DETERMINATION OF THE ELONGATIONAL BEHAVIOR OF POLYPROPYLENE MELTS FROM TRANSIENT SHEAR EXPERIMENTS USING WAGNER MODEL

This work deals with the nonlinear viscoelastic behavior of commercial polypropylenes on the basis of Wagner's constitutive equation. First, shear dynamic experiments are performed to determine the relaxation spectrum assuming a log-normal law which is in agreement with the theory of linear viscoelasticity. Secondly, shear and elongation experiments are performed at constant strain rates to obtain the damping function, while checking the separability assumption of the memory function into a time-dependent linear function and a strain-dependent nonlinear part. The damping function is described by a single law, partially independent of molecular parameters and which can be used either for shear or elongation flows through a general invariant of the relative Finger strain tensor.