Local correlations in the orientation of neighboring molecules have been shown to exist both experimentally and theoretically for polymer melts, blends and networks. Such nematic interactions alter the stress-optic coefficient, but predict no change in the overall stress in long time scales in the linear viscoelastic regime. The impact of nematic effects on the extensional stress-strain response of concentrated polymer solutions has not been experimentally investigated. In this work, we consider the influence of several solvents on the linear and nonlinear rheological responses of concentrated polymer solutions in extensional flow. We prepared three polystyrene (PS) solutions with identical concentrations of the same PS sample (with the molecular weight M = 545k), but diluted with three different solvents, oligomeric styrene (OS) with M = 1k, 2k, and 4k. The three solutions have exactly the same physical tube model parameters when compared in the same, normalized time scale. Although the three solutions behave identically in small amplitude oscillatory shear flow, their behavior is markedly different in extensional flow covering large strains. The solution in OS 1k solvent is significantly more strain hardening than the solution in OS 4k under similar conditions. The experimental observations presented here directly demonstrate that the tube model and its governing parameters are insufficient to describe the nonlinear extensional behavior of entangled polymer solutions. We propose a hypothesis that the nematic interactions among the polymers and between polymer and solvent are in part responsible for the nonlinear rheological response of concentrated polymer solutions in strong extensional flow.
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85th Annual Meeting of The Society of Rheology, 2013