Effect of extensional and shearing strains on molecular orientation of a polymer melt

Abstract

AbstractThe flow of two grades of polydimethysiloxane (Dow Corning type 200 grades 60 Pa's and 300 Pa's) was studied in a 60° wedge flow cell. The stress‐optical law was validated along the center‐line (extensional strains only) and at off centerline locations (extensional and shearing strains combined). Values for stressoptical coefficient were 8.01 × 10−11 Pa−1 and 9.61 × 10−11 Pa−1 respectively, the differences apparently being due to experimental error. Orientation angles along a fixed radius at 20° to the cell center‐line were almost constant. Center‐line orientation angles were zero, as expected. Orientation angles at a constant radius for various angular positions varied from zero at the centerline to 54° near the wall. A method for testing constitutional relations for stress is presented. The power law model and linear viscoelastic models are shown to be inadequate in describing orientation effects caused by rotation in the flow. The Goddard‐Miller model was shown to express this but accuracy was not very good, perhaps because of the single Maxwell unit used.