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MCR 702 Characterizing samples with edge fracture

Introduction

When characterizing the rheological properties of a sample, a number of external influences need to be taken into consideration, such as temperature and pressure. Furthermore, sample-specific properties must be taken into account when choosing a suitable measuring geometry and defining a measuring regime, in order to avoid any misinterpretation of the measuring results. For example wall-slip effects and shear banding but also turbulent flow may result in erroneous measuring results. [1]

A main limitation for characterizing polymer melts and concentrated polymer solutions at large deformation and/or high shear rates is known as edge fracture. This kind of sample instability is characterized by a deformation of the sample's surface at the free edges between the upper and lower part of the geometry.

Furthermore, secondary flow effects may develop within the sample at the edge. Deformation of the surface and secondary flow propagate radially as a function of both time and applied deformation. Hence, edge fracture results in increasing measuring errors within standard cone/plate or plate/plate geometries when presetting large deformations and/or high shear rates. Consequently, the accuracy of start-up shear measurements and flow curves at high shear rates as well as for large-amplitude oscillatory shear (LAOS) measurements can be strongly influenced by edge fracture.

In order to reduce the influence of edge fracture in measurements, a specific measuring system has been recommended (e.g. [2]–[5]). This measuring system consists of a cone and a partitioned plate and will be hereafter called cone partitioned plate (CPP).

The aim of the following measurements is to highlight the difference in the measuring performance when using such a CPP in comparison to conventional cone/plate geometries.

[1] Mezger, T.G. (2014). The Rheology Handbook. 4th Ed., Vincentz Network, Hannover.

[2] Meissner, J., Garbella, R.W., Hostettler, J. (1989). Measuring normal stress differences in polymer melt shear flow. J. Rheol. 33, 843-864.

[3] Schweizer, T. (2002). Measurement of the first and second normal stress differences in a polystyrene melt with a cone and partitioned plate tool. Rheol. Acta 41, 337–344.

[4] Schweizer, T. (2003). Comparing cone partitioned plate and cone standard-plate shear rheometry of a polystyrene melt. J. Rheol. 47, 1071-1085.

[5] Snijkers, F., Vlassopoulos, D. (2011). Cone partitioned plate geometry for the ARES rheometer with temperature control. J. Rheol. 55, 1167-1186.

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