Novel biobased, flexible blocky copolyesters based on poly(lactic acid) and poly(ethylene azelate)†

Abstract

The synthesis and characterization of a series of novel, high molecular weight poly(lactic acid)-b-poly(ethylene azelate) (PLA-b-PEAz) blocky copolyesters are reported for the first time. The copolyesters were synthesized by the ring-opening polymerization (ROP) of L-lactide, using oligo(ethylene azelate) as a macroinitiator. Four different comonomer mass ratios were used in the feed, namely 97.5-2.5, 95-5, 90-10, and 80-20, the minor comonomer being PEAz. Gel permeation chromatography (GPC) and intrinsic viscosity measurements [η] confirmed the high number average molecular weight of the materials, ranging from 10 to 80 kg mol⁻¹, while the chemical structure was studied via nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). NMR analysis indicated the formation of block copolymers; however, confirming the presence of triblock structures proved challenging. Therefore, a system consisting of PLA-b-PEAz block copolyesters along with PLA segments was proposed and described as blocky copolyesters. According to differential scanning calorimetry (DSC), the melting temperatures of the copolymers exhibited only slight shifts toward lower values, whereas the glass transition and cold crystallization temperatures decreased significantly, indicating enhanced flexibility. Furthermore, isothermal crystallization experiments from the melt suggested that the crystallization ability of the PLA-based copolyesters was improved compared to PLA. The thermal stability of most copolyesters was enhanced. The mechanical performance was assessed via tensile and flexural measurements, revealing high elongation and Young's modulus values, indicating tough and strong materials. Moreover, during the three-point bending tests, none of the copolyesters fractured.