Exploring Origins of Interfacial Yielding and Wall Slip in Entangled Linear Melts during Shear or after Shear Cessation

We study yielding at interface between a solid wall and entangled polymer melts. At a average rate higher than the terminal relaxation rate, a combination of rheometric and particle-tracking velocimetric (PTV) measurements reveals apparent wall slip (or interfacial yielding) at a strain gamma(iy), just beyond the stress overshoot. PTV observations show that the interfacial yielding occurs at a higher strain in the presence of a faster rate of shear. Shear cessation at a strain lower than gamma(iy) is observed to also result in interfacial failure, suggesting that the finite adhesion can be overcome in quiescence by the residual elastic retraction force. These two kinds of interfacial failure can be theoretically understood in terms of the intrachain elastic retraction forces overcoming the melt adhesion arising from interchain entanglement interactions between adsorbed and bulk chains.