Effect of molecular weight of polyethylene glycol on crystallization behaviors, thermal properties and tensile performance of polylactic acid stereocomplexes

In this work, the poly(d-lactic acid)-polyethylene glycol-poly(d-lactic acid) (PDLA-PEG-PDLA) triblock copolymer as a novel modification agent was incorporated into poly(l-lactic acid) (PLLA) to improve the thermal and mechanical properties of PLLA. The influences of molecular weight of PEG in the triblock copolymer on the structure, crystallization behaviors, heat resistance and tensile properties of PDLA-PEG-PDLA/PLLA blends were investigated by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), polarized optical microscopy (POM), thermogravimetric analysis (TGA) and tensile testing. The results from FTIR, XRD and DSC confirm the formation of a polylactide stereocomplex in the PLLA blends. The structure and properties of the stereocomplex crystals are different from those of pure PLLA. The melting temperature (T-m) of the stereocrystal is near 200 degrees C, which is significantly higher than that of the homogeneous crystal of PLLA. The effect of molecular weight of PEG on the crystal morphology of PLLA blends is also obvious. The improvement of tensile properties for PLLA blends is attributed to the crystal morphological features, which will potentially enhance the utility of the PLLA based polymer.