Theory and Simulation of Fiber Texture Formation and Rheology of Carbonaceous Mesophase Fibers
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Theory and Simulation of Fiber Texture Formation and Rheology of Carbonaceous Mesophase Fibers A. D. Rey Department of Chemical Engineering, Mcgill University, 3610 University Street. Montreal, PQ H3A 2B2, Canada Abstract Carbonaceous mesophases are discotic nematic liquid crystals that are spun into high performance carbon fibers using the melt spinning process. The spinning process produces a wide range of different fiber textures and cross-sectional shapes. Circular planar polar (PP), circular planar radial (PR) textures, ribbon planar radial (RPR), and ribbon planar line (RPL) textures are ubiquitous ones. This paper presents, solves, and validates a model of mesophase fiber texture formation based on the classical Landau-de Gennes theory of liquid crystals, adapted here to carbonaceous mesophases. The effects of fiber cross-sectional shape and elongational flow on texture formation are characterized. Emphasis is on qualitative model validation using existing experimental data [1,2]. The results provide additional knowledge on how to optimize and control mesophase fiber textures. Introduction Carbonaceous mesophases (CMs), such as coal tar and petroleum pitches, are used in the industrial manufacturing of high performance carbon fibers. The thermodynamic phase that describes CMs is the discotic nematic liquid crystal (DNLC) state [3]. The industrial fabrication of mesophase carbon fiber using the conventional melt spinning process typically produces micrometer-sized cylindrical and ribbon filaments whose cross sectional area displays a variety of transverse textures [4], that is, different spatial arrangements of the average orientation n on the plane perpendicular to the fiber axis. The selection mechanisms that drive the texture formation pattern are at present not well understood, but due to the strong structure-properties correlations, they are essential for product optimization [2,4], and are the main topic of this paper. CMs are anisotropic viscoelastic textured materials, and fiber textures are dominated by the interactions of elastic and viscous modes with topological defects. CMs have three elastic storages modes ( splay, twist, and bend ) [5], and the twist mode tends to be avoided due to large energy penalties. Viscous mechanisms also affect texture formation and structure. When considering flow-induced orientation in complex processing flows, the orienting effects of shear and extensional flow have to be taken taken into account. The theoretical rheology of discotic nematic liquid crystals has been widely studied in the past [5-8]. Flow-aligning discotic nematic liquid crystals subjected to simple shear flow orient the director within the shear plane and close to the velocity gradient direction. Furthermore, the effect of uniaxial elongation flow is to align the director along the compression plane. These simple rules are sufficient to qualitatively predict flow-induced orientation. Structural and Geometric Analysis of Observed Carbonaceous Mesophase Fiber Textures Circular cross-section CMs fiber te
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