J. Non-Newtonian Fluid Mech. 2136 (2002) 115

A comparison of the stress and birefringence growth of dilute, semi-dilute and concentrated polymer solutions in uniaxial extensional flows

Jonathan P. Rothstein, Gareth H. McKinley
Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Corresponding author: rothstei@ecs.umass.edu (J.P. Rothstein).


Filament stretching experiments are used to follow the evolution in the flow induced Birefringence and the tensile force in polymeric fluid threads undergoing uniaxial elongation. The tests are performed with several monodisperse polystyrene solutions spanning concentrations from 0.025 wt.% to 12 wt.%, and the measurements are used to investigate the extent of hysteresis in the polymeric stress and molecular conformation during imposed stretching and subsequent stress relaxation. For Deborah numbers above De > 0.5 (based on the longest or Zimm relaxation time), a pronounced stress-conformation hysteresis is observed in the dilute polystyrene solution at large Hencky strains, and the magnitude of the deviation from the quasi-static FENE connector force increases with imposed deformation rate. However, tests with a semi-dilute entangled polystyrene solution and a concentrated entangled polystyrene solution show that although the stress-optical rule is violated at large deformation rates and large strains, there is no evidence of any hysteresis during extension and relaxation. These observations are interpreted in terms of the relative magnitude of the stretching rate as compared to the reptation time, and also as compared to the Rouse time scales for stretching of the entire chain or for a single entangled segment. These distinct time scales depend on the number of entanglements in the fluid and govern the different relaxation mechanisms that exist in entangled polymer solutions for the polymeric stress and the molecular conformation.