Investigating Thermomechanical Behavior, Molecular and Dynamics of Rigid-Flexible Bio-based Copolyesters

Abstract

Petroleum-based plastics contribute significantly to global pollution due to their poor degradability, highlighting an urgent need for green alternatives that simultaneously offer thermomechanical performance comparable to commercially available plastics and biodegradability after their end of life. To address the issue, a novel series of lignin-based aromatic-aliphatic copolyesters (M1–M4) was synthesized via polymer chain tuning polymerization. This process utilized methyl 4-(oxybenzoate)benzoate (MBOB), a diester containing free phenolic hydroxyl groups synthesized via Williamson etherification, hydroquinone bis(2-hydroxyethyl) ether (HQEE) as a chain extender, and various aliphatic diacids (succinic-C4, adipic-C6, suberic-C8, dodecanedioic-C12) to adjust the physicochemical properties. The copolyesters exhibited well-controlled molecular weight characteristics (Mw: 4.25–4.65 ×104 g/mol, PDI: 2.22–2.48), while demonstrating semi-crystalline behaviors (18–36%). Their thermal (glass transition temperature: 53–91 °C and melting temperature: 142–178 °C) and mechanical properties (tensile strength: 50–67 MPa; elongation at break: 270–320%) were found to depend on the carbon chain length of aliphatic spacers, rivaling commercially available polyesters such as poly(ethylene terephthalate) (PET), poly(butylene succinate) (PBS), and poly(lactic acid) (PLA). Additionally, biochemical oxygen demand (BOD) assessments revealed satisfactory biodegradation rates (2.95–4.61% in soil, 3.05–7.18% in pond water, and 8.51–12.75% in seawater), with M4 showcasing a 45.7% weight loss, owing to accessible polymer chains enhanced via microbial activity and salt-catalyzed hydrolysis. Through molecular dynamics, the degradable chain segments (MBOB-HQEE, Al-MBOB, and Al-HQEE) conformations and kinetics were revealed, with their free energy increasing (1.2-fold) in seawater due to induced hydrolysis. These results indicate that M1–M4 copolyesters effectively combine high thermal properties, toughness, and good biodegradability, showcasing their significant potential for applications in food packaging and related fields.