In-situ SAXS study of the plastic deformation behavior of polylactide upon cold-drawing

The cold drawing behavior of PLA has been investigated with special attention given to the identification of the elementary plastic deformation mechanisms involved. A combination of SAXS and AFM studies proved to be the most adapted and complementary approaches for this purpose, providing relevant information at a pertinent mesoscopic scale. Regarding the mechanical response, the present work shows that a brittle to ductile transition occurs when the draw temperature is increased and/or when the stretching speed is decreased. Morphological studies reveal that the brittle behavior of PLA is correlated with the occurrence of crazing, whereas in the case of a ductile, several plastic deformation mechanisms are involved and interact in a rather unusual way. In particular, the formation of “shear band crazes” was identified and a tentative explanation of its origin is proposed. In addition this study shows shear banding, activated at high temperature, plays a dual role. While shear bands promote the formation of crazes, they also stabilize them during their growth. Finally some correlations between chain dynamics and the observed mechanical behavior of PLA are also discussed.