In this study, the relationship between amorphous structure and changes in tensile properties caused by thermal treatment of amorphous polylactic acid (PLA) injection moldings in the glassy state was investigated. The amorphous PLA injection moldings exhibited ductile fracture behavior in tensile testing before the thermal treatment, whereas their tensile elongation at break decreased after the thermal treatment. It was found that the higher the thermal treatment temperature, the faster the rate of the decrease in the elongation at break. Specimens heated at 33 ℃ or 43 ℃ for 4 hours showed brittle fracture behavior. In contrast, for specimens treated at 53 ℃, a slight decrease in elongation at break was observed after 30 minutes, but no further decrease occurred with longer treatment times. The specimens heated at 53 ℃ for 4 hours exhibited ductile fracture behavior. Comparing the progress of enthalpy relaxation by the thermal treatment with the change in the elongation at break, it was clarified that the enthalpy relaxation progressed further in the specimens heated at 53 ℃ for 4 hours than those heated at 33 ℃ for 4 hours. Moreover, the results of dynamic mechanical analysis indicated that differences in sort-time-scale structural relaxation during physical aging were related to the observed differences in tensile fracture behavior. From these results, it was concluded that the variation in tensile properties of amorphous PLA during the physical aging cannot be explained by enthalpy relaxation.
Tao et al. (Fri,) studied this question.