Structure–Property Relationships of Renewable Ternary Polyesters Derived from Vanillin-Based Methyl Ester, Aliphatic Diacids, and Phenol Ether-Based Diols

Abstract

As sustainable polymer development gains urgency, vanillin, a lignin-derived renewable compound, has emerged as a promising aromatic building block. Capitalizing on phenyl structure, a novel biphenyl diester monomer (DEBM) was prepared from bio-sourced methyl vanillate. Subsequently, via melt polymerization DEBM along with phenol ether-based diols (hydroquinone bis(2-hydroxyethyl)ether (HQEE) and 1,3-bis(2-hydroxyethoxy)benzene (HBE)) and aliphatic diacids (succinic acid and adipic acid), afforded a series of ternary aromatic-aliphatic copolyesters (P₁–P₄). The chemical structures were characterized using Fourier transform infrared (FT-IR) and nuclear magnetic resonance (¹H NMR) spectroscopy, molecular weight via gel permeation chromatography (GPC), thermal transitions and stability through differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermo-mechanical properties assessed by dynamic mechanical analysis (DMA) and tensile testing. From the results, the copolyesters were semi-crystalline in nature, with weight-average molecular weights (M_w) ranging from 3.55 to 4.9 × 10⁴ g/mol and polydispersity (PDI) within 1.58–2.13. The glass transition temperatures (T_g) varied between 57.5 and 77.1 °C, melting point (T_m) from 166.5 to 190.5 °C, and initial decomposition temperatures (T_{d, 5%}) within 370.3–389.5 °C, highlighting the satisfactory thermal stability of the copolyesters. Tensile testing demonstrated robust mechanical strength (42–47 MPa) along with elongation at break (284–316%), outperforming conventional polyesters and comparable to poly(ethylene terephthalate) (PET). Furthermore, hydrolytic degradation under varying pH (7 and 10) over 16 weeks revealed significantly advanced decomposition rates at pH 10 compared to pH 7, owing to rapid hydrolysis of ester and ether functionality, resulting in modest weight loss between 2.9 and 4.5%. By integrating flexible aliphatic diacid segments with varying aromatic components, one can tune the aromaticity and physicochemical properties of copolyesters, offering a sustainable alternative to petroleum-derived counterparts. These copolyesters could potentially become of great interest to industrial automobiles, specifically friction plates, while significantly addressing ecological concerns related to material decomposition after their end of life.