Bond Exchange Research Articles

Exchange Rates and Capital Market Instruments – Analysis Based on the Example of Switzerland before and during the Covid-19 Pandemic

The purpose of the article. The aim of the article is to present factors influencing foreign exchange rates and to examine the relationship between them and stock and bond indices in Switzerland in the period before and during the COVID-19 pandemic. The hypothesis that the studied relationships changed as a result of the turmoil caused by the pandemic crisis was verified.
 Methodology. To verify the hypothesis, linear and non-linear functions were analyzed, and Pearson's correlation indices between the examined instruments were calculated.
 Results of the research. As a result of the analysis, it was found that the relationship between the CHF exchange rate index and stock indices is non-linear, and the relationship between the CHF exchange rate index and bond indices is linear. Correlation coefficients before the pandemic indicated strong relationships between the CHF exchange rate index and the examined capital market indices, and moreover, the relationship with stock indices was positive and with bond indices negative. It was found that during the pandemic, the correlation with bond indices changed and became positive. The motivation of the analysis is related to the spotlight on a Swiss franc as a result of numerous financial decisions related to this currency.

Self-Consistent Theory for Structural Relaxation, Dynamic Bond Exchange Times, and the Glass Transition in Polymeric Vitrimers

Self-Consistent Theory for Structural Relaxation, Dynamic Bond Exchange Times, and the Glass Transition in Polymeric Vitrimers

Coordinate mechanical property and self‐healing performance by hydrogen bonding promoting microphase separation structures in waterborne polyurethanes

AbstractSelf‐repairing materials aim to replicate nature's repair mechanisms, but their current properties are inadequate for commercialization due to the trade‐off between repair and mechanical properties. An environmentally responsive low‐temperature self‐healing aqueous polyurethane with different sequence structures was developed in which the WPU‐MD/APDS system exhibited excellent performance. Statically, the incorporation of MD (1,8‐diamino‐P‐menthane) enhances hydrogen bonding within the system and facilitates reversible weak dynamic bonding; microphase separation enables aggregation of disulfide bonds in the hard‐phase domain, thereby improving disulfide bond exchange rates. Dynamically, the inclusion of MD introduces a rigid ring structure and promotes microphase separation with increased hydrogen bonding, leading to enhanced mechanical properties of the materials. The material exhibited tensile strength of 27.37 MPa, elongation at break of 1353%, excellent elastic recovery ability, and high tensile toughness of 136 MJ m−3; a sample with thickness of 0.2 mm lifted weight greater than its own weight into 40,000 times without damage recovery at 40°C. Rheological analysis confirmed its self‐healing property at room temperature.

Seamless, Self-Transformation of Thermoplastic Polyesters into Vitrimers Through Bond Exchange-Triggered Cross-Linking.

In this study, a seamless, self-transformation system of linear thermoplastic polyesters into the sustainable cross-linked polymers, vitrimers, is demonstrated. The key is the use of polyesters bearing abundant hydroxyl side groups, which are synthesized via the reaction using dithiol molecules bearing ester units and diepoxy molecules. The polymerization reaction progresses efficiently at relatively low temperature due to the click nature of the thiol-epoxy reaction, which provides the hydroxyl side groups along the polyester chain. The tin catalyst (stannous octoate) is added in the initial polymerization, and the catalyst also works to cross-link the polyesters via intermolecular transesterification bond exchange simply by heating at high temperatures. By adjusting the degrees of cross-linking, the mechanical properties as well as the thermal properties are well tuned. The bond exchange can still be activated in the final cross-linked sample; and thus, the material behaves as vitrimers, exhibiting mechanical recyclability. The application of a new type of hot melt adhesive, where the post-coating tuning/enhancement of adhesion strength is realized, is also demonstrated. On the whole, the present system is very simple but proposes a new application window of bond exchange concept into self-transformation polymers.

Tertiary amine-based autocatalytic transesterification epoxy vitrimer: Effect of tertiary amine structure and covalent incorporation on the properties of vitrimers

Since its first report in 2011, researches on vitrimer transesterification catalyst systems had been gradually focused on three aspects: low toxicity and low corrosion, good compatibility with resin matrix, and high catalytic performance for bond exchange reactions. However, it is difficult to obtain a catalytic system that can simultaneously meet the above three expectations. This research conducted the preparation of autocatalytic transesterification epoxy vitrimers by directly mixing small tertiary amine molecules with epoxy-anhydride systems and covalent incorporating tertiary amine structures into curing systems. And the influence of tertiary amine structures and covalent incorporation on the various properties of vitrimers was also discussed. A tertiary amine promoter composed of isofunctional group ratio reaction from di-n-hexamine and epoxy resin provided by this research overcame the problem of poor compatibility between tertiary amine and epoxy resin, introduced a highly active catalytic structure into vitrimer curing system. And the result indicated that the vitrimer prepared according to our new strategy had excellent comprehensive performance. In conclusion, a new idea of covalent incorporation was proposed to provide ideas and inspiration for the development of new tertiary amine-based autocatalytic transesterification epoxy vitrimers.

A Design of Transesterification Vitrimers with Variable Fractions of Network Defects and the Creep Characteristics

AbstractVitrimer materials have attained great interest owing to their sustainable functions and distinctive physical properties. Compared with conventional cross‐linked materials, the thermoplastic‐like nature, such as the stress relaxation and creep at high temperatures, is outstanding, which is owing to the activation of the bond exchange in the cross‐linked network. The activation temperature of the bond exchange, so‐called topology freezing temperature (Tv), are treated as the important material parameters, and non‐isothermal (i.e., temperature‐ramp) creep test is one of the method to determine the Tv. This study investigates effects of network defects, that is, the breaking point of network connectivity, on the creep and Tv. The transesterification vitrimer samples with varying fraction of the network defects are designed using thiol‐epoxy click reaction, where the overall fraction of the bond exchange units, that is, the ester bond and hydroxyl group, is kept constant. Non‐isothermal creep tests are conducted with varying temperature‐ramp rate and applied stress levels, and the difference of Tv and creep behaviors at T > Tv are discussed in terms of network strand diffusion during bond exchange and chain elongation under stress.

Hybrid Chemomechanical Yielding of Covalent Adaptable Networks for Toughening Ionic Semi-Crystalline Epoxy

Covalent adaptable network (CAN) in thermoset epoxy shows good recyclability due to its bond exchange reactions (BERs), which result in low mechanical strength and toughness. Many experimental studies on using the semi-crystalline structures and ions with the CAN have shown to improve the mechanical strength and toughness of these thermoset epoxies. However, their toughening mechanisms and coupling effects have not been well understood. Herein, we propose an extended Maxwell model to explore toughening mechanisms and coupling effects of the semi-crystallization process and formation of ion phases on CANs, which enables the thermoset epoxy with hybrid chemomechanical yielding. A constitutive stress–strain relationship is formulated to characterize the mechanical toughening and hybrid chemomechanical yielding behavior, where the first stage of yielding is originated from the effect of ions on amorphous phases, and the second stage of yielding is due to the coupling effect of semi-crystallization and formation of ions. The proposed models have been verified using the experimental results, and provide a fundamental approach to understanding the toughening mechanisms and coupling effects of semi-crystallization and ions in thermoset epoxy.

Peridynamic modeling of seepage in multiscale fractured rigid porous media

AbstractThe numerical simulation of seepage in fractured porous media holds significant relevance for subsurface energy development. The presence of fractures at various scales profoundly influences the hydraulic properties of porous media during seepage. A peridynamic (PD)‐based frame is proposed for the seepage problem analysis of multiscale fractured porous media, in which micro‐fractures are implicitly represented like the porous media, macro‐fractures are explicitly represented. For fluid flow modeling, the peridynamic bonds are divided into three types: pore seepage bond, fracture seepage bond, and fluid exchange bond. The micro‐permeabilities of different bonds are derived by Darcy's law, Poiseuille's law, and fluid exchange model, respectively. The proposed peridynamic model can naturally simulate the seepage problem of porous rock with multiscale fractures in a unified framework. Then, four cases of pore seepage and fracture seepage are analyzed using the proposed model, fractured porous media seepage with micro‐fractures and macro‐fractures are respectively considered. The predicted results are compared with the available theoretical solutions, the close agreement shows that the wide validity and applicability of the proposed model in multiscale seepage problems.

Platform for Multiple Isotope Labeling via Carbon-Sulfur Bond Exchange.

In this work, we explore a nickel-catalyzed reversible carbon-sulfur (C-S) bond activation strategy to achieve selective sulfur isotope exchange. Isotopes are at the foundation of applications in life science, such as nuclear imaging, and are essential tools for the determination of pharmacokinetic and dynamic profiles of new pharmaceuticals. However, the insertion of an isotope into an organic molecule remains challenging, and current technologies are element-specific. Despite the ubiquitous presence of sulfur in many biologically active molecules, sulfur isotope labeling is an underexplored field, and sulfur isotope exchange has been overlooked. This approach enables us to move beyond standardized element-specific procedures and was applied to multiple isotopes, including deuterium, carbon-13, sulfur-34, and radioactive carbon-14. These results provide a unique platform for multiple isotope labeling and are compatible with a wide range of substrates, including pharmaceuticals. In addition, this technology proved its potential as an isotopic encryption device for organic molecules.

Resolving the relaxation complexity of vitrimers: Time-temperature superpositions of a time-temperature non-equivalent system

Vitrimers are polymer networks that, thanks to covalent bond exchange, combine desirable properties of thermoplastic and thermosets, such as flowability and insolubility. For this reason, vitrimers are considered to be good candidates for a number of innovative applications from self-healing soft robots to hard reprocessable materials. All these applications are related to the unusual thermomechanical behavior of vitrimers, consequence of the non-trivial interplay between the polymer network dynamics and the thermally activated chemical link exchange. Here we use solid-state rheology to investigate the properties of a recently developed epoxy-based vitrimer. The rheological analysis demonstrates that the mechanical spectrum is composed of two relaxation processes with distinct activation energies that are associated with glass dynamics and covalent bond exchange, respectively. This makes the material thermo-rheologically complex and time temperature equivalence does not apply. Nonetheless, thanks to mechanical spectral analysis in a wide range of stiffness, time and temperature, we are able to depict the time-temperature-relaxation landscape in an enough precise way to account for the two dynamical processes and recombine them to predict the mechanical moduli in a wide interval of frequencies (21 decades), from low temperatures (close to room temperature) to high temperatures (above the glass transition temperature).

A phase-transition model of reprocessible thermadapt shape memory polymer

A new type of thermadapt shape memory polymer (SMP) has not only the processability of thermoplastic SMP, but also the excellent shape fixation of thermosetting SMP. To enhance the application of this thermadapt SMP within industrial sectors, a comprehensive constitutive model based on phase transition is being proposed as an indicative descriptor of the semi-crystalline thermadapt SMP’s salient features, predominantly related to its two-way shape memory effect (SME) and thermal reprocessability. The concept of cooling elongation is also introduced in this model for modeling the two-way SME during the crystallization process. The molecular mechanism of chain-packing has been studied and used to establish phenomenological formulas. In addition, to systematically assess the temperature-time dependence of the crystallization process, the Avrami equation is improved by incorporating the distribution of polymer chain segments. This strategy provides a detailed investigation into the evolving pattern of the crystallization process in response to various temperature and time conditions. Compared with the experimental results, it is found that our model can well capture mechanical behavior in multiple shape memory cycles, including the two-way SME and reshaping process caused by bond exchange reaction. Furthermore, the potential application of SMP in smart mandrels is explored because the cooling elongation feature is able to endow it with self-adaptive expansion ability.

Front Cover

The cover image by Simon van Hurne depicts the formation of a dynamic covalent boronate‐TetraAzaADamantane bond, which could be integrated in a covalent adaptable network. As a result of the on‐going dynamic bond exchange reactions within this network, the material could undergo stress relaxation. At the same time, the more robust triple bonding motif allowed for stronger materials compared to their conventional boronate ester‐based counterparts. DOI: 10.1002/pol.20230446 image

Rapid tooling for rubber extrusion molding by digital light processing 3D printing with dual curable vitrimers

AbstractFor the manufacture of extrusion dies, three‐dimensional (3D) printing with photopolymers offers numerous advantages including flexibility, high surface quality, decent build speed, low costs and a reduced amount of waste. However, the majority of photocurable resins used in vat photopolymerization 3D printing rely on acrylates, which entail 3D‐printed objects with poor mechanical properties. In particular, the high brittleness limits their application in rapid tooling, for which tough materials with high glass transition temperatures (Tg) are required. In the present study, we highlight the use of dual curable acrylate‐epoxy resins with dynamic covalent bonds for the direct fabrication of extrusion dies. During digital light processing (DLP) 3D printing the acrylate network is formed, whose toughness and thermal stability are significantly enhanced by the thermoactivated formation of a second network. By following a postbaking procedure, aminoglycidiyl monomers are cured with an anhydride hardener bearing bulky norbornene groups yielding interpenetrating polymer networks with a Tg > 100°C. The tertiary amine groups present in the structure of the aminoglycidyl derivatives do not only accelerate the ring‐opening reaction but also act as internal catalysts and activate bond exchange reactions between free –OH groups and ester moieties available in the photopolymer. This is confirmed by rheometer studies showing a distinctive stress relaxation at elevated temperature and giving rise to a possible reprocessability of the 3D‐printed dies. With a selected resin formulation, a set of dies is printed by DLP 3D printing, with which a highly filled rubber compound is successfully extruded. The results clearly show that dual curable resins with dynamic covalent bonds are a promising class of material for rapid tooling and pave the way towards a customized and convenient fabrication of extrusion dies for rubber processing.

Dynamic borate ester bond-based 3D printing fluorescence polysiloxane with self-healing, antimicrobial, and shape memory

Developing a shape memory polysiloxanes possessing superior mechanical properties, antibacterial qualities, and exceptional self-healing capabilities has become a significant obstacle for the development of smart materials. We tackled this challenge by modifying polysiloxanes using N-acetylcysteine (NACL) and photosensitive molding with a dynamic borate ester crosslinker (S-BDB). The resultant photosensitive polysiloxane exhibited remarkable mechanical properties, with a stress up to 4.7 MPa. By utilizing the hydrogen bonds of grafted NACL and dynamic borate ester bonds of S-BDB to heal the damaged polymer network, the efficient self-healing performance (99.2 %) of photosensitive polysiloxane was obtained. Additionally, the material exhibited typical shape memory properties due to the rapid exchange of hydrogen bonds at elevated temperatures and the fixation of borate esters, enabling shape editing of the material. The material obtained a phosphorescent effect due to the introduction of B-O bonding, making it possible to have application value in the field of complex pattern fluorescent anti-counterfeiting marking manufacturing. Simultaneously, due to the amphiphilic ionic nature of NACL, the resin material obtained exhibited excellent antimicrobial properties.

High strength, self-healing and hydrophobic fully bio-based polybenzoxazine reinforced pine oleoresin-based vitrimer and its application in carbon fiber reinforced polymers

The traditional carbon fiber reinforced polymers (CFRPs) matrix is a permanent 3D cross-linked network that is difficult to degrade and reprocess. In order to solve the closed-loop recycling of carbon fibers (CFs) and develop green economy. We synthesized aldehyde-containing bio-based benzoxazine (VD) from vanillin and 1,10-diaminodecane, the epoxy resin vitrimer matrix (P-AE-MV) was prepared by curing acrylpimaric acid diglycidyl ester (AE) with a Schiff base (MV) formed by 1,8-menthane diamine (MDA) and VD. Under the double dynamic covalent bond of Schiff base and β-hydroxy ester, P-AE-MV has a very low dynamic bond exchange activation energy Ea = 48.1 kJ mol−1, which can realize the rapid topological rearrangement of polymer network, and give P-AE-MV excellent self-healing, shape memory, reprocessability and degradability. In addition, the introduction of polybenzoxazine (PBz) structure greatly improved the mechanical properties, thermal decomposition temperature, carbon yield and hydrophobicity of epoxy resin vitrimer, which is very suitable for CFRPs matrix. The CFRP (P-AE-MV-CF) prepared with P-AE-MV as matrix resin have good mechanical properties, reprocessability, shape memory and self-adhesion. Especially, it can be rapidly ammonolysis with n-butylamine under mild conditions (60 °C), achieving efficient and non-destructive recycling of CFs. This work provides an effective solution to promote the closed-loop recycling of CFs and the sustainable development of CFRPs using fully bio-based resources as raw materials.

Preparation of Chitosan-modified Biochar and Its Adsorption Mechanism for Cd2+ in Aqueous Solution

Chitosan-modified biochar （CBC） was prepared as a low-cost and highly efficient adsorbent for Cd2+ in aqueous solutions. Batch adsorption experiments were conducted to evaluate the adsorption performance. Characterization experiments with SEM-EDS， FTIR， and XPS were used to analyze the surface microstructure and chemical composition of the adsorbent. The results showed that the adsorption performance of CBC was remarkably improved by the introduction of surface functional groups （-OH， -C=O， and -NH2）. The pseudo-second-order kinetic model and Langmuir model were better for describing the kinetics and isotherms for Cd2+ adsorption onto CBC， indicating that the adsorption rate was determined by the active sites and controlled by monolayer chemisorption. The adsorption process was endothermic spontaneous， and the key mechanisms involved complexation， precipitation， cation exchange， and cation-π bonds. After five instances of adsorption-desorption cycles， the adsorption capacity of CBC for Cd2+ still remained above 80% of the initial adsorption capacity， indicating that CBC had a favorable recyclability. The current work embodies the concept of green chemistry， and the prepared chitosan-modified biochar was a promising adsorbent for the removal of Cd2+ in wastewater and soil.

Relief of Residual Stress in Bulk Thermosets in the Glassy State by Local Bond Exchange.

The covalently cross-linked network gives thermosets superior thermal, mechanical, and electrical properties, which, however, squarely makes the large residual stress that is inevitably induced during preparation hardly relieved in the glassy state. In this work, an incredible reduction in residual stress is successfully achieved in bulk thermosets in the glassy state through introducing highly dynamic thiocarbamate bonds by "click" reactions of thiols and isocyanates. Due to the excellent dynamic behaviors of thiocarbamate bonds, local network rearrangement is achieved through thermal stimulation, while the strong 3D cross-linked network is well maintained. Ultimately, a decrease by 44% in residual stress is detected by simply annealing samples at 30°C below glass transition temperature (Tg), during which they could well maintain more than 98.4% of the storage modulus. After the annealing, more uniform residual stress distribution is also observed, showing a 32% decline in sample standard deviation. However, the residual stress of epoxy resin, a typical thermoset as a reference, changes little even after annealing at Tg. The results prove it a feasible strategy to reduce residual stress in bulk thermosets in the glassy state by introducing proper dynamic covalent bonds.

Vulcanization of polypropylene

AbstractDynamic covalent crosslinking of commodity thermoplastics is a desirable target in material development, as it promises to combine the enhanced mechanical properties and thermal/solvent stability of thermosets with reprocessability and plastic flow under certain conditions activating the bond exchange. Many attempts of this development suffer from the same two problems: enhanced cost due to complex and often toxic chemicals, and the effective melt‐flow index being too low for practical use. Here we return to the origins of polymer networks, and mimic the vulcanization of natural rubber in the commodity polypropylene using elemental sulfur initiated by peroxide. Forming sulfur bridges allows easy catalyst‐free reprocessability based on the disulfide bond exchange. We study a broad range of compositions and reaction conditions, finding optimal balance between the crosslinking and chain scission in the melt compounder, and demonstrating much enhanced characteristics of the resulting materials. We specifically discuss and evaluate the balance between the rubber‐elastic network response at high temperatures and the plastic flow enabled by disulfide exchange, responsible for the reprocessing of our vitrimers.

Facile Bond Exchanging Strategy for Engineering Wet Adhesion and Antioxidant/Antibacterial Thin Layer over a Dynamic Hydrogel via the Carbon Dots Derived from Tannic Acid/ε-Polylysine.

Adhesive hydrogels, playing an essential role in stretchable electronics, soft robotics, tissue engineering, and so forth, upon functioning often need to adhere to various substrates in wet conditions and simultaneously exhibit antibacterial/antioxidant properties while possessing the intrinsic stretchability and elasticity of the hydrogel network intact. Therefore, simple approaches to conveniently access adhesive hydrogels with multifunctional surfaces are being pursued. Herein, a facile strategy has been proposed to construct multifunctional adhesive hydrogels via surface engineering of a multifunctional carbon dot (CD)-decorated polymeric thin layer by dynamic bond exchange. By this strategy, a double cross-linked network hydrogel of polyacrylamide (PAM) and oxidized dextran (ODA) was engineered with a unique dense layer over the Schiff base hydrogel matrix by aqueous solution immersion of PA-120, versatile CDs derived from tannic acid (TA) and ε-polylysine (PL). Without any additional agents, the PA-120 CDs with residual polyphenolic/catechol and amine moieties were incorporated into the surface structure of the hydrogel network by the combined action of the Schiff base and hydrogen bonds to form a dense surface layer that can exhibit high wet adhesive performance via the amine-polyphenol/catechol pair. The armor-like dense architecture also endowed hydrogels with considerably enhanced tensile/compression properties and excellent antioxidant/antibacterial abilities. Besides, the single-sided modified Janus hydrogel and completely surface-modified hydrogel can be flexibly developed through this approach. This strategy will provide new insights into the preparation and application of surface-modified hydrogels featuring multiple functions and tunable interfacial properties.

Ultra-wide damping temperature range polyurethane elastomer enabled by an asymmetrically structured chain extender with double disulfide bonds

Conventional polymer damping materials are extensively utilized to mitigate vibration and noise in various fields, yet suffer from the drawback of a narrow effective damping temperature range. Herein, we innovatively synthesized an asymmetrically structured chain extender (AIS) with double disulfide bonds that provided polyurethane elastomer (PUE) with ultra-high energy dissipation over a wide temperature range, which was unattainable by conventional chain extenders. This was achieved by enhancing the bond exchange rate of dynamic disulfide bonds in the PUE. Specifically, the asymmetric structure of AIS was strategically designed to disrupt the regularity of the hard segments of the PUE, enabling the disulfide bonds to break and reorganize efficiently, thus depleting mechanical energy. The results demonstrated that the effective damping temperature range (Tan δ > 0.3) of the PUE cured by AIS can reach 130 °C (−15 °C–115 °C), which is 210 % higher than that of the matrix, and a stable damping plateau was observed in the region from room temperature to high temperature. Moreover, the introduction of AIS significantly enhanced the mechanical properties of the PUE. This study encourages further investigation and application of disulfide bonds in the field of damping.