On the Natural Draw Ratio of Semi‐Crystalline Polymers: Review of the Mechanical,Physical and Molecular Aspects

Drawing of semi‐crystalline polymers generally involves a necking phenomenon, which is often referred to as plastic instability. The draw ratio in the stable neck is called natural draw ratio. A review is made of papers dealing with the phenomenology of necking and endeavors to identify its physical origin. The destruction of the spherulitic structure that consists of chain‐folded lamellae involves a strain‐softening accompanied with a localization of the plastic deformation. In turn, the fibrillar transition that results from the lamellar fragmentation and subsequent rearrangement of the crystal blocks into microfibrils brings about a strain‐hardening, which stabilizes the plastic deformation. These competitive processes give rise to the natural draw ratio. The macromolecular network that consists of both chain entanglements and intercrystalline tie molecules appears to be a major factor of the neck stabilization. Theoretical approaches of the phenomenon are critically reviewed. Finally, the practical usefulness of the natural draw ratio is discussed with regard to predicting the long‐term mechanical behavior of high‐density polyethylene.