Elasticity-driven texture selection mechanism in mesophase carbon fibers

Abstract

In this Brief Report, we identify the parametric conditions and elasticity-driven mechanism that give rise to a characteristic texture commonly found during the fiber melt spinning of carbonaceous mesophase precursors; this mesophase consists of a biphasic isotropic-uniaxial discotic nematic liquid crystal mixture. The essential feature of the characteristic texture consists of a radial zig-zag molecular arrangement on a cross section of the mesophase carbon fiber. Using the Williams elastic-driven mechanism [D.R.M. Williams, Phys. Rev. E 50, 1686 (1994)] that gives rise to periodic director oscillations in cylindrical geometries, here adapted to discotic nematic fibers subjected to extensional flow, we show that when the bend ${\mathit{K}}_3$ constant becomes larger than the splay ${\mathit{K}}_1$ constant, the radial trajectories of the molecular planes become unstable in the presence of spatially periodic perturbations, leading to the experimentally observed radial zig-zag texture.