Steady states in extensional flow of strain hardening polymer melts and the uncertainties of their determination

The existence of a plateau, a maximum or even an overshoot of the elongational viscosity of polymer melts as a function of time or extension, respectively, is a hot topic of scientific discussions. Although not essential for the relation of elongational properties to applications, the feature is crucial for a verification of various theories. From a critical review of sophisticated comparative stressing and creep experiments on a low density polyethylene (LDPE), the existence of a pronounced stationary elongational viscosity becomes very probable. These conclusions are supported by investigations on a long-chain branched polypropylene. For an LDPE of higher molar mass with stress maxima caused by local necking within the sample, the maximum viscosities agree with the corresponding steady-state viscosities obtained from creep experiments performed by the same rheometer. The decisive role of the uniformity of the sample deformation for the accuracy of elongational experiments is pointed out. From the findings presented, it becomes evident that a careful control of the sample uniformity is the basic condition for a critical assessment of differing results from the various experimental methods found in the literature. (C) 2013 The Society of Rheology.