RHEOLOGICAL BEHAVIOR OF DEGRADED POLYPROPYLENE MELTS - FROM MWD TO DYNAMIC MODULI

The linear viscoelastic behavior of polydisperse polypropylenes in the melt is predicted using the molecular weight distribution (MWD) as de termined from gel permeation chromatography, on the basis of simplifie d molecular dynamics: single exponential form of the relaxation modulu s of narrow fractions, double reptation, tube renewal, and constraint release. Owing to a few approximations, the calculation only requires a few parameters, namely the scaling law for the zero shear viscosity of narrow fractions eta(0) = f(M), the plateau modulus G(N)(0), and th e value of the molecular weight between entanglements M(e). Using this method a relaxation spectrum of Maxwellian contributions with a large number of modes is obtained. This spectrum well predicts the rheologi cal behavior in the terminal zone of samples obtained by controlled pe roxydic degradation of polypropylene with polydispersity ranging from 4 to 10. Attention is focused on the zero shear rate viscosity, freque ncy, and modulus of the crossover of the storage and loss moduli from experiments and calculations, because these parameters are generally t hought to be sensitive to both average molecular weight and polydisper sity and are relatively easy to get from dynamic experiments. How the initial spectrum can be conveniently reduced to a more simple spectrum with only a few modes, without significant loss of information, is sh own. This spectrum may be useful and time saving in calculations, for example, to describe the memory function in nonlinear constitutive equ ations while keeping its physical meaning in relation to the MWD. (C) 1996 John Wiley & Sons, Inc.