COMPARISON OF OPTICAL AND MECHANICAL MEASUREMENTS OF 2ND NORMAL STRESS DIFFERENCE RELAXATION FOLLOWING STEP STRAIN

Relaxation of the second normal stress difference (Nz) following step strain of a concentrated monodisperse polystyrene solution has been st udied using mechanical and optical rheometry. Measurements of normal t hrust in a parallel plate geometry are corrected for strain inhomogene ity and combined with independent measurements of the first normal str ess difference (N-1) to determine N-2. Optical experiments were perfor med using a novel configuration where flow birefringence data collecte d using multiple light paths within the shear plane are combined with the stress-optical law to determine all three independent stress compo nents for shearing deformations. This technique eliminates end effects , and provides an opportunity to oversample the stress tensor and deve lop consistency checks of experimental data. N-2 is found to be nonzer o at all accessible times, and relaxes in roughly constant proportion to N-1. This reflects nonaffine distribution of chain segments, even w ell within the regime of chain retraction at short times. Data collect ed with the two techniques are reasonably consistent with each other, and with results of previous studies, generally lying between the pred ictions of the Doi-Edwards model with and without the independent alig nment approximation. The normal stress ratio-N-2/N-1 shows pronounced strain thinning in the nonlinear regime.