Effect of poly(lactic acid) crystallization on its mechanical and heat resistance performances

Abstract

Poly (lactic acid), a promising biodegradable polymer, is restricted by poor heat resistance, which is determined by the crystallization. It is important to expound the relationship between crystal structure and thermal, mechanical performance. Several PLA specimens were prepared by fully crystallized at different temperature (100 °C, 110 °C, 115 °C, 120 °C, and 130 °C) from melt state. The morphology, crystallization behaviors, thermal and mechanical properties were evaluated by polarizing optical microscope (POM), wide-angle X-ray diffraction (WAXD), differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA), and Vicat softening temperature (VST), respectively. It was found that the PLA crystal shows the characterization of spherulite and many nuclei are generated at low temperature (100 °C and 110 °C). The nuclei gradually decrease and the spherulites grow as the crystallization temperature increases. Compared with the amorphous PLA specimens, the tensile strength of crystallized samples gradually decreases from 29.4 MPa to 25.8 MPa and the VST is improved from 51.6 °C to 64.9 °C. It confirms that the crystallization is an effective way to enhance the heat resistance. However, when the temperature reaches 130 °C, the crystalline and amorphous region aggregate, forming the distinct two phases, which leads to a decrease in VST (53.2 °C). Meanwhile, the spherulites become irregular, larger, and imperfect at high temperature, resulting in the further deterioration of mechanical performance. In addition, the molecular dynamics was further used to simulate the structural evolution during the crystallization and reveals the mechanism behind the relationship.