Mechanism of Improved Crystallizability of Poly(L‐Lactic Acid) by Plasticizers

Poly(L‐lactic acid) (PLLA) is one of the most popularly utilized plant‐based polymers, as PLLA can be synthesized from renewable sources such as sugar, carbohydrate, etc. In recent several decades, scientists have been interested in PLLA because of its environment‐friendly characteristics. However, there exist some big problems with PLLA, which are low crystallization rate and low crystallinity. In order to overcome such drawbacks, nucleation agents have been incorporated into PLLA. On the contrary, we have reported that plasticizers (both of biobased (organic acid mono‐glyceride; OMG) and oil‐based (dioctylphthalate)) can improve the crystallizability of PLLA [Polymer Journal 2019; 51(2): 283–294]. The results are significant because plasticizers are believed to delay crystallization of polymers by reducing the thermodynamical driving force of crystallization. We now recognize that the lowering of the activation energy for the PLLA crystallization may be the main effect of a plasticizer with small‐amount loading. In this study, we confirm that in the presence of the plasticizer molecules, better folding of the PLLA chains results, which has been experimentally confirmed by the fact that the surface free energy ( σ e ) of the crystallites plane, on which the folds of the PLLA chains exist, becomes lower upon the loading of plasticizers. The free energy change upon the formation of a primary nucleus is then drawn as a function of the thickness of the primary nucleus using the values of σ e (which were evaluated in the framework of the Hoffman‐Lauritzen theory through the growth rate of spherulite from the polarizing optical microscopic observation) and it was found that the energy barrier (activation energy) for the crystallization is decreased by the loading of plasticizers. This explains the enhancement of the crystallizability by the presence of plasticizer.