Synthesis and properties of biodegradable aliphatic–aromatic polyesters derived from 4-hydroxybenzaldehyde

A novel biphenyl monomer, diethyl 6,6′-dimethoxy-[1,1′-bipheny]-3,3′-dicarboxylate (DDBD), was synthesized from 4-hydroxybenzaldehyde, a phenolic compound derived from lignin. A series of polyesters were prepared by polycondensation of the biphenyl monomer with hydroquinone bis(2-hydroxyethyl) ether and aliphatic diacids via a two-step melt polycondensation method. It was shown that the properties of the polyesters can be tuned by varying the length and the structure of the spacer in the aliphatic diacid monomers. Structure–property relations for these materials were investigated by Fourier-transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, gel permeation chromatography (GPC), differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical analysis. GPC analysis showed that the weight-average molecular weight ( M w ) of the samples varied from 4.2–5.0 × 10 4 g/mol, while thermal analysis yielded T g values of 51–120°C, and 5% decomposition temperatures ( T d,5% ) of 327–389°C. All the samples exhibited a high yield strength (56–87 MPa). After 30 weeks of degradation in soil, the copolyesters had mass losses of up to 4.4%, and an investigation of the ecotoxicity of the copolyesters with Eisenia fetida yielded a survival rate of over 80% after 14 days for the different copolyesters, suggesting low toxicity in the environment. Overall, these copolyesters have good properties and would be suitable to replace petroleum-based commercial poly(ethylene terephthalate) packaging materials.