Synthesis and Properties of Degradable Polyesters Based on a Lignin Derivative, 4-Hydroxybenzoic Acid

A novel diester monomer synthesized from 4-hydroxybenzoic acid, a compound which can be derived from lignin, was used to obtain aliphatic-aromatic copolyesters (P1–P4) by melt polymerization with 1,4-cyclohexanedimethanol, and either 1,4-butanedioic acid, 1,6-hexanedioic acid, 1,8-octanedioic acid, or 1,12-dodecanedioic acid as aliphatic diacid. Structure-property relations for the copolyesters were established using FTIR and ¹H NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis and tensile testing. The weight-average molecular weight (M_w) of the samples ranged from 41,400 to 48,300 g/mol. As the spacer length in the aliphatic diacid was increased, the glass transition temperature (T_g) decreased from 90 to 51 °C, the melting point (T_m) from 175 to 147 °C, the tensile modulus from 1800 to 980 MPa, and the yield strength from 76 to 54 MPa, while the elongation at break increased from 270 to 320%. The thermal stability of the copolyesters also decreased for longer aliphatic diacid spacers. The copolyester derived from 1,12-dodecanedioic acid displayed the highest degradation rate in artificial soil, with 4.4% degradation after 30 weeks, and low ecotoxicity in an earthworm viability test. These findings contribute to understanding structure-property relationships and the environmental impact of aliphatic-aromatic copolyesters as potential sustainable materials.