MOLECULAR CHARACTER OF SHARKSKIN PHENOMENON IN METALLOCENE LINEAR LOW-DENSITY POLYETHYLENES

The effect of molecular weight on sharkskin dynamics is systematically studied using three metallocene-catalyzed linear low density polyethy lenes (mLLDPE) of different molecular weights. A new experimental meth od is developed to enable in-situ characterization of the time scale t au on which sharkskin forms on the extrudate. This laser scattering te chnique directly measures the periodic variation of the refracted ligh t of the rough sharkskin-like extrudate surface. It is found that tau increases with molecular weight as strongly as the overall molecular c hain relaxation time, tau, determined from oscillatory shear measurem ents. The observed molecular weight dependence provides additional sup port for the recently proposed interfacial stress growth/relaxation me chanism for sharkskin formation.