MECHANISM OF SHEAR MODIFICATION OF LOW-DENSITY POLYETHYLENE

In this paper we address the aspects associated with shear modificatio n of low density polyethylene by extrusion. By successive passes of th e polymer through an extruder it can be shown that both the melt visco sity and melt elasticity are reduced by shear modification. These reve rsible variations are not accompanied by significant changes in molecu lar weight distribution. The major effects of extrusion are to decreas e the amount of elastically effective material. Study of proton relaxa tion times at 150-degrees-C shows that the melt comprises regions of r elatively ordered, entangled, and unentangled polymer. Shearing reduce s the proportion of more ordered material. The elastically effective a nchors that are reduced by shearing are regions with restricted segmen tal mobility rather than conventional entanglements or intermolecular knots. Successive extrusions produce a relatively small decrease in th e number of such constraints to molecular motion. Repeated shearing ha s a diminishing effect on the proportion of more ordered material in t he polyethylene melt. This parallels observations of the effects of re peated shear modification on the properties of polyethylene films and other extrudates.