Flow birefringence studies of a concentrated polystyrene solution in a two‐roll mill. I. Steady flow and start‐up of steady flow

Abstract

AbstractWe have studied a 4.8 volume percent solution of a narrow distribution polystyrene with molecular weight 3.84 × 106 in flows generated by a co‐rotating two‐roll mill. These flows have a stagnation point at the midpoint between the roller axes. Further, they are linear, two‐dimensional, and the magnitudes of the strain‐rates are greater than the vorticity. The overall objective of our studies is to explore the dynamics of concentrated polymer solutions which are in the highly deformed state that is generated in the two‐roll mill. Birefringence data are presented for both steady flow and start‐up of steady flow in the two‐roll mill. The steady and transient data are used to analyze the linear and nonlinear viscoelastic regions of material behavior. In the nonlinear regime, the birefringence upon start‐up shows an initial overshoot followed by a strong undershoot that is enhanced as the ratio of elongation to rotation is increased (i.e., the flow becomes increasingly extensional in character.) We attribute this undershoot, which does not seem to appear in simple shear flows, or flows close to simple shear flow, to polymer segment stretching following an initial period of segment reorientation. Model studies are currently underway to test this notion. © 1992 John Wiley & Sons, Inc.