Relationship Rheological Behavior and Molecular Architecture of LDPE’s Designed for Extrusion Coating

In this paper the relationship between molecular structure and behavior of the LDPE melt is discussed from a scaling perspective on the bases of results from GPC-MALLS analysis. Long chain branches introduce a characteristic length scale that marks a transition in rheology behavior of the melt with decreasing length scale of deformation i.e. with increasing process rate or decreasing temperature. The characteristic length scale is associated with intramolecular entanglements that cause a change in swell behavior of the polymer coil in solution or the melt. This change in swell behavior affects interaction with neighboring molecules in the melt. The characteristic length scale can be expressed in terms of a molecular weight, referred to as the “cross-over” molecular weight. Autoclave LDPE can be characterized by a single “cross-over” molecular weight while for tubular products a range of cross-over molecular weights appears to be required. In describing the flow behavior of the polymer melt, “cross-over” molecular weight gains importance with increasing length scale of deformation (slow processes, high temperature) while the effect of the overall molecular weight distribution expressed in one or more molecular weight moments, loses importance. With decreasing length scale of deformation (fast processes, low temperature), the situation is reversed. Experimental results are presented for the melt index measurement and for the behavior of the contraction of the polymer web (neck-in) in extrusion coating.