Ultrafast Self-Healing Elastomer with Closed-Loop Recyclability.

The loss of function after prolonged periods of use is inevitable for all materials including plastics. Hence, self-healing capabilities are a key development to prolong the service lifetime of materials. One of such self-healing capabilities can be achieved by integrating dynamic bonds such as boronic ester linkages into polymeric materials, however the rate of self-healing in these materials is insufficient and current methods to accelerate it are limited. In this study, we report the rational design, synthesis and characterization of a fluorinated elastomer (FBE15) that utilizes enhanced interaction between polymer chains afforded by strong dipole-dipole interactions from -CF3, which showed a significant increase in binding energy to -7.71 Kcal/mol from -5.51 Kcal/mol, resulting in increased interaction between the boronic ester linkages and improving self-healing capabilities of boronic ester materials, drastically reducing the time required for stress relaxation by 900 %. The bulk elastomer is capable of ultrafast self-healing in a one-click fashion that can happen in mere seconds, which can then be stretched to 150 % of its original length. By utilising the dynamic cross-linking, FBE15 is also capable of both mechanical reprocessing into the same materials and chemical recycling into its starting materials, respectively, further allowing reconstruction of the elastomers that have comparable properties to the original ones at the end of its service lifespan.

Knoevenagel C═C Metathesis Enabled Glassy Vitrimers with High Rigidity, Toughness, and Malleability.

Thermosets, characterized by their permanent cross-linked networks, present significant challenges in recyclability and brittleness. In this work, we explore a polarized Knoevenagel C═C metathesis reaction for the development of rigid yet tough and malleable thermosets. Initial investigation on small molecule model reactions reveals the feasibility of conducting the base-catalyzed C═C metathesis reaction in a solvent-free environment. Subsequently, thermosetting poly(α-cyanocinnamate)s (PCCs) were synthesized via Knoevenagel condensation between a triarm cyanoacetate star and a dialdehyde. The thermal and mechanical properties of the developed PCCs can be easily modulated by altering the structure of the dialdehyde. Remarkably, the introduction of ether groups into the PCC leads to a combination of high rigidity and toughness with Young's modulus of ∼1590 MPa, an elongation at break of ∼79%, and a toughness reaching ∼30 MJ m3. These values are competitive to traditional thermosets, in Young's modulus but far exceed them in ductility and toughness. Moreover, the C═C metathesis facilitates stress relaxation within the bulk polymer networks, thus rendering PCCs excellent malleability and reprocessability. This work overcomes the traditional limitations of thermosets, introducing groundbreaking insights for the design of rigid yet tough and malleable thermosets, and contributing significantly to the sustainability of materials.

Recyclable, Malleable, and Strong Thermosets Enabled by Knoevenagel Adducts.

Plastic recycling is critical for waste management and achieving a circular economy, but it entails difficult trade-offs between performance and recyclability. Here, we report a thermoset, poly(α-cyanocinnamate) (PCC), synthesized using Knoevenagel condensation between terephthalaldehyde (TPA) and a triarm cyanoacetate star, that tackles this difficulty by harnessing its intrinsically conjugated and dynamic chemical characteristics. PCCs exhibit extraordinary thermal and mechanical properties with a typical Tg of ∼178 °C, Young's modulus of 3.8 GPa, and tensile strength of 102 MPa, along with remarkable flexibility and dimensional and chemical stabilities. Furthermore, end-of-life PCCs can be selectively degraded and partially recycled back into one starting monomer TPA for a new production cycle or reprocessed through dynamic exchange aided by cyanoacetate chain-ends. This study lays the scientific groundwork for the design of robust and recyclable thermosets, with transformative potential in plastic engineering.

Catalyst-Free α-Acetyl Cinnamate/Acetoacetate Exchange to Enable High Creep-Resistant Vitrimers.

Vitrimers represent an emerging class of polymeric materials that combine the desirable characteristics of both thermoplastics and thermosets achieved through the design of dynamic covalent bonds within the polymer networks. However, these materials are prone to creep due to the inherent instability of dynamic covalent bonds. Consequently, there are pressing demands for the development of robust and stable dynamic covalent chemistries. Here, we report a catalyst-free α-acetyl cinnamate/acetoacetate (α-AC/A) exchange reaction to develop vitrimers with remarkable creep resistance. Small-molecule model studies revealed that the α-AC/A exchange occurred at temperatures above 140 °C in bulk, whereas at 120 °C, this reaction was absent. For demonstration in the case of polymers, copolymers derived from common vinyl monomers were crosslinked with terephthalaldehyde to produce α-AC/A vitrimers with tunable thermal and mechanical performance. All resulting α-AC/A vitrimers exhibited high stability, especially in terms of creep resistance at 120 °C, while retaining commendable reprocessability when subjected to high temperatures. This work showcases the α-AC/A exchange reaction as a novel and robust dynamic covalent chemistry capable of imparting both reprocessability and high stability to cross-linked networks.

Crystallizable Aliphatic Chains Enhanced Covalent Adaptable Networks: Fast Reprocessing and Improved Performance.

Covalent adaptable networks (CANs) exhibit recyclability such as reprocessing, but it's a challenge to address the contradiction between reprocessing rate and performance. Here, pendent aliphatic chain anhydride monoesters are innovatively introduced into epoxy CANs based on transesterification, which efficiently accelerates the reprocessing without sacrificing thermal and mechanical properties. The transesterification rate is raised on account of the flexible aliphatic chain-promoted segment movement and dynamic transfer auto-catalysis. When the carbon number reflecting the length of the pendent chain is 12, the epoxy CAN exhibits the fastest stress relaxation or reprocessing. Computation via molecular dynamics simulation demonstrates that the increased segmental mobility from the pendent aliphatic chains contributes to enhanced reprocessability. Interestingly, the crystallization of the pendent aliphatic chains maintains or even improves the thermal and mechanical properties. Thus, introducing a flexible and crystallizable aliphatic side chain is an innovative and efficient approach to accelerate dynamic reactions and network arrangement while improving performance.

Fast-Reprocessing, Postadjustable, Self-Healing Covalent Adaptable Networks with Schiff Base and Diels-Alder Adduct.

Covalent adaptable networks (CANs) are a new type of polymers, which possess excellent performance of thermosets and reprocessability of thermoplastics. Nevertheless, it is still a challenge to realize rapid reprocessing and postadjusting (adjust properties after preparation). Herein, for the first time, a method of combining Schiff base and Diels-Alder adduct in one network is developed to achieve rapid reprocessing and postadjusting. Through the dissociation of the Diels-Alder adduct at high temperatures, the cross-link densities of the networks are reduced, thereby accelerating the rearrangement of the networks and realizing the rapid reprocessing and self-healing. Moreover, the reconnecting degree of network after dissociation of Diels-Alder adduct can be easily controlled by annealing; as a result, the properties of the obtained CANs are postadjustable. This work provides a simple and promising approach of achieving excellent reprocessing and postadjusting for CANs via the synergism of an associative dynamic chemistry with a dissociative dynamic chemistry.