Waste-PET-derived rigid–flexible copolyesters with tunable ordered structures and enhanced mechanical and gas-barrier properties

Poly(ethylene terephthalate) (PET)-based copolyesters are promising materials for engineering films and packaging applications, yet simultaneously improving mechanical toughness and gas-barrier performance remains challenging because flexible comonomers typically increase chain mobility at the expense of ordered chain packing. Herein, a series of waste-PET-derived rigid–flexible copolyesters, P(ET-co-Pedol) n , were synthesized by glycolysis of waste PET to bis(2-hydroxyethyl) terephthalate (BHET), followed by melt repolycondensation with an aromatic polyether diol (Pedol, M n  ≈ 800 g mol −1 ). Combined WAXD, SAXS, DSC, DMA, and rheological analyses revealed that Pedol incorporation regulated the crystallization behavior, thermal transitions, and nanoscale morphology of the copolyesters, producing a rigid–flexible architecture composed of PET-rich ordered domains and Pedol-containing flexible regions. The optimized composition, P(ET-co-Pedol) 6 , maintained WAXD-detectable local ordering while retaining sufficient segmental mobility and melt processability required for film fabrication. In situ variable-temperature SAXS further revealed thermally induced nanoscale structural evolution before the primary DSC crystallization event, suggesting that local ordering or density fluctuation may precede conventional lamellar crystallization in this copolyester system. Consequently, P(ET-co-Pedol) 6 exhibited a tensile strength of 84.2 MPa and an elongation at break of 582.7%, representing increases of 59.7% and 860% relative to neat PET, respectively. Meanwhile, oxygen and carbon dioxide permeabilities were reduced by 63% and 33%, respectively, which is consistent with the combined effects of preserved PET-rich ordered domains, local ordering, film drawing-induced orientation, and increased gas-diffusion tortuosity. Glycolysis–repolymerization further enabled regeneration of the copolyester with approximately 90% tensile-strength retention. These results demonstrate that rigid–flexible segment engineering provides an effective strategy for developing PET-based copolyesters with coupled mechanical toughness, gas-barrier performance, melt processability, and chemical recyclability.