Linear viscoelasticity of textured carbonaceous mesophases

Abstract

High performance carbon fibers can be produced from anisotropic petroleum pitch using the fiber spinning process. The mesophase pitch is a thermotropic discotic nematic liquid crystal possessing orientational order. Liquid crystals are textured, anisotropic, viscoleastic materials. An important phenomenon of liquid crystals is the interaction between orientational textures and rheology, where the texture is due to the presence of orientational defects. The fiber spinning process used in the carbon fiber manufacture from nematic precursors uses a cascade of processing flows that involves capillary, convergent, and extensional flows, which promotes the formation of a variety of textures that will give specific properties to the resultant carbon fiber. Hence a fundamental understanding of the impact of textures on rheology is needed for eventual process control and product optimization. In this paper we study the interaction of rheology and texture and develop a model of anisotropic linear viscoelasticity of textured mesophases. The model is able to predict all the features observed experimentally, including the frequency dependency of the loss and storage modulus and their temperature sensitivity, and provides a better understanding of the role of defects and anisotropy on pitch viscoelasticity.