Nonlinear viscoelasticity of concentrated polystyrene solutions: Sliding plate rheometer studies

Because of the degree to which the linearity and polydispersity of polystyrene samples can be controlled in laboratory polymerizations, the rheological properties of melts and solutions of this polymer have been extensively studied. However, in previous work the maximum strain or strain rate was limited to small values due to edge effects in the rotational rheometers used. We used a sliding plate rheometer equipped with a shear stress transducer and a birefringence apparatus to measure simultaneously the shear stress and the third normal stress difference, N₃ (≡σ₁₁−σ₃₃), during step strain, start‐up of steady shear, and exponential shear. Depending on the strain history, the maximum strain achieved was between 20 and 80. The steady‐state shear stress in steady shear flow was found to be nearly independent of shear rate over a range of shear rates with a strong suggestion of a maximum, a phenomenon predicted by the Doi–Edwards theory. The relaxation modulus for the shear stress was found to be superposable except at short times, and the shift factor was used to determine the damping function. The maxima in both shear stress and N₃(t) during start‐up experiments were found to occur at fixed values of strain over a range of shear rates. In exponential shear, the solutions were found to be strongly shear thinning.