Thermodynamic investigation of renewable block copolymers based on poly(lactic acid) and poly(ethylene azelate)

Abstract

We studied a recently synthesized series of diblock copolymers based on poly(ethylene azelate) (PEAz) and poly(lactic acid) (PLA), prepared via in situ ring-opening polymerization of L-lactide in the presence of low-molecular-weight PEAz (5 kg mol^-1). The initial PEAz amount varied from 2.5 up to 20%. The materials are envisaged for use in biomedical applications, so we aimed to manipulate the overall properties (glass transition, crystallizability) and improve the compostability of PLA, with the latter being required for green chemistry and the circular economy. A series of structural and thermodynamic techniques were employed. Regarding novelty, molecular dynamics mapping for PEAz-b-PLA and neat PEAz is presented here for the first time. The PEAz-b-PLA copolymers were generally found to be quite homogeneous systems with respect to their thermal transitions. The crystal nucleation and fraction are suppressed in the copolymers, most probably due to the reduction in M_n, while alterations in the semicrystalline morphology were recorded. As for the amorphous polymer mobility, the glass transition temperature (calorimetric and dielectric, from ∼60 to ∼25 °C) and the fragility index of PLA (hard component) both fall systematically in the presence of PEAz. It is estimated that the in situ copolymerization, the presence of PEAz (soft component), and the simultaneous reduction in the average molecular weight (12-76 kg mol^-1) lead to an increase in the free volume in the copolyesters. The overall results provide firm indications of the plasticizing role of PEAz on PLA, which is one of the general goals for such copolymers. Overall, using these copolyesters, there is an opportunity for targeted structural manipulation (thermochemically mild) connected to the material's macroscopic performance.