Liquid crystal surface anchoring of mesophase pitch Original Research Article

Mesophase pitch, in common with other liquid crystalline substances, exhibits preferred angles of molecular orientation at its boundaries with other phases. These orientations, or surface anchoring states, are important because they influence the ultimate graphene layer arrangement in a variety of carbon materials where the pitch precursor encounters a composite filler, a free surface, a bubble cavity, or the surfaces of processing equipment such as a fiber spinneret. This paper presents experimentally determined anchoring states for two mesophase pitches at free surfaces, and on twenty solid substrates. Edge-on anchoring is found to be the most common state, occurring on the free surface, on some metals, on PTFE, and on all oxides with the exception of the lamellar material mica. The optical texture associated with the edge-on films is observed to be stable during carbonization up to 1200 °C. Face-on anchoring is observed on carbon graphene planes, mica and the metals Pt, Ni, and Ag. Trends in the data are discussed in terms of the strength of pitch/substrate intermolecular forces relative to π–π bonding between large discotic mesogens within the pitch. The implications for the structure and properties of carbon materials are discussed.