Relationships between molecular structure and sharkskin defect for linear polymers
Résumé
This work is focused on the sharkskin defect encountered during the extrusion of linear polymers. This defect is characterized by the development of surface cracks, perpendicular to the flow direction. Our purpose is to establish relationships between sharkskin onset and rheological and molecular properties of the extruded polymer. Starting from the elasticity theory of Griffith, we show that the period of the defect is proportional to its amplitude, and that the critical stress for the onset of sharkskin is a function of the plateau modulus, the weight average molecular weight and the molecular mass between entanglements. On the other hand, we show that the cracks propagation velocity is controlled by the extrudate velocity and the Rouse time. We explain that the critical shear rate for the onset of sharkskin depends on the longest time of the distribution of relaxation times, i.e. the tube renewal. Finally, the present approach allows to clarify the ambiguity of simultaneous apparition of sharkskin defect and wall slip, as reported in the literature.