Optical Anisotropy in Viscoelastic Carbon Nanotube Suspensions
D. Fry and E. K. Hobbie
Polymers Division, Materials Science and Engineering Laboratory
Carbon nanotubes, having remarkably high tensile strength, large aspect ratio, and enhanced electrical and thermal properties, continue to hold much promise as "filler" particles for new classes of composite materials. Processing of these materials, such as plastic composites, often subjects nanotube/polymer suspensions to simple shear flow. Structural characterization of flowing suspensions has been done with in situ microscopy and small angle light scattering. However, relatively little is known about the intrinsic optical properties of the tubes themselves, namely the degree of tube polarizability. By extending a previously developed metrology to include rheo-optic measurement of the flowing suspension, combined with a first principles macroscopic theory, we have gained insight into the relative difference in tube polarizability normal and parallel to the tube axis. Measurements have been performed on several different concentrations of multi-walled carbon nanotubes suspended in PIB Boger fluids. Over a range of shear rate, samples exhibit optical anisotropy, via a measured birefringence and dichroism, substantially larger than the pure fluid. The magnitude of optical anisotropy is dependent on tube concentration and shear rate. The shear rate dependence is non-linear, and appears be coupled to ordering of the tubes in the flow direction, and linked to the rheological response of the fluid phase.
Materials Science and Engineering Laboratory
Building 224, Room A219
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