Cross-linked Polymer Networks Research Articles

Mechanically triggered on-demand degradation of polymers synthesized by radical polymerizations.

Polymers that degrade on demand have the potential to facilitate chemical recycling, reduce environmental pollution and are useful in implant immolation, drug delivery or as adhesives that debond on demand. However, polymers made by radical polymerization, which feature all carbon-bond backbones and constitute the most important class of polymers, have proven difficult to render degradable. Here we report cyclobutene-based monomers that can be co-polymerized with conventional monomers and impart the resulting polymers with mechanically triggered degradability. The cyclobutene residues act as mechanophores and can undergo a mechanically triggered ring-opening reaction, which causes a rearrangement that renders the polymer chains cleavable by hydrolysis under basic conditions. These cyclobutene-based monomers are broadly applicable in free radical and controlled radical polymerizations, introduce functional groups into the backbone of polymers and allow the mechanically gated degradation of high-molecular-weight materials or cross-linked polymer networks into low-molecular-weight species.

A Vitrimer Acts as a Compatibilizer for Polyethylene and Polypropylene Blends

AbstractPolymer compatibilization plays a critical role in achieving polymer blends with favorable mechanical properties and enabling efficient recycling of mixed plastic wastes. Nonetheless, traditional compatibilization methods often require tailored designs based on the specific chemical compositions of the blends. In this study, we propose a new approach for compatibilizing polymer blends using a dynamically crosslinked polymer network, known as vitrimers. By adding a relatively small amount (1–5 w/w%) of a vitrimer made of siloxane‐crosslinked high‐density polyethylene (HDPE), we successfully compatibilized unmodified HDPE and isotactic polypropylene (iPP). The vitrimer‐compatibilized blend exhibited enhanced elongation at break (120 %) and smaller iPP domain sizes (0.4 μm) compared to the control blend (22 % elongation at break, 0.9 μm iPP droplet size). Moreover, the vitrimer‐compatibilized blend showed significantly improved microphase stability during annealing at 180 °C. This straightforward method shows promise for applications across various polymer blend systems.

Recent Advances in Environmentally Friendly Dual‐crosslinking Polymer Networks

Environmentally friendly crosslinked polymer networks feature degradable covalent or non‐covalent bonds, with many of them manifesting dynamic characteristics. These attributes enable convenient degradation, facile reprocessibility, and self‐healing capabilities. However, the inherent instability of these crosslinking bonds often compromises the mechanical properties of polymer networks, limiting their practical applications. In this context, environmentally friendly dual‐crosslinking polymer networks (denoted EF‐DCPNs) have emerged as promising alternatives to address this challenge. These materials effectively balance the need for high mechanical properties with the ability to degrade, recycle, and/or self‐heal. Despite their promising potential, investigations into EF‐DCPNs remain in their nascent stages, and several gaps and limitations persist. This Review provides a comprehensive overview of the synthesis, properties, and applications of recent progress in EF‐DCPNs. Firstly, synthetic routes to a rich variety of EF‐DCPNs possessing two distinct types of dynamic bonds (i.e., imine, disulfide, ester, hydrogen bond, coordination bond, and other bonds) are introduced. Subsequently, complex structure‐ and dynamic nature‐dependent mechanical, thermal, and electrical properties of EF‐DCPNs are discussed, followed by their exemplary applications in electronics and biotechnology. Finally, future research directions in this rapidly evolving field are outlined.

An Electrode-Crosstalk-Suppressing Smart Polymer Electrolyte for High Safety Lithium-Ion Batteries.

Electrode crosstalk between anode and cathode at elevated temperatures has been identified as a real culprit triggering the thermal runaway of lithium-ion batteries. Herein, to address this challenge, a novel smart polymer electrolyte is prepared through in-situ polymerization of methyl methacrylate and acrylic anhydride monomers within a succinonitrile-based dual-anion deep eutectic solvent. Owing to the abundant active unsaturated double bonds on the as-obtained polymer matrix end, this smart polymer electrolyte can spontaneously form a dense crosslinked polymer network under elevated temperatures, effectively slowing down the crosstalk diffusion kinetics of lithium ions and active gases. Impressively, LiCoO2/graphite pouch cells employing this smart polymer electrolyte demonstrate no thermal runaway even at the temperature up to 250°C via accelerating rate calorimeter testing. Meanwhile, because of its abundance of functional motifs, this smart polymer electrolyte can facilitate the formation of stable and thermally robust electrode/electrolyte interface on both electrodes, ensuring the long cycle life and high safety of LIBs. In specific, this smart polymer electrolyte endows 1.1 Ah LiCoO2/graphite pouch cell with a capacity retention of 96% after 398 cycles at 0.2 C. This article is protected by copyright. All rights reserved.

Tunable H-bond effects enabled ionic conductive gels with high stretchability, transparency, self-adhesion and wide environmental tolerance for e-skin

It is a high demand but a great challenge to construct all-in-one ionic conductive hydrogels with excellent mechanical performances, transparency, self-adhesion, conductivity and environmental tolerance. Herein, a versatile gel platform was developed via a facile and rapid photopolymerization of acrylic acid (AA) in deep eutectic solvent (DES) with multiple roles simultaneously as water-free solvent, ionic conductor, regulator of hydrogen bond density. Benefiting from the formed crosslinking networks including covalently cross-linked polymer network and multiple dynamic interactions, the resultant PAA-DES gels exhibited high stretchability of 884.7 ± 4.9%, toughness of 31.2 ± 1.4 MJ/m3, fatigue resistance and recovery ability, transparency of 93%, self-adhesive strength of 472 kPa. Moreover, the inherent advantages of DES enabled the gels with good ionic conductivity (2.5 ± 0.2 mS/cm), wide working window and biocompatibility. The PAA-DES as self-adhesive ionic sensors not only possessed wide strain range (∼400%) and stability, but also displayed excellent humidity (30–80%), temperature (−40–60 °C), and sweat tolerance which could precisely and steadily discern and monitor human motions and weak physiological signals. Thus, this work not only provides a strategy to construct all-in-one ionic conductive gels with high elasticity, fatigue resilience, transparency, self-adhesion and wide environmental tolerance, but also exploits versatile platform for intelligent gel sensor and e-skin.

Superabsorbent Hydrogels in the Agriculture and Reclamation of Degraded Areas

Superabsorbent materials (SAPs) are crosslinked polymer networks composed of ionic and non-ionic monomers. SAPs can absorb and retain water solutions up to several hundred times their own weight. As a result of swelling, they form a gel that is insoluble in water, considered safe and decomposes over time. This review focuses on the synthesis, degradation and chemical composition of SAP materials, with particular emphasis on chemical substances that are soluble in water and can migrate into the environment. Numerous applications of natural and synthetic hydrogels in agriculture and the reclamation of degraded areas in preventing erosion, retention water, reducing leaching of colloidal soil components and plant protection products, fertilisers and mineral salts into surface waters have been described. The influence of SAPs on the microbial activity of soils is described. New trends in the search for environmentally friendly SAPs made of modified biopolymers and waste materials are presented, which not only increase yields, but also ensure sustainable agro-environmental development.

Two-Dimensional Nanoporous Cross-linked Polymer Networks as Emerging Candidates for Gas Adsorption.

This paper illustrates the gas adsorption properties of newly synthesized nanoporous cross-linked polymer networks (CPNs). All synthesized CPNs possess N-rich functional groups and are used for the utilization of carbon dioxide and methane. Good gas adsorption and selectivities are obtained for all of the samples. Among the materials, HEREON2 outperforms better selectivity for methane separation from nitrogen rather than zeolites, activated carbons, molecular sieves, covalent organic frameworks, and metal-organic frameworks (MOFs). The accessibility of the N-rich functionalities makes these materials potential candidates for the separation of hydrocarbons via increased polarizabilities. High-pressure adsorption experiments showed that the synthesized two-dimensional nanoporous materials also have a high affinity toward carbon dioxide. HEREON2 powders showed an increased experimental CO2/N2 selectivity of ∼25,000 at 50 bar due to the presence of nitrogen groups in the structure. Fourier-transform infrared spectroscopy (FTIR), solid-state NMR, X-ray diffraction, thermogravimetric analysis, energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) were applied for the characterization of the synthesized nanoporous CPNs. The results show a potential new pathway for future CPN membrane development.

Effect of penetrant-polymer interactions and shape on the motion of molecular penetrants in dense polymer networks.

The diffusion of dilute molecular penetrants within polymers plays a crucial role in the advancement of material engineering for applications such as coatings and membrane separations. The potential of highly cross-linked polymer networks in these applications stems from their capacity to adjust the size and shape selectivity through subtle changes in network structures. In this paper, we use molecular dynamics simulation to understand the role of penetrant shape (aspect ratios) and its interaction with polymer networks on its diffusivity. We characterize both local penetrant hopping and the long-time diffusive motion for penetrants and consider different aspect ratios and penetrant-network interaction strengths at a variety of cross-link densities and temperatures. The shape affects the coupling of penetrant motion to the cross-link density- and temperature-dependent structural relaxation of networks and also affects the way a penetrant experiences the confinement from the network meshes. The attractive interaction between the penetrant and network primarily affects the former since only the system of dilute limit is of present interest. These results offer fundamental insights into the intricate interplay between penetrant characteristics and polymer network properties and also suggest future directions for manipulating polymer design to enhance the separation efficiency.

A Crown-Ether-Based Elastomer Bearing Loop Structures with Dissipating Characteristics and Enhanced Mechanical Performance.

Loops are prevalent topological structures in cross-linked polymer networks, resulting from the folding of polymer chains back onto themselves. Traditionally, they have been considered as defects that compromise the mechanical properties of the network, leading to extensive efforts in synthesis to prevent their formation. In this study, we introduce the inclusion of cyclic dibenzo-24-crown-8 (DB24C8) moieties within the polymer network strands to form CCNs, and surprisingly, these loops enhance the mechanical performances of the network, leading to tough elastomers. The toughening effect can be attributed to the unique cyclic structure of DB24C8. The relatively small size and the presence of rigid phenyl rings provide the loops with relatively stable conformations, allowing for substantial energy dissipation upon the application of force. Furthermore, the DB24C8 rings possess a broad range of potential conformations, imparting the materials with exceptional elasticity. The synergistic combination of these two features effectively toughens the materials, resulting in a remarkable 66-fold increase in toughness compared to the control sample of covalent networks. Moreover, the mechanical properties, particularly the recovery performance of the network, can be effectively tuned by introducing guests to bind with DB24C8, such as potassium ions and secondary ammonium salts.

DESIGN OF JACKFRUIT GUM-BASED GENIPIN CROSSLINKED NANOPARTICLES FOR SUSTAINED RELEASE OF CURCUMIN: OPTIMIZATION AND IN VITRO CHARACTERIZATION

The present work aims to design jackfruit gum-based curcumin-loaded nanoparticles (CUR-NPs) for improved drug entrapment and modified release of CUR using ionotropic gelation. Briefly, the optimization of CUR-NPs was confirmed using a 32 response surface methodology. The diffractogram and thermogram of CUR-NPs confirmed reduction of crystallinity of CUR (optimized batch: F5) due to jackfruit gum and genipin cross-linked polymeric network. The particle size and zeta potential analysis confirmed formation of nanosized and stable CUR-NPs, respectively. Also, the nanoparticles demonstrated 83.99 ± 1.23% entrapment efficiency, whereas they showed 98.36 ± 0.96% of CUR release within 12 h at pH 7.4. The CUR-NPs exhibited good mucoadhesive properties due to the presence of jackfruit gum. Finally, the MTT assay showed a decrease in colorectal cancer cell viability due to tailored CUR release from CUR-NPs. In conclusion, jackfruit gum-genipin-based CUR-NPs offered high entrapment efficiency, tailored releases of CUR, good mucoadhesive property and improved anticancer activity.

Highly conductive and mechanically robust composite cathodes based on 3D interconnected elastomeric networks for deformable lithium‐ion batteries

AbstractDeformable lithium‐ion batteries (LIBs) can serve as the main power sources for flexible and wearable electronics owing to their high energy capacity, reliability, and durability. The pivotal role of cathodes in LIB performance necessitates the development of mechanically free‐standing and stretchable cathodes. This study demonstrates a promising strategy to generate deformable cathodes with electrical conductivity by forming 3D interconnected elastomeric networks. Beginning with a physically crosslinked polymer network using poly(vinylidene fluoride‐co‐hexafluoropropylene) and 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]), subsequent exchange with a 1 M LiPF6 electrolyte imparts elastic characteristics to the cathodes. The resulting LiFePO4 composite electrodes maintained their resistance under 500 consecutive bending cycles at an extremely small bending radius of 1.8 mm and showed high discharge capacity of 158 mAh g−1 with stable potential plateaus in charging and discharging curves. Moreover, flexible cells utilizing the composite electrodes exhibited superior operational stability under rolling, bending, and folding deformations.image

Swelling behavior of sodium lignosulfonate grafted polyacrylic acid highly absorbent hydrogels with two-phase structure

There exist two theories on the swelling behavior of cross-linked polymer networks treated by statistical thermodynamic and scalar methods, respectively. These two theories predict inconsistent network chain length dependence of the swelling ratio. The swelling of biopolymer gels is analysed with scaling laws from polymer physics, as an alternative for the classical Flory-Rehner theory. In this paper, the swelling data of sodium lignosulfonate grafted polyacrylic acid cross-linked networks in good solvents are reanalyzed, and it is pointed out that these networks are a typical model network conforming to the two-phase structural picture. For the swelling behavior of gels in the semi-dilute regime we derive the scaling theory based swelling equations, which make them compact and universal character.

Mussel inspired sequential protein delivery based on self-healing injectable nanocomposite hydrogel

Polysaccharide based self-healing and injectable hydrogels with reversible characteristics have widespread potential in protein drug delivery. However, it is a challenge to design the dynamic hydrogel for sequential release of protein drugs. Herein, we developed a novel mussel inspired sequential protein delivery dynamic polysaccharide hydrogel. The nanocomposite hydrogel can be fabricated through doping polydopamine nanoparticles (PDA NPs) into reversible covalent bond (imine bonds) crosslinked polymer networks of oxidized hyaluronic acid (OHA) and carboxymethyl chitosan (CEC), named PDA NPs@OHA-l-CEC. Besides multiple capabilities (i.e., injection, self-healing, and biodegradability), the nanocomposite hydrogel can achieve sustained and sequential protein delivery of vascular endothelial growth factor (VEGF) and bovine serum albumin (BSA). PDA NPs doped in hydrogel matrix serve dual roles, acting as secondary protein release structures and form dynamic non-covalent interactions (i.e., hydrogen bonds) with polysaccharides. Moreover, by adjusting the oxidation degree of OHA, the hydrogels with different crosslinking density could control overall protein release rate. Analysis of different release kinetic models revealed that Fickian diffusion drove rapid VEGF release, while the slower BSA release followed a Super Case II transport mechanism. The novel biocompatible system achieved sequential release of protein drugs has potentials in multi-stage synergistic drug deliver based on dynamic hydrogel.

Bio‐Functional Hydrogel Microspheres for Musculoskeletal Regeneration

AbstractThe worldwide prevalence of musculoskeletal disorders is expected to rise due to an accelerating aging population and increasing associated factors. Despite the high rate of disability induction, clinical options for the treatment of musculoskeletal disorders remain limited. Hydrogel microspheres (HMSs), which are colloidal particles with crosslinked polymer networks at the nanometer scale, are characterized by excellent biocompatibility, easy‐to‐tune structures (e.g., stiffness, porosity, and composition), and high encapsulation efficiency of therapeutic agents (e.g., drugs and cells). At present, multiple techniques are utilized for the fabrication of HMSs and encapsulation of therapeutic agents. The HMSs can be produced and used in different states to match specific utilizations. These desirable properties, along with their injectability, bestow HMSs with mutifunctionalities that can be utilized for a variety of biomedical applications (e.g., delivery of biologics, tissue regeneration, and bio‐lubrication), and thus enable them to be promising tools for tackling a variety of clinical diseases. Herein, the focus is the fabrication and application of therapeutic HMSs for i) bone disorders, ii) cartilage and osteochondral disorders, iii) intervertebral disc (IVD) disorders, and iv) neurovascular disorders, with the aim of overviewing the recent progress to summarize key contributions and findings and highlight the limitations and opportunities in this field.

Single-atom tailored atomically-precise nanoclusters for enhanced electrochemical reduction of CO2-to-CO activity

The development of facile tailoring approach to adjust the intrinsic activity and stability of atomically-precise metal nanoclusters catalysts is of great interest but remians challenging. Herein, the well-defined Au8 nanoclusters modified by single-atom sites are rationally synthesized via a co-eletropolymerization strategy, in which uniformly dispersed metal nanocluster and single-atom co-entrenched on the poly-carbazole matrix. Systematic characterization and theoretical modeling reveal that functionalizing single-atoms enable altering the electronic structures of Au8 clusters, which amplifies their electrocatalytic reduction of CO2 to CO activity by ~18.07 fold compared to isolated Au8 metal clusters. The rearrangements of the electronic structure not only strengthen the adsorption of the key intermediates *COOH, but also establish a favorable reaction pathway for the CO2 reduction reaction. Moreover, this strategy fixing nanoclusters and single-atoms on cross-linked polymer networks efficiently deduce the performance deactivation caused by agglomeration during the catalytic process. This work contribute to explore the intrinsic activity and stability improvement of metal clusters.

A Vitrimer Acts as a Compatibilizer for Polyethylene and Polypropylene Blends.

Polymer compatibilization plays a critical role in achieving polymer blends with favorable mechanical properties and enabling efficient recycling of mixed plastic wastes. Nonetheless, traditional compatibilization methods often require tailored designs based on the specific chemical compositions of the blends. In this study, we propose a new approach for compatibilizing polymer blends using a dynamically crosslinked polymer network, known as vitrimers. By adding a relatively small amount (1-5 w/w%) of a vitrimer made of siloxane-crosslinked high-density polyethylene (HDPE), we successfully compatibilized unmodified HDPE and isotactic polypropylene (iPP). The vitrimer-compatibilized blend exhibited enhanced elongation at break (120 %) and smaller iPP domain sizes (0.4 μm) compared to the control blend (22 % elongation at break, 0.9 μm iPP droplet size). Moreover, the vitrimer-compatibilized blend showed significantly improved microphase stability during annealing at 180 °C. This straightforward method shows promise for applications across various polymer blend systems.

External Stimuli-Induced Welding of Dynamic Cross-Linked Polymer Networks.

Thermosets have been crucial in modern engineering for decades, finding applications in various industries. Welding cross-linked components are essential in the processing of thermosets for repairing damaged areas or fabricating complex structures. However, the inherent insolubility and infusibility of thermoset materials, attributed to their three-dimensional network structure, pose challenges to welding development. Incorporating dynamic chemical bonds into highly cross-linked networks bridges the gap between thermosets and thermoplastics presenting a promising avenue for innovative welding techniques. External stimuli, including thermal, light, solvent, pH, electric, and magnetic fields, induce dynamic bonds' breakage and reformation, rendering the cross-linked network malleable. This plasticity facilitates the seamless linkage of two parts to an integral whole, attracting significant attention for potential applications in soft actuators, smart devices, solid batteries, and more. This review provides a comprehensive overview of dynamic bonds employed in welding dynamic cross-linked networks (DCNs). It extensively discusses the classification and fabrication of common epoxy DCNs and acrylate DCNs. Notably, recent advancements in welding processes based on DCNs under external stimuli are detailed, focusing on the welding dynamics among covalent adaptable networks (CANs).

Theory of expansion and compression of polymeric materials: Implications for membrane solvent flow under compaction

We extend the classical Flory-Rehner theory for the expansion and compression of porous materials such as cross-linked polymer networks and membranes. The theory includes volume exclusion, affinity with the solvent, and finite stretching of the polymer chains. We also modify this equilibrium theory, which applies to equal expansion of a material in all directions, to the situation where a material can only expand in a single direction, as is the case when a thin polymer layer is tightly bound to a support structure. We further extend this equilibrium model to the case where a pressure is applied across a thin polymer layer, such as a membrane, and liquid flows across the membrane. The theory describes how in the direction of liquid flow the membrane is increasingly compacted and becomes less porous, and this effect increases when the applied pressure goes up. We present results of example calculations for a thick membrane showing significant changes in compaction across its thickness, and a thin membrane where compaction due to flow is minor. Lastly, we model the dynamics of the change in size of a porous material in time after a step change in the solvent-polymer attraction parameter, which may be relevant, for instance, when the attraction parameter is highly temperature-dependent, and we change the temperature. The developed theory has direct implications for pressure-driven membrane separation processes ranging from reverse osmosis to organic solvent nanofiltration.

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.