Nature Of Cross-links Research Articles

Well-Defined Block Copolymer Vitrimer Membranes.

A well-defined α,ω-dialdehyde polyisoprene-b-polystyrene block copolymer, synthesized using anionic polymerization high-vacuum techniques, is employed to prepare vitrimers with tris(2-aminoethyl)amine as the cross-linking agent. The vitrimer network, featuring dynamic imine cross-links, results in robust, flexible, and solvent-resistant films, which are applicable in thin film composite membranes. These vitrimer membranes, with molecular weight cut-offs in the nanofiltration range, are successfully used for organic solvent separation and evaluated for gas separation. The cross-linking density, controlled by the cross-linker, affects the material's gas permeability and affinity for CO₂. The dynamic nature of the imine cross-links enables the vitrimer's self-healing ability, activated by heat treatment at temperatures as low as 50°C. Additionally, the vitrimer membranes can be reprocessed through solvent dissolution in the presence of the excess cross-linking agent.

Chemical and Solvent-Based Recycling of DGEBA-Based Epoxy Thermoset and Carbon-Fiber Reinforced Epoxy Composite Utilizing Imine-Containing Secondary Amine Hardener.

Epoxy systems are essential in numerous industrial applications due to their exceptional mechanical properties, thermal stability, and chemical resistance. Yet, recycling epoxy networks and reinforcing materials in epoxy composites remains challenging, raising environmental concerns. The critical challenge is the recovery of well-defined molecules upon depolymerization. To address these issues, an innovative strategy is developed utilizing imine-containing secondary amine hardener (M1). The reaction of M1 with DGEBA produced high-performance epoxy thermoset P1, which exhibits Young's modulus of 2.18 GPa and tensile strength of 63.4 MPa, and excellent stability in neutral aqueous conditions. Upon carbon-fiber reinforcement, Young's modulus and tensile strength are significantly elevated to 10.99GPa and 328.3MPa, respectively. The reactive secondary amine functionalities enabled the tailored network to display a well-defined growth pattern, yielding only well-defined molecules and intact carbon fibers upon acidic depolymerization. Consequently, the recycled polymers retained properties identical to those of P1. Notably, it is discovered that despite the cross-linked nature of the epoxy networks, complete dissolution in dichloromethane facilitated straightforward solvent-based recycling, allowing the recovery of undamaged carbon fibers and an epoxy thermoset with properties matching the virgin material. Presented novel monomer design and approach showcased two important and efficient recycling options for epoxy systems.

Water as a Structural Marker in Gelatin Hydrogels with Different Cross-Linking Nature.

We have studied the molecular properties of water in physically and chemically cross-linked gelatin hydrogels by FTIR-spectroscopy, NMR relaxation, and diffusivity and broadband dielectric spectroscopy, which are sensitive to dynamical properties of water, being a structural marker of polymer network. All experiments demonstrated definite reinforcement of the hydrogel net structure and an increase in the amount of hydrate water. FTIR experiments have shown that the chemical cross-linking of gelatin molecules initiates an increase in the collagen-like triple helices "strength", as a result of infused restriction on protein molecular mobility. The "strengthening" of protein chains hinders the mobility of protein fragments, introducing complex modifications into the structural properties of water which are remained practically unchanged up to up to 30-40 °C.

Synthesis and Comprehensive Characterization of Amino Acid-Derived Vinyl Monomer Gels.

In this study, we report an easy synthetic pathway to vinyl monomers derivatized with amino acids. Tyrosine-, phenylalanine-, tryptophan-, leucine-, and methionine-based monomers were synthesized, and their polymerization in the presence of cross-linking agents led to the formation of amino acid-based gels. The nature of cross-linker, the time of polymerization, and the type of initiation (photopolymerization or thermopolymerization) were investigated. The obtained gels were characterized using a combination of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), rheology, scanning electron microscopy (SEM), and solid-state nuclear magnetic resonance (NMR) spectroscopy. These novel amino acid-based gels could find applications in various areas such as drug delivery, biosensing, and biotechnology.

Bispillar[5]arene-Based Slide-Ring Polyrotaxanation Enables Enhanced Toughness, Recyclability, Impact, and Puncture Resistance of Polyisoprene Elastomers.

A series of slide-ring polyrotaxanes (SRPs) have been constructed by the solvent-free blending of a ditopic pillar[5]arene (DP5A) and polyisoprene (PIP) after thermal annealing. Solid-state 13C NMR experiments supported the fact that the pillar[5]arene rings of DP5A were threaded by PIP chains to afford physically interlocked networks. Tensile tests revealed that 1% of DP5A can improve the elongation at break from 50 to 239%, the tensile modulus from 2.1 to 3.9 MPa, and the toughness from 0.35 to 4.5 MJ/m3. Impact and puncture resistance experiments show that the DP5A-doped materials exhibit remarkable enhancement of protective and impalement-resistant performance. The samples can be also recycled repeatedly due to their physical crosslinking nature. The important stress delocalization effects have been attributed to the pulley effect of DP5A in the SRP materials, which represents a supramolecular approach for improving the performance of PIP elastomers.

Polysquaramides.

Thermoplastics, while advantageous for their processability and recyclability, often compromise thermochemical stability and mechanical strength compared to thermosets. Addressing this limitation, we introduce an innovative approach employing reversibly cross-linked polymers, utilizing squaramide moieties to reconcile recyclability and robustness. Herein, we detail the synthesis of supramolecularly cross-linked polysquaramides through the condensation polymerization of diethyl squarate with primary and secondary diamines. This methodology embeds hydrogen-bonding squaramide motifs into the polymer chains, yielding materials with significantly enhanced storage moduli, reaching up to 1.2 GPa. Material characterization via dynamic mechanical analysis, creep-recovery, and stress relaxation experiments delineate a distinctive rubbery plateau across a broad temperature range, excellent creep resistance, and multimodal viscoelastic flow, respectively, attributable to the dynamic nature of the supramolecular cross-links. Additionally, the study showcases the modulation of glass transition temperature (Tg) by altering the monomer composition and stoichiometry, demonstrating the tunability of polymer viscoelastic properties through precise control over hydrogen bonding interactions. Overall, the incorporation of squaramide motifs not only provides the structural integrity and mechanical performance of these thermoplastics but also leads to engineering materials with tailored viscoelastic characteristics.

Glucose-responsive, self-healing, wet adhesive and multi-biofunctional hydrogels for diabetic wound healing

Diabetic wounds are serious clinical complications which manifest wet condition due to the mass exudate, along with disturbed regulation of inflammation, severe oxidative stress and repetitive bacterial infection. Existing treatments for diabetic wounds remain unsatisfactory due to the lack of ideal dressings that encompass mechanical performance, adherence to moist tissue surfaces, quick repair, and diverse therapeutic benefits. Herein, we fabricated a wet adhesive, self-healing, glucose-responsive drug releasing hydrogel with efficient antimicrobial and pro-healing properties for diabetic wound treatment. PAE hydrogel was constructed with poly(acrylic acid-co-acrylamide) (AA-Am) integrated with a dynamic E-F crosslinker, which consisted of epigallocatechin gallate (EGCG) and 4-(2-acrylamidoethylcarbamoyl)-3-fluorophenylboronic acid (AFPBA). Due to the dynamic crosslinking nature of boronate esters, abundant catechol groups and hydrogen bonding, PAE hydrogel demonstrated excellent mechanical properties with about 1000 % elongation, robust adhesion to moist tissues, fast self-healing, and absorption of biofluids of 10 times of its own weight. Importantly, PAE hydrogel exhibited sustained and glucose-responsive release of EGCG. Together, the bioactive PAE hydrogel had effective antibacterial, antioxidative, and anti-inflammatory properties in vitro, and accelerated diabetic wound healing in rats via reducing tissue-inflammatory response, enhancing angiogenesis, and reprogramming of macrophages. Overall, this versatile hydrogel provides a straightforward solution for the treatment of diabetic wound, and shows potential for other wound-related application scenarios.

Biomass-based monomer design and closed-loop recycling strategy development for epoxy resin thermoset

Designing plastic materials derived from renewable sources with inherent recyclability is of great significance for sustainable plastics development. However, commonly used epoxy resin thermosets (ERTs) present significant challenges. Their highly cross-linked nature generally makes them non-recyclable, and their raw materials are typically derived from fossil-based bisphenol A. In a recent study published in Science, Barta et al. introduced a fully lignocellulose-derived epoxy resin (DGF/MBCA) with a novel structure that exhibited excellent properties. Additionally, an methanolysis-acetolysis tandem recycling strategy was developed to regenerate the pristine monomers.

Recovery of gaseous fuels through CO2-mediated pyrolysis of thermosetting polymer waste

Thermosetting polymers are used in a wide range of applications due to their robust mechanical strength and superior flame retardancy. Despite these technical benefits, recycling of thermosetting polymers has been challenging because of their crosslinking nature. Moreover, their disposal through conventional methods (landfill and combustion) poses environmental concerns, such as microplastics and air pollutants. To address these issues, this study introduces a thermo-chemical disposal platform for thermosetting polymer wastes that employs carbon dioxide (CO2) as a reactive medium. In this work, melamine-formaldehyde was used as model compound of thermosetting polymers. In single-stage pyrolysis, it was revealed that CO2 plays a crucial role in controlling in the compositional matrices of pyrolytic gases, liquid products, and wax. These compositional changes were attributed to the homogeneous reactions between CO2 and the volatile compounds released from the thermolysis of MF. To enhance the thermal cracking of the MF, a double-stage pyrolysis process was tested, which increased the production of pyrolytic gases and eliminated wax formation. However, the slow kinetics governing the reactivity of CO2 limits the occurrence of homogeneous reactions. A nickel-based catalyst was used to accelerate reaction kinetics. The catalytic pyrolysis under CO2 conditions led to substantial increases in syngas (H2 and CO) production of 880% and 460%, respectively, compared with double-stage pyrolysis. These findings demonstrate that thermosetting polymer wastes can be valorized into gaseous fuels through thermo-chemical process, and CO2 enhances the recovery of energy and chemicals. Therefore, this study presents an innovative technical platform to convert thermosetting polymer wastes and CO2 into syngas.

Natural Polyelectrolyte Hydrogels with Two Types of Cross-Links with Different Energy

For the first time, gels of a polymer cross-linked by two types of ionic cross-links of different energies, namely trivalent chromium (III) ions and divalent iron (II) ions, were prepared. The negatively charged polysaccharide xanthan, a polyelectrolyte of natural origin, was used as a polymer. A study of xanthan gels cross-linked with each cross-linker separately was carried out to identify the optimal concentrations of each cross-linker for the production of a double cross-linked gel. The mechanical properties of hydrogels under oscillatory shear deformations were then studied. It was demonstrated that the simultaneous use of two cross-linkers resulted in a synergistic increase in the elastic modulus (plateau storage modulus) compared to gels cross-linked with each cross-linker separately. For a gel with two types of cross-links, the elastic modulus was 16 Pa. In contrast, for gels cross-linked with either chromium (III) cations or iron (II) cations, the elastic modulus was approximately 0.6 and 0.5 Pa, respectively. The strong effect can be attributed to the different nature of cross-linking between xanthan macromolecules and chromium (III) and iron (II) cations, as well as the different strength of the cross-links formed. The optimal range of pH values was determined in which a synergistic increase in the elastic modulus of gels with two types of cross-links was observed compared to the corresponding gels cross-linked with each cross-linker separately. Consequently, the concurrent utilisation of two cross-linking agents that generate cross-links with disparate energy levels represents an efficacious strategy to improve the tensile strength of polymer hydrogels.

Strain-Stiffening Mechanoresponse in Dynamic-Covalent Cellulose Hydrogels.

Mechanical stimuli such as strain, force, and pressure are pervasive within and beyond the human body. Mechanoresponsive hydrogels have been engineered to undergo changes in their physicochemical or mechanical properties in response to such stimuli. Relevant responses can include strain-stiffening, self-healing, strain-dependent stress relaxation, and shear rate-dependent viscosity. These features are a direct result of dynamic bonds or noncovalent/physical interactions within such hydrogels. The contributions of various types of bonds and intermolecular interactions to these behaviors are important to more fully understand the resulting materials and engineer their mechanoresponsive features. Here, strain-stiffening in carboxymethylcellulose hydrogels cross-linked with pendant dynamic-covalent boronate esters using tannic acid is studied and modulated as a function of polymer concentration, temperature, and effective cross-link density. Furthermore, these materials are found to exhibit self-healing and strain-memory, as well as strain-dependent stress relaxation and shear rate-dependent changes in gel viscosity. These features are attributed to the dynamic nature of the boronate ester cross-links, interchain hydrogen bonding and bundling, or a combination of these two intermolecular interactions. This work provides insight into the interplay of such interactions in the context of mechanoresponsive behaviors, particularly informing the design of hydrogels with tunable strain-stiffening. The multiresponsive and tunable nature of this hydrogel system therefore presents a promising platform for a variety of applications.

Closed-loop recyclability of a biomass-derived epoxy-amine thermoset by methanolysis.

Epoxy resin thermosets (ERTs) are an important class of polymeric materials. However, owing to their highly cross-linked nature, they suffer from poor recyclability, which contributes to an unacceptable level of environmental pollution. There is a clear need for the design of inherently recyclable ERTs that are based on renewable resources. We present the synthesis and closed-loop recycling of a fully lignocellulose-derivable epoxy resin (DGF/MBCA), prepared from dimethyl ester of 2,5-furandicarboxylic acid (DMFD), 4,4'-methylenebis(cyclohexylamine) (MBCA), and glycidol, which displays excellent thermomechanical properties (a glass transition temperature of 170°C, and a storage modulus at 25°C of 1.2 gigapascals). Notably, the material undergoes methanolysis in the absence of any catalyst, regenerating 90% of the original DMFD. The diamine MBCA and glycidol can subsequently be reformed by acetolysis. Application and recycling of DGF/MBCA in glass and plant fiber composites are demonstrated.

Chemically Recyclable and Biodegradable Vulcanized Rubber.

The cross-linked nature of vulcanized rubbers as used in tire and many other applications prohibits an effective closed-loop mechanical or chemical recycling. Moreover, vulcanization significantly retards the material's biodegradation. Here, we report a recyclable and biodegradable rubber that is generated by the vulcanization of amorphous, unsaturated polyesters. The elastic material can be broken down via solvolysis into the underlying monomers. After removal of the vulcanized repeat units, the saturated monomers, constituting the major share of the material, can be recovered in overall recycling rates exceeding 90%. Respirometric biodegradation experiments by 13CO2 tracking under environmental conditions via the polyesters' diol monomer indicated depolymerization and partial mineralization of the vulcanized polyester rubbers.

Fabrication and Characterization of Non-Conventional Starch Based Biofilms Employing Different Plasticizers

Increasing stress on environmental awareness has promoted the use of alternative strategies for common petrochemical based plastic sources to environmentally friendly biofilms and packaging materials. Food by products and underutilised indigenous plants sources are rich in biopolymers which can naturally decompose. The present study focusses on preparations of bioplastics from unconventional starch sources of litchi seed and churkha tuber as novel initiative to combat environmental pollution along with valorisation of undervalued plant sources and agrobased wastes. Ternary blend films of poly vinyl alcohol, starch, plasticizers like (glycerol, sorbitol, mannitol, propylene glycol, polyethylene glycol), citric acid prepared by casting technique produced best result. Comparative study of test samples (TH11, TH12, TH13, TH14 etc) with standardized thin films (TH1, TH2, TH3 etc.) showed variation in appearance, water activity, thickness, since the starch differ in their compositions, gelatinization and gelation time and temperature. High amylose containing litchi seed starch forms firm and more flexible films in comparison to low amylose containing churkha tuber starch. Maximum hygroscopicity was recorded in case of TH1 (77.4%) while thickness was maximum in case of TH12 (0.18 mm). Micro structural analysis clearly showed the nature of crosslinking between starch molecule and different plasticizers. The sensory analysis of food packed with the best among the formularized thin films showed mild change in taste while the other parameters remain same as normal. Better acceptability was in case of TH12 wrapped food rather than TH1. Thus, fabrication of biodegradable packaging is considered as the most sustainable alternative for food preservation. Int. J. Appl. Sci. Biotechnol. Vol 12(1): 29-38.

Atomistic insights into the mechanical properties of cross-linked Poly(N-isopropylacrylamide) hydrogel

Poly(N-isopropylacrylamide) (PNIPAM) is a highly versatile thermo-responsive hydrogel with immense potential for applications in biomedicine and tissue engineering. While PNIPAM has been extensively researched, there remains a significant gap in understanding its mechanical behavior at the atomistic scale. To address this challenge, this study employs molecular dynamics (MD) simulations to delve into the mechanical response of cross-linked PNIPAM hydrogels. The uniqueness of this research lies in our emphasis on providing atomic-level insights into the influence of chemical cross-linking and physical entanglements, as well as quantifying their effects during the application of strain. This represents a novel contribution to the study of hydrogels. MD simulations enable us to scrutinize the behavior of individual polymer chains and their intricate interactions in unprecedented detail, shedding light on microscale phenomena that are often inaccessible through experiments alone. To tackle the complexities associated with atomistic simulations of cross-linked hydrogels, we introduce a dynamic cross-linking algorithm. This innovative approach incorporates a nonreactive force field to construct realistic three-dimensional hydrogel microstructures, employing a stepwise bond formation strategy. Our findings underscore the remarkable impact of increasing the Degree of Cross-linking (DoC) and/or the Degree of Polymerization (DoP) on enhancing the robustness and elastic modulus of PNIPAM hydrogels. Additionally, we uncover the stochastic nature of chemical cross-linking, leading to the formation of anisotropic super-fragments when DoC exceeds a critical threshold. Furthermore, this research highlights the pivotal role of low strain rates on the order of 106 s−1 in accurately modeling the mechanical properties of hydrogels using MD simulations. This approach enables us to obtain meaningful quantitative mechanical properties that align with experimental results reported in the existing literature. In summary, this paper offers an unprecedented level of insight into the intricate interplay of chemical cross-linking and physical entanglements, providing a foundation for future research in this domain.

Efficient removal of antibiotics from water by highly crosslinked metal-alginate particles: Preparation, isotherms, kinetics, and microbiological assay

Traces of antibiotics reaching aquatic environment lead to the emergence of antimicrobial resistance (AMR). The efficient removal of antibiotics (ATBs) traces from wastewater is essential to tackle the AMR. In this study, a novel solid-state crosslinking method of alginate (ALG) was developed and applied to specifically remove ATBs from water. A wide range of crosslinkers (Ca2+, Zn2+, Cu2+, Ni2+, Fe3+ and Al3+) was used and the crosslinking nature, density, and distribution were evidenced by FTIR, ICP-MS, and SEM-EDS. Compared with ionotropic gelation, the novel solid-state crosslinking method proved superior in term of ease of production, high crosslinking degree, and ATBs removal capacity. Fe-ALG and Zn-ALG showed high removal capacity of ciprofloxacin (356.5 mg/g and 928.6 mg/g) and doxycycline (90 mg/g and 690 mg/g), however, they were less effective toward amoxicillin (11.5 mg/g and 6 mg/g). Removal isotherms and kinetics followed type I and pseudo-second order suggesting a chemisorption removal mechanism. Fe-ALG was successfully regenerated with no loss in ATB removal capacity. The microbiological assay showed significant reductions of antibacterial activities after ATBs removal from water. Overall, metal-ALG systems obtained by solid-state crosslinking are promising for ATBs removal from wastewater giving the ease of production, high efficiency, regenerability, and scalability potential.

Dynamic Crosslinking of Polyethylene by Hydrogen Bonding Grafted Motifs: A Molecular Dynamics Simulation Study

AbstractThe recyclability of polyethylene‐based is an important social and environmental issue. Dynamic crosslinking of polyethylene by multiple hydrogen bonding motifs provides an opportunity for complete recyclability. This approach is based on producing reversible covalent bonds in the crosslinking polyethylene networks to produce vitrimers. It is employed molecular dynamics simulation to study the effect of mole percent of randomly grafted 1‐(7‐oxo‐7,8‐dihydro‐1,8‐naphthyridin‐2‐yl) poly(ethylene‐co‐[2‐hydroxyethyl methacrylate])s (ODIN‐PEHEMAs), on thermal stability of the vitrimers. The crosslinking nature and dynamics of these grafted polyethylene chains are analyzed focusing on free volume, specific volume, radial distribution function, and mean squared displacement. The addition of hydrogen bonding units increased the free volumes, but the mole percent of side chains is not enough to affect the dynamics of the main chains in vicinity of the glass transition temperature. The simulation results show that for 1.67‐mole percent of randomly grafted ODIN‐PEHEMAs, the mean squared displacement, MSD, dramatically increases in vicinity of 312.5 K, which is in good agreement with the experimental results due to the physical cross‐linking breakage at room temperature. With increasing the mole percent of ODIN (4.33, 8.33, and 10.67 mole percent), the breakage of physical cross‐links shifted to higher temperatures.

Highly Stretchable yet Degradable and Recyclable Conductive Composites with Liquid Metal Nanodroplets as Physical Crosslinks

AbstractLiquid metal (LM) droplet‐embedded polymer composites with exceptional thermal, electrical, and mechanical characteristics are attractive for applications in soft electronics and thermal management. Generally, a high volume fraction of LMs is required for desirable thermal and electrical properties, and thus recycling of LMs is favorable and cost effective. However, recycling LMs from composites remains a formidable challenge because the LM droplets are typically dispersed into a permanent covalently crosslinked polymer matrix. Here, a facile synthesis of LM‐polymer composites with high electrical conductivity (1800 S cm−1), high stretchability (1400% tensile strain), as well as degradability and recyclability is reported. The composite comprises a poly(2‐hydroxyethyl acrylate) hydrogel that is physically crosslinked by LM nanodroplets as a polymer matrix and either LM microdroplets or silver microflakes as a conductive filler. A postdrying process improves the mechanical performance of the composite while creating percolating pathways via the rearrangement of the conductive fillers, imparting high electrical conductivity to the composite. Moreover, the composites are readily degraded and dissolved in aqueous alkali solutions at room temperature thanks to the reactive nature of LM crosslinks, enabling high recycling efficiency of ≈80% and 95% for composites with LM microdroplets and silver microflakes as conductive fillers, respectively.

Applications of Degradable Hydrogels in Novel Approaches to Disease Treatment and New Modes of Drug Delivery.

Hydrogels are particularly suitable materials for loading drug delivery agents; their high water content provides a biocompatible environment for most biomolecules, and their cross-linked nature protects the loaded agents from damage. During delivery, the delivered substance usually needs to be released gradually over time, which can be achieved by degradable cross-linked chains. In recent years, biodegradable hydrogels have become a promising technology in new methods of disease treatment and drug delivery methods due to their many advantageous properties. This review briefly discusses the degradation mechanisms of different types of biodegradable hydrogel systems and introduces the specific applications of degradable hydrogels in several new methods of disease treatment and drug delivery methods.

Fundamental Insights Into Recrosslinkable Granular Hydrogels for Fracture Remediation

Summary The concentration of prior recrosslinkable granular hydrogels was based on the assessments of their properties and petrophysical performance. However, there is no reported correlation of microstructural information of the hydrogels with their macroscopic bulk properties, hindering the deployments of such hydrogels to complex geological reservoirs. Herein, we report that the relationship of elastic modulus with angular frequency can assess the crosslinking nature of hydrogels from our reliable experiments. The covalently crosslinked hydrogels showed independence from angular frequency, whereas the ionically crosslinked sample had a strong dependence. We established a robust database of commonly used chemical crosslinkers for hydrogel synthesis with their thermal stabilities, where organic covalently crosslinked hydrogels showed much better thermal robustness than their physically bridged counterparts. Moreover, we found that oven aging is a more appropriate strategy to analyze structural integrity compared with thermal gravimetric analysis (TGA). Finally, we demonstrated structural differences between the heterogeneous interlinking approach and homogeneous void-free regenerative strategy and their influences on petrophysical properties of the in-situ reformed bulk materials. We give an in-depth analysis of fundamental insights into crosslinking assessments, thermal stabilities, and recrosslinking approaches for laboratory studies and field applications. The fundamentals illustrated herein offer a robust method to appraise granular hydrogels with crosslinkable function for fracture treatments in the petroleum industry.