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 s pectrum assuming a log-normal law which is in agreement with the theor y of linear viscoelasticity. Secondly, shear and elongation experiment s are performed at constant strain rates to obtain the damping functio n, while checking the separability assumption of the memory function i nto a time-dependent linear function and a strain-dependent nonlinear part. The damping function is described by a single law, partially ind ependent of molecular parameters and which can be used either for shea r or elongation flows through a general invariant of the relative Fing er strain tensor.