Shear-Enhanced Structure and Dynamics in Semidilute Polymer Solutions
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SHEAR-ENHANCED STRUCTURE AND DYNAMICS IN SEMIDILUTE POLYMER SOLUTIONS P. K. DIXON*, D. J. PINE* AND X.-L. WU** * Exxon Research and Engineering Co., Annandale, NJ 08801. ** Department of Physics, University of Pittsburgh, Pittsburgh, PA 15260.
ABSTRACT We present elastic light scattering measurements of a semidilute polymer solution under shear flow. The basic symmetries observed in the shear-enhanced structure factor agree with the predictions of recently proposed theoretical models. The enhancement appears to be due to a coupling between the polymer concentration and shear flow through the concentration-dependent viscosity and normal stress coefficients. We also found an unexpected long wavelength peak in the shear-enhanced structure factor. Transient scattering measurements upon the cessation of shear have revealed the origin of this peak. The polymer solution demonstrates two modes of relaxation for concentration fluctuations in equilibrium due to a coupling between collective diffusion and stress relaxation. The transient scattering measurements indicate that the mechanism that couples the concentration fluctuations to the shear flow, and thereby enhances concentration fluctuations, selectively enhances only the slower of the two equilibrium modes. This selective enhancement generates the long wavelength peak in the shear-enhanced structure.
INTRODUCTION Semidilute polymer solutions subjected to laminar shear flow typically demonstrate a dramatic increase in turbidity.[l] This increased turbidity is due to scattering from enhanced concentration fluctuations. Although the effect is still present far above the phase separation temperature (AT - 50 OC) and at very low shear rates (< 10 s-I), this enhanced scattering has been interpreted as evidence of a shear-induced phase transition.[1,2] Recently, this interpretation has been called into question by a number of investigators.[3-8] In particular, Helfand and Fredrickson (HF) [3] have proposed a mechanism in which concentration fluctuations are enhanced by a coupling between the polymer concentration and shear flow through the concentration-dependent viscosity and normal stress coefficients. In the first section of this paper, we present measurements of the non-equilibrium steady-state structure factor, S(q, Y), that support that the HF theory. [9-11] We see no evidence of a shear-induced shift in the phase boundary at low rates of shear. Nevertheless, a shear-induced nonequilibrium transition remains a possibility at higher rates of strain. While the nonequilibrium steady-state structure of these systems has been found to agree with the basic symmetries of the HF theory, our measurements reveal a distinct peak in the structure that is not accounted for within the theory. Milner has suggested that the model fails in this regard because it neglects the dynamics of the sheared polymer solution.[12] In the second section of this paper, we present transient scattering measurements of the induced structure that
Mat. Res. Soc. Symp. Proc. Vol. 248. 01992 Materials R
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