Chapter 15 - Recyclable thermoset polymers: beyond self-healing

Increasing the cross-link density in a dual dissociative and associative polythiourethane covalent adaptable network improves both creep resistance and extrudability

High-level hierarchical morphology reinforcing covalent adaptable networks

Covalent adaptable networks (CANs), unlike typical thermosets or other covalently crosslinked networks, possess a unique, often dormant ability to activate one or more forms of stimuli-responsive, dynamic covalent chemistries as a means to transition their behavior from that of a viscoelastic solid to a material with fluid-like plastic flow. Upon application of a stimulus, such as light or other irradiation, temperature, or even a distinct chemical signal, the CAN responds by transforming to a state of temporal plasticity through activation of either reversible addition or reversible bond exchange, either of which allows the material to essentially re-equilibrate to an altered set of conditions that are distinct from those in which the original covalently crosslinked network is formed, often simultaneously enabling a new and distinct shape, function, and characteristics. As such, CANs span the divide between thermosets and thermoplastics, thus offering unprecedented possibilities for innovation in polymer and materials science. Without attempting to comprehensively review the literature, recent developments in CANs are discussed here with an emphasis on the most effective dynamic chemistries that render these materials to be stimuli responsive, enabling features that make CANs more broadly applicable.

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.

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

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.

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.

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.

Sustainable Green Polymerizations and End-of-Life Treatment of Polymers.

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

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

This work estimates that if the growth of polymer production continues at its current rate of 5% each year, the current annual production of 395 million tons of plastic will exceed 1000 million tons by 2039. Only 9% of the plastics that are currently produced are recycled while most of these materials end up in landfills or leak into oceans, thus creating severe environmental challenges. Covalent adaptable networks (CANs) materials can play a significant role in reducing the burden posed by plastics materials on the environment because CANs are reusable and recyclable. This review is focused on recent research related to CANs of polycarbonates, polyesters, polyamides, polyurethanes, and polyurea. In particular, trends in self-healing CANs systems, the market value of these materials, as well as mechanistic insights regarding polycarbonates, polyesters, polyamides, polyurethanes, and polyurea are highlighted in this review. Finally, the challenges and outlook for CANs are described herein.

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

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.

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.