Complex transients in the capillary flow of linear polyethylene

The spurt how behavior of a linear high-density polyethylene was carefully studied in capillary experiments. Depending on the flow conditions, new typ es of complex transients were observed. The study pointed out a second osci llating area, described by a second hysteresis cycle, close to or inside th e classical one. In one case, the second oscillating area was situated betw een two parts of the stable second branch, and pressure oscillation pattern s for the two distinct oscillating areas were clearly reported. In another case, the smaller secondary oscillations occur within the cycle of the prim ary oscillations, showing double instability pressure patterns. The transie nts in the capillary flow of linear polyethylene put in evidence a complexi ty, which cannot be explained only in terms of melt compressibility associa ted with a simple relationship between slip and shear stress. (C) 2000 The Society of Rheology. [S0148-6055(00)01305-5].