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

Unique Ligand Exchange Dynamics of Metal–Organic Polyhedra for Vitrimer-like Gas Separation Membranes

High-level hierarchical morphology reinforcing covalent adaptable networks

Fabrication of well-dispersed cellulose nanocrystal reinforced biobased epoxy composites using reversibility of covalent adaptable network

Thermally switchable polymers: From thermo-reversibly self-healing hybrid polymers to irreversibly crosslinked flame-retardant networks

ABSTRACTObjective: Modifying linear polyolefin elastomer (POE) into covalent adaptable networks (CANs) leads to improved mechanical properties and stability, while retaining good processability.Methods: In this work, commercially available POE is converted into a dynamically cross‐linked network through free‐radical reactive processing. Two POE‐based CANs are fabricated with free radical initiator dicumyl peroxide (DCP) and two different disulfide‐containing cross‐linkers, namely, one is aromatic (BM‐DS) and the other is aliphatic (DTME).Results: The optimal conditions for CAN formation are investigated, revealing that successful network formation requires the simultaneous addition of both cross‐linker and 0.1 wt.% DCP. Compared to pure POE with a tensile strength of 15 MPa, CANs with 1 wt.% BM‐DS or DTME exhibit enhanced tensile strengths of 23 MPa and 21 MPa, and the best 20‐cycle elastic recovery of 63.5% and 61.9%, respectively. Moreover, the CANs achieve significantly shorter stress relaxation times of 38.8 and 27.8 s, compared to 99 s for the permanently cross‐linked network at 70°C.Conclusion: These results demonstrate that incorporating 1 wt.% BM‐DS or DTME effectively improves both the mechanical strength and dynamic relaxation properties of CANs, while maintaining good processability. This work provides an effective strategy for the development of recyclable polyolefin‐based materials.

Reprocessable and degradable bio-based polyurethane by molecular design engineering with extraordinary mechanical properties for recycling carbon fiber

Recent advances in recyclable thermosets and thermoset composites based on covalent adaptable networks

Aromatic polyimine covalent adaptable networks with superior water and heat resistances

Thermosets are known to be very reliable polymeric materials for high-performance and light-weight applications, due to their retained dimensional stability, chemical inertia and rigidity over a broad range of temperatures. However, once fully cured, they cannot be easily reshaped or reprocessed, thus leaving still unsolved the issues of recycling and the lack of technological flexibility. Vitrimers, introduced by Leibler et al. in 2011, are a valiant step in the direction of bridging the chasm between thermoplastics and thermosets. Owing to their dynamic covalent networks, they can retain mechanical stability and solvent resistance, but can also flow on demand upon heating. More generally, the family of Covalent Adaptable Networks (CANs) is gleaming with astounding potential, thanks to the huge variety of chemistries that may enable bond exchange. Arising from this signature feature, intriguing properties such as self-healing, recyclability and weldability may expand the horizons for thermosets in terms of improved life-span, sustainability and overall enhanced functionality and versatility. In this review, we present a comprehensive overview of the most promising studies featuring CANs and vitrimers specifically, with particular regard for their industrial applications. Investigations into composites and sustainable vitrimers from epoxy-based and elastomeric networks are covered in detail.

Compared to classic thermosets, the reprocessability of covalent adaptable networks (CANs) endowed by dynamic covalent bonds (DCBs) often comes at the expense of mechanical performance and thermomechanical stability. Herein, we report a "High Activity & Low Content" strategy for CANs to achieve superior thermomechanical stability, which is enabled by dynamic N-hydroxyphthalimide-urethane bonds (NUBs). The catalyst-free addition reaction between N-hydroxyphthalimides and isocyanates proceeds to a near-quantitative conversion within 2 h at room temperature in dimethyl sulfoxide, while the formed bonds dissociate even up to ~ 28% at 120 °C. The dual high activity, characterized by a high degree of dissociation and fast association kinetics, allows for an effective reduction in DCB content within CANs while preserving their dynamic characteristics. We incorporate only 5 mol% of dynamic units to develop poly(N-hydroxyphthalimide-urethanes) (PNU) networks. The "High Activity & Low Content" design endows PNUs with superior mechanical properties, exceptional crack tolerance, and remarkable mechanical stability at high temperatures. Furthermore, even with minimal DCB participation, the PNUs exhibit excellent reprocessability and mild degradability in neutral aqueous conditions. This study proposes a compelling strategy that enables CANs to achieve excellent reprocessability while retaining their mechanical strength and thermomechanical robustness-overcoming the traditional trade-off between these properties.

ABSTRACTReprocessable covalent adaptable networks (CANs) offer potential improvements in polymer life‐cycle management, yet balancing mechanical strength with reprocessability remains a critical challenge. Herein, the inherent structural features of a lignin‐derivable precursor, bisguaiacol A (BGA), were leveraged to develop a stiff, non‐isocyanate polyurethane (NIPU) CAN with (thermo)mechanical and reprocessability performance comparable to a bisphenol A (BPA)‐based counterpart. The NIPU CANs were prepared by reacting either a BGA‐ or BPA‐derived cyclic carbonate (BGACC or BPACC) with a trifunctional amine and cystamine to form crosslinked networks containing dynamic disulfide bonds. The methoxy groups in the BGACC‐CAN enabled significantly faster stress relaxation (∼3–5 times) than the BPACC‐CAN, with no loss in creep resistance. Furthermore, the aromatic lignin‐derivable monomer produced a CAN with tensile strength (∼45 MPa) and modulus (∼1.7 GPa) comparable to the petroleum‐derived analog (tensile strength ∼40 MPa, modulus ∼1.5 GPa). These BGA/BPA‐based CANs showed complete recovery of crosslink densities and essential (thermo)mechanical properties (e.g., glass transition temperatures, thermal stabilities, tensile strengths, moduli) after four reprocessing cycles via compression molding at 130 °C for 45 min. Overall, the lignin‐derivable cyclic carbonate explored in this work serves as a promising building block to design sustainable NIPU CANs that achieve structural robustness and reprocessability.

Covalent adaptable networks (CANs) are widely used in the field of self-repair materials. They are a group of covalently cross-linked associative polymers that undergo reversible chemical reactions, and can be further divided into dissociative CANs (Diss-CANs) and associative CANs (Asso-CANs). Self-repair refers to the ability of a material to repair itself without external intervention, and can be classified into self-adhesion and self-healing according to the utilization of open stickers. Unlike conventional materials, the viscoelastic properties of CANs are influenced by both the molecular structure and reaction kinetics, ultimately affecting their repair performance. To gain deeper insight into the repair mechanism of CANs, we conducted simulations by using the hybrid MC/MD algorithm, as previously proposed in our research. Interestingly, we observed a significant correlation between reaction kinetics and repair behavior. Asso-CANs exhibited strong mechanical strength and high creep resistance, rendering them suitable as self-adhesion materials. On the other hand, Diss-CANs formed open stickers that facilitated local relaxation, aligning perfectly with self-healing processes. Moreover, the introduction of crosslinkers in the form of small molecules enhanced the repair efficiency. Theoretically, it was found that the repair timescale of Asso-CANs is slower than that of Diss-CANs with identical molecular structures. Our study not only clarifies the similarities and differences between Diss-CANs and Asso-CANs in terms of their self-repairing capabilities, but more importantly, it provides valuable insights guiding the effective utilization of CANs in the development of self-repair materials.

Thermoset polymers show favorable material properties, while bringing about environmental pollution due to non-reprocessing and unrecyclable. Diels–Alder (DA) chemistry or reversible exchange boronic ester bonds have been employed to fabricate recycled polymers with covalent adaptable networks (CANs). Herein, a novel type of CANs with multiple dynamic linkers (DA chemistry and boronic ester bonds) was firstly constructed based on a linear copolymer of styrene and furfuryl methacrylate and boronic ester crosslinker. Thermoplastic polyurethane is introduced into the CANs to give a semi Interpenetrating Polymer Networks (semi IPNs) to enhance the properties of the CANs. We describe the synthesis and dynamic properties of semi IPNs. Because of the DA reaction and transesterification of boronic ester bonds, the topologies of semi IPNs can be altered, contributing to the reprocessing, self-healing, welding, and shape memory behaviors of the produced polymer. Through a microinjection technique, the cut samples of the semi IPNs can be reshaped and mechanical properties of the recycled samples can be well-restored after being remolded at 190 °C for 5 min.

Covalent adaptable networks (CANs) can replace classical thermosets, as their unique dynamic covalent bonds enable recyclable crosslinked polymers. Their creep susceptibility, however, hampers their application. Herein, an efficient strategy to enhance creep resistance of CANs via metal coordination to dynamic covalent imines is demonstrated. Crucially, the coordination bonds not only form additional crosslinks, but also affect the imine exchange. This dual effect results in enhanced glass transition temperature (Tg ), elasticmodulus (G') and creep resistance. The robustness of metal coordination is demonstrated by varying metal ion, counter anion, and coordinating imine ligand. All variations in metal or anion significantly enhance the material properties. The Tg and G' of the CANs are correlated to the coordination bond strength, offering a tunable handle by which choice of metal can steer material properties. Additionally, large differences in Tg and G' are observed for materials with different anions, which are mostly linked to the anion size. This serves as a reminder that for coordination chemistry in the bulk, not only the metal ion is to be considered, but also the accompanying anion. Finally, the reinforcing effect of metal coordination is proved insensitive to the metal-ligand ratio, emphasizing the robustness of the applied method.

The development of polymeric nanocomposites with dynamic covalent adaptable networks and biobased nanomaterials has been a promising approach toward sustainable advanced materials, enabling reprogramming and recycling capabilities. Herein, a core-shell nanohybrid of functionalized cellulose nanocrystals (CNCs) is explored to provide crucial interfacial compatibility for improving the covalent adaptable networks of epoxy-thiol vitrimers in fracture resistance. The poly(ε-caprolactone) (PCL) shells grafted from CNC surfaces can be cross-linked with the covalent adaptable networks via a hot-pressing transesterification process. According to the additive concentration and annealing temperature, the stress relaxation behavior of nanohybrid vitrimer composites can be effectively regulated by the core-shell PCL-grafted CNC (CNC-PCL) nanohybrids from a dispersed to cross-linked interaction. The addition of 15 wt % of the core-shell CNC-PCLs exhibits the reinforced improvement of nanohybrid vitrimer composites in the average Young's modulus of 2.5×, fracture stress of 5.4×, and fracture strain of 2.0×. The research findings might have profound implications for developing synergistic interfacial compatibility between dynamic vitrimer networks and functional nanoparticles for advanced polymeric nanocomposites.