Toughened aromatic poly-(decylene terephthalate) copolyesters with two renewable eugenol-based components via a random copolymerization method

Abstract

Poly-(decylene terephthalate) PDT homopolyester and two series of thermoplastic PDT_1−xM1_x and PDT_1−xM2_x copolyesters with varying molar compositions (x = 0%, 20%, 40%, 60%, 80%, 100%) were prepared via random copolymerization of dimethyl terephthalate (DMT) and 1,10-decanediol, with renewable eugenol-based subunits providing toughener properties. These eugenol-based tougheners were synthesized via a thiol–ene click reaction and subsequent nucleophilic substitutions. The obtained copolyesters exhibit averaged molecular weights (M_ws) ranging between 26 800 and 55 300 g mol⁻¹, together with dispersity (D) values between 1.8 and 2.2. ¹H NMR, ¹³C NMR and FTIR spectroscopic analyses were carried out as structural characterization techniques. The quantitative ¹³C NMR spectra of PDT_1−xM1_x indicate that the copolyesters feature random microstructures. The results from thermogravimetric analysis (TGA) show that thermal stabilities of both PDT_1−xM1_x and PDT_1−xM2_x species gradually decrease with increasing toughener contents. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were carried out in order to study the thermo-mechanical properties of the polyesters. The toughener content proves to have no significant influence on the glass transition temperatures (T_gs) of the copolyesters, despite the fact that an inconspicuous T_g value was observed for the PDT homopolyester. Furthermore, the gradually diminishing melting enthalpy (ΔH_m) results from a decreasing crystallinity upon increase of the toughener unit content, suggesting that the incorporation of eugenol-based tougheners into PDT reduces the crystallinity in a significant manner. Isothermal crystallization and wide X-ray diffraction (WXRD) techniques provide evidence for this conclusion as well. The amount of eugenol-based units significantly influences the observed toughening effect. Strong tenacity of the brittle PDT was realized from the tensile and impact assays, with the content of the tougheners up to 20%. Specifically, the impact strength increases up to 16.4 and 15.8 kJ m⁻² for PDT_80%M1_20% and PDT_80%M2_20%, respectively. Simultaneously, the ultimate strength decreases 2.6–4.3 MPa from 16.8 MPa to 12.5–14.2 MPa and elongation at break increases from approximately 5% to 18–20%. The findings described herein prove to be particularly important from a commercial point of view to meet the strict requirements for a variety of potential applications of such polyester species in structural materials.