Concept of Stretch/Orientation-Induced Friction Reduction Tested with a Simple Molecular Constitutive Equation

Recent experiments have established the thinning feature of the uniaxial steady state elongational viscosity η E for entangled monodisperse linear polystyrene (PS) melts and the thickening feature for equally entangled PS solutions, both occurring in the same range of strain rate ε4 higher than the equilibrium Rouse relaxation frequency, wR. The classical Doi-Edwards tube theory (DE theory) assuming a constant chain length cannot describe the thickening of the solutions, whereas the extended tube theory of Marrucci and Grizzuti considering the chain stretch does not reproduce the monotonic thinning for the melts. Thus, several molecular mechanisms have been proposed for consistent description of the behavior of the melts and solutions, as explained below. One of the important molecular mechanisms under elongational flow is the finite extensible nonlinear elasticity (FENE). Ye et al. investigated the elongational behavior of PS solutions using a modified Mead-Larson-Doi (MLD) model that incorporates reptation, contour length fluctuation, thermal constraint release (TCR), convective constraint release (CCR) and chain stretch associated with FENE. They showed quantitative agreement of the model with the solution data and reported that η E is sensitive to the maximum stretch ratio. Leygue et al. have developed another tube model to show that the magnitude of elongational thickening decreases with a decrease of the maximum stretch ratio. From this result, one may expect that the difference of the maximum stretch ratio between melts and solutions results in the difference of their elongational behavior. However, the reported value of the maximum stretch ratio to attain the thinning was unrealistically small. Yaoita et al. performed multi-chain slip-link (PCN) simulations with reasonable maximum stretch ratios for polystyrene melts and solutions. They showed quantitative agreement of the simulation with the solution data and confirmed the importance of the maximum stretch ratio for thinning/thickening of η E. However, they also found that a reasonable value of this ratio for melts still leads to the thickening and thus tuning of the FENE factor alone cannot reproduce the difference between the melts and solutions. Marrucci and Ianniruberto focused on the interchain Concept of Stretch/Orientation-Induced Friction Reduction Tested with a Simple Molecular Constitutive Equation