Rheological measuring techniques and their relevance for the molecular characterization of polymers

The zero shear viscosity η0, the linear steady-state compliance and the elongational viscosity are rheological quantities, which can preferably be used for a molecular characterization of polymers. The techniques applied to obtain reliable results are described. It is demonstrated that linear polyethylenes of high-density fulfill the relationship η0 ∼ Mw3.6 independently of the polydispersity. Mw is the absolute mass-average molar mass. Long-chain branched polyethylenes significantly deviate from this relation. The results from polyethylenes are used to characterize electron beam irradiated polypropylenes. It is clearly shown that long-chain branches are introduced by irradiation the topography of which changes with the dose. Measurements of the elongational viscosities support the conclusions from shear experiments. Creep tests in extension show the existence of steady-state elongational viscosities for the irradiated polypropylenes. The viscosities run through a maximum as a function of the elongational rates reached at steady state. The shape of the curves depends on the dose indicating once more a change in molecular structure by irradiation.