Nonisothermal Crystallization Behavior and Enhanced Heat Resistance and Impact Toughness of Poly(l-lactic acid) with Bimodal Molecular Weight Distribution

Poor heat resistance and toughness put some limitations on the applications of poly(lactic acid), one of the most promising eco-friendly polymers. Herein, bimodal poly(l-lactic acid) (PLLA) was prepared by melt-blending high- and low- molecular-weight PLLAs, and the nonisothermal crystallization behavior, heat resistance, and mechanical properties of bimodal PLLA were investigated. Low-molecular-weight PLLA (L-PLLA) improves the crystallization capability of bimodal PLLA. The double melting behavior is reasonably explained by the melting–recrystallization mechanism. Octamethylenedicarboxylic dibenzoylhydrazide, a typical heterogeneous nucleating agent, significantly enhances the crystallization rate and crystallinity of PLLA. The maximum crystallinity of 100% L-PLLA is as high as 54.4%, and the heat deflection temperature (HDT) of the bimodal PLLA containing 95% L-PLLA is up to 151 °C, while the impact strength drops sharply to 17.2 kJ/m2 due to the large crystallite size. d-Sorbitol (DS) exhibits a completely different mechanism from heterogeneous nucleation, that is, a lower crystallization rate but elevated crystallinity and crystallization temperature. The DS-modified sample realizes an excellent balance between heat resistance (HDT up to 136 °C) and impact strength (47.9 kJ/m2) via a self-toughening effect by increasing the crystallinity and reducing the crystallite size.