On the plastic behavior of homogeneous ethylene copolymers compared with heterogeneous copolymers

The plastic behavior of homogeneous ethylene-octene copolymers and heterogeneous ethylene-butene copolymers has been compared on uniaxial tensile draw. True-stress-strain curves have been determined for various draw temperatures and strain-rates. Significant differences in yield behavior and strain-hardening have been revealed between the two types of materials, at equivalent crystallinity. In parallel, thermal analysis gave an indication that homogeneous copolymers have much narrow distributions of lamellae thicknesses together with lower values of the most probable thickness. Quantitative data from small-angle X-ray scattering support the latter point. An estimation of the surface free energy has revealed a more disordered chain-folding topology for the homogeneous copolymers. Interpretation of the mechanical behavior is based on the framework of a previous model for the plastic deformation of polyethylene involving competition of homogeneous and heterogeneous crystal slip processes. The former process governs the ability of the material to develop macroscopically homogeneous plastic deformation; the second process is a precursor to necking. It is proposed that the nucleation of dislocations, which is the basic mechanism of the homogeneous crystal slip, is favored for homogeneous copolymers, owing to the lower value of the most probable lamellae thickness. More tie molecules and chain entanglements in the disordered amorphous phase may also favor stable homogeneous plastic deformation in the case of homogeneous copolymers.