Description of uniaxial, biaxial, and planar elongational viscosities of polystyrene melt by the K-BKZ model

The three types of Kaye-Bernstein-Kearsley-Zapas (K-BKZ) equations: Wagner- Demarmels (WD) model, Papanastasiou-Scriven-Macosko (PSM) model, PSM-Luo-Ta nner (PSMLT) model have been examined to describe experimental results of u niaxial, biaxial, and planar elongational viscosities of polystyrene (PS) m elt. In these equations, the viscosity was calculated from the relaxation s pectrum and damping function. The relaxation spectrum was determined by dyn amic shear experiment. The damping functions were obtained by stress relaxa tion experiments under biaxial, planar and shear deformations. The shear an d biaxial damping functions were described better by the PSM and PSMLT mode ls than by the WD model. However, the experimental planar damping function was not described well. This poor fitting to the planar damping function ca me from the result that the planar damping function gave more strain-soften ing than shear. Tn the theoretical predictions, the damping function of she ar and planar elongations should be identical with each other. Our result i s inconsistent with this prediction. The elongational viscosities were calc ulated from the relaxation spectrum and the damping functions, and then com pared to experimental data. Elongational viscosities for planar and biaxial deformations were measured by the lubricated squeezing method. In the case of uniaxial deformation, it was measured by a Meissner-type rheometer. Exp erimental results of uniaxial and biaxial elongational viscosities agreed w ith the prediction of the PSMLT model. Although the PSMLT model underpredic ted the planar elongational viscosity, it was concluded that this model was the best of three models. (C) 2000 Elsevier Science B.V. All rights reserv ed.