Thermodynamical Investigation in Renewable Block Copolymers based on Poly(lactic acid) and Poly(ethylene azetate)
Abstract
We study a recently synthesized series of diblock copolymers based on poly(ethylene azetate) (PEAz) and poly(lactic acid) (PLA), prepared via in-situ ring opening polymerization of l-lactide at the presence of low molecular weight PEAz (5 kg/mol). The initial PEAz amount varied from 2.5 up to 20 %. The materials are envisaged for use in biomedical applications, whereas it is aimed to manipulate the overall properties (glass transition, crystallizability) and improve the compostability of PLA, the latter being wanted in the ‘green and circular’ chemistry and economy. A series of structure and thermo-dynamical techniques are employed for this study. Regarding novelty, the molecular dynamics mapping for the PEAz-b-PLA, moreover, for neat PEAz, is presented here for the first time. The PEAz-b-PLA copolymers were found in general quite homogeneous systems, with respect to the thermal transitions. The crystal nucleation and fraction are suppressed in the copolymers, most probably due to Mn drop, while alternations in the semicrystalline morphology were recorded. As for the amorphous polymer mobility, the glass transition temperature (calorimetric and dielectric, from ~60 to ~25 oC) as well as the fragility index of PLA (hard component) drop systematically at the presence of PEAz. It is estimated that the in-situ copolymerization, the presence of PEAz (soft component) and the simultaneous drop in the average molecular weight (12-76 kg/mol), lead to an increasing of 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, by these copolyesters, there seems to be a chance for targeted structure manipulations (thermochemically mild) being connected to the materials macroscopic performance.