MICROSTRUCTURE IN A SERIES OF MESOPHASE PITCH-BASED FIBERS FROM DU-PONT - ZONES, FOLDS, AND DISCLINATIONS

The microstructure of seven types of Du Pont fibers were studied using light, scanning, and transmission electron microscopy. The dominant microstructure in transverse section is of an oriented core region and a banded sheath. The orientation of layering within the sheath zig-zags about the radial direction in a regular pattern of folding corresponding to the banded structure. A transition zone occurs between the core and the sheath, and layering is often quite convoluted. An irregular pattern of finer scale folding, and a high density of splays and disclinations, also occur throughout the fiber. The microporous phase is absent and there is no evidence of abutments in the layering. It is concluded that these fibers were spun from mesophase containing no isotropic phase. The pattern of flow during spinning probably causes the regularly folded structure and modifies any pre-existing fine-scale folds and disclination structure. Consideration of the effects of all of these fine-scale microstructures upon fiber strength suggests that fold separation is the feature which critically controls strength. The microstructures in the Du Pont fibers may all help to give improved strength compared to other fibers, but other factors, such as reduced size and density of inherent flaws, cannot be discounted.