Wall slip in polymer melts: A pseudo-chemical model

A chemical-type theory for wall slip in polymer melts is developed by modeling the exchange of bridging sites between two opposing polymeric and solid surfaces. Kinetic equations, describing surface coverage by bridging monomers, are formulated and analyzed to evaluate the stability of adhesive contact and slip characteristics of the viscoelastic melt. Order of magnitude estimates of the kinetic coefficients suggest that the polymer-solid interface is always at equilibrium, even under slip. The model displays the following features. The polymer slips at all stresses; the slip velocity, v(s), obeys time-free volume superposition and depends on both sheer and normal stresses. At small stresses, v(s) is linear in shear stress and proportional to a function of the work of adhesion; the slip parameter b (the slip extrapolation length scale) takes on the same form as that proposed by de Gennes, but displays an additional dependence on adhesive energy. At constant v(s) the shear stress is proportional to the adhesive free energy. A catastrophic loss of adhesion occurs at a critical stress that depends on the difference between the work of adhesion (polymer-solid) and the work of cohesion (polymer-polymer). Predictions compare favorably with literature data for slip of linear low-density polyethylene on metal. (C) 1998 The Society of Rheology.