Transient elongational viscosities of aqueous polyacrylamide solutions measured with an optical rheometer |
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Authors: | Stefan Schuberth Helmut Münstedt |
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Institution: | (1) Institute of Polymer Materials, Friedrich-Alexander-University Erlangen–Nürnberg, Martensstr. 7, 91058 Erlangen, Germany |
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Abstract: | An anionic polyacrylamide solution was characterized in elongational flow by combining laser-Doppler velocimetry to determine
the strain rate in the flow direction and the two-color flow-induced birefringence method to measure the first normal stress
difference along the axial centerline of a hyperbolic die. The elongational rate was constant along the axial centerline of
the planar hyperbolic die as long as vortices at the die entrance did not occur. The transient elongational viscosity μ
+ was determined as a function of the elongational rate. The parameters varied are the Hencky strain rate and the polymer concentration.
μ
+ showed a pronounced increase over the linear viscoelastic behavior above critical Hencky strains of 1.2 to 1.5; that is, a significant strain hardening could be observed for polyacrylamide
solutions. This strain hardening is stronger the higher the elongational rate. A slight enhancement of strain hardening was
found by increasing the concentration from 0.5 to 1 g/l. The stress optical coefficient was determined as 1.8 × 10−7 Pa−1 (0.5 g/l) and 1.2 × 10−7 Pa−1 (1 g/l).
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Keywords: | Flow-induced birefringence Laser-Doppler velocimetry Extensional viscosity Polyacrylamide solution Hyperbolic die |
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