Improving the mechanical and high-frequency dielectric properties of the bio-based thermoplastic copolyesters using fatty acid dimer diol and aromatic diester monomers

Thermoplastic copolyesters (TPCs) are crucial structural components in a wide range of high-performance applications where exceptional processing temperatures and remarkable flexibility are essential. Among them, fatty acid dimer diol (DDO), a bio-based compound, has been used as the soft segment in bio-based TPCs through various synthesis methods. However, few studies have compared the mechanical and dielectric properties of TPCs with DDO and aromatic diester monomers of varying structures to evaluate their suitability for soft electronic devices. The objective of this study is to investigate the impact of structural isomerism and rigidity on the properties of TPCs by incorporating three different aromatic diester monomers with distinct structures into the materials. The copolymerization of a series of TPCs via melt polycondensation, catalyzed by titanium butoxide and butyltin hydroxide oxide, was carried out using alkyl diols and aromatic diesters such as DDO, 1,4-butanediol, dimethyl terephthalate, dimethyl isophthalate (DMI), and dimethyl-2,6-naphthalenedicarboxylate (NDC), resulting in promising outcomes. The results show that introducing a higher proportion of DDO not only increases the bio-based content of TPCs but also enhances their flexibility, effectively reducing both the dielectric constant (D_k) and dissipation factor (D_f) at high frequencies. Notably, under the exact condition of adding 20 mol% DDO, copolymerizing with 20 mol% meta-positioned DMI significantly improves the flexibility of TPCs, with the elongation at break increasing to 607 % from 109 % before copolymerization, although the D_k and D_f slightly increase. On the other hand, when 20 mol% DDO is added, copolymerizing with the more rigid NDC increases the mechanical strength. It effectively lowers the dielectric property, as the D_k of the copolymerized 20 mol% NDC decreases from 2.43 to 2.32 at 29 GHz. These results are closely related to the solid-state stacking revealed by their distinct refractive index and crystallinity. This study, through the development of a series of bio-based TPCs, effectively explores the relationship between the mechanical and dielectric properties of these materials, providing valuable insights for the design of flexible electronic components and the development of bio-based materials.