Rheological Characterization and Constitutive Modeling of Two LDPE Melts

Experimental data of two low‐density polyethylene (LDPE) melts at 200° C for both shear flow (transient and steady shear viscosity as well as steady first normal stress coefficient) and elongational flow (transient and steady‐state elongational viscosity) as published by Pivokonsky et al. [1] were analyzed by use of the Molecular Stress Function (MSF) model for broadly distributed, randomly branched molecular structures. For quantitative modeling of melt rheology in both types of flow and in a very wide range of deformation rates, only three nonlinear viscoelastic material parameters are needed: While the rotational parameter, $a2,$ and the structural parameter, β, are found to be equal for the two melts considered, the melts differ in the parameter $fmax2$ describing maximum stretch of the polymer chains.