Ligand-Regulated Fe-coordinated Dynamic Networks in Epoxidized Natural Rubber for Mechanical Recovery and Relaxation Recovery

Ferric chloride (FeCl 3 ) coordination networks were fabricated in epoxidized natural rubber (ENR) containing 50 mol% epoxide groups (E50) to regulate the effective crosslink density and re-bonding behavior. X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) revealed the coexistence of ferrous ion (Fe 2+ ) and ferric ion (Fe 3+ ) species in the E50 matrix with different compositions after the processing. The network structure was regulated by incorporating imidazole (IM) and 2,6-diaminopyridine (DP). IM grafted onto ENR chains during mixing, generating ionic bonds along the backbone and introducing reversible interactions that facilitate network rearrangement and relaxation behavior. In contrast, DP increased interchain spacing and free volume, facilitating segmental mobility without disrupting Fe–O coordination crosslinks. The combined IM/DP system created a hybrid network integrating coordination crosslinks, ionic interactions, and controlled chain spacing. Temperature scanning stress relaxation (TSSR) showed that the elastic modulus scaled with effective crosslink density, while high temperature relaxation was governed by thermally activated coordination dissociation. The IM/DP system exhibited a broader relaxation spectrum, indicating a wider distribution of interaction strengths and more controlled stress dissipation. Thermogravimetric analysis confirmed reversible bond dissociation and reformation in the 260–330°C region. Mechanically, the hybrid network achieved a balanced combination of stiffness and extensibility and demonstrated the highest recovery of Young’s modulus and tensile strength after damage. These findings establish that coordinated regulation of crosslinks, ionic bonding, and chain spacing enables mechanical properties and dynamically reversible ENR elastomers.