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Yogesh M. Joshi
Studies on Wall-Slip in Entangled Polymeric Liquids
Appl. Rheol. 11:5 (2001) 277-280
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In this work, it is attempted to theoretically understand the phenomenon of wall slip through empirical and molecular
models. Initially, we use the framework a transient network theory. We show that the severe disentanglement
in the interfacial region can give rise to non-monotonic flow curve locally in that region. Further, we
generalize this model into a unified slip model, which predicts wall slip by either disentanglement or by debonding
mechanism, depending upon the adhesive energy of the wall-polymer pair. The model predictions of the
critical wall shear stress are in good agreement with experiments for various adhesive energies of the wall-polymer
pair. The model predicts that the temperature dependence of the critical wall shear stress for debonding is
different than that of disentanglement mechanism under certain experimental conditions. To validate the predictions
of unified model, we measure the critical stress for sudden slip due to debonding for various temperatures
using cone and plate viscometer with fluoroelastomer-coated cone. The temperature dependence of the
critical stress for instability (slip) on a coated cone is found out to be inversely dependent on temperature, which
expected for the case of debonding. In the final part of this thesis, we develop a parameter-free tube model for
predicting the stick-slip phenomenon. The model, which is based on the contour variable model [Mead et al.,
1998, Macromolecules, 31, 7895], considers the dynamics of the tethered chains, which are grafted on a highenergy
wall and which are entangled with the bulk chains flowing past them. We show that the restricted relaxation
modes of the tethered molecule give rise to discontinuous slip instability. More specifically, the slow relaxation
of the tethered chain due to the restricted convective constraint release is unable to randomize its
flow-induced orientation above a critical shear rate or stress. This decreases the resistance to flow for the bulk
chains, which suddenly slip past the oriented tethered chains.
► Cite this publication as follows:
Joshi YM: Studies on Wall-Slip in Entangled Polymeric Liquids, Appl. Rheol. 11 (2001) 277.
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