In this article, we compare and contrast the theological and microstructural entanglement properties of a series of linear-chain polyethylene liquids under both planar Couette and planar elongational flow. We measure and compare the viscosities of the liquids in the two types of flow, and notice that both exhibit thinning behavior with increasing strain rate as the chains elongate and orient within the flow field. From the microstructural perspective, we examine the contributions of the chain energetics, such as bond-bending and bond-torsion, to the stress tensor and the degree of extension of the chains, as well as to the overall chain flexibility. Furthermore, entanglement characteristics, such as the shortest primitive path length, and the network configurations, are investigated - for the first time - as functions of strain rate in both vastly different flow fields. We expect the qualitative findings to be relevant to the understanding of entangled polymer melts, even though the available molecular weights are still below, or on the order of, the entanglement molecular weight. (C) 2007 Elsevier B.V. All rights reserved.