Balancing Strength and Ductility in Poly(butylene succinate) Copolyesters through BHET Incorporation

Poly(butylene succinate) (PBS) is a sustainable polyester with attractive processability; however, achieving an optimal balance between strength and ductility remains a key challenge for broader applications. In this study, PBS-based copolyesters were synthesized by phosphoric acid-catalyzed melt polycondensation using bis(2-hydroxyethyl) terephthalate (BHET), a key intermediate derived from the chemical recycling of polyethylene terephthalate (PET). Mg(2-ethylhexanoate)₂ was introduced during the final stage of polymerization to induce phosphate-metal ionic interactions and modulate the polymer architecture.

Structural analyses confirmed the successful incorporation of BHET-derived units into the PBS backbone. ³¹P NMR revealed that increasing the BHET content increased the phosphate triester fraction from 17% for BHET-free PBS to 48%, 60%, and 70% for copolymers containing 5, 10, and 15 mol% BHET, respectively, indicating an increased extent of phosphate-induced long-chain branching. Residual phosphate monoester and diester species are also expected to promote ionic aggregation through coordination with Mg²⁺.

The mechanical properties were highly dependent on BHET content. BHET-free PBS exhibited a yield strength of 34 MPa, a Young's modulus of 370 MPa, and an elongation at break of 180 ± 130%. Incorporation of 5 mol% BHET provided the most balanced mechanical performance, exhibiting a yield strength of 26 MPa, a Young's modulus of 290 MPa, and an improved elongation at break of 370 ± 30%. Further increasing the BHET content led to a continued increase in ductility at the expense of strength. These results demonstrate that controlled BHET incorporation offers an effective strategy for tuning the mechanical properties of PBS copolyesters while maintaining a desirable balance between strength and ductility.