Viscosity or the flow of a material is measured through the study of rheology. However, with rheology, there is an imposed stress that is typically designed to create a shear.
With extensional viscosity or extensional rheology, there is no shear but there is a force pulling on the test material. In many ways, it is the same as stretching a rubber band or pulling on any elastic material. There is an elongation force but without any shear. Just to be precise, it is the absence of pressure or strain when the layers in the material are shifted in a lateral direction in relation to each other.
For most materials, the forces needed to create extensional viscosity will be more than the forces or pressure needed to create shear. With polymer materials that have very long, repeating molecular structures, this stress can be magnified. To add to this, the long individual chains of molecules in polymers can intertwine and tangle with each other, creating shapes that are even more resistant to deformation and extension.
The higher the resistance to deformation due to extension, the greater the viscosity of a particular polymer will be. Polymers are specifically designed to have high levels of viscosity to create very durable and strong materials under high levels of pressure and temperature.
Measurement Options
There are several different types of devices and testers that have been developed to accurately measure extensional viscosity. These are also known as tensile testing devices. They can be used to determine specific types of polymer melts with very high degrees of accuracy based on the flow behavior of the sample.
It is important to realize that these are not the same types of devices used for general rheology testing. The standard devices will all use shear, which is not the same type of force needed to measure elongational flow behavior.
These machines can be designed to be continuous in operation, providing continual quality control and monitoring options for manufacturers. Additionally, these online systems can be used in conjunction with other traditional types of capillary rheometers to provide a full testing profile for materials including both extensional and shear data.
Different equipment can offer variable speeds in linear or exponential operations as well as different functions and features. Choosing the correct testing system will be a factor of the specific requirements for the manufacturing or testing facility as well as the types of materials to be tested.