An experimental appraisal of the Cox–Merz rule and Launʹs rule based on bidisperse entangled polystyrene solutions

The empirical relationship provided by the Cox–Merz rule or Launʹs rule has frequently been employed for linking rheological data collected in steady-state shear experiment to those in dynamic oscillatory experiment for entangled polymer liquids. Good applicability of these empirical rules has generally been found, especially for nearly monodipserse systems; for polydisperse systems, however, discrepancies have occasionally been reported without explanations. To throw some light on the impact of blending on the applicability of these empirical rules for entangled polymer liquids, we scrutinize simultaneously the steady state and dynamic oscillatory data on a series of bidisperse entangled polystyrene solutions. The primary finding is that deviations from the predictions of these empirical rules are generally observable as the long chain becomes markedly dilute, especially for the case of Launʹs rule. The phenomenological effects of blending are tentatively interpreted in view of the contribution of chain stretching.