Melt viscosity of polyethylene at zero shear

Abstract

Abstract Melt viscosities of polyethylene fractions were measured at low rates of shear in a cone‐and‐plate rotational viscometer and extrapolated to zero shear viscosity. More accurate molecular weight determinations could be made on these fractions since they were free from extra large molecules. Such molecules are known to be present in unfractionated polyethylenes, and they interfere with light‐scattering molecular weight determinations. The zero shear viscosity of both high‐density and low‐density polyethylene was found to obey the equation where M̄ w is the light‐scattering molecular weight. At 165°C. the zero shear viscosities of both types of polyethylene could be correlated by one single equation, but at higher temperatures and more branched low‐density polyethylene has a lower melt viscosity. The activation energy of flow was found to be 7.5 kcal./mole and 14.6 kcal./mole for high‐density and low‐density polyethylene, respectively. This difference activation energy is attributed to long‐chain branches in low‐density polyethylene rather than short‐chain branches. A discrepancy was observed between molecular weights of many high‐density polyethylene fractions determined by light‐scattering methods and the molecular weight of the same fractions calculated from inherent viscosity using a known intrinsic viscosity–molecular weight relationship. Examination of the data showed that the existence of a small amount of long‐chain branches in these high‐density polyethylene fractions was the probable explanation for this discrepancy.