Developing materials that can adjust their mechanical properties based on external stimuli is crucial in preventing failure and prolonging their service life. However, existing mechanically adaptive polymers are still limited by issues such as insufficient load-bearing capacity, difficulty in achieving reversible changes, high cost, and lack of multiple responsiveness. To address these issues, Cheng-Hui Li’s group utilized dynamic coordination bonds to design novel mechanically adaptive materials with dual stimulus responsiveness (Figure 1).
Figure 1.Schematic and synthetic route of the 3D crosslinking polymer networks based on dynamic coordination bonds under external stimuli and release.
Since coordination interaction usually occurs in equilibrium between its associated and dissociated states, BPA-Fe and BPA-Co coordination complexes are investigated to dissociate dynamically under external stimuli. Based on the reversible fracture-recombination of coordination dynamic crosslinking points under external stimuli, which induces fracture or slip behavior of entangled polymer chain segments and leads to topological rearrangement of the polymer networks, the designed polymers exhibit stimulus-responsive mechanical adaptability. When the impact velocity is lower than the dissociation rate of the coordination bonds, the coordination bond can be regarded as an active bond. On the contrary, the coordination bond is regarded as an inactive bond when the impact velocity exceeds the dissociation rate, thereby affecting the mechanical properties of the polymer. In rheological testing, the modulus of the prepared coordinated crosslinked polymer changed by six orders, indicating significant temperature responsiveness of mechanical strength. In addition, through tensile and compressive stress-strain tests, the Young's modulus and tensile toughness of the polymer also exhibit significant rate-dependent behavior (Figure 2).
Figure 2. Temperature and rate dependent mechanical adaptability of the coordination crosslinking polymers.
Furthermore, the coordinated crosslinking polymers exhibit excellent energy dissipation and damping properties, which demonstrated significant dissipation and elastic recovery under cyclic compression and tension with different strains. The energy dissipation ratios of polymers remain above 40%, with a maximum value of 78.67%. Similar trends were also observed in cyclic tensile tests, with energy dissipation ratios exceeding 80%. In addition, rheological studies showed that polymers can maintain their damping ability over a wide range of frequencies and temperatures (Figure 3). Therefore, the prepared polymers exhibit impact resistance. The steel ball dropping impact tests showed that the obtained polymers PBMBD-Fe and PBMBD-Co can significantly attenuate the impact force of steel balls on the substrates. The polymers can also serve as toughening agents for the mechanical toughening of hard and brittle epoxy resins. Moreover, the temperature responsiveness and mechanical adaptability of the coordination crosslinking polymers make them an ideal choice for 3D printing. By utilizing the 3D printing properties of damping polymers, various customized impact resistant materials can be obtained, which can provide potential applications in different fields, such as the manufacturing of personal protective equipment, aerospace and automotive industries.
Figure 3. Energy dissipation and damping capacities of polymers.
Figure 4. The measurements of anti-impact property of polymers and their toughening and 3D printing abilities.
In summary, the developed materials exhibit temperature-sensitive controllable strength regulation and rate-induced impact hardening behavior. Based on dynamic coordination linkages, the polymers display impressive energy dissipation, damping ability (loss factors of 1.15 and 2.09 at 1.0 Hz, respectively), self-healing and 3D printing capabilities, endowing them with durable and customizable impact resistance and protection properties. This work not only proposes a new strategy of introducing coordination equilibrium in mechanically adaptive materials, but also develops a new generation of impact-resistant materials with comprehensive performance for the field of intelligent protection.
This work innovatively introduces dynamic coordination equilibrium into polymers, obtaining mechanically adaptive materials with excellent comprehensive performance, which provides a new direction for developing sustainable materials. This work entitled Mechanically Adaptive Polymers Constructed from Dynamic Coordination Equilibria was published in Angewandte Chemie International Edition (DOI: 10.1002/anie.202400758). Dr. Zi-Han Zhao is the first author of the article, and Professor Cheng-Hui Li is the corresponding author. This work was supported by the National Natural Science Foundation of China and the Fundamental Research Funds for the Central Universities.