Stress and birefringence measurements during the uniaxial elongation of polystyrene melts

A rheometer for generating uniaxial elongations in molten polymers has been modified to allow for the simultaneous measurement of stress and flow-indu ced birefringence. Tensile stress sigma and birefringence Delta n' data in flows at constant strain rates up to 1 s(-1) were collected on a polydisper se polystyrene melt at temperatures of 160 and 170 degrees C. From these da ta, the stress-optic rule was followed for stresses below roughly 1 MPa. Fo r stresses less than 1 MPa, the stress-optic coefficient \C\ = \Delta n'\l sigma was found to have a value of 4.8x10(-9) Pa-1 which was independent of strain, strain rate, and temperature. At stress levels higher than 1 MPa, \C\ decreased indicating a failure of the stress-optic rule. A criteria for failure of the stress-optic rule was formulated using simple arguments fro m network models and characteristic times suggested by the tube model. This criteria, which is based on the hypothesis that failure of the stress-opti c rule is the result of significant chain stretching, was found to be consi stent with the data reported in this study and with data from previous stud ies on polystyrene melts. (C) 1999 The Society of Rheology. [S0148-6055(99) 01003-2].