Steady state measurements in stress plateau region of entangled polymer solutions: Controlled-rate and controlled-stress modes

Despite decades of efforts, reliable measurements of nonlinear flow behavior of well-entangled polymers in continuous shear have been challenging to obtain. The present work attempts to accomplish three important tasks: (A) overcome this challenge by adopting a strategy of decoupling theological measurements from the outer meniscus region in a cone-partitioned plate (C/PP) setup; (B) determine whether well-entangled solutions indeed undergo a flow transformation under creep that can be taken to phenomenologically define an entanglement-disentanglement transition (EDT); (C) provide the velocity profiles of such solutions undergoing either controlled-stress or controlled-rate shear by carrying out in situ particle-tracking velocimetric (PTV) measurements. Upon removing any influence of edge fracture and sample loss, we are able to reach steady state during continual shear and elucidate more reliably the nonlinear flow behavior of well entangled polymer solutions with little ambiguity. Three well-entangled solutions with Z=40 entanglements per chain exhibited overlapping continuous and monotonic flow curves both in controlled-rate and controlled-stress modes. The OPP based experiments qualitatively confirm the report by Tapadia and Wang [Macro molecules 37, 9083-9095 (2004)] that at a given applied shear stress the solution evolves over time from a state of high viscosity to that of substantially reduced viscosity, i.e., EDT occurs ubiquitously in absence of any edge effects. These results, made possible by a combination of C/PP and PTV, cause us to reevaluate the objectives of polymer rheology, and are expected to impact the future development in the field of rheometry for entangled polymers and other viscoelastic materials. (C) 2008 The Society of Rheology.