FULL TENSOR OPTICAL RHEOMETRY OF POLYMER FLUIDS

A novel three-dimensional flow birefringence technique has been develo ped to measure the real time, three-dimensional, optical anisotropy in flowing polymer solutions. Full tensor optical rheometry utilizes thr ee polarization-modulated beams to probe all independent elements of t he refractive index tensor and is, thus, used as a quantitative probe of time-dependent, three-dimensional average molecular orientation. We present results for shear flow of polystyrene in a 6% semidilute Solu tion of tricresyl phosphate over a range of shear rates. Although the time-dependent behavior of N-2 has not been widely reported, probably because of experimental difficulties, it can be a powerful means of te sting rheological models. Experimental measurements of the transient b ehavior of N-2 and -N-2/N-1 on inception and cessation of simple shear flow are compared with the predictions of the Doi-Edwards and Gieseku s models. The Giesekus model is in qualitative agreement with most of the time-dependent features observed for N-2 and -N-2/N-1 On-flow ince ption overshoots in N-2 and -N-2/N-1 are observed; Further, on-flow ce ssation, N-2 relaxes more slowly than N-1 and -N-2/N-1 shows a strong nonexponential increase to a constant value of similar to 0.9 at long times. The Doi-Edwards model is less successful in predicting transien t results but agrees quite well with observed steady-state dependence of N-2 with shear rate. (C) 1997 The Society of Rheology.