Crystallinity Enhances the Tensile Ductility of Polyethylene

The crystalline fraction of polyethylene is controlled by crystallization conditions, molar mass and comonomer content. It has long been known that higher crystallinity leads to greater elastic stiffness and greater yield strength. Intrinsic flow properties beyond the yield point are often obscured by deformation (neck formation and propagation). We present tensile true stress-true strain behavior of a series of isotropic polyethlenes with crystalline fraction ranging from 0.27 to 0.74. Tests were at room temperature and corrected to a constant local strain rate of 2.15×10^-3/sec. Unanticipated was the observation that the lowest crystallinity materials strain harden abruptly, failing at a true strain of ca. 1.5 because of finite extensitility of amorphous network chain segments. More crystalline polyethylenes flow to true strains approaching 3 with modest strain hardeing; here crystal plasticity dominates. The roles of morphology and entanglement are considered.