Elongational rheology of polyethylene melts

Effective elongational viscosities were measured for high- and low-density polyethylene samples using a capillary rheometer fitted with semihyperbolic dies. These dies establish a purely elongational flow field at constant el ongational strain rate. The effective elongational viscosities were evaluat ed under the influence of the process strain rate, Hencky strain, and tempe rature. Enthalpy and entropy changes associated with the orientation develo pment of semihyperbolic-processed melts were also estimated. The results sh owed that elongational viscosities were primarily affected by differences i n the weight-average molecular weight rather than in the degree of branchin g. This effect was process-strain-rate- as well as temperature-dependent. A n investigation of the melt-pressure relaxation and the associated first de cay time constants revealed that with increasing strain rate the molecular field of the melt asymptotically gained orientation in approaching a limit. As a result of this behavior, molecular uniqueness became much less distin ct at high process strain rates, apparently yielding to orientation develop ment and the associated restructuring of the melt's molecular morphology. ( C) 2001 John Wiley & Sons, Inc.