Effect of molecular structure on polyethylene melt rheology. III. Effects of long‐chain branching and of temperature on melt elasticity in shear

Abstract

AbstractThe effects of long‐chain branching and of temperature on the melt elasticity in shear of polyethylene were investigated using die swell measurements and relating them to recoverable shear strain, normal stress, and shear modulus. Die swell measurements, as a function of shear rate, were obtained for high‐ and low‐density polyethylenes at temperatures ranging from 130° to 225°C. The samples were characterized by GPC and intrinsic viscosity for molecular weight distributions and degrees of long‐chain branching. The importance of annealing the extrudates at temperatures above the polymer melting temperature to achieve equilibrium, or strain‐free, values of die swell was demonstrated. The effect of long‐chain branch was to decrease elastic deformation. At constant shear stress, the melt elasticity of both high‐ and low‐density polyethylene was found to be essentially independent of temperature. Thus, at constant shear rate, elastic deformation decreased with increasing temperature, and it was demonstrated that this decrease could be quantitatively defined in terms of previously determined shear rate–temperature viscosity superposition shift factors.