Phase transition in high molecular weight polyethylene during capillary extrusion: the reversibility of the temperature window

We present results confirming the existence of discontinuity in extrusion pressure, with temperature, within a very narrow temperature window (150-152°C) during the extrusion of high molecular weight polyethylene melts. This pressure minimum effect, the appearance of which is critical in both molecular weight (M_w) and shear rate (γ), is now found to be reversible with temperature. Further, our work shows, in a quantitatively documented manner, that die swell is much reduced in the temperature window, corroborating the previously proposed interpretation that the effect is associated with a new phase with liquid crystal characteristics, which is the mobile hexagonal phase. Finally, we report on a structure-related memory effect, observed during the cooling cycle, which is interpreted in terms of structures created during the preceding heating cycle adhering to the flow vessel and persisting in the melt regime up to 180°C.