Next-Generation High-Performance Biobased Naphthalate-Modified PET for Sustainable Food Packaging Applications

We report a series of novel poly(ethylene terephthalate) (PET) copolymers with improved properties through the incorporation of bioadvantaged dimethyl 2,7-naphthalenedicarboxylate (2,7-N) as a comonomer. PET is among the most commonly used engineering thermoplastics, ubiquitous in the food packaging industry. However, its application is limited by poor thermal (low Tg) and oxygen barrier performance. A series of poly(ethylene terephthalate-stat-2,7-naphthalate) copolymers were synthesized from ethylene glycol (EG), terephthalic acid (TPA), and 2,7-N via a standard two-step melt polycondensation reaction. The 2,7-N significantly improved the thermal, mechanical, and barrier properties. The glass transition temperature (Tg > 75.4 °C) and thermal stability (Td,5% > 405.1 °C) of the copolymers increase monotonically with 2,7-N content, exceeding those of PET (Tg = 69.7 °C, Td,5% = 401.4 °C). Moreover, the mechanical properties and the crystallization behaviors are tunable through the 2,7-N loading. Composition-optimized copolymers showed an increase of 70% and 200% in elongation at break and tensile strength, respectively. In addition, the oxygen permeability value of the copolymers containing 20% 2,7-N loading fell to PO2 = 0.0073 barrer, a 30% improvement over that of PET. These results illustrate that the novel substitution patterns offered by biobased chemicals can translate to performance advantages in packaging materials. Finally, the fundamental structure–property relationships connecting the bioadvantaged chemicals as the comonomers to the product performance were constructed as a guide for value-added renewable polymers in the future.