Plastic deformation of polyethylene studied by infrared dichroism

Infrared dichroism measurements have been carried out to investigate the pl astic deformation mechanisms of homogeneous and heterogeneous ethylene copo lymers issued from the metallocene and the Ziegler-Natta catalyses, respect ively. The two copolymers having the same density display significantly dif ferent plastic behavior that should result from different mechanisms of cry stal slip. Such mechanisms are analyzed on the basis of the main crystal sl ip systems (100)[001] and (010)[001] having the higher shear compliances. A ttention is focused on the Schmid factor, S, of the slip planes, which is t he ratio of the resolved shear stress acting on each slip plane to the appl ied tensile stress. From the orientation functions of the three crystallogr aphic axes of the orthorhombic unit cell, the Schmid factor of the two slip systems is determined as a function of plastic strain. The heterogeneous c opolymer exhibits somewhat higher data for the (010)[001] slip, which sugge st a preferred activation of this system during the yielding stage. In cont rast, the homogeneous copolymer displays close values for the two slip syst ems, indicating that plastic flow is more likely governed by a combined act ivation of the two slip systems, and with eventual cross slip owing to the common [001] slip direction.