Cross-linked Polymer Networks Research Articles

Dual–network carboxymethyl chitosan conductive hydrogels for multifunctional sensors and high–performance triboelectric nanogenerators

With the development of technology, there is a growing demand for wearable electronics that can fulfill different application scenarios. Hydrogel–based sensors are considered ideal candidates for realizing multifunctional wearable flexible devices. However, there are great challenges in preparing hydrogel–based sensors with both superior mechanical and electrical properties. Herein, we report a composite conductive hydrogel prepared by using a dynamically crosslinked carboxymethyl chitosan network and a covalently crosslinked polymer network, and carboxylated carbon nanotubes as conductive filler. The carboxymethyl chitosan–based hydrogels had excellent mechanical properties and strength (tensile strength of 475.4 kPa, and compressive strength of 1.9 MPa) and ultra–high conductivity (0.19 S·cm−1). Based on the above characteristics, the hydrogel could accurately identify the movement signals of the human body and different writing signals, and achieve encrypted transmission of signals, broadening the application scenarios. In addition, a triboelectric nanogenerator (TENG) was fabricated based on the hydrogel, which had an outstanding output performance with open–circuit voltage of 336 V, short–circuit current of 18 μA, transferred charge of 52 nC and maximum power density of 340 mW·m−2, and could power small devices. This work is expected to provide new ideas for the development of self–powered, multi–functional wearable, and flexible polysaccharide–based devices.

4D printing of semi-crystalline crosslinked polymer networks with two-way shape-memory effect

This paper introduces a novel approach to 4D printing tailored structures with reversible two-way shape-memory effect (SME) through material extrusion technology. To this aim, methacrylated poly(ε-caprolactone) (PCL) was synthesized and evaluated from a rheological perspective to determine its suitability for extrusion-based printing. Following a printability assessment, an optimal set of parameters was identified to fabricate 3D structures, UV-crosslinked during printing. Subsequently, a physical and thermo-mechanical characterization of the printed structures was conducted to deepen the understanding of the fabrication process and properties of the obtained structures. To assess the shape-memory properties of the printed structures, both the one-way and two-way SME under load were investigated. Overall, this study opens the floodgates to implementing 4D printing via material extrusion technology, specifically targeting PCL-based semi-crystalline chemically crosslinked polymer networks with two-way SME. Because of its cost-effectiveness, versatility, and user-friendly nature, extrusion-based printing offers noteworthy advantages over other additive manufacturing approaches when reversible behavior of the printed structures is needed. Lastly, a glimpse of potential 4D printed structures from PCL-based semi-crystalline chemically crosslinked polymer networks is presented. The approach described holds significant promise across multiple research and industrial domains, including but not limited to smart actuators, soft robotics, and medical devices.

Mechanically Triggered Polymer Deconstruction through Mechanoacid Generation and Catalytic Enol Ether Hydrolysis.

Polymers that amplify a transient external stimulus into changes in their morphology, physical state, or properties continue to be desirable targets for a range of applications. Here, we report a polymer comprising an acid-sensitive, hydrolytically unstable enol ether backbone onto which is embedded gem-dichlorocyclopropane (gDCC) mechanophores through a single postsynthetic modification. The gDCC mechanophore releases HCl in response to large forces of tension along the polymer backbone, and the acid subsequently catalyzes polymer deconstruction at the enol ether sites. Pulsed sonication of a 61 kDa PDHF with 77% gDCC on the backbone in THF with 100 mM H2O for 10 min triggers the subsequent degradation of the polymer to a final molecular weight of less than 3 kDa after 24 h of standing, whereas controls lacking either the gDCC or the enol ether reach final molecular weights of 38 and 27 kDa, respectively. The process of sonication, along with the presence of water and the existence of gDCC on the backbone, significantly accelerates the rate of polymer chain deconstruction. Both acid generation and the resulting triggered polymer deconstruction are translated to bulk, cross-linked polymer networks. Networks formed via thiol-ene cross-linking and subjected to unconstrained quasi-static uniaxial compression dissolve on time scales that are at least 3 times faster than controls where the mechanophore is not covalently coupled to the network. We anticipate that this concept can be extended to other acid-sensitive polymer networks for the stress-responsive deconstruction of gels and solvent-free elastomers.

Designing Configurable Soft Microgelsomes as a Smart Biomimetic Protocell.

Self-assembly is an intriguing aspect of primitive cells. The construction of a semipermeable compartment with a robust framework of soft material capable of housing an array of functional components for chemical changes is essential for the fabrication of synthetic protocells. Microgels, loosely cross-linked polymer networks, are suitable building blocks for protocell capsule generation due to their porous structure, tunable properties, and assembly at the emulsion interface. Here, we present an interfacial assembly of microgel-based microcompartments (microgelsomes, MGC) that are defined by a semipermeable, temperature-responsive elastic membrane formed by densely packed microgels in a monolayer. The water-dispersible microgelsomes can thermally shuttle between 10 and 95 °C while retaining their structural integrity. Importantly, the microgelsomes exhibited distinct properties of protocells, such as cargo encapsulation, semipermeable membrane, DNA amplification, and membrane-gated compartmentalized enzymatic cascade reaction. This versatile approach for the construction of biomimetic microcompartments augments the protocell library and paves the way for programmable synthetic cells.

Bio-based conductive polyurethane composites derived from renewable castor oil with enhanced self-healing ability for flexible supercapacitors

To alleviate the pressure on the petrochemical industry and address environmental concerns, the utilization of polyurethane (PU) derived from castor oil (CO) (as an ester polyol replacement of petrochemical-based materials) has garnered significant attention in recent decades. Extending the service life of materials requires imparting self-healing properties to vegetable oil-based polymers, an aspect that has received limited attention in current studies. However, low self-healing efficiency still poses a significant challenge, and non-conductivity also remains an obstacle in current research, crucial for their application in electronic devices. In this work, we present the first series of electrically self-healing biopolymer composites constructed by incorporating carbon nanotube (CNT) networks into crosslinked castor oil-based polyurethane (BPU) through a simple curing process. These materials address the challenges mentioned above and exhibit improved mechanical, electrical, and self-healing capabilities compared to other bio-based self-healing materials. The resulting BPU/CNT composite demonstrated exceptional repeated self-healing capacity, restoring both mechanical properties and electrical performance even after experiencing severe mechanical damage. Notably, this composite served as a conductive substrate in flexible solid-state supercapacitor (FSSC) devices. Consequently, the FSSC derived from the composite conductive substrate achieved an impressive 92.4 % self-healing efficiency even after undergoing 7 cutting/healing cycles. The device remained virtually unchanged even after being bent at a 180° angle with a bending radius of 1.6 mm, indicating excellent repeatability and durability. The exceptional self-healing ability, with ∼98 % electrical recovery at 100 °C for 70 s and 93 % at 80 °C after 6 min, of these composites was attributed to the synergistic interactions of the dynamic exchange reactions of disulfide bonds and dense hydrogen bonds within the BPU matrix, which provide a reversible dynamic polymer network. The healing efficiency of these dynamic bonds was evaluated by adjusting the composition ratio of the long linear chain of pTHF in hybrid polyols of the crosslinked polymer network. Overall, this work highlights a series of green, simple, and highly efficient self-healing polymer composites derived from renewable castor oil, and it establishes an essential framework for future sustainable polymer composite design.

Educational series: characterizing crosslinked polymer networks

Characterization of polymer networks presents unique challenges due to the insolubility of the materials, but this also enables characterization by new techniques that take advantage of the bulk network structure.

Solvent-triggered shape change in gradient-based 4D printed bilayers: case study on semi-crystalline polymer networks.

We propose an approach to 4D print solvent-triggered, gradient-based bilayers made of semi-crystalline crosslinked polymer networks. Out-of-plane bending is obtained after immersion in the solvent, exploiting the different swelling degrees of the layers resulting from crosslinking gradients. Lastly, a beam model of the shape transformation is applied and experimentally validated.

Crosslinker energy landscape effects on dynamic mechanical properties of ideal polymer hydrogels.

Reversible crosslinkers can enable several desirable mechanical properties, such as improved toughness and self-healing, when incorporated in polymer networks for bioengineering and structural applications. In this work, we performed coarse-grained molecular dynamics to investigate the effect of the energy landscape of reversible crosslinkers on the dynamic mechanical properties of crosslinked polymer network hydrogels. We report that, for an ideal network, the energy potential of the crosslinker interaction drives the viscosity of the network, where a stronger potential results in a higher viscosity. Additional topographical analyses reveal a mechanistic understanding of the structural rearrangement of the network as it deforms and indicate that as the number of defects increases in the network, the viscosity of the network increases. As an important validation for the relationship between the energy landscape of a crosslinker chemistry and the resulting dynamic mechanical properties of a crosslinked ideal network hydrogel, this work enhances our understanding of deformation mechanisms in polymer networks that cannot easily be revealed by experiment and reveals design ideas that can lead to better performance of the polymer network at the macroscale.

Renewable whey-based hydrogel with polysaccharides and polyvinyl alcohol as a soil amendment for sustainable agricultural application

This work describes the preparation of a novel biopolymer hydrogel based on acid whey, cellulose derivatives and polyvinyl alcohol (PVA). The hydrogel was prepared and characterized with the aim of producing an environmentally-friendly soil amendment to increase water retention capacity of the soil. The findings showed considerable swelling properties of the hydrogels depending on the PVA content and crosslinking density. The samples with PVA in a concentration 2.5 % and 5 % were more rigid, the gel fraction increased with a subsequently decrease in their swelling capacity. The hydrogels crosslinked with 15 % of citric acid demonstrated a constant swelling ratio (SR) of around 500 % within 10 swelling/drying cycles. The hydrogels crosslinked with 10 % citric acid and supplemented with 1 % of PVA showed SR of 1000–1400 % caused by less crosslinked polymer network and increased pore volume for water uptake. It was found that hydrogel with a higher gel fraction had a stable structure. Supplementing PVA at 5 % extended the period of decomposition of the hydrogel material by almost 60 % in the soil environment and soil humidity was maintained for longer. Applying 2 % of the hydrogel 5PVA to soil increased the water retention capacity by 19 %.

Injectable microcapillary network hydrogels engineered by liquid-liquid phase separation for stem cell transplantation

Injectable hydrogels are promising carriers for cell delivery in regenerative medicine. However, injectable hydrogels composed of crosslinked polymer networks are often non-microporous and prevent biological communication with host tissues through signals, nutrients, oxygen, and cells, thereby limiting graft survival and tissue integration. Here we report injectable hydrogels with liquid-liquid phase separation-induced microcapillary networks (μCN) as stem cell-delivering scaffolds. The molecular modification of gelatin with hydrogen bonding moieties induced liquid-liquid phase separation when mixed with unmodified gelatin to form μCN structures in the hydrogels. Through spatiotemporally controlled covalent crosslinking and dissolution processes, porous μCN structures were formed in the hydrogels, which can enhance mass transport and cellular activity. The encapsulation of cells with injectable μCN hydrogels improved cellular spreading, migration, and proliferation. Transplantation of mesenchymal stem cells with injectable μCN hydrogels enhanced graft survival and recovered hindlimb ischemia by enhancing material-tissue communication with biological signals and cells through μCN. This facile approach may serve as an advanced scaffold for improving stem cell transplantation therapies in regenerative medicine.

A Fully Amorphous, Dynamic Cross-Linked Polymer Electrolyte for Lithium-Sulfur Batteries Operating at Subzero-Temperatures.

Solid-state lithium-sulfur batteries have shown prospects as safe, high-energy electrochemical storage technology for powering regional electrified transportation. Owing to limited ion mobility in crystalline polymer electrolytes, the battery is incapable of operating at subzero temperature. Addition of liquid plasticizer into the polymer electrolyte improves the Li-ion conductivity yet sacrifices the mechanical strength and interfacial stability with both electrodes. In this work, we showed that by introducing a spherical hyperbranched solid polymer plasticizer into a Li+ -conductive linear polymer matrix, an integrated dynamic cross-linked polymer network was built to maintain fully amorphous in a wide temperature range down to subzero. A quasi-solid polymer electrolyte with a solid mass content >90 % was prepared from the cross-linked polymer network, and demonstrated fast Li+ conduction at a low temperature, high mechanical strength, and stable interfacial chemistry. As a result, solid-state lithium-sulfur batteries employing the new electrolyte delivered high reversible capacity and long cycle life at 25 °C, 0 °C and -10 °C to serve energy storage at complex environmental conditions.

Probing the Solubility of Imine-Based Covalent Adaptable Networks.

Covalent adaptable networks (CANs) are polymer materials that are covalently cross-linked via dynamic covalent bonds. The cross-linked polymer network is generally expected to be insoluble, as is seen for traditional thermosets. However, in recent years, it has become apparent that-under certain conditions-both dissociative and associative CANs can be dissolved in a good solvent. For some applications (e.g., those that require long-term (chemical) stability), the solubility of CANs can be problematic. However, many forget that (selective) solubility of CANs can also be applied advantageously, for example, in recycling or modification of the materials. In this work, we provide results and insights related to the tunable solubility of imine-based CANs. We observed that selected CANs could be fully dissolved in a good solvent without observing dissociation of imines. Only in an acidic environment (partial) dissociation of imines was observed, which could be reverted to the associated state by addition of a base. By adjusting the network composition, we were able to either facilitate or hamper solubility as well as control the size of the dissolved particles. DLS showed that the size of dissolved polymer particles decreased at lower concentrations. Similarly, decreasing cross-linking density resulted in smaller particles. Last, we showed that we could use the solubility of the CANs as a means for chemical recycling and postpolymerization modification. The combination of our studies with existing literature provides a better understanding of the solubility of CANs and their applications as recyclable thermosets.

Herbal nanogels: Revolutionizing skin cancer therapy through nanotechnology and natural remedies

The escalating global incidence of skin cancer demands innovative therapeutic approaches that balance efficacy and reduced side effects. The convergence of nanotechnology and medicinal research has unveiled herbal nanogels as promising candidates for advancing skin cancer therapy. Skin cancer, comprising diverse malignancies, necessitates novel strategies to enhance treatment outcomes. Nanogels, intricate networks of cross-linked polymers at the nanoscale, have emerged as versatile vehicles for targeted drug delivery. Their ability to encapsulate a range of therapeutic agents and respond to stimuli offers a platform to revolutionize treatment paradigms. The taxonomy of nanogels encompasses physical cross-linked gels, liposome-modified nanogels, micellar nanogels, hybrid nanogels, and chemically cross-linked nanogels, each holding distinctive characteristics suitable for specific applications. The synthesis of nanogels is a precise craft involving techniques like photolithography, micro-moulding, and emulsion methods, enabling control over particle size, morphology, and drug-loading capacity. Stimuli-responsive nanogels present a dynamic facet of this field, exhibiting potential for targeted drug delivery. Noteworthy examples include LHRH-targeted nanogels and nanogels responsive to the intricate microenvironment of bacterial-accumulated tumors. One of the most intriguing developments lies in the fusion of herbal medicine and nanotechnology, paving the way for herbal nanogels. By incorporating bioactive plant-derived compounds into nanogel matrices, these constructs offer a holistic therapeutic approach. The convergence of nanotechnology and herbal medicine in the form of herbal nanogels presents a promising avenue for elevating skin cancer therapy. Through precise drug delivery mechanisms and responsiveness to complex microenvironments, nanogels exhibit a potential to reshape treatment norms. This comprehensive review provides an in-depth roadmap of the evolving landscape of herbal nanogels, illuminating their potential as effective, targeted, and minimally invasive tools for combating skin cancer.

Precise Rheological Analysis of Permanently and Transiently Crosslinked Polymer Networks with Well-Controlled Structures

Precise Rheological Analysis of Permanently and Transiently Crosslinked Polymer Networks with Well-Controlled Structures

Microfluidic Fabrication of Monodisperse and Recyclable TiO2-Poly(ethylene glycol) Diacrylate Hybrid Microgels for Removal of Methylene Blue from Aqueous Medium.

Nearly monodisperse titanium oxide-polyethylene glycol diacrylate [TiO2-P(EGDA)] hybrid microbeads containing 0.5 wt % TiO2 nanoparticles entrapped within a P(EGDA) cross-linked polymeric network were synthesized using a modular Lego-inspired glass capillary microfluidic device. TiO2-P(EGDA) hybrid microgels were characterized by optical microscopy, scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, and thermogravimetric analysis. The fabricated TiO2-P(EGDA) hybrid microgel system showed 100% removal efficiency of methylene blue (MB) from its 1-3 ppm aqueous solutions after 4 h of UV light irradiation at 0.2 mW/cm2 at the loading of 25 g/L photocatalyst beads in the reaction mixture, corresponding to the loading of naked TiO2 of just 0.025 g/L. No decrease in photocatalytic efficiency was observed in 10 repeated runs with recycled photocatalyst using a fresh 1 ppm MB solution in each cycle. The rate of photocatalytic degradation was controlled by the UV light irradiance, catalyst loading, and the initial dye concentration. Physical adsorption of MB onto the surface of composite microgel was also observed. The adsorption data was best fitted with the Langmuir adsorption isotherm and the Elovich kinetic model. TiO2-P(EGDA) microgel beads are biocompatible, can be prepared with a tunable size in the microfluidic device, and can easily be separated from the reaction mixture by gravity settling. The TiO2-P(EGDA) system can be used for the removal of other toxic dyes and micropollutants from industrial wastewater.

Gel Polymer Electrolytes for Lithium-Ion Batteries Enabled by Photo Crosslinked Polymer Network.

We demonstrate a gel polymer electrolyte (GPE) featuring a crosslinked polymer matrix formed by poly(ethylene glycol) diacrylate (PEGDA) and dipentaerythritol hexaacrylate (DPHA) using the radical photo initiator via ultraviolet (UV) photopolymerization for lithium-ion batteries. The two monomers with acrylate functional groups undergo chemical crosslinking, resulting in a three-dimensional structure capable of absorbing liquid electrolytes to form a gel. The GPE system was strategically designed by varying the ratios between the main polymer backbone (PEGDA) and the crosslinker (DPHA) to achieve an optimal gel polymer electrolyte network. The resulting GPE exhibited enhanced thermal stability compared to conventional liquid electrolytes (LE) and demonstrated high ionic conductivity (1.40 mS/cm) with a high lithium transference number of 0.65. Moreover, the obtained GPE displayed exceptional cycle performance, maintaining a higher capacity retention (85.2%) comparable to the cell with LE (79.3%) after 200 cycles.

Advances in hydrogel materials applied to pancreatic-related diseases

The pancreas is a glandular organ that maintains internal homeostasis through its endocrine and exocrine functions. These functions are vital for overall well-being. However, environmental and lifestyle changes have led to an increasing incidence of pancreatic diseases, including pancreatic cancer, pancreatitis, and pancreatic neuroendocrine tumors. These conditions profoundly impact the health and quality of life of affected individuals. The existing diagnostic and treatment approaches for pancreatic diseases exhibit limitations and shortcomings, necessitating exploring novel strategies. In recent years, advancements in medicine and bioengineering have fostered multidisciplinary and interdisciplinary innovations, introducing fresh avenues for diagnosing and treating pancreatic diseases. Hydrogels, emerging as a biomaterial, represent highly hydrated cross-linked hydrophilic polymer networks. Their exceptional biodegradability and biocompatibility have rendered them instrumental in various medical applications. Hydrogels, with their macromolecular porous structures, are efficient carriers for drug delivery and controlled release. Hydrogel materials have garnered substantial attention for their unique properties and diverse applications in the context of pancreatic diseases. This article aims to provide an overview of the current limitations in diagnosing and treating pancreatic diseases while highlighting the latest trends and advancements in developing hydrogel carrier materials tailored for these conditions, primarily focusing on strategies for pancreatitis, pancreatic cancer, and pancreatic neuroendocrine tumors. The article endeavors to give researchers and clinicians a comprehensive grasp of this field, offering valuable insights into prospective research directions and emerging trends.

Insights into nanomechanical and nanotribological characterization of cross-linked polymer nanocomposites via molecular dynamics simulation

Polymer nanocomposites (PNCs) representing the cutting-edge domain of research have been exploited through indentation with experimentation and simulation. However, comprehending the underlying deformation mechanism via indentation in PNCs is still an open question. The present study emphasis on the influence of incorporating different sizes of nanoparticles (NPs) with varied volume fraction to regulate and improve mechanical and tribological properties of PNCs through molecular dynamics (MD) simulations. The coarse-grained MD simulations for nanoindentation and nanoscratching of crosslinked polymer network without and with NPs are performed. The identification of static and dynamic properties reveals the domains of dispersion and agglomeration with structural conformations in the PNCs. The NPs act as promising reinforcements that significantly strengthen the polymer system through improvement in mechanical and tribological properties. Further the analysis of curing percentage and bond breakage play vital role in locating stress induced sites during indentation. From MD simulations, the prominent improvement in deformation resistance is shown with particles exhibiting least radius of 1.5σ and 2.5σ which enhances with increase in volume fraction of NPs. The observations evoke the fact that presence of NPs reduce coefficient of friction (cof) and diminishes the resistance against scratch which generates higher shear strain in polymer chains. The main aim of this study is to provide design rule for incorporation of NPs coordinated towards tuning mechanical and tribological performance of PNCs. Additionally, insights into the deformation and nanoscratching mechanisms are highlighted providing directions to confer multi-functionalities to advanced PNCs.

Anion-Immobilized Gel Polymer Electrolyte with a High Ion Transference Number for High-Performance Lithium/Sodium Metal Batteries.

Due to their high energy density, lithium/sodium metal batteries (LMBs/SMBs) are expected to be the next generation of energy storage systems. However, the further application of alkali metal batteries based on liquid electrolytes is limited due to increasing safety concerns. Gel polymer electrolytes (GPEs), which combine the advantages of the high ionic conductivity of liquid electrolytes and excellent mechanical properties of solid polymer electrolytes, are considered to play an irreplaceable role in the realization of high-performance alkali metal batteries. In this work, a flexible boron-containing GPE (B-GPE) with a cross-linked polymer network structure is prepared by a UV-induced process. The as-prepared B-GPE exhibits good ionic conductivity and has an extremely high ion transference number due to the electron-withdrawing effect of the boron moiety and the facile electrolyte uptake ability of the ethylene oxide chain. Furthermore, a "gentle" electrode/electrolyte contact is designed by a one-step in situ polymerization method, which can enhance ion transport within the electrode and at the electrode/electrolyte interface due to the presence of a continuous polymer phase for ion conduction. Therefore, LMBs and SMBs containing B-GPE are able to effectively inhibit the growth of dendrites while exhibiting excellent cycling stability. These comprehensive results indicate that this novel B-GPE possesses potential applications for high-performance alkali metal batteries.

Mechanochemical Reactivity of a 1,2,4-Triazoline-3,5-dione-Anthracene Diels-Alder Adduct.

Force-responsive molecules that produce fluorescent moieties under stress provide a means for stress-sensing and material damage assessment. In this work, we report a mechanophore based on Diels-Alder adduct TAD-An of 4,4'-(4,4'-diphenylmethylene)-bis-(1,2,4-triazoline-3,5-dione) and initiator-substituted anthracene that can undergo retro-Diels-Alder (rDA) reaction by pulsed ultrasonication and compressive activation in bulk materials. The influence of having C-N versus C-C bonds at the sites of bond scission is elucidated by comparing the relative mechanical strength of TAD-An to another Diels-Alder adduct MAL-An obtained from maleimide and anthracene. The susceptibility to undergo rDa reaction correlates well with bond energy, such that C-N bond containing TAD-An degrades faster C-C bond containing MAL-An because C-N bond is weaker than C-C bond. Specifically, the results from polymer degradation kinetics under pulsed ultrasonication shows that polymer containing TAD-An has a rate constant of 1.59×10-5 min-1 , while MAL-An (C-C bond) has a rate constant of 1.40×10-5 min-1 . Incorporation of TAD-An in a crosslinked polymer network demonstrates the feasibility to utilize TAD-An as an alternative force-responsive probe to visualize mechanical damage where fluorescence can be "turned-on" due to force-accelerated retro-Diels-Alder reaction.