Plastic deformation of polyethylene studied by infrared dichroism

Infrared dichroism measurements have been carried out to investigate the plastic deformation mechanisms of homogeneous and heterogeneous ethylene copolymers issued from the metallocene and the Ziegler-Natta catalyses, respectively. The two copolymers having the same density display significantly different plastic behavior that should result from different mechanisms of crystal slip. Such mechanisms are analyzed on the basis of the main crystal slip systems (100)[001] and (010)[001] having the higher shear compliances. Attention is focused on the Schmid factor, S, of the slip planes, which is the ratio of the resolved shear stress acting on each slip plane to the applied tensile stress. From the orientation functions of the three crystallographic axes of the orthorhombic unit cell, the Schmid factor of the two slip systems is determined as a function of plastic strain. The heterogeneous copolymer exhibits somewhat higher data for the (010)[001] slip, which suggest a preferred activation of this system during the yielding stage. In contrast, the homogeneous copolymer displays close values for the two slip systems, indicating that plastic flow is more likely governed by a combined activation of the two slip systems, and with eventual cross slip owing to the common [001] slip direction.