Temperature-dependent instabilities in the capillary flow of a metallocenelinear low-density polyethylene melt

The capillary flow behavior of a metallocene linear low-density polyethylen e was studied in a wide temperature range. The critical shear stress for th e onset of the unstable spurt flow was found to be dependent on temperature in a nonlinear fashion and it showed a minimum value at a critical tempera ture, at which unusually long period pressure oscillations were observed. F or temperatures above the critical one, the observed decrease of the critic al shear stress with decreasing temperature is explained on the basis of an increase in the distance between entanglements. At temperatures below the critical one, the increase in the critical shear stress and the eventual su ppression of pressure oscillations as the temperature is further decreased are suggested to be the result of a flow-induced phase change that ends on complete crystallization and suppression of flow. The flow-induced crystall ization phenomenon and the extrudate quality were dependent on the contract ion ratio. Elimination of surface extrudate distortions took place at low t emperatures when using a contraction ratio of 30, this fact can be attribut ed to the flow-induced phase change. Finally, a decrease in the activation energy for flow or "easy flow" was observed at temperatures below the criti cal one. Such ''easy flow'' is perhaps the precursor of the ''temperature w indow'' of low-flow resistance reported by Keller and co-workers [Kolnaar, J. W. H. and A. Keller, Polymer 35, 3863-3874 (1994); Waddon, A. J. and A. Keller, J. Polym. Sci. Polym. Part B: Polym. Phys. 28, 1063-1073 (1990)]. ( C) 2000 The Society of Rheology. [S0148-6055(00)00203-0].