Scheme 2. The proposed curing process of ESO. (a) The reaction between phenol and epoxy via anion addition; (b) the reaction between 2,4-substituted imidazole and epoxy via two-step nucleophilic addition; (c) initiation by an MZ/phenol complex and a 1 : 1 adduct/phenol complex generated from ionic interactions between TA and the pyridine-type nitrogen in histidine (H) and a 1 : 1 adduct of H and epoxy, respectively, and (d) a schematic diagram of the curing networks of ESO and TA at the presence of H.