Slip in entangled polymer melts. 2. Effect of surface treatment

The influences of various physical and chemical characteristics of solid su bstrates on apparent slip violations in entangled polybutadienes are invest igated in plane-Couette shear flow. Slip measurements are reported for a ra nge of substrates: (i) clean silica glass, (ii) uncleaned silica glass, (ii i) fluorocarbon-coated silica, (iv) clean roughened silica, and (v) polybut adiene-grafted silica. Though the magnitude and details of the measured sli p velocities vary dramatically from substrate to substrate, our findings al l appear consistent with a shear-induced polymer disentanglement explanatio n for apparent slip violations in entangled polymers. On uncleaned silica, at shear stresses beyond the critical shear stress sigma* for macroscopic s lip, we find extensive void and/or shear fracture formation near the polyme r-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 instabilitie s reported in Part 1 for smooth silica. This difference in slip dynamics is attributed to differences in the surface relaxation dynamics of polymer ch ains on the two substrates. On low-energy smooth silica, very large slip ve locities are found over the entire range of shear stresses investigated. Th ese slip velocities are, however, at least an order of magnitude lower than current expectations for slip over ideal nonadsorbing surfaces. Finally, s lip velocity versus shear stress diagrams obtained for polybutadiene-grafte d silica are found to be remarkably similar to those observed on bare silic a. This finding provides additional support to an earlier contention that s urface friction characteristics of long polybutadiene molecules spontaneous ly adsorbed onto a neutral substrate are reasonably approximated by those o f entangled, end-grafted polymer chains.