EXTENSIONAL BEHAVIOR OF ENTANGLED POLYMERS

Recent particle-tracking velocimetry (PTV) has revealed inhomogeneous elastic breakdown of entangled polymer networks during shear. Such cohesive failures indicate a many-bodied network picture for entangled polymers. The Doi-Edwards tube model appears to be inadequate to capture the crucial physics of this behavior. Observations of shear deformation have motivated the study of the extensional rheology of a series of SBR melts and blends. During step and startup extension, localized failures similar to those found in simple shear have been observed. After a large step extension, entangled polymers exhibit elastic breakdown, when high retraction forces overcome cohesion imparted by chain entanglement. In startup extension, engineering stress maxima indicate yielding just as stress overshoots in the start-up of simple shear mark the onset of yielding. The present work clarifies confusion regarding strain-hardening and shows that the basic physics of shear and extensional flow are similar. The yield-to-rupture transition in melts at high rates of uniaxial extension has als