A rigid and healable polymer cross-linked by weak but abundant Zn(II)-carboxylate interactions.

Achieving a desirable combination of solid-like properties and fast self-healing is a great challenge due to slow diffusion dynamics. In this work, we describe a design concept that utilizes weak but abundant coordination bonds to achieve this objective. The designed PDMS polymer, crosslinked by abundant Zn(II)-carboxylate interactions, is very strong and rigid at room temperature. As the coordination equilibrium is sensitive to temperature, the mechanical strength of this polymer rapidly and reversibly changes upon heating or cooling. The soft-rigid switching ability σ, defined as G'max /G'min, can reach 8000 when ΔT = 100 °C. Based on these features, this polymer not only exhibits fast thermal-healing properties, but is also advantageous for various applications such as in orthopedic immobilization, conductive composites/adhesives, and 3D printing.

Distinct Mechanical and Self-Healing Properties in Two Polydimethylsiloxane Coordination Polymers with Fine-Tuned Bond Strength.

Coordination bonds are effective for constructing highly efficient self-healing materials as their strength is highly tunable. To design self-healing polymers with better performance, it is important to get a profound understanding of the structure-property relationships. However, this is challenging for self-healing polymers based on coordination bonds, because many parameters, such as bond energy, bond dynamics, and coordination number will have an essential effect on the mechanical and self-healing properties of the polymer. In this work, we synthesized two poly(dimethylsiloxane) (PDMS) polymers cross-linked by different Zn(II)-diiminopyridine coordination complexes (denoted as PDMS-NNN-Zn, PDMS-MeNNN-Zn respectively). The two cross-linking Zn(II)-diiminopyridine complexes are similar in coordination modes, but differ in coordination dynamics. As manifested by ITC, rheology, and tensile experiments, we confirm that the coordination bond in PDMS-MeNNN-Zn polymer films is weaker but more dynamic. Consequently, the PDMS-MeNNN-Zn polymer has poorer mechanical strength but higher stretchability and better self-healing properties. The inflicted cracks on PDMS-MeNNN-Zn polymer films can be completely healed after healing at room temperature for only 30 min with healing efficiencies higher than 90%. Such fast self-healing properties have never been achieved in self-healing polymers based on coordination bonds. Our results also demonstrate the important impact of the thermodynamic stability and kinetic lability of coordination complexes on the mechanical and self-healing properties of polymers. Such a comprehensive understanding is helpful for further design of novel synthetic polymers, which can achieve an optimal balance between the mechanical strength and self-healing performance.