Effect of Methoxy Groups on the Structure–Properties Relationship of Lignin-Derived Recyclable Epoxy Thermosets

A series of renewable lignin-derived imine hardeners (LDIHs) with different contents of methoxy-group (n = 0–2) were successfully prepared using lignin-derivative aldehydes (hydroxybenzaldehyde─M0, vanillin─M1, and syringaldehyde─M2) and cyclohexane diamine, used to cure epoxy monomers. The structural confirmation of LDIHs was performed by Fourier transform infrared spectroscopy (FTIR), gas chromatography–mass spectrometry (GCMS), and NMR spectroscopy. The curing behavior and mechanical and thermal properties of the resulting epoxy thermosets were evaluated by differential scanning calorimetry (DSC), tensile analysis, thermogravimetric analysis (TGA), and dynamic mechanical analyzer (DMA). Enhancing the number of methoxy groups lowered the curing reactivity between the LDIHs and epoxy monomers. Interestingly, the cured epoxy thermosets exhibited significantly distinct mechanical (108–106 MPa of flexural strength and 2.44–2.35 GPa of flexural moduli, 87–83 MPa of tensile strength and 2.6–2.4 GPa of tensile moduli), thermo-mechanical (3.1–2.9 GPa of glassy moduli and 94–82 °C of Tg), and thermal (307–288 °C of Td5%) properties due to the incorporation of varying number of methoxy groups in LDIHs. This is because of the oxygen content and electron-donating effects of high-methoxy-content thermosets. Moreover, these three cured LDIHs can be reprocessed and repaired several times without losing their mechanical and thermal properties due to incorporating dynamic imine bond into their molecular structures. Notably, the pendant methoxy groups attached in LDIHs significantly influences the dynamic performances of the cured thermosets and M2 displays the quickest stress relaxation level. Finally, all of the cured products showed excellent chemical stability in various solvents. These remarkable findings offer important clues for comprehending the structure–property link of recyclable epoxy thermosets with methoxy groups.