Slip in Entangled Polymer Melts. 2. Effect of Surface Treatment

Abstract

The influences of various physical and chemical characteristics of solid substrates on apparent slip violations in entangled polybutadienes are investigated in plane-Couette shear flow. Slip measurements are reported for a range of substrates: (i) clean silica glass, (ii) uncleaned silica glass, (iii) fluorocarbon-coated silica, (iv) clean roughened silica, and (v) polybutadiene-grafted silica. Though the magnitude and details of the measured slip velocities vary dramatically from substrate to substrate, our findings all appear consistent with a shear-induced polymer disentanglement explanation for apparent slip violations in entangled polymers. On uncleaned silica, at shear stresses beyond the critical shear stress σ* for macroscopic slip, we find extensive void and/or shear fracture formation near the polymer−silica interface. We show that these observations result from nonuniform slip velocities on the heterogeneous, unclean substrate. On clean roughened silica glass surfaces, no evidence is found of the stick-slip instabilities reported in Part 1 for smooth silica. This difference in slip dynamics is attributed to differences in the surface relaxation dynamics of polymer chains on the two substrates. On low-energy smooth silica, very large slip velocities are found over the entire range of shear stresses investigated. These slip velocities are, however, at least an order of magnitude lower than current expectations for slip over ideal nonadsorbing surfaces. Finally, slip velocity versus shear stress diagrams obtained for polybutadiene-grafted silica are found to be remarkably similar to those observed on bare silica. This finding provides additional support to an earlier contention that surface friction characteristics of long polybutadiene molecules spontaneously adsorbed onto a neutral substrate are reasonably approximated by those of entangled, end-grafted polymer chains.