Renewable Epoxy Thermosets from Fully Lignin-Derived Triphenols

A series of fully renewable triphenols (TPs) with various number of methoxy group substituents (n = 0–6) were synthesized using lignin-derived phenols (guaiacol and 2,6-dimethoxyphenol) and aldehydes (4-hydroxybenzaldehyde, vanillin, and syringaldehyde). The structural evolution from TPs to epoxy thermosets was followed by nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. Thermomechanical properties of the resulting epoxy thermosets were investigated by dynamic mechanical analysis (DMA), tensile analysis (TA), and thermogravimetric analysis (TGA). Increasing the content of methoxy groups decreased the glass transition temperature (132–118 °C) and glassy modulus (2.7–2.2 GPa). Thermal stability of high-methoxy-content thermosets was reduced due to electron-donating effects and higher oxygen content. Conversions and isolated yields of TPs significantly decreased as the number of methoxy substituents increased, which markedly determined the feasibility of TPs as precursors for polymers. This work widens the synthesis route of fully lignin-derived polyphenols, yielding polymers with thermomechanical properties comparable to bisphenol A (BPA) based materials. Finally, evaluation of methoxy substitution provides insight for the selection of lignin-derived monomers.