Flow-induced microstructures in the spinning of carbonaceous mesophase, a discotic nematic liquid crystal

arbon fibers produced by spinning the carbonaceous mesophase, a discotic liquid crystal, can attain exceptional levels of orientation of the graphitic microstructure, and thus reach high levels of elastic modulus and thermal conductivity. The objectives of this thesis are to study the formation of mesophase microstructures in fiber spinline and to explore the limits to which the flow memory of mesophase pitch can be utilized to manipulate the microstructure of mesophase carbon fibers. The approach has been primarily experimental. Mesophase flowing in an expendable spinneret was quenched to trap flow structures in place. Reflected polarized-light micrography on polished sections was applied to analyze the flow-induced microstructures, and to trace structural features to their origin in the spinline. Dark-field TEM was used to carry the micrographic analysis to the level of spun filaments.;The rheological memory of mesophase is very strong, as observed in spinning experiments in which an orthogonal grid-like pattern was imposed by a plain-weave screen placed above the spinneret capillary. The grid pattern results from the merging of adjacent streams through the screen, leading to the formation of weld boundaries of lamellar microstructure. Dark-field TEM observations showed that the grid pattern can be retained with little change to spun fiber, where the microstructures can be resolved in terms of their constituent disclinations.;Shear flow, in the capillary or in passage through a screen introduces instabilities that misorient mesophase layers from the flow direction. A fine-banded microstructure with the layers in zig-zag radial arrays forms in the capillary. The bands tend to narrow in the high-shear-rate region near the capillary wall.;In sharp contrast to shear flow, elongational flow in the entrance to the capillary or in the draw-down cone reorients the mesophase layers to the flow direction and imposes a radial preferred orientation. Shear in the capillary modulates the radial preferred orientation to a set of rings that grow in number as mesophase flows down the capillary. Micrographic analysis of the preferred orientation rings shows the structures to consist of alternating rings of...