Extensional stress growth and stress relaxation in entangled polymer solutions. P.K. Bhattacharjee (1), D.A. Nguyen, T. Sridhar (1), G.H. McKinley (2) (1) Monash University, Department of Chemical Engineering, Clayton, Victoria 3800, Australia (2)Massachusetts Institute of Technology, Department of Mechanical Engineering, Cambridge, MA 02139, U.S.A. We report an evaluation of the double constraint release model with chain stretch (DCR-CS) suggested by Ianniruberto and Marrucci [J. Rheol. 45 (2001)], in predicting the transient stress growth and stress relaxation behaviour of two well-characterised entangled polymer solutions undergoing homogeneous uniaxial extensional flow. The experiments are conducted using a filament stretching rheometer. The DCR-CS model belongs to a family of simplified single-segment models that incorporate constraint release, double reptation and segmental stretching into the basic reptation mechanism proposed in the original Doi-Edwards theory and seeks to extend the predictive capacity of the theory to more complex flow fields. We show that the single-mode DCR-CS differential model performs well in predicting the transient extensional stress growth and steady-state extensional viscosity over a range of stretch rates. The model also predicts the observed stress relaxation following cessation of stretching satisfactorily. We further show that the model predicts shear-thickening even in steady shear flow.