On estimating the zero-shear-rate viscosity: Tests with PIB and PDMS

The zero-shear-rate viscosity η0 is a limiting value that cannot be measured directly; rather, it must be estimated by extrapolation. The value determined by extrapolation will depend on the model used, and the range and quality of the data used for the extrapolation. The error can be surprisingly large. The ongoing study described in this paper has several objectives including development of methods for bracketing the true value, and for improving the estimation process with less-than-ideal sets of data. These methods have been evaluated using data for two room-temperature polymer melts: poly(dimethyl siloxane) (PDMS) and poly(isobutylene) (PIB).