Rheology of molten polymers

Abstract

TheCross equation describes the flow of pseudoplastic liquids in terms of an upper and a lower Newtonian viscosity corresponding to infinite and zero shear,η∞ and η0, and of a third material constantα related to the mechanism of rupture of linkages between particles in the intermediate, non-Newtonian flow regime, Calculation ofη∞ of bulk polymers is important, since it cannot be determined experimentally. The equation was applied to the melt flow data of two low density polyethylenes at three temperatures. Using data in the non-Newtonian region covering 3 decades of shear rate to extrapolate to the zero-shear viscosity resulted in errors amounting to about onethird of the measuredη 0 values. The extrapolated upper Newtonian viscosity was found to be independent of temperature within the precision of the data, indicating that it has a small activation energy. Theη 0 values were from 100 to 1,400 times larger than theη∞ values at the corresponding temperatures. The values ofα were large compared to theα values found for colloidal dispersions and polymer solutions, but decreased with increasing temperature. This shows that shear is the main factor in reducing chain entanglements, but that the contribution of Brownian motion becomes greater at higher temperatures.