Rheology of molten polymers

This chapter focuses on the rheology of molten polymers. A central concept in rheology is that force applied to materials results in deformations for solids or flow for fluids. The relative deformation is called strain and the applied force per unit area is called stress. The main objective of rheology is measurement, examination, and development of relations between stresses and the corresponding strains and the change of strains per unit time (strain rates). There are two rheological phenomena that manifest themselves during the flow of immiscible polymer melts through dies, which include layer nonuniformity and interfacial instability. Layer nonuniformity in coextrusion flows is caused mainly by the less viscous polymer going to the high shear region (i.e. the wall), thereby producing encapsulation. Layer nonuniformity can also arise in feedblock coextrusion, in which melt streams are merged into a single stream in a feedblock prior to entering the flat die. Uneven flow leakage from the flat die manifold to the downstream die sections can lead to encapsulation of the more viscous polymer by the less viscous, or even the reverse. The commercially available instruments for measuring the various rheological properties can be classified into melt indexers, capillary viscometers, rotational rheometers, and torque rheometers. Melt indexers are inexpensive and they are used for quality control. Capillary instruments are equipped with one or two pistons for simultaneous measurements through a long and a very short (orifice) die. Rotational instruments that have either cone-and-plate or parallel disk geometry are operated in either steady or dynamic (oscillatory) modes.