Cross-linked Polymer Networks Research Articles

A renewably sourced, circular photopolymer resin for additive manufacturing.

The additive manufacturing of photopolymer resins by means of vat photopolymerization enables the rapid fabrication of bespoke 3D-printed parts. Advances in methodology have continually improved resolution and manufacturing speed, yet both the process design and resin technology have remained largely consistent since its inception in the 1980s1. Liquid resin formulations, which are composed of reactive monomers and/or oligomers containing (meth)acrylates and epoxides, rapidly photopolymerize to create crosslinked polymer networks on exposure to a light stimulus in the presence of a photoinitiator2. These resin components are mostly obtained from petroleum feedstocks, although recent progress has been madethrough the derivatization of renewable biomass3-6 and the introduction of hydrolytically degradable bonds7-9. However, the resulting materials are still akin to conventional crosslinked rubbers and thermosets, thus limiting the recyclability of printed parts. At present, no existing photopolymer resin can be depolymerized and directly re-used in a circular, closed-loop pathway. Here we describe a photopolymer resin platform derived entirely from renewable lipoates that can be 3D-printed into high-resolution parts, efficiently deconstructed and subsequently reprinted in a circular manner. Previous inefficiencies with methods using internal dynamic covalent bonds10-17 to recycle and reprint 3D-printed photopolymers are resolved by exchanging conventional (meth)acrylates for dynamic cyclic disulfide species in lipoates. The lipoate resin platform is highly modular, whereby the composition and network architecture can be tuned to access printed materials with varied thermal and mechanical properties that are comparable to several commercial acrylic resins.

Recent Advances in Environmentally Friendly Dual‐crosslinking Polymer Networks

AbstractEnvironmentally 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.

Amphiphilic polymer Co-networks based on cross-linked tetra-PEG-b-PCL star block copolymers

A new class of AB4-type amphiphilic polymer co-networks (ACN) was prepared by hetero-complementary 2-(4-nitrophenyl)-benzoxazinone/amine end group cross-linking chemistry in dimethyl sulfoxide (DMSO). For this purpose, a well-defined star block copolymer (tetra-PEG-b-PCL) consisting of a hydrophilic tetra-arm PEG core (Mn ∼ 5 kg mol−1) and hydrophobic PCL wings with a total molar mass of Mn ∼ 10 kg mol−1 was synthesized by ring-opening polymerization (ROP). This tetra-PEG-b-PCL star block copolymer was functionalized either with 2-(4-nitrophenyl)-benzoxazinone or with amino terminal groups. The ACN synthesis was carried out by conversion of these two star block copolymers. The overlap concentration of c* ∼76 g L−1 determined by viscosity measurements of both stars showed no significant temperature dependence. The network syntheses carried out at different concentrations (0.5 c* to 2.5 c*) resulted in conversion degrees in the range of p ≈ 95 % or higher as detected by HR MAS NMR spectroscopy. As in previous work on the synthesis of A4B4 networks using the same cross-linking chemistry, deviations from the expected network structure (increased proportion of double links between two stars) were detected also here by multi quantum NMR spectroscopy. The expected environmentally sensitive behavior of the ACNs was confirmed in terms of their equilibrium swelling in solvents of different quality to PEG and PCL. The scaling of the swelling and viscoelasticity data as a function of preparation concentration showed deviations from the expectations of a perfectly cross-linked star polymer network. Finally, a two-step gelation process was observed by rheology at 30 °C in DMSO, which is related to associations between the 2-(4-nitrophenyl)-benzoxazinone groups surrounded by PCL blocks that become apparent on the time scale of rheology when the molar mass is increased as a result of cross-linking.

Surface modified molybdenum disulfide nanosheets for corrosion resistance improvement on polyurethane coatings

Abstract Organic–inorganic hybrid coating has been applied on metallic corrosion protection effectively. Molybdenum disulfide (MoS2) nanosheets with graphene-like two-dimensional lamellar structure were an anticorrosion inorganic additive, rendering the organic coating better corrosion resistant. However, the aggregation and poor solubility are still current issues that should be addressed. Functionalization MoS2 nanosheets with surface modified by polydopamine (PDA) and silane coupling agent (KH560) were prepared in polyurethane (PU) composite coatings to obtain dense and intact multilayer network structure coatings for corrosion protection. KH560-PDA-MoS2/PU coating with crosslinked polymer network structure has a high impedance modulus, large contact angle, and strong hydrophobicity. The coating meets the national technical standards for salt spray testing and nitric acid (HNO3) titration testing, demonstrating excellent corrosion resistance.

A wearable strain sensor based on self-healable MXene/PVA hydrogel for bodily motion detection

Developing flexible, stretchable, and self-healing wearable electronic devices with skin-like capabilities is highly desirable for healthcare and human-machine interaction. Hydrogels as a promising sensing material with crosslinked polymer networks have received widespread attention for decades. However, sensors based on hydrogels suffer from low sensitivity and stability due to their poor electrical conductivity or the movement of nanofillers in hydrogel networks. Herein, a stable, sensitive, and self-healing strain sensor is fabricated by the Ti3C2Tx MXene nanosheets/polyvinyl alcohol (PVA) hydrogel (T-hydrogel). The introduction of MXene increases the number of H-bonds in the PVA hydrogel network and enhances the conductivity, resulting in high sensitivity, stability, and self-healing character. The self-healing T-hydrogel-based strain sensor has a performance close to that of the original sensor. In addition, the device is capable of detecting bodily motions, indicating the potential application in the field of human health monitoring and human-computer interaction.

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

Electrode crosstalk between anode and cathode at elevated temperatures is 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 1.1AhLiCoO2/graphite pouch cell with a capacity retention of 96% after 398 cycles at 0.2 C.

Iterative SuFEx approach for sequence-regulated oligosulfates and its extension to periodic copolymers

The synthesis of sequence-regulated oligosulfates has not yet been established due to the difficulties in precise reactivity control. In this work, we report an example of a multi-directional divergent iterative method to furnish oligosulfates based on a chain homologation approach, in which the fluorosulfate unit is regenerated. The oligosulfate sequences are determined by high resolution mass spectrometry of the hydrolyzed fragments, and polysulfate periodic copolymers are synthesized by using oligomeric bisfluorosulfates in a bi-directional fashion. The synthetic utility of this iterative ligation is demonstrated by preparing crosslinked network polymers as synthetic adhesive materials.

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.

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.

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.

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.

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.

Performance evaluation of biodegradable polymer PHBV and PBAT blends with adjustable melt flow behaviour, heat deflection temperature, and morphological transition

AbstractMelt blending is a reliable and well‐demonstrated strategy for improving the mechanical, thermal, rheological, and surface properties of biopolymers. Poly(hydroxy‐3‐butyrate‐co‐3‐hydroxyvalerate) (PHBV) and poly(butylene adipate‐co‐terephthalate) (PBAT) are the two popular choices for blending polymers due to their diverse properties and complementary soil biodegradable behaviour. Due to their immiscibility, however, blending with the help of processing additives is necessary to reap the most significant benefits from this process and to avoid immiscibility issues. This study utilized the additives (peroxides and epoxy‐based chain extender) to compatibilize the biodegradable polymers PHBV and PBAT in a 60:40 blending ratio. The tensile strength and Young's modulus of the PHBV/PBAT(60/40) blend were improved by 32% and 64%, respectively, after adding a combination of peroxide (0.02 phr) and chain extender (0.3 phr) due to the formation of a complex network structure with increased chain length. The positive effect of an additive addition was also reflected by a 30°C increment in heat deflection temperature of biodegradable blend due to its high modulus value as supported by mechanical properties. The combined action of a peroxide and chain extender demonstrated a significantly higher complex viscosity of the PHBV/PBAT(60/40) blend due to the formation of a crosslinked polymer network as analyzed by rheological analysis. Our research demonstrated the effect of additives and their combined impact on analytical properties of PHBV/PBAT(60/40) blend to guide future work in improving their candidature to serve as a drop‐in solution in replacing non‐biodegradable petro‐based plastic products.

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.

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.