Incorporation of lactyl unit to PBAT for enhanced gas barrier property and biodegradability by direct polycondensation via alcoholysis of cyclic anhydride with lactic acid

The preparation of copolymers containing lactyl unit by direct melt polymerization of lactic acid has been a big challenge, especially for those with high molecular weight (M_n). In this paper, lactic acid was converted to bifunctional monomer (i.e. SLA) suitable for direct polycondensation via alcoholysis of cyclic anhydride. A series of PBALT copolyesters were synthesized by replacing adipic acid with SLA in order to introduce lactyl unit to poly(butylene adipate-co-terephathalate) (PBAT) for enhanced barrier property and biodegradability. Results showed that a series of PBALT copolyesters with high M_n of more than 41 000 g/mol can be successfully synthesized. The elongation at break remained high as the SLA content increased, while the melting temperature, the tensile modulus and strength gradually decreased. More importantly, the water vapour and gas barrier (towards CO₂ and O₂) property were increased by a factor of up to 1.62 and 4.3–4.4, respectively. In addition, the compost and enzymatic degradation rates of the copolyesters were accelerated by the introduction of the lactyl unit, which is mainly due to the reduction of the crystallinity. This study provides a facile strategy for the direct polycondensation of lactic acid, rather than using lactide, and thus establishes a new method for the development of biodegradable copolyesters containing lactyl unit with enhanced properties.