Exploring Dynamic Disulfide Bonds in Terpene-Derived Covalent Adaptable Network: A Trade-Off between Non-Reprocessable and Reprocessable Elastomeric Epoxy Thermoset

The present study describes the two-step synthesis of thioether-bonded epoxy prepolymer derived from terpene feedstocks such as limonene and geraniol using epichlorohydrin. The synthesized epoxy monomer was thermally cured with various aliphatic diamines, including 1,2-ethanediamine (EDA), 1,4-butanediamine (BDA), and 1,6-hexanediamine (HDA) as conventional curing agents, and cystamine (CYS) as a disulfide-containing biobased diamine to fabricate non-reprocessable and reprocessable elastomeric epoxy thermosets, respectively. In this Perspective, here we have demonstrated the pivotal role of disulfide linkage toward the establishment of covalent adaptable networks (CANs), commonly known as vitrimer. The thermal transition (DSC) of all the materials exhibited single glass transition temperatures ( T g ) in the range of ∼4 < T g < 30 °C and ∼7 < T g < 32 °C, respectively, and displayed reliable thermal stability in the temperature range of 220–250 °C. Furthermore, LME_ECH-EDA and GME_ECH-EDA, as non-reprocessable counterparts, exhibited the highest tensile strengths of 11.6 and 17.9 MPa, respectively, suggesting the influence of the shorter diamine chain length over other conventional diamines in the formation of an irreversible network. In contrast, the reprocessable complement, namely, LME_ECH–CYS and GME_ECH–CYS, exhibited moderate tensile strengths of 3.1 and 3.7 MPa, respectively, accompanied by enhanced elongation at break, indicating a balance between flexibility and mechanical performance. In addition, due to the presence of dynamic disulfide linkage, LME_ECH–CYS and GME_ECH–CYS displayed excellent reprocessing efficiency and weldability performance compared to the irreversible elastomeric thermosets. Moreover, Angell’s fragility plot also emphasized the vitrimeric behavior of reprocessable counterparts elucidated by the slow and linear decrease of viscosity with temperature, like silica as “strong glass former”. Briefly, this work presents a comparative study of terpene-based non-reprocessable and reprocessable elastomeric epoxy thermosets, aiming to create sustainable CANs by integrating dynamic disulfide bonds into the polymer networks, thereby promoting environmental stewardship and resource conservation.