Effect Of Nucleating Agents LBPA And PONA On The Crystallization Properties Of Poly(Lactic Acid)

Poly(lactic acid)(PLA) is a biodegradable thermoplastic polyester that can bederived from renewable plant resources, and it exhibits the superior mechanicalproperties and biocompatibility comparable to those traditional polymers. However,the key bottlenecks in extending the fields of application for PLA are the lowcrystallization rate and crystallinity. One of the easiest ways to improve thecrystallization kinetics of PLA is by adding nucleating agents.The effects of selected nucleation agents LBPA and PONA on the crystallizationbehavior and kinetics of PLA were investigated by using Differential ScanningCalorimetry (DSC), Polarized Optical Microscope (POM), Wide-angle X-RayDiffraction (WAXD) and Fourier Transform Infrared (FTIR) in this paper. The crystalmorphology of PLA was observed via Scanning Electron Microscopy (SEM)technique through chemical etching.It was found that both LBPA and PONA were the effective nucleating agents forPLA, but their effects of nucleation were different. The LBPA has the function ofheterogeneous nucleation for PLA melt-crystallization while the PONA could increasethe finial crystallinity by promoting the cold-crystallization. The mixed LBPA/PONAshown a good synergic effect on the crystallization kinetics improvement. Theaddition of blended nucleating agents could reduce the nucleation induction periodand crystal size but significantly increased the nucleation density.The micrographs of POM and SEM revealed that the PLA crystals were the mostcommon spherulites. The POM images illustrated that the crystals showed an obviousmaltesecross while SEM presented the three-dimensional spherical crystal afteretched by sodium hydroxide (0.03mol/L) at50℃for5h.The crystallization kinetics results indicated that t1/2shows an U-shapedtemperature dependence, the fastest growth rate was achieved in the temperaturerange of105~110℃. However, the mechanisms of nucleation and crystal growth weresimilar since the Avrami exponent values, n, were all around3for PLA samplescrystallized at different temperatures, indicating that a three-dimensional crystalgrowth (spherulite structure) that results from an instantaneous athermal nucleationoccurred.The data of WAXD diffractogram and FTIR spectra indicated that nocharacteristic band of PLA β-form was found. Hence, all these PLA samples were formed the most common crystal modification (α-form). On the other hand, thecrystallinities of PLA samples were increased with the temperature increasing. Thisled to the intensities of band at921cm-1increases, while those of the955cm-1banddecreases due to the cold-crystallization process, and their changes are synchronized.