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

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 poly(ethylene azelate) oligomers as macroinitiators. 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 (Mn-) of the materials, ranging from 10 to 80 kg/mol, while the chemical structure was studied via Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared Spectroscopy (FTIR). NMR findings suggested the synthesis of block copolymers, but it was difficult to prove the formation of triblock copolymers; thus, a system comprised of PLA-b-PEAz block copolyesters and PLA segments was proposed and referred to as blocky copolyesters. According to Differential Scanning Calorimetry (DSC), the melting temperatures of the copolymers only shifted slightly towards lower temperatures, while the glass transition temperatures and the cold crystallization temperatures decreased sharply, revealing their flexible nature. 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 almost all copolyesters was enhanced. The mechanical performance was assessed via tensile and flexural measurements, revealing high elongation and Young’s modulus values for all copolyesters, indicating tough and strong materials, while during the 3-point bending tests, the copolyesters did not break.