Cross-linked Polymer Research Articles

Interfacially Assembled Anion Exchange Membranes for Water Electrolysis.

High-performance and durable anion exchange membranes (AEMs) are critical for realizing economical green hydrogen production through alkaline water electrolysis (AWE) or AEM water electrosysis (AEMWE). However, existing AEMs require sophisticated fabrication protocols and exhibit unsatisfactory electrochemical performance and long-term durability. Here we report an AEM fabricated via a one-pot, in situ interfacial Menshutkin reaction, which assembles a highly cross-linked polymer containing high-density quaternary ammoniums and nanovoids inside a reinforcing porous support. This structure endows the membrane with high anion-conducting ability, water uptake (but low swelling), and mechanical and thermochemical robustness. Consequently, the assembled membrane achieves excellent AWE (0.97 A cm-2 at 1.8 V) and AEMWE (5.23 A cm-2 at 1.8 V) performance at 5 wt % KOH and 80 °C, significantly exceeding that of commercial and previously developed membranes, and excellent long-term durability. Our approach provides an effective method for fabricating AEMs for various energy and environmental applications.

Construction of Slide-Ring Polymers Based on Pillar[5]Arene/Alkyl Chain Host-Guest Interactions.

Slide-ring polymers exhibit distinctive mechanical properties, making them highly promising for applications in emerging fields such as energy storage devices and smart sensing. However, existing slide-ring polymer systems primarily rely on hydrophilic-hydrophobic interactions to achieve ring-axle interlocking in aqueous phases. This reliance limits the construction of slide-ring networks mainly to water-soluble polymers, excluding a diverse range of lipophilic polymers. Therefore, it is crucial to introduce efficient construction strategies that facilitate interpenetration in organic solvents, enabling the development of diverse slide-ring polymers and expanding their range and applications. Herein, by utilizing the pillar[5]arene/alkyl chain host-guest interactions, we successfully facilitated the interpenetration of a pillar[5]arene and poly(caprolactone), enabling the efficient construction of two slide-ring polymer networks in organic solvents. One of these two slide-ring polymers demonstrates a unique network deformation mechanism along with outstanding mechanical properties compared with the control covalently cross-linked polymer network, including maximum stress (4.43 vs 1.98 MPa), maximum strain (1285 vs 330 %), and toughness (35.4 vs 3.92 MJ/m3). More importantly, this strategy of making slide-ring polymers is highly versatile, given the wide range of macrocyclic arenes and alkyl chain-containing polymers it can accommodate.

Efficient Cross-Linking through C-H Bond Insertion of Unfunctionalized Commodity Materials Using Diazirine-Containing Polymers.

The synthesis and application of multifunctional diazirine-containing polymers for on-demand cross-linking of unfunctionalized commodity polymers through C-H bond insertion is demonstrated. While small-molecule diazirine cross-linkers have seen important applications such as plastic compatibilization and photopatterning, the high degree of functionalization of polymer-based diazirine cross-linkers offers promise for enhanced compatibility based on polymer blending and increased efficiency due to controllable multivalency. As a demonstrative example, unfunctionalized linear poly(n-butyl acrylate) (PnBA) can be cross-linked using various polymeric cross-linkers with diazirine contents as low as 0.8 wt % in 1 min under photochemical conditions. With gel fractions up to 95%, tunable rheological behavior is observed with increasing cross-linker loadings, consistent with a transition from entangled branched polymers to a cross-linked network. Moreover, the synthetic stability of the diazirine units can be exploited to prepare diazirine-containing polymers based on a variety of different backbones, from vinyl copolymers to poly(dimethylsiloxane) (PDMS), which allows successful photopatterning using a commercial 3D printer.

Study of mechanical and rheogical properties of HDPE/iPP cross-linking polymer blends

The rapidly developing subject of polymer mix design presents prospects for the creation of materials with precisely customised qualities. It's crucial to comprehend the morphology that results from blending immiscible polymers for obtaining the required mechanical qualities. This study explores the complex interactions between structure and characteristics in both cross-linked and unmodified blends of HDPE and iPP. Our study provides important new information by combining mechanical testing and rheological evaluations. It is observed that materials that are cross-linked have a significantly higher viscosity, which suggests the establishment of a three-dimensional network and improved stability. Furthermore, HDPE exhibits enhanced behaviour after crosslinking. After crosslinking, the melt flow index decreases, indicating increased resistance to flow and deformation because more interchain links are formed. Higher PP concentration in unmodified blends results in greater flexibility and ductility, but cross-linking causes rigidity. On the other hand, cross-linking improves flexibility by reducing rigidity in mixes with a lower iPP concentration. Additionally, the combination of iPP and crosslinking agents results in a significant improvement in mechanical strength, which reinforces structural integrity. Crosslinking also improves impact resistance, making the materials appropriate for uses requiring strong performance in demanding circumstances.

Uniform Polymer Microspheres by Photoinduced Metal-Free Atom Transfer Radical Precipitation Polymerization.

Herein, a photoinduced method is introduced for the synthesis of highly cross-linked and uniform polymer microspheres by atom transfer radical polymerization (ATRP) at room temperature and in the absence of stabilizers or surfactants. Uniform particles are obtained at monomer concentrations as high as 10% (by volume), with polymers being exempt from contamination by residual transition metal catalysts, thereby overcoming the two major longstanding problems associated with thermally initiated ATRP-mediated precipitation polymerization. Moreover, the obtained particles have also immobilized ATRP initiators on their surface, which directly enables the controlled growth of densely grafted polymer layers with adjustable thickness and a well-defined chemical composition. The method is then employed successfully for the synthesis of molecularly imprinted polymer microspheres.

Development of aryl ether-free cross-linked polymer membranes for sustainable electrochemical energy conversion and storage applications

Development of aryl ether-free cross-linked polymer membranes for sustainable electrochemical energy conversion and storage applications

Effect of Surface Polymeric Organosilicon Nanolayers on the Electrochemical and Corrosion Behavior of Copper.

The formation of polymeric self-organizing organosilicon surface nanolayers on copper occuring as a result of modification of the metal surface with organosilane-based formulations has been studied. The anticorrosive effect of such surface layers in corrosive chloride-containing electrolytes as well as in artificial and natural atmospheres has been studied in detail. It has been found that the maximum protective effect is observed at a thickness of 3.8 molecular layers, where the densest cross-linked polymer layers are formed that hinder the adsorption of chloride ions and other corrosive agents on the metal surface, thus significantly reducing the rate of their reactions with the surface copper atoms, and, as a result, inhibiting the corrosion and local anodic dissolution of the metal.

Solvent Choice during Flow Assembly of Photocross-Linked Single-Chain Nanoparticles and Micelles Affects Cellular Uptake.

Polymeric micelles have widely been used as drug delivery carriers, and recently, single-chain nanoparticles (SCNPs) emerged as potential, smaller-sized, alternatives. In this work, we are comparing both NPs side by side and evaluate their ability to be internalized by breast cancer cells (MCF-7) and macrophages (RAW 264.7). To be able to generate these NPs on demand, the polymers were assembled by flow, followed by the stabilization of the structures by photocross-linking using blue light. The central aim of this work is to evaluate how the type of solvent affects self-assembly and ultimately the structure of the final NP. Therefore, a library of copolymers with different sequences, including block copolymers (AB, ABA, BAB), and statistical copolymers (rAB and rAC) was synthesized using PET-RAFT with A denoting poly(ethylene glycol) methyl ether acrylate (PEGMEA), B as 2-hydroxyethyl acrylate (HEA), and C as 4-hydroxybutyl acrylate (HBA). The polymers were conjugated with a quinoline derivative to enable the formation of cross-linked structures by photocross-linking during flow assembly. Using water as the dispersant for photocross-linking led to the preassembly of these amphiphilic polymers into compact SCNPs and cross-linked micelles, resulting in a quick photoreaction. In contrast, acetonitrile led to fully dissolved polymers but a low rate of the photoreaction. These intramolecularly cross-linked polymers were then placed in water to result in more dynamic micelles and looser SCNPs. Small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and size exclusion chromatography (SEC) coupled with a viscosity detector show that cross-linking in acetonitrile results in better-defined NPs with a shell rich in PEGMEA. Cross-linking in acetonitrile led to NPs with significantly higher cellular uptake. Interestingly, passive transport was identified as the main pathway for the delivery of our NPs on MCF-7 cells, confirmed by the uptake of NPs on cells treated with inhibitors and by red blood cells. This work underscored the importance of the polymer precursor's structure and the choice of solvent during intramolecular cross-linking in determining the drug delivery efficiency and biological behavior of SCNPs.

Understanding the role of chain stiffness in the mechanical response of cross-linked polymer: Flexible vs. semi-flexible chains

Cross-linked polymers are widely used in structural, engineering, and biomedical applications due to their lightweight and superior properties. Although chain bending stiffness has been recognized to play an essential role in their thermodynamical and mechanical properties, how it influences these properties of cross-linked polymers with flexible or semi-flexible chains remains under debate. Here, we systematically explore its influences utilizing coarse-grained (CG) molecular dynamics (MD) simulations based on a bead-spring CG model. It is found that with chain bending stiffness increasing, both density and elastic moduli (i.e., shear modulus and tensile modulus) of cross-linked polymers first decrease slightly and then decrease significantly followed by a gradual increase, along with the polymer transition from a dense cross-linked thermoset to a highly porous fibrous network. The moduli of cross-linked polymers with flexible and semi-flexible chains exhibit distinct scaling laws with the density. For cross-linked polymers with flexible chains, their moduli increase significantly with increasing strain rate, which correlates to the change in potential energy of interchain interaction during deformation. However, the moduli display slight dependence on strain rate for porous cross-linked polymers with sufficiently stiff chains, where the intrachain interactions (i.e., bond stretching and angle bending energies) become dominant and independent of strain rate. Moreover, the elastic moduli exhibit scaling laws with Debye-Waller factor for both dense cross-linked thermosets with flexible chains and highly porous networks with stiff backbones. Our work facilitates a better understanding for mechanical properties and deformation mechanism of cross-linked polymers with variable chain bending stiffness at molecular level, shedding light on tailoring mechanical properties of cross-linked polymers via chain engineering.

Multisite Crosslinked Poly(ether-urethane)-Based Polymer Electrolytes for High-Voltage Solid-State Lithium Metal Batteries.

Utilizing solid-state polymer electrolytes (SPEs) in high-voltage Li-metal batteries is a promising strategy for achieving high energy density and safety. However, the SPEs face the challenges such as undesirable mechanical strength, low ionic conductivity and incompatible high-voltage interface. Here, a novel crosslinked poly(ether-urethane)-based SPE with a molecular cross-linked structure is fabricated to create high-throughput Li+ transport pathway. The amino-modified Zr-porphyrin-based metal-organic frameworks (ZrMOF) are introduced as multisite cross-linking nodes and polymer chain extenders. The abundant ether/ketonic-oxygen and Lewis acid sites in the SPE achieve high Li+ conductivity (5.7 × 10-4 S cm-1 at 30°C) and Li+ transference number (0.84). The interpenetrating cross-linked structure of SPE with robust mechanical strength results in a record cycle life of 8000h in Li||Li symmetric cell. The high structural stability of ZrMOF and abundant electron-withdrawing urethane/ureido groups in the SPE with high oxidation potential (5.1V) enables a discharge capacity of 182 mAh g-1 at 0.3 C over 500 cycles in a LiNi0.8Co0.1Mn0.1O2||Li cell. Remarkably, a high energy density of 446Wh kg-1 in a 1.5-Ah pouch cell is obtained with high loading cathode (≈4 mAh cm-2), demonstrating a great prospect of the current SPE for practical application in solid-state, high-voltage Li-metal batteries.

Highly Stretchable Sensitive Multiscale Hydrogel Inspired by Biological Muscles for Wearing Sensors.

Hydrogels have attracted substantial research interest for application in wearable electronics due to their stretchability, elasticity, and compliance. However, most hydrogels could not satisfy the application requirements for high-performance wearable sensors due to their poor sensitivity, low mechanical properties, and sensing detection range until this day. Inspired by the fascia in biological muscles, we propose a strategy to form entangled "clusters" through the dense entanglement between highly cross-linked elastic hydrogel microspheres and polymer segments, and prepared a multiscale hydrogel with high sensitivity and mechanical toughness. This strategy embedded highly swollen hydrogel microspheres (with different pore sizes) to act as the microregions of dense entanglement in the soft matrix to adjust the microstructure of multiscale gel. When pressure was applied, this structure could provide a fast response due to the stack layer formed by microspheres and soft matrix produced effective stress distribution, resulting in the outstanding sensitivity of the multiscale hydrogel (S = 1.1 kPa-1) in the pressure range of 0-50 kPa. The distinct microspheres functioning as microscale joint areas significantly augment energy dissipation, culminating in exceptional mechanical stability, ultrastretchability (≈1050%), and high strength of the multiscale hydrogel. The most notable progress was that the synthesized multiscale hydrogel not only combined the above advantages but also simultaneously solved multiple dilemmas of tedious synthesis steps, high cost, and poor durability. Besides, the multiscale hydrogel also had excellent antibacterial properties and biocompatibility, which enabled them to have large-scale application potential in wearable and implantable electronic devices. Our research could provide a universal approach to the creation of robust, flexible, wearable, and sensitive sensors, significantly increasing the uses of stress sensors in wearable technology.

A Novel Approach for Nanosponge: Wool Waste as a Building Block for the Synthesis of Keratin-Based Nanosponge and Perspective Application in Wastewater Treatment.

Wool waste is a huge environmental problem that needs to be addressed in order to avoid the continuous accumulation of biohazardous waste in landfills. In recent years, wool has proven to be an excellent source of keratin that can be used for various purposes. But never before has keratin from wool waste been used as a building block to synthesize a well-known class of biopolymers called nanosponges. Typically, nanosponges are produced by the reaction of cyclodextrins with an appropriate cross-linker to obtain an insoluble hyper-cross-linked polymer, which has applications in various fields. For this reason, a novel, affordable approach for the synthesis of a novel class of nanosponge using wool keratin as the building block has been presented. The keratin nanosponge was synthesized by reacting keratin with pyromellitic dianhydride as a cross-linking agent. The formation of a cross-linked polymer was successfully confirmed by CHNS-elemental analysis, TGA, DSC, FTIR-ATR, SEM, and water absorption capacity measurements. Surprisingly, the keratin-based nanosponge showed ∼50% uptake of heavy metals after only 24 h of contact time. The adsorption kinetics was also evaluated, indicating a pseudo-second-order model fit and the mechanism is predominantly the intraparticle diffusion process. The novel synthesized nanosponge proved to be a possible alternative for wastewater treatment.

Flame retardant epoxy vitrimers employing a Meldrum's acid-functionalized and phosphorus-containing bisphenol compound as the cross-linking agent for conventional epoxy

In addition to recycling and reprocessing features, designs of vitrimers possessing critical properties required for conventional cross-linked polymers are of interest. In this work, a newly designed Meldrum's acid (MA)-functionalized and phosphorus-containing bisphenol compound (P-MADV) compound is synthesized. Employing P-MADV as a curing agent, simultaneous formation of permanent and dynamic covalent linkages through one-pot reaction has been achieved in formation of flame retardant epoxy resin vitrimers. The newly synthesized P-MADV compound is an effective cross-linking agent for commercially-available epoxy resins for synthesis of flame-retardant epoxy vitrimers through the conventional cross-linking conditions and processes. The cross-linked epoxy vitrimer passes the flame test under UL 94V-0 specification contributed from the phosphorus-containing groups. The flame retardant vitrimer also exhibits a storage modulus of 1.2 GPa at 50 °C and a Tg (tanδ peak) of 185 °C, which are comparable to the values found with conventional phenolic novolac-cross-linked epoxy resin (storage modulus: 1.6 GPa; Tg: 155 °C). Without addition of catalysts for dynamic bond exchanging reaction, the cross-linked epoxy vitrimer shows stress relaxation time of 397 s at 230 °C and an activation energy of stress relaxation of 97.8 kJ mol−1. The dynamic features warrant the epoxy vitrimer being recyclable under thermally pressing at 230 °C and 16 MPa for 1 h. The recycled samples exhibit thermal and mechanical properties being comparable to the pristine resin. This work has demonstrated an attractive approach for preparation of flame retardant epoxy resin vitrimers in viewpoints of both manufacturing processes and resins properties.

Recyclable sulfur cured natural rubber with controlled disulfide metathesis

Traditionally, sulfur-cured natural rubber compounds exhibit limited recyclability due to a significant drop in mechanical performance after reprocessing. Maintaining physical and chemical properties after recycling of a cross-linked polymer is an essential requirement for the global rubber industry to become more sustainable. Here, we demonstrate that tuning the curing process to favour a reversible cross-linked network based on disulfide and polysulfide bonds enables recyclability. We use a sulfur-based vulcanization system optimized with copper (II) methacrylate at concentrations of 2.47, 4.94, and 9.89 phr to control disulfide metathesis at low temperatures and enhance recyclability. Mechanical characterization identifies 2.47 phr as optimal for maintaining mechanical properties after initial moulding and full recovery after recycling. Additionally, we demonstrate that copper (II) methacrylate can be incorporated into existing rubber waste streams to promote recyclability.

Judd-Ofelt analysis and temperature sensing properties of polyethylmethacrylate (PEMA) networks doped with CdNb2O6: Er3+/Yb3+ phosphors

Research on the sensitivity of temperature measurements and optical thermometry involving rare earth ions has been an area of interest in the field of photonics and materials science. Introducing polymer networks as a new host material has an important role due to their properties including a versatile and stable environment for rare earth ions and rapid response in temperature detection. In this work, linear and crosslinked polyethylmethacrylate (PEMA) networks doped with Er3+/Yb3+ (1.5 mol % Er3+, 2 mol% Yb3+) synthesized by free-radical crosslinking polymerization with 0.1 EMA (weight %) at 60 °C were used to investigate direct and indirect optical bandgap energies and Urbach energy from UV–Visible spectra. The Judd-Ofelt (JO) approach was employed to analyze parameters Ωt (t = 2,4,6), spontaneous transition probabilities (Α), branching ratios (β) and radiative lifetimes (τ) as a function of linear and crosslinked PEMA doped nano-crystalline CdNb2O6: Er3+/Yb3+. The stimulated emission cross-sections of the transitions 2H11/2⟶4I15/2, 2S3/2⟶4I15/2 and 2F9/2⟶4I15/2 of Er3+/Yb3+ were calculated by two different methods; Fuchtbauer-Ladenburg formula and modified theory, respectively. The gain bandwidth cross-section product for the 2H11/2⟶4I15/2 was found to be 162.06. 10−28cm3, 260.94. 10−28cm3 and 461.20. 10−28cm3 of Er3+/Yb3+ embedded in linear, low and high crosslinked PEMA samples, respectively. JO parameters and stimulated emission cross-sections increased; therefore, radiative lifetimes decreased by increasing crosslinking content. In addition, the temperature dependence of upconversion (UC) luminescence was monitored under 975 nm excitation. The fluorescence intensity ratio (FIR) method examined the temperature sensing under two thermally coupled levels at 525 and 548 nm. The maximum sensitivities obtained from the FIR technique within the 300–650 K temperature range shifted to lower temperatures with increasing crosslinker content. Hence, linear and crosslinked polymer hosts doped rare-earth ions can be candidates for remote temperature sensors across various fields.

A Numerical Framework of Simulating Flow-Induced Deformation during Liquid Composite Moulding

Fibre deformation (or shearing of yarns) can develop during the liquid moulding of composites due to injection pressures or polymerisation (cross-linking) reactions (e.g., chemical shrinkage). On that premise, this may also induce potential residual stress–strain, warpage, and design defects in the composite part. In this paper, a developed numerical framework is customised to analyse deformations and the residual stress–strain of fibre (at a micro-scale) and yarns (at a meso-scale) during a liquid composite moulding (LCM) process cycle (fill and cure stages). This is achieved by linking flow simulations (coupled filling–curing simulation) to a transient structural model using ANSYS software. This work develops advanced User-Defined Functions (UDFs) and User-Defined Scalers (UDSs) to enhance the commercial CFD code with extra models for chemorheology, cure kinetics, heat generation, and permeability. Such models will be hooked within the conservation equations in the thermo-chemo-flow model and hence reflected by the structural model. In doing so, the knowledge of permeability, polymerisation, rheology, and mechanical response can be digitally obtained for more coherent and optimised manufacturing processes of advanced composites.

Flexible and Flame Retardant Cotton Fiber-reinforced CS/CNF/EP Composites For a Sensitive Fire Warning

Enhancing fire safety performance in resin-based composites is critical for mitigating fire hazards. Composites that provide early warnings during initial fire stages and maintain prolonged alarm capabilities under flame exposure are essential for minimizing injuries and property damage. This study employs chitosan (CS) as a cross-linking and char-forming agent, while carbon nanofiber (CNF) acts as a flame retardant and conductive component. Cotton fiber-reinforced chitosan/carbon nanofiber composites with varying epoxy and CNF contents (CS/CNF/EP) were prepared through a straightforward low-temperature curing process (40 °C, 4 h). Crosslinking polymerization between chitosan and epoxy was confirmed via Fourier Transform Infrared Spectroscopy (FTIR). The presence of the tough structure of CS and the rigid structure of EP results in a synergistic toughening and reinforcement effect in the CS/CNF/EP composites. Additionally, due to the carbon facilitating properties of CS and the enhanced conductivity of CNF, the composites exhibit a sensitive fire alarm functionality. At 35 % epoxy content, the CS/CNF/EP composites demonstrated exceptional properties, including mechanical flexibility (tensile strength of 7.09 MPa and breaking elongation of 129.05 %) and outstanding flame retardancy (LOI of 32.3 % and UL-94 V-0 rating). These composites provided a rapid alarm (∼1 s), sustaining a 263 s warning under flame exposure and over 35 min post-flame removal. These findings indicate that CS/CNF/EP composites, integrating mechanical flexibility, flame retardancy, and superior fire warning properties, present significant potential applications in fire safety engineering.

Fluoroamphiphiles for enhancing immune response of subunit vaccine against SARS-CoV-2

In recent decades, protein-based therapy has garnered valid attention for treating infectious diseases, genetic disorders, cancer, and other clinical requirements. However, safeguarding protein-based drugs against degradation and denaturation during processing, storage, and delivery poses a formidable challenge. Herein, we designed a novel fluoroamphiphiles polymer to deliver protein. Two different formulations of nanoparticles, cross-linked (CNP) and micelle (MNP) polymer, were prepared rationally by disulfide cross-linked and thin-film hydration techniques, respectively. Two prepared formulations were characterized for size, zeta potential, and morphology. The delivery efficacy of both in vitro and in vivo was also assessed. The in vitro findings demonstrated that both formulations effectively facilitated protein delivery into various cell lines. In vivo experiments revealed that intramuscular administration of the two formulations loaded with a SARS-CoV-2 recombinant receptor-binding domain (RBD) vaccine induced robust antibody responses in mice without adding another adjuvant. These results proved it may be employed as a safe and effective carrier for the delivery of subunit vaccines in vivo.

Dynamic Covalent Bonds Enabled Carbon Fiber Reinforced Polymers Recyclability and Material Circularity.

Due to their remarkable features of lightweight, high strength, stiffness, high-temperature resistance, and corrosion resistance, carbon fiber reinforced polymers (CFRPs) are extensively used in sports equipment, vehicles, aircraft, windmill blades, and other sectors. The urging need to develop a resource-saving and environmentally responsible society requires the recycling of CFRPs. Traditional CFRPs, on the other hand, are difficult to recycle due to the permanent covalent crosslinking of polymer matrices. The combination of covalent adaptable networks (CANs) with carbon fibers (CFs) marks a new development path for closed-loop recyclable CFRPs and polymer resins. In this review, we summarize the most recent developments of closed-loop recyclable CFRPs from the unique paradigm of dynamic crosslinking polymers, CANs. These sophisticated materials with diverse functions, oriented towards CFs recycling and resin sustainability, are further categorized into several active domains of dynamic covalent bonds, including ester bonds, imine bonds, disulfide bonds, boronic ester bonds, and acetal linkages, etc. Finally, the possible strategies for the future design of recyclable CFPRs by combining dynamic covalent chemistry innovation with materials interface science are proposed.

Design of sulfonic group based hyper cross-linked polymer as adsorbent for efficient enrichment of endocrine disrupters

Design of sulfonic group based hyper cross-linked polymer as adsorbent for efficient enrichment of endocrine disrupters