Revisiting stress-oscillation in cold drawing of poly(ethylene terephthalate)

This paper investigates the unusual phenomenon of stress-oscillation (SO) where a polymer exhibits periodic oscillatory neck propagation during drawing. It has been suggested that heat generation at the neck is critical, while other theories propose that an unstable neck stress is the controlling mechanism. Here, we study colddrawing of poly(ethylene terephthalate) at 100 mm min-1 where SO has been monitored by thermal analysis. The morphology of the transparent and opaque bands were investigated using DSC, X-ray scattering and microscopy. It is revealed that heat release is a consequence of the plastic deformation as opposed to being the cause of the yielding. Neck geometry plays a key role in the mechanism. Temperature rise at the neck is low (9 degrees C) and does not lead to a change in crystalline morphology between the transparent and opaque bands. It is concluded that stress-induced crystallization plays no role in the SO process for cold drawn PET and that the opaque bands result from axial voiding during high strain-rate yielding.