Rheological Properties of Isotactic and Atactic Propylene-Ethylene Copolymers

The shear viscosity and extrudate swell of the isotactic and atactic fractions of propylene-ethylene copolymers produced with different ethylene content were measured at various temperatures and shear rates, using a capillary rheometer. In the isotactic fractions of these copolymers, the theoretical ethyl ene content was in the range of 4 to 10 wt%. The results indicate that the shear viscosity is not sensitive to composition for the isotactic copolymers. The tem perature dependence of the viscosity seems to be about the same for both the isotactic and atactic fractions of these copolymers. The extensional viscosity of these copolymers was calculated using Cogswell's analysis of converging flows and it seems that the isotactic copolymers with higher ethylene content exhibit slightly higher extensional viscosity. Finally, results from extrudate swell measurements show that the isotactic copolymers exhibit higher swelling than the atactic fractions tested. Extrudate swell measurements were also carried out after annealing of the samples in an oil bath. No strong effect of the anneal ing process was observed.