Recent advances in extensional rheology: controlled flows and fracture

Extensional deformation and flow occur in a number of polymer processing operations such as fiber spinning and film blowing. To understand and analyze material behavior in such operations, accurate and quantitative measurements of the rheological properties in well-defined extensional deformations are needed. However, while shear flows may in principle easily be generated in geometries such as cone-and-plate and plate-and-plate, the generation of controlled extensional flows has proven to be a challenge [1]. This presentation will focus on the application of Filament Stretching Rheometry (FSR) to measure extensional flow properties of polymeric liquids. It will be demonstrated, that the FSR technique enables measurements in start-up of constant extensional rate flow to sufficiently large Hencky strain so that steady flow is attained for several polymer systems. In addition the FSR may be used to study the relaxation of stress after cession of flow and the transient extensional creep under conditions of constant stress.
Combination of the FSR technique with other methods gives further possibilities. For example high speed imaging may be used to study and analyze crack profiles under filament fracture [2]. Likewise the deuterium labeling of selected molecules in combination with the FSR allows for the study of molecular relaxation by small angle neutron scattering [3].