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We report an elastic-light-scattering experiment for a semidilute polymer solution under a uniform laminar shear flow. The concentration fluctuations are greatly enhanced by shear flow and their structure is highly anisotropic in both the weak (yr 1) shear regimes, where y is the shear rate and Xd is the longest relaxation time of the system. The anisotropy is qualitatively different from binary liquid mixtures under a shear flow as a result of the strong coupling between the concentration fluctuations and the hydrodynamic flow.PACS numbers: 82.70. Kj, 66.90.+r, 82.70.Dd It is well known that entangled polymer solutions near phase separation undergo significant changes when subjected to shear flow. The most notable effect is a dramatic increase in turbidity which, since its discovery nearly twenty years ago, has been interpreted as evidence of a shear-induced phase transition. 1,2 There are welldocumented cases of shear-induced changes in the phase behavior of other systems: simple binary liquid mixtures, 3,4 binary polymer melts, 5 and binary polymer solutions. 6 However, in these systems, shear flow suppresses concentration fluctuations, turbidity, and phase separation. Furthermore, the shear-induced effects are much smaller than in polymer solutions. In binary liquid mixtures, for example, the turbidity is reduced by shear only within -0.1 °C of the critical temperature and only for shear rates exceeding y-1000 s -1 . 3 By contrast, in polymer solutions, the turbidity can be enhanced for temperatures of more than 50 °C above the phaseseparation temperature at shear rates of less than 10Recently, the existence of a shear-induced change of the phase transition in semidilute polymer solutions has been called into question by a number of investigators. 7 "" 9 Part of the controversy centers around questions of how to treat the thermodynamics of these nonequilibrium systems. An even more pressing issue is the question of the mechanism for the shear-induced enhancement of the concentration fluctuations which leads to the increased turbidity. Several conflicting mechanisms have been proposed. 2,7,8 However, progress on these problems has been hampered by a lack of detailed experimental information about the structure of sheared polymer solutions.In this paper, we present light-scattering measurements of the nonequilibrium steady-state structure factor S(q,y).Our results support a mechanism proposed by Helfand and Fredrickson 7 in which concentration fluctuations are enhanced by a coupling between the poly-mer concentration and shear flow through the concentration-dependent viscosity and normal stress coefficients. 7,9~n In contrast to previous experiments, we do not find evidence of a shear-induced shift in the phase boundary at low rates of shear. At higher rates of shear, a shear-induced transition remains an intriguing possibility.Our samples consisted of a volume fraction 0=0.04 of polystyrene (PS) dissolved in dioctylphthalate (DOP). The polystyrene has a molecular weight of M W = \....

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The low quantum yield (∼10−5) has restricted practical use of photoluminescence (PL) from MoS2 composed of a few layers, but the quantum confinement effects across two-dimensional planes are believed to be able to boost the PL intensity. In this work, PL from 2 to 9 nm MoS2 quantum dots (QDs) is excluded from the solvent and the absorption and PL spectra are shown to be consistent with the size distribution. PL from MoS2 QDs is also found to be sensitive to aggregation due to the size effect.