Rheologica Acta Rheol Acta 32:1-8 (1993) Original Contributions Flow-induced concentration fluctuations in polymer solutions: Structure/property relationships P. Moldenaers 1, H. Yanase 1, J. Mewis l, G.G. Fuller 2, C.-S. Lee 3 and J.J. Magda 3 ~Department Chemische Ingenieurstechnieken, Katholieke Universiteit Leuven, Leuven, Belgium 2Department of Chemical Engineering, Stanford University, Stanford, California, USA 3Department of Chemical Engineering, University of Utah, Salt Lake City, Utah, USA Abstract." Mechanical and optical rheometric measurements are reported on solutions of polystyrene dissolved in dioctyl phthalate, a solution that can undergo an apparent phase separation upon the application of shear. Solutions prepared using three molecular weights ranging from one to four million were studied. Time-temperature superposition was observed to apply for these solu- tions up to and including the onset of an apparent shear thickening of the steady shear and first normal stresses. Optical measurements employing turbidity and scattering dichroism determined that concentration fluctuations were enhanced by flow and grew parallel to the vorticity axis below the critical velocity gradient for the onset of the apparent shear thickening effect. Prior to the onset of thickening, the fluctuations were observed to rearrange and orient parallel to the flow direction. Second normal stress difference measurements indicate these solutions have a negative ratio of the second to the first normal stress dif- ferences. It is interesting to point out that the ratio tends to zero in the vicinity of the shear rate range at which shear thickening occurs. Key words." Phase behavior - rheo-optics - second normal stress 1. Introduction In a previous paper (Yanase et al., 1991), evidence of flow-induced growth of concentration fluctuations was offered using a combination of optical and mechanical rheometric methods. That work com- plements several other studies on solutions formed by polystyrene (PS) and dioctyl phthalate (DOP) where it has been observed that application of flow results in a reversible loss of transparency (ver Strate and Philippoff, 1974; Rangel-Nafaile et al., 1984) and pronounced, anisotropic structure factors as mea- sured by scattered light (Yanase et al., 1991; Hashimoto and Fujioka, 1990; Wu et al., 1991). This specific polymer/solvent mixture is characterized by a theta temperature of 22°C (Park and Berry, 1989) and a cloud point temperature in the vicinity of 9 °C to 12 °C, depending on molecular weight and concen- tration (Rangel-Nafaile et al., 1984). In Yanase et al. (1991), measurements of scattering dichroism in the (1,3) plane for a shear field described by ui = 7x2&il demonstrated that concentration fluc- tuations grow parallel to the vorticity axis. Birefrin- gence measurements, probing local segmental orienta- tions, on the other hand, revealed that the chain segments were oriented in the direction of flow in that plane. This observation of the orientation of concen- tration fluctuations is consistent with small angle light scattering (SALS) measurements reported in Yanase et al. (1991) and Hashimoto and Fujioka (1990). In both studies, the SALS patterns were found to be in the form of elongated lobes arranged parallel to the flow direction. Such patterns infer scattering struc- tures aligned in an orthogonal orientation, parallel to the vorticity axis. The scattered light structure factors presented by Wu and coworkers (1991) were made in the plane of the flow (the (1,2) plane). These patterns bear a qual- itative similarity to the SALS patterns measured in the (1,3) plane, being also formed of two lobes. In the (1,2) plane the lobes achieve specific orientations relative to the flow direction and for weak flows have a limiting orientation of 45 ° relative to the xl axis. As the magnitude of the flows is increased, the lobes