Effect of Solid-State Extrusion on Melting Behavior and Morphology of High-Density Polyethylene

Abstract

The effect of solid-state extrusion on the melting behavior of high-density polyethylene has been investigated in detail. It was found that ΔTm, the increase in melting temperature, and ΔHf, the increase in the heat of fusion, both tend to saturate for values of the extrusion ratio R>20. We have also determined the orientation in the crystalline and the amorphous phases through X-ray and birefringence measurements. It was found that the orientation function fam for the amorphous phase also saturates for R>20. A careful analysis of our results has brought out that, irrespective of the temperature at which the extrudate is prepared, a universal relationship exists between ΔTm, ΔHf, and fam. It was further found that when an oriented sample is completely etched, it undergoes fusion at the normal melting temperature of the unoriented polymer. All these findings point to the conclusion that the changes in the melting behavior of high-density polyethylene are brought about essentially by the orientation produced in the amorphous phase.