Structure memory effects and rheological behaviors of polyethylenes in processing temperature window

Abstract

AbstractIn this work, both hexene copolymerized polyethylene of TR‐571 and homopolymerized polyethylene of 5000s were ram extruded using a temperature window effect. The rheological results show that the extrusion pressures abruptly drops at very narrow extrusion temperature windows for the two types of polyethylenes, but the window range (150–155°C) of TR‐571 are higher about 6°C than that of 5000s, and the threshold melt velocity value (where the window effect set in) for TR‐571 is remarkably lower than that of 5000s. Besides, the differential scanning calorimeter experiments indicate that the melting points (Tm) of the extrudates obtained inside the temperature window are lower 3.74–4.56°C than those obtained outside, whereas the crystallization peak temperatures (Tc) of the former are higher 3.29–3.57°C than those of the later. The crystal orientation, shown in the photographs of wide‐angle x‐ray diffraction and scanning electron microscope, indicates that the chain alignment induced by elongational flow in the temperature window can at least be partially maintained and leave a structure memory in the melt state. Successive self‐nucleation and annealing fractionation results suggest that these well‐aligned chains as the result of structure memory effect can act as a nucleating agent in the crystallization process. Thus, the number of the lamellae increases and the thickness decrease, which finally leading to higher Tc and lower Tm of extrudates obtained inside a temperature window. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013