Shear small-angle light scattering studies of shear-induced concentration fluctuations and steady state viscoelastic properties

We aimed at elucidating the influence of shear-induced structures (shear-enhanced concentration fluctuations and/or shear-induced phase separation), as observed by rheo-optical methods with small-angle light scattering under shear flow (shear-SALS) and shear-microscopy, on viscoelastic properties in semidilute polystyrene (PS) solutions of 6.0 wt % concentration using dioctyl phthalate (DOP) as a solvent and tricresyl phosphate (TCP) as a good solvent. In order to quantify the effects of the shear-induced structures, we conducted a numerical analysis of rheological properties in a homogeneous solution based on the constitutive equation developed by Kaye-Bernstein, Kearsley, and Zapas (K-BKZ). In the low-to-intermediate shear rate γ region between τw -1 and τe -1, where τw and τe are, respectively, terminal relaxation time and the relaxation time for chain stretching, the steady state rheological properties, such as shear stress 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 K-BKZ equation, in spite of the fact that there is a significant difference in the shear-induced structures as observed by shear-SALS and shear-microscopy. This implies that the contribution of the concentration fluctuations built up by shear flow to the rheological properties seems very small in this γ region. On the other hand, once γ exceeds τe -1, and N1 for both PS/DOP and PS/TCP start to deviate from the predicted values. Moreover, when γ further increases and becomes higher than γa,DOP (sufficiently higher than τe -1), above which rheological and scattering anomalies are observed for PS/DOP, and N1 for PS/DOP and PS/TCP are significantly larger than those predicted by K-BKZ. Particularly, a steep increase of and N1 for PS/DOP above γa,DOP is attributed to an excess free energy stored in the system via the deformation of interface of well-defined 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 shear-induced structures should be well considered on the behavior of and N1 at the high γ region above τe -1 in semidilute polymer solutions.