Effects of Loading Amount of Plasticizers on Improved Crystallization of Poly(L-lactic acid)

Poly(L-lactic acid) (PLLA) is one of the most popularly utilized biobased polymers, as it is derived from renewable sources (sugar, carbohydrate, etc). Although PLLA bears environmentally benign characteristics, there are crucial faults of PLLA such as slow crystallization and insufficient crystallinity which can be finally attained even after the fully thermal annealing. To improve crystallizability of PLLA, nucleation agents have been utilized. Although typical nucleation agents work at the solid state, we have reported that a special plasticizer (organic acid monoglyceride; OMG) can partly improve the crystallizability of PLLA. The results are significant because plasticizers are believed to delay crystallization of polymers by reducing the thermodynamical driving force of crystallization. We speculated that the lowering of the activation energy for the PLLA crystallization may be the main effect of the plasticizer. In this study, we confirmed that the enhanced crystallization of PLLA can be also attained by using a general plasticizer which is dioctyl phthalate (DOP). To identify the effects of loading amount of plasticizers on improved crystallization of PLLA, we conducted time-resolved small- and wide-angle X-ray scattering (SWAXS) and differential scanning calorimetric (DSC) measurements to follow, respectively, the isothermal and non-isothermal crystallization of PLLA/plasticizer specimens. Moreover, polarizing optical microscopic (POM) observation was conducted to evaluate the growth rate of a spherulite and the number of nuclei formed as a function of time.