Higher Crosslinking Density Research Articles

Effect of Molecular Organization on the Properties of Fractionated Lignin-Based Thiol-Ene Thermoset Materials.

In this study, the combination of sequential solvent fractionation of technical Kraft lignin was followed by allylation of most OH functionalities to give highly functional thermoset resins. All lignin fractions were highly functionalized on the phenolic (≥95%) and carboxylic acid OH (≥85%) and to a significant extent on the aliphatic OH moieties (between 43 and 75%). The resins were subsequently cross-linked using thiol-ene chemistry. The high amount of allyl functionalities resulted in a high cross-link density. Dynamic mechanical analysis measurements showed that the thioether content, directly related to the allyl content, strongly affects the performance of these thermosets with a glass transition temperature (Tg) between 81 and 95 °C and with a storage modulus between 1.9 and 3.8 GPa for all thermosets. The lignin fractions and lignin-based thermosets' morphology, at the nanoscale, was studied by wide-angle X-ray scattering measurements. Two π-π stacking interactions were observed: sandwich (≈4.1-4.7 Å) and T-shaped (≈5.5-7.2 Å). The introduction of allyl functionalities weakens the T-shaped π-π stacking interactions. A new signal corresponding to a distance of ≈3.5 Å was observed in lignin-based thermosets, which was attributed to a thioether organized structure. At the same time, a lignin superstructure was observed with a distance/size corresponding to 7.9-17.5 Å in all samples.

Effect of polyelectrolyte architecture on chemical stability in alkaline medium of anion exchange membranes

The chemical stability of anion exchange membranes (AEM) is crucial for the durability of the electrochemical devicesusing them. In this context, poly(epichlorohydrin)-based AEM were subjected to chemical degradation in the presence of a corrosive alkaline medium. Linear and cross-linked polyelectrolytes grafted with ammonium cations based on Quinuclidine and/or DABCO were first synthesized. They were characterized by an IEC comprising between 1.3 and 3.1 mmol/g, an ionic conductivity above 2 mS/cm and transport numbers between 0.71 and 0.89. Their chemical stability was then evaluated after immersion in 1 M to 8 M KOH solutions for 672 h at 50 °C. The evolutions of these characteristics as well as the molecular structure of the materials during this period inform on the chemical resistance of the polymer backbone as well as that of the cationic sites. The main degradation reactions have been identified as chains scission and Hofmann elimination, leading to the loss of the cationic groups. Nevertheless, these reactions can be limited by a high crosslinking density of the membrane, enhancing then its chemical stability despite a high ammonium group content.

High solid and low viscosity waterborne polyurethane acrylate with excellent anti-corrosion and anti-bacterial performances

The design and preparation of great corrosion resistance and high-solid waterborne polyurethane dispersion with low viscosity is still a challenge in the field of the polyurethane industry. Herein, we developed a simple method involving a core-shell process to prepare a core-shell structured waterborne polyurethane acrylate dispersion (WPUAD). First, isobornyl acrylate, a derivative of natural product borneol, dihydroxylated by a thiol-ene click reaction was used to prepare oil-based polyurethane acrylate as prepolymer A (core) and waterborne polyurethane acrylate as prepolymer B (shell). Then, the core-shell WPUAD was obtained by mixing and emulsifying the two prepolymer at a certain ratio. The study reveals that the core-shell structured WPUAD not only has a low viscosity (~1330 mPa.s) but also holds a high-solid content (~ 60 wt%). At the same time, the highly solid WPUAD has excellent stability with no breast breakage even after more than 1 year of storage. In addition, various electrochemical measurements, water contact angles, and water absorption tests were performed to determine the anti-corrosive properties of the WPUAD-derived coatings. The results show that the WPUA coatings exhibit significantly enhanced anticorrosion with respect to the single prepolymer B derived coatings, which is attributed to their high cross-linking density resulting from the core-shell structure and the introduction of hydrophobic isobornyl acrylate. In addition, because of the introduction of the dihydroxylated isobornyl acrylate as chain extenders, the WPUA coating has great anti-bacterial adhesion properties with an anti-bacterial efficiency of 82.3 %. We believe that our work provides a generic preparation method to obtain a commercially available high-solid WPUAD with excellent anti-corrosion properties.

A highly-crosslinked phthalonitrile modified bismaleimide-triazine resin for PCB substrates: The synergistic effect on curing behavior and properties

In view of the problems of difficult processing and poor toughness, bismaleimide-triazine (BT) resin was modified by the 4-(4-amino-phenoxy)phthalonitrile (4-APN). The effect of 4-APN on the curing behavior of BT resin was investigated, followed by the calculated activation energy. The possible curing mechanism of this ternary system was also proposed, with the further study of the relationship between chemical structures and properties. The results showed that 4-APN can increase the reactivity and decrease the curing temperature of BT resin. Meanwhile, due to the fact that 4-APN exerted a thermal synergistic curing effect on the BT resin, poly(BT/APN)s with a high crosslinking density exhibited the superior overall performance. Concretely, the high heat-resistance of poly(BT/APN)s was displayed, especially the glass transition temperature up to 347 ℃ of poly(BT/30APN). More intriguingly, poly(BT/APN)s with a high breakdown strength possessed better insulation properties, and the dielectric constant and loss reduced to 3 and 0.005 (1 MHz), respectively, while remaining a good stability at high temperature. Additionally, the moisture resistance and toughness of poly(BT/APN)s were also significantly improved. Based on these merits, poly(BT/APN)s can be considered as an ideal material for microelectronics.

Reinforcing behavior of reclaimed rubber filled natural rubber composites

AbstractWith the growing volume of waste rubber products, the pressures on the environment and resources become increasingly serious. The reclamation of waste rubber products is one of the practical methods to meet this challenge. Therefore, the application of reclaimed rubber (RR) attracts researchers’ attention. In this work, different contents of natural rubber (NR) matrix are replaced by RR and the reinforcing behavior of the RR phase is discussed. The results show that the crosslinking density, Payne effect and 100% modulus increase with increase in RR content in the NR/RR blends when the RR content is not more than 60 phr, and then drop when the RR content gets to 80 phr. It is inferred that both the NR and the RR constitute co‐continuous phases when the RR content is 60 phr, and the RR phase is the only continuous phase when its content reaches 80 phr, which can be confirmed by SEM observation. Besides, the curing rate is enhanced with increase of the RR content. It is also found that structures with high crosslinking density appear when the RR is incorporated into the NR matrix. Finally, it is proposed that carbon black in the RR, a structure with high crosslinking density and a continuous rubber matrix are important factors influencing the reinforcement behaviour in RR filled rubber composites. © 2023 Society of Chemical Industry.

Thin film composite membranes prepared from diaminoguanidine hydrochloride for Mg2+/Li+ separation

This study aims to prepare and investigate thin film composite (TFC) membranes with a highly positively charged rejection layer for effective separation of Mg2+/Li+. The aqueous phase monomer employed in fabricating the TFC membranes was a highly ionic and hydrophilic guanidinium, namely 1,3-diaminoguanidine hydrochloride (DAGH), using an interfacial polymerization method. Two methods were used to prepare the TFC membranes: conventional interfacial polymerization and support-free interfacial polymerization. The surface properties and morphologies of the TFC membranes were characterized by FESEM, AFM, surface zeta-potential, and water contact angle. The XPS was used to analyze the crosslinking structure of the interfacial layer. The study results revealed that the SF-TFC membrane had a higher crosslinking density and zeta-potential compared to the C-TFC membrane. Notably, SF-TFC exhibited a MgCl2 rejection of 95.09% at 7 bar, and for Mg2+/Li+ separation, the SF-TFC membrane had a Li+/Mg2+ separation factor of 23.32 at the Mg2+/Li+ mass ratio of 10. Additionally, the strong ionic and hydrophilic properties of DAGH led to the proposal of an aqueous-phase polymerization mechanism, substantiated by AFM and ESI-MS analysis.

Physico-mechanical characterisation of basecoats for tailored UV-cured multilayered wood coating systems

The durability and aesthetic properties of wood flooring arise from the properties of the coating applied on its surface. Due to their fast-curing speed, high crosslinking density and high durability, UV-curable coatings are often preferred for flat surfaces such as hardwood flooring, tabletops and doors. In the case of hardwood flooring, several types of deteriorations on the coating surface can shatter the perception of the whole product. In the present work, UV-curable formulations with various monomer-oligomer couples were prepared and used as a basecoat within a multilayered wood finishing system. While the topcoat is designed to endure most of the in-use loads, elastic and plastic stresses can reach deeper layers. The aim of this study is to analyze the influence of basecoat composition and thickness on the mechanical behavior of a UVcurable multilayered wood finishing system. First, physical properties such as the average theoretical segment length, glass transition temperature and crosslinking density, of standalone films of the various monomer-oligomer couples were investigated. Then, indentation and scratch resistance tests were performed to understand the role of the basecoats in the overall mechanical response of the multilayered coatings. The thickness of the basecoat applied was found to have a great influence on the mechanical resistance of the finishing system. No direct correlation was established between the basecoat as standalone films and within the multilayered coatings, considering the complexity of such systems several behaviors were detected. A finishing system able to promote overall good scratch resistance and good indentation modulus was obtained for a formulation displaying an equilibrium between the network density and the elasticity.

Chitosan as a Polyfunctional Crosslinker for a Renewable-Based Resorcinol Diglycidyl Ether

Chitosan is an abundant renewable biopolymer, insufficiently valorized, which is suitable for the development of sustainable and fully biobased materials. In this study, we combined it with the resorcinol diglycidyl ether, a biobased monoaromatic diepoxide. Formulations with a high chitosan content of up to 44 wt % were investigated and showed high reactivity and appropriate intervals of polymerization, ranging from 75 to 125 or 155 °C. The in situ Fourier transform infrared (FTIR) spectroscopy analysis confirmed that the two compounds reacted via an anionic polymerization mechanism combined with polyaddition reactions, proving that the chitosan acts as a polyfunctional crosslinker. The obtained thermoset resins have tan δ values ranging between 79 and 73 °C and storage moduli increasing with chitosan content (E′ ∼3.5–4.2 GPa), together with high crosslinking densities (15.65–27 mmol·cm–3).

Design and characterization of dexamethasone phosphate -loaded microcapsules obtained by a double-emulsion method

Polymeric microcapsules are extensively investigated as drug delivery systems for a broad range of applications. In the present study, Dexamethasone-loaded carboxylated chitosan (CCS)/poly (vinyl alcohol) (PVA)-based microcapsules were prepared in view of their potential administration by inhalation for the treatment of lung diseases. The crosslinking between PVA and CCS was activated by [4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride] (DMT-MM) and the FTIR results proved the formation of ester bonds between the two polymers. The sizes of the obtained microcapsules are influenced by the ratio between the polymers but also by the concentration of the DMT-MM activator. Moreover, the amount of PVA in the system has an important influence on swelling degree, encapsulation efficiency, drug release degree, biodegradation and protein adsorption. The sample with the highest amount of PVA has the highest crosslinking density and thus the lowest swelling degree and encapsulation efficiency. However, an encapsulation degree of 61.3% was obtained for the sample MCP-6 with the lowest PVA content. The same sample showed the lowest BSA adsorption. A controlled and sustained Dexamethasone release of around 90% was observed in PBS at pH 7.4 and 37 °C during 24 h. All the obtained samples were hemocompatibles and thus can be used as efficient drug delivery systems.

In-Vivo fluorescent nanosensor implants based on hydrogel-encapsulation: investigating the inflammation and the foreign-body response

Nanotechnology-enabled sensors or nanosensors are emerging as promising new tools for various in-vivo life science applications such as biosensing, components of delivery systems, and probes for spatial bioimaging. However, as with a wide range of synthetic biomaterials, tissue responses have been observed depending on cell types and various nanocomponent properties. The tissue response is critical for determining the acute and long term health of the organism and the functional lifetime of the material in-vivo. While nanomaterial properties can contribute significantly to the tissue response, it may be possible to circumvent adverse reactions by formulation of the encapsulation vehicle. In this study, five formulations of poly (ethylene glycol) diacrylate (PEGDA) hydrogel-encapsulated fluorescent nanosensors were implanted into SKH-1E mice, and the inflammatory responses were tracked in order to determine the favorable design rules for hydrogel encapsulation and minimization of such responses. Hydrogels with higher crosslinking density were found to allow faster resolution of acute inflammation. Five different immunocompromised mice lines were utilized for comparison across different inflammatory cell populations and responses. Degradation products of the gels were also characterized. Finally, the importance of the tissue response in determining functional lifetime was demonstrated by measuring the time-dependent nanosensor deactivation following implantation into animal models.

Effects of MOFs-derived Ni@NC/CNT nanocomposites on impermeability and microwave absorption of modified epoxy resin coatings on cement-based materials

Human health and information security are harmed by electromagnetic wave (EMW) energy’s constant increase and various reflections. In this study, Metal-organic frameworks (MOFs) derived metal nickel@nitrogen-doped porous carbon/carbon nanotube (Ni@NC/CNT) nanocomposites with excellent EMW absorption performance were utilized to modify epoxy resin coatings on the cement-based materials. Surprisingly, when the dosage is 3 wt%, the cement-based materials coated with the modified epoxy resin coatings show outstanding permeability resistance and EMW absorption properties. The capillary water absorption coefficient and the chloride ion diffusion coefficient are 37.583 and 2.42 × 10-12 m2 s−1, which are 88.05% and 73.2% respectively lower than that of the uncoated specimen. The excellent EMW absorption performance was concretely manifested that the minimum reflection loss (RLmin) value is −45.76 dB, the effective absorption bandwidth (EAB) value is 12.88 GHz, and the effective practical application bandwidth (EPB) value is 15.04 GHz. Additionally, the modified epoxy resin coatings also exhibit exceptional thermal stability and mechanical properties, which are further analyzed with TGA and SEM results. These outstanding properties are attributed to the good compatibility between Ni@NC/CNT nanocomposites and epoxy resin, higher cross-linking densities, favorable nano-size effect, and the excellent EMW performance of Ni@NC/CNT nanocomposites. This work is extremely helpful for improving EMW absorption and other properties of epoxy resin coatings on cement-based materials.

Rosin side chain type catalyst-free vitrimers with high cross-link density, mechanical strength, and thermal stability

Rosin side chain type catalyst-free vitrimers with high cross-link density, mechanical strength, and thermal stability

Synthesis of chemically-crosslinked multi-arm star-shaped polyurethane with triple-shape memory effect

In this study, chemically-crosslinked multi-arm star-shaped polyurethanes (SPUs) were prepared using three, four, and six-arm polycaprolactone, hexamethylene diisocyanate, and 1, 4-butanediol. The hydrogen bonding indices of soft and hard segments were calculated using Fourier transform infrared spectra. The results indicated that the phase separation among hard and soft segments increased with the increment of PCL arm numbers. Moreover, the results of X-ray diffraction and differential scanning calorimetry showed that the crystallization ability of the three and four-arm SPUs were lower than that for six-arm SPU (6SPU), which is due to their higher crosslinking densities. In addition, the results of the mechanical studies showed that the crosslinking density and degree of crystallinity are the main effective parameters controlling the mechanical properties, by which 6SPU showed higher Young's modulus and lower elongation at break compared to other SPUs. Cyclic shape memory studies showed that 6SPU could fix approximately all the temporary shapes during three cycles and recover 100% of its original shape. Moreover, 6SPU could show triple-shape memory effect (TSME) by which it could fix two different temporary shapes. These results show that 6SPU has a high potential for practical applications due to its good mechanical properties, shape memory fatigue resistance, and TSME.

Dual-crosslinked starch−poly(ester urethane)−oligochitosan films with high starch content: Application as biodegradable food packaging

This work offered a dual-crosslinking strategy to prepare starch−poly(ester urethane)−oligochitosan (SPEO) composites with high starch content (> 46 wt%). The crosslinking agent polycaprolactone-based polyurethane prepolymer (PPUP) with terminal −NCO groups and pendant −CHO groups was first synthesized. Then, the starch was modified by PPUP to form a crosslinked structure linked by urethane bonds. Oligochitosan (OCS) was subsequently introduced to produce another crosslinked structure linked by imine bonds, and the films were fabricated by compression molding. The chemical structures and physical performance of SPEO were characterized. Crystallinity, micromorphology and thermal analysis demonstrated that the dual-crosslinking structures enhanced the composite compatibility, which improved their mechanical properties. The surface hydrophilicity, water absorption, and in vitro degradation rate increased slowly with increasing OCS content, while a high crosslinking density provide a good barrier to water vapor permeation and therefore decreased the water vapor permeability (< 128 g/(m2∙24 h)). Due to the residual −NH2 groups of OCS, the composite film of SPEO-6 exhibited certain bacteriostatic activity against E. coli and S. aureus. Furthermore, the cytotoxicity of the composite films was assayed by the MTT method, and relative growth rates were much higher than 75% after incubation for 72 h, indicating high cytocompatibility and non-toxicity to food. The dual-crosslinked high-starch-content composites possessing high security, superior mechanical properties, good biodegradability, low water vapor permeability, and certain bacteriostatic capacity hold great potential to substitute petroleum-derived plastics for fresh-keeping food packaging applications.

Improved strength, creep resistance and recyclability of polyisoprene vitrimers by bottom-up construction of inhomogeneous network

Vitrimer elastomers bring huge chance to solve the recycling problem of conventional rubbers. However, their application is extremely limited by their lack of creep resistance. Although the crosslinking density can significantly affect the relaxation behavior of dynamic covalent networks, but limited by the tradeoff between mechanical strength and creep resistant properties. Here, we introduce inhomogeneous dynamic covalent network with regions of different crosslinking densities, in which the low crosslinking density mainly contribute to the creep resistance and the high crosslinking density contribute to the mechanical strength. In detail, block copolymers with polymyrcene and polyisoprene blocks and its control sample polyisoprene were prepared and epoxidized. The polymyrcene block is a double bond-rich block compared to polyisoprene block, which will become an epoxy group-rich block after epoxidation and a high cross-link density phase region after crosslinking with a dicarboxylic acid agent. Therefore, the cross-linked block copolymer will have an inhomogeneous dynamic covalent network, which were confirmed by the Multi-quantum (MQ) NMR tests, AFM images and tanδ measurement. And the heterogeneous structure becomes more obvious with the increased length of polymyrcene block. WAXD results show that cross-linked block copolymers have lower strain-induced crystallization onset strain and higher crystallinity, making them far stronger and tougher. In addition, block copolymer samples demonstrate lower creep and lower permanent set due to slow relaxation of the low crosslinking density regions. The recycling efficiencies of mechanical strength of block copolymer samples were higher than that of homopolymer. This work provides a useful approach for improving the mechanical properties and dimensional stability of vitrimer elastomers simultaneously.

An efficient strategy triggered crosslinking of polyethylene and its application in degassing‐free ultrahigh voltage power cables

AbstractPolyethylene has excellent electrical insulation, but its low thermal stability and breakdown strength limit its application in the field of high‐voltage (HV) and ultrahigh voltage (UHV) power cables. In this work, we have developed a new organic peroxide, 2,5‐dimethyl‐2,5‐di(tert‐butylperoxy) hexane (Bi25), which can efficiently trigger the crosslinking of polyethylene (Bi25‐XLPE). Bi25‐XLPE does not produce gas byproducts and has a longer scorching time. All performance of Bi25‐XLPE sheets fulfill the standard of the 500 kV power cable insulation due to the void‐free nature, higher crosslinking density and crystallinity. In addition, one 500 kV power cable model was constructed by COMSOL, which confirmed the breakdown strength (up to 61.66 kV/mm) and stability of Bi25‐XLPE.

Castor oil-based, robust, non-leaching and durable antibacterial waterborne polyurethane/polyhexamethylene guanidine composites prepared via an electrostatic self-assembly strategy

Antibacterial materials with durable antibacterial properties and high mechanical strength from renewable resources recently have attracted extensive attention. In this study, a facile and environmentally friendly electrostatic self-assembly strategy was adopted to prepare a series of castor oil-based waterborne polyurethane/polyhexamethylene guanidine (WPU/PHMG) composites. The relationship between performance (antibacterial properties, mechanical properties, etc.) and the chemical structures of these resulting composite films has been systematically investigated. The results showed that the antibacterial rate of these antibacterial composite films against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) was more than 99.9%. Interestingly, the obtained composite films still demonstrated long-lasting antibacterial properties after 42 days of immersion in water and six cycles of antibacterial testing due to the synergistic effect of electrostatic interaction and intermolecular hydrogen bonds. In addition, the tensile strength of the obtained composite films was significantly improved up to 30 MPa, 2 times higher than the original polyurethane films, and the toughness was high up to 70 MJ/m3, which could easily pull up to 19,000 times their own weight. Furthermore, the water absorption of the obtained composite films was greatly reduced from 56.4% to 6.6% due to their high crosslinking densities and intermolecular interaction. The technique reported in this study provides a facile and simple strategy for development of high mechanical properties and durable antibacterial properties of bio-based polymer composites, which would find promising applications in medical devices, product packaging, and so on.

Open-cell PDMS polyHIPEs prepared using polymethylvinylsiloxane to prevent pore collapse

Polymerized high internal phase emulsions (PolyHIPEs) are porous polymers made using an emulsion template. Our group has previously shown that polydimethylsiloxane (PDMS) polyHIPEs prepared using a thiol-ene crosslinking reaction could obtain a maximum total porosity of ∼63%. Attempting to increase total porosity by increasing the volume fraction dispersed phase resulted in collapse of the porous material due to the soft nature of PDMS. In this work, polymethylvinylsiloxane (PMVS) was synthesized to impart high crosslinking density in the network. This polymer can be blended with divinyl-PDMS in the continuous phase to obtain a large range of storage moduli (25–900 kPa) in the final polyHIPEs with higher total porosity (74–77%). Finally, the use of these PDMS PolyHIPEs as separation media for oil/water mixtures has been demonstrated.

A strategy for dual-networked epoxy composite systems toward high cross-linking density and solid interfacial adhesion

Epoxy composites (ECs) are used as epoxy molding compound (EMC), which significantly protects the integrated circuits from the humid environment. One of the major limitations with ECs is their phenomena of moisture absorption in humid environments, which drastically reduces their mechanical properties and hinders them from gaining widespread industrial applications. The low cross-linking density of conventional epoxy composites that can lead to intrinsically providing hydrophilic sites and low activation energy in water molecule diffusion. Herein, a dual-networked epoxy composite system as a platform technique by adopting strategically designed poly (methyl methacrylate)-b-(dimethyl aminoethyl methacrylate) (PMD) as polymer compatibilizer to improve the water absorption resistance and mechanical properties derived from good interfacial adhesion and high cross-linking density of epoxy/silica composites were investigated. The as-prepared dual-networked epoxy composite system demonstrates a high-water absorption suppression of about 166.78% and a high tensile modulus of 4.6 GPa, compared to other epoxy composites. In addition, the current strategy has excellent expandability to various fillers also applicable to diverse epoxy composite systems.

The study of composition-properties relationships for composite hydrogels based on poly(methacrylic acid) and high concentrations of MFI zeolite

High concentrations of hydrophobic MFI zeolite (26.0–51.3 wt%) were successfully embedded in poly(methacrylic acid) (PMAA) network in simple, one-pot synthesis of PMAA-MFI composite hydrogels. The effects of zeolite concentration regarding pure PMAA xerogel, for all of the investigated properties (primary structural parameters, morphological properties, water swelling ability, thermal degradation mechanism and stability, and mechanical properties) were determined and discussed. The PMAA-MFI composite hydrogels had remarkably higher density than PMAA xerogel (1660/1420 kg m−3), higher crosslinking density (29/1.0 × 10−4), improved mechanics in the dry and swollen state (up to 4.9 times higher storage modulus) and lower swelling degree (42/420 g g−1), average molar mass between the network crosslinks (3.6/110 × 104 g mol−1), and distance between the macromolecular chains (64/710 nm). Despite high zeolite concentration, the composites kept porous hydrogel structure with uniformly dispersed zeolite particles. FTIR spectra disclosed the existence of functional groups of PMAA network and MFI zeolite that interacted mutually and led to increased crosslinking density. The effect of zeolite concentration on the thermal degradation mechanism was established. The composition-properties correlations were determined and described by the adequate linear or power law equations revealing the additive nature of the composites and the fractal structure of the networks. These correlations enable the achievement of desired properties by simple adjustment of the initial composition, i.e. zeolite concentration. Therefore, this work presents the base for further development of tailor-made PMAA-MFI composite hydrogels aimed for specific application.