Shear flow induced microstructure of a synthetic mesophase pitch

Abstract

Carbon fibers and composites derived from mesophase pitch exhibit ultrahigh stiffness and thermal conductivity due to a high degree of graphitic content, which is generated by the liquid crystalline state of the precursor and the molecular orientation that is developed during melt processing steps. To understand the flow and its effect on microstructure, this paper presents an integrated experimental and modeling approach for a synthetic discotic mesophase pitch (AR-HP). Careful control of shear rate and strain was exercised throughout the rheological studies. Wide-angle x-ray diffraction studies were conducted on carefully solidified rheological specimens to obtain azimuthal profiles for layer plane orientation. Cross-polarized microscopy was conducted in the reflected mode using a first-order red plate to examine the orientation in three orthogonal planes. Under the influence of steady shear, the microstructure of mesophase pitch became flow-aligned, which is similar to the fibrous structure reported in the literature. Transient stress response, however, displayed a nonmonotonic behavior. Microscopic observations indicate that the local maximum in the shear stress is likely caused by the deformation of the initial microstructure. To develop a better understanding of these complex flow dynamics, simulations based on the Landau–de Gennes nematodynamics adapted to discotic mesophases were performed, and a qualitative agreement between simulations and experiments was found. The experimentally obtained rheostructural results and the numerical simulations provide a systematic understanding of flow-microstructure relationships during transient and steady shear flow.