Wall slip and shear stresses of polymer melts at high shear rates without pressure and viscous heating effects

We present experimental data demonstrating wall slip of high temperature po lymer melts at high shear rates (up to approximate to 70 s(-1)) without pre ssure and viscous heating effects. Results for polydisperse (linear low den sity polyethylene) and monodisperse (polystyrene) melts are presented. For the monodisperse case two distinct regimes of slip are seen: (1) a low shea r rate regime in which the slip velocity increases slowly with shear rate ( weak slip) and (2) a high shear rate regime in which the increase is dramat ic (strong slip). Through scaling of our shear stress/shear rate data we de monstrate that the crossover between the two regimes occurs when the bulk p olymer chains are effectively disentangled. These findings are in qualitati ve agreement with molecular models of slip which invoke an entanglement-dis entanglement scenario for the transition from weak to strong slip. The poly disperse system shows less critical sip behaviour and the slip velocity inc reases; yet, the increase is less dramatic than the monodisperse system. (C ) 2000 Elsevier Science B.V. All rights reserved.