Measurements of Flow Fields of Polymer Melts by Laser-Doppler Velocimetry
Capillary rheometers and processing machines are “black boxes” regarding the knowledge of the flow fields of polymer melts within them. With the laser-Doppler velocimetry it is possible to quantitatively measure velocity fields. The flow behavior of vario
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Measurements of Flow Fields of Polymer Melts by Laser-Doppler Velocimetry
16.1 Motivation The formulas for the evaluation of material functions from rheological experiments as presented in the foregoing sections are based on certain assumptions concerning the underlying flow fields. Adhesion to the wall is the fundamental condition for all the derivations. But not all polymer melts stick to the wall and there are cases for the processing of polymer melts in which slippage on the walls of a tool is deliberately generated (cf. Sect. 17.3). Therefore, it is of fundamental interest to know the conditions under which the flow of a polymer melt takes place. Besides that, there are phenomena particularly connected with capillary flow which can only be understood by looking at the flow fields in detail. Examples are the extrudate swell and the various appearances of melt fracture. In general, the flow in a capillary rheometer, which is frequently applied to determine viscosity functions and which is the base of the widely used melt indexer, has to be regarded as very complex as it is composed of an entry flow region and the shear flow in the capillary. Some attempts were undertaken to get an insight into the flow pattern by observing differently dyed polymer layers during or after extrusion. A big disadvantage of this method is that the single fluid elements cannot be distinguished from each other and, therefore, a quantitative evaluation of the velocities is difficult. Another visualization method is the streak photography. It is based on tracer particles added to the fluid the motion of which is followed up by photography or more recently by particle image velocimetry. From these methods, in principle, particle velocities can be determined, but the results from the literature show that the accuracy is not high enough for getting reliable quantitative results for geometrically complex flows. A method to measure velocity fields with a much higher accuracy is the laserDoppler velocimetry (LDV). Developed for the determination of velocities in gases, it was used in the 1960’s for measurements in water [1]. Later, first LDV measurements were performed on polymer solutions, e.g., [2, 3, 4, 5]. H. Münstedt and F. R. Schwarzl, Deformation and Flow of Polymeric Materials, DOI: 10.1007/978-3-642-55409-4_16, Ó Springer-Verlag Berlin Heidelberg 2014
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Measurements of Flow Fields of Polymer Melts by Laser-Doppler Velocimetry
LDV-measurements on polymer melts are very scarce in the literature. First results were reported 1980 on a low density polyethylene [6], and later ones on low and high density polyethylenes [7], on a high density polyethylene [8] and a polybutadiene [9]. Particularly for polymer melts, the accuracy of the measurements was not good enough to get a quantitative picture of the velocity distributions in the secondary flow regions which are of special interest, however, with respect to the features of the entry flow and their dependence on the molecular structure of the polymers. One reason for this is
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