Beginning with a recently proposed expression for the drag force on a single macromolecule pulled with constant velocity through a fluid of long-entangled molecules (V. R. Mhetar and L. A. Archer, Macromolecules 1998, 31, 6639), we investigate the effect of entanglement loss on polymer dynamics in steady shearing flows. At steady-state, a balance between the elastic restoring force and viscous drag acting on entangled polymer segments reveals a critical molecular strain ␥ m,c beyond which the drag force exerted on polymer molecules by their neighbors is insufficient to support arbitrarily small orientation angles. Specifically, we find that in fast steady shear flows
, polymer orientation in the shear plane approaches a limiting angle c Ϸ atau(1/(1 ϩ ␥ m,c )) beyond which flow becomes incapable of producing further molecular alignment. Shear flow experiments using a series of concentrated polystyrene/diethyl phthalate solutions with fixed entanglement spacing, but variable polymer molecular weight 0.94 ϫ 10 6 Յ M w Յ 5.48 ϫ 10 6 , reveal a limiting steady-state orientation angle between 6°and 9°over a range of shear rates; confirming the theoretical result. Orientation angle undershoots observed during start-up of fast steady shearing flows are also explained in terms of a transient imbalance of elastic restoring force and viscous drag on oriented polymer molecules. Our findings suggest that the Doi-Edwards affine orientation tensor (Q) is not universal, but rather depends on deformation type and deformation history through a balance of elastic force and viscous drag on polymer molecules.