Closed-loop recycling of tough epoxy supramolecular thermosets constructed with hyperbranched topological structure

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

High-level hierarchical morphology reinforcing covalent adaptable networks

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

Programming degradation and closed-loop recycling of ultra-stable bio-based thermosetting polyurethane relied on double-locked covalent adaptable networks

Covalent adaptable networks (CANs), which can reconfigure on-demand under photo- or thermal stimuli, have recently been pursued as an alternative to the traditional thermosetting polymers. While these materials have demonstrated excellent recyclability and reprocessability, the majority of them reported to date are based on non-renewable resources. Meanwhile, material recycling highly counts on the collection system, and any materials that inevitably escape from the collection system will eventually go to the environment, challenging nature's ability to break down these materials. Therefore, CAN materials that possess both recyclability and degradability are highly desirable. In this work, we seek to simultaneously address the recyclability, renewability, and degradability of CAN materials. Spiro diacetal building blocks are derived from bio-based benzaldehyde and erythritol and then subjected to the curing process using bio-based epoxy soybean oil as crosslinkers, yielding fully biobased CAN materials. Owing to the dynamic and degradable features of acetal motifs, our CAN materials exhibit both good recyclability and acid degradability, and the degraded products are reusable for preparation of new CANs. In addition, by tuning the steric hindrance adjacent to the reactive phenol site, we are able to control the mechanical properties of CANs using different bio-based benzaldehydes (vanillin, ethyl vanillin, and syringaldehyde). The outcome of the current research provides a strategy for the design of recyclable and degradable bio-based CANs, which will extend the development of CANs.

Self-healing covalent adaptable networks (CANs) are not only of fundamental interest but also of practical importance for achieving carbon neutrality and sustainable development. However, there is a trade-off between the mobility and cross-linking structure of CANs, making it challenging to develop CANs with excellent mechanical properties and high self-healing efficiency. Here, we report the utilization of a highly dynamic four-arm cross-linking unit with an internally catalyzed oxime-urethane group to obtain CAN-based ionogel with both high self-healing efficiency (>92.1%) at room temperature and superior mechanical properties (tensile strength 4.55 MPa and toughness 13.49 MJ m-3). This work demonstrates the significant potential of utilizing the synergistic electronic, spatial, and topological effects as a design strategy for developing high-performance materials.

Covalent adaptable networks (CANs) are being developed as future replacements for thermosets as they can retain the high mechanical and chemical robustness inherent to thermosets but also integrate the possibility of reprocessing after material use. Here, covalent adaptable polyimine-based networks were designed with methoxy and allyloxy-substituted divanillin as a core component together with long flexible aliphatic fatty acid-based amines and a short rigid chain triamine, yielding CANs with a high renewable content. The designed series of CANs with reversible imine functionality allowed for fast stress relaxation and tailorability of the thermomechanical properties, as a result of the ratio between long flexible and short rigid amines, with tensile strength (σb) ranging 1.07-18.7 MPa and glass transition temperatures ranging 16-61 °C. The CANs were subsequently successfully reprocessed up to three times without determinantal structure alterations and retained mechanical performance. The CANs were also successfully chemically recycled under acidic conditions, where the starting divanillin monomer was recovered and utilized for the synthesis of a recycled CAN with similar thermal and mechanical properties. This promising class of thermosets bearing sustainable dynamic functionalities opens a window of opportunity for the progressive replacement of fossil-based thermosets.

Traditional epoxy thermosets cannot be reprocessed or recycled due to their permanent covalent cross-linking network. Covalent adaptable networks (CANs) emerge as a solution, endowing epoxy thermosets with recyclability, reprocessability and self-healing ability to tackle the recycling issue. Nevertheless, the existing covalent adaptable epoxy network exhibits low mechanical robustness, glass transition temperature and thermal stability. Herein, we have developed a covalent adaptable epoxy network based on dynamic amine terminated hyperbranched polyamide (AHPA) to fabricate catalyst-free and high-performance epoxy vitrimers. The incorporation of thermoactivated rearrangement of AHPA enables the obtained epoxy vitrimers to possess remarkable reprocessability, along with good thermal stability, high glass transition temperature and excellent creep resistance. The epoxy vitrimers can be easily reprocessed without compromising thermal and mechanical properties even after multiple cycles, presenting a promising design of dynamic hyperbranched polymers for constructing adaptive and recyclable epoxy thermosets for sustainable engineering applications.

Catalyst-free readily dual-recyclable acetal-based covalent adaptable cellulose networks

Covalent adaptable networks (CANs) relying on dynamic cross-links have been developed to make the cross-linked polymeric materials and composites degradable. However, due to the reversibility of dynamic bonds, the CANs and composites suffer accidental degradation and failure upon a certain stimulus (moisture, acid/base, reductant/oxidant, etc.) in the application environment. Herein, inspired by parallel circuits, interlocked covalent adaptable networks (ICANs) were prepared by one-pot reactions from epoxy monomers and two curing agents that contained different dynamic bonds of aromatic disulfide and aromatic imine bonds, resulting in dual dynamic parallel cross-links in homogeneous epoxy networks. The ICANs exhibited outstanding mechanical properties and improved stability, relying on the topological interlocking structure. The ICANs could be unlocked and became degradable only when two stimuli were both applied to completely break the cross-links of disulfide bonds and imine bonds. When applying ICANs as a matrix to form carbon fiber-reinforced polymer (CFRP) composites, the resulted CFRP inherited the interlocking properties from the ICANs, exhibiting improved stability and nondestructive recyclability. Maintaining the degradable properties, the interlocking structure of networks provided a facile way to optimize the stability of CANs and their composites.

Covalent adaptable networks (CANs) are polymer materials that are covalently cross-linked via dynamic covalent bonds. The cross-linked polymer network is generally expected to be insoluble, as is seen for traditional thermosets. However, in recent years, it has become apparent that-under certain conditions-both dissociative and associative CANs can be dissolved in a good solvent. For some applications (e.g., those that require long-term (chemical) stability), the solubility of CANs can be problematic. However, many forget that (selective) solubility of CANs can also be applied advantageously, for example, in recycling or modification of the materials. In this work, we provide results and insights related to the tunable solubility of imine-based CANs. We observed that selected CANs could be fully dissolved in a good solvent without observing dissociation of imines. Only in an acidic environment (partial) dissociation of imines was observed, which could be reverted to the associated state by addition of a base. By adjusting the network composition, we were able to either facilitate or hamper solubility as well as control the size of the dissolved particles. DLS showed that the size of dissolved polymer particles decreased at lower concentrations. Similarly, decreasing cross-linking density resulted in smaller particles. Last, we showed that we could use the solubility of the CANs as a means for chemical recycling and postpolymerization modification. The combination of our studies with existing literature provides a better understanding of the solubility of CANs and their applications as recyclable thermosets.

Most covalent adaptable networks (CANs) based on citric acid (CA) exhibit low thermostability, low glass-transition temperatures (Tg), and poor mechanical properties. Moreover, their slow relaxation rates necessitate prolonged reprocessing time, resulting in issues such as degradation or side-reactions. Herein, a bicyclic tetracarboxylate (DMTE) derived from CA was prepared. A series of polyester CANs were prepared via catalyst-free melt-polycondensation of DMTE, CA, and 1,6-hexanediol (HDO). To our delight, the highly rigid DMTE simultaneously improved the Tg values and mechanical properties of the CANs based on CA. Furthermore, benefiting from its dissociative transesterification reaction (TER), the introduction of DMTE significantly improved their relaxation rates. This allows their reprocessing time to be significantly shortened to merely 10 s at 180 °C, compared to the long reprocessing time needed for the CAN solely based on CA (30 min). Lastly, these materials can be closed-loop recycled by catalyst-free methanolysis to recover the initial monomers in high yields.

In the state-of-the-art membrane industry, membranes have linear life cycles and are commonly disposed of by landfill or incineration, sacrificing their sustainability. To date, little or no thought is given in the design phase to the end-of-life management of membranes. For the first time, we have innovated high-performance sustainable membranes, which can be closed-loop recycled after long-term usage for water purification. By synergizing membrane technology and dynamic covalent chemistry, covalent adaptable networks (CANs) with thermally reversible Diels-Alder (DA) adducts were synthesized and employed to fabricate integrally skinned asymmetric membranes via the nonsolvent-induced phase separation technique. Due to the stable and reversible features of CAN, the closed-loop recyclable membranes exhibit excellent mechanical properties and thermal and chemical stabilities as well as separation performance, which are comparable to or even higher than the state-of-the-art nonrecyclable membranes. Moreover, the used membranes can be closed-loop recycled with consistent properties and separation performance by depolymerization to remove contaminants, followed by refabrication into new membranes through the dissociation and reformation of DA adducts. This study may fill in the gaps in closed-loop recycling of membranes and inspire the advancement of sustainable membranes for a green membrane industry.

Fusion of immiscible covalent adaptable networks rooted in Enamine-One and β-Amino ester bonds with robust mechanically and excellent re-processing properties

Covalent adaptable networks (CANs) are cross-linked polymers that have mechanical properties similar to thermosets at operating conditions yet can be reprocessed by cross-link exchange reactions that are activated by a stimulus. Although CAN exchange dynamics have been studied for many polymer compositions, the tensile properties of these demonstration systems are often inferior compared to those of commercial thermosets. In this study, we explore toughening CANs capable of forming covalent bonds with a reactive filler to characterize the trade-off between improved toughness and longer reprocessing times. Polycarbonate (PC) and polyurethane (PU) CANs were toughened by incorporating cellulose modified with cyclic carbonate groups as a reactive filler with loadings from 1.3 to 6.6 wt %. The addition of 6.6 wt % of the cellulose derivative resulted in a 3.2-fold increase in average toughness for the PC CANs, yet it only increased the characteristic relaxation time of stress relaxation (τ*) via disulfide exchange at 180 °C from 63 to 365 s. The cellulose-containing samples also showed >80% recovery in crosslinking density and mechanical properties after reprocessing. The addition of 3.2 wt % of the functionalized cellulose into a polyethylene glycol-based PU CAN led to a 2.3-fold increase in toughness while increasing τ* at 140 °C from 106 to 157 s. These findings demonstrate the promise of functionalized cellulose as an inexpensive, renewable, and sustainable filler that toughens CANs containing hydroxyl groups.