Tensile yield and strain hardening of homogeneous ethylene copolymers compared with heterogeneous copolymers

The plastic flow of a chemically homogeneous ethylene copolymer and a heterogeneous copolymer having nearly equivalent crystallinity is investigated comparatively upon uniaxial tensile drawing. The homogeneous copolymer displays lower yield stress and more complex strain-hardening behaviour than the heterogeneous copolymer. Furthermore, this latter exhibits a greater propensity for plastic instability. Competition of uniform and localized crystal slip processes in relation to the higher crystal thickness of the heterogeneous copolymer is proposed to account for this phenomenon. The strong dependence of the strain-hardening coefficient of the homogeneous copolymer on temperature and strain rate is also analyzed. Its sigmoidal evolution with strain is ascribed to three steps in the occurrence of the homogeneous slip, i.e. initiation, propagation and exhaustion.