The orientation of mesophase pitch during fully developed channel flow

Abstract

Mesophase pitch-based carbon fibers are ideally suited for applications where the rapid dissipation of heat is important (structural composites, printed circuit substrates, etc.). However, control of texture is essential in order to optimize fiber properties, especially thermal conductivity. Because ribbon-shaped mesophase pitch-based carbon fibers possess a highly linear texture, these fibers are very promising for high thermal conductivity applications. The observed texture of these ribbon-shaped fibers and the radial texture commonly observed in circular fibers can be predicted from liquid-crystal flow theory, where the precursor molecules are modeled as rigid disks.In order to determine the orientation of flowing mesophase pitch, a capillary rheometer was modified to quench capillaries during extrusion. Aluminum capillaries were constructed with rectangular channels down the center (with a 9:1 aspect ratio). The capillaries containing the mesophase samples were mounted in low-viscosity epoxy resin and cured. The desired mesophase surface (transverse or longitudinal) was isolated by grinding and polishing the samples to 3 gmm. Then the pitch surface was observed optically under cross-polarized light. The transverse surfaces of mesophase pitch indicated an orientation pattern where the aromatic rings lie perpendicular to the channel walls. This result is consistent with that predicted by the liquid-crystal flow theory, as well as with the textures of carbonized fibers.