2ND NORMAL STRESS DIFFERENCE RELAXATION IN A LINEAR POLYMER MELT FOLLOWING STEP-STRAIN

Flow birefringence is used to study stress relaxation following step s train deformations of a well entangled polyisoprene melt. The optical method employs multiple light paths to fully sample the three-dimensio nal stress tensor, and hence provides measurements of all three indepe ndent shear material functions (shear stress and both first and second normal stress differences). Experiments are complicated by multiple o rders in retardation. However, data show that the ratio of the second to the first normal stress difference is a strain thinning function, w ith magnitude intermediate between the predictions of the Doi-Edwards model with and without independent alignment. (C) 1998 John Wiley & So ns, Inc.