| Creep experiments with a solution of polystyrene (M-w= 2.6 MDa, 16 vol.%, 25 degrees C) in diethyl phthalate are reported for stresses between 100 and 2,500 Pa (approximate to 3G(N) (0)/4). The aim was to look for a flow transition as reported for strongly entangled poly( isobutylene) solutions. The experiments with the polystyrene solution were repeated for cone angles of 2, 4, and 6 ( radius 15 mm) and showed no dependence on cone angle. The Cox-Merz rule was not fulfilled for stresses beyond about 800 Pa. The tangential observation with a CCD camera showed that the edge took a concave shape because of the second normal stress difference. Beyond 1,000 Pa, the concave edge develops into a crevice, thus substantially reducing the effective cross-section. This leads to runaway in a constant torque experiment. At p(21)= 800 Pa, head-on particle tracking confirms that the originally linear velocity profile takes a gooseneck shape, thus revealing shear banding. When the creep stress is stepped down to 100 Pa, this velocity profile evolves back to a linear one. The conclusion from this work is that even if nonlinear creep experiments are reproducible and a steady state is reached, this does not mean that the flow field is homogeneous.
for LaTeX users @article{TSchweizer2007-46,
author = {T. Schweizer},
title = {Shear-banding during nonlinear creep with a solution of monodisperse polystyrene},
journal = {Rheol. Acta},
volume = {46},
pages = {629-637},
year = {2007}
}
\bibitem{TSchweizer2007-46} T. Schweizer,
Shear-banding during nonlinear creep with a solution of monodisperse polystyrene,
Rheol. Acta {\bf 46} (2007) 629-637.TSchweizer2007-46
T. Schweizer
Shear-banding during nonlinear creep with a solution of monodisperse polystyrene
Rheol. Acta,46,2007,629-637 |
mk@mat.ethz.ch
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