Interfacial molecular instability mechanism for sharkskin phenomenon in capillary extrusion of linear polyethylenes

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Abstract
A comprehensive study of sharkskin behavior in linear polyethylene extrusion is carried out to explore its molecular origin. Experimental characteristics are analyzed as a function of temperature, applied stress, and die surface condition. The experimental data favor an interfacial 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 proposed IMI mechanism. The IMI mechanism states that sharkskin occurs because of a local conformational transition at the die exit wall where the adsorbed chains entrap a layer of interfacial chains. This layer oscillates between entanglement and disentanglement states due to a reversible coil↔stretch transition. The corresponding oscillation of the exit wall boundary condition leads to cycles of local stress relaxation and growth and to periodic perturbation of the extrudate swell in the form of sharkskinlike surface roughening on the extrudate.