Microscopic Origin of the Brittle-to-Ductile Transition Induced by Macromolecular Orientation in Glassy Polymers: From Crazes to Shear Bands

In glassy amorphous polymers, a brittle-to-ductile transition of macroperformances can be induced by macromolecular orientation, while the physical origin remains unclear. Herein, this work is focused on elucidating quantitatively the influence of molecular orientation induced by biaxial stretching on the initiation of elementary plastic deformation mechanisms involved in uniaxial stretching, mainly in the case of glassy poly(lactic acid) (PLA). As a result, crazing is preferentially activated in isotropic PLA and is gradually underprivileged in biaxially preoriented PLAs, which tend to deform rather by shear banding. Based on in situ and post-mortem scanning small-angle X-ray scattering (SAXS) analyses, it is found that systematically the critical nucleation stress of crazes increases with the orientation function while the geometry of the fibrils into the crazes remains unchanged. By contrast, the critical nucleation stress of shear bands is found to be roughly independent of the macromolecular orientation. These different sensitivities of critical nucleation stresses of craze/shear bands toward macromolecular orientation function lead to the higher energy barrier for craze than shear bands to be initiated in an oriented polymer network. In other words, increasing orientation function triggers a craze–shear band transition, which finally leads to a brittle–ductile transition. Based on a theoretical model, the increase in the craze initiation energy barrier could originate from modifications of entanglement architecture during the preorientation process.