Structure–property of copolyesters with potential applications in optical communication

A series of novel biobased ternary copolyesters (P₁–P₄) was synthesized via two-step melt polycondensation process using bis(2-(methoxycarbonyl)phenyl) terephthalate (PMPE), derived from methyl salicylate combined with varying aliphatic chain diols including 1,4-butanediol (BDO), 1,6-hexanediol (HDO), 1,8-octanediol (ODO), and 1,4-cyclohexanedimethanol (CHDM). Chemical structures were characterized using Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (¹H NMR), while molecular weights were confirmed via gel permeation chromatography (GPC), and thermomechanical properties were investigated via differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA). The copolyesters exhibited molecular weights (M_w) ranging from 3.9–4.1 × 10⁴ g/mol and PDI from 2.17–2.31. Glass transition temperatures (T_g) varied from 49–88 °C, influenced by the incorporation of stable aliphatic and cyclic chain segments within the polyester backbone, with 5% (T_{d, 5%}) and maximum (T_{d, max}) decomposition temperatures varying between 329–347 °C and 396–412 °C, respectively. Tensile testing showed strength values ranging from 1030–1740 MPa, with yield strengths of 47–58 MPa, comparable to polyethylene terephthalate (PET). Interestingly, the copolyesters exhibited excellent optical transparency with a transmission rate of 88% and low absorbance peaks (0.4–1.1%), indicating strong potential for optical communication applications. Although, biodegradation over 15 weeks of incubation showed modest degradation rates of 2.74–4.03%, ecotoxicity assessment using earthworms confirmed low toxicity with survival rates exceeding 80% even at saturated copolyester concentrations (2000 mg/kg). These findings elucidate the structure–property relationships of methyl salicylate-based copolyesters as promising sustainable alternatives to conventional PET in optical communication technologies.