Shear smallangle light scattering studies of shearinduced concentration fluctuations and steady state viscoelastic properties
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Abstract
We aimed at elucidating the influence of shearinduced structures (shearenhanced concentration fluctuations and/or shearinduced phase separation), as observed by rheooptical methods with smallangle light scattering under shear flow (shearSALS) and shearmicroscopy, on viscoelastic properties in semidilute polystyrene (PS) solutions of 6.0 wt % concentration using dioctyl phthalate (DOP) as a Theta solvent and tricresyl phosphate (TCP) as a good solvent. In order to quantify the effects of the shearinduced structures, we conducted a numerical analysis of rheological properties in a homogeneous solution based on the constitutive equation developed by KayeBernstein, Kearsley, and Zapas (KBKZ). In the lowtointermediate shear rate gammadot region between tauw−1 and taue−1, where tauw and taue are, respectively, terminal relaxation time and the relaxation time for chain stretching, the steady state rheological properties, such as shear stress sigma and the first normal stress difference N1, for the PS/DOP and PS/TCP solutions are found to be almost same and also well predicted by the KBKZ equation, in spite of the fact that there is a significant difference in the shearinduced structures as observed by shearSALS and shearmicroscopy. This implies that the contribution of the concentration fluctuations built up by shear flow to the rheological properties seems very small in this gammadot region. On the other hand, once gammadot exceeds taue−1, sigma and N1 for both PS/DOP and PS/TCP start to deviate from the predicted values. Moreover, when gammadot further increases and becomes higher than gammadota, DOP (sufficiently higher than taue−1), above which rheological and scattering anomalies are observed for PS/DOP, sigma and N1 for PS/DOP and PS/TCP are significantly larger than those predicted by KBKZ. Particularly, a steep increase of sigma and N1 for PS/DOP above gammadota, DOP is attributed to an excess free energy stored in the system via the deformation of interface of welldefined domains, which are aligned into the stringlike structure developed parallel to the flow axis, and stretching of the chains connecting the domains in the stringlike structures. Thus, we advocate that the effect of shearinduced structures should be well considered on the behavior of sigma and N1 at the high gammadot region above taue−1 in semidilute polymer solutions.
Journal

 JOURNAL OF CHEMICAL PHYSICS

JOURNAL OF CHEMICAL PHYSICS 128(16), 20080428
American Institute of Physics