The relation between melt flow properties and molecular weight of polyethylene

The non‐Newtonian behavior of commercial linear polyethylene samples and their fractions were studied at 190°C. The viscosity η versus shear rate \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} curves of whole polymers could be superimposed onto a single master curve despite the variations of their molecular weights and molecular weight distributions. For fractions, however, the same master curve was inapplicable, and the sensitivity of the viscosity to shear rate was found to be greater than those of the whole polymers. The zero‐shear viscosities η 0 of fractions were related to the 3.42 power of the weight‐average molecular weight M u as follows:\documentclass{article}\pagestyle{empty}\begin{document}$ \eta _0 = 2.39 \times 10^{ - 15} M_{w^{3.42} } . $\end{document} For whole polymers, the zero‐shear viscosities were found to be considerably higher at the same M w and markedly lower at the same z ‐average molecular weight M z than those of the fractions. Thus, it was concluded that η 0 corresponds to an average of molecular weight between M w and M z . It was found that the molecular relaxation time τ is proportional to M z 5.3 for whole polymers and to η 0 M w for fractions. Using these relations it was possible to relate the flow ratio, the ratio of flow rates at two different shear stresses, with the molecular weight distribution.