Phase-contrast microscopy, small-angle light scattering, and fluorescence microscopy have been combined in situ to study domain deformation, breakup, and homogenization in unstable mixtures of polystyrene (PS) and polybutadiene (PB) under shear flow. Close to the critical point, mixing of the two components toward a single homogeneous phase occurs via repeated deformation and fragmentation of minority-phase droplets, and the data are in good agreement with the mode-coupling renormalization-group (MCRG) theory of a simple binary mixture under shear flow. Well into the two-phase region of diluted high-molecular-weight PS/PB blends, however, the data suggest a tendency for T c(γ) to saturate at very high shear rates.
The miscibility of a diluted polymer blend under steady shear has been investigated in the twophase region using fluorescence and phase-contrast microscopy. Critical exponents describing the shape of the coexistence curve and the shift of the critical temperature DT c ͑ ᠨ g͒ were compatible with expectations based on renormalization group and mode-coupling theories, but the introduction of a reduced variable description of DT c suggested a tendency for T c to saturate at high shear rates.[S0031-9007 (97)02774-9] PACS numbers: 83.80.Es, 64.75. + gThe effects of shear on the critical temperature T c has been studied in small molecule binary mixtures as well as polymeric blends [1][2][3][4]. Because of the relatively large influence of shear and the slow dynamics of polymeric systems, it has been possible to perform more comprehensive measurements on bulk [3] and diluted [4] polymer blends using a combination of small angle neutron scattering and dynamic light scattering techniques. A comparison [5] of experimental results obtained in the one phase region with the theory of Onuki and Kawasaki [6] indicates a remarkable consistency. However, many questions remain about the influence of shear on the phase stability of mixtures. For example, the combined influence of additives (e.g., solvent for the blend) and shear on the phase stability of mixtures and the nature of mode-coupling effects in the unstable regime have received limited attention. The present paper considers these problems based on the existing Onuki-Kawasaki framework.Previous measurements [2] on diluted polymer blends in the unstable regime under conditions of steady shear showed a marked decrease of scattering intensity at high rates of shear. This shear-induced "homogenization" effect was interpreted in terms of a shift of the phase boundary with shear, as in earlier shear-scattering measurements on small molecule mixtures in the one-phase region [1]. The effect of droplet distortion on the shear-scattering measurements was minimized by considering the projection of the scattering data along the vorticity direction. Estimates of the integrated intensity along this direction from shear light scattering measurements showed a tendency to strongly decrease and then to saturate at high shear rates. The characteristic shear rate ᠨ g c , corresponding to this saturation point, was then employed along with theoretical arguments to estimate the shift of the phase boundary with shear. While this procedure led to estimates of the critical temperature shifts with shear that were apparently consistent with the Onuki-Kawasaki (OK) theory [6], it seems unclear to us whether the diminished scattering intensity ("homogenization") reflected a true shift of the phase boundary or merely the breakup of droplets under shear to a scale undetectable by light scattering measure-ments. Since this kind of scattering intensity measurement depends on the history of the shearing measurement, the procedure of extrapolation, and the sensitivity of the detection system, it seems nece...
A shear light scattering photometer with an optical microscope was constructed for the study of polymer blends in a simple shear field. This instrument utilizes a cone and plate or parallel plate geometry for the generation of shear field. The shear rates are controlled by a microstepping motor. The controllable range of shear rate is between 0.002 and 1000 s−1. The bottom plate of the shear cell has a special design to accommodate the microscope objective and a thin disk-type heater for temperature control. The accessible q range is from 0.7 to 4.6 μm−1 with a 632.8 nm He–Ne laser or from 0.9 to 6.0 μm−1 with a 488 nm Ar ion laser. The temperature can be controlled from ambient temperature to 250 °C with ±0.1 °C accuracy. A phase contrast microscope and a fluorescence microscope are built into this photometer for the in situ morphological study of materials of interest. The optics for light scattering and microscopy can be switched back and forth by a simple translational movement of a rail-mounted optical platform, without any realignment, for comparison of data from reciprocal space with that from real space. A bulk polystyrene/polybutadiene blend and a polystyrene/polybutadiene/dioctylphthalate blend were used to demonstrate the performance and versatility of this instrument.
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