Investigation of Non-linear Rheological Behaviors of Entangled Polymer Melts in Complex Geometries

Download or read book Investigation of Non-linear Rheological Behaviors of Entangled Polymer Melts in Complex Geometries written by Xiangyang Zhu and published by . This book was released on 2012 with total page 137 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nonlinear rheology of polymer melts has been of central interest in polymer science and engineering for the last four decades. Recent particle-tracking velocimetric (PTV) observations have revealed strain localization either during startup shear, beyond the stress overshoot, or after stepwise shear of entangled solutions and melts. Strain localization under rate-controlled shear shows that yielding, i.e., transition from elastic deformation to flow, has taken place. Under controlled stress, i.e. creep, entangled polymers also exhibit breakdown of the entanglement network in the form of an entanglement-disentanglement transition (EDT). Characterization of structural changes during creep has been performed for entangled polymer solutions undergoing EDT. The PTV method was employed to probe yielding and strain localization under large, fast external deformation of polymeric melts in uniaxial extension and capillary extrusion. Shear yielding through chain disentanglement is anticipated despite the fact that these two modes of deformation are not simple shear. During uniaxial extension, we are able to identify different failure modes of tensile decohesion and necking failure initiated by shear yielding. During capillary extrusion, entry pressure loss due to shear yielding also has been systematically studied in the fast flow region. Through Bagley extrapolation or flow curves obtained in a Couette device, the magnitude of pressure loss was determined. It was found to be proportional to one third power of the volumetric flow rate.