Nonlinear viscoelasticity of entangled polymeric liquids

The Doi-Edwards (D-E) theory for entangled polymer dynamics is extended to account for polymer chain stretching in fast flows. Our analysis is based o n a new estimate for the non-equilibrium size of the tube under a macroscop ic deformation and yields an optimum primitive chain contour length [\E . u \(1/2)] times greater than the equilibrium (no-flow) value. This finding im plies that polymer chains subjected to nonlinear step strain will only retr act partially and should therefore maintain significant chain stretch up to the time the primitive chain escapes the deformed tube. Primitive chain re traction is also shown to reduce the number of entanglements between a defo rmed polymer chain and its neighbors, yielding significant enhancement of m olecular relaxation at high deformations. Various steady-state and transien t stress predictions based on this partial strand extension concept are com pared with existing experimental data. (C) 1999 Elsevier Science B.V. All r ights reserved.