Observations on a low-angle X-ray diffraction peak for AR-HP mesophase pitch

Using wide angle x-ray diffraction on AR-HP mesophase pitch, a diffraction peak was detected at approximately 7° (2Θ). The low-angle peak was reproducible using different diffractometers and different sample-to-detector distances, and was observed for all pitch samples irrespective of the flow history, but not for carbon fibers derived from the same precursor fibers. The interplanar 002 peak at ∼25° was observed at the equatorial position, whereas, the peak at 7° was in the meridional position, i.e., the peaks are nominally 90° apart in an azimuthal scan. This signifies that the low angle peak was due to diffraction from planes that are orthogonal to (002) planes. The 7° peak is equivalent to a d-spacing of 1.25 nm, which corresponds to the nominal lateral dimension of the mesogen molecules that constitute the mesophase pitch. A typical MALDI analysis showed predominance of molecular weights of ∼500 au. Edges of such a constituent of mesophase pitch molecules, which lead to a lateral ordered spacing of approximately 1.1 nm, can lead to a diffraction peak at approximately 7°.