Rheological behavior of degraded polypropylene melts: From MWD to dynamic moduli

Abstract

SYNOPSISThe linear viscoelastic behavior of polydisperse polypropylenes in the melt is predicted using the molecular weight distribution (MWD) as determined from gel permeation chromatography, on the basis of simplified molecular dynamics: single exponential form of the relaxation modulus 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 vo = f ( M ) , the plateau modulus Gk, and the value of the molecular weight between entanglements Me. Using this method a relaxation spectrum of Maxwellian contributions with a large number of modes is obtained. This spectrum well predicts the rheological behavior in the terminal zone of samples obtained by controlled peroxydic degradation of polypropylene with polydispersity ranging from 4 to 10. Attention is focused on the zero shear rate viscosity, frequency, and modulus of the crossover of the storage and loss moduli from experiments and calculations, because these parameters are generally thought to be sensitive to both average molecular weight and polydispersity 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 shown. This spectrum may be useful and time saving in calculations, for example, to describe the memory function in nonlinear constitutive equations while keeping its physical meaning in relation to the MWD. 0 1996

Continuous soluble Ziegler‐Natta ethylene polymerizations in reactor trains. I. Mathematical modeling

“…As the viscosity of the polymer solution depends basically on the MWD of the polymer produced, as polymer concentrations do not vary very much, it may be related to the melt flow index of the polymer as 17 :…”

Polydispersity index from linear viscoelastic data: unimodal and bimodal linear polymer melts

“…where k E is the constant of proportionality and generally lies between 3.3 and 3.7, except for extremely broad blends, for which higher values are observed [13][14][15][16]. is often taken as 3.4, since this is the value usually obtained for most linear polymers investigated.…”

Nitroxide‐mediated controlled degradation of polypropylene

“…Because of that, these products are referred to as Controlled-Rheology Polypropylenes (CRPP). The importance of this can be shown by referring to Several researchers have studied MWD of CRPP (Tzoganakis (1988, Malz (1995), Bonilla -Rios et al (1995), Carrot et al (1996), BonillaRios et al (1998), Asteasuain et al (2003)). In general, it has been concluded that CRPP resins have lower molecular weight, narrower MWD and reduced viscosity.…”