FIRST AND 2ND NORMAL STRESS DIFFERENCE RELAXATION IN REVERSING DOUBLE-STEP STRAIN FLOWS

Reversing double-step strains provide a severe test of constitutive eq uations and have often been used to test the Doi-Edwards (DE) model wi th and without the independent alignment approximation. We report meas urements of the full stress tensor in a concentrated monodisperse poly styrene solution subjected to reversing double-step strains using flow birefringence. Shear stress and first normal stress difference result s agree with previous studies. In flows where the second strain is hal f the magnitude of the first (''specialized type-B''), certain rheolog ical models predict that normal stresses should be independent of time between strains (t(1)), and equal to those measured in single-step st rain. Both first and second normal stress differences follow this beha vior at long times, but deviations are found at short times, where cha in retraction is occurring. In flows with equal and opposite step stra ins (''type-C''), the ratio N-1/sigma is found to be equal to the stra in. In both of these flows, the DE model predicts the normal stress ra tio -N-2/N-1 to be independent of t(1). In specialized type-B flows, t he experimental normal stress ratio is nearly independent of t(1), whi le in type-C flows the ratio depends strongly on t(1) and is found to be substantially larger than that predicted by the DE model or observe d in single-step strains. DE calculations for more general flow protoc ols predict that the normal stress ratio may depend on t(1) and on the time following the second deformation. Experiments show qualitative a greement with both forms of the DE model for these other flows, where the major influence of the independent alignment approximation is on t he magnitude of the predicted normal stress ratio. (C) 1996 Society of Rheology.