Studies on Wall-Slip in Entangled Polymeric Liquids

In this work, it is attempted to theoretically understand the phenomenon of wall slip through empirical and mol-ecular models. Initially, we use the framework a transient network theory. We show that the severe disentan-glement 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 debond-ing 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-poly-mer 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 pre-dictions of unified model, we measure the critical stress for sudden slip due to debonding for various tempera-tures 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 high-energy wall and which are entangled with the bulk chains flowing past them. We show that the restricted relax