Domain structure in MP (mesophase pitch)-based fibres

Abstract

The microstructure of a number of commercially available (Union Carbide, Thornel, or Amoco) mesophase pitch-based (MP) carbon fibres was investigated using light, scanning and transmission electron microscopy. Size, shape, and distribution of structure were examined in longitudinal and transverse sections. Because microtome sectioning methods produced considerable structural damage, a technique was developed for preparing ion-milled transverse sections for TEM. A domain structure is very common, perhaps ubiquitous, in MP fibres. Domains are long, thin needle-like or ribbon-like structural components of fibres that extend up to 100 μm parallel to the fibre axis and may be elongate in transverse section, the longer axis being 0.5 to 3 μm while the shorter axis is typically 0.2 μm. The fabric of domains is identified from textures in transverse polished and fractured sections. Many MP fibres examined have domains arranged in an oriented-core texture. This texture consists of polar zones where layering of domains is chaotic separated by an equatorial zone of layering continuous across each filament. PAC-man morphology can develop in some oriented-core filaments by radial failure in the flat equatorial regions. Not all domains are of the same kind. In dense domains, graphene layers are densely stacked with comparatively little void space, while microporous domains consist of thin walls of graphene sheets enclosing a high proportion of void space. Graphene layers within dense domains are oriented predominantly parallel both to the long axis of the domain in transverse section and to the fibre axis. Graphene layers in microporous domains have no preferred orientation. Dense domains represent former mesophase, while microporous domains represent a phase inferred to have been capable of forming at least some mesophase with generation of volatile substances. In some MP fibres, ordering in dense domains is turbostratic. In others, true graphite with three-dimensional crystal structure occurs, not as a separate domain type, but as a component of the domains described. All domains appear to originate at the time of pitch maturation (mesophase forming) and are considerably modified during spinning. Domains and their distribution influence mechanical properties. Factors such as the state of ordering within domains, volume fraction and distribution of microporous material and presence of folded and kinked layering and of flaws will modify processes like initiation and propagation of fracture and affect properties such as elastic modulus.