Preferred orientation of pitch precursor fibers and carbon fibers prepared from isotropic pitch

Abstract

The preferred orientation of pitch precursor fibers and carbon fibers from an isotropic pitch was studied using x-ray diffraction techniques. The φ scan profile of a pitch precursor fiber was divided into two parts: φ scan-dependent (anisotropic) and φ scan-independent (isotropic) profiles. The half width at half maximum (HWHM) intensity of the anisotropic part became smaller as the pitch viscosity increased during spinning and as the filament diameter decreased. The area ratio of the anisotropic part to the total area increased when spinning at higher pitch viscosities and when making the diameter thinner; no preferred orientation existed for pitch precursor fibers with very large diameters. A thermodynamic model is proposed for the preferred orientation formation when spinning an isotropic pitch. The carbon fiber exhibited no skin-core difference in preferred orientation. A crystallite with a larger crystallite size L_c(002) in the carbon fiber was shown to be better aligned along the fiber axis than that with a smaller L_c(002). Furthermore, a well-aligned crystallite possessed a larger L_c(002) than one that was misaligned. The pitch precursor fiber also exhibited such a correlation between L_c(002) and the preferred orientation, but the correlation was weak.