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.