Local versus integral measurements of the extensional viscosity of polymer melts

A simple method to measure the local extensional properties of polymer melts using a modified Munstedt extensional rheometer is demonstrated. Real time imaging of the specimen during the extension process provides the local rates of strain as a function of the coordinate along the specimen. Simultaneously, the local stresses are assessed by measurements of the sample diameter along the sample and synchronous measurements of the force exerted on the bottom plate of the rheometer. The method provides a quantitative measure of the homogeneity of the deformation states via a kinematic and a dynamic fingerprint of the process. Test experiments using geometrically homogeneous samples reveal a good level of agreement between local and integral measurements of the transient extensional viscosity and thus validate the method. Furthermore, this technique is employed to assess quantitatively the impact of inhomogeneities of the deformation on measurements of the transient extensional viscosity. It is concluded that in the case of initially inhomogeneous samples, the extensional properties depend strongly on the position along the specimen and the usual integral measurements of the transient extensional viscosity are highly unreliable and unrepeatable. Higher Hencky strain measurements indicate that, although the sample inhomogeneity increases systematically during experiments, the integral viscosity data remain reliable as long as the samples were reasonably homogeneous in their initial state. (C) 2009 The Society of Rheology. [DOI: 10.1122/1.3237024]