajanus cajun biochar polymer composite for Cu (II) removal: mechanism, modification, and application.

This study introduces a cost-effective approach to fabricating a porous and ionically surface-modified biochar-based alginate polymer networks composite (SBPC) through air drying. The study critically analyzes the role and concentrations of various components in the success of SBPC. Characterization techniques were employed to evaluate the microstructure and adsorption mechanism, confirming the ability of the adsorbent's carboxyl and hydroxyl groups to eliminate various heavy metal ions in water simultaneously. The SBPC demonstrated high copper binding capacities (937.4mg/g and 823.2mg/g) through response surface methodology (RSM) and column studies. It was also influential in single and natural systems, exhibiting competitive behavior and efficient removal of Cu2+. The Langmuir isotherm and pseudo-second-order kinetics strongly correlate with experimental data, with R2 values of 0.98 and 0.99, respectively. SBPC showed remarkable stability, up to 10 desorption cycles, and achieved 98% Cu2+ adsorption efficiency and 91.0% desorption. Finally, the cost analysis showed a cost of 125.68 INR/kg or 1.51 USD/kg, which is very low compared to the literature. These results highlight the potential of SPBC and show that it provides an efficient and cost-effective solution for removing Cu2+ from a real system.

Advanced Pyrene Copolymer/rGO Hydrogels for Efficient Congo Red Removal from Aqueous Systems

AbstractThis study investigates the use of hydrogel semi‐interpenetrating polymer networks (semi IPN) as efficient adsorbents for removing the dye congo red (CR) from aqueous solutions. Specifically, three hydrogel semi IPN, namely AH0–AH2, were synthesized through redox polymerization of acrylamide, polyethylenimine, polyhydroxylethylmethacrylate and (pyren‐1‐yl)methyl methacrylate/2‐acrylamido‐2‐methylpropane‐1‐sulfonic acid copolymer (pyrene−AMPS) using a reduced graphene oxide (RGO)‐N,N’‐methylene‐bis(acrylamide) (MBA)‐based crosslinker (GABC). Results indicated that these hydrogels were efficient in removing CR at pH 6.73, with the adsorption behaviour conforming to the Langmuir isotherm. This suggests a monolayer adsorption pattern, indicating a chemical nature. Thermodynamic analysis supports the practical feasibility of this process. The negative Gibbs free energy of adsorption (ΔG̊) indicates that adsorption is spontaneous and favorable. Conversely, the positive standard entropy of adsorption (ΔS̊) suggests an increase in molecular randomness during the process. Additionally, the standard enthalpy change (ΔH̊) suggests an endothermic activation energy for the binding process. Response Surface Methodology (RSM), utilizing four variables was employed to optimize the removal of CR. The model demonstrated high R2‐values and strong agreement between experimental and projected results, affirming its reliability. AH1 and AH2 showed significant efficacy in removing CR dye from aqueous solutions, underscoring their potential application in wastewater remediation.