ELONGATIONAL BEHAVIOR OF A LOW-DENSITY POLYETHYLENE MELT .2. TRANSIENT-BEHAVIOR IN CONSTANT STRETCHING RATE AND TENSILE CREEP EXPERIMENTS - COMPARISON WITH SHEAR DATA - TEMPERATURE-DEPENDENCE OF THE ELONGATIONAL PROPERTIES

In addition to earlier findings of Meißner on a low density polyethylene that in the linear range of deformation the time-dependent elongational viscosity is three times the shear viscosity the validity of the relationship {Mathematical expression} could be demonstrated where {Mathematical expression} and {Mathematical expression} describe the compliances in elongation and shear, respectively. In the nonlinear range J(t) was found to increase with stress whereas D(t) decreases. The nonlinear viscosity-time behaviour is different in shear and elongation, too. The shear viscosity obtained by relating the stress to the rate of viscous strain decreases with shear from the zero shear viscosity to its steady-state value. The elongational viscosity goes up with time from 3 η0 to a higher steady-state value. The temperature dependence which was determined by measuring the steady-state elongational viscosities at different constant stresses as a function of temperature and by shifting the tensile creep compliances D(t) measured at constant stress with respect to time, was found to be exactly the same as in shear. This result could indirectly be verified by some constant stretching rate tests. It was shown experimentally that the recoverable strain in the steady-state is independent of temperature if measured at constant stress. © 1979 Dr. Dietrich Steinkopff Verlag.