MEASUREMENT AND MODELING OF UNIAXIAL AND PLANAR EXTENSIONAL VISCOSITIES FOR LINEAR ISOTACTIC POLYPROPYLENES

In this work, novel rectangular and circular orifice (zero-length) dies were used to measure planar and uniaxial extensional viscosities as a function of strain rate for various linear isotactic polypropylene melts by using Cogswell methodology. The obtained experimental data were combined with shear and uniaxial extensional viscosity data determined at very high strain rates. The ability of the molecularized Generalized Newtonian Fluid (mGNF) [M. Zatloukal and J. Drabek, "Generalized Newtonian fluid constitutive equation for polymer liquids considering chain stretch and monomeric friction reduction for very fast flows modeling," Physics of Fluids 33(8), 083106 (2021)], Giesekus and explicit Yao constitutive equations to describe the measured data was tested. It has been shown that including the effect of the chemical environment (i.e., the role of the oligomeric solvent) using a simplified version of the mGNF constitutive equation (instead of the commonly used Newton's law) can significantly improve the ability of the Giesekus and Yao viscoelastic constitutive equations to describe the measured experimental data, especially at very high strain rates with using adjustable parameters with a clear physical meaning.