Elongational viscosity at a constant elongational strain rate of polypropylene melt

The measurement of elongational viscosity has been carried out with an improved elongational rheometer, sensitive to small elongational forces and measuring simultaneously the diameter of elongating samples. Force distribution was evaluated for various conditions. The method of reduced variables, successful in linear viscoelastic properties, can be applied to the temperature dependence of elongational viscosity only at small elongational strain rate. For large elongational strain rates, the elongational viscosity was found to be non-linear and a new method characterizing non-linearity was proposed. The non-linearity parameter in elongational viscosity was independent of elongational strain rate and almost independent of temperature. Non-linearity in elongational viscosity is discussed in terms of a structural change in the polymer melt.