Anomalous nonlinearities in steady shear of polyethylene melts

High-density polyethylene (HDPE) melts have been previously identified as having an unusually high degree of molecular order, but the rheological implications of this have heretofore not been explored. This has been a serious oversight, inasmuch as HDPE melt processing is one of the major tasks of the world's polymer industry. Here, we investigate several industrial samples of HDPE, covering a range of molecular weight (8.5 10 < Mw < 13.7 10) and temperature (1908 T 2508C) using a Rheometrics Mechanical Spectrometer 800. Remarkable manifestations of liquid-state order are found that agree with theoretical predictions for a liquid-crystal polymer (LCP). These include indications of an approach to a sign change in N1( _ ) at low values of the steady-shear rate, _ , and a kink in the Non-Newtonian viscosity ( _ ). Some evidence on transient normal stress and on dynamic properties G0(!) and 0(!) is also presented. The collective data appear to confirm that the HDPE melt may be considered to be a LCP, and also that the LCP rheological theories can be applied. # 1999 Elsevier Science B.V. All