USE OF COUPLED BIREFRINGENCE AND LDV STUDIES OF FLOW-THROUGH A PLANARCONTRACTION TO TEST CONSTITUTIVE-EQUATIONS FOR CONCENTRATED POLYMER-SOLUTIONS

Laser Doppler velocimetry and flow-induced birefringence are used to m easure the rate of deformation and the principal components of the ref ractive index tensor in a 5% polyisobutylene (PIB) solution in tetrade cane (C14) flowing along the centerplane of an abrupt 3.97:1 planar co ntraction. The stress optical law is used to interpret the birefringen ce data in terms of the normal stress difference, which is used to cal culate a transient elongational viscosity defined along the centerplan e. These measurements are compared directly to predictions of six mult imode, differential constitutive models (Oldroyd-B, White-Metzner, Aci erno et al., Giesekus, Bird-DeAguiar, and Phan-Thien-Tanner) that are fit to steady and small amplitude oscillatory shear flow data for the PIB/C14 solution. The fluid exhibits slight elongational thickening fo llowed by apparent extensional thinning at higher elongation rates. We believe that this ''thinning'' behavior is due to the decreased resid ence time of the polymer molecules in the high-strain-rate region as t he flow rate (and maximum elongation rate) is increased. The nonlinear constitutive equations, except for the White-Metzner model, are virtu ally indistinguishable in their description of the dynamical response of the fluid in this experiment; however, the Phan-Thien-Tanner model gives the best quantitative fit to the data. These results point to th e need for experiments in which the fluid flowing along the centerline is subjected to a greater total elongational strain. (C) 1995 Society of Rheology