Extensional or elongational viscosity are two terms used to describe the same thing. This is the stress that pulls the sample in a straight line, so the force is elongated, without the shear force.
Every polymer has an extensional viscosity that is consistent within a sample or the same type of polymer. The elongational viscosity is significant for most polymers because of their structure. These tend to be made up of chained molecules that are intertwined and wrapped around other chains. The more intertwining that occurs in a polymer, the greater the force that will be needed to create the stretching action. The more that the polymer resists stretching, the higher the viscosity will be for the polymer.
How it is Measured
Many polymers, in their room temperature state, cannot be stretched or elongated because of their molecular structure. However, by taking a small sample of the polymer and heating it to the point of melting or a molten state, the stretching can occur.
This is not done by stretching the liquid on its own, but rather using a drawing stem of wheels, sometimes called a haul-off, to create filaments of the polymer. This can be done with multiple sets of wheels to create the optimal filament for testing.
This is done at a precise speed, and the different result can accurately determine the polymer in the sample. In effect, three speeds have to be controlled including the linear pull-off speed, the acceleration, and the exponential speed. The force required to elongate the strand is measured, which provides measurements for extensional viscosity, elongational stress, draw ratio and the rate of elongation.
Often extensional viscosity is measured in conjunction with other types of tests for a given polymer. When this is a factor, look for systems that can be integrated to streamline the testing process.