Low shear stress wall slip in polymer melts : an experimental investigation

Abstract

Wall slip in flowing polymer melts is a controversial proposal and is thought to be intimately linked with melt flow defects. An abundance of confusing experimental data currently exists, severely limiting understanding of both phenomenon. Investigations into the low shear stress wall slip behaviour in polymer melts using a previously overlooked reducing gap experimental technique have been made. A modern Force Rebalance Transducer, parallel plate torsional rheometer was used to measure the rheology of monodisperse polystyrene (MDPS) standards. After calibration and preparation procedures, the rheology of a Newtonian mineral oil was measured. The necessities of meticulous preparation were vindicated by constant viscosity to separations as small as 2 microns. Previously unsubstantiated claims of drift, poor alignment and separation performance which have limited experimental development in this direction were shown to be false. The terminal region rheology of various molecular weights MDPS standards at 210 °C was ascertained at separations as small as 5 microns. A gap dependency was found below separations of approximately 100 microns. Stress levels dropped to less than 20% of their ‘normal4 values at gaps of approximately 10 microns. The observed dependency was independent of both tool surface topology or material of construction, and shear rate within the linear terminal region. The gap dependency was more severe for higher molecular weight samples. Samples below the critical molecular weight for entanglements showed similar behaviour to that of the Newtonian oil. Geometry dependent flow properties indicate wall slip, either apparent or true. An analysis considering apparent wall slip, initiated by lubricating layering or surface structure formation mechanisms, poorly predicted observed events and these propositions were deemed infeasible. The gap dependency was analysed according to deGennes’ theory of true wall slip for flowing polymer melts. The degree of slip was found independent of the shear rate and the characteristic slip dimension, b, proportional to molecular weight to the 1.11 power. deGennes theory correctly predicted the shear rate dependency of observed wall slip, but failed to describe the relationship with molecular weight. Since no wall slip occurred for the Newtonian oil under identical conditions, it was concluded that true wall slip was being measured for polymer melts. True wall slip was measured in flowing polymer melts at stress levels approximately two orders of magnitude below purported critical levels observed using a capillary rheometer. Furthermore, capillary rheometers lack the resolution to measure these small slip values. Temperature and molecular weight dependencies exhibit rich and complicated behaviour, with local maximums in the slip length being observed as a function of both temperature and molecular weight on occasion. Overall, slip decreased with temperature and molecular weight. High stress slip behaviour (capillary rheology) can be adequately modelled using Eyring’s activation theory. This slip velocity is proportional to the hyperbolic sine of the shear stress, resulting in characteristic ‘S’ shaped curves. At large shear stresses, this behaviour may be easily mistaken for power law behaviour over small stress ranges.