ABSTRACT
The objective of this chapter is to give a critical review of existing models which can be used to estimate the elongational viscosity of polymer melts. These models will be compared to uniaxial elongational viscosity measurements. The processing of polymer melts involves both shear and extensional deformations. In simple shear flow the velocity changes in the direction perpendicular to the flow direction. Material elements are not stretched along the streamlines contrary to elongational flow. Two types of axisymmetrical elongational flows exist. The most investigated type of extensional flow is simple extension. In this type of flow a cylinder is stretched as shown in Figure I (a). The cylinder becomes elongated in the direction of stretching. The other axisymmetrical flow is biaxial extension. In this flow the same initial cylinder is extended into a disk as indicated in Figure !(b). Planar extension on the other hand is a non axisymmetrical elongational flow since a block of material is deformed only in two directions (Figure I( c)). All these flows are shear free which greatly complicates the design of extensional rheometers to replicate these flows since the polymer must not adhere to any surfaces other than those which transmit the stretching force. Many extensional flows involved in polymer processing operations are not exactly of the type shown in Figures 1 (a) to 1 (c).
