Stick and slip phenomena during extrusion of polyethylene melts as investigated by laser-Doppler velocimetry

The flow behavior of polyethylene melts in a slit die was investigated usin g laser-Doppler velocimetry. Two polyethylenes, a linear and a long-chain b ranched sample, were compared in order to get an insight into the influence of branching on stick and slip phenomena. For the long-chain branched poly ethylene, velocity profiles were obtained which do not give any indication to wall slip in the range of shear rates applied. For the linear polyethyle ne, the velocity distributions in the three regions, commonly distinguished by different pressure-output relations in the literature, were measured. P ronounced wall slip velocities are detected at low apparent shear rates. Th e importance of this finding for the determination of viscosity functions i s discussed. At higher output rates the well-known pressure oscillations ar e accompanied by velocity fluctuations of the same frequency but totally di fferent shapes of amplitudes. Significant periodic features of the time dep endence of the velocity are tried to be interpreted by a hypothesis of enta nglement and disentanglement of the molecules close to the wall. Exceeding an upper critical output value, a flow region is reached which is distingui shed by a constant pressure reading as a function of time again. Within thi s region a nearly ideal plug flow is observed, i.e., there is an indication for strong slip at the wall. (C) 2000 The Society of Rheology. [S0148-6055 (00)01002-6].