Oscillatory Normal Stresses in Dilute Polymer Solutions

The Rouse–Zimm theories of polymer solution dynamics have been extended to include normal stresses as well as shear stresses and have been evaluated for low-amplitude oscillatory shear. Two normal stress coefficients (ζd, characterizing a displacement function, and ζ*, characterizing the oscillation about the displacement) appear as analogs to the complex viscosity η* for shear stress. Theoretical predictions are compared with data obtained with a Weissenberg rheogoniometer on five dilute solutions of monodisperse polystyrene (M = 1.8 × 106 and 4.1 × 105) in Aroclor 1254. The models are found to describe the measured η* and ζd with equal good success, but uncertainties in ζ* measurements preclude complete verification. Strong evidence is presented to support the model-independent prediction that ωζd = η″.