Comparison of simple constitutive equations for polymer melts in shear and biaxial and uniaxial extensions.

Experimental melt data are compared with differential constitutive equations that contain only a single adjustable parameter, besides the parameters describing the linear relaxation spectrum. These include the equations of Giesekus, Phan-Thien and Tanner, Johnson- Segalman, White-Metzner, Larson, and Acierno et al. The deformations are step shear, step biaxial extension and steady uniaxial extension of HOPE without long side branches and LDPE with long side branches. The Johnson-Segalman and White-Metzner models fail to predict experimental data accurately. The Acierno equation shows strong oscillations in elongation and shear. The Giesekus equation fits the uniaxial extension and shear data but fails to do so for biaxial extension regardless of the choice of the adjustable parameter. The models of Phan- Thien and Tanner, and Larson seem to fit the data best.