Slip in entangled polymer melts. 1. General features

Apparent violations of the no-slip boundary condition are studied using a s eries of narrow molecular-weight distribution polybutadiene melts (67300 le ss than or equal to (M) over bar(n) less than or equal to 650000), subjecte d to plane-Couette shearing over clean silica glass surfaces. Simultaneous measurements of slip velocity and shear stress reveal several new molecular characteristics of slip in entangled polymers. log-log plots of slip veloc ity versus shear stress display three distinct power-law regimes: (i) A wea k slip regime at low shear stresses that is characterized by extrapolation/ slip lengths b of the order of a few micrometers; (ii) A stick-slip regime at intermediate shear stresses marked by periodic oscillations in slip velo city and shear stress; (iii) A strong slip regime beyond a defined critical shear stress sigma*. Slip violations in this last regime are characterized by large slip velocities and massive extrapolation lengths (b(infinity) si milar to 100-1500 mu m). For all polymers studied the critical stress sigma * for the weak-to-strong slip transition is found to be proportional to the plateau modulus G(e) of the bulk polymer, sigma* approximate to (0.2 +/- 0 .02) G(e). This finding is consistent with a shear-induced polymer disentan glement explanation for apparent slip violations in entangled polymers. Our experimental observations are also found to be in good agreement with a re cently proposed scaling theory for friction and slip in entangled polymers, which assumes noninteracting surface chains. We rationalize this last resu lt in terms of a polymer adsorption model in which a single macromolecule s pontaneously attaches to numerous surface sites, yet offers a sufficiently long tail to resist relative motion of a chemically identical bulk polymer that attempts to slide over it.