Double-network Polymer Research Articles

Conductive-Polymer-Based Double-Network Hydrogels for Wearable Supercapacitors.

In the field of contemporary epidermal bioelectronics, there is a demand for energy supplies that are safe, lightweight, flexible and robust. In this work, double-network polymer hydrogels were synthesized by polymerization of 3,4-ethylenedioxythiophene (EDOT) into a poly(vinyl alcohol)/poly(ethylene glycol diacrylate) (PVA/PEGDA) double-network hydrogel matrix. The PEDOT-PVA/PEGDA double-network hydrogel shows both excellent mechanical and electrochemical performance, having a strain up to 498%, electrical conductivity as high as 5 S m-1 and specific capacitance of 84.1 ± 3.6 mF cm⁻2. After assembling two PEDOT-PVA/PEGDA double-network hydrogel electrodes with the free-standing boron cross-linked PVA/KCl hydrogel electrolyte, the formed supercapacitor device exhibits a specific capacitance of 54.5 mF cm⁻2 at 10 mV s-1, with an energy density of 4.7 μWh cm-2. The device exhibits excellent electrochemical stability with 97.6% capacitance retention after 3000 charging-discharging cycles. In addition, the hydrogel also exhibits great sensitivity to strains and excellent antifouling properties. It was also found that the abovementioned hydrogel can achieve stable signals under both small and large deformations as a flexible sensor. The flexible and antifouling PEDOT-PVA/PEGDA double-network hydrogel-based supercapacitor is a promising power storage device with potential applications in wearable electronics.

Polymer hydrogel electronics with adhesive, self-healing, and anti-ultraviolet capabilities for machine learning-promoted electrophysiological detection

While adjustable mechanical properties, diverse biochemical features, and good ionic conductivity enabled by molecular engineering, polymer hydrogels as ionic skins (i-skins) are still limited by multiple challenges in the practical human health monitoring and health diagnosis, such as undesirable ductility and contact adhesion, poor functionality, and insufficient sensing ability. In view of this, the strategy of multiple dynamic interactions was proposed, and a mussel characteristic double-network polymer composite hydrogel (PG-BT2) with tannic acid and borax as dynamic media was reasonably designed. Due to the dynamic reversible interactions from the crosslinked network combined with special functional groups, the obtained PG-BT2 hydrogel attains remarkable stretchability with >1200 % elongation and good toughness of 268.6 kJ/m−3, and also possesses favorable versatility (e.g., self-healing time <2 s, adhesion strength of 4.62 kPa to pigskin, and UV transmittance of 15.3 % at 365 nm). Additionally, the i-skin derived from the PG-BT2 hydrogel exhibits excellent sensing properties, including a low interface contact resistance of 8.3 kΩ at 1 kHz, fast response/recovery time of 0.657 s/0.348 s, and a wide sensing range of 0–1100 % strain. By combining the hydrogel-based i-skin patch with the wireless circuit connected to a Bluetooth module, the constructed sensor system is capable of accurately monitoring micro-expressions and multi-joint movements. The portable system is also demonstrated to capture high-fidelity signals of electrocardiograms available for training artificial intelligence, achieving differentiation and recognition of blood pressure status, which will greatly benefit health management and intelligent medicine.

Real-time monitoring the formation of polymer double network for gel polymer electrolyte by means of dielectric spectroscopy

A non-destructive method of detecting the internal information of the system is attempted in a polymer double network for gel polymer electrolyte in lithium-ion battery. The real-time monitoring for polymerization of 1,3-dioxolane (DOL) and cross-linking of cyanoethyl poly(vinyl alcohol) (PVA-CN) is carried out in a coin cell at 0–80 ℃ by means of dielectric spectrum. A widely distributed dielectric relaxation varying with monitoring time and temperature is observed in the frequency range of 3–103 kHz, and the relaxation parameters of the three sub-relaxations are obtained by fitting the dielectric spectrum with the HN equation. It is found that the relaxation intensity Δε and relaxation time τ are obviously dependent on the monitoring time and temperature: Δε and τ change obviously near the respective gel points of DOL and PVA-CN (DOL at 25 ℃, PVA-CN at 60 ℃), suggesting gelation at two gel points respectively. The different structural changes of the system before and after gelling temperature are speculated, and the possible microstructure of the system in each temperature range is proposed after analyzing the relaxation parameters. The applicability of the real-time dielectric monitoring in the study of the structure of gel polymer electrolytes is preliminarily concluded.

Superior compressive and tensile bi-directional strain sensing capabilities achieved using liquid metal Hybrid-Hydrogels empowered by Machine learning algorithms

Hydrogel-based flexible strain sensors have great potentials for applications in wearable electronics, human–machine interfaces, and soft robotics. Although great efforts have been made on enhancing tensile strain sensing capability of hydrogels, their sensing capabilities under compressive deformation have seldom been explored. Herein, we report a liquid metal-based hybrid hydrogel which achieves high stretchability (2300 %), superior bi-directional responses to both compressive strains (with a gauge factor of 33.43) and tensile strains (with a gauge factor of 9.59), fast response time (190 ms), and excellent durability (>1500 cycles). This conductive hybrid hydrogel with multi-interpenetrating networks is constructed by incorporating liquid metal, graphene oxide, poly-dopamine, and potassium chloride into a polymer double-network of polyacrylamide and poly(3,4-ethylenedioxythiophene): poly (styrene sulfonate). By integrating machine-learning algorithm with the hybrid hydrogel sensors, an intelligent dual-mode handwriting recognition system is developed for perceiving finger touch signals (in compressive-strain mode) and finger bending signals (in tensile-strain mode), with high accuracy (>93 %) and fast recognition time (<1s) when recognizing numbers, letters, and signature signals. Furthermore, this handwriting recognition system demonstrates advanced performance for human–machine interactions (e.g., playing music) and virtual reality interactions. This study offers a promising strategy to develop hydrogel electronics with bidirectional sensing capabilities for diversified practical applications.

MXene‐based dual‐network conductive organic hydrogel for wearable sensor to monitor human motions

AbstractConductive hydrogel is a novel material that can be used to prepare flexible wearable strain sensors. It paves the way for future uses in electronic skin, human‐computer interaction, and personalized medical monitoring. However, because the majority of conductive hydrogels use pure water as their medium, they will freeze at low temperatures. It will inevitably lose water at room temperature. We developed a PAM/SA/MXene organic hydrogel (denoted as PSMOH). First, MXene nanosheets were added as conductive filler into the double network polymer hydrogel, which was composed of polyacrylamide (PAM) and sodium alginate (SA), we denoted as PSMH. And then immerse the prepared PSMH in a glycerol solution using a solvent displacement method to obtain PSMOH. Even if at extreme temperatures (−30°C), the prepared PSMOH has a strong antifreezing ability and can retain moisture for 8 days. The sensor also has excellent mechanical properties (tensile strain is 1579%), self‐adhesive property, excellent sensitivity (gauge factor is 49.92), low detection limit (1% strain), and excellent fatigue resistance (800 cycles at 30% strain). As a result, the hydrogel can be assembled into a flexible wearable sensor for real‐time monitoring of human motions and other activities.

Molecularly imprinted self-buffering double network hydrogel containing bi-amidoxime functional groups for the rapid hydrolysis of organophosphates.

The development of high-performance catalyst materials with high catalytic activity for the hydrolysis of organophosphorus toxicants without additional pH buffer conditions has become an urgent need for practical application. Here, a multifunctional molecularly imprinted polymer double network hydrogel (MIP-DN) material has been prepared by integrating the first polymer network containing the functional group of bi-amidoxime as the catalytic active center and the cationic polymer polyethyleneimine (PEI) with pH buffer function as the main component of the second network. Advantageously, the resultant MIP-DN hydrogel showed excellent catalytic performance without additional pH buffer conditions, exhibiting a half-life of 25min for the hydrolysis of paraoxon in pure water. Together with multi-functions of high catalytic activity, self-buffering function and excellent processability, the MIP-DN hydrogel prepared in this work provides a new strategy for the preparation of catalytic materials with practical application value toward toxic organophosphates.

Squid/synthetic polymer double-network gel: elaborated anisotropy and outstanding fracture toughness characteristics

The hierarchical anisotropy of a biotissue plays an essential role in its elaborate functions. To mimic the anisotropy-based functions of biotissues, soft and wet synthetic hydrogels with sophisticated biotissue-like anisotropy have been extensively explored. However, most existing synthetically manufactured anisotropic hydrogels exhibit fundamental anisotropy and poor mechanical toughness characteristics. In this paper, natural/synthetic hybrid double-network (DN) hydrogels with hierarchical anisotropy and high toughness characteristics are reported. These DN gels are prepared directly by using a squid mantle as an anisotropic soft bioproduct for the primary network and polyacrylamide (PAAm) as a synthetic polymer for the secondary network. The obtained squid/PAAm DN gel maintains the complex orientation of the muscle fibers of the squid mantle and exhibits anisotropic, enhanced mechanical properties and excellent fracture resistance due to its unique composite structure. This hybrid strategy provides a general method for preparing hydrogels with elaborated anisotropy and determining functions derived from the anisotropy.

Polyzwitterion-SiO2 Double-Network Polymer Electrolyte with High Strength and High Ionic Conductivity.

The key to developing high-performance polymer electrolytes (PEs) is to achieve their high strength and high ionic conductivity, but this is still challenging. Herein, we designed a new double-network PE based on the nonhydrolytic sol-gel reaction of tetraethyl orthosilicate and in situ polymerization of zwitterions. The as-prepared PE possesses high strength (0.75 Mpa) and high stretchability (560%) due to the efficient dissipation energy of the inorganic network and elastic characteristics of the polymer network. In addition, the highest ionic conductivity of the PE reaches 0.44 mS cm-1 at 30 °C owning to the construction of dynamic ion channels between the polyzwitterion segments and between the polyzwitterion segments and ionic liquids. Furthermore, the inorganic network can act as Lewis acid to adsorb trace impurities, resulting in a prepared electrolyte with a high electrochemical window over 5 V. The excellent interface compatibility of the as-prepared PE with a Li metal electrode is also confirmed. Our work provides new insights into the design and preparation of high-performance polymer-based electrolytes for solid-state energy storage devices.

Double-Network Polymer Electrolytes with Ionic Liquids for Lithium Metal Batteries.

Solid-state polymer electrolytes have become promising candidates for high-energy-density lithium metal batteries (LMBs). However, they suffer from low ionic conductivities at room temperature. In this work, two types of composite polymer electrolytes based on a double-network polymer, an ionic liquid (IL) of 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI) or 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl) imide (EmimTFSI), and bis(trifluoromethane)sulfonamide lithium salt (LiTFSI) were prepared by a facile one-pot method. The two types of CPEs possess good mechanical properties, excellent thermal stability, and high ionic conductivities greater than 10−4 S cm−1 at 20 C with 26 wt% IL. The performance diversity of the CPEs was also carefully investigated through a series of electrochemical measurements. Although the CPEs containing EmimTFSI show higher ionic conductivities than those of CPEs with Pyr14TFSI, the latter ones have wider electrochemical stability windows and better resistance to the growth of lithium dendrites. Moreover, CPE with 34 wt% Pyr14TFSI leads to Li/LiFePO4 batteries with favorable rate capability and cycling stability and a columbic efficiency of 98.8% at 20 °C, which suggests that CPEs are promising for practical application in solid-state LMBs.

Construction of a novel double network polymer composite and evaluation of its highly efficient adsorption properties for copper ions

AbstractTo explore more efficient heavy metal copper ions adsorbents with double network polymers, this work has presented the novel fabrication processes of the polymer composite diatomaceous earth/polydopamine‐polyethyleneimine/alginate DE@PDA‐PEI@SA, which was characterized by FT‐IR, SEM, EDS, XPS, and was efficiently utilized for copper ions adsorption. The relevant adsorption behaviors of DE@PDA‐PEI@SA for copper ions such as the effect of pH, the effect of adsorbent dosage, adsorption kinetics, adsorption isotherm etc. have been investigated. The results reveal that the adsorption kinetics of DE@PDA‐PEI@SA for copper ions at different temperatures can be modeled by the pseudo second‐order rate equation, the experimental data fit well with the Langmuir isotherm model, indicating a homogeneous monolayer adsorption, and the maximum adsorption capacity of DE@PDA‐PEI@SA for copper ions could reach 454.55 mg/g at 35°C that is very high compared to other reported adsorbent materials. The research results indicate that DE@PDA‐PEI@SA is a novel efficient adsorbent with high efficiency in wastewater treatment and water purification, which could provide a facile processing for copper adsorption.

Sustainable Self-Cleaning Evaporators for Highly Efficient Solar Desalination Using a Highly Elastic Sponge-like Hydrogel.

Interfacial evaporation using light-absorbing hydrogels offers efficient solar evaporation performance under natural sunlight, ensuring an affordable clean water supply. However, achieving light-absorbing hydrogels with durable and efficient utilization is still a challenge due to inevitable salt accumulation, a difficult-to-control surface morphology, and poor mechanical properties on the surfaces of hydrogel-based evaporators. In this work, a photothermal sponge-like hydrogel with a 3D interconnected porous structure was constructed using low-cost activated carbon as a photothermal material, as well as a double-network polymer chain as the basic skeleton using a simple foaming polymerization strategy. The sponge-like hydrogel evaporator showed tailored surface topography, adequate water transport, excellent elasticity and toughness, good salt rejection, and thermal localization properties. Under the irradiation of simulated sunlight (1.0 kW/m2), a high evaporation rate of 2.33 kg·m-2·h-1 was achieved. Furthermore, efficient salt self-cleaning behavior was achieved due to the fast ion diffusion within the 3D interconnected porous structures. Even in highly concentrated brine of 15 wt %, continuous and efficient water evaporation was still achieved. The excellent evaporation and salt rejection properties of this photothermal sponge-like hydrogel indicated its promising long-term sustainable utilization in seawater desalination.

Antifreezing Zwitterionic-Based Hydrogel Electrolyte for Aqueous Zn Ion Batteries

Hydrogel electrolytes have attracted great attention because the properties of integrated conductivity and mechanical flexibility endow them with potential applications in flexible solid-state batteries. However, hydrogels are easy to freeze when the temperature drops below zero, resulting in the reduction of ionic conductivity and mechanical strength of hydrogels. Herein, an antifreezing polymer hydrogel electrolyte was developed by introducing a zwitterionic group and salt into the double-network polymer. The polymer hydrogel electrolyte possessed a perfect ionic conductivity of 10.38 mS cm–1 at −20 °C and could remain unfrozen until −50 °C. A flexible solid-state Zn//LiFePO4 battery assembled with the prepared hydrogel electrolyte exhibited a discharge capacity of 126.1 and 75.1 mAh g–1 at 20 and −20 °C at 1 C, respectively. In addition, it showed quite perfect cycling performance and maintained a capacity retention of 72.6% after 1000 cycles with a favorable Coulombic efficiency (∼100%) at −20 °C at 1 C. The hydrogel electrolyte with perfect antifreezing performance is supposed to broaden the range of working temperature and practical applications of the zinc ion battery.

Mussel Inspired Trigger-Detachable Adhesive Hydrogel.

Adhesion to many kinds of surfaces, including biological tissues, is important in many fields but has been proved to be extremely challenging. Furthermore, peeling from strong adhesion is needed in many conditions, but is sometimes painful. Herein, a mussel inspired hydrogel is developed to achieve both strong adhesion and trigger-detachment. The former is actualized by electrostatic interactions, covalent bonds, and physical interpenetration, while the latter is triggered, on-demand, through combining a thixotropic supramolecular network and polymer double network. The results of the experiments show that the hydrogel can adhere to various material surfaces and tissues. Moreover, triggered by shear force, non-covalent interactions of the supramolecular network are destroyed. This adhesion can be peeled easily. The possible mechanism involved is discussed and proved. This work will bring new insight into electronic engineering and tissue repair like skin care for premature infants and burn victims.

Lithium Salt-Induced In Situ Polymerizations Enable Double Network Polymer Electrolytes.

Structural design is an intriguing strategy to improve the physical and electrochemical performance of polymer electrolytes (PEs) for lithium-ion batteries. However, the complex synthetic process and introduction of nonelectrolyte composition severely limit the development and practical application of PEs. Here, a facile method is reported for the fabrication of a double network polymer electrolyte (DN-PE) through combining the lithium salt-accelerated thiol-Michael addition and lithium salt-catalyzed radical polymerization. By adjusting the reaction temperature, the double network with the cross-linking structure can be in situ formed step by step at room temperature and 80°C. Notably, using lithium salt as the accelerator and catalyst avoids the addition of extra species and the related side reactions in the electrolyte system. Compared with single network polymer electrolyte (SN-PE), DN-PE has a distinctly improved mechanical strength and a better interfacial compatibility with the electrode, which leads to a stable cycling of the symmetric Li|DN-PE|Li cell over 1000 h at a current density of 0.05mA cm-2 . In addition, the Li|DN-PE|LiFePO4 cell shows a high discharge specific capacity of 150.3 mAh g-1 at 0.1 C and coulomb efficiency of 99%.

Self-Powered Smart Arm Training Band Sensor Based on Extremely Stretchable Hydrogel Conductors.

The development of elastic electronic technology has promoted the application of triboelectric nanogenerators (TENGs) in flexible wearable electronics. However, most of the flexible electronics cannot achieve the requirements of being extremely stretchable, transparent, and highly conductive at the same time. Herein, we report a TENG constructed using a double-network polymer ionic conductor sodium alginate/zinc sulfate/poly acrylic-acrylamide (SA-Zn) hydrogel, which exhibited outstanding stretchability (>10,000%), high transparency (>95%), and good conductivity (0.34 S·m-1). The SA-Zn hydrogel TENG (SH-TENG) could harvest energy from typical human movements, such as bending, stretching, and twisting, which could light up 234 green commercial LEDs easily. Additionally, the SH-TENG can be used to prepare a self-powered smart training band sensor for monitoring arm stretching motion. This work may provide an innovative platform for accessing the next generation of sustainable wearable and sports monitoring electronics.

Low‐Molecular‐Weight Supramolecular‐Polymer Double‐Network Eutectogels for Self‐Adhesive and Bidirectional Sensors

AbstractIonic conducting eutectogels have attracted enormous attention as an alternative to the conventional temperature‐intolerant hydrogels and costly ionic liquid gels in constructing flexible electronic devices. However, current eutectogels prepared via cross‐linked polymer or low‐molecular‐weight gelators suffer from limited stretchability and insufficient surface‐adaptive adhesion. Herein, a low‐molecular‐weight supramolecular network is introduced into a covalent polymer network in a eutectogel architecture, and a novel supramolecular‐polymer double‐network (SP‐DN) strategy is demonstrated to fabricate conductive SP‐DN eutectogels with high stretchability (&gt;4000% elongation) and toughness (≈800 J m−2), as well as self‐healing, self‐adhesive and anti‐freezing/anti‐drying characteristics. These unique features lead to the successful realization of SP‐DN eutectogels in wearable self‐adhesive strain sensors, which can conformally deform with the skin and dynamically monitor body movements with high sensitivity and long‐term stability over a wide temperature range (−40 to 60 °C). Furthermore, the strain sensors can accurately detect body movements along two opposite directions (bend up or bend down), which are rarely reported in the literature. Distinct from the widely explored polymer double‐network (P‐DN) hydrogels, the developed SP‐DN eutectogel platform is capable of well‐regulating molecular‐scale noncovalent and covalent interactions, providing a paradigm for the creation of smart soft materials with versatile performance and high environmental adaptability.

A cytocompatible conductive polydopamine towards electrochromic energy storage device

Polydopamine (PDA), a quinone containing bioinspired material, is attractive for environmental and human-friendly energy storage systems because of its fast and reversible proton-involved redox activity. Herein, we describe a multifunctional porous double-network polymer based on tetraaniline and PDA. This material can take advantage of the redox activity and wettability from PDA, as well as the electronic conductivity and electrochromic capability provided by tetraaniline. It does not show cytotoxic effects as evidenced by conventional cell viability and imaging assays. An electrochromic battery-type device based on this material delivers a considerable capacity of 160 mAh g−1 together with a high coloring efficiency of 270 cm2/C and a large optical contrast of up to 37% at 700 nm. The device is capable of storing energy while indicating the energy status visually. This multifunctional porous conductive PDA material is promising for versatile energy storage applications.

Self-Healing Double Network Polymer Gels with Dynamic Crosslinks

Double network polymer gels in which macromolecules are crosslinked by two types of rapidly restoring dynamic crosslinks have been obtained for the first time. It has been shown that the material based on these double networks combines improved mechanical properties as compared with components and ability to fast self-healing at room temperature.

Pre‐Stretched Double Network Polymer Films Based on Agarose and Polyacrylamide with Sensitive Humidity‐Responsive Deformation, Shape Memory, and Self‐Healing Properties

AbstractA series of pre‐stretched double network (DN) polymer films based on agarose and polyacrylamide (Agar/PAM) are prepared. These films show the rapid and large deformation in response to humidity due to the release of contraction stress. The pre‐stretched dry film can curl toward the humidity direction into a scroll‐like structure in an extremely short time. The deformation direction can be also well controlled by the stretching direction. The released contraction stress increases with the increase of tensile strain. The DN Agar/PAM hydrogel film shows good water‐induced shape memory effect. It can be stretched to form various temporary shapes such as circle, square, and spiral. The shape fixing and recovery ratios are both close to 100%. The released large contraction stress accelerates the shape recovery process. The recovery time increases with the increase of film thickness. The DN dry film shows the good water‐induced self‐healing ability. The healed film maintains about 88% tensile strength (103 MPa) of the original film. Due to these good properties, the pre‐stretched DN dry films can be fabricated into a soft robot jack which can easily lift up to ≈20 times its own weight within 8 s.

A novel post-processed surface modified double-network polymer layer for a triboelectric nanogenerator

A high output performance triboelectric nanogenerator with a novel low-cost, rapidly yielding, post-processed micro-patterned double-network PDMS film.