Biologically Derived Metal–Cysteine Coordination Complexes Crosslink Carboxylated Nitrile Rubber and Enable Room Temperature Self-Healing, Stretchability, and Recyclability

In this study, we investigated a network of carboxylated nitrile rubber crosslinked by biologically derived coordination complexes that possesses good room temperature self-healing properties in addition to high tensile strength, stretchability, and recyclability. First, we showed the synthesis and analysis of coordination complexes composed of two metal salts (nickel nitrate and zinc nitrate), whose corresponding thiol and amino groups engage with the l-cysteine amino acid to produce the complex. Infrared spectrum, X-ray pattern, mass spectroscopy, energy-dispersive X-ray analyses (EDX), and morphology (SEM and TEM) analysis have all been used to characterize the metal–cysteine complexes. These Ni–cysteine and Zn–cysteine complexes have an apparent behavior after addition to XNBR rubber, as observed by several investigations (including swelling experiment, rubber process analysis, universal testing machine (UTM) analysis, and morphological analysis). Therefore, compared to the Ni-cysteine- and pristine cysteine-cured XNBR compounds, the Zn–cysteine complex-cured XNBR compound showed extreme stretchability, recyclability, and strong tensile strength of 3.8 ± 0.2 MPa. It also had a remarkable healing performance of 89.5%. This concept is strongly approved by the increased physico-mechanical properties of XNBR rubber and the recyclability with self-healing capability.