Dynamic Covalent Polymer Networks Research Articles

Fabrication and properties of recyclable tung oil–based polymer coatings based on dual cross-linked dynamic covalent polymer networks

The exploration of reprocessability and high-adhesion biomass polymer coatings is in line with the concept of green materials and is conducive to energy conservation and environmental protection. Here, a series of novel recyclable tung oil–based polymer coatings based on dual cross-linked dynamic covalent networks were reported. Tung oil polymerized maleic anhydride (TOA) was firstly prepared and further reacted with furfuryl alcohol. Then tung oil-polymerized maleic furanol ester (TOAFA) containing reactive conjugated double bond and carboxyl groups was prepared. FTIR and 1H NMR demonstrated the reactive monomer TOAFA was successfully synthesized. Then TOAFA was cross-linked with bismaleimide and different epoxy resins to fabricate target polymer networks based on dynamic Diels-Alder(D-A) bonds and dynamic transesterification. The shape memory, reprocessing, and mechanical and thermal stability of the copolymerized systems with different crosslinking states were investigated. The coating performances were characterized with solvent resistance, adhesion strength, hardness and glossiness tests. It was found that the dynamic D-A covalent bonds were perfectly combined with transesterification in the polymerization systems. The copolymerized systems possessed excellent thermal stability and mechanical properties. The rigid and flexible properties of the polymer networks can be controlled by adjusting the component weight ratios and the type of epoxy resin in the polymerization system. The rigid polymerization system had the highest tensile strength of 28 MPa, and its elongation at break was up to 57 % of the flexible polymerization system. Coating adhesion test showed the adhesion property reached level 1. The polymer coatings displayed excellent solvent corrosion resistance, and relative high gloss. The polymer networks also showed certain reprocessing performance and excellent shape memory effect.

Dynamic covalent polymer network enabled bio‐based vitrimers with excellent substrate bonding

AbstractIn this article, we report the synthesis of dynamic covalent polymer network (DCPN)‐enabled vitrimers derived from gum rosin—a pine tree extract. The concept of Diels Alder (DA) thermo‐reversibility has been used to introduce self‐healing/dynamic covalent ability into the polymer systems. For this purpose, the diene moieties were incorporated in the gum rosin‐based polymer network, and the bismaleimides served as the dienophiles. The thermo‐reversible crosslinks formed between the diene and dienophile moieties can enhance the service life of the materials by improving the durability, mechanical as well as chemical properties of the materials. Upon damage, the thermo‐reversible DA bonds de‐crosslink resulting in smaller chain segments, enabling the material to flow to the site of damage and seal the fractured edges. The formation of DA crosslinks has been confirmed by using Fourier transform‐infrared spectroscopic technique. Scanning electron microscopy analysis was used to verify the self‐healing ability of the resin matrix. The healing process was studied after manual scratches were made. The polymers' ability to heal multiple times is investigated using differential scanning calorimetry. The adhesion strength of the self‐healing material has been examined on variety of substrates such as glass, steel, and concrete. The materials exhibited superior adhesion strength on glass substrate.

Accessing pluripotent materials through tempering of dynamic covalent polymer networks.

Pluripotency, which is defined as a system not fixed as to its developmental potentialities, is typically associated with biology and stem cells. Inspired by this concept, we report synthetic polymers that act as a single "pluripotent" feedstock and can be differentiated into a range of materials that exhibit different mechanical properties, from hard and brittle to soft and extensible. To achieve this, we have exploited dynamic covalent networks that contain labile, dynamic thia-Michael bonds, whose extent of bonding can be thermally modulated and retained through tempering, akin to the process used in metallurgy. In addition, we show that the shape memory behavior of these materials can be tailored through tempering and that these materials can be patterned to spatially control mechanical properties.

Vinylogous Urea-Urethane Vitrimers: Accelerating and Inhibiting Network Dynamics through Hydrogen Bonding.

Vinylogous urethane (VUO ) based polymer networks are widely used as catalyst-free vitrimers that show rapid covalent bond exchange at elevated temperatures. In solution, vinylogous ureas (VUN ) undergo much faster bond exchange than VUO and are highly dynamic at room temperature. However, this difference in reactivity is not observed in their respective dynamic polymer networks, as VUO and VUN vitrimers prepared herein with very similar macromolecular architectures show comparable stress relaxation and creep behavior. However, by using mixtures of VUO and VUN linkages within the same network, the dynamic reactions can be accelerated by an order of magnitude. The results can be rationalized by the effect of intermolecular hydrogen bonding, which is absent in VUO vitrimers, but is very pronounced for vinylogous urea moieties. At low concentrations of VUN , these hydrogen bonds act as catalysts for covalent bond exchange, while at high concentration, they provide a pervasive vinylogous urea - urethane (VU) network of strong non-covalent interactions, giving rise to phase separation and inhibiting polymer chain dynamics. This offers a straightforward design principle for dynamic polymer materials, showing at the same time the possible additive and synergistic effects of supramolecular and dynamic covalent polymer networks.

Vinylogous Urea—Urethane Vitrimers: Accelerating and Inhibiting Network Dynamics through Hydrogen Bonding

AbstractVinylogous urethane (VUO) based polymer networks are widely used as catalyst‐free vitrimers that show rapid covalent bond exchange at elevated temperatures. In solution, vinylogous ureas (VUN) undergo much faster bond exchange than VUO and are highly dynamic at room temperature. However, this difference in reactivity is not observed in their respective dynamic polymer networks, as VUO and VUN vitrimers prepared herein with very similar macromolecular architectures show comparable stress relaxation and creep behavior. However, by using mixtures of VUO and VUN linkages within the same network, the dynamic reactions can be accelerated by an order of magnitude. The results can be rationalized by the effect of intermolecular hydrogen bonding, which is absent in VUO vitrimers, but is very pronounced for vinylogous urea moieties. At low concentrations of VUN, these hydrogen bonds act as catalysts for covalent bond exchange, while at high concentration, they provide a pervasive vinylogous urea ‐ urethane (VU) network of strong non‐covalent interactions, giving rise to phase separation and inhibiting polymer chain dynamics. This offers a straightforward design principle for dynamic polymer materials, showing at the same time the possible additive and synergistic effects of supramolecular and dynamic covalent polymer networks.

Design, fabrication and characterization of dynamic covalent polymer networks containing reversible Diels-Alder adducts derived from vegetable oil and furfural

The exploration of novel bio-based functional polymers has high application value and confirms to sustainable development strategies following the increasing shortage of fossil resources. In this research, several different recyclable bio-based dynamic covalent polymer networks (DCPNs) containing dynamic Diels-Alder bonds from vegetable oil and furan were designed and fabricated via a simple preparation process. The reprocessability, shape memory, molecular structures, thermal properties and mechanical properties of the DCPNs were all investigated. 1H nuclear magnetic resonance (1H-NMR) and Fourier transform infrared spectroscopy (FTIR) analysises demonstrated the target reactive products were all successfully synthesized. Variable-temperature FTIR and differential scanning calorimetry revealed the changing rule of dynamic Diels-Alder bond molecular structures with temperature. The DCPNs possess controllable tensile properties including superior ductility and certain rigidity via adjusting fatty chain component, which is in accordance with the result of dynamic mechanical analysis. Thermal gravimetric analysis demonstrated the DCPNs had excellent thermostability, and the main peak decomposition temperatures were all above 460 °C. The nature of the dynamic Diels-Alder bonds could make shattered DCPNs be reprocessed multiple times under relatively mild conditions. The DCPNs displayed certain shape memory properties, and the shape fixation and recovery ratio at the last two cycles were kept above 95.53% and 84.58%, respectively. This work will help with the development on this kind of bio-based polymers containing dynamic Diels-Alder covalent bonds.

Tough Epoxy Vitrimer with Hybrid Networks and Its Applications in Reprocessable Composites

Progress in the ever‐growing vitrimer chemistry opens up great opportunities for a sustainable society by producing reprocessable thermosets. However, most vitrimers suffer from the conflict between mechanical performance and dynamic reactivity, hindering the development of industrial applications. Herein, a facile but universal material design strategy is proposed to yield tough epoxy vitrimers while preserving excellent reprocessability. Copolymerization and bond exchange reactions can build covalent bridges between heterogeneous polymer networks, thus leading to a hybrid dynamic covalent polymer network containing hard and soft segments. Tailorable mechanical and thermal properties are obtained by controlling the ratio of hard‐to‐soft components. The resulting epoxy vitrimer can balance strength and stretchability while exhibiting brittle‐to‐ductile transformation in tensile tests. Enhanced fracture toughness is achieved compared to both pristine hard and soft vitrimers. The resulting vitrimers exhibit excellent processability and potential applications in composite fabrication, welding and reshaping, and multiple shape memory functions. This work presents a feasible approach to satisfying the demands of mechanical performance and dynamic reactivity in the design of epoxy vitrimers. The material design principle will be applicable to a wide range of vitrimer systems to pursue desirable properties.

Thermally Latent Bases in Dynamic Covalent Polymer Networks and their Emerging Applications.

A novel strategy allowing temporal control of dynamic bond exchange in covalently crosslinked polymer networks via latent transesterification catalysts is introduced. Obtained by a straightforward air- and water-tolerant synthesis, the latent catalyst is designed for an irreversible temperature-mediated release of a strong organic base. Its long-term inactivity at temperatures below 50 °C provides the unique opportunity to equip dynamic covalent networks with creep resistance and high bond-exchange rates, once activated. The presented thermally latent base catalyst is conveniently introducible in readily available building blocks and, as proof of concept, applied in a radically polymerized thiol-ene network. Light-mediated curing is used for 3D-printing functional objects, on which the possibility of spatially controlled reshaping and welding based on dynamic transesterification is illustrated. Since the catalyst is thermally activated, limitations regarding sample geometry and optical transparency do not apply, which facilitates a transfer to well-established industrial technologies. Consequently, fiber-reinforced and highly filled magneto-active thiol-ene polymer composites are fabricated by a thermal curing approach. The on-demand activation of dynamic transesterification is demonstrated by (magneto-assisted) reshaping experiments, highlighting a wide range of potential future applications offered by the presented concept.

Tough and Thermostable Polybutylene Terephthalate (PBT)/Vitrimer Blend with Enhanced Interfacial Compatibility.

Polymer blending is an efficient way to obtain extraordinary polymeric materials. However, once permanently cross-linked thermosets are involved in blending, there are challenges in designing and optimizing the structures and interfacial compatibility of blends. Vitrimer with dynamic covalent polymer networks provides an innovative opportunity for blending thermoplastics and thermosets. Herein, a reactive blending strategy is proposed to develop thermoplastic-thermoset blend with enhanced compatibility on the basis of dynamic covalent chemistry. Specifically, polybutylene terephthalate (PBT) and polymerized epoxy vitrimer can be directly melt blended to obtain tough and thermostable blends with desirable microstructures and interfacial interaction. Bond exchange facilitates the grafting of PBT and epoxy vitrimer chains, thus enhancing the interfacial compatibility and thermal stability of blends. The obtained blend balances the strength and stretchability of PBT and epoxy vitrimer, resulting in enhanced toughness. This work offers a new way for designing and fabricating new polymeric materials by blending thermoplastics and thermosets. It also suggests a facile direction towards upcycling thermoplastics and thermosets. This article is protected by copyright. All rights reserved.

3D printing of dynamic covalent polymer network with on-demand geometric and mechanical reprogrammability

Delicate geometries and suitable mechanical properties are essential for device applications of polymer materials. 3D printing offers unprecedented versatility, but the geometries and mechanical properties are typically fixed after printing. Here, we report a 3D photo-printable dynamic covalent network that can undergo two independently controllable bond exchange reactions, allowing reprogramming the geometry and mechanical properties after printing. Specifically, the network is designed to contain hindered urea bonds and pendant hydroxyl groups. The homolytic exchange between hindered urea bonds allows reconfiguring the printed shape without affecting the network topology and mechanical properties. Under different conditions, the hindered urea bonds are transformed into urethane bonds via exchange reactions with hydroxyl groups, which permits tailoring of the mechanical properties. The freedom to reprogram the shape and properties in an on-demand fashion offers the opportunity to produce multiple 3D printed products from one single printing step.

Circularity in polymers: addressing performance and sustainability challenges using dynamic covalent chemistries.

The circularity of current and future polymeric materials is a major focus of fundamental and applied research, as undesirable end-of-life outcomes and waste accumulation are global problems that impact our society. The recycling or repurposing of thermoplastics and thermosets is an attractive solution to these issues, yet both options are encumbered by poor property retention upon reuse, along with heterogeneities in common waste streams that limit property optimization. Dynamic covalent chemistry, when applied to polymeric materials, enables the targeted design of reversible bonds that can be tailored to specific reprocessing conditions to help address conventional recycling challenges. In this review, we highlight the key features of several dynamic covalent chemistries that can promote closed-loop recyclability and we discuss recent synthetic progress towards incorporating these chemistries into new polymers and existing commodity plastics. Next, we outline how dynamic covalent bonds and polymer network structure influence thermomechanical properties related to application and recyclability, with a focus on predictive physical models that describe network rearrangement. Finally, we examine the potential economic and environmental impacts of dynamic covalent polymeric materials in closed-loop processing using elements derived from techno-economic analysis and life-cycle assessment, including minimum selling prices and greenhouse gas emissions. Throughout each section, we discuss interdisciplinary obstacles that hinder the widespread adoption of dynamic polymers and present opportunities and new directions toward the realization of circularity in polymeric materials.

Strong and Tough Supramolecular Covalent Adaptable Networks with Room-Temperature Closed-Loop Recyclability.

Development of closed-loop chemically recyclable plastics (CCRPs) that can be widely used in daily life can be a fundamental solution to the global plastic waste crisis. Hence, it is of great significance to develop easy-to-recycle CCRPs that possess superior or comparable material properties to the commodity plastics. Here, a novel dual crosslinked CCRP, namely, supramolecular covalent adaptable networks (supra-CANs), is reported, which not only displays mechanical properties higher than the strong and tough commodity polycarbonate, but also exhibits excellent solvent resistance as thermosets. The supra-CANs are constructed by introducing reversible noncovalent crosslinks into the dynamic covalent polymer networks, resulting in highly stiff and strong thermosets that also exhibit thermoplastic-like ductile and tough behaviors as well as reprocessability and rehealability. In great contrast, the analogs that do not have noncovalent crosslinks (CANs) show elastomeric properties with significantly decreased mechanical strength. Importantly, the developed supra-CANs and CANs can be converted back into the initial monomers in high yields and purity at room temperature, even with additives, which enables the sustainable polymer-monomer-polymer circulation. This work provides new design principles for high-performance chemically recyclable polymers as sustainable substitutes for the conventional plastics.

A Highly Dynamic Covalent Polymer Network without Creep: Mission Impossible?

Dynamic covalent polymer networks provide an interesting solution to the challenging recyclability of thermosets and elastomers. One of the remaining design constraints, however, is balancing thermal reprocessability in the form of material flow with dimensional stability during use. As a result, many chemistries are being investigated in order to improve bond reactivity control and material robustness. This Minireview highlights a number of promising concepts, with a particular emphasis on disconnecting chemical reactivity in low and high temperature regimes to obtain creep resistant, yet highly dynamic polymer networks. In addition, we will highlight the impact of sharp reactivity changes when applying extrapolation-based approaches during rheological analysis. As a result, we are confident that abandoning the myth of "permanent" reactivity will aid in the development of sustainable polymeric materials that can truly combine the benefits of thermoplastic and thermoset behaviour.

A Highly Dynamic Covalent Polymer Network without Creep: Mission Impossible?

AbstractDynamic covalent polymer networks provide an interesting solution to the challenging recyclability of thermosets and elastomers. One of the remaining design constraints, however, is balancing thermal reprocessability in the form of material flow with dimensional stability during use. As a result, many chemistries are being investigated in order to improve bond reactivity control and material robustness. This Minireview highlights a number of promising concepts, with a particular emphasis on disconnecting chemical reactivity in low and high temperature regimes to obtain creep resistant, yet highly dynamic polymer networks. In addition, we will highlight the impact of sharp reactivity changes when applying extrapolation‐based approaches during rheological analysis. As a result, we are confident that abandoning the myth of “permanent” reactivity will aid in the development of sustainable polymeric materials that can truly combine the benefits of thermoplastic and thermoset behaviour.

Thermadapt Shape Memory Polymers Enabling Spatially Regulated Plasticity.

Converting planar polymer films into sophisticated 3D structures with a facile and effective method is highly challenging yet desirable for device applications in the real world. Dynamic covalent polymer networks enable permanent shape transformations from 2D sheets to 3D structures, but either sophisticated molecular design or a complex fabrication method is required. Here, we report a shape memory polymer cross-linked by ester bonds, which can be activated upon heating after photoexposure to release the catalyst for the transesterification. The region that is activated via the bond exchange can be patterned due to the spatial-temporal selectivity of the photoexposure. Accordingly, the material presents a localized heterogeneity in stress relaxation upon stretching. The exposed and the unexposed regions show respectively plastic deformation and elastic recovery after removal of the external force, which finally make the 2D sheet transform into a 3D structure. The decoupling of the activated region (photoexposure) and activated condition (heating) enables facile chemical design and fabrication for 2D-to-3D shape morphing.

Impact of crosslinker on stereochemistry of a dynamic covalent polymer network: A molecular dynamics simulation

We studied the effect of crosslinking density, ϕc, crosslinker length, m, and crosslinker flexibility on endo to exo adduct ratio in Diels–Alder polymer networks. The fraction of endo attachments, ψendo, increased with an increase of ϕc due to suppression of exo adducts at the early time-limiting stage of the network formation, as also observed in our recently published experimental study [1]. Polymer networks formed with flexible crosslinkers demonstrated a linear growth of ψendo with ϕc for all values of m. ψexo of networks with stiff crosslinkers sharply grew in the range of lower ϕc (ϕc<0.6) and plateaued at ϕc>0.6.

Force–reversible chemical reaction at ambient temperature for designing toughened dynamic covalent polymer networks

Force-reversible C-N bonds, resulting from the click chemistry reaction between triazolinedione (TAD) and indole derivatives, offer exciting opportunities for molecular-level engineering to design materials that respond to mechanical loads. Here, we displayed that TAD-indole adducts, acting as crosslink points in dry-state covalently crosslinked polymers, enable materials to display reversible stress-responsiveness in real time already at ambient temperature. Whereas the exergonic TAD-indole reaction results in the formation of bench-stable adducts, they were shown to dissociate at ambient temperature when embedded in a polymer network and subjected to a stretching force to recover the original products. Moreover, the nascent TAD moiety can spontaneously and immediately be recombined after dissociation with an indole reaction partners at ambient temperature, thus allowing for the adjustment of the polymer segment conformation and the maintenance of the network integrity by force-reversible behaviors. Overall, our strategy represents a general method to create toughened covalently crosslinked polymer materials with simultaneous enhancement of mechanical strength and ductility, which is quite challenging to achieve by conventional chemical methods.

A Strong and Rigid Coordination Adaptable Network that Can Be Reprocessed and Recycled at Mild Conditions

A Strong and Rigid Coordination Adaptable Network that Can Be Reprocessed and Recycled at Mild Conditions

Molecular Dynamics Simulation of the Structural, Mechanical, and Reprocessing Properties of Vitrimers Based on a Dynamic Covalent Polymer Network

Recyclability and reprocessability of permanently cross-linked polymeric materials have received considerable scientific and technological attention in view of the environmental pollution and sustainable development. By introducing dynamic covalent bonds, vitrimers are emerging as a promising attempt to address this pressing challenge. However, there is still a lack of thermodynamic and kinetic understanding of the bond exchange reactions (BERs) of vitrimers at the molecular level. Herein, by employing coarse-grained molecular dynamics simulations, we successfully construct a model vitrimer system composed of a polymer network formed from linear chains, which can rearrange the network topology via BERs. In this study, we examine the effect of the bond swap energy barrier (ΔEsw) on a variety of mechanical properties. We find that ΔEsw critically controls the dynamics of the linear chains and the reactive beads located on the linear chain. Our results indicate that the best mechanical performance characteristics are achieved at an intermediate value of ΔEsw. Meanwhile, stress relaxations are examined for different ΔEsw systems. By performing a triaxial deformation to induce the cavities, the vitrimer exhibits excellent self-healing capability by decreasing ΔEsw, as well as increasing the self-healing time and temperature. Lastly, extrusion of polymer vitrimer is simulated, and we find that the extrusion rate tends to increase linearly as ΔEsw decreases. In general, our results provide rational guidelines for designing high-performance vitrimers with good mechanical properties, excellent self-healing ability, and good reprocessability.

An Orthogonal Dynamic Covalent Polymer Network with Distinctive Topology Transformations for Shape‐ and Molecular Architecture Reconfiguration

AbstractBond exchange in a typical dynamic covalent polymer network allows access to macroscopic shape reconfigurability, but the network architecture is not altered. An alternative possibility is that the network architecture can be designed to switch to various topological states corresponding to different material properties. Achieving both in one network can expand the material scope, but their intrinsically conflicting mechanisms make it challenging. We design a dynamic covalent network that can undergo two orthogonal topological transformations, namely transesterification on the branched chains and olefin metathesis on the mainframe. This allows independent control of the macroscopic shape and molecular architecture. With this design, we illustrate a bottlebrush network with programmable shape and spatially definable mechanical properties. Our strategy paves a way to on‐demand regulation of network polymers.