INTERFACIAL MOLECULAR-INSTABILITY MECHANISM FOR SHARKSKIN PHENOMENON IN CAPILLARY EXTRUSION OF LINEAR POLYETHYLENES

A comprehensive study of sharkskin behavior in linear polyethylene ext rusion is carried out to explore its molecular origin. Experimental ch aracteristics are analyzed as a Function of temperature, applied stres s, and die surface condition. The experimental data favor an interfaci al molecular instability (IMI) mechanism for sharkskin formation over a noninterfacial continuum mechanical mechanism. The effect of a local cooling of the die exit is demonstrated to be predictable by the prop osed IMI mechanism. The IMI mechanism states that sharkskin occurs bec ause of a local conformational transition at the die exit wall where t he adsorbed chains entrap a layer of interfacial chains. This layer os cillates between entanglement and disentanglement states due to a reve rsible coil ti stretch transition. The corresponding oscillation of th e exit wall boundary condition leads to cycles of local stress relaxat ion and growth and to periodic perturbation of the extrudate swell in the form of sharkskinlike surface roughening on the extrudate. (C) 199 8 The Society of Rheology.