Plasticization of Polylactide after Solidification: An Effectiveness and Utilization for Correct Interpretation of Thermal Properties

Polylactide/triethyl citrate (PLA/TEC) systems were prepared in two ways by introducing TEC to solidified polymer matrix (SS) and by blending in a molten state (MS) to investigate the effectiveness of the plasticization process after solidification of polylactide. The plasticization processes, independent of the way of introducing the TEC into PLA matrix, leads to systems characterized by similar stability, morphology, and properties. Some differences in mechanical properties between MS and SS systems result primarily from the difference in the degree of crystallinity/crystal thickness of the PLA matrix itself. Based on the presented results, it was concluded that the plasticization process after solidification of polylactide is an alternative to the conventional method of modification-blending in a molten state. Then, this new approach to plasticization process was utilized for the interpretation of thermal properties of PLA and PLA/TEC systems. It turned out that double melting peak observed at differential scanning calorimetry (DSC) thermograms does not result from the melting of a double population of crystals with different lamellar thickness, or the melting of both the α′ and α crystalline phase (commonly used explanations in literature), but is associated with the improvement of perfection of crystalline structure of PLA during heating process.