Rheo-Optiics for Multicomponent Liquids
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RHEO-OPTIICS FOR MULTICOMPONENT LIQUIDS G. Fuller, J. van Egmond, J. Zawada, and L. Archer Department of ChemicaB Engineering Stanford University Stanford, CA 94305-5025 ABSTRACT The application of techniques in optical rheometry for the study of multicomponent systems is reviewed. Small angle light scattering (SALS) patterns are related to the structure of concentration fluctuations with length scales of the order of the wavelength of light. Scattering techniques such as SALS and scattering dichroism have been applied to monitor the transient evolution of anisotropic concentration fluctuation enhancement during simple shear induced phase separation in a semi-dilute solution of polystyrene (PS) in dioctyl phthalate(DOP). Furthermore, the OnukiDoi theory relating scattering dichroism and structure factor has been used to verify the consistency between scattering dichroism and anisotropy in structure factor. Infrared polarimetry is a useful technique in probing the transient microstructural orientation of individual chemical species in multicomponent systems. The simultaneous measurement of intrinsic infrared dichroism and birefringence is particularly effective and has been employed to monitor component relaxation dynamics in miscible blends of poly(ethylene oxide) and poly(methyl methacrylate). Polarization Modulated Laser Raman Scattering (PMLRS) has been successfully employed to study the orientation dynamics of a polymer melt subjected to transient uniaxial extension. PMLRS provides quantitative information about the time evolution of both the second and fourth moments of the orientation distribution function of molecular segments. .LOINTRODUCTION This paper reviews methods in optical rheometry designed to measure the dynamics of multicomponent liquids. Such liquids are characterized by dynamics associated with specific structural constituents that should be separated to properly understand the response of these materials to external fields. For the purpose of this paper, this separation can refer to the isolation of the dynamics as a function of length scales existing in the liquid, or associated with specific chemical components. The former class of systems would include phase separated blends where the response of the domain structure must be isolated. Other examples would include the grain structure in block copolymers, suspensions and polymer liquid crystals. Scattering techniques are the obvious choices for these cases and for structural length scales in the range of 0.5 to 20 4tm, small angle light scattering and scattering dichroism can be used for this purpose. Spectroscopic techniques are required when the dynamics of individual chemical elements must be identified. Here infrared polarimetry and Raman scattering are discussed and applied to single phase polymer blends and melts. 2.0 TRANSMISSION EXPERIMENTS: THE REFRACTIVE INDEX TENSOR The interaction of light with a sample can be measured in transmission, scattering and reflection. In transmission it is the refractive index tensor that is normally mea
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