Superfluid-Like Stick−Slip Transition in Capillary Flow of Linear Polyethylene Melts. 1. General Features

This paper explores rheological characteristics of and molecular mechanism for a superfluid-like stick−slip transition occurring under controlled pressure in capillary flow of a series of highly entangled linear polyethylene (PE) melts and establishes its connection with the spurt flow phenomenon. The transition is signified by a large discontinuity in the flow rate at a critical stress, resulting in a double value within the flow curve. The magnitude of the transition can be quantified in terms of an extrapolation length, b . In particular, the superfluid-like flow transition occurs throughout a range of temperatures from T = 180 to 260 °C as long as a critical stress, σ c, is exceeded. It is found that σ c increases linearly with T, and b c at the transition remains around 1.7 mm at all the temperatures for the PE (MH20) of weight-average molecular weight M w = 316 600. Thus the observed remarkably large interfacial slip is believed to be due to complete disentanglement of the adsorbed chains from free chains at the melt/wall interface at and beyond the transition. The amount of wall slip, as described by b, diminishes quickly with decreasing M w, in qualitative agreement with a simple scaling relation for noninteracting interfaces. The flow transition depends on the surface condition of the die wall and occurs at a considerably lower critical stress when the wall is treated by depositing a fluorocarbon elastomer to weaken the PE adsorption. Application of both controlled-pressure and controlled-piston speed conditions demonstrates that spurt flow instability originates from indeterminacy of the hydrodynamic boundary condition at the PE/die wall interfaces.