Wall Slip of Bidisperse Linear Polymer Melts

We have characterized the effect of molecular weight distribution on slip of linear 1,4-polybutadiene samples sandwiched between cover glass and silicon wafer. Monodisperse polybutadiene samples with molecular weights in the range of 4-195 kg/mol and their binary mixtures were examined at steady state in planar Couette flow using tracer particle velocimetry. Slip velocity was measured at shear rates over the range of similar to 0.1-15 s(-1). Our results revealed that weakly entangled short chains play a crucial role in wall slip and flow dynamics of linear polymer melts. It was found that the critical shear stress for the onset of the transition to the strong slip regime is significantly reduced when a small amount of weakly entangled chains is added to a sample of highly entangled polymer. Within the same range of shear stresses, binary mixtures of long and short chains exhibit significantly enhanced slip compared to the moderate slip of the individual long chains. This is attributed to the reduced friction coefficient at the polymer-solid interface which is likely a complicated function of the nature of entanglement, chain adsorption, and relaxation dynamics of chains at the interface.